late archean prograde amphibolite to granulite facies relations in the fiskenaesset region southern west greenland the tasiusarsuaq terrane in the fiskenaesset nuuk region of southern west greenland is interpreted as a late archean tilted crustal section modified by deformation the northern part of the terrane was affected by granulite facies metamorphism at about 2810 ma but large areas were subsequently retrogressed to amphibolite facies in the prograde amphibolite to granulite facies transition in inner bjornesund a relatively sharp but highly irregular front that crosses structures and lithological boundaries on outcrop scale separates orthopyroxene free from orthopyroxene bearing rocks outcrop relations resemble in some ways examples of arrested charnockite formation described from high grade regions in southern india and sri lanka relations in the transition zone in bjornesund appear to be the result of late local influx of a fluid presumably rich in co 2 that permitted orthopyroxene to grow at the expense of hornblende regional considerations suggest a genetic connection between granulite faci

petrology and geochemistry of mafic rocks in the acasta gneiss complex implications for the oldest mafic rocks and their origin abstract the acasta gneiss complex located in the western part of the slave province canada is widely recognized as the oldest eoarchean terrane in addition to felsic gneisses with the ages of 3 6 4 0 ga minor mafic rocks occur as rounded to elliptical enclaves and inclusions within the felsic gneisses despite serving as potential sources of geochemical information on the hadean mantle the mafic rocks have received less attention in previous studies thus we conducted a comprehensive geological petrological and geochemical investigation on the acasta mafic rocks to constrain their petrogenesis and geodynamic setting the mafic rocks comprise massive to weakly foliated amphibolite garnet amphibolite and hornblendite with variable abundances of hornblende plagioclase chlorite and quartz and subordinate clinopyroxene garnet and cummingtonite they commonly underwent high grade metamorphic recrystallization under amphibolite to upper amphibolite facies conditions the observed variations in mineral assemblages abundances and compositions reflect large differences in whole rock compositions likely caused by crustal anatexis during the eoarchean thermal events responsible for the generation of the surrounding felsic gneisses infiltration or extraction of felsic melts formed due to partial melting of precursor rocks can account for an overall negative correlation between al 2 o 3 and mgo contents and variable enrichments in the incompatible elements despite the widespread influence of anatexis on the geochemistry of acasta mafic rocks we identified the precursor compositions of the least modified amphibolites as basaltic magmas they are characterized by intermediate al 2 o 3 and mgo contents on the observed array and by near chondritic patterns for incompatible trace elements except for slightly negative nb and ta anomalies we considered two scenarios to explain the origin of eoarchean basaltic rocks with nb ta anomalies 1 generation of nb ta deficient basaltic magma in a suprasubduction setting analogous to modern arcs derived magmas and 2 generation of nb ta deficient basaltic magma from the melting of a nb ta deficient primitive mantle possible if the core contains significant proportions of the earth s nb and ta budget although the operation of plate tectonics and the presence of subduction zones at the end of hadean may be an attractive explanation for the observed nb ta depletions the chondritic relative proportions of other immobile trace elements for acasta mafic rocks leave open the possibility of their formation from an nb ta deficient primitive mantle

neoarchaean volcanic rocks central hearne supracrustal belt western churchill province canada geochemical and isotopic evidence supporting intra oceanic supra subduction zone extension abstract the kaminak segment of the central hearne supracrustal belt chsb western churchill province canada comprises a diverse sequence of neoarchaean volcanic and less abundant metasedimentary rocks that were emplaced as two assemblages between 2695 2711 ma and 2681 2686 ma respectively these are intruded by uncommon pre tectonic diorites and tonalites ca 2691 ma voluminous syn tectonic tonalites and granodiorites ca 2679 2686 ma and rare post tectonic potassic monzogranite and syenite ca 2659 2666 ma metasedimentary rocks include turbidites epiclastic tuffs minor iron formation oxide and rare volcanic conglomerates volcanic rocks of assemblage i include abundant pillowed and massive basalts to andesites with less common silicic lavas tuffs and volcaniclastic debris flow deposits assemblage ii contains voluminous silicic tuff and volcaniclastic debris flow deposits but fewer basaltic to andesitic flows the critical diagnostic feature of the chsb is the stratigraphic intercalation of compositionally diverse basaltic andesitic and felsic volcanic rocks throughout both assemblages mapping u pb geochronology and lithogeochemistry suggest that an initial morb like basaltic plain containing widespread intercalations of dacite to rhyolite was replaced at ca 2688 ma by a relatively short lived dacite to rhyolite dominated magmatic environment characterized by localized felsic volcanic centres and a bloom of 2686 2679 ma tonalitic to granodioritic plutons basaltic to andesitic rocks are dominated by iron rich tholeiites although the proportion of calc alkaline rocks increases with silica content felsic volcanic rocks all exhibit calc alkaline affinities the wide range in chemistry of the basaltic to andesitic rocks of both volcanic assemblages implies diverse mantle sources capable of generating voluminous morb with less common arc neb oib and rare babb like rocks similarly the variable composition of the felsic volcanic rocks indicates both anatexis of eclogitic to garnetiferous mafic crust and also extensive fractionation of mafic precursors in crustal magma chambers two geochemically distinct arc like mafic suites were generated through contamination of primary mantle derived magmas by juvenile ca 2700 ma silicic crust either in their mantle source or through assimilation upon ascent ndt isotopic data are comparable to contemporaneous depleted mantle with only local evidence for incorporation of older 2700 ma crust the chsb may have formed via tectono magmatic processes comparable to those of the eocene infant arcs of the sw pacific whereby the formation of a thick sequence of coeval intercalated compositionally diverse mantle and crustal derived rocks are generated in an extensional supra subduction setting the cessation of supra subduction zone extension at ca 2688 ma was followed by the short lived development of felsic volcanic edifices incipient arc the extrusion of mafic to felsic magmas and the concomitant intrusion of voluminous syn kinematic tonalitic plutons this accompanied a major change in the tectono magmatic setting accompanying and presumably following the termination of extensional supra subduction zone processes

evolution of crystallizing interstitial liquid in an arc related cumulate determined by la icp ms mapping of a large amphibole oikocryst high resolution chemical mapping of a large single oikocrystic introduction amphibole grain from the eriksberg gabbro by laser ablation the most commonly studied cumulus rocks are those inductively coupled plasma mass spectrometry la icp ms formed by crystallization of relatively dry basaltic liquids documents the progressive evolution of interstitial liquid in a hydrous e g parsons 1986 cawthorn 1996 these include most basaltic system the amphibole has a nearly constant mg number layered intrusions ophiolites oceanic cumulates and and only minor variations in most major elements in part because most cumulates recovered from the moon and in metof the resorption of olivine buffering the liquid composition however eorite collections studies of these cumulates have innon buffered trace elements including sc v nb zr th u and cluded efforts to understand how the crystals are the rare earth elements ree show at least an order of magnitude segregated from the liquid cumulus processes as well variation for example v varies from 1000 ppm zr as how the resulting pile of crystals plus liquid evolves from 4 3 to 1000 ppm nb from 0 14 to 12 3 ppm and the during slow cooling postcumulus processes by comree patterns range from bowed up with a negative eu anomaly parison very little effort has been directed toward underto bowed down with a positive eu anomaly all within a single standing these processes in the crystallization of relatively crystal the distribution of amphibole compositions indicates that hydrous basaltic liquids such as those associated with arc the interstitial liquid was not uniformly distributed as crystallization magmatism the presence of a significant amount of proceeded rather the compositional variations reflect progressively water in the liquid is especially important for the postmore channelized flow of interstitial liquid during compaction when cumulus evolution of a crystal pile as it will stabilize the interstitial liquid evolved so that the crystallizing amphibole had hydrous minerals and cause vapor saturation while a 300 ppm v vapor saturation was reached and the behavior of the significant amount of interstitial liquid remains incompatible trace elements changed markedly the final amphibole to the importance of fractional crystallization in procrystallize is enriched in soluble incompatible elements and depleted ducing compositional variability in crystallizing silicate in zr and nb a relationship that is consistent with crystallization liquids has been well established since bowen s exfrom a fluid where compaction returns such evolved liquid or fluid perimental work bowen 1928 during perfect fractional to an overlying magma reservoir resulting fractionation trends will crystallization the solids are isolated from the liquid before the liquid evolves appreciably and thus they proshow the relative depletions in ti nb and zr seen in arc related vide an instantaneous record of the liquid evolution basalts incorporation of liquid in a crystal pile can change both the composition of the solids and the evolution of the supernatant liquid initially the parental liquid if the

weathering mechanisms and their effects on the landsliding of ignimbrite subject to vapor phase crystallization in the shirakawa pyroclastic flow northern japan abstract ignimbrite which is consolidated by vapor phase crystallization is weathered in humid regions to form a special type of weathering profile that consists of a hydrated zone an exfoliated zone and a disintegrated zone from the depth to the ground surface with each zone having a basal front the ignimbrite is hydrated first and loses a significant amount of phosphorous at the hydration front where rock forming tridymite is dissolved and cristobalite is precipitated the ignimbrite further loses its alkali and alkali earth components at the top of the hydrated zone by reacting with reactive water from the exfoliated zone then the leached layers are exfoliated and become part of the exfoliated zone and then they soften significantly at the top of the exfoliated zone rock is disintegrated so completely that rock texture disappears water from rainstorms infiltrates down to the exfoliation front but penetrates only slightly further downward thus saturating the weathered rock in the exfoliated and disintegrated zones and leading to a landslide with a slip surface within the exfoliated zone

alteration and geochemical patterns in the 3 7 3 8 ga isua greenstone belt west greenland abstract volcanic rocks of the 3 7 3 8 ga isua greenstone belt have undergone heterogeneous poly phase post magmatic alteration resulting in mobilisation of many elements including lree and th in contrast nb ta ti zr y and hree were relatively immobile during alteration processes the degree of element mobility appears to have been controlled by chemical composition primary volcanic structures carbonate and silica enrichment and proximity to ultramafic units and shear zones on the basis of the geochemical characteristics of the least altered samples two distinct associations have been recognised in structurally separated lithotectonic sequences these magmatic associations are a low hfse association tio2 0 20 0 40 wt zr 12 30 ppm nb 0 13 0 80 ppm y 6 14 ppm in the central arc sequence resembling tertiary boninites and a high hfse association tio2 0 50 1 14 wt zr 34 77 ppm nb 1 2 2 7 ppm y 11 28 ppm in the outer and inner arc sequences comparable to phanerozoic island arc basaltic to picritic volcanic rocks these two geochemically distinct sequences were likely to have been juxtaposed as a consequence of phanerozoic style plate tectonic processes operating in the early earth the isua volcanic rocks are characterised by sub chondritic nb ta 10 16 ratios post magmatic alteration can be ruled out as the cause of these low ratios rather these ratios reflect a compositional characteristic of the paleoarchean mantle source s the sub chondritic nb ta ratios may have resulted from either preferential sink of nb into the core as a weakly siderophile element or preferential retention of nb in the subducted oceanic crust during subduction zone petrogenetic processes alternatively the building blocks of the earth were not composed of ci like chondritic material

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

amphibole plagioclase and clinopyroxene geochemistry of the archean fiskenaesset complex at majorqap qâva southwestern greenland implications for archean petrogenetic and geodynamic processes abstract despite multiple phases of deformation and metamorphism the archean ca 2 97 ga fiskenaesset complex southwestern greenland contains well preserved igneous olivine pyroxene amphibole predominantly hornblende and plagioclase this study presents new major and trace element data for hornblende plagioclase and clinopyroxene from the majorqap q va stratigraphic section of the complex petrographic observations and coherent trace element patterns of hornblende clinopyroxene and plagioclase grains in samples within and between stratigraphic units indicate that trace elements including lile ree and hfse in these minerals were generally immobile during amphibolite to granulite facies metamorphism their mineral chemistry therefore should reflect the trace element composition of the melt from which they grew two distinct petrogenetic suites of rocks are recognized using the trace element composition of hornblende the earlier suite a is characterized by hree depleted hornblende whereas the later suite b contains hree enriched hornblende within suite a hornblendes can be grouped into type a1 type a2 and type a3 hornblendes type a1 hornblendes characterized by low ree nb and ta contents and the absence of eu anomalies most likely have derived from a primitive liquid type a2 hornblendes which have high concentrations of incompatible elements including nb zr and ree and negative eu anomalies appear to have precipitated from a residual liquid following the extensive crystallization of plagioclase type a3 hornblendes are slightly enriched in mree compare to lree and hree displaying negative nb zr and ba anomalies suite a typifies a hydrous crystallization sequence of olivine clinopyroxene orthopyroxene hornblende plagioclase hornblende intercumulus this crystallization sequence played an important role in the formation of anorthosite layers in the fiskenaesset complex within suite b hornblendes are grouped into four types type b1 type b2 type b3 and type b4 hornblendes irrespective of eu anomalies type b1 type b2 and type b3 hornblendes display parallel patterns with coherent hree enrichment over lree in contrast type b4 hornblendes are enriched in lree suite b illustrates a crystallization of sequence of olivine plagioclase hornblende hornblende the presence and chemistry of the igneous hornblendes suggest the fiskenaesset complex formed in an archean convergent plate margin setting

