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

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

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

initiation of subduction zones along transform and accreting plate boundaries triple junction evolution and forearc spreading centres implications for ophiolitic geology and obduction

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

moisture supply for northern ice sheet growth during the last glacial maximum during the last ice age the barents sea ice sheet began to grow 22 kyr ago1 only 8 kyr before it began to disintegrate2 this implies that the ice must have grown very rapidly from the coast to the edge of the continental shelf such rapid growth of a large ice sheet requires significant amounts of moisture3 but the origin of this moisture has been unclear particularly as the climap climate reconstruction suggests4 5 that the greenland iceland norwegian gin seas were perennially ice covered during this period here we present data from deep sea sediment cores from the fram strait which suggest that relatively warm water from the north atlantic ocean was advected into the gin seas in two short term events 27 22 5 and 19 5 14 5 kyr ago we suggest that the resulting seasonally ice free waters were an important regional moisture source for the barents sea ice sheet and that the gin seas played a much more active role in climate during the last glaciation than has previously been supposed

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

geology u pb zircon geochronology and geochemistry of pge bearing neoarchean and paleoproterozoic gabbroic rocks of the peter lake domain southern hearne craton canada the peter lake domain a component of the hearne craton in saskatchewan includes several intrusive complexes ranging in age from 2 70 to 1 83 ga two gabbroic complexes swan river and porter bay together represent one of the largest accumulations of precambrian mafic intrusion both metamorphosed at amphibolite facies sulfide poor pge occurrences are found in both complexes and share many textural and lithological characteristics with magmatic contact type pge ni cu deposits such as the chaotic lac des iles pluton and with layered stratiform deposits of large layered intrusions such as bushveld or stillwater lithologically both complexes are dominated by leucocratic gabbronorite and gabbro locally characterized by cumulate layering cross bedding brecciated and pegmatitic textures u pb zircon crystallization ages between 2562 and 2560 ma were obtained for the swan river complex sulfide formation is interpreted to be of magmatic origin rather than remobilized as had been speculated by explora

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

complex calc alkaline volcanism recorded in mesoarchaean supracrustal belts north of frederikshab isblink southern west greenland implications for subduction zone processes in the early earth we present new geochemical data for three mesoarchaean supracrustal belts ravns storo bjornesund and perserajoorsuaq situated north of frederikshab isblink in southern west greenland for which we propose the collective name the ikkattup nunaa supracrustal association they comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc alkaline andesite composition both lithological units are cut by aplite sheets of tonalite trondhjemite granodiorite ttg composition with u pb zircon ages of c 2900 ma lu hf and sm nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 41 ma and 3020 78 ma respectively which are within error of the age of the fiskenaesset complex situated 5 25 km to the north leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and ttg gneisses as the end members by assimilation fractional crystallisation afc modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a ttg like end member and another group of high p2o5 la and nb leucoamphibolites can be explained by contamination involving a hypothetical low silica adakite end member however the leucoamphibolites are juvenile with epsilon nd 2970 ma from 2 1 to 3 5 and epsilon hf 2970 ma of 3 5 to 4 3 thus the mafic source of the felsic contaminant melts must have been derived from a depleted mantle source more or less at the same time 60 ma as the volcanism took place our preferred interpretation of the geochemical and isotope data is that the protoliths of the supracrustal rocks formed in an island arc setting where early tholeiitic volcanism gave way to calc alkaline volcanism in a maturing arc the apparent afc trends are thus explained by in situ partial melting of basaltic arc crust to form juvenile ttg and adakite melts that mixed with mafic magmas or contaminated their mantle source to produce the calc alkaline leucoamphibolite protolith this model has important implications for the general interpretation of other archaean supracrustal belts because afc and geochemical mixing trends towards a ttg end member are not uniquely diagnostic of crustal contamination but may rather reflect processes operating at lower crustal or source levels in arcs such as melting assimilation storage homogenisation mash or slab melt low silica adakite metasomatism of the mantle source respectively c 2012 elsevier b v all rights reserved less

complex 3670 3500 ma orogenic episodes superimposed on juvenile crust accreted between 3850 and 3690 ma itsaq gneiss complex southern west greenland abstract the itsaq gneiss complex of the nuuk region southern west greenland is dominated by 3850 3690 ma tonalites intruded into and intercalated with lesser amounts of different 3850 3700 ma supracrustal units published whole rock sr and nd isotopic studies demonstrate that the tonalites are juvenile crustal additions from a depleted mantle source from our field studies and shrimp u pb zircon dating we argue that this juvenile crustal accretion was spread over ca 170 m yr 3850 3690 ma following 3850 3690 ma juvenile crustal accretion events the evolution of the itsaq gneiss complex continued with numerous crustal reworking events between 3670 and 3500 ma examples of these reworking events are as follows 1 there is intrusion of several generations of geochemically diverse granites sensu stricto with subordinate gabbros and diorites 2 there are superimposed multiple episodes of amphibolite to granulite facies metamorphism illustrated with akilia island samples by detailed dating of

in situ u pb granulite hosted zircon dates kapuskasing structural zone ontario a late archean large igneous province lip as a substrate for juvenile crust abstract a geochronological study of the kapuskasing structural zone ksz a section of late archean lower crust within the southern superior province canadian shield was undertaken to test a plate tectonic model for the late archean abitibi opatica terrane as a modified large igneous province lip new geochronological data allow us to interpret 1 construction of the lip in an environment where inherited zircons were derived from an older sialic crust and preserved in the granulite grade lower crust and 2 high grade metamorphism in the lower crust related to crustal refinement of the lip u pb and pb pb dates were obtained by la icp ms with beams of 8 30 m the study yielded 21 interpretable zircon dates spanning 500 m yr from one mafic granulite and two garnetiferous metatonalites nine of the dates are significantly older than the oldest known magmatic event recorded in the upper crust of the abitibi opatica terrane and indicate 2760 ma proto crustal material in the ksz lower crust the rema

