2 61 ga potassic granites and crustal reworking in the western dharwar craton southern india tectonic geochronologic and geochemical constraints abstract we present combined field structural data and spot image analysis petrographic data u pb zircon ages nd isotopes and whole rock geochemical data for the late archaean granite plutons from arsikere banavara ab suite and chitradurga jampalnaikankote hosdurga cjh suite in the western dharwar craton wdc ab plutons are syn kinematic with respect to the development of dome and basin patterns resulting from partial crustal diapiric overturn d1 whilst cjh suite plutons show deformation patterns resulting from the superimposition of large scale regional strike slip shearing d2 onto d1 syn emplacement fabrics sims u pb zircon ages of 2614 10 ma for the chitradurga pluton cjh and of 2617 3 ma for the arsikere pluton ab define the minimum age of the d1 strain pattern the cjh and ab suites are mainly high k granites whose origin is to be sought in partial melting of an old 3 0 ga depleted lower crust of intermediate composition the cjh and ab suites correspond mainly to the ca1 and ca2 types of archaean calc alkaline granites and some facies to the transitional ttg their heterogeneous chemical compositions indicate involvement of various sources and also significantly different depths of partial melting to produce these granites these results allow documenting regional partial reworking of the wdc 2615 ma ago requiring an ht event to have taken place before 2 61 ga and thus different from the 2 55 to 2 51 ga granulite facies metamorphic episode associated to regional scale strike slip shearing d2 that affected the entire craton partial reworking of the older lower crust at 2 61 ga and associated deformation would result from heat advection and or crustal thickening that may relate to voluminous mafic greenstone volcanism and moderate shortening at that time

geochemistry and nd isotope systematics of archaean granites of the eastern goldfields yilgarn craton australia implications for crustal growth processes abstract the archaean granites and granitic gneisses of the eastern goldfields in the yilgarn craton western australia can be divided into two major groups high ca and low ca and three minor groups high hfse high field strength elements mafic and syenitic the high ca group 68 77 sio 2 with high al 2 o 3 na 2 o and sr and low y shares many features with typical archaean tonalite trondhjemite suites but has higher k 2 o rb and th contents the low ca group 70 76 sio 2 differs from the high ca group in having lower al 2 o 3 cao and na 2 o but higher k 2 o rb th zr y la and ce contents granites of the high hfse mafic and syenitic groups form a minor component 10 20 of the eastern goldfields granites the siliceous 74 77 sio 2 high hfse granites are restricted to a narrow nnw trending zone and are characterised by high tio 2 total feo mgo y zr and ce but only moderate rb th and pb contents the a type syenites 50 68 sio 2 are distinguished by their high total alkalies and mainly occur along tectonic lineaments the mafic group 55 70 sio 2 is lithologically diverse and exhibits a wide range of k 2 o rb th la and ce contents granite emplacement occurred between 2 69 and 2 60 ga contemporaneous with or post dating greenstone formation nd values for the high ca mafic and syenitic groups are similar mostly 0 0 to 2 5 whereas those for the low ca group show a pronounced polarity from 4 5 in the west to 2 0 in the east nd depleted mantle model ages for the low ca granites range from 3 2 ga in the west to 2 75 ga in the east model ages for the high ca group 2 75 2 9 ga overlap with ages for inherited zircons from these granites the high ca granites appear to have been derived at high pressures by partial melting of a mafic to intermediate source subducted oceanic crust crustal underplate or thickened crust or remelting of an older tonalitic source derived by such a process if the high ca granites were largely crust derived then their extensive source rocks must have formed less than 250 ma prior to granite formation the low ca granites appear to be the products of crustal reworking probably from a tonalitic to granodioritic protolith that was progressively younger towards the east perhaps in the order of several hundred million years younger or involved an additional younger component the localisation of the crustally derived high hfse granites suggests either geochemical zonation of the crust or specific tectonic processes such as rifting the mafic granites and possibly the syenites imply a significant mantle contribution and new crustal growth although the extent is equivocal the crust of the eastern goldfields appears to have been generated by both lateral and vertical accretion over a significant period from or before 3 0 ga until the time of granite emplacement

garnet orthopyroxene bearing granulites from enderby land antarctica metamorphic pressure temperature time evolution of the archaean napier complex

forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

phanerozoic sanukitoids from caledonian scotland implications for archean subduction sanukitoids represent a volumetrically minor but important series of neoarchean granitoids enriched in large ion lithophile elements e g ba sr and light rare earth elements and with relatively high compatible elements e g mg ni and cr petrogenetic models have favored an origin in the suprasubduction mantle wedge so their sudden appearance ca 2 95 2 68 ga has made them central to the ongoing debate about the beginnings of modern plate tectonics caledonian high ba sr granites from the northern highlands of scotland are petrological and compositional equivalents the caledonian examples have not undergone subsequent reworking and their petrogenetic setting can be used to inform genetic models for sanukitoids prolonged subduction preceded emplacement of the high ba sr granites in a short interval 10 ma at the end of the caledonian orogeny sediment subduction was responsible for elemental and isotopic enrichments and slab breakoff triggered melting in the subcontinental lithospheric mantle this implies that during the neoarchean the evolving tectonothermal regime moved the locus of melting from the basaltic slab to the sediment infused mantle wedge creating the first subcontinental lithospheric mantle sanukitoids may then have resulted from widespread closely subsequent slab breakoff events producing evolved magmas by familiar fractionation mechanisms careful study of secular variations in high ba sr magmas could constrain the evolution of the subcontinental lithosphere

