the geochemical fingerprints of different tectonomagmatic environments using hygromagmatophile element abundances of tholeiitic basalts and basaltic andesites abstract using hygromagmatophile element abundances normalized to primordial mantle composition it is possible to discriminate between the tectonic settings of tholeiitic basalts and basaltic andesites 47 0 53 5 sio2 based on field and chemical evidence data from provinces or grouped localities have been compiled in order to establish chemical reference data sets for ocean island continental ocean ridge and floor subduction zone and back arc environments the disturbing effects of crustal contamination and hydrothermal alteration on the hygromagmatophile h element patterns are of minor significance in the recognition of plate tectonic settings on the basis of the data sets magmas from a number of ambiguous tectonic settings have been recognized using their h element patterns attention is directed to problems concerning rifting and drifting related basalts in an apparent continental setting and to the origin of ophiolite complexes using available data the pillow lavas of the troodos ophiolite complex cyprus are recognized as basalts erupted in a subduction zone environment

further investigations of geomagnetic diurnal variations associated with the 2011 off the pacific coast of tohoku earthquake mw 9 0 abstract as one of the most promising candidates for short term earthquake forecasting the seismo electromagnetic phenomena have been intensively studied for several decades recently xu et al 2013 have reported unusual behaviors of geomagnetic diurnal variations in the vertical component prior to the 2011 off the pacific coast of tohoku earthquake mw 9 0 to validate this result further investigations have been applied in this study geomagnetic data of 16 years long term observation have been analyzed using the same method in xu et al 2013 ratios of diurnal variation range between the target station esashi esa which is about 130 km from the epicenter and the remote reference station kakioka kak about 300 km distant to the epicenter have been computed after removing seasonal variations revealed by wavelet transform analysis the 15 day mean values of the ratios in the vertical component shows a clear anomaly exceeding the statistical threshold about 2 months before the mega event this anomaly is unique over a 16 year long background and further discussions indicate that this anomaly is unlikely caused by strong geomagnetic storms or a statistical fluke therefore this study has provided a strong support to the previous results in xu et al 2013

accretion and post accretion metamorphism in subduction complex terranes of the new england fold belt eastern australia regional metamorphic rocks that form late palaeozoic subduction complexes in central queensland australia are products of two metamorphic episodes synaccretion metamorphism m1 gave rise to prehnite pumpellyite and greenschist facies rocks whereas a subsequent episode m2 at about 250 ma formed upper greenschist to upper amphibolite facies rocks of both intermediate and low pressure type probably in a compressive arc or back arc setting a similar pattern can be recognized for 1000 km along the new england fold belt although at several localities where higher grade rocks are exposed metamorphism was essentially continuous over the m1 m2 interval with a rapid rise in geothermal gradient at the end of accretion where out stepping of tectonic elements has occurred at long lived convergent margins elsewhere similar overprinting of high by lower pressure facies series is anticipated complicating the tectonic interpretation of metamorphism the discrete character of metamorphic events may be blurred where conditions giving rise to a major episode of accretion and out stepping are followed by the subduction of a major heat source

evolution of santorini volcano dominated by episodic and rapid fluxes of melt from depth santorini volcano in greece was thought to be continually charged by small injections of magma measurements of surface deformation show that magma equivalent to 10 50 of that emitted in previous small eruptions has been injected beneath santorini since january 2011 implying that the volcano is instead charged by rapid episodic fluxes of melt

trace element composition of single glass shards in distal minoan tephra from sw turkey volcanic glass from the seventeenth century bc c 3600 bp minoan eruption of santorini thera has been recovered from a 4 cm thick tephra deposit at golhisar golu a small lake in sw turkey analysis of single grains of this glass by laser ablation inductively coupled plasma mass spectrometry la icp ms provides for the first time an accurate determination of the trace element composition of the juvenile glass component of this eruption free from contaminants such as phenocrysts or detrital material individual glass shards 150 m show a large degree of variation in their trace element composition indicative of substantial small scale compositional heterogeneity in the magma this is a result of the proximity of crystallizing phases and provides insights into the chemical evolution of the minoan magma the range of elemental compositions displayed by individual glass shards combined with the consistency of many trace element ratios are characteristics of the minoan glass these features provide a trace element signature for the minoan tephra which will be of value in stratigraphic correlation across the eastern mediterranean

petrography and geochemistry of basaltic rocks from the conrad fracture zone on the america antarctica ridge intrusive and extrusive basaltic rocks have been dredged from the conrad fracture zone transecting the slow spreading america antarctica ridge the majority of rocks recovered are holocrystalline with the dominant mineral assemblage being plagioclase plus clinopyroxene with or without minor fe ti oxides olivine occurs in only three samples and many of the samples show evidence of extensive alteration secondary minerals include chlorite actinolite k and na feldspar analcite and epidote in terms of bulk chemistry the rocks are characterized by their generally evolved and highly variable compositions e g mg 0 65 0 35 tio2 0 7 3 6 zr 31 374ppm nb 3 21ppm y 17 96ppm ni 100 9ppm but with respect to the immobile incompatible element ratios e g zr nb y nb la sm are similar to normal or depleted mid oceanic ridge basalts quantitative major and trace element modelling indicate that most of the variation observed can be attributed to low pressure fractional crystallization of plagioclase plus clinopyroxene in approximately equal proportions with or without minor fe ti oxides the range in composition can be accounted for by up to 76 fractional crystallization although ferrobasalts have not frequently been associated with slow spreading ridges the extreme differentiation observed in the conrad fracture zone basalts implies some additional constraint other than spreading rate on the formation of ferrobasalt and reaffirms the importance of extensive crustal differentiation during the production of this basalt type

the new england batholith eastern australia geochemical variations in time and space the new england batholith australia is part of the upper paleozoic new england fold belt with most plutons intruded into the deformed trench complex metasedimetary rocks in the southeast part of the fold belt the batholith was emplaced in two major periods of plutonism the first during the upper carboniferous and the second during the upper permian and triassic with a major phase of metamorphism and deformation including westward overthrusting of the trench complex sedimentary rocks between the two periods on the basis of petrography geochemistry and isotopic characteristics the granitoids of the batholith are subdivided into five named intrusive suites and a group of leucoadamellites the differences between the six groups are considered to reflect differences in their source rock types the carboniferous granitoids are peraluminous s type and are divided into the bundarra plutonic suite a belt of very coarse grained adamellites with cordierite garnet and the hillgrove plutonic suite a belt of biotite rich garnet deformed adamellites and granodiorites both suites have 18o greater than 10 negative 34s 87sr 86sr initial ratios about 0 706 are ilmenite bearing and have low feo3 ratios the bundarra plutonic suite however is consistently sio2 rich greater than 70 contains cordierite and has higher 18o than the hillgrove plutonic suite the two s type suites are inferred to have formed by partial melting of the deepest parts of a wedge of trench complex sedimentary rocks oceanwards of an andean volcanic chain to the west analagous to the lower tertiary s type plutons of the sanak baranof plutonic belt of southern alaska of the remaining four groups of plutons emplaced during the upper permian and triassic two are metaluminous i type granitoids one the clarence river plutonic suite new name a group of k poor granodiorites and tonalites with low 87sr 86sr initial ratios average 0 7035 and the other the moonbi plutonic suite a group of k rich adamellites with low 87sr 86sr initial ratios average 0 7045 18o less than 10 and 34s positive the clarence river plutonic suite is inferred to have formed from the partial melting of a large ion lithophile element poor gabbro source region and the moonbi plutonic suite from a large ion lithophile element rich shoshonite source region a third group the uralla plutonic suite is less metaluminous has higher 87sr 86sr initial ratios average 0 706 18o only just less than 10 and negative 34s and is considered to have formed in a region containing a physical mixture of metaluminous and peraluminous source rocks gabbro and metasedimentary rocks plutons of the fourth group are leucoadamellites largely emplaced after the other suites with low 18o indicating an i type source it is argued that this group has resulted from low degrees of dry partial metling of an i type source region that had undergone an earlier melting event these four groups of plutons are largely confined to a narrow belt parallel to but 100 km west of the present rifted margin of australia their origin may be related to crustal thickening attendant on the overthrusting that occurred early in the permian and or a temperature increase in the lower crust due to the recovery of normal continental geotherms following the cessation of subduction

continental crust melting induced by subduction initiation of the south tianshan ocean insight from the latest devonian granitic magmatism in the southern yili block nw china abstract the tianshan belt of the southwestern central asian orogenic belt was generated by paleozoic multi stage subduction and final closure of several extinct oceans including the south tianshan ocean between the kazakhstan yili and tarim blocks however the subduction initiation and polarity of the south tianshan ocean remain issues of highly debated this study presents new zircon u pb ages geochemical compositions and sr nd isotopes as well as zircon hf isotopic data of the latest devonian to early carboniferous granitic rocks in the wusun mountain of the yili paleozoic convergent margin which together with the spatial temporal distributions of regional magmatic rocks are applied to elucidate their petrogenesis and tectonic linkage to the northward subduction initiation of the south tianshan ocean our zircon u pb dating results reveal that these granites were emplaced at the time interval of 362 0 1 2 360 3 1 9 ma suggesting a marked partial melting event of the continental crust in the latest devonian to early carboniferous these granites based on their mineral compositions and textures can be categorized as monzogranites and k feldspar granites geochemically both monzogranites and k feldspar granites have characters of i type granites with high k 2 o contents 4 64 4 83 wt and the k feldspar granites are highly fractionated i type granites while the monzogranites have features of unfractionated i type granites whole rock sr nd isotopic modeling results suggest that ca 20 40 mantle derived magmas may be involved in magma mixing with continental crust partial melts to generate the parental magmas of the granites the mantle derived basaltic magmas was inferred not only to be a major component of magma mixture but also as an important heat source to fuse the continental crust in an extensional setting which is evidenced by the high zircon saturation temperatures 713 727 c and 760 782 c of the studied granites the latest devonian to early carboniferous extensional setting in the wusun mountain region of the yili paleozoic convergent margin is addressed by the subduction initiation of the south tianshan ocean and constituted a late paleozoic nascent arc back arc system in the southwestern caob

pre earthquake signals underlying physical processes abstract prior to large earthquakes the earth sends out transient signals sometimes strong more often subtle and fleeting these signals may consist of local magnetic field variations electromagnetic emissions over a wide range of frequencies a variety of atmospheric and ionospheric phenomena great uncertainty exists as to the nature of the processes that could produce such signals both inside the earth s crust and at the surface the absence of a comprehensive physical mechanism has led to a patchwork of explanations which are not internally consistent the recognition that most crustal rocks contain dormant electronic charge carriers in the form of peroxy defects o 3 si oo sio 3 holds the key to a deeper understanding of these pre earthquake signals from a solid state physics perspective when rocks are stressed peroxy links break releasing electronic charge carriers h known as positive holes the positive holes are highly mobile and can flow out of the stressed subvolume the situation is similar to that in a battery the h outflow is possible when the battery circuit closes the h outflow constitutes an electric current which generates magnetic field variations and low frequency em emissions when the positive holes arrive at the earth s surface they lead to ionization of air at the ground air interface under certain conditions corona discharges occur which cause rf emission the upward expansion of ionized air may be the reason for perturbations in the ionosphere recombination of h charge carriers at the surface leads to a spectroscopically distinct non thermal ir emission

crystallization of apatite in natural magmas under high pressure hydrous conditions with particular reference to orogenic rock series two separate series of hydrous experiments involving 1 imposing apatite saturation on a series of igneous rock compositions from basanite to rhyolite and 2 crystallizing similar natural rock compositions progressively until apatite appears demonstrate a close dependence between apatite saturation and silica content of the magma and determine p2o5 levels at a given silica value and temperature at which that composition may be expected to crystallize apatite the effect of pressure on apatite solubility is not great and is most significant for silicic compositions

anomalous behaviors of geomagnetic diurnal variations prior to the 2011 off the pacific coast of tohoku earthquake mw9 0 there have been many reports on ultra low frequency ulf electromagnetic phenomena associated with earthquakes in a very wide frequency range in this study unusual behaviors of geomagnetic diurnal variations prior to the 2011 off the pacific coast of tohoku earthquake mw9 0 have been reported ratios of diurnal variation range between the target station esashi esa which is about 135 km from the epicenter and the remote reference station kakioka kak about 302 km distant to the epicenter have been computed the results of 10 day running mean of the ratios showed a clear anomaly exceeding the statistical threshold in the vertical component about 2 months before the mega event this anomaly is unique over a 3 year background and the further stochastic test indicates that this anomaly is unlikely a random anomaly which is highly suggestive of correlation with the mega event the original records of geomagnetic fields of the esa station also exhibit continuous anomalous behaviors for about 10 days in the vertical component approximate 2 months prior to the mw9 0 earthquake