sm nd and rb sr isotope systematics of an archean anorthosite and related rocks from the superior province of the canadian shield sm nd and rb sr isotopic data for the bad vermilion lake anorthosite complex bvl in the rainy lake area of the superior province of northwestern ontario show that direct ages of archean anorthosites can be obtained with these isotopic systems despite the effects of low grade metamorphism there is sufficient spread in sm nd between plagioclase megacrysts and coexisting mafic groundmass to allow the determination of reasonably precise internal sm nd isochrons anorthosite samples from bvl show an unusually large range in lil concentrations such that there is sufficient spread in rb sr for a whole rock isochron 2 69 0 10 ga isr 0 70079 8 this variability may have been caused by rb introduction during hydrothermal alteration and or low grade metamorphism the sm nd isochron for bvl 2747 58 ma end 2 0 1 4 includes data for anorthosite gabbro and metabasalt and is consistent with the consanguineity of these units in the rainy lake area the age is interpreted as the time of crystallization of the anorthosite complex and related mafic plutonics and volcanics visibly altered samples show evidence for disturbance of the sm nd and rb sr isotopic systems in one altered porphyritic dike plagioclase appears to have exchanged light ree with the relatively ree rich basaltic matrix this sample yields an internal sm nd age of 2 16 0 05 ga which may correspond to the time of local heating or represent a partial resetting from a still younger event initial isotopic ratios of nd and sr determined here add to the growing body of data indicating that the superior province is underlain by depleted mantle

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

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

origin of mesoarchaean arc related rocks with boninite komatiite affinities from southern west greenland we report whole rock elemental and sm nd isotope geochemical data from mafic ultramafic supracrustal rocks from the nunatak 1390 area in southern west greenland additionally we report the metamorphic temperature history for these rocks as derived from tourmaline thermometry on a tourmalinite inlier as well as in situ u pb hf and o isotopic data from zircons extracted from tonalite trondhjemite granodiorite ttg gneisses that intruded the mafic ultramafic sequence the supracrustal rocks from the nunatak 1390 area have a minimum age of c 2900 ma defined by u pb zircon ages of cross cutting aplite sheets of ttg composition the supracrustal sequence comprises mafic rocks with pillow structures and ultramafic rocks with no evidence of their protolith they all have amphibolite facies mineral assemblages and a peak metamorphic temperature of approximately 550 degrees c the mafic sequence has relatively flat trace element patterns la n sm n of 0 70 2 4 and mostly negative nb anomalies nb nb of 0 30 1 0 and resembles modern island arc tholeiites the mafic sequence can be divided into a high and low ti group where the former group has lower mgo and significantly higher contents of incompatible elements such as tio2 p2o5 zr nb and th the ultramafic rocks have major and trace element compositions similar to ti enriched karasjok type komatiites described in the literature however there are no textural indications that the ultramafic rocks from nunatak 1390 are komatiites sensu stricto the low ti group of the mafic sequence appears to have been derived from a n morb source whereas the high ti group and the ultramafic rocks appear to have been derived from a mantle source that is more enriched than the n morb source however there is no difference in the initial epsilon nd of the mafic and ultramafic rocks additionally assimilation fractional crystallisation afc modelling is consistent with this enrichment being caused by introduction of juvenile low silica adalcite slab melt into the mantle source region accordingly we propose that the mafic and ultramafic rocks were derived from a similar type of mantle source but that the ultramafic rocks were derived from a previously depleted mantle source that was refertilised by slab melts in a subduction zone setting the high mgo contents of the ultramafic rocks could thus reflect a second stage of partial melting of a refractory mantle in a process similar to that which is suggested for the formation of modern boninites we propose that the mafic ultramafic sequence represents an island arc that evolved initially as a juvenile complex c 3000 ma however inherited zircon grains in aplites and hf isotope data recorded by the second intrusive ttg phase c 2850 2870 ma show that mixing with older pre existing crust occurred during this event because the regional crust is dominated by ttgs of this younger age our data suggests that it likely formed by accretion and melting of arcs of different ages and or contamination of juvenile arcs by pre existing continental crust rather than entirely by juvenile arc differentiation or melting our data thus supports melting of thickened mafic crust in an accretionary setting rather than direct slab melting as a mechanism for archaean crust formation c 2012 elsevier b v all rights reserved

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

heterogeneously hydrated mantle beneath the late archean yilgarn craton abstract archean mafic ultramafic melts crystallized as layered intrusions in the upper crust and extruded as komatiitic flows are primary probes of upper mantle chemistry however the message from their primary chemical composition can be compromised by different modes of contamination contaminants are typically cryptic in terms of their geochemical and isotopic signals but may be related to metasomatised mantle sources ambient crustal assimilation or subduction related inputs in this work we present critical evidence for both dry and wet archean mantle sources for two juxtaposed layered intrusions in the australian yilgarn craton the ca 2813 ma windimurra and ca 2800 ma narndee igneous complexes in western australia are two adjacent layered intrusions and would be expected to derive via similar mantle sections a key difference in their chemistry is the presence of crystal bound water in the narndee igneous complex represented primarily by abundant hornblende such a primary hydrous phase is notably absent in the windimurra igneous complex new 40ar 39ar plateau ages for fresh narndee hornblende weighted mean 2805 14 ma mswd 1 8 probability 0 18 agrees with the published u pb age of 2800 6 ma for the complex and is consistent with a magmatic origin for this phase zircon hf and whole rock hf and nd isotopes for the narndee igneous complex indicate only minor crustal contamination in agreement with h and o isotope values in amphibole and o isotope values in rare zircon crystals plagioclase and pyroxene within both complexes these findings illustrate a fast temporal transition in proximal bodies from anhydrous to hydrous mantle sources with very minor crustal contamination these large layered mafic ultramafic intrusions are igneous bodies with a primitive chemical bulk composition that requires large degrees of mantle melting this has been attributed by many workers to mantle plume activity yet not without dispute as subduction related flux melting may also generate large melt fractions we conclude that the source of the magmatic water at narndee is the mantle which in conjunction with its absence in the adjacent windimurra igneous complex argues for a heterogeneous hydration of mantle source regions under the yilgarn craton in the mesoarchean