geochemistry and geodynamic origin of the mesoarchean ujarassuit and ivisaartoq greenstone belts sw greenland abstract the ivisaartoq ca 3075 ma and ujarassuit ca 3070 ma greenstone belts are the largest mesoarchean supracrustal lithotectonic assemblages in the nuuk region sw greenland both greenstone belts underwent polyphase deformation and amphibolite facies metamorphism and were in due course variously dismembered pillow lavas pillow breccia magmatic layering and relic sedimentary structures are well preserved in the ivisaartoq belt volcanic rocks include basalts and minor andesites and picrites boninite like rocks occur in the ujarassuit belt metasedimentary rocks in the ujarassuit belt occur as thin layers 0 5 1 0 m of biotite schists and in the ivisaartoq belt as 500 m thick biotite schists intercalated with minor quartzitic gneisses there is no field evidence indicating that the ivisaartoq and ujarassuit supracrustal rocks were deposited on older continental basement and their volcanic rocks do not exhibit any geochemical trends indicating contamination by archean upper continental crust four groups of amphibolites metavolcanic rocks were recognized in the ujarassuit greenstone belt on the basis of their ree and hfse characteristics 1 group 1 is characterized by near flat ree patterns la smcn 0 77 1 14 la ybcn 0 84 1 24 and moderate negative nb anomalies nb nb 0 60 0 79 2 group 2 displays lree depleted patterns la smcn 0 53 1 02 la ybcn 0 32 0 61 and pronounced negative nb anomalies nb nb 0 32 0 67 3 group 3 consists of lree depleted patterns la smcn 0 69 0 84 la ybcn 0 55 0 91 and absence of significant nb anomalies nb nb 0 92 1 15 and 4 group 4 has concave upward ree patterns la smcn 1 64 2 42 gd ybcn 0 57 1 01 and large negative nb anomalies nb nb 0 28 0 42 the depletion of lree in group 2 amphibolites appears to have resulted from mobility of these elements during metamorphism group 1 3 and 4 amphibolites retain their near primary geochemical signatures and their trace element patterns are comparable to those of phanerozoic oceanic island arc tholeiites iat normal mid ocean ridge basalts n morb and boninites respectively the trace element patterns of the least altered meta ultramafic rocks la ybcn 2 48 1 35 nb nb 0 31 0 60 are comparable to those of modern subduction related picrites amphibolites with an andesitic composition show large negative nb anomalies nb nb 0 21 0 55 enriched lree patterns la ybcn 6 29 15 64 and fractionated hree gd ybcn 2 61 3 12 implying deep melting in equilibrium with residual garnet in the source biotite schists and quartzitic gneisses have low chemical indexes of alteration values cia 46 to 62 and trace element characteristics indicating volcaniclastic sedimentary protoliths derived from poorly weathered felsic to mafic source rocks collectively the geochemical features of metavolcanic and metasedimentary rocks suggest that the ivisaartoq and ujarassuit greenstone belts represent dismembered fragments of mesoarchean supra subduction zone oceanic crust formed in an arc forearc backarc tectonic setting

rapid formation of river terraces in non welded ignimbrite along the hishida river kyushu japan abstract river terraces are well preserved in the middle to lower reaches of the hishida river that flows through the extensive ignimbrite field produced by the large scale ito eruption about 24 500 years ago from aira caldera in southern kyushu japan the terraces are erosional and formed during dissection of the initial topography of the ito ignimbrite these occur within the upper non welded zone of the ito ignimbrite about 20 60 m below the depositional surface and usually more than 20 m above the welded zone streams associated with terrace formation were ephemeral because perennial streams are considered highly unlikely within the porous non welded ignimbrite all tephra layers on the original ignimbrite depositional surface also cover the terraces this indicates that the formation of terraces preceded the accumulation of the oldest tephra the age of which is about 23 000 years old high erodibility of the non welded ito ignimbrite and absence of vegetation on the initial surface would have permitted a rapid dissection of the initial topography and the formation of the river terraces thus it is interpreted that the river terraces were formed very rapidly in a period of far less than 1500 years immediately after the deposition of the ito ignimbrite and before cover of new vegetation on the depositional surface

the fiskenaesset anorthosite complex stable isotope evidence for shallow emplacement into archean ocean crust oxygen and hydrogen isotope ratios indicate that unusual rocks at the upper contact of the archean fiskenaesset anorthosite complex at fiskenaesset harbor southwest greenland are the products of hydrothermal alteration by seawater at the time of anorthosite intrusion subsequent granulite facies metamorphism of these ca poor and al and mg rich rocks produced sapphirine and kornerupine bearing assemblages because large amounts of surface waters cannot penetrate to depths of 30 km during granulite facies metamorphism the isotopic signature of the contact rocks must have been obtained prior to regional metamorphism the stable isotope and geochemical characteristics of the contact rocks support a model of shallow emplacement into archean ocean crust for the fiskenaesset anorthosite complex 45 refs 3 figs 2 tabs

a vestige of earth s oldest ophiolite a sheeted dike complex within the 3 8 billion year old isua supracrustal belt isb in southwest greenland provides the oldest evidence of oceanic crustal accretion by spreading the geochemistry of the dikes and associated pillow lavas demonstrates an intraoceanic island arc and mid ocean ridge like setting and their oxygen isotopes suggest a hydrothermal ocean floor type metamorphism the pillows and dikes are associated with gabbroic and ultramafic rocks that together make up an ophiolitic association the paleoarchean isua ophiolite complex these sheeted dikes offer evidence for remnants of oceanic crust formed by sea floor spreading of the earliest intact rocks on earth

deep weathering of a group of thick argillaceous limestone rocks near three gorges reservoir central china rock mass characterization is one of the most basic and important tasks in rock mechanics and engineering literature review indicates that although there are many publications available on weathered rocks there are few reports in english on weathered argillaceous limestone rocks and their chemical and mechanical properties this paper presents a case study of characterizing a group of highly weathered thick argillaceous limestone rocks most importantly they can be easily and quickly decomposed into soils after they are exposed due to excavation consequently they would substantially lose their mechanical strengths geohazards such as landslides and sinkholes could occur in uncovered highly weathered argillaceous limestone rocks the problematic weathered rocks were found during the development of a new town in upper mountains above the three gorges reservoir on yangtze river in central china in the past 20 years the factual data are presented on the changes of chemical compositions and mechanical properties of the weathered argillaceous limestone rocks due to chemical weathering some empirical correlations are given to quantify the changes in their uniaxial compressive strength and deformation moduli in terms of their contents of calcium carbonate the data and findings presented in the paper can be useful to future urban development in geologically poor mountainous environments occupied by highly weathered argillaceous limestone rocks in the world

the chimalpahad anorthosite complex and associated basaltic amphibolites nellore schist belt india magma chamber and roof of a proterozoic island arc abstract new major and trace element data on the proterozoic chimalpahad layered anorthositic complex and associated basaltic amphibolites of the nellore schist belt of south india provide new constraints on their petrogenesis and geodynamic setting the complex consists of layered anorthosites leucogabbros gabbros ultramafic rocks and is spatially associated with basaltic amphibolites despite deformation and metamorphism primary cumulate textures and igneous layering are locally well preserved throughout the complex whereas the amphibolites display diverse ree systematics the chimalpahad anorthositic gabbroic rocks are characterized by moderately depleted to strongly enriched lree patterns and by flat to depleted hree patterns the field relations major and trace element compositions of the basaltic amphibolites suggest that they are petrogenetically related to the anorthositic gabbroic rocks by fractional crystallization the anorthositic rocks and the basaltic amphibolites share the depletion of nb relative to th and la on primitive mantle normalized diagrams they exhibit signatures of arc magmatic rocks such as high lile and lree relative to the hfse and hree as well as high ba nb ba zr sr y la yb ratios that mimic chondrite normalized ree and primitive mantle normalized trace element patterns of arc magmas similarly on log transformed tectonic discrimination diagrams the chimalpahad rocks plot within the field of phanerozoic magmatic arcs consistent with a subduction zone origin on the basis of field relations and geochemical characteristics the chimalpahad complex is interpreted as a fragment of a magma chamber of an island arc which is tectonically juxtaposed against its original volcanic cover a new preliminary sm nd date of anorthosite from the chimalpahad complex indicates a model age of 1170 ma

weathering mechanisms and their effects on landsliding in pelitic schist physical chemical and mineralogical analyses of undisturbed drill cores of pelitic schist from a landslide area in japan clarified the mechanisms of chemical weathering of pelitic schist oxidizing surface water percolates downward and reaches an oxidation front where chlorite is altered to al vermiculite graphite and pyrite are oxidized and depleted and goethite precipitates oxidation of pyrite also occurs just below the oxidation front probably by ferric iron pyrite oxidation yields sulphuric acid which penetrates further downward interacting with and weakening the rocks in addition to this chemical weakening stress release and shearing along schistosities form an incipient shear zone which propagates to a sliding zone that forms the rupture surface of a landslide once a sliding zone has developed it inhibits downward groundwater flow across it because of its low permeability slowing the downward propagation of the weathering zone until this filtration barrier is broken by landslide movement copyright 2010 john wiley sons ltd

relationship between the initiation of a shallow landslide and rainfall intensity duration thresholds in japan abstract the empirical rainfall intensity and duration i d threshold for the initiation of shallow landslide is newly defined for japan where heavy rainfalls frequently occur during the east asian summer monsoon season the rainfall causes sediment related disasters annually this paper presents an examination of 1174 rainfall induced shallow landslides that occurred during 2006 2008 their i d conditions were analyzed objectively from rainfall data radar raingauge analyzed precipitation to derive the i d threshold using the quantile regression method i 2 18 d 0 26 where i is measured in millimeters per hour and d in hours as measured from the beginning of rainfall to the landslide occurrence rainfall events are separated by the absence of rainfall for 24 h we then established a rescaled i d threshold by dividing the rainfall intensity by the mean annual precipitation map as imap 0 0007 d 0 21 where imap is the rescaled average per hour rainfall intensity these thresholds were defined by the second percentile regression line for d of 3 537 h the new thresholds are considerably lower than those previously reported for the world humid subtropical regions the asian monsoon region and japan the result suggests that japan is highly prone to rainfall induced shallow landslides because of its high relief topography geologic conditions human interference and rainfall characteristics during the east asian summer monsoon season

chemical assembly of archean anorthosites from amphibolite and granulite facies terranes sw greenland we report whole rock major and trace element compositions obtained by xrf and ina methods for the amphibolite facies buksefjorden and granulite facies nordland anorthosites sw greenland in a previous petrologic study on the same sample suite we documented differences in texture mineralogy and mineral compositions between these two anorthosite bodies chemical analyses confirm differences in composition between the two bodies but these differences cannot be explained by variations in metamorphic conditions and point towards differences in the nature of their protoliths analyzed nordland samples are anorthosites and leucogabbros with 88 98 normative plagioclase whereas those from buksefjorden include anorthosite leucogabbro and gabbro with 55 95 normative plagioclase two or more compositional groupings can be recognized at each site which correspond to differences in color and mineralogy of the hand samples samples from buksefjorden are mainly quartz normative whereas those from nordland are olivine nepheline normative other differences include higher ni co ratio and ree contents in the granulite facies anorthosites from nordland ree pattern shapes are similar however being moderately fractionated at 0 5 10 chondrites with positive eu anomalies calculated equilibrium melt patterns are similar for both anorthosites being relatively flat at 50 150 chondrites suggesting unfractionated but evolved parental magmas olivine must have been present in the protoliths of the nordland rocks compared with buksefjorden otherwise the protoliths contained plagioclase with variable an content an62 an92 and a mafic component with variable fe mg mg 0 3 0 8 this mafic component was either hornblende or a combination of ortho and clinopyroxene in fixed proportions plus a small amount of magnetite mixing calculations demonstrate that some buksefjorden anorthosites contain two varieties of plagioclase a calcic type that may correspond to cumulus crystals and a sodic type that may correspond to a trapped melt component on plots of normative whole rock an versus mg compositions of the buksefjorden and nordland anorthosites form crude negative arrays that differ from the generally positive trends of mafic layered intrusions kiglapait skaergaard and from the generally flats trends of plagioclase rich cumulate rocks st urbain and stillwater anorthosites this difference provides further evidence for the distinctive nature of archean calcic anorthosite complexes compared with other types of mafic intrusions moreover this distribution of data points is consistent with the assembly of the protolith of the sw greenland anorthosites mainly as mixtures of plagioclase and hornblende finally the field for the buksefjorden and nordland anorthosites overlaps only slightly with that for the fiskenaesset complex thus extending the known range of compositions for archean anorthosites in west greenland

no water no granites no oceans no continents water is essential for the formation of granite and granite in turn is essential for the formation of continents earth the only inner planet with abundant water is the only planet with granite and continents the moon and the other inner planets have little or no water and no granites or continents

new age ca 2970 ma mantle source composition and geodynamic constraints on the archean fiskenaesset anorthosite complex sw greenland the archean fiskenaesset complex sw greenland consists of an association of ca 550 meter thick layered anorthosite leucogabbro gabbro and ultramafic rocks peridotite pyroxenite dunite hornblendite the complex was intruded by tonalite trondhjemite and granodiorite ttg sheets now orthogneisses during thrusting that was followed by several phases of isoclinal folding the trace element systematics of the fiskenaesset complex and associated volcanic rocks are consistent with a supra subduction zone geodynamic setting the fiskenaesset anorthosites leucogabbros gabbros and ultramafic rocks collectively yield an sm nd errorchron age of 2973 28 ma mswd 33 with an average initial epsilon nd 3 3 0 7 consistent with a long term depleted mantle source regression of pb isotope data define an age of 2945 36 ma mswd 44 and the regression line intersects the average growth curve at 3036 ma slightly lower pb pb errorchron age is interpreted as reflecting partial disturbance of the u pb system in gabbros leucogabbros and ultramafic rocks during intrusion of ttgs complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca 3200 and 2650 ma zircon ages peak at about 3200 3100 3000 2950 2820 and 2750 ma the 3200 3000 ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks 2950 ma is considered as an approximate intrusion age of sampled ttgs the 29402650 ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the fiskenaesset region on the basis of recently published trace element data and new nd and pb isotope and u pb zircon age data a three stage geodynamic model is proposed to explain the evolution of the fiskenaesset complex stage 1 represents the formation of depleted shallow mantle source 3000 ma epsilon nd 3 3 0 7 for the complex stage 2 corresponds to the development of an intra oceanic island arc between 3000 2950 ma stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950 2940 ma c 2010 elsevier b v all rights reserved less