chronology of multiphase emplacement of the salmi rapakivi granite anorthosite complex baltic shield implications for magmatic evolution the u pb dating of 18 samples representing the principal rock types of the 4000 km2 salmi anorthosite rapakivi granite complex and its satellite uljalegi pluton southeastern baltic fennoscandian shield reveals that six temporally distinct episodes of igneous activity occurred in a timespan of 17 million years from oldest to youngest they are 1 gabbronorite and monzonite at 1546 7 ma 2 syenogranite at 1543 4 ma 3 early wiborgite and pyterlite at 1540 6 1537 9 ma 4 biotite granite and more evolved granite at 1538 4 1535 ma 5 late pyterlite at 1535 2 ma 6 olivine gabbro and biotite amphibole granite at 1530 ma the resolvable intervals between magmatic episodes are 3 5 5 0 million years early wiborgite and pyterlite 3 above and biotite granite 4 above probably crystallized from multiple magma intrusions age differences of 3 4 1 5 million years between zircon and baddeleyite in olivine gabbro 6 above are probably a result of xenocrystic origin of baddeleyite extracted from an earlier mafic phase of the salmi complex the ages and chemical features of early and late zircon populations together with our modeling of magma crystallization and zircon growth show that the duration of magma crystallization and pb diffusion in zircon was short lived and insignificant compared to the precision of dating of about 1 2 million years hence the range of u pb ages for each of the major rock types may approximate the emplacement intervals of their respective magmas average rate of magma emplacement was about 0 01 km3 year for the most voluminous phase of early biotite amphibole rapakivi granite and about 0 0024 km3 year for the salmi complex as a whole compositional changes of the salmi magmas over time are in agreement with the model of magmatism related to lithospheric extension

petrogenetic constraints for the genesis of archaean sanukitoid suites geochemistry and isotopic evidence from karelia baltic shield new sm nd isotope data are presented for late archaean sanukitoid suites of the karelian granite greenstone terrane of the baltic shield within the terrane there are regional variations in the nd isotope compositions of the sanukitoids sanukitoids in the youngest central karelian domain have nd isotope characteristics similar to depleted mantle whereas those from the older west karelian and vodlozero domains have lower initial end values the isotopic heterogeneity of the sanukitoids is interpreted to result from variations in the time interval between mantle source enrichment and the partial melting that produced the sanukitoid magmas we therefore suggest a two stage model of the generation of sanukitoid magmas during first stage the mantle was metasomatised by fluids and or melts generated during subduction or tectonic underplating later between 2 74 and 2 70 ga a tectonothermal anomaly generated the sanukitoid magmas by melting of the previously metasomatised mantle most of the sanukitoid intrusions are cut by calc alkaline lamprophyre dykes that have the similar geochemical signatures to their sanukitoid wall rocks new sm nd isotope data for these lamprophyres suggest a genetic kinship between the sanukitoids and the lamprophyres comparison between chemical signatures of sanukitoids and modern subduction related magmas shows that archaean sanukitoids actually have no modern analogues sanukitoid melts can be modelled as partial melts of mantle that had been metasomatised a short time previously the melts could have been in equilibrium with residues containing ol cpx plus minor phl and grt at a pressure of about 2 5 gpa and a temperature of 1150 1230 8c the small percentage of residual phlogopite would retain rb in the mantle and explain the peculiar signature of low rb and high ba and sr characteristic of the sanukitoids we envisage a temporal evolution of the composition of the mantle in this region this pattern of evolution controlled production of the sanukitoid melts and explains why the sanukitoids have such a restricted range of ages sanukitoid magma production must have followed from a major global mantle enrichment event these magmas could not have been produced earlier because the archaean mantle had not become sufficiently enriched before 2 7 ga we suggest that their scarcity after the major period of sanukitoid magmatism may be due to the increased thickness of overlying crust which can inhibit mantle enrichment

high precision 207pb206pb zircon geochronology using a small ion microprobe abstract a small geometry ion microprobe cameca ims 4f has been successfully used for the routine measurement of 207 pb 206 pb ratios in individual zircons with a spatial resolution of 20 m in spite of the lower sensitivity of the small ion microprobe resulting from operating at high mass resolution m m 4600 in order to resolve isobaric molecular interferences we can discern archean components whose ages differ by 50 ma a single measurement requires 90 min of data acquisition thus a comprehensive study of a zircon population can be accomplished in a few days three sets of precambrian zircons analyzed earlier by other techniques yielded ages which are within 2 of their true values as our first unknown we analyzed zircons from a sample of orthogneiss collected from the aravalli craton of northwestern india for this sample we obtained a minimum age of 3281 3 ma 1 which we believe closely approximates the time of crystallization of the gneiss s igneous protolith

formation of paleoarchean continental crust through infracrustal melting of enriched basalt abstract paleoarchean rocks of the tonalite trondhjemite granodiorite ttg series require a basaltic source region more enriched in k lile th and lree than the low k tholeiites typical of archean supracrustal sequences most ttg of the pilbara craton in northwestern australia formed between 3 5 and 3 42 ga through infracrustal melting of a source older than 3 5 ga basaltic to andesitic rocks of the 3 51 ga coucal formation at the base of the pilbara supergroup are amongst the only well preserved remnants of pre 3 5 ga supracrustal material on earth and may have formed a large proportion of pre 3 5 ga pilbara crust these rocks are significantly enriched in k lile th and lree compared to post 3 5 ga paleoarchean basalts and andesites and form a compositionally suitable source for ttg enrichment in these basalts was not the result of crustal assimilation but was inherited from a mantle source that was less depleted than modern morb source and was enriched in recycled crustal components we suggest that the formation of paleoarchean ttg and of their voluminous mafic source regions reflects both a primitive stage in the thermal and compositional evolution of the mantle and a significant prehistory of crustal recycling

stabilization of the aravalli craton of northwestern india at 2 5 ga an ion microprobe zircon study abstract in order to establish the stabilization age of the aravalli craton of northwestern india we have used an ion microprobe to measure 207 pb 206 pb ages of zircons from six samples collected from the southern aravalli mountains our analyses gave precise results for four granitoids whose minimum crystallization ages ranged between 2562 6 and 2440 8 ma 1 a single sample of gneiss gave an age of 2502 4 ma which is unexpectedly younger than an age of 3281 ma which we previously obtained from another gneissic sample we interpret the 2502 ma age as the time of emplacement of the sample s igneous protolith suggesting that the mewar gneiss contains multiple metamorphic components of differing ages based on our data we conclude that the southern segment of the aravalli craton had broadly stabilized by 2 5 ga this stabilized landmass formed the basement on which the younger aravalli supergroup was unconformably deposited this study represents the first application of a small geometry ion microprobe for routine geochronology