new pieces to the archaean terrane jigsaw puzzle in the nuuk region southern west greenland steps in transforming a simple insight into a complex regional tectonothermal model in the south of the nuuk region of west greenland our 1980s mapping recognized four archaean gneiss terranes f ringehavn tre brodre tasiusarsuaq and akia terranes with different protolith ages and separate early tectonothermal histories later in the archaean these were juxtaposed and then experienced the same 2700 2500 ma tectonothermal events here we abandon extrapolation of only these four terranes across the whole region and distinguish two new terranes in the ne the northernmost isukasia terrane previously regarded as the northernmost exposure of the f ringehavn terrane consists of palaeoarchaean rocks 3600 ma tectonically bounded on its south by the 3075 2960 ma kapisilik terrane these were juxtaposed and metamorphosed together by 2950 ma the previously recognized f ringehavn terrane to the sw is another separate entity of palaeoarchaean rocks that was juxtaposed with adjacent terranes only after c 2800 ma hence in an increasingly complex regional model there were several mid to neoarchaean terrane assembly events with superimposed orogenies from c 2950 ma until after 2700 ma although the f ringehavn and isukasia terranes were incorporated into the later archaean terrane collage at different times they might be fragments from a larger palaeoarchaean complex rifted apart from c 3500 ma onwards

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

dynamics of deformation and sedimentation in the northern sierras pampeanas an integrated study of the neogene fiambala basin nw argentina reply the thick skinned sierras pampeanas morphotectonic domain of western and northwestern argentina 27 s 33 s is characterized by reverse fault bounded basement blocks that delimit internally deformed neogene sedimentary basins foreland basin evolution in this part of the andes is still not very well understood for example challenging questions exist as to how thick skinned deformation develops if there are distinct spatiotemporal trends in deformation and exhumation how such deformation styles influence sedimentation patterns and whether or not broken foreland basins are related to regional plate tectonic processes such as flat slab subduction the fiambala basin of the northwestern sierras pampeanas is the largest of several intermontane basins in the transition to the southern margin of the puna plateau this basin preserves a thick continental neogene sequence that provides information on the dynamics of thick skinned deformation and resulting sedimentation the fiambala basin contains 4 km of fluvial alluvial sedimentary rocks that comprise the tamberia guanchin and punaschotter formations u pb geochronology of ashes intercalated within the fiambala stratigraphic sequence demonstrates that these sedimentary rocks are late miocene to pliocene 8 2 0 3 ma to 3 05 0 4 ma in age sedimentology and provenance data indicate that the source of the tamberia formation was located to the west of the modern western basin bounding range the guanchin and punaschotter formations record input from local sources including the modern basin bounding range to the west and the southern puna plateau to the north suggesting reorganization of the catchment area at ca 5 5 ma the coarsening upward trends recorded by the fluvial tamberia and guanchin formations indicate enhanced tectonics and relief during sedimentation the punaschot ter conglomerates record alluvial fan sedimentation and local sources fault kinematic data document a contractional regime characterized by e w and ne sw shortening active throughout the middle late miocene and pliocene furthermore a comparison between the fiambala basin and similar sedimentary basins in the sierras pampeanas e g bermejo foreland basin and the eastern cordillera leads us to propose that the study area originally constituted an integral part of a continuous and more extensive foreland basin system thin skinned for much of its early history our data suggest coeval intrabasin deformation along strike from the bermejo region northward to the eastern cordillera the coeval change at ca 6 ma from a regional to more compartmentalized thick skinned tectono sedimentary environment in the regions adjacent to the eastern cordillera the southern puna margin and other sectors within the sierras pampeanas domain may thus reflect a regional tectonic process related to flat subduction our data combined with existing sedimentological and petrological evidence imply that the passage from steep to flat subduction occurred synchronously from 30 s to 26 s

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

late quaternary contourites and glaciomarine sedimentation in the fram strait two sites in the eastern fram strait the vestnesa ridge and the yermak plateau have been surveyed and sampled providing a depositional record over the last glacial interglacial cycle the fram strait is the only deep water connection from the arctic ocean to the north atlantic and contains a marine sediment record of both high latitude thermohaline flow and ice sheet interaction on the vestnesa ridge the western svalbard margin a sediment drift was identified in 1226 m of water gravity and multicores from the crest of the drift recovered turbidites and contourites 14c dating indicates an age range of 8287 to 26 900 years bp early holocene to late weichselian the yermak plateau is characterized by slope sediments in 961 m of water gravity and multicores recovered contourites and hemipelagites 14c ages were between 8615 and 46 437 years bp early holocene to mid weichselian downcore dinoflagellate cyst analyses from both sites provide a record of changing surface water conditions since the mid weichselian suggesting variable sea ice extent productivity and polynyas present even during the last glacial maximum four layers of ice rafted debris were also identified and correlated within the cores these events occurred ca at 9 24 to 25 26 to 27 and 43 ka asynchronous with heinrich layers in the wider north east atlantic and here interpreted as reflecting instability in the svalbard barents ice sheet and the northward advection of warm atlantic water during the late weichselian the activity of the ancestral west spitsbergen current is interpreted using mean sortable silt records from the cores on the vestnesa ridge drift the modern mass accumulation rate calculated using excess 210pb is 0 076 g cm 2 year 1 on the yermak plateau slope the modern mass accumulation rate is 0 053 g cm 2 year 1

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

geochemistry and petrogenesis of the fiskenaesset anorthosite complex southern west greenland nature of the parent magma abstract the petrogenesis of the fiskenaesset anorthosite body has been investigated using major and trace element data for a large range of rock types from each zone of the complex the chemistry of these ultramafic to anorthositic cumulates is interpreted in terms of crystal fractionation of a parental trace element impoverished tholeiitic magma involving crystallisation of the cumulus phases olivine orthopyroxene clinopyroxene and dominant plagioclase feldspar amphibole appears not to have been a significant cumulus phase at any stage of crystallisation of the body the abundant amphibole found in the rocks of the complex being produced by primary intercumulus crystallisation supplemented by secondary metamorphic recrystallisation similarly magnetite is unlikely to have been a significant early cumulus phase although together with chromite it crystallised as a cumulus phase at high stratigraphic levels in the complex the metamorphism appears to be largely isochemical although sub solidus metamorphic re equilibration of the ree can be demonstrated on a grain size scale the spatial and temporal association between the anorthosite complex and the bordering metavolcanic amphibolites is matched by a strong similarity between the observed trace element chemistry of the amphibolites and the trace element chemistry of calculated successive liquids for the complex this is taken to suggest a genetic relationship between the evolution of the anorthosite complex and enclosing amphibolites the presence of trace element impoverished amphibolites which are not cumulates with trace element abundances comparable to those of the suggested parental liquid to the anorthosite complex is used to derive a major element composition for the primary fiskenasset magma this composition approximates a moderately aluminous tholeiitic basalt which may have been generated by hydrous fusion of previously depleted mantle this primary magma underwent crystal fractionation under low pressure conditions allowing the development of extensive plagioclase cumulates the fiskenaesset anorthosite and similar bodies cannot represent a cumulate residue complementary to the enclosing voluminous tonalitic gneisses which have a calc alkaline chemistry controlled by high pressure crystal liquid fractionation rather the association between the cumulate layered complex and bordering supracrustal sequence may imply an ancient ocean crust analogue for the development of this component of archaean high grade terrains it is suggested that slices of such archaean ocean floor may be emplaced laterally into the base of the continental crust during subduction of oceanic lithosphere at cordilleran type continental margins