weathering profile of non welded ignimbrite and the water infiltration behavior within it in relation to the generation of shallow landslides the mechanism underlying rain induced shallow landslides of non welded ignimbrite is found to be a special type of weathering profile and the behavior of water infiltrating through that profile according to our study of ito ignimbrite in southern kyushu japan rhyolitic volcanic glass the primary component of ignimbrite is first hydrated and dissolved forming halloysite halloysite near ground surface is then transported through the ignimbrite by infiltrating water and becomes clogged in interstices to form clay bands suction monitoring across a weathering profile indicated that downward infiltration of water is disrupted once by a zone of less permeable clay bands and again at the weathering front this disruption at the front is caused by a capillary barrier effect caused by the structure where finer weathered material overlies coarser fresh material this results in a well defined weathering front particularly beneath a slope where water flux is parallel to the front whereas the front is transitional beneath a ridge top where the front is nearly horizontal and the water flux is normal to the front infiltrating water from rain increases the weight of weathered material and decreases the suction within the material which is the final trigger of a shallow landslide of non welded ignimbrite long term weathering which proceeds on the order of years provides slide material

a juvenile oceanic island arc origin for the archean ca 2 97 ga fiskenaesset anorthosite complex southwestern greenland evidence from oxygen isotopes abstract the archean ca 2 97 ga fiskenaesset layered intrusion southwestern greenland consists of an association of anorthosite leucogabbro gabbro hornblendite pyroxenite peridotite and dunite the intrusion is characterized by well preserved igneous layering cumulate texture and primary igneous minerals including olivine pyroxene plagioclase hornblende and chromite we use new whole rock n 36 and mineral n 32 oxygen isotopic data for all major lithologic units from the best preserved stratigraphic section of the fiskenaesset complex at majorqap q va to revisit geodynamic and petrogenetic hypotheses proposed for the origin of archean terranes the fiskenaesset complex has modern mantle like whole rock o isotope compositions o 18 5 8 0 5 average o 18 values increase from peridotite o 18 5 0 through hornblendite o 18 5 7 gabbro o 18 5 8 pyroxene hornblendite o 18 6 0 and leucogabbro o 18 6 3 to anorthosite o 18 6 3 these whole rock isotopic compositions reflect the approximate modal abundances of olivine average o 18 4 9 hornblende average o 18 5 7 clinopyroxene average o 18 6 4 and plagioclase average o 18 6 4 in each rock type as a consequence of mineral fractionation in the magma chamber s field relationships and the absence of crustal contamination suggest that the fiskenaesset complex formed in an oceanic setting subduction zone like whole rock trace element signatures and mantle like o 18 and initial e nd values are consistent with formation of these rocks in a juvenile oceanic island arc setting field and geochemical data from the fiskenaesset region and adjacent terranes suggest that the origin of archean crust in southwestern greenland is consistent with phanerozoic like plate tectonic processes rather than density driven sinking delamination and diapiric processes requiring formation of greenstone belts and anorthosite complexes on pre existing continental crust mantle like o 18 values in the ca 2 97 ga fiskenaesset complex are inconsistent with substantial recycling of continental crust indicating that the fiskenaesset rocks originated distal from continental sources

geotechnical and landslide aspects in weathered granitoid rock masses serre massif southern calabria italy abstract the weathering processes produce important chemical and physical transformations in rock masses producing a decay of their strength and then stability in this study different methodologies were employed to define the physical mechanical properties and the minero petrographical changes at each grade of weathering useful to determine a geological model of the weathered granitoids involved in mass movements the paper focuses on a multidisciplinary research on the weathering profiles of the paleozoic granitoid rocks surrounding the village of fabrizia located in the serre massif southern calabria italy fabrizia lies on an old erosion surface composed of basement rocks made up of weathered granitoids belonging to the serre massif granitoid complex and overlain by thin eluvial colluvial deposits the tectonic uplift of the area combined with mediterranean climatic conditions both active at least since the quaternary are the main causes of the deep weathering of the plutonic rocks the weathered profiles of the granitoid complex have been studied through field observations borehole explorations with geotechnical tests and seismic surveys coupled to minero petrographical analysis and laboratory tests weathered profiles are mainly composed by rock masses varying from completely weathered and residual soil classes v vi to moderately weathered rocks class iii characterized by a progressive increase of geotechnical properties e g seismic wave velocity rock quality designation internal friction angle uniaxial compressive strength with a decrease of weathering the weathered rocks display a wide variety of mass movement landslide types are rockfalls for the fresher rocks class iii translational slides occur between different weathering degrees classes iv and v and rotational slides dominate at the top of the slopes and generally involve the weathering grades from class iv to classes v vi the shallow horizons are composed of thin eluvial colluvial deposits and more weathered granitoids classes v vi that may generate dangerous debris flows or debris avalanches during intense and prolonged rainfall the landslide surface is usually at a depth lower than 2 m in the debris flow type while deeper landslides depth 2 m are rotational and translational types with sliding surfaces occurring mainly in the transition between the classes iv and v the field survey coupled to geotechnical geophysical and minero petrographical data allowed the recognition of the different weathered horizons and relative geotechnical properties useful to the reconstruction of the reliable geological model the model represents an important support for the study of the landslide processes and for a sustainable management of the territory the used approach provided results to set a general reference framework for establishing more precise correlations between mass movement phenomena and weathering processes

chromite silicate chemistry of the neoarchean sittampundi complex southern india implications for subduction related arc magmatism the neoarchean layered anorthositic complex at sittampundi in southern india is known for its chromitite layers that are mostly associated with anorthosite an 90 100 the chromitites contain feal rich chromites concentrated in layers between amphibole rich layers with a dominant mineralogy of amphibole spinel plagiocase sapphirine the chromite rich layers contain only amphibole and plagioclase mineral compositions illustrated by x ray composition maps and profiles show subtle chemical differences the chrome spinels are of refractory grade with cr2o3 and al2o3 contents varying between 34 40 wt and 23 28 wt the chromite compositions are noticeably different from those in layered igneous intrusions of the bushveld stillwater type the existence of original highly calcic plagioclase feal rich chromite and magmatic amphibole is consistent with derivation from a parental magma of hydrous tholeiitic composition that was most likely generated in a supra subduction zone arc setting in terms of mineralogy and field relations the sittampundi chromitites are remarkably similar to anorthosite hosted chromitites in the neoarchean fiskensset anorthositic complex greenland we propose that the sittampundi chromitites formed by partial melting of unusually aluminous harzburgite in a hydrated mantle wedge above a subduction zone this melting process produced hydrous aluminous basalt which fractionated at depth to give rise to a variety of high alumina basalt compositions from which the anorthositic complex with its cumulate chromite rich and amphibole rich layers formed within the magma chamber of a supra subduction zone arc c 2011 elsevier b v all rights reserved

multiple mafic magmatic and high grade metamorphic events revealed by zircons from meta mafic rocks in the daqingshan wulashan complex of the khondalite belt north china craton abstract meta mafic rocks in the daqingshan wulashan complex in the central segment of the paleoproterozoic khondalite belt of the north china craton are composed predominantly of garnet bearing mafic granulites mafic granulites and amphibolites they occur mainly as irregular lenses and as a set of deformed dyke sill swarms within dioritic granitic orthogneisses and al rich paragneisses an integrated study involving laser raman and scanning electron microscope analyses of mineral inclusions from zircons cathodoluminescence imaging and la icp ms u pb dating of zircons shows that magmatic zircons in these meta mafic rocks record four age groups of 2500 2450 2300 2100 1970 1930 and ca 1850 ma indicating multiple mafic magmatic events occurred within the daqingshan wulashan complex in addition four groups of metamorphic zircons with ages of ca 2450 ca 1950 ca 1900 and ca 1850 ma are present within these meta mafic rocks zircons of the first oldest group contain medium to low pressure mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz and yield ages of ca 2450 ma representing the timing of the early mlp granulite facies metamorphic event within the daqingshan wulashan reworked basement complex in contrast zircons of the second group are free of mineral inclusions and yield ages of ca 1950 ma which is considered to be the timing of the regional high grade metamorphic and structural event probably caused by the collision of the yinshan and ordos blocks zircons of the third group contain mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz similar to the zircons of the first group but yield younger ages of ca 1900 ma interpreted as the timing of the regional mlp granulite facies metamorphism that most likely occurred during the early stage of post orogenic exhumation of the daqingshan wulashan complex zircons of the fourth group contain mlp amphibolite facies mineral inclusions of clinopyoxene amphibole plagioclase quartz and yield the youngest ages of ca 1850 ma which probably represent the timing of the regional amphibolite facies metamorphism during the late stage of post orogenic exhumation of the daqingshan wulashan complex combined with previous geological data of the khondalite belt it is speculated that the multiple mafic magmatism and contemporaneous high grade metamorphism of the daqingshan wulashan complex from the khondalite belt were most likely generated by an orogenic process involving early subduction collision and late exhumation in an active continental margin arc

weathering indices for rhyolitic tuff and granite in hong kong a series of chemical analyses and dry density tests of rhyolitic tuff and granitic samples have been carried out the test results have been interpreted using a newly developed theoretical weathering model for calculating a mobility index of major elements in rock and soil it is found that this index is useful for describing changes of rock and soil during weathering and classifying the degrees of weathering based on the theoretical model a new parameter called the volume index which combines chemical and physical data is suggested this index appears to be a good indicator for classifying the degree of weathering in hong kong a quantitative classification scheme is suggested

volcanic evolution of the upper onverwacht suite barberton greenstone belt south africa abstract the volcanic stratigraphy of the upper onverwacht suite in the southeastern part of the paleoarchean barberton greenstone belt has been investigated in 18 sections through parts of the hooggenoeg kromberg and lower mendon complexes the ca 2700 m thick volcanic sequence of the lowest tectonostratigraphic unit the hooggenoeg complex hc can be subdivided into 9 stratigraphic units representing major eruptive phases each separated by silicified sedimentary and volcaniclastic rocks the thicknesses of the units vary from the lavas of the hc and kc are predominantly pillowed and massive flows whereas those of the ncakini section are exclusively massive the massive and pillowed lava are commonly organized in cyclic units from 3 m to 32 m thick and consisting of massive lava or large pillows that are succeeded by progressively smaller pillows cyclic units are inferred to have resulted from individual eruptions with decreasing rates of effusion the pillow lavas of both the hc and kc contain

detrital zircon sedimentary provenance ages for the eoarchaean isua supracrustal belt southern west greenland juxtaposition of an imbricated ca 3700 ma juvenile arc against an older complex with 3920 3760 ma components abstract the amphibolite facies eoarchaean isua supracrustal belt northern part of the nuuk region southern west greenland is dominated by strongly deformed metabasalts with chert banded iron formation felsic volcanic and volcano sedimentary rocks and minor gabbro and sedimentary carbonates it comprises a suture zone between a northern terrane formed at ca 3700 ma and a southern one formed at ca 3800 ma at the junction between these two terranes is a strongly tectonised thin unit of metachert bif and carbonate bearing rocks with minor detrital components named the dividing sedimentary unit away from the belt the northern terrane is dominated by ca 3700 ma tonalites and the southern one by ca 3800 ma tonalites the isua supracrustal belt chemical metasedimentary rocks give low yields of small commonly 50 m long prismatic oscillatory zoned zircons with less common somewhat rounded zircons of volcanic and or detrital origin the most siliceous of the felsic schists give a larger yield of volcanic or detrital zircons the ages of these zircons are distinct in different parts of the belt in the northern terrane most have ages of 3710 3690 ma with a few slightly older 3750 ma detrital zircons suggests that these volcanic assemblages evolved remote from 3800 ma crust in contrast the up to 3940 ma detrital zircons from the dividing sedimentary unit and the southern fuchsite quartzite shows that the southern ca 3800 ma terrane although dominated by arc related rocks might contain older pre 3800 ma crust we propose that the dividing sedimentary unit was deposited on top of already deformed southern terrane rocks and then acted as a decollement when the northern terrane ca 3700 ma arc complex collided with the southern terrane by ca 3660 ma from previous age constraints thus the detrital zircon age data is compatible with proposals from other evidence for juxtaposition of ca 3700 and 3800 ma terranes in the isua area some thin siliceous metasedimentary rocks along post assembly mylonite zones in the isua supracrustal belt yield some