the record of magma chamber processes in plagioclase phenocrysts at thera volcano aegean volcanic arc greece lavas and pyroclastic rocks throughout the volcanic stratigraphy of the tertiary quaternary volcanic complex of thera in the aegean island arc display inhomogenous plagioclase populations and phenocryst resorption textures interpreted as indicative of magma mixing plagioclase zoning characteristics studied by nomarski and laser interferometry techniques establish three main categories of plagioclase i inherited plagioclase nucleated in endmember prior to initial mixing event ii in situ plagioclase nucleated in mixed or hybrid magma and iii xenocrystic plagioclase nomarski contrast images and linearized compositional zoning profiles reveal striking differences between calcic and sodic plagioclases depending on the composition of the lava in which they are hosted these differences reflect the contrasting effects of changes in physical chemical parameters in basic vis a vis more acidic melts during magma mixing and or influx of new magma into the subvolcanic magma chamber as well as the influence of magma chamber dynamics on plagioclase equilibration variations in bulk major and trace element abundances of thera volcanic products reflect the dominant overprint of crystal fractionation but decoupling between major and trace element fractionation models and variations in incompatible trace element distributions are all indicative of magma mixing processes consistent with compositional and textural zoning in plagioclases

geochemistry and evolution of the calc alkaline volcanic complex of santorini aegean sea greece abstract volcanic activity on santorini started in upper pliocene times and is still in progress the last eruption occurred in 1950 at least seven eruptive centers were active at various times and some centers were active contemporaneously the overall chemical variations displayed by lavas and pyroclastics from the volcanic complex are largely explicable in terms of fractional crystallization involving removal of the observed phenocryst phases together with variable amounts of magma mixing and assimilation comparison of the chemistry of volcanic products from different eruption centers indicates long term variations in magma chemistry progressive depletion in lil elements and enrichment in compatible elements with time that cannot be explained in terms of fractionation mixing and assimilation such variations are attributed to processes occurring within the upper mantle trace element data require that the primitive lavas erupted on santorini were derived from an upper mantle source which was relatively depleted in lil elements compared with other calc alkaline volcanics and it is proposed that the source region contained a dominant refractory component which had experienced earlier events and a component derived from the subducted slab as magmatism progressed the contribution to the source region from the slab decreased due to either 1 cessation of subduction and hence a decrease in the proportion of slab derived material in the source region 2 a rise in geothermal gradient during melting so that the earliest melts contained a higher proportion of low melting slab derived material or 3 a combination of these processes we conclude that it is possible to see through intracrustal processes and to evaluate the processes which occur in the upper mantle beneath island arcs and we also conclude that the time integrated history of the mantle wedge above the subducted slab beneath island arcs must be taken into account when constructing viable models for are magmatism

oceanic ridge metamorphism of the patuki volcanics d urville island new zealand abstract metamorphic assemblages in ophiolitic rocks of the patuki volcanics are grouped into four facies types which represent recrystallization under widely diverse p t conditions prehnite pumpellyite and greenschist facies assemblages characterize the lavas metasediments dykes and high level gabbros which have undergone extensive spilitization hydrothermal alteration affecting the lavas is strongly non isochemical and is related to the degree of crystallinity of the original rock epidote amphibolite facies assemblages are developed in completely recrystallized rocks in shear zones metamorphism of patuki intrusive rocks was at a higher grade but less extensive and more nearly isochemical than in the lavas early formed amphibolite facies assemblages characterize the metacumulate suite and many cross cutting dykes greenschist facies assemblages are common in late stage intrusives and disequilibrium textures are often observed reflecting a series of reactions at progressively lower temperatures in a tectonically active environment hydrothermal alteration affecting the 1 to 2 km thick patuki ophiolite sequence implies a steep geothermal gradient at least 250 350 c km metamorphism at a slow spreading mid oceanic ridge is indicated and the sequential nature of metamorphic reactions occurring at the ridge site is demonstrated

lithology of the basement underlying the campi flegrei caldera volcanological and petrological constraints abstract a geologically reasonable working hypothesis is proposed for the lithology of the basement underlying the campi flegrei caldera in the ca 4 8 km depth range in most current geophysical modeling this portion of crust is interpreted as composed of meso cenozoic carbonate rocks underlain by a ca 1 km thick sill of partially molten rock thought to be a main magma reservoir shallower magma reservoirs likely occur in the 3 4 km depth range however the lack of carbonate lithics in any campi flegrei caldera volcanic rocks does not support the hypothesis of a limestone basement considering the major caldera forming eruptions which generated widespread and voluminous ignimbrites during late quaternary times including the campanian ignimbrite and neapolitan yellow tuff eruptions the total volume of trachytic to phonolitic ejected magma is conservatively estimated at not less than 350 km 3 results of least squared mass balance calculations suggest that this evolved magma formed through fractional crystallization from at least 2500 km 3 of parent shoshonitic magma in turn derived from even more voluminous more mafic k basaltic magma calculations suggest that shoshonitic magma likely emplaced at ca 8 km depth must have crystallized about 2100 km 3 of solid material dominated by alkali feldspar and plagioclase with a slightly lower amount of mafic minerals during its route toward shallower magma reservoirs before feeding the campi flegrei large volume eruptions the calculated volume of cumulate material likely syenitic in composition at least in its upper portions is more than enough to completely fill the basement volume in the 4 8 km depth range beneath the campi flegrei caldera estimated at ca 1250 km 3 thus it is proposed that the basement underlying the campi flegrei caldera below 4 km is composed mostly of crystalline igneous rocks as for many large calderas worldwide syenite sensu lato would meet physical properties requirements for geophysical data interpretations explain some geochemical and isotopic features of the past 15 ka volcanics and justify the carbon isotopic composition of fumaroles at the campi flegrei caldera this implies that meso cenozoic limestones if still present today beneath the campi flegrei caldera no longer constitute significant portions of its basement

reconstruction of eastern gondwanaland substantial additions of large continental blocks to the smith et al 1 model of eastern gondwanaland are proposed here largely on the basis of the distribution of key land floras tropical and subtropical marine faunas lithofacies patterns and the identification of a triassic magmatic arc that characterized all the eastern margin of gondwanaland the continental fragments that must have been rifted from northern australia new guinea in the jurassic are identified as south tibet burma thailand malaya and sumatra the original site of deposition of the subtropical permian limestones and tropical late triassic limestones overthrust onto the northern margin of australia by the late cenozoic collision2 is located in this greater gondwanaland palaeomagnetic data except those from malaya3 support this reconstruction

episodic behavior of gondwanide deformation in eastern australia insights from the gympie terrane the mechanisms that drove permian triassic orogenesis in australia and throughout the cordilleran type gondwanan margin is a subject of debate here we present field based results on the structural evolution of the gympie terrane eastern australia with the aim of evaluating its possible role in triggering widespread orogenesis we document several deformation events d d in the gympie terrane and show that the earliest deformation d occurred only during the final pulse of orogenesis 235 230 ma within the broader gondwanide orogeny in addition we found no evidence for a crustal suture suggesting that terrane accretion was not the main mechanism behind deformation rather the similar spatiotemporal evolution of permian triassic orogenic belts in australia antarctica south africa and south america suggest that the gondwanide orogeny was more likely linked to large scale tectonic processes such as plate reorganization in the context of previous work our results highlight a number of spatial and temporal variations in pulses of deformation in eastern australia suggesting that shorter cycles of deformation occurred at a regional scale within the broader episode of the gondwanide orogeny similarly to the cenozoic evolution of the central and southern andes we suggest that plate coupling and orogenic cycles in the late paleozoic to early mesozoic gondwanide orogeny have resulted from the superposition of mechanisms acting at a range of scales perhaps contributing to the observed variations in the intensity timing and duration of deformation phases within the orogenic belt

petrology and geochemistry of the paliza canyon formation and the bearhead rhyolite keres group jemez mountains new mexico basaltic andesite andesite dacite and rhyodacite of the paliza canyon formation 9 1 to 8 5 m y and the bearhead rhyolite 7 1 to 6 5 m y belong to a high k calc alkaline association in the southern jemez mountains new mexico the majority of the rocks of the paliza canyon formation contain augite hypersthene and plagioclase phenocrysts olivine is an important phenocryst in the basaltic andesite but it is absent in the other rock types in the rhyodacite hornblende is a major phenocryst at the expense of the pyroxenes characteristically a very restricted range in the fe mg composition is found for the pyroxenes augite from 0 33 0 29 and hornblende 0 41 0 47 throughout the whole range of rock types from basaltic andesite 56 sio 2 to rhyodacite 67 sio 2 on the other hand the bearhead rhyolite lacks pyroxene and it has two feldspars sanidine and plagioclase and bipyramidal quartz and biotite its sio 2 content ranges between 76 5 and 77 3 the bearhead rhyolite lies on the two feldspar surface or below it in the sanidine field the paliza canyon rocks plot within the plagioclase volume of the granite system both major and trace element trends in the paliza canyon formation do not project to the bearhead rhyolite composition major and trace element modeling suggests that the sequence from basaltic andesite to rhyodacite in the paliza canyon formation can be formed by simple fractional crystallization the bearhead rhyolite cannot be formed from the paliza canyon rhyodacite by this process however removal of the observed phenocrysts found in the rhyodacite would form a rhyolite enriched in ba hf th u and light rare earth elements lree and depleted in v and sc the bearhead rhyolite however is depleted in the former group and enriched in the latter these conclusions are supported by initial 87 sr 86 sr ratios three samples of bearhead rhyolite have initial ratios of 0 7056 0 7073 and 0 7076 two samples of paliza canyon basaltic andesite on the other hand have ratios of 0 7041 and 0 7044 the most likely source for the bearhead rhyolite is lower crustal material which may have been fused partially from the heat provided by the andesitic magmas of the paliza canyon formation

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

deep mantle diamonds from south australia a record of pacific subduction at the gondwanan margin diamonds from jurassic kimberlites at eurelia south australia contain coexisting inclusions of ferropericlase and mgsi perovskite that provide evidence for their deep 670 km lower mantle origin eurelia diamonds formed from mixed carbon sources likely including subducted carbonate as indicated by a trend toward isotopically heavy carbon compositions 13 c 0 and low nitrogen concentrations

quantifying crustal thickness variations in evolving orogens correlation between arc basalt composition and moho depth a compilation of geochemical data from 50 active volcanoes where seismically defined moho depth measurements are available representing most volcanic arcs around the world indicates a relation exists between crustal thickness moho depth and the trace element composition of basalts the best correlation exists for maximum light heavy rare earth element ree ratios and we use maximum ce y r2 0 90 to show the exponential function with increasing depth to 50 km application to new zealand part of the eastern gondwanan margin through much of the phanerozoic shows that higher ce y ratios in arc basalts correspond to two periods of recognized crustal thickening plus an additional event at ca 240 ma which links new zealand to the gondwanide orogeny conversely low ce y ratios in basalts correlate with periods of recognized extension in eastern gondwana very high ce y ratios in ca 120 ma rocks imply an unusually thickened 50 km arc crust consistent with independent geological estimates of crustal thickness at that time the correlation shows that crustal thickness variations during orogenesis can be conservatively estimated to within 3 km

modal composition and detrital clinopyroxene geochemistry of lithic sandstones from the new england fold belt east australia a paleozoic forearc terrane the new england fold belt of southeast australia is divided by the peel fault system into two time equivalent zones showing contrasting depositional structural and metamorphic histories these are inferred to represent a western magmatic arc frontal arc succession and an eastern subduction complex assemblage the composition of the early to middle paleozoic clastic rocks from both east and west of the peel fault system indicates derivation from an undissected volcanic arc detrital quartz is absent or present only in minor amounts and always as monocrystalline grains lithic fragments are almost exclusively volcanic and normally dominant over feldspar which is invariably plagioclase now largely albitized to the west of the fault the distribution of lathwork microlitic felsitic and vitric rock fragments as well as variation in the amount of detrital quartz indicates changes in provenance through time and two cycles of basaltic to silicic volcanic activity have been identified the composition of detrital calcic clinopyroxene grains particularly their low ti and al contents and their ti mn na ratios indicates that they crystallized mainly from subalkaline probably calc alkaline volcanic arc magmas the dominantly volcanogenic provenance of sandstone from east of the fault and the low ti content of most detrital clinopyroxene grains indicate that the volcanic chain that lay to the west of the peel fault was the most likely source for the major portion of the clastic debris in the eastern rocks however the presence of ti rich detrital clinopyroxenes identical in composition to those in the metabasalts of the subduction complex indicates that uplifted segments of the complex provided an additional sediment source