revealing hidden structures the application of cathodoluminescence and back scattered electron imaging to dating zircons from lower crustal xenoliths abstract the use of cathodoluminescence cl and or back scattered electron bse imaging techniques with in situ ion probe analyses of zircons can help unravel complex crustal histories of metamorphic rocks that otherwise might remain elusive using these techniques we have imaged zircons from three lower crustal xenolith suites that have previously been dated by shrimp sensitive high resolution ion microprobe in all three cases the zircons are featureless in transmitted light but cl and bse reveal internal structures that correlate with distinct growth events generally cl and bse images reveal similar structures with cl showing finer detail neither imaging technique is capable of delineating all growth features in every sample and the best results are obtained using a combination of the two techniques igneous cores in zircons commonly emit a different color cl emission in many cases zoning in the cores is truncated indicating that the zircons either spent time in the supracrustal environment after their initial crystallization and prior to the granulite facies event s or that part of the core zircon was resorbed during the subsequent metamorphic event metamorphic rims when present are commonly 10 to 30 m thick and are nearly always unzoned and featureless igneous cores and metamorphic overgrowths commonly have distinctive cl emission spectra and trace element concentrations however the cl spectra can only be used to qualify chemical differences as a linear relationship has not been shown to exist between cl intensity and trace element concentration in natural zircons in many cases the hf y p u and the heavy rare earth elements hrees concentrations can be correlated to igneous and metamorphic growth using a combination of cl and bse imaging techniques and in situ trace element analyses with either the electron microprobe or pixe particle induced x ray emission

seabed erosion on the lomonosov ridge central arctic ocean a tale of deep draft icebergs in the eurasia basin and the influence of atlantic water inflow on iceberg motion 1 the submarine lomonosov ridge which bisects the arctic ocean is covered by 450 m of hemipelagic sediments high resolution chirp sonar and side scan data have documented erosion of a section of the ridge here we present multichannel seismic and chirp sonar data which show removal of unconsolidated sediments along the eurasia basin side of the lomonosov ridge where the central ridge is shallower than 1000 m the incomplete erosion of the hemipelagic drape and probable topographic control of ice motion suggest that the deepest draft glacier ice which reached the central arctic ocean from the eurasia basin was in the form of armadas of large icebergs embedded in sea ice rather than a single continuous floating ice shelf massive rapid discharge of glacier ice was probably required to produce the deepest draft 800 m icebergs eastward and northward drift trajectories from a likely iceberg source area on the kara barents sea margin would reflect the relative strength of atlantic inflow through the fram strait however only icebergs with drafts exceeding bathymetric thresholds on the lomonosov ridge would leave an erosive imprint on the seabed advection of atlantic water may have been relatively strong at times of discharge from the saalian ice sheet icebergs exiting the arctic ocean caused massive erosion and redeposition of the hemipelagic sediments on top of yermak plateau

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

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

asynchronous deposition of ice rafted layers in the nordic seas and north atlantic ocean instabilities in ice stream flow within the north american laurentide ice sheet leading to the periodic release of armadas of icebergs into the north atlantic ocean over the past 60 000 years have produced extensive layers of coarse grained iceberg rafted debris heinrich layers in north atlantic sediments1 2 correlation of these layers with iceberg discharge events from the ice sheets on greenland iceland and scandinavia suggested in previous studies for some heinrich layers and in some areas3 4 5 would imply that ice sheet instability had been synchronous across the north atlantic presumably in response to a common environmental cause here we show a lack of widespread systematic correlations both between ice rafted debris layers in different sediment cores from the nordic seas and between the nordic layers and the north atlantic heinrich layers this suggests that the full glacial nordic ice sheets did not exhibit unstable behaviour coincident with iceberg discharge from the vast hudson bay drainage basin of the laurentide ice sheet6 7 off the hudson strait significant ice sheet discharge of melt water is indicated by size sorted sandy and muddy turbidite sediments different from the poorly sorted debris flows which dominate sedimentation on the margins of the nordic seas8 9 10 together these results suggest that the dynamics of quaternary ice sheets surrounding the nordic seas were different from the outlet glacier draining the hudson bay basin and they provide evidence against a common circum north atlantic mechanism driving the discharge of icebergs

hafnium isotope evidence for a transition in the dynamics of continental growth 3 2 gyr ago earth s lithosphere probably experienced an evolution towards the modern plate tectonic regime owing to secular changes in mantle temperature radiogenic isotope variations are interpreted as evidence for the declining rates of continental crustal growth over time with some estimates suggesting that over 70 of the present continental crustal reservoir was extracted by the end of the archaean eon patterns of crustal growth and reworking in rocks younger than three billion years gyr are thought to reflect the assembly and break up of supercontinents by wilson cycle processes and mark an important change in lithosphere dynamics in southern west greenland numerous studies have however argued for subduction settings and crust growth by arc accretion back to 3 8 gyr ago suggesting that modern day tectonic regimes operated during the formation of the earliest crustal rock record here we report in situ uranium lead hafnium and oxygen isotope data from zircons of basement rocks in southern west greenland across the critical time period during which modern like tectonic regimes could have initiated our data show pronounced differences in the hafnium isotope time patterns across this interval requiring changes in the characteristics of the magmatic protolith the observations suggest that 3 9 3 5 gyr old rocks differentiated from a 3 9 gyr old source reservoir with a chondritic to slightly depleted hafnium isotope composition in contrast rocks formed after 3 2 gyr ago register the first additions of juvenile depleted material that is new mantle derived crust since 3 9 gyr ago and are characterized by striking shifts in hafnium isotope ratios similar to those shown by phanerozoic subduction related orogens these data suggest a transitional period 3 5 3 2 gyr ago from an ancient 3 9 3 5 gyr old crustal evolutionary regime unlike that of modern plate tectonics to a geodynamic setting after 3 2 gyr ago that involved juvenile crust generation by plate tectonic processes