seismic evidence for preservation of the archean uchi granite greenstone belt by crustal scale extension abstract in the westernmost superior province of canada the east west alignment of granite greenstone belts and the adjacent highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2 5 ga ago with the belts thrust against one another by east west oriented subduction zones here we present seismic reflection data which demonstrate that in this region the present juxtaposition of the uchi granite greenstone belt and the north caribou gneiss terrane occurred along a late southeast dipping extensional shear zone that extends from the surface into the lower crust the preservation of the uchi belt and probably the english river metasedimentary belt is directly related to their dropping along extensional shear zones which limited subsequent erosion the relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust nor preserved by vertical crustal movements as might occur in the absence of plate tectonics extension may have been associated with the emplacement of mantle derived magmas at 2700 ma which has been linked to slab break off or lithospheric delamination making the extension approximately coeval with local gold mineralisation since crustal scale faults can facilitate the circulation of gold bearing fluids we suggest that greenstone rocks preserved in the hanging walls of syn to post accretion extensional shear zones may preferentially host archean lode gold deposits in the westernmost superior province our seismic observations imply that some of the late structures in the well developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes

the affinity of archean crust on the yilgarn albany fraser orogen boundary implications for gold mineralisation in the tropicana zone abstract craton margins can be subject to a wide array of gold genesis and redistribution processes although high grade terrains on craton margins are frequently viewed as less prospective than lower grade counterparts in contrast to this the high grade tropicana zone a newly defined archean crustal component on the eastern margin of the yilgarn craton within the albany fraser orogen afo contains a significant proterozoic gold deposit this deposit and zone comprise mid amphibolite to granulite facies gneissic rocks with evidence of partial melting and granite injection the tropicana zone contains significant low si lile enriched granites classed as sanukitoids along with the distinctive compositions the rarity of these rocks within any archean craton suggests that the granitoid protoliths represent a single suite emplaced during one event due to the intense granulite facies overprinting of the tropicana zone rocks determination of the magmatic protolith age for these sanukitoids is challenging nonetheless the best age estimate for magmatism is 2692 16 ma based on the youngest zircons preserving textural evidence of growth within a viscous silicate melt this age is older than compositionally similar magmatism found within the yilgarn craton although a sanukitoid in the northern foreland of the afo has a similar age furthermore the granulite facies metamorphic zircon growth in the tropicana zone at 2718 2554 ma was prolonged compared to that in the yilgarn craton nonetheless the hf isotopic signature of the tropicana zone zircon shares strong similarity to that from the eastern goldfields superterrane of the yilgarn craton this implies that the tropicana zone reflects a deeper crustal level of the yilgarn craton exhumed and thrust nw an unknown distance over the craton edge in addition we observe that the granulite facies zircons have a less radiogenic hf isotope signature than the preserved pre metamorphic zircon cores based on correlations with alpha dose u and th content and 176hf 177hf we suggest this reflects the preferential destruction and release of unradiogenic hf from inherited zircon whereas the protolith sanukitoid zircon with lower u and th content was more resistant to mobilisation during high grade metamorphism we note this situation may be a more general response of the hf isotopic system in which zircon grown in a more mafic melt is less likely to contribute to the metamorphic hf reservoir than its felsic counterpart juvenile granitic veins dated at c 1780 ma intruded into the tropicana zone indicate that the tropicana zone was structurally emplaced at or before c 1780 ma given similar proterozoic magmatic events are well documented from para autochthonous adjacent units re os dating of pyrite coeval with one generation of gold in these rocks indicates model ages of c 2100 ma supportive of a palaeoproterozoic age of mineralisation this mineralisation event is distinct from major proterozoic tectonothermal events elsewhere in the afo sanukitoid magmas are well known for gold fertility and were likely the original source of gold in the tropicana zone which was subsequently concentrated into brittle structures during several episodes gold mineralisation post dated peak metamorphic conditions and is significantly younger than gold mineralisation within other parts of the adjoining yilgarn craton

sm nd evidence for 3 3 ga old rocks in rajasthan northwestern india abstract tonalitic to granodioritic gneisses from the basement gneiss of the aravalli region northwestern india define a sm nd isochron corresponding to an age of 3 31 0 07 ga and an initial nd isotopic ratio that is close to chondritic lenticular bodies of mafic amphibolites within the gneisses however define an isochron corresponding to a younger age of 2 83 0 05 ga and small positive initial nd this age is similar to that reported for a large intrusive granite the untala granite in this area and combined with field evidence suggests that mafic volcanism may have been associated with formation of the granite a highly hree enriched garnet separate from one of the tonalitic gneisses defines a two point garnet wholerock isochron age of 2 45 0 02 ga which also is similar to the age of a known thermal event in the region intrusion of the berach and bundelkhand granites at approximately 2 5 ga these results document in broad outline archean crustal formation events in the northwestern segment of the indian subcontinent and demonstrate the need for more detailed studies of the complex terranes mapped by heron 1953 as a single unit the banded gneiss complex

neoarchean crustal recycling and mantle metasomatism hf nd pb o isotope evidence from sanukitoids of the fennoscandian shield abstract the growing knowledge of the geological record reveals evidence of crustal recycling in the archean eon in order to assess the role of recycling of crust into the mantle we have analyzed radiogenic hf in zircon as well as whole rock nd and pb isotope compositions of mantle derived granitoids of the sanukitoid series in the neoarchean karelian province fennoscandian shield homogeneous initial hf and nd values suggest that sanukitoids are derived from well mixed source s limited scatter in the initial pb isotope data within intrusions as well as in other radiogenic isotopes point to effective mixing processes in the mantle o isotope data for zircon from karelian sanukitoids indicate a dominant mantle source 18 o 5 5 6 1 with minor contributions from sources with elevated 18 o 7 0 to 8 5 such as sediments and altered upper oceanic crust our results suggest that the parental melts of sanukitoids are mixtures of depleted mantle and recycled crustal components a slab breakoff at the end stage of subduction is a plausible trigger mechanism for sanukitoid magmatism and it explains variable contributions from different 18 o sources