unexpected changes in resistivity monitoring for earthquakes of the longmen shan in sichuan china with a fixed schlumberger sounding array abstract an electrical resistivity monitoring experiment using a fixed schlumberger sounding array was conducted at pixian in sichuan province china from january 1984 to june 1989 for the purposes of better understanding some geoelectrical phenomena and detecting earthquake associated changes ancillary data of well water fluctuations and rainfall changes along with seismicity pattern fault map atmospheric humidity and temperature have been collected a very peculiar resistivity behavior was observed the direction of the change in apparent resistivity for large electrode expanders was opposite to that of the resistivity change for the overburden the geoelectrical structure beneath the monitoring array could be inferred from resistivity sounding data and available geological information a method of data analysis applying singular value decomposition to the first order sensitivity matrix yields the estimated change in true resistivity for the constituent medium in the sense of generalized least squares the sensitivity analysis shows that for the pixian station the abnormal annual variations in resistivity monitoring with large electrode spacings are related to the multilayered resistivity section with conductive substratum and are caused by the resistivity changes of overlying layer because the sensitivity coefficient for the topmost layer for such a section is negative the resistivity changes of the overlying layer could be attributed mainly to seasonal rainfall changes during the experiment no significant resistivity changes are correlated with the earthquakes of ml 5 4 or less in the longmen shan and adjacent regions however a detailed interpretation of the observed phenomenon is helpful to improve our understanding of possible resistivity precursors to earthquakes

dredged basalts from the western nazca plate and the evolution of the east pacific rise abstract ocean floor basalts and glasses were recovered from three stations along the western nazca plate from a sublinear topographic feature believed to represent the proto east pacific rise epr and include abyssal tholeiites feti basalts and glasses as well as transitional and little fractionated compositions when compared with their coexisting fresh glasses the feti basalts have higher total alkalies tio2 and mgo and lower feo suggesting that they have also been affected by non oxidative post magmatic alteration processes the feti glasses form a remarkably uniform compositional group through space and time a little fractionated composition having anmg number 73 similar to those reported from the mathematician ridge has higher na2o and tio2 and slightly lower cao than similar compositions from the slowly accreting mid atlantic ridge the basalts and glasses reported here exhibit the compositional diversity expected for propagating rifts and probably represent more than one volcanic episode both geochemical and geophysical interpretations support the inference that the epr grew from miocene times by the progressive growth and propagation of mantle perturbations leaving a remnant sublinear zone of rough topography characteristic of slower accretion as the trace of the proto epr continuing translations and rotations of axial segments are occurring along the epr probably in response to self reorganizations of mantle flow patterns arising from rapid melting and depletion of the source regions the data allow the inference that the youthful rift systems of the eastern pacific are far from thermodynamic equilibrium as might be expected if such systems were to drive fundamental life processes

isotope geochemistry of recent magmatism in the aegean arc sr nd hf and o isotopic ratios in the lavas of milos and santorini geodynamic implications in this comparative study of variations in the isotopic compositions sr nd o and hf of the calc alkaline magmas of the largest two volcanoes milos and santorini of the aegean arc eastern mediterranean we demonstrate the complexity of the processes governing the evolution of the magmas on the scale both of the arc and of each volcano on santorini the crustal contamination processes have been limited effecting the magma gradually during its differentiation the most differentiated lavas rhyodacite and pumice are also the most contaminated on milos by contrast these processes are very extensive they are expressed in the143nd 144nd vs 87sr 86sr diagram as a continuous mixing curve between a mantle and a crustal end member pole defined by schists and metavolcanic rocks outcropping on these volcanoes in contrast with santorini the least differentiated lavas on milos are the most contaminated these isotopic singularities can be correlated with the geodynamic evolution of the aegean subduction zone consisting of alternating tectonic phases of distension and compression the genesis of rhyolitic magmas can be linked to the two phases of distension and the contamination of the calc alkaline mantle derived magmas with the intermediate compressive phase the isotopic characteristics of uncontaminated calc alkaline primitive magmas of milos and santorini are directly comparable to those of magmas generated in subduction zones for which a contribution of subducted sediments to partial melts from the mantle is suggested such as in the aleutian sunda and lesser antilles island arcs however in spite of the importance of the sediment pile in the eastern mediterranen oceanic crust 6 10 km the contribution of the subducted terrigenous materials remains of limited amplitude

subduction related shoshonitic and ultrapotassic magmatism a study of siluro ordovician syenites from the scottish caledonides syenites are important or predominant components of several plutonic complexes emplaced between 456 and 415 ma along the nw margin of the caledonian orogenic belt adjacent to the lewisian foreland in w and nw scotland although there are in detail chemical differences between the syenites from each centre they form a well defined compositional group overall ratios amongst their trace elements especially very high values of la nb are quite different from those trachytes and syenites formed by fractional crystallisation of ocean island basalts and their continental equivalents emplaced in regions of anorogenic crustal tension instead the scottish caledonian syenites closely resemble chemically the fractional crystallisation residua of potassic subduction related magmas such as the shoshonitic series a comendite minor intrusion from a swarm associated with the loch borralan and loch ailsh syenitic complexes is remarkably similar in composition to recent obsidian from the shoshonitic volcano of lipari in the aeolian arc published sr and pb isotopic ratios preclude a significant component of either upper proterozoic moine schists or lower crust granulite facies archaean lewisian or proterozoic grenvillian gneisses in all these syenites except in local syenitic facies of the glenelg ratagain complex fractional crystallisation appears to be the mechanism by which the liquids which formed these syenites evolved from basic parental magmas the phases involved in this process may have included plagioclase alkali feldspar pyroxene amphibole biotite garnet fe ti oxide sphene allanite apatite zircon and zirconolite and therefore all the ratios amongst even the so called incompatible elements may have changed during the evolution of the leucocratic magmas nevertheless a detailed study of the glen dessarry complex shows that the changes are insufficient to disguise the geochemical nature of the parental magmas these appear to be picritic shoshonite mgo 15 ni 400ppm la yb 20 la nb 5 for the glen dessarry loch borralan and loch ailsh syenites and picritic ultrapotassic magmas mgo 15 ni 400 ppm la yb 60 la nb 7 such as would crystallise to minettes for the loch loyal and glenelg ratagain syenites mafic shoshonites were erupted amongst the 410 ma lome lavas of this region and also occur as widespread approximately contemporaneous volcanic feeder plugs minettes of similar age are also common as dykes in nw scotland numerous large tonalite granite complexes with minor diorites and gabbros were emplaced in w and nw scotland between 435 and 400 ma these are generally acknowledged to be mixtures of magmas from crustal and mantle sources their more mafic members show compositional features such as very high ba and sr which group them with the rare syenites and the lorne lavas of the same region into a distinctive geochemical province within which shoshonitic and allied magmas were the mantle derived component two models are presented to account for the generation of strongly potassic subduction related magmas several hundred km behind a nw directed subduction zone or during the period immediately following continental collision

geoelectric signals in china and the earthquake generation process we present original apparent resistivity records for stations located in the chinese provinces of hebei and sichuan along with seismicity patterns fault maps and limited hydrologic data these records are part of a 30 year database of geoelectric signals that have been recorded in china by a nationwide network of more than 100 stations set up by the state seismological bureau of the people s republic of china the database includes the time period during which the m 7 8 1976 tangshan and m 7 2 1976 songpan earthquakes occurred resistivity data recorded in and around the epicentral region of the tangshan event show coherent changes both temporal and in areal distribution years and months before and after the earthquakes a regional change in water table occurred more than 3 years prior to the tangshan earthquake the resistivity and water table signals appear to correlate in the 2 to 3 year period prior to the earthquake rainfall records from the region around tangshan extending to the cities of beijing and tianjin do not appear to account for the resistivity changes regional seismicity and active fault patterns in north china indicate that deformation occurs mainly along the borders of large tectonic blocks the source region for the geoelectric signals appears to lie within a large shear zone at the edge of a tectonic block undergoing rotation the tangshan epicentral area is thus only part of a larger zone of deformation we suggest that dilational strain associated with the opening of fractures may lower resistivity and water table for shallow crustal depths such as those monitored by the chinese geoelectric network dilation and resistivity changes are expected to begin at stress levels well below the peak values of the seismic cycle our study suggests that in some cases the areal extent of geoelectric signals may be significantly larger than that of the eventual earthquake source dimension the chinese geoelectric database offers an opportunity to improve our understanding of possible geoelectric precursors to earthquakes

caldera like structures related to permo carboniferous volcanism of the catalan pyrenees ne spain abstract the permo carboniferous volcanism of the catalan pyrences produced calc alkaline orogenic rocks with a predominance of rhyolitic and rhyodacitic pyroclastic deposits on the basis of the regional stratigraphic tectonic relationships and facies analysis of sedimentary and pyroclastic deposits the existence of four permo carboniferous basins which acted as volcanotectonic depressions has been deduced the origin of these basins seems to be related to the movement of regional strike slip faults which formed during the permo carboniferous compressive deformation caldera like structures which in some cases affected the entire basin occurred in each basin these caldera like structures seem to follow a similar pattern in three of these basins each taking place when the basin structure was already developed and continental sedimentation had started volcanism caused a high subsidence rate during the most important explosive periods by a mechanism similar to that of caldera collapse but after the subsidence structure has been established moreover a caldera structure formed by the collapse of a central volcanic edifice is also suggested in the western margin of the fourth basin

investigating the behaviour of mg isotopes during the formation of clay minerals abstract we present elemental and isotopic data detailing how the mg isotope system behaves in natural and experimentally synthesized clay minerals we show that the bulk mg isotopic composition 26 mg of a set of natural illite montmorillonite and kaolinite spans a 2 range and that their isotopic composition depends strongly on a balance between the relative proportions of structural and exchangeable mg after acid leaching these natural clays become relatively enriched in isotopically heavy mg by between 0 2 and 1 6 results of exchange experiments indicate that the mg that has adsorbed to interlayer spaces and surface charged sites is relatively enriched in isotopically light mg compared to the residual clay the isotopic composition of this exchangeable mg 1 49 to 2 03 is characteristic of the isotopic composition of mg found in many natural waters further experiments with an isotopically characterized mgcl 2 solution shows that the clay minerals adsorb this exchangeable mg with little or no isotopic fractionation although we cannot discount the possibility that the uptake of exchangeable mg does so with a slight preference for 24 mg to characterize the behaviour of mg isotopes during clay mineral formation we synthesized brucite mg oh 2 which we consider to be a good analogue for the incorporation of mg into the octahedral sheet of mg rich clay minerals or into the brucitic layer of clays such as chlorite in our experiment the brucite mineral becomes enriched in the heavy isotopes of mg while the corresponding solution is always relatively enriched in isotopically light mg the system reaches a steady state after 10 days with a final fractionation factor solid solution of 1 0005 at near neutral ph this result is consistent with the general consensus that secondary clay minerals preferentially take up isotopically heavy mg during their formation however our results also show that exchangeable mg is an important component within bulk clay minerals and can have an important influence over the bulk clay 26 mg value modeling shows that in certain soils or sediments where the percentage of exchangeable mg is 30 and the isotopic composition of the exchangeable mg is around 2 the generation of bulk 26 mg values of 26 mg values that are closer in composition to the ucc despite this results of modeling together with experimental observation suggests that the uptake of exchangeable mg into these clay rich sediments could cause a decrease in the bulk 26 mg value by up to 0 3 0 4 this should be accounted for when assessing the 26 mg value of sediments on a crustal scale

spatiotemporal characteristics of the geomagnetic diurnal variation anomalies prior to the 2011 tohoku earthquake mw 9 0 and the possible coupling of multiple pre earthquake phenomena abstract xu et al 2013 and han et al 2015 have reported unusual behaviors of geomagnetic diurnal variation gdv in the vertical component prior to the 2011 off the pacific coast of tohoku earthquake mw 9 0 to make a better understanding of this phenomenon temporal spatial analyses of gdv have been applied in this study geomagnetic data of long term observations at 17 stations in japan have been analyzed using the same method in han et al 2015 ratios of diurnal variation range between the reference station kak and the target stations have been computed after removing seasonal variations the 15 day backward running mean values of the ratios in the vertical component shows a clear anomaly exceeding the statistical threshold about 2 months before the mega event at both esa and miz stations in the tohoku region locations of anomalies in spatial distribution show a good correlation with the epicenter of the mw 9 0 earthquake these spatiotemporal results are consistent with those obtained from other independent observations such as groundwater level and gps displacements the coupling of multiple pre earthquake phenomena may help to understand the preparation process of a mega earthquake in the subduction zone