holocene periodicity in north atlantic climate and deep ocean flow south of iceland climate fluctuations during the past millennium are relatively well documented1 on a longer timescale there is growing evidence of millennial scale variability of holocene climate at periodicities of 2 500 and 950 years possibly caused by changes in solar flux 2 3 and 1 500 years maybe related to an internal oscillation of the climate system 4 5 6 but the involvement of deep water masses in these holocene climate changes has yet to be established here we use sediment grain size data from the iceland basin to reconstruct past changes in the speed of deep water flow the study site is under the influence of iceland scotland overflow water isow the flow of which is an important component of the thermohaline circulation that modulates european climate flow changes coincide with some known climate events the little ice age and the mediaeval warm period and extend over the entire holocene epoch with aquasi periodicity of 1 500 years the grain size data indicate afaster isow flow when the climate of northern europe is warmer however a second mode of operation is observed in the early holocene when warm climate intervals are associated with slower isow flow at that time the melting remnant of land based glacial age ice may have provided a sufficient source of fresh water to the ocean to reduce isow flow south of iceland

meso and neoarchaean geological history of the bjornesund and ravns storo supracrustal belts southern west greenland settings for gold enrichment and corundum formation to understand the mesa and neoarchaean geological history of the bjornesund region southern west greenland ca 62 63 degrees n we have undertaken an integrated study combining field observations with u pb zircon geochronology by la icp ms metamorphic petrology and microstructural analyses the bjornesund region links the southern part of the fiskenaesset anorthosite complex to the well preserved metavolcanic rocks of the ravn story supracrustal belt and the bjornesund supracrustal belt so the development of this region is of key interest to unravelling the archaean geology of the entire region in this study we couple the observed phases of gold enrichment and corundum formation to the geological history the leucogabbroic rocks of the bjornesund region which are part of the fiskenaesset complex intruded at ca 2 95 ga into the tholeiitic and calc alkaline metavolcanic rocks of the two studied belts following isoclinal folding the rocks were intruded by diorite 2 92 ga and granodiorite 2 91 and 2 88 ga which are now texturally orthogneisses the area was affected by three major phases of folding f1 undated f2 at 2 85 2 83 ga and f3 at 2 71 2 70 ga the former deformation event is associated with the gold enrichment in alteration zones in shear folds in metavolcanic amphibolite near the contact with the orthogneiss intrusion of granitic sheets during the f3 deformation triggered the formation of corundum which is locally of gem quality at the contact between leucogabbroic and ultramafic rocks peak metamorphic conditions were calculated as 580 630 degrees c and 6 4 kbar at 2 85 2 83 ga later retrogression in the southern part of the area at ca 500 450 degrees c and 4 5 3 kbar and extensional fault reactivation overprinted some of the older structures c 2014 elsevier b v all rights reserved less

mantle and crustal sources of archean anorthosite a combined in situ isotopic study of pb pb in plagioclase and lu hf in zircon isotopic analyses of ancient mantle derived magmatic rocks are used to trace the geochemical evolution of the earth s mantle but it is often difficult to determine their primary initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration we present in situ analyses by la mc icpms for the pb isotopic compositions of igneous plagioclase an75 89 megacrysts and the hf isotopic compositions of bse imaged domains of zircon grains from two mantle derived anorthosite complexes from south west greenland fiskenaesset and nunataarsuk which represent two of the best preserved archean anorthosites in the world in situ la icpms u pb geochronology of the zircon grains suggests that the minimum crystallization age of the fiskenaesset complex is 2 936 13 ma 2 mswd 1 5 and the nunataarsuk complex is 2 914 6 9 ma 2 mswd 2 0 initial hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ehf units in terms of pb isotopic compositions of plagioclase both anorthosite complexes share a depleted mantle end member yet their pb isotopic compositions diverge in opposite directions from this point fiskenaesset toward a high more radiogenic pb crustal composition and nunataarsuk toward low less radiogenic pb crustal composition by using hf isotopes in zircon in conjunction with pb isotopes in plagioclase we are able to constrain both the timing of mantle extraction of the crustal end member and its composition at fiskenaesset the depleted mantle melt interacted with an eoarchean 3 700 ma mafic crust with a maximum 176lu 177hf 0 028 at nunataarsuk the depleted mantle melt interacted with a hadean 4 200 ma mafic crust with a maximum 176lu 177hf 0 0315 evidence from both anorthosite complexes provides support for the long term survival of ancient mafic crusts that although unidentified at the surface to date could still be present within the fiskenaesset and nunataarsuk regions

interpretation of ancient epikarst features in carbonate successions a note of caution abstract in shallow marine carbonate successions of the phanerozoic evidence for shorter lived subaerial exposure stages of ancient carbonate seafloors is commonly found in the presence of small scale epi karst solution pits in discontinuity surfaces under favorable conditions these solution features are accompanied by soil features including calcretes or root traces alveolar septal structures petrographic evidence such as pendant cements circumgranular cracks or pisoliths bleaching and staining of carbonate rocks or circumstantial geochemical evidence perhaps more often however ancient carbonate successions lack undisputable evidence for meteoric karsting using a well exposed case example from the aptian of oman we here document that the superficial visual field inspection of solution pits in discontinuity surfaces may lead to erroneous interpretations outcrops at jabal madar a diapiric structure allow for an in depth analysis of dissolution features in the regionally extensive top shu aiba discontinuity the solution pits discussed here were investigated for their stratigraphic position their orientation relative to bedding planes diagenetic and petrographic features and their potential relation to extensional fractures related to the updoming of jabal madar the main message brought forward is that under burial conditions spatially localized hypogenic carbonate leaching may form features that are easily mistaken for ancient meteoric epikarst these features preferentially form in interstratal positions where fractured massive carbonate rocks are capped by a major discontinuity surface overlain by non fractured argillaceous sediments thus while dissolution reprecipitation processes in the burial phreatic realm are omni directional in permeable carbonates low permeability argillaceous fines are not or to a much lesser extent prone to chemical corrosion as a consequence carbonate aggressive burial fluids leach out pits at the carbonate shale interface these appear to protrude perpendicular as bowl shaped depressions into the underlying limestones and are in the case examples documented here preferentially aligned along factures these findings have significance for the interpretation of ancient epikarst features in shallow marine carbonate successions