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

trace element systematics of the neoarchean fiskenaesset anorthosite complex and associated meta volcanic rocks sw greenland evidence for a magmatic arc origin abstract new major and high precision icp ms trace element data on the neoarchean ca 2970 ma layered fiskenaesset complex and associated volcanic rocks southern west greenland provide new constraints on the petrogenesis and geodynamic setting of the complex the complex appears to have been emplaced as multiple sills of magma and crystal mush into oceanic crust tholeiitic basalts and gabbros forming an association of ca 550 m thick anorthosite leucogabbro gabbro and peridotite layers the fiskenaesset complex and the associated volcanic rocks were intruded by neoarchean tonalite trondhjemite and granodiorite ttg sheets during thrusting that was followed by several phases of isoclinal folding despite the intense deformation and amphibolite to granulite facies metamorphism primary cumulate textures and igneous layering are locally well preserved throughout the complex the presence of calcic plagioclase an75 95 and igneous amphibole in anorthosites gabbros and leucogabbros and hornblendite veins in peridotites suggests a hydrous magma source s the major and trace element compositions of tholeiitic basalts amphibolites suggest that they are petrogenetically related to the fiskenaesset complex by fractional crystallization the trace element systematics of the least altered anorthosites gabbros leucogabbros peridotites tholeiitic basalts and calc alkaline high magnesian andesites hma is collectively consistent with a supra subduction zone geodynamic setting on the log transformed tectonic discrimination diagram including la th sm th yb th and nb th ratios tholeiitic basalts display a trend projecting from mid ocean ridge basalt morb field to island arc basalt iab field this trend is interpreted as reflecting a transition from the neoarchean depleted upper mantle to a subarc mantle wedge following the initiation of intra oceanic subduction and arc migration collectively on the basis of field relationships petrographic features and geochemical characteristics the fiskenaesset complex is interpreted as a fragment of a neoarchean oceanic island arc on the basis of ree patterns anorthosites are divisible into four major groups 1 group 1 displays moderately depleted to slightly enriched lree and hree patterns 2 group 2 possesses strongly enriched lree and moderately depleted hree patterns 3 group 3 has strongly enriched lree and depleted hree patterns and 4 group 4 garnet anorthosite exhibits concave upward ree patterns that appear to have resulted from contamination of anorthositic magma by basaltic rocks or magma the different ree patterns in group 1 2 and 3 anorthosites are interpreted to reflect various depths of partial melting and variably enriched in incompatible elements source compositions the majority of anorthosites and leucogabbros were derived from depths above the garnet stability field

petrology and tectonic significance of gabbros tonalites shoshonites and anorthosites in a late paleozoic arc root complex in the wrangellia terrane southern alaska plutonic rocks intrusive into the late paleozoic tetelna formation of southern alaska are the underpinnings of the late paleozoic skolai arc of the wrangellia terrane there are four groups of intrusive rocks within the skolai arc 1 gabbro diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition 2 silicic intrusions including tonalite granodiorite and granite 3 monzonitic to syenitic plutonic rocks of the ahtell complex and related dikes and sills 4 fault bounded bytownite anorthosite of uncertain age and association these anorthosites may be related to post skolai nikolai greenstone magmatism the silicic rocks yield discordant u pb zircon ages of 290 320 ma early to late pennsylvanian relative age relations suggest that the oldest intrusive rocks are the gabbro diorite plutons the youngest are the monzonitic rocks and that the silicic rocks span this range the gabbro diorite pluto

water percolating behavior indicated by the water chemistry within a decomposed granite slope central japan abstract monitoring rainfall discharge amount and the chemistry of water from just beneath the groundwater table yielded a conceptual model of water percolation behavior within a decomposed granite slope in central japan providing a clue to evaluate a landslide hazard potential by rainstorms when rainfall is strong enough more than about 50 mm a day in this case air pressure in the vadose zone probably increases temporarily because of the downward movement of a wetting front pushing down the water of the capillary fringe the capillary water has sio2 concentrations controlled by the precipitation of cristobalite the water is rich in ca which is supplied from plagioclase with sio2 after the pushing down of the capillary water rainwater with soil water in the unsaturated zone reaches the groundwater surface the cation concentrations in the unsaturated soil water and the capillary water are affected by ion exchange

centroid moment tensor solutions for april june 1993 abstract centroid moment tensor solutions are presented for 196 earthquakes that occurred during the second quarter of 1993 the solutions are obtained using corrections for aspherical earth structure represented by a whole mantle shear velocity model sh8 u4l8 of dziewonski and woodward

smnd age of the fisken sset anorthosite complex west greenland abstract the fisken sset anorthosite complex of southern west greenland is a sheet like layered igneous body which has been fragmented by granitoid intrusions multiply deformed and folded metamorphosed as high as granulite facies and in some places retrogressed to lower amphibolite or greenschist facies in some localities however such as at majorqap qa va some 20 km inland from the coast these effects are minimal here metamorphic grade reached only amphibolite facies with no apparent retrogression igneous textures and structures are spectacularly preserved detailed stratigraphic sequences have been established and in some samples primary mineralogy remains we report a sm nd isotopic study on samples from this locality attempting to constrain the age of crystallization of the fisken sset complex this complex represents a useful marker unit with which the age relationships of surrounding units may be compared a five point isochron including data for whole rock samples of anorthosite metagabbro metaperidotite and separates of calcic plagioclase and mafic matrix from a coarse megacrystic leucogabbro corresponds to an age of 2 86 0 05 ga mswd 2 5 with initial end of 2 9 0 4 the sm nd age is significantly older than previous pb pb whole rock ages of the complex but is sufficiently close to the 2 8 ga high grade metamorphic event which affected the area to cause partial resetting of the sm nd system to be considered however based on the integrity of the whole rock sm nd isotopic system in similar anorthositic complexes of higher metamorphic grade and the apparent preservation of magmatic ree patterns in the analysed plagioclase megacryst and coexisting mafic matrix we conclude that 2 86 0 05 ga represents the time of crystallization of the fisken sset complex this would imply a relatively short time interval on the order of 70 ma during which anorthosite formation tonalite emplacement and high grade metamorphism took place and this must be accounted for in any tectonic model for late archean events in southern west greenland in any case the positive end value of the fisken sset complex requires that it was derived from a source with long term depletion in light ree these results add to the increasing body of data indicating the widespread presence of depleted mantle sources during the archean

petrology and geochemistry of the guyang hornblendite complex in the yinshan block north china craton implications for the melting of subduction modified mantle abstract the hornblendite complexes hosted in the guyang granite greenstone terrane form part of the neoarchean basement in the western block of the north china craton in this study we focus on the largest one from this block previously named the guyang komatiite and present results from lithological geochronological and geochemical studies the dominant lithology in the guyang hornblendite complex is greenschist facies hornblendite and can be divided into clinopyroxene hornblendite 75 and olivine orthopyroxene hornblendite 25 the oldest calculated re depletion model ages t rd of these hornblendites is of 2454 ma and the zircon u pb age of the wallrock is 2480 ma with single stage depleted mantle nd model ages t dm1 nd varying from 2 61 to 2 88 ga these data suggest that the guyang hornblendite complexes formed during neoarchean paleoproterozoic time the hornblendites show low sio 2 high mgo contents and are enriched in cr ni and lree with negative ce anomalies and depleted in ti nb and ta their 187 os 188 os ratios range from 0 11145 to 0 11279 with os 2 5ga varying from 3 9 to 1 4 geochemically the olivine orthopyroxene hornblendite shows little variation with typical cumulate feature in contrast the clinopyroxene hornblendite shows a large range of chemical variation in the cao vs mgo and cao al 2 o 3 vs mgo diagrams the clinopyroxene hornblendite shows opx fractionation trend combined with lithological information we infer that the hornblendite magma was emplaced as a crystal mush with orthopyroxene as cumulus phase the os and nd isotope composition negative nb ta ce anomalies and the relationships in th yb nb yb diagram suggest that the guyang hornblendites formed from a source mantle that was modified by subduction related melts fluids derived from a seawater altered basaltic slab compared with typical archean komatiites the rocks of present study show significant differences in lithological and geochemical characteristics and thus it cannot be named as komatiite to explain these characteristics we propose a geodynamic model involving ridge subduction and slab window mechanism to account for the formation of the guyang hornblendite complex

komatiite flooding of a rifted archean rhyolitic arc complex geochemical signature and tectonic significance of the stoughton roquemaure group abitibi greenstone belt canada the 0 2 2 km thick archean stoughton roquemaure group srg in the northern volcanic zone of the abitibi greenstone belt quebec canada is composed of tholeiitic basalt komatiitic basalt and komatiite the mafic and ultramafic rocks are pillowed brecciated and massive columnar jointed flows the srg conformably overlies the 2730 ma hunter mine group a volcanic complex dominated by calc alkaline felsic rocks the tholeiitic basalts of the srg resemble morb the komatiitic basalts and komatiites have positive nd values overlapping those of the tholeiitic basalts komatiitic basalts with low al2o3 tio2 ratios 10 and fractionated heavy ree patterns are similar to al depleted komatiites in contrast the komatiites have high al2o3 tio2 20 unfractionated heavy ree patterns and resemble al undepleted munro type komatiites the al depleted komatiitic basalts occur at the base of the srg whereas the al undepleted komatiites are prevalent higher up in the stratigraphy the association of calcalka

palaeomagnetic geochronological and geochemical study of mesoproterozoic lakhna dykes in the bastar craton india implications for the mesoproterozoic supercontinent abstract palaeomagnetic analysis of the lakhna dykes bastar craton india yields a palaeopole at 36 6 n 132 8 e dp 12 4 dm 15 9 and the u pb zircon age obtained from one of the rhyolitic dykes is 1466 4 2 6 ma mswd 0 21 concordia age based on two analyses with identical pb u ages similar to previously published u pb ages major and trace element analyses of the lakhna dykes show shoshonitic and high k calc alkaline affinities consistent with a subduction related characteristics suggesting an active continental margin setting this is in keeping with the palaeo to mesoproterozoic tectonic environments in the eastern indian margin the new 1460 ma indian palaeopole was used to test possible palaeopositions of india within the mesoproterozoic supercontinent columbia of the four palaeomagnetically permissible reconstructions juxtaposing western india against south west baltica is geologically the most reliably constrained and best fitting model our preferred reconstruction implies a long palaeo to mesoproterozoic accretionary orogen stretching from south eastern laurentia through south western baltica to south eastern india breakup of india and baltica probably occurred in the late mesoproterozoic but additional constraints are needed

geochemistry of neoarchean mafic volcanic rocks and late mafic dikes in the zanhuang complex central orogenic belt north china craton implications for geodynamic setting abstract an archean melange belt has been recognized in the zanhuang complex central orogenic belt cob north china craton ncc all units of the melange belt are intruded by a ca 2 5 ga granite pluton and cross cut by undeformed 2 5 ga pegmatites which constrains the archean formation age of the melange some exotic mafic ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the melange and mainly consist of metabasaltic rocks deformed pillow lavas with epidosite cores gabbros and ultramafic rocks both metabasalts and epidosites are characterized by enrichment in light rare earth elements lree and pronounced negative nb and zr anomalies on chondrite and primitive mantle normalized diagrams these geochemical characteristics are consistent with supra subduction zone geochemical signatures based on the pillow structures and seafloor hydrothermal alteration combined with the geochemical characteristics an intra oceanic arc forearc setting is proposed for the formation of these exotic blocks furthermore a west dipping intra oceanic subduction zone is proposed to have formed in an ocean between the eastern block and an arc terrane prior to 2 5 ga the collision between the arc and the eastern block of the ncc happened at ca 2 5 ga resulting in the formation of the archean melange belt in the zanhuang complex all of the units of the melange belt were intruded by late mafic dikes which are present as boudinaged or lenticular shapes in the melange and surrounding gneisses and are intruded by younger ca 2 5 ga pegmatites geochemical characteristics of the late mafic dikes are consistent with an arc related mantle source region rather than an oib like source accordingly pending determination of their precise age an arc polarity reversal event is proposed to have occurred following the arc continent collision resulting in east dipping subduction beneath the newly accreted arc and eastern block forming the swarm of arc affinity mafic dikes the new subduction zone further resulted in ponded magma below the accreted arc block which induced the partial melting of thickened crust and the intrusion of ca 2 5 ga granitic and pegmatitic melts into the melange and the mafic dikes the mafic dikes were then deformed and metamorphosed when the remaining open part of the ocean between the arc collision modified margin of the eastern block collided with the western block of the ncc

weathering of submerged stressed calcarenites chemo mechanical coupling mechanisms long portions of the apulian coast are steep cliffs in carbonate soft rocks these especially the calcarenite are affected by weathering processes that markedly alter their mechanical properties with time potentially leading to instability of coastal geomorphological structures such alterations are mainly due to chemical reactions between the solid and fluid phases and are driven by chemical variables which are internal variables and hence uncontrollable in a search for the variables that drive the process of rock weakening recourse is made to the micro scale at which most of the chemical processes are observed and quantified observations using scanning electron microsope thin sections and x ray computed tomography analyses appear to be crucial for the understanding interpretation and definition of the degradation mechanisms of the material a chemo mechanical coupled model at the meso scale of the chemically reactive stressed porous system is presented and framed in the context of a multi sca

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

the neoarchean ultramafic mafic complex in the yinshan block north china craton magmatic monitor of development of archean lithospheric mantle abstract neoarchean ultramafic mafic complexes are critical for characterizing the embryonic continental lithosphere and hence for arbitrating the extrapolation of plate tectonics into the archean geology this geochronological and geochemical study documents the newly identified amphibolite to granulite facies xiwulanbulang xwlbl ultramafic mafic complex in the yinshan block north china craton ncc it is composed mainly of meta gabbro hornblendite and serpentinite sims u pb zircon dating on the later granite constrains a minimum formation age of 2581 ma for the complex whereas the metamorphic zircons from the hornblendite and coexisting kyanite garnet gneiss yield a metamorphic age of 2 50 ga the dominant meta gabbros in the complex show an morb composition and characterized by an lree depleted to flat ree pattern yet possessing prominent nb zr and ti depletions which suggest dual geochemical features combined with their depleted nd t isotope compositions 1 77 to 2 23 and regional high temperature high mg basaltic magmatism we suggest that they may have been produced by interaction of the upwelling asthenosphere with the metasomatized lithospheric mantle minor hornblendites pyroxenites that occurred in the complex display high mgo 13 6 20 2 wt cao 7 5 13 8 wt cr 475 2049 ppm and ni 154 859 ppm contents reflecting they have originated either as a crystal mush magma or high degree partial melts of a mantle source they show moderately fractionated ree patterns la yb n 2 9 10 7 but depleted nd t 1 48 to 3 53 and zircon hf t up to 7 3 values considering their negative anomalies for nb zr and ti relative to neighboring ree we suggest that the mantle source of the hornblendites was refractory depleted peridotite that experienced prior basaltic magma extraction followed by metasomatism of subduction related fluids systematic variations trend in both major and trace elements between the hornblendites and the associated serpentinites support their genetic linkage high cao 0 7 6 9 wt and al2o3 3 9 6 9 wt contents but low mg 79 85 imply that the serpentinites were likely the cumulates segregated from the coexisting hornblendite pyroxenite magma instead of mantle peridotite taking into full account the regional tectono thermal events we conclude that the xwlbl ultramafic mafic complex was produced in a neoarchean subduction related zone which was more likely trigged by the upwelling asthenosphere mantle as a result of rollback of subducted oceanic lithosphere