subduction and vertical coastal motions in the eastern mediterranean convergence in the eastern mediterranean of oceanic nubia with anatolia and the aegean is complex and poorly understood large volumes of sediment obscure the shallow structure of the subduction zone and since much of the convergence is accommodated aseismically there are limited earthquake data to constrain its kinematics we present new source models for recent earthquakes combining these with field observations published gps velocities and reflection seismic data to investigate faulting in three areas the florence rise sw turkey and the pliny and strabo trenches the depths and locations of earthquakes reveal the geometry of the subducting nubian plate ne of the florence rise a bathymetric high that is probably formed by deformation of sediment at the surface projection of the anatolia nubia subduction interface in sw turkey the presence of a strike slip shear zone has often been inferred despite an absence of strike slip earthquakes we show that the gps derived strain rate field is consistent with extension on the orthogonal systems of normal faults observed in the region and that strike slip faulting is not required to explain observed gps velocities further sw the pliny and strabo trenches are also often interpreted as strike slip shear zones but almost all nearby earthquakes have either reverse faulting or normal faulting focal mechanisms oblique convergence across the trenches may be accommodated either by a partitioned system of strike slip and reverse faults or by oblique slip on the aegean nubia subduction interface the observed late quaternary vertical motions of coastlines close to the subduction zone are influenced by the interplay between 1 thickening of the material overriding the subduction interface associated with convergence which promotes coastal uplift and 2 subsidence due to extension and associated crustal thinning long wavelength gravity data suggest that some of the observed topographic contrasts in the eastern mediterranean are supported by mantle convection however whether the convection is time dependent and whether its pattern moves relative to nubia are uncertain and its contribution to present day rates of vertical coastal motions is therefore hard to constrain the observed extension of the overriding material in the subduction system is probably partly related to buoyancy forces arising from topographic contrasts between the aegean anatolia and the mediterranean seafloor but the reasons for regional variations are less clear

kaolin a most diverse rock in genesis texture physical properties and uses the rock kaolin exhibits wider diversity in genesis texture physical properties and uses than probably any other common rock its diverse textures shown in scanning electron micrographs are correlative with and indicative of genesis and industrial use of kaolin kaolin originates by weathering by hydrothermal alteration or by crystallization from colloid size sediment whose chemical composition is congruent with that of kaolinite some kaolins freely slake into clay sized particles when wetted with water and likewise develop natural plasticity their texture is characterized by loosely expanded and packed crystals industrial uses of this type of kaolin exploit its free slaking property other kaolins of diametrically opposite properties do not slake and do not become plastic when immersed in water their texture is one of compact tightly interlocking crystals they are appropriately used in industry where selected size shape and original bulk density in crushed clay particles must be maintained despite wetting

physical properties of the variations of the electric field of the earth preceding earthquakes i abstract in an earlier paper we described the physical properties of the preseismic transient variations in the electrotelluric field the present letter clarifies a few points that were not properly emphasized in our previous publications furthermore recent results like that of the destructive kalamata earthquake on september 13 1986 indicate that the duration of a preseismic electric signal may reach considerably high values lasting up to several hours

rare earth element geochemistry of the island arc volcanic rocks of rabaul and talasea new britain the island arc volcanic rocks of rabaul and talasea new britain range in composition from basalt through rhyolite rare earth elements have been determined by mass spectrometric isotope dilution in 16 samples chondrite normalized rare earth element patterns are distinct for each volcanic center but all are relatively flat ce yb e f 1 1 to 3 0 within each center rare earth element concentrations increase from basalt to dacite and eu anomalies become progressively more negative from basalt to dacite eu eu 1 0 to 0 8 lavas inferred to contain cumulate phenocrysts show positive eu anomalies eu eu of up to 1 2 the observed variations are consistent with fractional crystallization of basalt or basaltic andesite by removal or accumulation of the observed phenocrysts including olivine plagioclase pyroxene and opaque minerals quantitative trace element models for talasea lavas indicate precipitation of 50 wt percent of phenocrysts from basalt to produce andesite and an additional 22 wt percent to produce dacite a total equilibrium model provides the best approximation to observed concentrations implying that the process took place at a depth sufficient to allow slow cooling of the magma as precipitation proceeded the least fractionated basalts and basaltic andesites are similar in rare earth element pattern to those of corresponding type suites in other oceanic island arcs implying a widespread uniformity of source and process in the production of each magma type

geological mineralogical and chemical characterization of devonian kaolinite bearing sediments for further applications in the ceramic tiles industry in la paz bolivia clay minerals are widely distributed in bolivia among them illitic clays are most common in the altiplano where they are mined for use in the ceramic industry that has been growing in the last few years in addition to illitic sediments kaolinitic sediments have been recently discovered in sedimentary units in the bolivian altiplano residual ball clay occurrences in devonian sedimentary units were studied as part of this work geological mapping and geophysical studies ert and gpr were done for better understanding the origin of the deposits and were part of a preliminary study of the mineral potential to define the relationship with the host rock chemical and mineralogical techniques such as x ray diffraction scanning electron microscopy and inductively coupled plasma analyses were performed in samples from the studied area to verify the presence of kaolinite atterberg limits and behaviour of the raw material in ceramic specimens supported by chemical analyses show that this material is suitable for manufacturing tile ceramics this study provides fundamental knowledge for deposit exploitation and future generation of an alternative source of employment for the inhabitants of micaya

uncertainties in the relative positions of the australia antarctica lord howe and pacific plates since the late cretaceous we determined parameters that describe finite rotations and their uncertainty regions for relative plate motion at the spreading centers between the pacific and antarctica plates between australia and antarctica and between the lord howe rise and australia we combined these to yield a range of possible finite rotations describing the relative positions of the pacific australia antarctica and lord howe plates since the late cretaceous if the pacific australia plate boundary has had its present trend since anomaly 18 time reconstructions show 330 110 km of motion of the pacific plate relative to the lord howe rise since anomaly 5 time 9 8 m y 420 110 km since anomaly 6 time 19 5 m y 770 330 km since anomaly 13 time 35 6 m y and 820 260 km since anomaly 18 time 43 0 m y we examined two cases for times prior to anomaly 18 assuming a late cretaceous age of australia antarctica separation if a plate boundary existed between the lord howe rise and pacific plates since the late cretaceous with no plate boundary in antarctica reconstructions with the lord howe rise fixed predict 610 200 km of westward motion of the pacific plate between the times of anomalies 31 and 22 followed by 260 100 km of northward motion between the times of anomalies 22 and 18 if the lord howe rise was fixed to the pacific plate until the eocene but a plate boundary existed between east and west antarctica reconstructions show very little motion across this boundary between the times of anomalies 31 and 22 followed by convergence between the times of anomalies 22 and 18 this second case also brings 70 80 m y paleomagnetic poles from the pacific and east antarctica plates into better agreement than the first case but large uncertainties in the reconstructions do not allow the first case to be conclusively eliminated

ferrobasalts from the spiess ridge segment of the southwest indian ridge highly vesicular microporphyritic basaltic rocks have been dredged from the slow spreading spiess ridge segment of the southwest indian ridge all the samples recovered are hyalocrystalline with plagioclase clinopyroxene and olivine as phenocryst and microphenocryst phases titanomagnetite occurs as euhedral microphenocrysts in some of the more evolved samples in terms of bulk rock and quench glass chemistry the lavas are characterised by highly evolved compositions e g feo 10 3 14 2 tio2 2 0 3 4 k2o 0 50 1 1 mgo 6 0 3 5 zr 160 274ppm nb 14 32ppm and can be classified as ferrobasalts isotopic and incompatible element ratios of the lavas e g 87sr 86sr 0 70325 0 70333 zr nb 8 4 11 3 y nb 2 3 1 4 indicate their strongly enriched nature see also dickey et al 6 quantitative major and trace element modelling indicates that most of the compositional variations observed can be attributed to low pressure fractional crystallisation of plagioclase clinopyroxene and minor olivine and titanomagnetite the range in composition can be accounted for by up to 65 fractional crystallisation we suggest that the extreme differentiation of the spiess ridge lavas is related not to spreading rate but to rate of magma supply the basaltic melts appear to have evolved in a newly established zone of magmatic activity associated with the most recent northward jump of the bouvet triple junction where they were effectively isolated from significant admixture of primitive magmas

retrospective investigation of geophysical data possibly associated with the ms8 0 wenchuan earthquake in sichuan china abstract the ms8 0 wenchuan earthquake is one of the most catastrophic earthquakes occurred recently although the geophysical network around the epicentral region is not dense enough partially due to the inactivity of the longmenshan fault the retrospective investigation of geophysical data in and around the epicentral region would be helpful for strengthening the understanding of the seismogenic process of the wenchuan earthquake this paper summarized some preliminary results of the retrospective investigation especially focusing on the seismic and electromagnetic data possibly associated with the wenchuan earthquake and its aftershocks the spatio temporal investigation of the seismicity changes indicated that there is an intermediate term seismic quiescence around the epicenter during 2006 2007 the electromagnetic data also showed some anomalous changes including co seismic electromagnetic changes which may be related with the wenchuan earthquake and its aftershocks although further work is definitely required to enhance the reliability of the relationship between the above geophysical changes and the seismogenic process the above preliminary results would provide some useful information for the related study and stimulate further validation study on the precursors of the wenchuan earthquake

a suggestion for the origin of mesoscopic fabric in accretionary melange based on features observed in the chrystalls beach complex south island new zealand the chrystalls beach complex a probable accretionary melange of triassic age cropping out in south island new zealand contains a distinctive set of mesoscopic structures all of which contribute to the typically chaotic appearance of the melange these structures are 1 soft sediment bedding pull aparts 2 weakly developed slaty cleavage 3 folds in dismembered bedding with slaty cleavage as the axial surface foliation and 4 quartz fiber lineated shear surfaces analysis of overprinting relations indicates that these structures formed during a single progressive deformation rather than in several discrete events as such they record a change in response to incremental strain from initial ductile behavior toward one dominated by brittle failure it is suggested that this variation resulted from progressive dewatering during shallow level accretion at the base of an inner trench slope

clay mineral assemblages from late quaternary deposits on vancouver island southwestern british columbia canada resume dans le secteur de l ile de vancouver les formations de dashwood drift cowichan head quadra sand et vashon drift se sont deposees au cours des periodes glaciaires et non glaciaires du pleistocene terminal elles presentent des variations importantes du cortege argileux dont certaines sont relatives aux fluctuations climatiques les periodes glaciaires sont caracterisees par des chlorites riches en fer des illites toutes deux bien cristallisees et des smectites avec une morphologie rappelant celle de smectites developpees a partir de roches volcaniques la mineralogie des argiles des depots non glaciaires est plus complexe elle est marquee en outre par la presence de vermiculite kaolinite halloysite et de mineraux interstratifies irreguliers ces assemblages sont mediocrement preserves et montrent une degre d alteration pedologique plus eleve par rapport aux assemblages glaciaires les variations importantes enregistrees dans les depots nonglaciaires sont egalement dues a des facteurs secondaires tels la sedimentation differentielle les sources sedimentaires variantes et des possibles modification diagenetiques toutefois ces facteurs ne semblent pas obliterer de maniere significative les empreintes climatiques des mineraux argileux cette etude permet egalement de differencier des sediments remanies sous regime glaciaire de determiner les relations entre les unites lithologiques d une meme coupe et d etablir l origine des mineraux argileux rencontres

composition diagenesis and weathering of the sediments and basement of the callabonna sub basin central australia implications for landscape evolution abstract the evolution of the callabonna sub basin central australia is described in terms of sediment provenance stratigraphy sedimentology and postdepositional mineral and chemical alteration six mineral exploration drillholes in the mundi mundi plain were studied using a combination of qualitative and quantitative xrd pimatm portable infrared mineral analyzer xrf normative analysis and sem the sedimentary cover consists of 160 210 m of diagenetically modified fluvio lacustrine tertiary clay silt and sand comprising sandy mottled and gray clay units overlain by quaternary soil the basement consists of weathered proterozoic schist gneiss siltstone and shale quartz kaolinite smectite and muscovite are the dominant minerals within the sediments whereas the weathered bedrock contains quartz feldspar kaolinite illite and muscovite characteristic wavelength absorption features from pimatm show that the youngest sandy clay unit exhibits the lowest reflectance intensities because of its lower diagenetic maturity weathering of primary minerals within the basement produced an illite kaolinite rich saprolite whose clays exhibit strong reflectance intensities and well developed crystallinity the micromorphology of all secondary products reflects their environment of crystallization low porosity and anoxic conditions within the sediments resulted in poorly developed clays of variable textures colors and compositions conversely the weathered bedrock developed over protracted periods of in situ weathering resulting in better developed secondary products this study distinguishes between preburial weathering of the basement detrital mineralogy postdepositional alteration of the sedimentary cover and subsequent weathering of previously formed mineral assemblages this has widespread implications for paleotopographic weathering reconstitution and landscape evolution thus 1 the extent of weathering of basement relative to diagenesis and maturity of sediments and gradational nature of their unconformity suggests that most of the alteration of basement occurred prior to and was sustained after deposition of sediments and 2 morphotectonic evolution of the region controlled the composition of transported materials by changing the style and patterns of fluvial systems further influencing the distribution and preservation of sediments and weathered bedrock and hence regional landscape morphology