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

geochemistry and petrogenesis of archaean metavolcanic amphibolites from fiskenaesset s w greenland abstract the meta volcanic amphibolites closely associated with the fiskenaesset anorthosite complex can be subdivided on the basis of trace element patterns or mineral chemistry by far the most abundant type has light rare earth element lree depleted ree patterns and displays a wide range in trace element abundances chemically comparable amphibolites can be recognised throughout the ca 2800 m yr west greenland terrain the geochemistry of the basaltic amphibolites is dominantly controlled by fractional crystallisation processes although variable degrees of partial melting may also be important the required crystal extract plagioclase dominated in the proposed fractionation scheme is very similar to the primary mineralogy of cumulates of the fiskenaesset complex and trace element models support a genetic relationship between the anorthosite complex and enclosing host amphibolites the application of trace element discrimination to assign tectonic environment in the archaean is arguable however details of the trace element chemistry especially chondritic la ta ratios are taken to suggest out of a range of likely tectonic environments an ocean floor rather than island arc affinity for the fiskenaesset amphibolites the large ion lithophile lil elements display erratic distribution patterns but are generally enriched relative to the ree this appears not to be related to high grade metamorphism but may be a relict feature of seafloor alteration the association of the cumulate sequence with meta volcanic amphibolites and metasediments probably represents an ocean floor assemblage emplaced into the lower crust during crustal accretion

adjacent terranes with ca 2715 and 2650 ma high pressure metamorphic assemblages in the nuuk region of the north atlantic craton southern west greenland complexities of neoarchaean collisional orogeny abstract in the gneiss complex of the nuuk region of the north atlantic craton in southern west greenland terrane juxtaposition was followed by neoarchaean folding under amphibolite facies conditions with widespread low pressure recrystallisation 5 kbar and 550 700 c the complex metamorphic overprinting requires that the p t history related to actual terrane assembly has to be extracted from very small relicts of older metamorphic assemblages combined with the u pb dating petrology and geochemistry of metamorphic zircons in the south of the region the faeringehavn terrane eoarchaean orthogneisses is tectonically overlain by a supracrustal package of amphibolites and paragneisses ca 2840 ma felsic volcano sedimentary protoliths this package is juxtaposed against a higher tectonic level represented by the tre brodre terrane 2825 ma orthogneisses and the tasiusarsuaq terrane 2920 2810 ma orthogneisses the terranes were assembled by 2710 2720 ma as shown by dating of granitic sheets intruded along the terrane boundary mylonites in the faeringehavn terrane and in the overlying 2840 ma supracrustal package relict early high pressure assemblages 12 8 kbar 700 750 c are clinopyroxene garnet plagioclase quartz hornblende in mafic rocks and garnet kyanite rutile bearing assemblages in paragneisses these are commonly replaced by lower pressure assemblages 7 5 kbar such as cordierite sillimanite garnet in paragneisses and hornblende plagioclase quartz garnet in mafic rocks in situ partial melting took place during both low and high pressure regimes metamorphic zircon in the high and low pressure assemblages yields dates of ca 2715 ma mostly with errors of the kapisilik terrane 3050 2960 ma orthogneisses and another supracrustal assemblage of amphibolites and ca 2800 ma quartzo feldspathic metasedimentary rocks are exposed north of the faeringehavn terrane and within fold cores along its western margin are bounded by folded neoarchaean mylonites the kapisilik terrane and this supracrustal assemblage include high pressure metamorphic remnants in amphibolites and metasediments metamorphic segregations with garnet clinopyroxene and kyanite respectively that formed at ca 2650 ma these remnants are overprinted by high temperature but lower pressure metamorphic events at ca 2630 2610 and 2580 ma thus remnants of early metamorphism reveal mutually exclusive ca 2715 and 2650 ma high pressure events in adjacent tectonically juxtaposed terranes repeated high pressure metamorphism in adjacent terranes is observed in younger complex collisional orogens

crustal evolution and the temporality of anorthosites abstract types of rock such as komatiite which formed entirely or dominantly during restricted time periods in solar system history are important indicators of how planetary bodies evolved in deep time in this paper we characterize three different types of temporally restricted anorthosites and discuss their significance to the broad scale evolution of planetary processes primordial anorthosites which constitute the bulk of the lunar crust were sampled during apollo and luna missions and have subsequently been identified in our meteorite collections they are characterized by very calcic plagioclase an93 98 and all have magmatic crystallization ages 4 3 ga suggesting that the earliest planetary crust forming processes on the moon and possibly elsewhere involved substantial if not total melting the magma ocean hypothesis which has endured for nearly 50 years argues that the lunar anorthositic rocks represent global flotation cumulates of plagioclase which crystallized after extensive precipitation and sinking of olivine and pyroxene another possibility revitalized by critical filtering of age data involves serial magmatism whereby the lunar crust was constructed by younger smaller episodic magmatic events on earth two distinct types of temporally restricted anorthosites offer opportunities to understand the evolution of terrestrial geodynamics tectonics and magmatic processes archean megacrystic anorthosites formed only between 3 73 and 2 49 ga as small bodies