collision driven slab breakoff magmatism in northwestern anatolia turkey abstract postcollisional eocene magmatism in northwestern anatolia produced two e w trending linear belts of plutons along and north of the izmir ankara erzincan suture zone iaesz whose geochemical features and age relations support a slab breakoff model for their petrogenetic oinevolution the suture zone granitoids szgs in the southern belt have ages around 54 48 ma are intrusive into blueschist rocks of the iaesz and are composed of diorite quartz diorite granodiorite and syenite the marmara granitoids mgs in the northern belt are slightly younger 48 35 ma intrusive into the paleozoic mesozoic crystalline basement rocks of the sakarya continent and composed of monzogranite granite and granodiorite both szgs and mgs have moderately to highly evolved medium to high k calc alkaline compositions and are predominantly metaluminous i type granitoids nd sr isotope systematics documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepacka

the archaean sanukitoid series of the baltic shield geological setting geochemical characteristics and implications for their origin abstract archaean high mg granitoids sanukitoids occur in the karelian granite greenstone terrain in the baltic shield in two distinct zones in the west of the shield sanukitoid intrusions formed between 2700 and 2720 ma and consist of a single igneous phase that varies in composition from diorite to granite in the eastern part of the shield sanukitoid intrusions formed between 2730 and 2745 ma and are strongly differentiated varying in composition from ultramafic to felsic all the sanukitoids are enriched in light rare earth elements lree la 80 ppm ce 150 ppm la n yb n 30 40 sr 2000 ppm ba 2500 ppm p 2 o 5 1 5 alkalis na 2 o k 2 o 5 10 possess high mg values 0 50 0 65 and show a negative nb ta anomaly they are spatially and temporally related to syenite intrusions and lamprophyre dykes sanukitoid intrusions in the western and eastern zones differ in composition in the west they have higher sio 2 mainly 60 and lower alkalis sr ba lree than in the eastern zone intrusions the most differentiated intrusion the panozero intrusion in the eastern zone was formed in two magmatic cycles separated by ductile deformation in the first cycle ultramafic to monzonitic rocks formed whereas in the second cycle the magmas were monzodioritic to quartz monzonite ultramafic and mafic rocks make up about 10 of the outcrop and occur as enclaves in monzonites and monzodiorites and as dykes implies a number of discrete magmatic events all rocks of the panozero intrusion have high k 2 o and the composition of the initial melt calculated from the weighted average of the first cycle magmas is monzodiorite sio 2 52 mg 0 55 na 2 o k 2 o 6 the presence of magmatic hornblende and biotite a high carbonate content widely distributed explosive breccias and evidence of liquid immiscibility are consistent with a high h 2 o co 2 content in the sanukitoid melt the geodynamic model which most satisfactorily explains our geological and geochemical observations is the formation of the karelian sanukitoids in a cratonic environment adjacent to the belomorian karelian suture

late archaean 2550 2520 ma juvenile magmatism in the eastern dharwar craton southern india constraints from geochronology nd sr isotopes and whole rock geochemistry the results of field geochronologic geochemical and isotopic studies are presented for the granitoids that occur east of the closepet batholith up to the kolar schist belt ksb field data such as common foliation strong shear deformation occasionally leading to mylonitization together with petrographic data including reduction in grain size with corroded borders show characteristics of the syn kinematic emplacement of the granitoids single zircon evaporation ages define a minimum age of 3127 ma for the tonalitic trondhjemitic granodioritic ttg basement and 2552 2534 ma plateau ages for the emplacement of the granitoids which slightly predate 20 30 ma the emplacement of the 2518 ma closepet batholith major and trace element data together with isotopic data suggest at least four magmatic suites from closepet batholith to the east which have independent magmatic evolution histories the observed data are compatible with magma mixing for the closepet batholith melting of ttg and assimilation fractional crystallization processes for bangalore granites either melting of heterogeneous source or different degree of melting of the same source for the granitoids of hoskote kolar and fractional crystallization for the western margin of the ksb isotopic nd sr and geochemical data lree and lil elements suggest highly enriched mantle and ancient ttg crust for the closepet batholith enriched mantle and ttg crust for the bangalore granites c a chondritic mantle source for the granitoids of hoskote kolar and the quartz monzonites of the western margin of the ksb and slightly depleted mantle for granodiorites of the eastern margin of the ksb we interpret all these geochronologic geochemical and isotopic characteristics of granitoids from the closepet batholith to the east up to the ksb in terms of a plume model the centre of the plume would be an enriched hot spot in the mantle that lies below the present exposure level of the closepet batholith melting of such an enriched mantle hot spot produces high temperature magmas closepet that penetrate overlying ancient crust where they strongly interact and induce partial melting of the surrounding crust these magmas cool very slowly as the hot spot maintains high temperatures for a long time thus they appear younger 2518 ma on the contrary to the east the plume induces melting of c a chondritic or slightly depleted mantle that produces relatively colder and less enriched magmas which show less or no interactions with the surrounding crust and cool rapidly and appear slightly older 2552 2534 ma this plume model can also account for late archaean geodynamic evolution including juvenile magmatism heat source for reworking inverse diapirism and granulite metamorphism in the dharwar craton