the patuki intrusive suite closed system fractionation beneath a slow spreading ridge a wide range of mafic and ultramafic rock types together with cogenetic silicic plagiogranites form a structurally coherent intrusive sequence within the patuki volcanics at south d urville island new zealand in addition gabbroic rocks comprise abundant tectonic inclusions in highly sheared concordant serpentinite bands which intrude the patuki suite chemical evidence suggests many of the gabbros including those in which recrystallization has obliterated original textures represent magmatic cumulates and indicates extensive closed system fractionation analogous to that known to occur beneath slow spreading mid oceanic ridges dyke intrusion occurred throughout the generation of the suite an early stage of spreading is suggested by the anomalously low thickness of the sequence the non sheeted nature of the dyke suite and chemical characteristics of the lavas which comprise the extrusive component of the ophiolite

south aegean volcanic arc geochemical variations and geotectonic implications the aegean volcanic arc is one of the most important geological structure of the mediterranean area it is a belt of volcanic centers consisting of products ranging from basaltic andesitic dacitic to rhyolitic in composition all of them displaying a typical calc alkaline chemical character

an adakitic pluton on kyushu island southwest japan arc abstract an adakitic pluton is designated the shiraishino granodiorite ca 15 km2 in outcrop area on kyushu island southwestern japanese arc sr y and rare earth element concentrations and crystallization characteristics of the pluton are similar to those of adakites geochemical and tectonic considerations show that the pluton was derived from partial melting of a subducted slab with minor contamination of high 87sr 86sr material and subsequent fractional crystallization of plagioclase hornblende compiled sr and y data indicate that most cretaceous granitic rocks of southwestern japan generated by partial melting of mafic lower crust adakitic plutons derived by slab melting occur rarely and have older ages it is concluded that plutonism in the southwestern japanese arc during cretaceous time was began with the emplacement of small adakitic intrusions and progressed to more abandant granitic rocks of the type derived from the mafic lower crust

magmatic changes during the stabilisation of a cordilleran fold belt the late carboniferous triassic igneous history of eastern new south wales australia abstract in a 60 ma interval between the late carboniferous and the late permian the magmatic arc associated with the cordilleran type new england fold belt in northeast new south wales shifted eastward and changed in trend from north northwest to north the eastern margin of the earlier devonian late carboniferous arc is marked by a sequence of calcalkaline lava flows tuffs and coarse volcaniclastic sedimentary rocks preserved in the west of the fold belt the younger arc late permian triassic is marked by i type calcalkaline granitoids and comagmatic volcanic rocks emplaced mostly in the earlier forearc but extending into the southern sydney basin in the former backarc region the growth of the younger arc was accompanied by widespread compressional deformation that stabilised the new england fold belt during the transitional interval two suites of s type granitoids were emplaced the hillgrove suite at about 305 ma during an episode of compressive deformation and regional metamorphism and the bundarra suite at about 280 ma during the later stages of an extensional episode isotopic and ree data indicate that both suites resulted from the partial melting of young silicic sedimentary rocks probably part of the carboniferous accretionary subduction complex with heat supplied by the rise of asthenospheric material both mafic and silicic volcanic activity were widespread within and behind the fold belt from the onset of rifting ca 295 ma until the reestablishment of the arc these volcanic rocks range in composition from morb like to calcalkaline and alkaline the termination of the earlier arc and the subsequent widespread and diverse igneous activity are considered to have resulted from the shallow breakoff of the downgoing plate which allowed the rise of asthenosphere through a widening lithospheric gap in this setting division of the igneous rocks into pre syn and post collisional groups is of limited value

generation conditions of dacite and rhyodacite via the crystallization of an andesitic magma implications for the plumbing system at santorini greece and the origin of tholeiitic or calc alkaline differentiation trends in arc magmas andesitic arc volcanism is the most common type of subduction related magmatism on earth how these melts are generated and under which conditions they evolve towards silica rich liquids is still a matter of discussion we have performed crystallization experiments on a representative andesite sample from the upper scoriae 1 usc 1 eruption of santorini greece with the aim of understanding such processes experiments were performed between 1000 and 900 c in the pressure range 100 400 mpa at fo2 from qfm quartz fayalite magnetite to nno nickel nickel oxide 1 5 with h2omelt contents varying from saturation to nominally dry conditions the results show that the usc 1 andesitic magma was generated at 1000 c and 12 15 km depth 400 mpa migrated to shallower levels 8 km 200 mpa and intruded into a partially crystallized dacitic magma body the magma cooled to 975 c and generated the phenocryst assemblage and compositional zonations that characterize the products of this eruption an injection of basaltic magma problably subsequently triggered the eruption in addition to providing the pre eruptive conditions of the usc 1 magma our experiments also shed light on the generation conditions of silica rich magmas at santorini experimental of runs performed at fo2 nno 1 0 5 closely mimic the compositional evolution of magmas at santorini whereas those at reduced conditions qfm do not glasses from runs at 1000 975 c encompass the magma compositions of intermediate dominated eruptions whereas those at 950 900 c reproduce the silicic dominated eruptions altogether the comparison between our experimental results and natural data for major recent eruptions from santorini shows that different magma reservoirs located at different levels were involved during highly energetic events our results suggest that fractionation in deep reservoirs may give rise to magma series with a tholeiitic signature whereas at shallow levels calc alkaline trends are produced

geoelectric precursors to strong earthquakes in china abstract the main results of searching for electrical precursors to strong earthquakes in china for the last 25 yr are presented this comprises the continuous twenty year resistivity record before and after the great tangshan earthquake of 1976 spatial and temporal variations in resistivity anomalies observed at more than 6 stations within 150 km of the tangshan earthquake epicenter the travel time curve for the front of the resistivity precursor and a method of intersection for predicting the epicenter location these results reveal a number of interesting facts 1 1 resistivity measurements with accuracies of 0 5 or better for over 20 yr show that resistivity decreases of several percent which began approximately 3 yr prior to the tangshan earthquake were larger than the background fluctuations and hence statistically significant an outstanding example of an intermediate term resistivity precursor is given 2 2 the intermediate term resistivity precursor decrease before tangshan earthquake is such a pervasive phenomenon that the mean decrease in percent can be contoured on a map of the beijing tianjin tangshan region this shows the maximum decrease centered over the epicenter 3 3 the anomalies in resistivity and self potential which began 2 0 5 months before the tangshan main shock had periods equal to that of the tidal waves m 2 and ms f respectively so that the associated anomalies can be identified as impending earthquake precursors and a modal related to stress displacement weakening is proposed

structural relations in the subduction complex of the paleozoic new england fold belt eastern australia the new england fold belt of southeastern australia is divided by the peel fault system into a western magmatic arc frontal arc succession and an eastern subduction complex assemblage the western part of the subduction complex consists of repeated slices of a sequence which where most complete comprises dolerite overlain by basalt sometimes pillowed radiolarian chert siltstone and in the younger thicker slices sandstone the basal contact of this sequence is always faulted a planar veined surface separates overlying basic igneous rocks from a sheared mass composed mostly of sedimentary material in which are imbedded tectonically derived blocks of basic igneous rock this sheared zone passes gradationally down into less deformed sedimentary rocks of the underlying sequence slices each of which is usually a few hundred meters thick young constantly westward toward the magmatic arc forming a stacked steeplydipping sequence tens of kilometers thick the distribution and inter relationship of li

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

regional implications of u pb shrimp age constraints on the tectonic evolution of new caledonia abstract u pb shrimp ages for zircons from plagiogranites indicate a late carboniferous age of formation for ophiolitic basement of the koh terrane in the central chain mountains of new caledonia samples from ophiolites at koh and koua yielded ages of 302 7 ma and 290 5 ma respectively the similarity of these ages to those of plagiogranites in the dun mountain ophiolite belt maitai terrane of new zealand and the comparable structural positions of the two terranes potentially constrain reconstructions of the cretaceous sw pacific margin detrital zircons from late mesozoic sediments that overlap the koh terrane also provide potential constraints on the location of new caledonia prior to its break up and dispersal from eastern australia

petrological evolution of the crust and mantle in its simplest form crust mantle evolution can be thought of as a study of the processes by which material is lost from the mantle and forms new crust the overall goal is achievement of mass balance between the average composition of material known to have been added to the crust and the estimated average composition of the entire crust attainment of this goal for the earth has been elusive primarily because the materials available for study have generally been involved in several stages of evolution including fractionation within both the crust and mantle and recycling of material from crust to mantle thereby complicating the interpretation of available data although the overall composition of the continental crust generally has been modelled as andesitic taylor and mclennan 1981a the present upper continental crust is granodioritic and requires a more mafic lower crust it has been proposed taylor and mclennan 1981a that originally the archean crust had an andesitic composition throughout its volume and that partial melting in the lower crust developed more felsic melts that rose to enrich the upper crust leaving a more mafic residuum of lower crust this concept has been challenged by weaver and tarney 1981a who argue that the granulites of presumed lower crustal origin do not have the correct compositions for this model if one considers more recent additions to the crust from the mantle over the past several hundred million years it appears that these latest additions are basaltic with very little if any granitic additions in fact barker 1981 in his introduction to a special issue of jgr on granites and rhyolites states to date no one has demonstrated that such magmas may form in the mantle indeed much experimental and geochemical work indicates that such siliceous liquids cannot originate there therefore such magmas must be generated in the crust in island arc environments where the average composition of added material normally is thought to be andesitic a anderson 1982 has shown that the parent melt is basaltic not andesitic in addition kay 1980 has shown that the k content of the newly added material is less than the average k content of the previously existing crust in the oceanic crust the additions from the mantle are basaltic and according to o hara 1982 probably are derived from very mafic picritic melts as parents continental flood basalts which have also added material from mantle to crust also are clearly derived from basaltic parental melts basaltic volcanism study project 1981 thus the evidence for the nature of material subtracted from the mantle and added to the crust for the past several hundred million or perhaps a billion years points to very mafic melts this contrasts significantly with the overall andesitic composition of continental crust it implies furthermore that if the overall archean crustal composition was andesitic as estimated by taylor and mclennan 1981a then additions to the crust have become more mafic with time a concept in direct conflict with most models in which secular trends are from mafic to felsic with time veizer and jansen 1979 and mclennan et al 198o a question which is beginning to appear either explicitly or implicitly is whether crustal volcanic rocks or the crust as a whole really represents the parent melt that formed in the mantle a anderson 1982 o hara 1982 and nisbet and walker 1982

geochemical constraints on the origin of croisilles and patuki ophiolites implications for late paleozoic mesozoic tectonics in new zealand three stages of predominantly mafic sub seafloor magmatism accompanied the late paleozoic evolution of both the croisilles and patuki ophiolites nelson province new zealand stage 1 croisilles and patuki basaltic pillow lavas show strong light rare earth element lree enrichment and low zr nb ratios resembling alkali basalts from fracture zones and within plate e g ocean island settings analogous eruptives are not recognized in the dun mountain ophiolite or typical subduction related settings evolved stage 2 basic lava flows and volcanic breccias show pronounced lree depletion and high zr nb ratios similar to n type mid ocean ridge basalts cross cutting dikes and irregular intrusions stage 3 magmas that range from near primary basalts mg 74 al2o3 tio2 20 9 to silicic plagiogranites show decoupled rare earth element and high field strength element levels they display evidence of extensive polybaric closed system fractionation adhering closely to geochemical trends of subduction related magmas the composition of the most primitive dikes and the presence of highly refractory magnesian cr diopside and cr spinel indicate that stage 3 magmas were derived by second stage melting of depleted upper mantle in a suprasubduction zone environment differences in mantle melting regimes i e source composition plus minus multiple melting for the three magmatic episodes imply initial generation of the croisilles and patuki ophiolitic protoliths stage 1 and 2 magmas in an ocean basin most likely in a leaky transform related setting subsequent evolution stage 3 intrusives took place in a forearc location after a time interval 50 ma the transition from oceanic to forearc regime could have occurred due to stepping out of a west dipping benioff zone related to the brook street island arc trapping a segment of oceanic lithosphere and creating a tensional situation conducive to second stage mantle melting the distinctive style of tectonic intrusion of patuki ocean crust by subparallel bands of partially serpentinized ultramafic rocks residual mantle after extraction of stage 3 magmas and formation of their metamorphic border zones k ar age 202 216 ma also occurred in response to this event