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

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

origin of archean tonalite trondhjemite granodiorite ttg suites and granites in the fiskenaesset region southern west greenland implications for continental growth abstract mesoarchean to neoarchean orthogneisses 2 95 2 79 ga in the fiskenaesset region southern west greenland are composed of an older suite of metamorphosed tonalites trondhjemites and granodiorites ttgs and a younger suite of high k granites the ttgs are characterized by high al 2 o 3 14 2 18 6 wt na 2 o 3 4 5 13 wt and sr 205 777 ppm and low y 0 7 17 4 ppm contents on chondrite and n morb normalized trace element diagrams the ttgs have the following geochemical characteristics 1 highly fractionated ree patterns la yb cn 14 664 la sm cn 4 3 11 0 gd yb cn 1 5 19 7 2 strong positive anomalies of sr sr sr 1 0 15 9 and pb pb pb 1 4 34 9 and 3 large negative anomalies of nb nb nb 0 01 0 34 and ti ti ti 0 1 0 6 the geochemical characteristics of the ttgs and trace element modeling suggest that they were generated by partial melting of hydrous basalts amphibolites at the base of a thickened magmatic arc leaving a rutile bearing eclogite residue field observations suggest that spatially and temporarily associated tholeiitic basalts now amphibolites in the fiskenaesset region might have been the sources of ttg melts the high k granites have steep ree patterns la yb cn 3 8 506 la sm cn 2 7 18 9 gd yb cn 0 92 12 1 and display variably negative eu anomalies eu eu 0 37 0 96 and moderate sr 84 539 ppm contents four outlier granite samples have variably positive eu eu eu 1 0 12 anomalies given that the granodiorites have higher k 2 o na 2 o than the tonalites and trondhjemites it is suggested that the granites were derived from partial melting of the granodiorites it is speculated that the dense eclogitic residues left after ttg melt extraction were foundered into the sub arc mantle leading to basaltic underplating beneath the lower rust melting of the granodiorites in response to the basaltic underplating resulted in the production of high k granitic melts formation of the fiskenaesset ttgs the foundering of the eclogitic residues into the mantle and the emplacement of the high k granites led to the growth of archean continental crust in the fiskenaesset region

anorthosites from an archean continental arc in the dharwar craton southern india implications for terrane assembly and cratonization abstract anorthositic rocks have attracted attention in terms of their possible role as the primordial crust of our planet and also as markers of archean paleoproterozoic plate tectonic regimes here we investigate the konkanhundi gabbro anorthosite suite from peninsular india adjacent to the major collisional suture between the western and eastern dharwar cratonic blocks the anorthosites display high al2o3 contents with negative correlation against mgo high cao and sr contents and positive eu anomalies indicating plagioclase flotation in the magma chamber the pyroxenite and gabbroic rocks of the suite show high ni cr co contents and geochemical features typical of pyroxene cumulation la icpms u pb analyses of magmatic zircon grains yield weighted mean 207pb 206pb ages of 2601 12 ma for pyroxenite 2616 12 ma for gabbro 2615 13 ma and 2627 14 ma for anorthositic gabbro 2594 16 ma for anorthosite and 2605 27 ma for microgabbro the age data from the different rock types in the anorthosite suite are broadly consistent marking the emplacement time as ca 2 6 ga and providing insights on one of the oldest anorthosite complexes in peninsular india zircon grains in the surrounding ttg gneisses into which the gabbro anorthosite suite was emplaced show weighted mean 207pb 206pb age of 3321 11 ma suggesting mesoarchean basement the zircon grains display high th u values and ree patterns typical of magmatic crystallization with enriched hree positive ce and sm anomalies and negative pr nd eu anomalies zircon lu hf analysis yield negative ehf t values of 4 9 0 7 with t dm c two stage model ages of 3123 3376 ma for pyroxenite 4 5 1 7 and 3192 3366 ma for gabbro 5 3 0 9 and 3034 3410 ma for anorthositic gabbro and 3 9 2 4 and 3221 3311 ma respectively suggesting the incorporation of mesoarchean components within the neoarchean magmatic suite however zircon grains in the ttg gneiss possess more juvenile ehf t values in the range of 0 2 1 3 with t dm c in the range of 3554 3646 ma the zircon hf isotopes and trace element data together with the whole rock geochemical features suggest that the parent magma of the konkanhundi gabbro anorthosite suite was derived from subduction related depleted mantle source that also incorporated continental crustal components the time of emplacement of the gabbro anorthosite complex broadly correlates with widespread arc magmatism and crust production as well recycling in other domains of the dharwar craton we envisage multiple convergence of microblocks during craton assembly at the end of archean in dharwar with the greenstone belts marking zones of paleo ocean closure

lithological structural and geochemical characteristics of the mesoarchean târtoq greenstone belt southern west greenland and the chugach prince william accretionary complex southern alaska evidence for uniformitarian plate tectonic processes the mesoarchean t rtoq greenstone belt southern west greenland consists of tectonically imbricated slices of metamorphosed basalt gabbro peridotite and sedimentary rocks and is intruded by fel

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

geophysical and geological tests of tectonic models of the north china craton abstract the geometry and timing of amalgamation of the north china craton have been controversial with three main models offering significantly different interpretations of regional structure geochronology and geological relationships one model suggests that the eastern and western blocks of the ncc formed separately in the archean and an active margin was developed on the eastern block between 2 5 and 1 85 ga when the two blocks collided above an east dipping subduction zone a second presumes the eastern block rifted from an unknown larger continent at circa 2 7 ga and experienced a collision with an arc perhaps attached to the western block above a west dipping subduction zone at 2 5 ga and the 1 85 ga metamorphism is related to a collision along the northern margin of the craton when the ncc joined the columbia supercontinent a third model suggests two collisions in the central orogenic belt at 2 1 and 1 88 ga but recognizes an early undated deformation event recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models showing that subduction beneath the central orogenic belt was west directed and that there is a second west dipping paleosubduction zone located to the east of the cob dipping beneath the western block ordos craton the boundaries identified through geophysics do not correlate with the boundaries of the trans north china orogen suggested in the first model and the subduction polarity is opposite that predicted by that model high pressure granulite facies metamorphism at 1 85 ga is not restricted to the tnco as suggested by the first model but is documented across the ncc as predicted by the second model suggesting a major continent continent collision along the north margin of the craton at 1 85 ga further it has recently been shown that in the southern tnco there is no record of metamorphism at circa 1 85 ga but only at 2 7 2 5 ga showing that the tnco as defined as a circa 1 85 ga orogen does not exist this is further confirmed by recent re os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern cob is late archean in age and that a province in the northern ncc is circa 1 8 ga correlating with the proposed collision belt of the ncc with the columbia supercontinent across the entire ncc the cob is an archean convergent belt re worked in the paleoproterozoic and the paleoproterozoic tectonism is widespread across the ncc as predicted by the model whereby the previously amalgamated eastern and western blocks experienced a continental collision with columbia at circa 1 85 ga but uplift exhumation rates are slow necessitating a re evaluation of the tectonic models of the ncc