timing of deposition and tectonothermal events of banded iron formations in the anshan benxi area liaoning province china evidence from shrimp u pb zircon geochronology of the wall rocks abstract most of the algoma type bifs and associated volcanic suites in the north china craton ncc have undergone high grade metamorphism during the 2 5 ga tectonothermal event isotopic systems in high grade metamorphic rocks are often reset thereby losing the earlier records whereas zircons in low grade metamorphic rocks are potential candidates to evaluate earlier thermal events detailed textural relationship and internal structures of zircons and their age data for eight wall rock samples with metamorphic grades from greenschist to upper amphibolite facies of the algoma type bifs in the anshan benxi area are used to constrain the age of protolith zircons in the greenschist to lower amphibolite facies are generally metamorphism resistant whereas those in the upper amphibolite facies rocks show variable recrystallization shrimp zircon u pb dating of magmatic zircons in six samples constrains the peak of bif deposition at 2 56 2 53 ga which is consistent with 2 5 ga crustal growth in the ncc in addition the greenschist facies metamorphic rocks in the anshan area also record an event of volcanic activity at 2 72 2 65 ga we consider that low grade metamorphic wall rocks were not significantly affected by the 2 5 ga tectonothermal event and the 2 72 2 65 ga age group records another important magmatic event corresponding to 2 7 ga crustal growth the metamorphic zircons from these rocks record a thermal event during 1 88 1 84 ga

geochemistry and petrogenesis of the mesoarchean granites from the canaa dos carajas area carajas province brazil implications for the origin of archean granites four mesoarchean 2 93 to 2 83 ga granite units which encompass the canaa dos carajas bom jesus cruzadao and serra dourada granites were recognized in the canaa dos carajas area of the archean carajas province the mesoarchean units are composed dominantly of biotite leucomonzogranites they are compared with the neoarchean planalto suite 2 73 ga which encompasses biotite hornblende monzogranites to syenogranites the canaa dos carajas bom jesus and the variety of the cruzadao granite with higher la yb n are geochemically more akin to the calc alkaline granites whereas the other varieties of the cruzadao granite are transitional between calc alkaline and alkaline granites the serra dourada granite has an ambiguous geochemical character with some features similar to those of calc alkaline granites and other peraluminous granites the planalto granites have ferroan character are similar geochemically to reduced a type granites and show a strong geochemical contrast with the mesoarchean studied granites the mesoarchean granites described in the canaa dos carajas area are geochemically distinct to those of the rio maria domain of the carajas province the canaa dos carajas and bom jesus granites are similar to the high ca granites whereas the cruzadao and serra dourada are more akin to the low cao granites of the yilgarn craton the geochemical characteristics of the mesoarchean studied granites approach those of the biotite granite group of dharwar but the latter are enriched in hfse and hree compared to the canaa dos carajas granites the neoarchean planalto suite granite has no counterpart in the mesoarchean rio maria domain of the carajas province neither in the yilgarn and dharwar cratons geochemical modeling suggests that partial melting of a source similar in composition to an archean basaltic andesite of the carajas province could give origin to the bom jesus and cruzadao granites in the case of the bom jesus granite the residue of melting contained in similar proportions plagioclase clinopyroxene amphibole and garnet with subordinate ilmenite the variations in la yb n and sr y of the cruzadao granite are controlled by changes in the residual melting phases garnet and amphibole are abundant in the residue of the variety with higher la yb n whereas in the rocks with moderate to lower la yb n garnet is absent magnetite appears in the residue and amphibole initially increases but is replaced by orthopyroxene in the rocks with the lowest la yb n in the canaa dos carajas and serra dourada granites garnet was probably an absent phase in the residue of melting and the influence of amphibole was also apparently limited a crustal environment and a variable pressure from 10 to 7 8 kbar are estimated for the generation of the granite magmas the bom jesus and cruzadao granites of carajas derived from basaltic andesite sources and not of ttg and their geochemical contrasts can be explained by variation in the pressure of melting the sources of archean granite magmas are more diversified and could be more mafic than generally admitted

secular changes in tonalite trondhjemite granodiorite composition as markers of the progressive cooling of earth archean tonalite trondhjemite granodiorite associations ttg are classically thought to generate through partial melting of hydrous metabasalts however the chemical composition of the least differentiated ttg parental magmas evolved from 4 0 to 2 5 ga during this interval the mg as well as the ni and cr contents increased which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite similarly cao na2o and sr also increased over time thus reflecting an increase in the abundance of plagioclase in the melt residue the presence or absence of residual plagioclase is interpreted in terms of melting depth the demonstrated interaction between ttg parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material at 4 0 ga earth s geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable consequently due to the small thickness of the overlying mantle wedge felsic magmas interacted little with the mantle at 2 5 ga however owing to lower geothermal gradients the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab as a result felsic magmas reacted strongly with the mantle peridotite the changes of ttg composition during archean time can be thus interpreted as reflecting the progressive cooling of earth

the diversification of granitoids and plate tectonic implications at the archaean proterozoic boundary in the bundelkhand craton central india

late archaean granites a typology based on the dharwar craton india abstract extensive field work in the eastern dharwar craton associated with petrographic and geochemical major and trace elements investigations allows four main types of late archaean granitoids to be distinguished 1 na rich granitoids of trondhjemitic tonalitic and granodioritic composition ttg that are characterised by strongly fractionated ree patterns and low hree contents and generally interpreted as slab melts generated by partial melting of metamorphosed hydrated basalt most likely in a subduction environment 2 sanukitoids which are k and mg rich monzonites and granodiorites with ttg like ree patterns associated with marked lile enrichment and considered to result from the reaction of slab melts generated in a subduction environment with and assimilation of mantle wedge peridotite 3 uncommon high hfse mg and k granites with strongly ree and lile enrichment that probably formed by partial melting of an enriched mantle source unlike in the genesis of sanukitoids in this case the slab melt is considered to be wholly consumed by reaction with mantle minerals resulting in mantle enrichment subsequent melting of this enriched mantle probably in a post subduction setting gives rise to high hfse mg and k magmas as demonstrated for the closepet granite dharwar craton the hot mantle derived magma can induce melting of continental crust and then mix with the anatectic products 4 k rich mg poor anatectic biotite granites with ree patterns that are less fractionated and show negative eu anomalies these granites result from the remelting of old basement or recently accreted plutons both with ttg compositions such anatexis can occur either in a subduction or in a post subduction environment this typology based on the well exposed dharwar craton can be extended to late archaean granitoids from all over the world about such 500 analysis were compiled and a number of discriminant diagrams based on both major and trace elements are proposed the most likely tectonic setting for the observed rock types is an accretionary orogen with accretion of continental blocks above a subduction system followed by thermal reworking of the newly accreted continental material however it has been found that archaean subduction related granitoids are significantly different from their modern counterparts implying progressive changes in the modes of magma generation at convergent margins from the archaean to the present