petrogenesis of gympie group volcanics evidence for remnants of an early permian volcanic arc in eastern australia mafic ohenocryst rich basalt to basaltic andesite tuff breccias agglomerates and subordinate lavas together with sparsely quartz and plagiophyric dacitic tuffs and glassy flows comprise the tectonomorphically anomalous lower permian submarine volcanic sequence of the gympie group southeast queensland geochemical data imply three distinct crystallisation stages for the main volcanic series these are 1 initial olivine cr spinel clinopyroxene removal that caused rapid mgo cao ni and cr depletion in the ascending mantle derived magma 2 extensive clinopyroxene dominated olivine clinopyroxene plagioclase fractionation that gave rise to tholeiitic feo1 tio2 al2o3 p2o5 zr and y enrichment in the basalt basaltic andesite range and 3 plagioclase dominant augite magnetite apatite crystallisation that resulted in decreasing al2o3 tio2 feo1 p2o5 v nb and sr levels in the dacites identical degrees of rare earth element ree fractionation for the basalts and dacites suggest their comagmatic origin and preclude significant amphibole involvement in their petrogenesis low mg numbers mg1 ni cr ree and high field strength cation hfsc abundances in the gympie basalts along with low zry tiy and nbzr ratios are features typical of island arc tholeiites the gympie suite represents an immature submarine tholeiitic stage of development of a portion of a major intra oceanic arc that bordered gondwana but was fragmented by opening of the tasman sea the presence of coarse clinopyroxene phyric ankaramites together with highly differentiated dacites may relate to a phase of strike slip tectonics that exerted important control on magma supply and volumetric eruptive rates during the early permian evolution of the gondwana margin

cumulate gabbros from the southwest indian ridge 54 s 7 16 e implications for magmatic processes at a slow spreading ridge a diverse volcanic and plutonic rock suite was recovered from the center of the 80 km long ridge segment of the southwest indian ridge 54 s 7 16 e between the islas orcadas and shaka fracture zones the cumulus nature of the gabbroic rocks in the suite is indicated by phase modal and cryptic layering igneous lamination and low incompatible element abundances we present a mass balance model for calculating the proportions and compositions of cumulus phases and crystallized intercumulus liquid from bulk rock major element compositions the model is based on the ability to define a compositional array of basaltic liquids and on the assumption that cumulus minerals are initially in equilibrium with trapped liquid calculated proportions of trapped liquid range from 3 15 values that are characteristic of adcumulates to mesocumulates models of postcumulus crystallization indicate significant enrichments of incompatible elements and buffering of compatible elements in residual trapped liquids thus explaining the high tio2 contents observed in magnesian clinopyroxenes cumulus phase assemblages and compositions suggest solidification in shallow level magma chambers but disequilibrium plagioclase compositions suggest some crystallization at greater depth furthermore basalt compositions projected onto the olivine clinopyroxenequartz pseudoternary suggest magma generation over a range of pressures from less than 10 to greater than 20 kb as well as polybaric fractional crystallization we suggest that the southwest indian ridge is characterized by low magma supply with small batches of melt that either ascend directly to the surface having undergone limited polybaric crystallization or are trapped in shallow crustal magma chambers where they evolve and solidify to form cumulate gabbros the adcumulus nature of the gabbros investigated here suggests slow cooling rates typical of large intrusions implying relatively large but ephemeral magma chambers below segments of the southwest indian ridge

a systematic compilation of earthquake precursors article a survey of published scientific literature was undertaken to identify and catalog observed earthquake pre cursors the earthquake precursors selected for analysis included electric and magnetic fields gas emissions groundwater level changes temperature changes surface deformations and seismicity for each of these precursors the published scientific literature was searched to document the statistics of each reported earth quake precursor spatial extent time duration amplitude signal noise ratio to analyze dependence of the observable for each precursor on earthquake magnitude and to explore proposed physical models to explain eachearthquakeprecursor somegeneralcharacteristics wereobservedfortheseprecursoryphenomena first the largest amplitude precursory anomalies tend to occur before the largest magnitude earthquakes also the number of precursory anomalies tends to increase the closer in time to the occurrence of the earthquake finally theprecursoryanomaliestendtooccurclosetotheeventualepicenterof theearthquake ingeneral the physical models indicate that all of the precursory phenomena are related to deformation that occurs near the fault prior to the main earthquake while the models provide plausible physical explanations for the pre cursors there are many free parameters in the models that are poorly resolved

detrital zircon age patterns and provenance in late paleozoic early mesozoic new zealand terranes and development of the paleo pacific gondwana margin permian triassic strata from parapara peak in the takaka terrane western province new zealand are dominated by detrital zircon peaks at ca 240 ma comparable to the torlesse terrane of the eastern province this contrasts with lower paleozoic strata onekaka schist which is comparable to other lower paleozoic terranes including adjacent buller terrane from the gondwana margin that are characterized by detrital zircon patterns with characteristic peaks at ca 500 650 ma and ca 1000 1150 ma a less distinct peak around 1400 1700 ma and scattered archean ages these data indicate the close proximity of the eastern and western provinces by permian time and a probable source region of parapara peak strata and torlesse terrane in the new england fold belt of southeast australia

orogenesis without collision stabilizing the terra australis accretionary orogen eastern australia the neoproterozoic to end paleozoic terra australis orogen extended along the gondwana margin of the paleo pacific ocean and it now provides a detailed record of orogenic activity and continental stabilization within an ongoing convergent accretionary plate margin new geochronological data from end paleozoic plutonic and volcanic rocks associated with the gondwanide orogeny in the new england region of eastern australia integrated with information on the nature and timing of associated sedimentation deformation and metamorphism allow resolution of a high fidelity record of orogenesis at the end of the carboniferous around 305 ma convergent margin magmatism which had been active along the western margin of the new england region terminated and was followed by a short pulse of regional compressional deformation and metamorphism marking the commencement of the tablelands phase of gondwanide orogenesis deformation was almost immediately followed by the onset of clastic sedimentation and local calc alkaline volcanism dated at 293 ma in the extensional barnard basin emplacement of the two new england s type granitic suites the bundarra and the hillgrove suites along with localized high temperature low pressure metamorphism was essentially contemporaneous ranging in age from 296 to 288 ma and overlapped in time with i type magmatism and the switch from regional compression to extension and barnard basin rifting the hunter bowen phase of the gondwanide orogeny commenced with contractional deformation resulting in termination of sedimentation in the barnard basin and regional deformation and metamorphism across new england and into the sydney and gunnedah basins to the west at around 265 260 ma contractional loading of the sydney and gunnedah basins resulted in their conversion from extensional to foreland basins which received ongoing pulses of sediment from the new england orogenic welt until 230 ma the hunter bowen phase was associated with widespread i type plutonism and volcanic activity in new england that ceased around 230 ma marking the termination of gondwanide orogenesis orogenesis occurred in an evolving convergent plate margin setting s and i type magmatic activity ranging in age from ca 300 to 230 ma represents a stepping out of arc magmatism from the western margin of new england prior to 305 ma into the preexisting arc trench gap there is no evidence that deformation was related to the collision of the convergent margin with a major lithospheric mass and the widespread development of extensional basins in the eastern third of australia in the early permian indicates control by phenomena acting on a continental scale probably changing plate kinematics associated with the amalgamation of pangea

geochemistry of the 1989 1990 eruption of redoubt volcano part i whole rock major and trace element chemistry the 1989 1990 eruption of redoubt volcano produced medium k calc alkaline andesite and dacite of limited compositional range 58 2 63 4 sio2 and entrained quenched andesitic inclusions 55 sio2 which bear chemical similarities to the rest of the ejecta the earliest december 15 magmas are pumiceous often compositionally banded and the majority is relatively mafic 59 sio2 the most silicic magmas of the eruption are the late december to early january domes up to 63 4 sio2 subsequent magmas formed domes and rare pumices which converge on 60 sio2 chemical variations among ejecta comprise tight linear two component arrays for all elements for which the analytical uncertainty is much less than the compositional range the two component arrays are interpreted as mixing arrays between unrelated magmas because several of the arrays are at steep angles to the normal liquid line of descent additionally the felsic endmember cannot be easily related to the mafic endmember by normal high temperature igneous processes e g the silicic endmember has higher zr yet lower hf than the mafic endmember also relative enrichments of highly incompatible elements are dramatically different across the arrays the mixing event must have preceded eruption by a significant yet unspecified amount of time because groundmass glass compositions are homogeneous for all post december samples swanson et al 1994 this volume in spite of the whole rock chemical diversity this implies time for additional crystallization after the mixing event swanson et al 1994 this volume discuss evidence for a potentially different mixing event recorded only in december 15 magmas cognate cumulate xenoliths composed of pl cpx opx hb mt melt were recovered from january and april deposits these blocks differ from local batholithic country rock in their low concentrations of incompatible elements e g rb 60 wt they have mg cr ni sc and v contents higher than the andesites but lower than redoubt basalts and basaltic andesites thus they may be crystallization products of andesites but do not represent the cumulate residue of basalt fractionation the xenoliths were probably derived from a shallow or intermediate crustal chamber

geochemistry of mafic lavas in the early rio grande rift yarmony mountain colorado u s a abstract mafic lavas high k basalts and shoshonites erupted 21 24 ma ago early miocene near state bridge nw colorado and now exposed on yarmony mountain were related to the initial phase of extension in the northernmost section of the rio grande rift the volcanism was of small volume consisting of eleven flows on yarmony mountain although much more voluminous contemporaneous lavas occur 50 km to the west all eleven flows have been analysed comprehensively for major and trace elements and five of the flows for nd and sr isotopes some samples experienced post eruptive carbonation and leaching of alkali elements the effects of these processes on normative compositions are examined and it is suggested that major element abundances are sufficiently well preserved to be fairly sure that the magmas experienced fractional crystallization at pressures appropriate for the mid lower part of the crust most of the basalts are chemically closely related and have the following characteristics 1 they are depleted in nb and ta relative to lree a feature of volcanic arc magmas 2 they have lower abundances of rb and k and to a lesser extent ba and th for a given lree content than typical magmatic arc magmas and 3 they have nd and sr isotopic ratios similar to both certain low 143 nd 144 nd oceanic basalts notably from the kerguelen area indian ocean and subduction related magmas from mature volcanic arcs notably from the andes south america we argue that the magmas erupted at yarmony mountain were generated by partial melting of asthenospheric mantle which had been modified by subduction of oceanic lithosphere during the cenozoic the relatively low abundances of the alkali elements appears to be a consequence of depletion of these elements in the mantle source by a previous episode of melt extraction before that which generated the yarmony magmas an alternative is that the mantle source was depleted in alkali elements by a dehydration event possibly involving a flow of co2 one of the lavas in the yarmony sequence is elementally and isotopically distinctive having higher k zr ba k 2 o na 2 o la ta and k la and lower 87 sr 86 sr than the rest of the samples it is argued in detail that this is not the result of contamination by any reasonable crustal composition and that this magma contained a component 10 20 of ultrapotassic mafic magma derived by partial melting of subcontinental lithospheric mantle there is no petrographic evidence for mixing and the hybridization probably took place in magma chambers situated in the mid lower crust

metallogeny and tectonic development of the tasman fold belt system in new south wales abstract the northwestern corner of new south wales consists of the paratectonic late proterozoic to early cambrian adelaide fold belt and older rocks which represent basement inliers in this fold belt the rest of the state is built by the composite late proterozoic to triassic tasman fold belt system or tasmanides in new south wales the tasman fold belt system includes three fold belts 1 the late proterozoic to early palaeozoic kanmantoo fold belt 2 the early to middle palaeozoic lachlan fold belt and 3 the early palaeozoic to triassic new england fold belt the late palaeozoic to triassic sydney bowen basin represents the foredeep of the new england fold belt the tasmanides developed in an active plate margin setting through the interaction of east gondwanaland with the ur precambrian and palaeo pacific plates the tasmanides are characterized by a polyphase terrane accretion history during the late proterozoic to triassic the tasmanides experienced three major episodes of terrane dispersal late proterozoic cambrian silurian devonian and late carboniferous permian and six terrane accretionary events cambrian ordovician late ordovician early silurian middle devonian carboniferous middle late permian and triassic the individual fold belts resulted from one or more accretionary events the kanmantoo fold belt has a very restricted range of mineralization and is characterized by stratabound copper deposits whereas the lachlan and new england fold belts have a great variety of metallogenic environments associated with both accretionary and dispersive tectonic episodes the earliest deposits in the lachlan fold belt are stratabound cu and mn deposits of cambro ordovician age in the ordovician cu deposits were formed in a volcanic are in the silurian porphyry cu au deposits were formed during the late stages of development of the same volcanic are post accretionary porphyry cu au deposits were emplaced in the early devonian on the sites of the accreted volcanic arc in the middle to late silurian and early devonian a large number of base metal deposits originated as a result of rifting and felsic volcanism in the silurian and early devonian numerous sn w mo and base metal au granitoid related deposits were formed a younger group of mo w and sn deposits resulted from early middle carboniferous granitic plutonism in the eastern part of the lachlan fold belt in the middle devonian epithermal au was associated with rifting and bimodal volcanism in the extreme eastern part of the lachlan fold belt in the new england fold belt pre accretionary deposits comprise stratabound cu and mn deposits pre early devonian stratabound cu and mn and exhalite au deposits late devonian to early carboniferous and stratabound cu exhalite au and quartz magnetite late carboniferous s type magmatism in the late carboniferous early permian was responsible for vein sn and possibly au as ag sb deposits volcanogenic base metals when compared with the lachlan fold belt are only poorly represented and were formed in the early permian the metallogenesis of the new england fold belt is dominated by granitoid related mineralization of middle permian to triassic age including sn w mo w and au ag as sb deposits also in the middle permian epithermal au ag mineralization was developed during the above period of post orogenic magmatism sizeable metahydrothermal sb au w and au deposits were emplaced in major fracture and shear zones in central and eastern new england the occurrence of antimony provides an additional distinguishing factor between the new england and lachlan fold belts in the new england fold belt antimony deposits are abundant whereas they are rare in the lachlan fold belt this may suggest fundamental crustal differences