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

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

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

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

formation origin and geographic typing of corundum ruby and pink sapphire from the fiskenaesset complex greenland abstract metamorphic petrology observations on rubies found in situ in their host rock are combined with geochemical measurements and optical microscopy observations on the same rubies with the aim of connecting the ruby forming metamorphic reaction to a unique fingerprint for these minerals the fiskenaesset complex in greenland is used as an area of this case study isochemical pressure temperature sections were calculated based on electron microprobe and whole rock geochemistry analyses and compared to field observations rubies formed from reaction between olivine serpentine and anorthite triggered by the intrusion of a 2 71 ga pegmatite al is sourced from the anorthite reacting to calcic amphibole silica from the pegmatite reacts with olivine serpentine to anthophyllite cr3 is mobile in the pegmatitic fluid giving colour to the rubies the ruby forming reaction occurs at about 640 c and 7 kbar in order to establish the unique fingerprint for this ruby bearing ultramafic complex laser ablation inductively coupled plasma mass spectrometry trace element measurements oxygen isotope compositions optical microscopy and scanning electron microscopy were applied due to the setting in an ultramafic rock anorthosite leucogabbro complex the fingerprint of the rubies from the fiskenaesset complex is rather unique compared to rubies from other localities fiskenaesset complex rubies contain high cr intermediate fe and low v ga and ti concentrations low oxygen isotope values 1 6 4 2 and a rarely observed combination of optical growth features and mineral inclusions like anthophyllite biotite results for other greenland localities are presented and discussed as well even though these are derived from ultramafic rock settings too they record different trace element ratios and oxygen isotope values resulting from variations in the archaean ruby forming reaction

isotopic constraints on contamination processes in the tonian goias stratiform complex abstract the tonian goias stratiform complex tgsc goias central brazil is one of the largest mafic ultramafic layered complexes in the world emplaced during the geotectonic events that led to the gondwana accretion in this study we present trace elements and in situ u pb lu hf analyses of zircons and 87sr 86sr ratios of plagioclases from anorthosites and gabbros of the tgsc although formed by three isolated bodies cana brava niquel ndia and barro alto and characterized by a lower and upper sequence ls and us our new u pb zircon data confirm recent geochemical geochronological and structural evidences that the tgsc has originated from a single intrusive body in the neoproterozoic new hf and sr isotope ratios construe a complex contamination history for the tgsc with different geochemical signatures in the two sequences the low hf and high sr isotope ratios of the lower sequence ehf t from 4 2 down to 27 5 87sr 86sr 0 706605 0 729226 suggest the presence of a crustal component and are consistent with contamination from meta pelitic and calc silicate rocks found as xenoliths within the sequence the more radiogenic hf isotope ratios and low sr isotope composition of the upper sequence ehf t from 11 3 down to 8 4 87sr 86sr 0 702368 0 702452 suggest a contamination from mantle derived metabasalts in agreement with the occurrences of amphibolite xenoliths in the us stratigraphy the differential contamination of the two sequences is explained by the intrusion of the tgsc in a stratified crust dominated by metasedimentary rocks in its deeper part and metavolcanics at shallower levels moreover the differential thermal gradient in the two crystallizing sequences might have contributed to the preservation and recrystallization of inherited zircon grains in the us and total dissolution or magmatic overgrowth of the ls zircons via melt rock reaction processes

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

neoarchean suprasubduction zone arc magmatism in southern india geochemistry zircon u pb geochronology and hf isotopes of the sittampundi anorthosite complex abstract the sittampundi anorthosite complex sac in southern india is one of the well exposed archean layered anorthosite gabbro ultramafic rock associations here we present high precision geochemical data for the various units of sac coupled with zircon u pb geochronology and hf isotopic data for the anorthosite the zircon ages define two populations the older yield a concordia age of 2541 13 ma which is interpreted as the best estimate of the magmatic crystallization age for the sittampundi anorthosite a high grade metamorphic event at 2461 15 ma is suggested by the upper intercept age of the younger zircon population a neoproterozoic event at 715 180 ma resulted in pb loss from some of the metamorphic zircons the magmatic age of the anorthosite correlates well with the timing of crystallization of the arc related 2530 ma magmatic charnockites in the adjacent salem block while the metamorphic age is synchronous with the regional metamorphic event the geochemical data suggest that the rocks were derived from a depleted mantle source sub arc mantle metasomatism of slab derived fluids and subsequent partial melting produced hydrous aluminous basalt magma the magma fractionated at depth to produce a variety of high alumina basalt compositions from which the anorthositic complex with its chromite rich and amphibole rich layers formed as cumulates within the magma chamber of a supra subduction zone arc the coherent initial176hf 177hf ratios and positive ehf values 1 7 4 5 of the magmatic zircons in the anorthosite are consistent with derivation of a rather homogeneous juvenile parent magma from a depleted mantle source our study further confirms that the southern part of the dharwar craton was an active convergent margin during the neoarchean with the generation and emplacement of suprasubduction zone arc magmas which played a significant role in continental growth

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

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

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

origin of archean anorthosites evidence from the bad vermilion lake anorthosite complex ontario the bad vermilion lake anorthosite complex 2 700 m y is exposed over an area of about 100 km2 near rainy lake ontario as is typical of other archean anorthosites it is composed of coarse 1 30 cm across equidimensional euhedral to subhedral calcic an80 plagioclase in a finer grained mafic matrix the amount of mafic matrix in individual samples ranges from none to about 70 by volume the complex has been variably metamorphosed to greenschist facies zoisite chlorite and hornblende are abundant but primary plagioclase is preserved in many places