slab breakoff a model for caledonian late granite syn collisional magmatism in the orthotectonic metamorphic zone of scotland and donegal ireland abstract none of the existing models for calc alkaline late granite siluro devonian genesis in the metamorphic caledonian orogenic belt of ireland and scotland fully explains their spatial age or chemical character a consistent model must involve the closure of iapetus ocean where slab breakoff is a natural consequence of attempted subduction of continental crust expected outcome is a long linear belt of high k calc alkaline magmas some with characteristic trace element signatures specifically high ba sr and zr other features include the critical magmatic association of coeval appinite and granite rapid uplift erosion and the low grade regional metamorphism in the southern uplands the linear heat pulse on breakoff is spatially intensity and time limited producing small volume melts emplaced as separated plutons over a short time span magmatism in the caledonian metamorphic belt is accurately accounted for by slab breakoff on collision of baltica with the scoto greenland margin during the scandian orogeny following iapetus ocean closure the two chemically isotopically and areally distinctive suites in the metamorphic belt in scotland viz the argyll and cairngorm suites can be modelled by reference to the donegal granites where sequential partial melting of new lamprophyric underplated crust then shallower old crust as heat conduction moved up through the crust on slab breakoff produced magmas characteristic of the two suites

geochemical changes in baslts and andesites across the archean proterozoic boundary identification and significance abstract to identify accurately changes in rock composition across the archean proterozoic boundary it is necessary to compare rocks from similar lithologic associations to constrain the effect of tectonic setting most basalts and andesites from the greenstone association volcanic dominated submarine supracrustal rocks possess a geochemical subduction zone component similar to their counterparts from modern arc systems basalts with island arc geochemical affinities dominate in archean greenstones while those with calc alkaline affinities are most abundant in proterozoic greenstones basalts with morb or oceanic within plate geochemical characteristics are rare in precambrian greenstones of all ages preserved early archean greenstone basalts 3500 ma reflect mantle sources less depleted than late archean greenstone basalts 2500 3500 ma proterozoic greenstone basalts are derived from relatively enriched mantle sources compared to all archean sources a feature which may be due to recycling of continental sediments into the mantle following rapid late archean continental growth precambrian andesites are geochemically similar to andesites from modern arcs but archean andesites are unique in that they are depleted in heavy ree and y results are consistent with archean andesite production by partial melting of descending mafic crust with amphibole garnet remaining in the residue while proterozoic and younger andesites are produced by fractional crystallization of basalts

1700 ma shachang complex northeast china proterozoic rapakivi granite not associated with paleoproterozoic orogenic crust proterozoic rapakivi granites are found on all continents and are characteristically associated with paleoproterozoic crustal domains that predate the granites by

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

syntectonic granite emplacement at different structural levels the closepet granite south india abstract the closepet granite in south india is a large 400 km long but only 30 km wide elongate late archaean granitic body structural levels from deep crust to upper levels crop out as evidenced by a shallowing of paleo depths from north to south all along the closepet granite this allows the study of the emplacement of the same granitic body at various crustal levels four zones have been identified a root zone where magmas are collected in active shear zones a transfer zone featuring large scale magma ascent and crystal liquid partitioning in the granitic mush a gap where the mush was filtered allowing only the liquids to rise shallow intrusions filled with this liquid the closepet granite was emplaced syntectonically field work and anisotropy of magnetic susceptibility allowed documentation of steep foliations with subhorizontal lineations both in the root and transfer zones and in the shallow intrusions remote sensing evidenced a network of shear zones bounding the closepet granite in the porphyritic root and transfer zones magmas cooled slowly thus developing strong fabrics during large scale dextral shearing ascent of residual liquids amidst the crystallizing solid framework was not recorded in the fabrics however these liquids were channelised through the gap and infilled the homogeneous shallow intrusions where rapid cooling only permitted the development of feint although wholly consistent fabrics

geochemistry and petrogenesis of late archaean aravalli volcanics basement enclaves and granitoids rajasthan abstract major trace and rare earth element data are presented for the basal volcanics of the aravalli supracrustal sequence for the mafic enclaves within the banded gneiss complex bgc adjacent to the supracrustals and for granitoids within the bgc and their petrogenesis discussed with regard to the problematic relationship between basement and cover in terms of major elements the aravalli lavas range from magnesian 22 mgo komatiites or picrites to fe rich tholeftes but differ from a typical archaean greenstone volcanic sequence in that their trace element patterns are relatively enriched in incompatible and light rare earth elements moreover the range of patterns observed requires a number of distinct mantle sources as well as different degrees of melting there are two distinct types of mafic enclave within the bgc the larger lenses correspond quite well with the range of variation seen in the aravalli sequence and could be feeders the other type close to the bgc aravalli junction has more alkalic characteristics high nb zr and could represent rift volcanics or tectonically accreted fragments of ocean islands the data suggest that the aravalli sequence is not an accreted oceanic terrane but are more consistent with interaction of a deep mantle plume with rather mature sub continental lithosphere the early archaean age of parts of the bgc the chemistry of the granitoids and the alkaline character of the enclaves are also broadly compatible with this model the character of the aravalli sequence is intermediate between that of an archaean greenstone belt and a phanerozoic flood basalt province