monitoring shallow resistivity changes prior to the 12 may 2008 m 8 0 wenchuan earthquake on the longmen shan tectonic zone china abstract an active source measurement of shallow resistivity using fixed electrode quasi schlumberger arrays has been conducted at pixian jiangyou and wudu stations on the longmen shan tectonic zone in western china with the hope of detecting earthquake associated changes for the duration of the monitoring experiment a gradual decrease of apparent resistivity of up to 6 7 several years prior to the 12 may 2008 m 8 0 wenchuan earthquake had been recorded clearly at pixian station approximately 35 km from the epicenter the change of apparent resistivity was monitored with a fixed schlumberger array of ab mn spacings of 736 m 226 m in the direction of n57 5 e giving precisions in measured daily averages of 0 16 or less a coseismic resistivity drop of up to 5 3 was observed at jiangyou station using a schlumberger array of ab mn spacings of 710 m 90 m in the direction of n10 e no fluctuation of resistivity was detected at wudu station at the time of the wenchuan mainshock while the focus of this paper is on monitoring or tracking resistivity variations prior to during and after the wenchuan earthquake we also aim to compare resistivity records of the wenchuan earthquake to those of the m 7 8 tangshan and m 7 2 songpan earthquakes of 1976 attempts to explain the observed resistivity variations have been made the results show that the resistivity variations observed at all three stations are in approximate agreement with resistivity stress behavior deduced from in situ experiments focal mechanisms a simplified dynamical model static stress analyses and field investigations from along the longmen shan fault zone

basement geology from three kings ridge to west norfolk ridge southwest pacific ocean evidence from petrology geochemistry and isotopic dating of dredge samples abstract we present new petrographie x ray fluorescence electron microprobe ar ar radiometric fission track and sr nd and pb isotopic data for igneous and metamorphic rocks from 18 dredges from a number of basins and ridges in the southwest pacific ocean the dredge samples can be divided into four groups 1 permian to cretaceous gabbroids granitoids and hornfelses from the west norfolk and wanganella ridges 2 mainly cretaceous mafic igneous rocks from the reinga ridge and vening meinesz escarpment 3 subduction related late oligocene and early miocene basalts and shoshonites from the three kings and southern norfolk ridges and 4 back arc basin and intraplate basalts of early miocene age in the south and north norfolk basins respectively and intraplate basalts of pliocene age in the reinga basin our dredge data combined with regional magnetic and gravity data provide much needed tests of earlier tectonic interpretations of the region correlatives of the permian brook street terrane and mesozoic median tectonic zone of onshore new zealand clearly extend along the west norfolk and wanganella ridges the three kings ridge was active at least in the 19 to 20 ma period and was probably generated above a west dipping subduction zone the norfolk basin was a back arc basin to this arc and had opened by 18 20 ma the oldest cenozoic volcanic rocks in the region are 26 ma subduction related volcanic breccias on the southern norfolk ridge they probably predate opening of the norfolk basin and associated eastward migration of the three kings ridge

differentiation conditions of a basaltic magma from santorini and its bearing on the production of andesite in arc settings santorini volcano in the aegean region greece is characterized by andesitic to silicic dominated explosive activity and caldera forming eruptions sourced from magmatic reservoirs located at various structural levels beneath the volcano there is a good understanding of the silica rich magmatism of the island whereas the andesite dominated volcanism and the petrogenesis of the parental mafic magmas are still poorly understood to fill this gap we have performed crystallization experiments on a representative basalt from santorini with the aim of determining the conditions of differentiation pressure temperature volatile fugacities and the parental magma relationship with the andesitic eruptive rocks experiments were carried out between 975 and 1040 c in the pressure range 100 400 mpa fo2 from qfm to nno 3 5 where qfm is quartz fayalite magnetite and nno is nickel nickel oxide with h2omelt contents varying from saturation to nominally dry conditions the results show that basalt phenocrysts within the basalt crystallized at around 1040 c in a magma storage reservoir located at a depth equivalent to 200 400 mpa pressure with 3 5 wt dissolved h2o and fo2 around qfm comparison with the xenocryst and phenocryst assemblages of the upper scoria 1 andesite shows that andesitic liquids are produced by fractionation of a similar basalt at 1000 c and 400 mpa following 60 80 wt crystallization of an ol cpx plag ti mag opx 6 pig ilm assemblage with melt water contents around 4 6 wt at santorini the andesitic low viscosity and water rich residual liquids produced at these depths segregate from the parent basaltic mush and feed the shallow magma reservoirs eventually erupting upon mixing with resident magma changes in prevailing oxygen fugacity may control the tholeiitic calc alkaline character of santorini magmas explaining the compositional and mineralogical differences observed between the recent thyra and old eruptive products from akrotiri

sensitivity analysis of the schlumberger monitoring array application to changes of resistivity prior to the 1976 earthquake in tangshan china abstract prior to the 1976 tangshan earthquake of m 7 8 at nine out of fourteen geoelectric stations within 180 km of the epicenter resistivity decreases were observed during this same period drops in water levels were recorded in seventeen deep wells in the same region the sensitivity analysis shows that the resistivity decrease cannot be caused by a groundwater level decrease because all of the coefficients for the resistivity structure types a h and ha with a resistive substratum are positive some examples of a schlumberger monitoring array are used to illustrate the application of the analytical approach in order to detect resistivity changes associated with an earthquake such an approach is useful for survey planning checking or setting the search depth of a given array and facilitates the quantitative interpretation of resistivity data

trace element geochemistry of high alumina basalt andesite dacite rhyodacite lavas of the main volcanic series of santorini volcano greece trace element systematics throughout the cal calkaline high alumina basalt basaltic andesite andesite dacite rhyodacite lavas and dyke rocks of the main volcanic series of santorini volcano greece are consistent with the crystal fractionation of observed phenocryst phases from a parental basaltic magma as the dominant mechanism involved in generating the range of magmatic compositions marked inflection points in several variation trends correspond to changes in phenocryst mineralogy and divide the main series into two distinct crystallisation intervals an early basalt to andesite stage characterised by calcic plagioclase augite olivine separation and a later andesite to rhyodacite stage generated by plagioclase augite hypersthene magnetite apatite crystallisation percent solidification values derived from ratios of highly incompatible trace elements agree with previous values derived from major element data using addition subtraction diagrams and indicate that basaltic andesites represent 47 69 andesites 70 76 dacites ca 80 and rhyodacite ca 84 crystallisation of the initial basalt magma least squares major element mixing calculations also confirm that crystal fractionation of the least fractionated basalts could generate derivative main series lavas though the details of the least squares solutions differ significantly from those derived from highly incompatible element and addition subtraction techniques main series basalts may result from partial melting of the mantle asthenosphere wedge followed by limited olivine pyroxene cr spinel crystallisation on ascent through the sub aegean mantle and may fractionate to more evolved compositions at pressures close to the base of the aegean crust residual andesitic to rhyodacite magmas may stagnate within the upper regions of the sialic aegean crust and form relatively high level magma chambers beneath the southern volcanic centres of santorini the eruption of large volumes of basic lavas and silicic pyroclastics from santorini may have a volcanological rather than petrological explanation

magma mixing at nisyros volcano as inferred from incompatible trace element systematics abstract rocks of the calc alkaline volcanic complex of nisyros at the eastern end of the quaternary south aegean volcanic arc range in composition from basaltic andesite to rhyodacite major and compatible trace element systematics are dominated by crystal fractionation involving olivine clinopyroxene plagioclase and magnetite in the mafic to intermediate compositional range and plagioclase clinopyroxene hypersthene fe ti oxides and apatite in the intermediate to silicic range although there is a marked paucity of lavas with 60 66 wt sio 2 the occurrence of andesitic inclusions in pre and post caldera dacites as well as reversely zoned and or partly resorbed phenocrysts exotic mineralogy e g forsteritic olivine compositionally bimodal plagioclase phenocryst populations and the observation of two distinct glass phases in melt inclusions and pumices all indicate that magma mixing processes have played a role in the petrogenesis of nisyros incompatible trace element systematics are consistent with mixing between mafic and silicic end members major element binary mixing calculations between end members successfully reproduce the dacite composition the eruptive sequence at nisyros suggests the existence of a compositionally zoned magma chamber in which mixing between underlying more mafic magma and overlying rhyodacitic melt result in the formation of dacitic magmas inherent mineral assemblages in basaltic andesites indicate that mixing between mafic liquids and between mafic and more silicic liquids played also a role in their petrogenesis

neoproterozoic high k granites produced by melting of newly formed mafic crust in the huangling region south china abstract the huangling igneous complex in the central yangtze block south china consists of the 850 ma tonalite and trondhjemite which were intruded by the 826 13 ma high k granite the granites are composed of plagioclase 30 60 alkali feldspar 10 30 quartz 20 30 biotite 2 63 1 73 3 wt k 2 o 2 58 4 79 wt and na 2 o 3 90 5 89 wt and low mgo 0 49 1 78 wt and are magnesian alkali calcic in composition their chondrite normalized ree patterns are characterized by enrichment of lree la yb n 14 5 32 8 and mostly negative eu anomalies eu eu 0 5 1 1 they have moderately high sr 202 698 ppm and low y 6 13 18 7 ppm with sr y ratios of 14 64 these large elemental variations probably resulted from fractional crystallization they also have negative whole rock nd 9 75 to 11 73 and zircon hf values 7 1 to 15 8 which are similar to those of the neoproterozoic mafic rocks in the same region suggesting that the granites may have been produced by melting of newly emplaced mafic rocks tonalites and trondhjemites of the huangling complex previously were considered as products by differentiation of water rich basaltic magmas and melting of the thickened archean continental crust in an arc setting respectively therefore the crustal growth and reworking of the yangtze block were probably wholly controlled by the subduction system which resulted in formation of the stable continent during the neoproterozoic

petrogenesis of picrite and associated basalts from the southern mid atlantic ridge basalts dredged from the south wall of a fracture zone transecting the southern mid atlantic ridge smar at 54 s are unusual in that they include a suite of highly olivine phyric basalts sampled along with more normal sparsely plagioclase phyric basalts and a highly plagioclase phyric basalt four basalt types olivine phyric sparsely plagioclase phyric evolved sparsely plagioclase phyric and highly plagioclase phyric are readily distinguished on the basis of petrography mineralogy and bulk composition they range from primitive to evolved with the olivine phyric basalts having elevated mgo up to 15 5 and the plagioclase phyric basalt having elevated al2o3 19 3 and cao 13 1 contents compositional variations are extremely consistant with the olivine phyric basalts and the sparsely plagioclase phyric basalts defining coherent linear trends on the basis of the ratios and covariation of the incompatible trace elements zr nb y and ba distinct parental magmas for each basalt type are required an investigation of fe mg and mg ni distribution coefficients between olivine and magma indicates that olivines from the olivine phyric basalts are on average too forsteritic and too ni poor to have crystallized in a magma corresponding to the host bulk rock composition this implies that these basalts are enriched in xenocrystic olivine olivines from the other basalt types are mostly of equilibrium composition although there are some exceptions petrogenetic models for the formation of the different basalt types are quantitatively evaluated in terms of fractional crystallization crystal accumulation processes these indicate that 1 the olivine phyric basalts are the products of olivine and minor cr spinel accumulation and do not represent analogues of primary magma or a liquid fractionation trend 2 that the sparsely plagioclase phyric basalts were formed by polybaric fractional crystallization of olivine plagioclase and clinopyroxene and 3 that the evolved sparsely plagioclase phyric basalts are not readily related to one another the single highly plagioclase phyric basalt is unrelated to the other basalt types and is cumulus enriched in plagioclase