weathering distribution in a steep slope of soft pyroclastic rocks as an indicator of slope instability abstract weathering of rocks on steep slopes influences slope instability and sometimes leads to slope failure during heavy rainfall to determine the weathering intensity of hillslope rocks and its inward change from the slope surface measurements of the penetrative hardness of rocks which correlates with their strength were made in some slopes composed of soft pyroclastic rocks on kyushu island japan a gradual reduction in hardness from the interior to the slope surface was recognized within slopes the hardness change was almost concordant with changes in other weathering indices such as rock color clay minerals and apparent grain size the weathered zone indicated by the reduction in hardness was 1 2 m thick at the foot of steep slopes which are frequently subjected to gullying and surface failures whereas the zone was 7 8 m thick at slope summits which are gentle and stable for a long period although the inward change in hardness was gradual on all parts of a slope the change was marked on the foot slope where the strength contrast between weathered and non weathered rock was greater the thickness of the weathered zone on the foot slope therefore controls the volume of failed materials when a failure occurs the difference in weathered thickness between the foot and the summit of a slope much affects the style of failure in general and consequently shapes the characteristic configuration of slopes in this region the estimated weathering rate on a slope surface based on hardness distribution was on the order of 10 2 to 100 cm year 1 assuming a constant rate this rate is far greater than that of other rocks therefore the frequent occurrence of slope failures in this region was attributed to this rapid weathering

geomechanical and geochemical changes during early stages of weathering of karamu basalt new zealand abstract variations in geochemical and geomechanical characteristics were measured through a basalt weathering sequence exposed at an abandoned quarry at karamu near hamilton new zealand geochemical measurements show that at the early stages of weathering fresh to slightly weathered reductions in the concentrations cao and mgo are rapid although they are not accompanied by major mineralogical change during later stages of weathering secondary clay minerals form following well described mineralogical changes fresh basalt is strong uniaxial compressive strength 262 mn m 2 point load index 5 59 with well developed closely spaced columnar jointing slightly weathered material is distinguished in the field only by slight discolouration along joint planes but laboratory tests show a dramatic loss of strength in slightly weathered rock point load index 0 41 cbr 43 a causal relationship between the loss of cations and loss of strength in the early stages of weathering is suggested this stage of weathering is diffusion controlled whereby cations are lost from the constituent primary minerals and replaced by h this process disrupts the lattice structure and causes a marked loss of strength implications are that slightly weathered rock which appears competent in the field is likely to have much lower strength and durability than assumed from field evidence

a neoarchean dismembered ophiolite complex from southern india geochemical and geochronological constraints on its suprasubduction origin abstract ophiolites the remnants of ancient oceanic lithosphere have been described from collisional sutures of various ages with only few examples from archean terranes here we report the discovery of a neoarchean ophiolite suite from the southern margin of the dharwar craton in india tectonically intercalated within a neoproterozoic suture zone the metamorphosed and variably dismembered ophiolite suite exposed around devanur comprises altered ultramafic units websterite gabbros mafic dykes amphibolites trondhjemites and pegmatites associated with ferruginous metachert structural and petrographic studies indicate that the rocks represent a highly sheared and metamorphosed suite emplaced as a thrust sheet the major and trace element geochemistry of the mafic dykes indicate derivation from basaltic andesitic magmas with tholeiitic to calc alkaline characteristics the rocks display negative nb anomalies with enrichment of lile k rb ba th and depletion in hfse ti nb hf tb the tectonic discrimination of these rocks based on various geochemical plots suggests that they were generated in a suprasubduction zone setting we present new shrimp zircon u pb data for two trondhjemite samples from this complex which yield 238u 206pb ages of 2528 61 and 2545 56 ma the neoarchean age from the trondhjemites obtained in our study is closely comparable to similar ages obtained in recent studies from magmatic zircons in charnockites and orthogneisses in the area the suprasubduction zone assemblages and arc magmas suggest a neoarchean ocean closure along the southern margin of the dharwar craton

combined roles of saturated permeability and rainfall characteristics on surficial failure of homogeneous soil slope abstract both rainfall characteristics rainfall intensity and duration and saturated permeability of soil may influence the type and mechanism of surficial slope failures in general the failures can be initiated by two mechanisms i e loss of matric suction through propagation of wetting front and rise of water table up to date there are still no clear indicators to identify the dominant parameters that control the type of failure for these shallow landslides this paper investigates the hydraulic responses of soils to the variations of rainfall characteristics and soil permeability through numerical analyses the results showed that the hydraulic responses to rainfall for a homogeneous infinite slope underlain by an impermeable layer can be divided into two stages 1 the propagation of wetting front and 2 the rise of water table based on these hydraulic responses the type and mechanism of failures were deduced from the analytical analyses both the rainfall characteristics and saturated permeability were found to be predominant in controlling the hydraulic responses of soil and hence the occurrence time depth of failure plane and type of surficial slope failures

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

u pb zircon study of tectonically bounded blocks of 2940 2840 ma crust with different metamorphic histories paamiut region south west greenland implications for the tectonic assembly of the north atlantic craton abstract new fieldwork map interpretation petrography and single zircon u pb geochronology has allowed the identification of different crustal blocks in the paamiut region in the southern portion of the west greenland archaean craton changes of metamorphic grade from only amphibolite facies to granulite facies some subsequently retrogressed corresponds with zones of archaean high strain ductile deformation mylonites u pb zircon dates are presented for the ttg tonalite trondhjemite granodiorite protoliths from each block in the paamiut region and the southern portion of the previously identified tasiusarsuaq terrane lying to the north the southern part of the tasiusarsuaq terrane contains 2880 2860 ma ttg rocks and underwent amphibolite facies metamorphism structurally underneath the tasiusarsuaq terrane to the south is the sioraq block containing 2870 2830 ma ttg rocks partly retrogressed from granulite facies structurally underneath and to the south is the paamiut block dominated by 2850 2770 ma granodioritic rocks that have only undergone amphibolite facies metamorphism also structurally overlying the paamiut block but cropping out separately from the sioraq block is the neria block this appears to be dominated by 2940 2920 ma gneisses that have been totally retrogressed from granulite facies and strongly deformed in the southernmost part of the region the neria block overlies the greenschist to lowermost amphibolite facies sermiligaarsuk block that contains the 2945 ma tartoq group rocks from all the blocks record ancient loss of pb from zircons and some new zircon growth at 2820 ma interpreted to indicate a high grade metamorphic event at that time including granulite facies metamorphism in the sioraq and neria blocks the blocks of different metamorphic grade are interpreted to have moved to their current positions after the 2820 ma metamorphism explaining the change in metamorphic history across some mylonites and ductile shear zones which deform and retrogress granulite facies textures the juxtaposed blocks and their contacts were subsequently folded under amphibolite facies conditions the contacts are cut by undeformed palaeoproterozoic dolerite dykes which post date amphibolite facies metamorphism these results together with previously published data from the godthabsfjord region north of paamiut shows that the north atlantic craton in west greenland from ivittuut in the south to maniitsoq in the north 550 km consists of a mosaic of ductile fault bounded packages that attained their present relative positions in the late archaean

archaean andesite petrogenesis insights from the graedefjord supracrustal belt southern west greenland we present new whole rock major trace and platinum group element data as well as sm nd and lu hfisotope data for meta volcanic rocks from the mesoarchaean graedefjord supracrustal belt gsb locatedwithin the tasiusarsuaq terrane southern west greenland we also present new in situ zircon u pbisotope data by la icp ms for associated felsic rocks this region has experienced amphibolite to lowergranulite facies metamorphism causing re equilibration of most mineral phases including zircon an intrusive tonalite sheet with a zircon u pb age of 2888 6 8 ma yields a minimum age for the gsb the sm nd and lu hf isotope data do not provide meaningful isochron ages but the isotope compositionsof the mafic rocks are consistent with the ca 2970 ma regional volcanic event which is documented in pre vious studies of the tasiusarsuaq terrane the major and trace element data suggest a significant crustalcontribution in the petrogenesis of andesitic volcanic rocks in the gsb the trace element variation of theseandesitic leucoamphibolites cannot be explained by bulk assimilation fractional crystallisation afc processes involving local basement rather the observed patterns require binary mixing between basalticand felsic end member magmas with between 50 and 80 contributions from the latter depending onthe assumed felsic composition hf isotope constraints point to contamination with pre existing conti nental crust with an age of ca 3250 ma basement gneisses of this age were previously described at twolocalities in the tasiusarsuaq terrane which supports the mixing hypothesis thus the felsic end memberlikely represents melts derived from the local basement ultramafic rocks 18 35 22 80 wt mgo in gsb have platinum group element pge patterns that aresimilar to magmas derived from high degree melting of mantle but they have relatively enriched traceelement patterns we propose that the ultramafic rocks represent arc related picrites or alternativelywere derived by melting of metasomatised sub continental lithospheric mantle overall these new geochemical data from the mesoarchaean graedefjord supracrustal belt and thepetrogenetic mixing model in particular are similar to observations from modern continental subductionzone environments which also require large degrees of mixing with felsic basement melts therefore we propose that the metavolcanic rocks formed in a modern style subduction zone geodynamic setting which due to the hotter archaean mantle conditions allowed for substantial amounts of partial meltingand magma mixing rather than assimilating pre existing continental crust less

a neoarchean subduction polarity reversal event in the north china craton abstract subduction polarity reversal events following arc continent arc arc or continent continent collisions have been well documented from cenozoic mesozoic and paleozoic orogens but not from the archean we here document a neoarchean subduction reversal event after an arc continent collision between the eastern block of the north china craton ncc and the fuping arc using field geochemical and geochronological data we focus our work on the wangjiazhuang granite in the zanhuang massif located along the eastern margin of the central orogenic belt cob of the ncc and a regional tectonic comparison with other granitic rocks with similar ages geochemical and petrogenetic characteristics the ca 2 5 ga a type wangjiazhuang granite intrudes the neoarchean zanhuang melange belt and contains mafic and felsic inclusions it has positive end t values 0 12 to 1 13 and tdm2 ages between 2784 ma and 2869 ma this work shows clearly from field structural relationships geochemistry and geochronology that the wangjiazhuang granite formed after an arc continent collision between the eastern block which is defined as a continental block and the fuping arc after a subduction polarity reversal event placed a new slab beneath the collisionally modified margin of the eastern block and converted it to an andean type margin the subduction polarity reversal event at ca 2 5 ga resulted in melting of the enriched mantle meanwhile the rising magma induced partial melting of the old and thickened ttg crust leading to the intrusion of ca 2 5 ga wangjiazhuang granite into the neoarchean zanhuang melange there are other granitic rocks with similar ages and geochemical and petrogenetic features in the central orogenic belt and eastern block of the north china craton suggesting that they formed in a similar tectonic setting as the circa 2 5 ga granites across the eastern block the neoarchean subduction polarity reversal event and prior arc continent collision provide strong evidence that plate tectonics was operating by the end of the neoarchean

evidence for neoarchean hydrous arc magmatism the anorthosite bearing mayville intrusion western superior province canada abstract the neoarchean 2742 8 0 8 ma megacrystic anorthosite bearing mayville layered intrusion of the bird river greenstone belt in the superior province of canada is well preserved and well exposed given its good preservation we studied the mayville intrusion through a combination of extensive field and petrographic observations mineral chemistry and whole rock major and trace element and sm nd and u th pb isotopic analyses to unravel its petrogenesis and geodynamic setting and determine whether its amphibole is of igneous origin and indicative of hydrous magmatism the mayville intrusion intrudes pillow basalts and gabbros of the mayville assemblage which was erupted onto and emplaced into the mesoarchean phase of the maskwa lake tonalite trondhjemite granodiorite ttg batholith some of the anorthosites and leucogabbros of the mayville intrusion have undergone extensive shearing and hydrothermal alteration and had their sm nd and u th pb isotope systems reset despite metamorphism and deformation affecting the mayville intrusion it has largely retained its primary igneous minerals and textures and trace element and sm nd and pb isotope compositions abundant interstitial magmatic amphibole oikocrysts in the anorthosites and leucogabbros exhibit whole grain optical continuity and well preserved igneous grain boundaries with magmatic plagioclase indicating that the mayville intrusion crystallized from a hydrous magma the mayville intrusion has variably positive end 2743 ma values 0 7 to 3 9 that are indicative of its derivation from a depleted mantle source the anorthosites and leucogabbros of the mayville intrusion crystallized from a hydrous ca and al rich tholeiitic parental magma that fractionated from a hydrous primitive arc tholeiitic parental magma these lithologies were derived by partial melting of a depleted shallow hydrous sub arc mantle wedge source the parental magmas to the gabbros leucogabbros and anorthosites of the anorthosite to leucogabbro zone subsequently experienced minor contamination by mesoarchean ttgs maskwa lake ttg batholith the field relationships and trace element geochemistry of the mayville intrusion and its proximity to the continental arc maskwa lake ttg batholith indicate that it formed in a chilean style continental back arc geodynamic setting the findings of this study demonstrate that hydrous arc magmatism and modern style plate tectonic processes have been operating since at least the late archean