plate tectonics on the early earth limitations imposed by strength and buoyancy of subducted lithosphere the tectonic style and viability of modern plate tectonics in the early earth is still debated field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the archean continue to accumulate here we present the first numerical modeling results that address for a hotter earth the viability of subduction one of the main requirements for plate tectonics a hotter mantle has mainly two effects 1 viscosity is lower and 2 more melt is produced which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation our modeling results show that eclogitization significantly relaxes this limitation for a developed ongoing subduction process furthermore the lower viscosity leads to more frequent slab breakoff and sometimes to crustal separation from the mantle lithosphere unlike earlier propositions not compositional buoyancy considerations but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter earth these results suggest a new explanation for the absence of ultrahigh pressure metamorphism uhpm and blueschists in most of the precambrian early slabs were not too buoyant but too weak to provide a mechanism for uhpm and exhumation

geochemical characteristics and possible origins of the southern closepet batholith south india the polyphase closepet batholith in south india intrudes archean tonalitic and granitic gneisses in a terrane that grades from amphibolite to granulite grade at the southern end of the batholith two granite and two granodioritic suites are recognized in the batholith one granite suite from the southern terminus of the batholith has high sr ba la yb and eu eu and low sc y nb ta and total ree and is similar to other granites from along the amphibolite granulite transition zone that have the characteristics of a fractionally crystallized cumulate the other granite suite has lower sr and ba higher sc and y and no distinctive eu anomaly this latter group which is widespread in the southern closepet batholith can be derived by 20 equilibrium partial melting of a tonalite source containing hornblende and garnet that is compositionally similar to south india tonalites near the amphibolite granulite transition the two granite groups may have been produced by separate episodes of co 2 metam

the stability of sapphirine quartz and hypersthene sillimanite quartz assemblages an experimental investigation in the system feo mgo al 2 o 3 sio 2 under h 2 o and co 2 conditions phase relations and mineral chemistry involving the phases garnet gt spinel sp hypersthene hy sapphirine sa cordierite cd sillimanite sil and quartz qz have been experimentally determined in the system fmas feo mgo al2o2 sio2 under low fo2 and for various h2o co2 conditions several compositions were studied with 100 mg mg fe ratio ranging from 64 to 87 with excess quartz and sillimanite our data do not show any differences in gt cd stability and composition as a function of h2o co2 and h2o co2 ch4 content in good agreement with a previous experimental study at lower temperature aranovich and podlesskii 1983 at 1 000 c and 11 kbar under co2 saturated conditions cordierite grew from a crystalline mix unseeded with cordierite thus under water absent conditions cordierite will have a high p stability field in the presence of co2 if water has a pressure stabilizing effect on cordierite then our results would indicate that the effects of h2o and co2 are of the same magnitude at high temperature our data support the theoretical p t grid proposed by hensen 1986 for high t metapelites and are largely consistent with the high temperature experimental data of hensen and green 1973 the univariant boundary gt cd hy sil qz which marks the disappearance of hy sil qz assemblages has a negative dp dt slope above 1 000 c and a positive one below this temperature extrapolation of our data to iron free systems shows that the high p breakdown limit of mg cordierite has a negative slope in the range 1 025 1 300 c and probably positive below 1 000 c this indicates a maximum of stability for mg cordierite at around 1 000 c and 13 kbar because of the curvature of the univariant reactions en sil py qz mg cd en sil qz and gt cd hy sil qz the iron free invariant point involving the phases py en cd sil and qz probably does not exist sapphirine qz bearing assemblages are stable only at temperatures above 1 050 c at 1 075 c the joint gt sa is stable up to 11 kbar at higher pressure garnet sapphirine and quartz react according to the reaction gt sa qz hy sil reequilibrated sapphirines are more aluminous than the theoretical endmember mg2al4sio10 due to alal mgsi substitutions 100 al2o3 al2o3 feo mgo in experimental sapphirines ranges from 50 5 to 52 2 sapphirine in the assemblage sa cd sil qz shows a decrease in al content with decreasing temperature and pressure such that the alumina isopleths for sapphirine have a slight negative dp dt slope a similar decrease in al content of sapphirine with temperature is also observed in sa sil qz assemblages

discrimination and origin of the sanukitoid series geochemical constraints from the neoarchean western karelian province finland abstract neoarchean late to post tectonic sanukitoid intrusions are found in the western part of the karelian province in finland variable sized even grained to k feldspar porphyritic intrusions form a series of diorites tonalites and granodiorites that are calc alkaline to alkali calcic magnesian and mostly peraluminous the major and trace element geochemistry of the intrusions show typical sanukitoid affinities a mantle signature high content of mg ni cr and high mg and enrichment in lile especially k ba and sr the intrusions form a distinctive series that can be distinguished from the ttg series of the karelian province by their high ba sr signature low na 2 o k 2 o ratio and uniform hree pattern the sanukitoid series can be compositionally constrained as follows sio 2 55 70 wt na 2 o k 2 o 0 5 3 mgo 1 5 9 wt mg 45 65 k 2 o 1 5 5 0 wt ba sr 1400 ppm and gd er n 2 6 the probable source was enriched subcontinental lithosphere mantle with high lile concentrations in phlogopite and or sanidine the accumulation of k ba and sr in the mantle source may have occurred as a consequence of two separate metasomatic events 1 by subduction related fluids melts and 2 by metasomatism related to asthenospheric mantle upwelling caused by slab breakoff the slab breakoff at the end stage of subduction is also a viable trigger for the sanukitoid magmatism