implications of radiolarian research for analysis of subduction complex terranes in the new england orogen nsw australia radiolarians provide important age constraints on geologically complex strata in the new england orogen of eastern australia several terranes interpreted as subduction complexes were undated prior to radiolarian studies radiolarian faunas indicate that the djungati terrane is significantly younger than the previously inferred ordovician to sulurian age range basalt chert successions in the djungati terrane developed during the silurian to late devonian in an environment which was far from the influence of terrigenous sedimentation radiolarians from tuffs within overlying arc derived volcaniclastic sequences constrain the timing of tectonic assembly of the terrane at a convergent margin to the latest devonian early carboniferous the extensive but previously undated anaiwan terrane also contains radiolarians which indicate a latest devonian to early carboniferous age for siliceous oceanic sediments these sediments were accreted into a subduction complex volcaniclastic sediments within this subduction complex contain radiolarians which indicate that it developed during the early carboniferous radiolarians in the willowie creek beds provide the first age constraints late devonian for another lithotectonic entity the yugambal terrane which is located along the eastern edge of the southern new england orogen many of the recently determined radiolarian ages are at variance with age estimates used to develop models for the tectonic development of the orogen these new age data require major reappraisal of models detailed analysis of the geological evolution of individual terranes is required before an integrated model for the entire orogen can be developed

the development of the sw pacific margin of gondwana correlations between the rangitata and new england orogens prior to formation of the tasman sea the late paleozoic mesozoic rangitata orogen of new zealand and new caledonia abutted the paleozoic new england orogen of eastern australia comparison of the record of permian cretaceous igneous and deformational events from the two orogens suggests that their tectonic evolution was interrelated and is a consequence of convergent plate interaction along the southwest pacific margin of gondwana the following relations are proposed 1 termination of arc volcanism and widespread sedimentation in new england together with the onset of regional deformation and crustal anatexis were synchronous with the commencement of volcanism and sedimentation within the rangitata orogen 2 early permian andesitic volcanism in eastern new england represents an along strike extension of the brook street terrane of new zealand 3 late permian regional deformation in new england coincides with both a break in subduction related igneous activity in the new england and rangitata orogens and a shift in the locus of this activity 4 late permian triassic calc alkaline igneous activity in new england correlates with a phase of relatively continuous accumulation of pyroclastic material in the forearc basin of the rangitata orogen 5 cessation of plutonism in new england corresponds with commencement of formation of the esk head melange in new zealand and the probable commencement of juxtaposition of the te anau and alpine assemblage 6 late cretaceous epizonal plutons intruded into the new england orogen are similar in character and age to those emplaced during the final phases of rangitata orogenesis and both appear to mark initial stages of rifting associated with formation of the tasman sea the generation of permian and triassic igneous activity in eastern new england by convergent plate interaction results on present reconstructions of the gondwana margin in an excessively wide arc trench gap succession for the position of the trench is constrained to east of new caledonia from the permian onward this suggests there may have been some rearrangement of tectonic elements along the margin resulting in widening of the lord howe rise between australia and new caledonia in the period between termination of igneous activity in new england and formation of the tasman sea in the late cretaceous

tectono metamorphic evolution of high p t and low p t metamorphic rocks in the tia complex southern new england fold belt eastern australia insights from k ar chronology the tia complex in the southern new england fold belt is a poly metamorphosed late paleozoic accretionary complex it consists mainly of high p low t type pumpellyite actinolite facies rare blueschist facies schists phyllite and serpentinite t 300 c and p 5 kbar and low p high t type amphibolite facies schist and gneiss t 600 c and p 5 kbar associated with granodioritic plutons tia granodiorite white mica and biotite k ar ages distinguish carboniferous subduction zone metamorphism and permian granitic intrusions respectively the systematic k ar age mapping along a n s traverse of the tia complex exhibits a gradual change the white mica ages become younger from the lowest grade zone 339 ma to the highest grade zone 259 ma in contrast si content of muscovite changes drastically only in the highest grade zone the regional changes of white mica k ar ages and chemical compositions of micas indicate argon depletion from precursor high p low t type phengitic white mica during the thermal overprinting and recrystallization by granitoids intrusions our new k ar ages and available geological data postulate a model of the eastward rollback of a subduction zone in early permian the eastward shift of a subduction zone system and subsequent magmatic activities of high mg andesite and adakite might explain formation of s type granitoids hillgrove suite and coeval low p high t type metamorphism in the tia complex

permian evidence for trans tasman relationships between east australia new caledonia and new zealand abstract basaltic to basaltic andesitic tuff breccias agglomerates and subordinate lavas together with cogenetic dacitic tuffs and glassy flows comprise the tectonomorphically anomalous early permian submarine volcanic sequence of the gympie group southeast queensland the least accumulative gympie basalts and basaltic andesites show geochemical features which typify island arc tholeiites the absence of andesitic eruptives i e the essential bimodality of the volcanic sequence together with low levels of large ion lithophile elements lile and the absence of compelling evidence for amphibole or garnet fractionation are consistent with an immature submarine tholeiitic stage of arc development on relatively thin oceanic crust by contrast the mid carboniferous to permian calc alkaline camboon arc to the west is inferred to have developed at least in part on early carboniferous continental crust the volcanics and overlying sediments of the gympie group are compared with those of the volcanic arc and adjoining formations of the nelson eglinton takitimu areas of new zealand to reveal some striking geochemical and lithostratigraphic similarities dacitic volcanics in new caledonia may also have formed a relatively young part of the same volcanic arc we analyse the overlying permian sediments to show further similarities and facial differences between the three regions essentially we propose new zealand was locus for an actively spreading mid ocean ridge dun mountain ultramafics and patuki ophiolite complex which received a substantial thickness of marine sediment gympie lay towards the end of the mid ocean ridge and new caledonia close to the terminus of the volcanic arc and received more terrestrial sediment we analyse original and subsequent spatial interrelationships that developed before and during the late cretaceous opening of the tasman sea and suggest that the history of separation was complicated by strike slip and or transform movements

the distribution geochronology and geochemistry of early paleozoic granitoid plutons in the north altun orogenic belt nw china implications for the petrogenesis and tectonic evolution abstract abundant early paleozoic granitoid plutons are widely distributed in the north altun orogenic belt these rocks provide clues to the tectonic evolution of the north altun orogenic belt and adjacent areas in this paper we report an integrated study of petrological features u pb zircon dating in situ zircon hf isotope and whole rock geochemical compositions for the abei 4337 highland and kaladawan plutons from north to south in the north altun orogenic belt the dating yielded magma crystallization ages of 514 ma for the abei pluton 494 ma for the 4337 highland pluton and 480 460 ma for the kaladawan pluton suggesting that they are all products of oceanic slab subduction because of the age constraint the abei monzogranites derived from the recycle of paleoproterozoic continental crust under low pressure and high temperature conditions are products of subduction initiation the 4337 highland granodiorites have some adakitic geochemical signatures and are sourced from partial melting of thickened mafic lower continental crust the kaladawan quartz diorites are produced by partial melting of mantle wedge according to the positive e hf t values and the kaladawan monzogranite syenogranite are derived from partial melting of neoproterozoic continental crust mixing the juvenile underplated mafic material from the depleted mantle these results together with existing data provide significant information about the evolution history of oceanic crust subduction during the 520 460 ma the initiation of subduction occurred during 520 500 ma with formation of abei pluton subsequent transition from steep angle to flat slab subduction at ca 500 ma due to the arrival of buoyant oceanic plateaus which induces the formation of 4337 highland pluton with ongoing subduction the steep angle subduction system is reestablished to cause the formation of 480 460 ma kaladawan pluton meanwhile it is this model that account for the temporal spatial distribution of these early paleozoic magmatic rocks in the north altun orogenic belt

development of new zealand according to the fore arc model of crustal evolution abstract analysis of new zealand geology using a fore arc model crook 1980a leads to the recognition of four arc terrains the west facing tuhua volcanic arc was active from the late proterozoic until the middle or late cambrian post subduction sediments neritic in the east and flysch in the west accumulated on the tuhua accretionary prism from the late cambrian until the early devonian thermal equilibration metamorphism granitoid plutonism and penetrative deformation occurred in the middle to late devonian a small area of permian platform cover has escaped later erosion the east facing rangitata terrain records subduction from early permian to late early cretaceous much of its accretionary prism consists of a submarine fan complex derived from western antarctica and carried sideways into the trench the accretionary prism is thick and completely kratonized in southern new zealand but the thickness is more variable northwards there the overlying upper cretaceous to upper oligocene post subduction sequence comprises shelf sediments implying an intermediate thickness prism or flysch followed by shelf sediments implying a thin prism during the accumulation of this sequence the rangitata terrain was a passive continental margin the south facing jurassic late oligocene northland terrain collided with this passive margin in northern new zealand at the end of the oligocene forming the northland allochthon subduction then flipped and the oldest part of the kaikoura terrain volcanic arc formed on the outer part of the northland terrain originally this terrain faced northeast and consumed the southwestern part of the south fiji basin crust but during the miocene the arc migrated clockwise to assume its present northeastern orientation the fore arc model employed here satisfactorily explains most first order and many second order features of new zealand geology without requiring modification thus attesting to the model s versatility and robustness new zealand provides a basis for elaborating some aspects of the model particularly the transition from the syn to post subduction phases of fore arc evolution combination of this study with a similar study of the southeastern australian paleozoic yields insights into the phanerozoic evolution of the australian pacific plates active margin

late palaeozoic early mesozoic fore arc basin sedimentary rocks at the pacific margin in western antarctica we infer that the trinity peninsula group of the antarctic peninsula was deposited in a fore arc basin s of composite type located between the gondwanian arc massif consisting of plutonic and older gneissose basement rocks and a subduction complex now represented by greenschists blueschists and albite epidote hornblende bearing rocks of the pre jurassic metamorphic complex

structural metamorphic evolution of the tia complex new england fold belt thermal overprint of an accretion subduction complex in a compressional back arc setting rocks in the tia complex underwent a simple clockwise p t t deformation path moving from mid crustal levels at blueschist facies conditions d1 d2 6 kbar 200 c to upper crustal levels at high t amphibolite facies conditions d4 d5 2 5 kbar 600 c in a relatively short time 15 ma the rocks remained under these conditions for approximately 40 ma d6 before they cooled and were brought to the surface d7 d8 during d6 permian rift basins were opening as thermal gradients in the tia complex reached a maximum d1 2 deformation was associated with accretion subduction processes and was followed by progressively less intense mainly e w oriented compression and uplift during d3 5 bringing the rocks to within 2 kbars of the surface d6 is characterized by limited extensional deformation and final uplift took place during d7 8 along shear zones that have orientations similar to d3 5 thrusts the shift from subduction dominated processes to thermally dominated processes occurred during d3 4 uplift of 4 kbar around 310 300 ma this coincided with a change in tectonic setting of the tia complex from an accretionary prism to a back arc position as the subduction zone stepped eastward an increased heat flow associated with back arc processes gradually caused thermal weakening of the lithosphere which due to external subduction related compressive forces resulted in uplift d3 with continued heating of the crust thermal doming resulted and uplift stopped d4 5 whilst thermal weakening led to limited crustal collapse as permian basins opened d6 the extremely high heat flow that was maintained for 40 ma suggests a heat source external to the crust possibly associated with thermal relaxation dehydration and detachment of the remnant stab left beneath the new england orogen after the subduction zone shifted east a stable crustal thermal structure was re established by 260 ma as the crust strengthened and could further support subduction induced compressional structures d7 8

record of permian early triassic continental arc magmatism in the western margin of the jiamusi block ne china petrogenesis and implications for paleo pacific subduction in this paper we report zircon u pb ages hf isotopes and whole rock geochemical data for the permian to early triassic granitoids from the western margin of the jiamusi block wjb ne china the intermediate to felsic sio2 59 67 74 04 wt granitoids belong to calc alkaline series and are characterized by enrichments in light rare earth elements and large ion lithophile elements with pronounced negative nb ta and ti anomalies revealing typical continental magmatic arc geochemical signatures the zircon u pb determinations on the granodiorite monzogranite syenogranite and quartz diorite samples yielded ages between ca 275 245 ma which together with the published coeval intrusive rocks indicates that permian to early triassic continental arc magmatism occurred extensively in the wjb the low and mainly negative zircon e hf t values between 7 6 and 1 6 and the zircon hf model ages of 1 2 1 8 ga which are significantly older than their crystallization ages suggest that they were mainly derived from reworking of ancient crustal materials with a limited input of juvenile components the geochemical systematics and petrogenetic considerations indicate that the studied granitoids were generated from a zone of melting assimilation storage and homogenization i e a mashed zone at the base of paleo to mesoproterozoic continental crust where large portions of igneous rocks and minor clay poor sediments involved in the source region in combination with regional geological data we argue that the jiamusi block was unlikely the rifted segment of the songliao block and two possible geodynamical models were proposed to interpret the formation of the ca 275 245 ma granitoids in the wjb in the context of permian global plate reconstruction we suggest that paleo pacific plate subduction was initiated in the permian to early triassic beneath the jiamusi block and even whole eastern ne china