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

setting of the 2560 ma qorqut granite complex in the archean crustal evolution of southern west greenland the archean gneiss complex of west greenland contains packages of unrelated rocks created during relatively short periods of time in arc like magmatic environments and having similarities to rocks formed at phanerozoic convergent plate boundaries the terranes of new archean crust were amalgamated by collisional orogeny and then partitioned by post assembly tectonic processes having summarized the origin of west greenland archean crust in arc like environments this paper then focuses on new data concerning the latest neoarchean post terrane assembly intra continental tectonic and magmatic evolution of the region following the youngest documented high pressure metamorphism in a clockwise p t t loop at 2650 ma attributed to tectonic thickening of the crust there is in west greenland a 150 million year record of intermittent production of crustally derived granite shearing and folding under amphibolite facies conditions this is exemplified by the ssw nne orientated neoarchean qorqut granite complex qgc which forms a myriad of closely spaced coeval sheets ne of nuuk town shrimp u pb zircon dating of a homogeneous gray granite sheet gives a magmatic age of 2561 11 ma with 3800 to 3600 and 3070 to 2970 ma zircon xenocrysts the 40 km long fjringehavn straight belt a lower amphibolite facies vertical shear zone runs from the qgc s se margin and contains strongly deformed granite sheets with a u pb zircon age of 2565 8 ma but is cut by undeformed granite sheets dated at 2555 12 ma the 60 km long ivisaartoq fault consisting of lowermost amphibolite facies mylonite runs from the northwestern end of the main mass of granite it formed post 2630 ma because granites of that age are truncated by it near the qgc s northeastern extent 2559 3 ma granitic lithons in a folded mylonite are cut by 2521 72 ma granite sheets at a deep structural level at the northern end of the qgc deformed granitic neosomes give ages of 2567 9 and 2563 5 ma therefore at 2560 ma the ages within error for strongly deformed to non deformed granite bodies shows that the qgc is not a largely post kinematic intrusion as previously thought but was coeval with lowermost amphibolite facies metamorphism and shear zones with an important strike slip component late in the development of regional non cylindrical upright folds the main body of the qgc appears to be essentially post kinematic only because it was emplaced in a node of dilation during the heterogeneous predominantly strike slip deformation melting at this node may have been triggered by meteoric water percolating down dilational fractures causing metasomatism melting of these altered rocks gave rise to the low 18o signature of qgc igneous zircons due to the hydrous nature of the melting event the qgc was emplaced immediately above its migmatitic generation zone these late neoarchean shear zones of the nuuk region partition and disrupt the earlier formed mosaic of amalgamated terranes of unrelated rocks such tectonic patterns are seen more recently for example in holocene asian intra continental tectonics along the north side of the himalayas

geologic factors contributing to landslide generation in a pyroclastic area august 1998 nishigo village japan vertical contrasts in permeability particularly where permeable surface materials overlie impermeable materials that prohibit the downward infiltration of groundwater concentrate the groundwater and become an important focus of landslides that are triggered by intense rainfall just such a hydrogeological structure is present within the pyroclastics in nishigo village in fukushima prefecture japan where intense rainfall of 1200 mm in 6 days generated more than 1000 landslides in august 1998 three types of landslides occurred the first type occurred along the edges of small plateaus where horizontal beds of permeable ash scoria and pumice overlie impermeable mudflow deposits consisting of tuffaceous fines and andesite blocks and massive weakly consolidated ignimbrites the rainfall on the plateaus infiltrated downward first then laterally within the permeable beds finally gushing out at the plateau edges and triggering landslides the second type of landslide occurred where weathered tuff of the same ignimbrite was present with a slip surface at the base of the heavily weathered zone within this heavily weathered zone the tuff exfoliated into thin weak plates running parallel to the slope surface the third type of landslide involved failure of colluvium or ash that filled hollows this type occurred as a result of subsurface erosion caused by the groundwater infiltrating the superficial beds above the impermeable tuff

the geodynamic evolution of mesoarchean anorthosite complexes inferred from the naajat kuuat complex southern west greenland layered anorthosite complexes are typical components of archean crustal domains however the geodynamic settings in which they were emplaced are still discussed as geological relationships are oft

field recognition of rocks totally retrogressed from granulite facies an example from archaean rocks in the paamiut region south west greenland abstract since the middle of the 1970s field criteria have been used in the archaean high grade gneiss complexes of southern west greenland to distinguish amphibolite facies rocks retrogressed from granulite facies from amphibolite facies rocks that never reached granulite facies examples of the textural criteria are figured from different areas many of them are the result of growth during dehydration partial melting of large grains of orthopyroxene that were subsequently retrogressed to large grains or aggregates of hornblende an example of the use of this field technique is given from the paamiut region in the southern part of the archaean craton rocks metamorphosed to granulite facies but partly or totally retrogressed to amphibolite facies have been found to crop out over about half of the paamiut region the metamorphic grade in most of this geologically well mapped region was previously considered not to have exceeded amphibolite facies the paamiut region can now be shown to be made up of three tectonically bounded blocks the sioraq block in the north and the neria block in the south which were metamorphosed to granulite facies overlie the paamiut block which was not metamorphosed above amphibolite facies

dynamic earth system and ecological controls of rainfall initiated landslides abstract rainfall initiated landslides continue to inflict damages and loss of life throughout the world processes and mechanisms revealed from hydrological geomorphic geotechnical pedological geological hydrochemical and biological investigations have advanced our understanding of these effects on slope stability however the interactions amongst these processes and attributes as they affect the initiation and propagation of landslides are not as well understood too often landslide studies are conducted from only one or at most two of these perspectives moreover while precipitation and hydrology are recognized as dynamic influences the earth system and ecological effects are often assumed to be static we assess the interplay of these processes related to landslides triggered by positive pore water accretion and loss of soil suction each of these conditions arguably requires a different view on the processes that cause slope failure and predictive approaches this review starts from the perspective of dynamic adapting earth and ecological systems and discusses how these attributes relate to landslide initiation mode location and timing specifically the role that large and small scale preferential flow plays in both contributing to and mitigating instability is elucidated including effects of bedrock exfiltration we also examine how and under what conditions these pathways manifest in different soils lithology and landforms the multiple effects of rhizosphere processes on slope stability are discussed including root reinforcement evaporation from canopies and litter layers transpiration and the role of root structure affecting preferential flow paths rainfall initiated landslides involve highly dynamic hydrologic earth surface and ecological processes that persist over a range of spatial and temporal scales however guidance for overcoming these challenges has been elusive a conceptual framework is presented to shed light on these dynamic and interactive processes that should lend insights into why and when certain slopes fail during storms while other seemingly similar slopes do not fail such advances will benefit landslide hazard assessments and disaster responsiveness protocols

a 2 5 ga fore arc subduction accretion complex in the dengfeng granite greenstone belt southern north china craton abstract the dengfeng granite greenstone belt dggb located in the southern segment of the central orogenic belt cob of the north china craton ncc consists of a volcano sedimentary assemblage intruded by tonalite trondhjemite granodiorite ttg suite diorite granites and late mafic dikes the volcano sedimentary assemblage in the dggb mainly consists of tectonically imbricated basaltic amphibolites meta gabbroic rocks with minor ultramafic rocks and metagreywacke marble and quartzite consistent with characterisitics of typical phanerozoic subduction accretion complexes the basaltic amphibolites yield a metamorphic zircon 207pb 206pb age of 2507 26 ma interpreted to represent the peak age of amphibolite facies metamorphism that took place during subduction accretion of the basaltic protolith the basaltic amphibolites are characterized by a tholeiitic affinity and flat lree patterns with minor negative nb and zr anomalies based on mixed morb and arc affinities the basaltic amphibolites in the dggb are interpreted to have formed in a fore arc tectonic setting one late potassic granite dike cutting across the fabrics of the volcano sedimentary assemblage yields an intrusion age of 2492 35 ma constraining the minimum deformation age for tectonic assembly of the package the ttg gneisses and diorites intruded the western margin and the center of the subduction accretion complex respectively one ttg sample and one diorite sample yield igneous zircon 207pb 206pb ages of 2514 26 ma and 2518 36 ma respectively constraining their intrusion ages the ttg gneisses display high ratios of la yb cn and sr y and depletion in hfse with negative nb ta and ti anomalies consistent with those of typical archean ttgs the ttg gneisses are therefore considered to be generated from partial melting of a shallowly subducting oceanic slab and or accreted arc amphibolites the diorites have high concentrations of mgo 2 89 6 05 wt ni 148 178 ppm and cr 85 7 120 6 ppm and highly fractionated ree patterns and are depleted in hfses with negative nb ta zr hf and ti anomalies we suggest that the high mg diorites in the dggb may have been derived from a hydrated mantle wedge which was previously metasomatized by subduction derived melts and or fluids collectively a neoarchean subduction accretion collision event is therefore proposed to have generated the dggb the volcano sedimentary assemblage in the dggb represents a fore arc subduction accretion complex which is interpreted to be related to the suture zone of ca 2 5 ga arc continent collision between a ttg dominated arc terrane in the cob and the eastern block of the ncc we further propose a long n s striking neoarchean suture zone occurring in the eastern margin of the cob mainly consists of the ca 2 5 ga subduction accretion complex in the dggb to the south the ca 2 5 ga melange belt in the zanhuang complex in the central and the ca 2 5 ga ophiolitic melange belt in the zunhua dongwanzi structural belt to the north which separate an arc terrane in the cob from the eastern block of the ncc

low gradient permeability measurements in a triaxial system permeability measurements were conducted with the flow pump method on sand sandy silt and silty clay specimens in a conventional triaxial system by introducing and withdrawing water at known constant flow rates into the base of a specimen with a flow pump and by monitoring the head difference induced across the length of the specimen with a sensitive differential pressure transducer the results show that the previously reported advantages of the flow pump method compared with conventional constant head and falling head methods were realized for permeability measurements in conventional triaxial equipment these advantages are that direct flow rate measurements are avoided together with the associated errors that arise from the effects of contaminants on capillary menisci and the long periods of time involved in flow rate measurements permeability measurements can be obtained much more rapidly and at substantially smaller gradients errors from the small intercept in the otherwise linear flow rate ve

the dynamics of big mantle wedge magma factory and metamorphic metasomatic factory in subduction zones abstract the east asian continental margin is underlain by stagnant slabs resulting from subduction of the pacific plate from the east and the philippine sea plate from the south we classify the upper mantle in this region into three major domains a metasomatic metamorphic factory mmf subduction zone magma factory szmf and the big mantle wedge bmw whereas the convection pattern is anticlockwise in the mmf domain it is predominantly clockwise in the szmf and bmw along a cross section from the south here we define the mmf as a small wedge corner which is driven by the subducting pacific plate and dominated by h 2 o rich fluids derived by dehydration reactions and enriched in large ion lithophile elements lile which cause the metasomatism the szmf is a zone intermediate between mmf and bmw domains and constitutes the main region of continental crust production by partial melting through wedge counter corner flow large hydrous plume generated at about 200 km depth causes extensive reduction in viscosity and the smaller scale hydrous plumes between 60 km and 200 km also bring about an overall reduction in the viscosity of szmf more fertile and high temperature peridotites are supplied from the entrance to this domain the domain extends obliquely to the volcanic front and then swings back to the deep mantle together with the subducting slab the bmw occupies the major portion of upper mantle in the western pacific and convects largely with a clockwise sense removing the eastern trench oceanward sporadic formation of hydrous plume at the depth of around 410 km and the curtain flow adjacent to the trench cause back arc spreading we envisage that the heat source in bmw could be the accumulated ttg tonalite trondhjemite granodiorite crust on the bottom of the mantle transition zone the ongoing process of transportation of granitic crust into the mantle transition zone is evident from the deep subduction of five intra oceanic arcs on the subducting philippine sea plate from the south in addition to the sediment trapped subduction by the pacific plate and philippine sea plate the dynamics of mmf szmf and bmw domains are controlled by the angle of subduction a wide zone of mmf in sw japan is caused by shallow angle subduction of the philippine sea plate and the markedly small mmf domain in the mariana trench is due to the high angle subduction of pacific plate the domains in ne japan and kyushu region are intermediate between these two during the tertiary a series of marginal basins were formed because of the nearly 2000 km northward shift of the subduction zone along the southern margin of tethyan asia which may be related to the collision of india with asia and the indentation the volume of upper mantle under asia was reduced extensively on the southern margin with a resultant oceanward trench retreat along the eastern margin of asia leading to the formation of a series of marginal basins the western pacific domain in general is characterized by double sided subduction from the east by the oldest pacific plate and from the south by the oldest indo australian plate the old plates are hence hydrated extensively even in their central domains and therefore of low temperature the cracks have allowed the transport of water into the deeper portions of the slab and these domains supply hydrous fluids even to the bottom of the upper mantle thus a fluid dominated upper mantle in the western pacific drives a number of microplates and promote the plate boundary processes

comparative petrology of archaean anorthosites in amphibolite and granulite facies terranes sw greenland we report new field and petrographic observations and mineral chemical data on the amphibolite facies buksefjorden and granulite facies nordland anorthosites which occur in different tectonostratigraphic terranes within the archaean gneiss complex of sw greenland the buksefjorden body from the akulleq middle terrane is dominated by plagioclase and ca amphibole but shows widespread effects of retrograde hydration epidote chlorite most plagioclase compositions are in the an60 82 range with the majority of samples showing average core compositions an76 whereas rims or recrystallized margins are an65 most grains in the an70 82 range display optically visible huttenlocher intergrowths amphiboles at buksefjorden are mainly magnesio hornblende with xmg ranging from 0 70 to 0 45 the nordland anorthosite from the akia northern terrane is also dominated by plagioclase and ca amphibole but contains additional clinopyroxene ca47mg38fe15 as well as minor orthopyroxene en68 spinel and corundum plagioclase at nordland shows an equilibrated equigranular texture consistent with prolonged slow cooling from high temperatures despite this textural equilibration plagioclase at nordland shows a striking range of compositions from an28 to an97 most of which is found in single thin sections a distinctive feature is the presence of discrete anorthite spinel corundum domains in some samples although a number of explanations may apply we consider these domains to result from prograde mass transfer reactions involving ca amphibole and plagioclase amphibole compositions at nordland show similar xmg to those at buksefjorden but are more aluminous alkalic and titanian this shift to more pargasitic compositions is consistent with the contrasts in metamorphic grade between the two anorthosite bodies at buksefjorden there is no correlation between the amount of modal ca amphibole and plagioclase composition which would be expected if amphibole was produced solely through metamorphism our results suggest alternatively that the primary igneous mineralogy of these rocks may have been plagioclase an76 hornblende pyroxene magnetite the primary mineralogy at nordland is less certain but it is noteworthy that no rocks contain anorthite of unambiguous igneous origin in contrast to some other occurrences of archaean anorthosites

geochemistry of ultramafic rocks and hornblendite veins in the fiskenaesset layered anorthosite complex sw greenland evidence for hydrous upper mantle in the archean abstract the fiskenaesset complex sw greenland contains the world s best preserved archean 2970 ma layered anorthosite leucogabbro gabbro and ultramafic association the complex was emplaced into archean oceanic crust distal from continental lithosphere and later intruded by tonalites trondhjemites and granodiorites ttg constituting archean continental crust the complex and bordering ttg intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions this study presents new whole rock major and trace element petrographic and sem scanning electron microscope bse backscatter electron image data for a 45 60 m thick ultramafic sill and cross cutting hornblendite veins the ultramafic sill is composed of 75 olivine pyroxene hornblendite 15 pyroxene hornblendite and 10 hornblende pyroxenite despite granulite facies metamorphism and multiple phases of deformation the fiskenaesset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and alaskan type ultramafic complexes hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism formation of orthopyroxene magnetite symplectitic vermicular intergrowths mainly at the expense of olivine is attributed to chemical reactions between late stage residual hydrous melts and olivine field geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion high mgo 16 31 wt contents suggest that the sill was emplaced as a crystal mush rather than as melt only petrographic observations and bse images indicate that olivine orthopyroxene and clinopyroxene were the major crystal phases whereas hornblende represents the main interstitial liquid phase in the crystal mush the sill exhibits three different ree patterns corresponding to three cryptic layers although the origin of these ree patterns is not fully understood they may reflect the intrusion of three batches of magma containing different hornblende forming melt crystal ratios geochemical data indicate that the cross cutting hornblendite veins likely originated from evolved late stage melts but were not related to the melts of the ultramafic sill large negative nb anomalies nb nb 0 04 0 66 suggest that the fiskenaesset hornblendites hornblende pyroxenites and hornblendite veins were derived from a hydrous sub arc mantle peridotite phanerozoic hornblendites are typically associated with supra subduction zone ophiolites and magmatic arcs accordingly it is suggested that water was recycled to the source of the fiskenaesset hornblendites through subduction of altered oceanic crust recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites but also played an important role in the formation of ttg dominated archean continental crust therefore the field petrographic and geochemical data presented in this study suggest that the origin of the mesoarchean to neoarchean terrane in the fiskenaesset region sw greenland is consistent with subduction zone geodynamic processes rather than non uniformitarian density driven vertical crustal overturn and diapiric processes

silicic volcanism on mars evidenced by tridymite in high sio2 sedimentary rock at gale crater tridymite a low pressure high temperature 870 c sio2 polymorph was detected in a drill sample of laminated mudstone buckskin at marias pass in gale crater mars by the chemistry and mineralogy x ray diffraction instrument onboard the mars science laboratory rover curiosity the tridymitic mudstone has 40 wt crystalline and 60 wt x ray amorphous material and a bulk composition with 74 wt sio2 alpha particle x ray spectrometer analysis plagioclase 17 wt of bulk sample tridymite 14 wt sanidine 3 wt cation deficient magnetite 3 wt cristobalite 2 wt and anhydrite 1 wt are the mudstone crystalline minerals amorphous material is silica rich 39 wt opal a and or high sio2 glass and opal ct volatile bearing 16 wt mixed cation sulfates phosphates and chlorides perchlorates chlorates and has minor tio2 and fe2o3t oxides 5 wt rietveld refinement yielded a monoclinic structural model for a well crystalline tridymite consistent with high formation temperatures terrestrial tridymite is commonly associated with silicic volcanism and detritus from such volcanism in a lake gale catchment environment can account for buckskin s tridymite cristobalite feldspar and any residual high sio2 glass these cogenetic detrital phases are possibly sourced from the gale crater wall rim central peak opaline silica could form during diagenesis from high sio2 glass as amorphous precipitated silica or as a residue of acidic leaching in the sediment source region or at marias pass the amorphous mixed cation salts and oxides and possibly the crystalline magnetite otherwise detrital are primary precipitates and or their diagenesis products derived from multiple infiltrations of aqueous solutions having variable compositions temperatures and acidities anhydrite is post lithification fracture vein fill

formation of the neoarchean bad vermilion lake anorthosite complex and spatially associated granitic rocks at a convergent plate margin superior province western ontario canada abstract the bad vermilion lake anorthosite complex henceforth the bvla complex in western ontario is one of the well exposed anorthosite bearing archean layered intrusions in the superior province canada this study presents new whole rock major and trace element data for the various units of the complex oxygen isotope data for the anorthosite and major and trace element data for the spatially associated granitic rocks intruding the bvla complex to constrain their petrogenetic and geodynamic origin zircons from granitic rocks have yielded a 207pb 206pb age of 2716 18 ma constraining the minimum intrusion age of the complex despite deformation and greenschist facies metamorphism primary igneous textures are locally well preserved in the bvla complex its whole rock major and trace elemental compositions and the oxygen isotopic systematics appear not to have been substantially modified by deformation and metamorphism mantle like oxygen isotope signatures and major and trace element compositions are inconsistent with significant crustal contamination of the bvla complex during its emplacement the existence of primary calcic igneous plagioclase coherent negative nb anomalies nb nb 0 08 0 88 and geochemical similarities between gabbros from the bvla complex and gabbros from cenozoic arcs collectively suggest an intra oceanic subduction zone geodynamic setting for the complex near flat ree patterns in the various units of the bvla complex suggest that they were derived from melting of a shallow source beneath a subarc mantle wedge trends in immobile major e g mgo and trace e g zr element data indicate that the mineralogical composition of the complex can be explained by fractional crystallization and accumulation of olivine orthopyroxene clinopyroxene plagioclase and possibly amphibole compositionally the bordering granitic rocks are a2 type and strongly enriched in th and ree 100 times chondrite and depleted of ba sr eu and ti we suggest that they formed in a post collisional extensional tectonic regime following emplacement of the bvla complex in an oceanic arc

an overview of the geochemistry of eoarchean to mesoarchean ultramafic to mafic volcanic rocks sw greenland implications for mantle depletion and petrogenetic processes at subduction zones in the early earth abstract this study reviews the geochemical characteristics of eoarchean to mesoarchean ultramafic to mafic volcanic rocks now amphibolites in sw greenland and compares them with those of cenozoic oceanic island arc basalts to evaluate archean subduction zone petrogenetic processes emphasis is placed on the th ree hfse zr ti and nb systematics of the ca 3800 and ca 3700 ma arc suites in the isua greenstone belt the ca 3075 ma ivisaartoq ujarassuit greenstone belt and amphibolites associated with the ca 2970 ma fiskenaesset layered anorthosite complex on n morb normalized diagrams the isua ivisaartoq ujarassuit and fiskenaesset volcanic rocks are all characterized by depletion of nb relative to th and lree consistent with a supra subduction forearc arc backarc geodynamic setting similarly on the th yb nb yb projection these suites plot within the field of cenozoic oceanic island arc basalts on log transformed immobile trace element ratio nb th la th sm th and yb th diagrams they display a trend projecting from morb mid ocean ridge basalt to iab island arc basalt on the iab crb continental rift basalt oib ocean island basalt morb diagram as for cenozoic oceanic island arc basalts accordingly these trace element compositions are interpreted as reflecting the enrichment of archean depleted upper mantle morb source by subduction derived melts and fluids following the initiation of intra oceanic subduction and arc migration concentrations of mgo and ni in sw greenland archean basalts overlap with but extend to 2 to 4 times higher than those in cenozoic oceanic island arc counterparts in contrast the majority of archean basalts have ree and hfse concentrations 2 to 4 times lower than cenozoic oceanic island arc basalts consistent with more depleted sub arc mantle wedge peridotites in the archean than in cenozoic counterparts such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter archean mantle we infer that less refractory compositions of the present day depleted upper mantle the source of morb and conservative hfse in arc basalts have resulted from re mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post archean times archean basalts in sw greenland share the negative nb and ti anomalies of cenozoic oceanic island arc basalts on average archean basalts however have lower th nb la nb sm tio2 and gd tio2 ratios than cenozoic oceanic island arc counterparts given that rutile controls the nb and ti budgets in arc magmas the lower th nb la nb sm tio2 and gd tio2 ratios in sw greenland basalts are attributed to the mobility of nb and ti in slab derived melts involving rutile fusion in archean subduction zones these elements are less mobile in fluids originating from cenozoic subducting oceanic crust where rutile appears to be generally stable accordingly it is suggested that higher geothermal gradients in the archean may have provided optimized conditions for slab melting and metasomatism of the sub arc mantle wedge by slab derived melts

mesoarchean subduction processes 2 87 ga eclogites from the kola peninsula russia the nature of tectonic processes on the early earth is still controversial the scarcity of high pressure metamorphic rocks such as eclogite the high pressure equivalent of basalt in archean cratons has been used to argue that plate tectonics did not operate until earth had cooled to a critical point perhaps around the 2 5 ga archean proterozoic transition however eclogites occur as meter to kilometer sized lenses enclosed in archean gneisses of the belomorian province of the fennoscandian shield geochemistry and internal features suggest that the protoliths of the eclogites were interlayered olivine gabbros troctolites and fe ti oxide gabbros greenschist facies mineral parageneses are enclosed in prograde zoned eclogite garnets and peak metamorphic conditions define an apparent thermal gradient 12 15 c km consistent with metamorphism in a warm archean subduction zone we show here that these eclogites represent the oldest known high pressure metamorphic rocks u pb dating and hf isotope analyses of zircons from the eclogites and a crosscutting felsic vein define a minimum age of 2 87 ga for the uzkaya salma eclogite a 2 70 ga age for the shirokaya salma eclogite is interpreted as the age of a granulite facies overprint thermal overprinting and growth of new zircon also occurred during the svecofennian 1 9 1 8 ga orogeny these new data imply that plate tectonic processes operated at least locally in late mesoarchean time the adakitic nature of the felsic vein suggests that partial melting of hydrated eclogites could produce archean tonalite trondhjemite granodiorite type magmas

geology and tectonostratigraphy of the onverwacht suite barberton greenstone belt south africa abstract a synthesis of new and existing data reveals that at least seven major shear zones and an unconformity separate the rocks of the southern barberton greenstone belt into seven units with different geological histories current views that these units constitute a continuous stratigraphic sequence and their subdivision into successive formations cannot be maintained we argue here that the status of the traditional formations of the onverwacht group of the barberton greenstone belt be changed to complexes and collectively these complexes be referred to as the onverwacht suite the total age range and tectonostratigraphic thickness of the suite is about 120 million years and 15 km respectively the precise age range of the rocks present in each complex is largely unknown the original spatial relationships between the complexes can in most cases only be inferred all of the complexes include significant volumes of intrusions that were emplaced into the volcanic and volcaniclastic host rocks the ages of the intrusions in the different complexes vary in some complexes the intrusions and volcanic rocks have similar geochemistry and are probably co magmatic others provide stitching ages between complexes four of the complexes display internal relationships that resemble ophiolites basaltic pillowed and massive lavas are the most common rock types sheeted dykes are generally absent whilst multiple mafic sills are common and dominate one of the complexes the volcanology and geochemistry of the complexes suggest that they formed predominantly in oceanic spreading and intra oceanic arc environments at water depths of 1 7 4 km the complexes were tectonically stacked during the final stages of their development and during two subsequent orogenic events one complex the hooggenoeg complex was uplifted and deeply eroded between about 3460 and 3470 ma with a minimum uplift rate ca 0 5 mm yr similar to that during emplacement of phanerozoic ophiolites unconformably overlying sub aerial coarse clastic sedimentary rocks part of the newly distinguished noisy complex overlap in age with felsic intermediate igneous activity dated between 3445 and 3460 ma that in turn is genetically related to tonalite trondjemite plutons exposed farther to the south the emergence of the deep water hooggenoeg complex above sea level the start of its erosion and fluvial sedimentation on top of it at ca 3458 ma represents the onset of its obduction onto older complexes some of which are now preserved as enclaves in the granitoid terrains to the south other oceanic slabs the kromberg and the mendon complexes were subsequently emplaced onto the noisy complex during latitudinal displacements ca 12 cm yr that were not notably faster than today exhumation with up to 18 km of unroofing along the southern margin of the greenstone belt took place during a second orogenic episode of subduction and accretion between 3140 and 3230 ma this episode was associated with folding thrusting and molasse type sedimentation within the greenstone belt coupled to extension and ductile faulting at a lower crustal level during this period younger complexes were assembled and the older complexes re cycled and eroded at rates and with geotherms comparable to those in modern orogenic belts in the barberton region between 3 2 and 3 5 ga tectonic processes appear to have operated at rates similar to those within modern evolving plate boundaries

a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland a relict oceanic island arc complex in the eastern akia terrane godthabsfjord southern west greenland constitutes a magmatic and geotectonic link between c 3 05 3 0 ga tonalitic orthogneiss and enclaves of older supracrustal amphibolite the relict arc forms isoclinally folded panels of volcaniclastic meta andesite with major and trace element island arc signatures intercalated with volcano sedimentary schist tholeiitic amphibolite and opx rich cumulate rocks a zircon u pb age of 3071 1 ma obtained from a volcano sedimentary schist is marginally older than the main orthogneisses and is the first depositional age reported from a supracrustal rock within akia terrane granite sheets were emplaced at c 3005 2980 ma synkinematically with the isoclinal folding and were followed by peak metamorphism at 2990 2970 ma with substantial recrystallization of volcanic zircon and mobility of large ion lithophile elements the identification of the arc complex provides new insight into mid archaean continental crustal accretion in west greenland and substantiates previous ideas that the orthogneisses are products of slab melting in convergent plate tectonic settings the presence of the arc complex also implies that archaean high grade orthogneiss amphibolite associations may not represent plate tectonic environments distinct from granite greenstone associations but expose deeper sections of the same convergent systems

a review of structural patterns and melting processes in the archean craton of west greenland evidence for crustal growth at convergent plate margins as opposed to non uniformitarian models abstract the archean craton of west greenland consists of many fault bounded eoarchean to neoarchean tectonic terranes crustal blocks these tectonic terranes are composed mainly of tonalite trondhjemite granodiorite ttg gneisses granitic gneisses metavolcanic dominated supracrustal belts layered anorthositic complexes and late to post tectonic granites rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs consisting mainly of ttg plutons tholeiitic to calc alkaline basalts boninites picrites and cumulate layers of ultramafic rocks gabbros leucogabbros and anorthosites with minor sedimentary rocks the structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics suggesting that the archean craton of west greenland grew at convergent plate margins several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms the terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds geometrically these structures are similar to those occurring on regional scales suggesting that the archean craton of west greenland can be interpreted as a continental scale accretionary complex such as the paleozoic altaids melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of ttgs non uniformitarian models proposed for the origin of archean terranes have no analogs in the geologic record and are inconsistent with structural lithological petrological and geochemical data collected from archean terranes over the last four decades the style of deformation and generation of felsic rocks on outcrop scales in the archean craton of west greenland and the mesozoic sulu orogenic belt of eastern china are similar consistent with the formation of archean continental crust by subduction zone processes

early permian plutons from the northern north china block constraints on continental arc evolution and convergent margin magmatism related to the central asian orogenic belt recent zircon dating identified several late carboniferous to early permian hornblende gabbro diorite quartz diorite granodiorite tonalite granite plutons in lithological assemblages at the northern margin of the north china block ncb that were previously regarded as archaean to palaeoproterozoic our geochronological results indicate that emplacement of these plutons was a continuous process during the late carboniferous to early permian from 324 6 to 274 6 ma and lasted for at least 50 ma in this paper the early permian components with compositions from gabbro to granite within the intrusive complex were studied the early permian plutons exhibit calc alkaline or high k calc alkaline metaluminous geochemical features and highly variable sio2 contents they have no significant eu anomaly in their ree patterns and in primitive mantle normalized spidergrams they display depletion in th u nb ta p and ti and enrichment in ba k pb and sr the granitoid bodies within these plutons display i type and adakitic geochemical signatures the early permian rocks exhibit low whole rock initial 87sr 86sr ratios from 0 70520 to 0 70615 and have negative whole rock e nd t values ranging from 17 4 to 9 3 and zircon e hf t values of 23 2 to 10 5 the gabbros exhibit higher e nd t values from 11 1 to 9 3 and e hf t values from 16 5 to 10 5 and one granodiorite exhibits an even lower e nd t value of 17 4 and zircon e hf t values of 23 2 to 15 1 geochemical sr nd and in situ zircon hf isotopic compositions suggest that the hornblende gabbros were derived from a metasomatized lithospheric mantle and the diorite and quartz diorite were generated from a gabbroic magma by fractional crystallization coupled with differential assimilation of ancient lower crustal material the granodiorite was likely derived from partial melting of ancient lower crust with involvement of some mantle components involvement of both lithospheric mantle and ancient lower crust in the generation of the early permian plutons indicates strong crust mantle interaction in the northern ncb petrological associations as well as geochemical and sr nd hf isotopic results show that the early permian plutons were emplaced along an andean type active continental margin during southward subduction of the palaeo asian oceanic plate beneath the ncb integration of our results with previously published data for late carboniferous and late permian to middle triassic intrusions suggests that the continental arc on the northern margin of the ncb existed for at least 50 ma during the late palaeozoic and final amalgamation of the mongolian arc terranes with the northern ncb likely occurred during a period from 270 to 250 ma i e in the late permian to earliest triassic

archean greenstone belt magmatism and the continental growth mantle evolution connection constraints from th u nb lree systematics of the 2 7 ga wawa subprovince superior province canada abstract an extensive database including th u nb ree systematics for diverse magmatic and sedimentary lithologies of 2 7 ga wawa greenstone belts provide new constraints on the mechanism of crustal growth in the southern superior province and controls on its composition the greenstone belts are characterized by collages of oceanic plateaus oceanic island arcs and trench turbidites these lithotectonic fragments were tectonically assembled in a large subduction accretion complex following juxtaposition these diverse lithologies were collectively intruded by syn kinematic ttg tonalite trondhjemite granodiorite plutons and ultramafic to felsic dykes and sills with subduction zone geochemical signatures intra oceanic basalts are characterized by near flat ree patterns and nb u and nb th ratios generally greater than primitive mantle values consistent with positive nd values they are associated with komatiites the association being interpreted as an ocean plateau sequence erupted from a mantle plume bimodal arc volcanic sequences trench turbidites and contemporaneous ttg suites are characterized by fractionated ree with nb u and nb th ratios less than primitive mantle values mixing hyperbolae between oceanic plateau and magmatic arc sequences pass through the estimated composition of bulk continental crust suggesting that crustal growth in the late archean was by tectonic sedimentary and chemical mixing of oceanic plateau and arc sequences at convergent plate boundaries mixing calculations suggest that oceanic plateau and subduction zone components in the wawa continental crust are represented by 6 12 and 88 94 respectively high nb u and nb th ratios of plateau tholeiitic basalts are interpreted as a complementary reservoir to arc magmatism low nb u and nb th hundreds of millions of years prior to recycling of oceanic lithosphere through a subduction zone high nb u nb th and its incorporation into a mantle plume from which 2 7 ga plateau tholeiites erupted the variably high nb u ratios of the plateau basalts are consistent with early extraction of large quantities of the protoliths magmatic precursor of continental crust from the southern superior province asthenospheric mantle

application of an analytical hierarchical process approach for landslide susceptibility mapping in a luoi district thua thien hue province vietnam a luoi is a vietnamese laotian border district situated in the western part of thua thien hue province central vietnam where landslides occur frequently and seriously affect local living conditions this study focuses on the spatial analysis of landslide susceptibility in this 263 km2 area to analyze landslide manifestation in the study area causative factor maps are derived of slope angle weathering land use geomorphology fault density geology drainage distance elevation and precipitation the analytical hierarchical process approach is used to combine these maps for landslide susceptibility mapping a landslide susceptibility zonation map with four landslide susceptibility classes i e low moderate high and very high susceptibility for landsliding is derived based on the correspondence with an inventory of observed landslides the final map indicates that about 37 of the area is very highly susceptible for landsliding and about 22 is highly susceptible which means that more than half of the area should be considered prone to landsliding

geochemistry of the mesoarchean fiskenaesset complex at majorqap qâva sw greenland evidence for two different magma compositions abstract the mesoarchean fiskenaesset complex was emplaced as multiple sills and dykes of magma and crystal mush into oceanic crust forming an association of ca 550 meter thick layered anorthosites leucogabbros gabbros hornblendites pyroxenites dunites and peridotites the complex was intruded by tonalites trondhjemites and granodiorites ttg and underwent multiple phases of deformation the fiskenaesset complex and bordering ttg intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions this study presents new field petrographic and major and trace element geochemical data for the six major stratigraphic units of the fiskenaesset complex at majorqap q va sw greenland in addition new high precision trace element data are reported for hornblendes from three stratigraphic units the sampled major lithological units from bottom to top are lower gabbro ultramafic lower leucogabbro middle gabbro upper leucogabbro and anorthosite both whole rock and hornblende trace element data are consistent with crystallization of the complex from two different parental magmas originating from different mantle sources subsequent differentiation of these parental magmas in the oceanic arc crust resulted in two evolved geochemical rock suites suite a and suite b which are both characterized by depletions of hfse nb ta zr and hf relative to th u lree and lile rb ba and enrichments of sr and pb relative to mree these data are consistent with a magmatic arc geodynamic setting

the whole rock sm nd age for the 2825 ma ikkattoq gneisses greenland is 800 ma too young insights into archaean ttg petrogenesis abstract the ikkattoq gneisses of the archaean gneiss complex in the nuuk region southern west greenland are the orthogneiss component within the amphibolite facies tre brodre terrane they have mostly granodioritic compositions with a small amount of quartz diorite sm nd isotopic data for a quartz diorite and five granodiorite ikkattoq gneiss samples from within 5 km of the ikkattoq fjord type locality yielded a regression with a slope equivalent to 2005 52 ma mswd 0 72 regardless of the low mswd this cannot be the true age of the ikkattoq gneisses because all ikkattoq gneisses yield u pb zircon dates of c 2825 ma and they are cut by the undeformed 2560 ma qorqut granite complex this anomalously young regression age resulted instead from mixing of different nd components indicating that the ikkattoq gneisses are derived from mixed source materials taking the true age of the ikkattoq gneisses as 2825 ma from u pb zircon dating the range of initial e nd in the ikkattoq gneisses is 7 1 to 1 8 the negative initial e nd values mean that older light rare earth enriched sialic crust contributed to the igneous precursors of the ikkattoq gneisses this nd evidence for contribution of older sialic crust is supported by positive e sr values for the ikkattoq gneisses with e nd values as low as 7 1 this older crustal component has to be eoarchaean the presence of scarce quartz diorites low sio 2 high mgo suggests that ultramafic rocks upper mantle metasomatised by the passage of fluids or silicic melts were another contributing source the ikkattoq gneisses are proposed as a complex suite incorporating material derived from melting of much older sialic crust and probably upper mantle the intercalation of tectonostratigraphic terranes during collisional orogeny at c 2720 ma destroyed the architecture of this 2825 ma magmatic system and the ikkattoq gneisses now form a slice tectonically isolated from their source region in terms of trace element parameters the ikkattoq gneisses resemble phanerozoic volcanic arc granites thus an andean style arc setting for the generation of the ikkattoq gneiss precursors is possible other archaean ttg suites of the nuuk region are generally thought to represent predominantly juvenile additions to the crust in the broadest sense they do because isotopic work over the past 30 years has demonstrated that they do not represent wholesale recycling of considerably older crust however in detail within these broadly juvenile suites a contribution from older crust can be detected thus c 3000 ma type nuk gneisses from around nuuk town show a spread in e nd values down to 1 7 in this case the likely older crustal component was 3230 ma quartz diorite that occurs as enclaves in the c 3000 ma suite thus to a lesser or greater degree some meso to neoarchaean ttg suites in the nuuk region display the same internal complexities and evidence for mixed sources as modern arc suites developed near the margins of older crust

geochemistry of anorthositic differentiated sills in the archean 2970 ma fiskenaesset complex sw greenland implications for parental magma compositions geodynamic setting and secular heat flow in arcs abstract the fiskenaesset complex sw greenland is one of the best preserved layered archean intrusions in the world consisting of an association of ca 550 meter thick anorthosite leucogabbro gabbro and ultramafic rocks dunite peridotite pyroxenite and hornblendite despite poly phase deformation and amphibolite to granulite facies metamorphism primary cumulate textures and igneous layering are well preserved in the complex this study reports new major and trace element data for three variably thick 1 to 5 m differentiated dunite through peridotite pyroxenite gabbro leucogabbro to anorthosite sequences sequences 1 2 and 3 in the sinarssuk area of the fiskenaesset region on several variation diagrams samples from these sequences plot along a well defined liquid line of descent consistent with in situ fractional crystallization the average chemical compositions of these sequences are used to constrain their approximate parental magma compositions petrographic observations and geochemical data suggest that sequences 2 and 3 solidified from evolved magmas that underwent olivine fractionation prior to their intrusion in contrast sequence 1 appears to have been derived from a near primary parental magma sio2 43 wt mgo 20 wt al2o3 16 wt cao 9 3 wt ni 840 ppm mg number 80 the trace element patterns of this parental magma are comparable to those of phanerozoic boninites consistent with a supra subduction zone geodynamic setting if the relative thickness of ultramafic layers the sum of dunite peridotite and pyroxenite layers in differentiated sequences is taken as an analog for the original complex emplaced into archean oceanic crust the fiskenaesset complex might have had a minimum thickness of 1000 m with a 500 m thick ultramafic unit at the bottom the thickness of the ultramafic unit in the preserved complex is less than 50 m suggesting that more than 90 of the original ultramafic unit was either delaminated and recycled back into the mantle as a residual cumulate or was destroyed during thrusting and ttg intrusion both the fiskenaesset complex and associated tholeiitic basalts display similar th nb lree patterns and plot along the same differentiation trend on zr versus incompatible trace element diagrams suggesting a possible petrogenetic link between the two suites of rocks however basalts do not display the same differentiation trend as the complex on several major and trace element diagrams in addition basalts and parental magma to the complex do not plot along the same fractionation line on al2o3 tio2 versus incompatible trace element diagrams implying that the fiskenaesset layered intrusive rocks were not derived from tholeiitic basalts through fractional crystallization as previously thought accordingly we infer that the fiskenaesset complex and spatially associated basalts were derived from different mantle sources parental magmas to the fiskenaesset complex originated from a mantle source that was metasomatized by highly aluminous slab derived melts layered anorthosite complexes are mostly restricted to the archean and typically associated with tonalite trondhjemite granodiorite ttg gneisses the petrogenesis of both suites appears to have been controlled mainly by slab melting endorsing independent evidence for a secular change from slab dominated melting to wedge dominated melting in arc magmatism at the end of the archean reflecting higher geothermal gradients in archean arcs than post archean counterparts

conditions of archean granulite metamorphism in the godthab fiskenaesset region southern west greenland the nordlandet peninsula akia terrane and the tasiusarsuaq terrane in southern west greenland were metamorphosed to granulite facies at 3 0 and 2 8 ga respectively temperatures of metamorphism are estimated using magnetite ilmenite garnet orthopyroxene garnet clinopyroxene and garnet biotite thermometers barometry has been carried out in the two terranes using eight different garnet barometers a uniform set of activity models for all minerals including the garnet activity model of newton et al 1986 is applied to each barometer in order to permit comparison pressure estimates using the different barometers are generally quite consistent 1 5 kbar use of the newton et al 1986 garnet activity data results in pressures similar to those obtained using other garnet activity models peak metamorphic conditions on the nordlandet peninsula are estimated to have been 800 50 c 7 9 1 0 kbar values of logfo2 are estimated to have been 1 1 to 2 0 above the quartz magnetite fayalite buffer from assemblages of magnetite ilmenite and quartz magnetite ferrosilite peak metamorphic conditions in the tasiusarsuaq terrane are estimated to have been 780 50 c 8 9 1 0 kbar estimates of fh2o and fco2 using biotite amphibile grossular anorthite and grossular scapolite equilibria are low in both terranes these results suggest that granulite metamorphism was fluid absent in both terranes and that the metamorphism in the akia terrane and possibly also in the tasiusarsuaq terrane was initiated by the injection of large volumes of magma into the lower crust