neoarchean leucogranitoids of the kianta complex karelian province finland source characteristics and processes responsible for the observed heterogeneity abstract in this study we describe the geochemistry and discuss the genesis of neoarchean ca 2 7 ga leucogranitoids of the kianta complex within the karelian province of the fennoscandian shield the leucogranitoids occur extensively as dykes and intrusions of varying size and represent substantial recycling of older crust occurring as the last magmatic event before final cratonization of the karelian province geochemically the leucogranitoids resemble the leucosomes of migmatized ttgs both groups are compositionally heterogenous but are characterized by high silica contents mean sio2 73 4 wt and leucocratic appearance reflecting a paucity of mafic minerals mean mgo 0 37 wt mg 32 6 the majority of samples analyzed are weakly peraluminous mean a cnk 1 05 mineralogically the leucogranitoids form a series grading from trondhjemite to leucogranite which is observable as highly variable cao 0 07 3 84 wt mean 1 47 k2o 0 30 8 76 wt mean 3 53 and na2o 2 22 7 23 wt mean 4 49 we consider the local ttgs sensu lato which comprise both true ttgs k2o na2o 0 5 to be the predominant source material for the leucogranitoids however some samples overlap compositionally with local ttgs and could represent either partial melting of garnet amphibolites or alternatively products of nearly complete fusion of ttgs leucogranitoids and transitional ttgs can be differentiated to some extent on the basis of geochemistry but definitive discrimination requires demonstration of field relationships and or isotope dating the observed compositional scatter is interpreted to be a result of source heterogeneity retention of variable accessory minerals and accumulation processes melting is attributed to thickening of the crust in response to collisional tectonics and occurred dominantly under water fluxed conditions possible fluid sources are the sanukitoid and quartz diorite suites plus dehydration reactions and partial melting of the lower crust

fe2 mg partitioning experiments between orthopyroxene and spinel using ultrahigh temperature granulite from the napier complex east antarctica

geochemical and isotopic nd o evidence bearing on the origin of late to post orogenic high k granitoid rocks in the western superior province implications for late archean tectonomagmatic processes abstract the granitoid rock dominated central wabigoon subprovince of the superior province records low k trondhjemite tonalite granodiorite ttg type magmatic episodes at

late archean high mg granitoids sanukitoids in the southern karelian domain eastern finland pb and nd isotopic constraints on crust mantle interactions late archean ca 2 73 ga porphyritic high k and high mg granitoids in the nilsia and lieksa areas eastern finland have geochemical signatures similar to those of high mg sanukitoid series from other parts of the karelian domain of the fennoscandian baltic shield and the archean sanukitoid suites from canada the nilsia and lieksa granitoids have low sio2 contents 62 7 67 0 and high mg numbers 0 45 0 52 they are enriched in k2o 2 40 4 73 wt p2o5 0 25 0 42 wt sr 610 850 ppm ba 1200 2300 ppm cr 40 80 ppm and lree la yb n 19 65 they are characterized by a negative eu anomaly eu eu 0 53 0 81 and show strong depletion in u high th u ratio low u pb ratio high 2 values for the source 9 positive nd 2 7 ga values of 0 3 1 4 and nd depleted mantle model ages 2 75 2 86 ga the high 2 values pointing to a significantly older crustal source apparently contradicts the positive nd 2 7 ga values supporting a mantle origin the geochemical and isotopic data suggest that the nilsia and lieksa granitoids originated at ca 2 73 ga from a mantle wedge source enriched in lile u th and pb by recycling of continental material in subduction related slab dehydration processes shortly up to 200 ma before melting a considerably large amount of crustal lead was contributed from subducting sediments into the overlying mantle wedge consequently the isotopic composition of mantle wedge lead was overprinted by crustal lead isotope signatures it is suggested that crustal recycling through subduction zone processes played an important role for the enrichment of the mantle wedge and generation of mantle derived high granitoids in the archean

zircon u pb ages trace elements and nd hf isotopic geochemistry of guyang sanukitoids and related rocks implications for the archean crustal evolution of the yinshan block north china craton abstract the yinshan block is a unique coherent neoarchean basement terrane of the western block of the north china craton in this paper three archean plutons namely the dajitu banshentu and haolaigou plutons have been investigated based on the field observations the major difference among these plutons is that many ultramafic mafic enclaves are enclosed in the dajitu and banshentu plutons whereas none occurs in the haolaigou pluton geochemically the dajitu and banshentu plutons are characterized by low sio2 contents 56 67 wt high mgo 1 9 7 1 wt cr 10 947 ppm ni 2 274 ppm and mg 44 64 contents negative nb ta and ti anomalies high large ion lithophile elements liles with no or slightly negative eu anomalies similar to a typical archean sanukitoid new geochronological data indicate that the dajitu and banshentu plutons were emplaced at 2523 7 ma and 2480 20 ma with inherited ages of 2606 19 ma and 2647 19 ma respectively in contrast the haolaigou pluton shows high sio2 68 71 wt mgo mg 46 55 cr and ni contents negative nb ta and ti anomalies and high liles with strongly positive eu anomalies high sr and ba concentrations and sr y and la yb ratios and low y and hrees abundances it was emplaced at 2466 15 ma with an inherited age of 2638 21 ma the whole rock nd t and zircon hf t values of the haolaigou pluton are positive the whole rock tdm nd and zircon tdm1 hf values range between 2 6 and 2 8 ga with a peak at 2 7 ga indicating that the melt was generated by the partial melting of a subducted slab interacting with the overlying mantle wedge the dajitu pluton has nd and hf isotope characteristics identical to those of the haolaigou pluton implying that the two plutons have similar sources it should be noted however that the dajitu pluton displays sanukitoid characteristics which are the result of interaction with ultramafic mafic enclaves the whole rock nd t and zircon hf t values of the banshentu pluton are mostly negative the whole rock tdm nd and zircon tdm1 hf values of the pluton range between 2 7 and 2 9 ga with a peak at 2 8 ga indicating that the pluton was generated from the melting of a lower crust the sanukitoid characteristics of the pluton resulted from interactions with an ultramafic enclave taken together these results with the geochronological geochemical data for other archean rocks in this area suggest that the yinshan block experienced significant crustal growth at 2 8 2 6 ga and further crustal reworking that led to strong magmatism at 2 5 ga

petrogenesis of early permian sanukitoids from west junggar northwest china implications for late paleozoic crustal growth in central asia abstract sanukitoids and their equivalents are rare subduction related rock types that have been found in modern arc settings and in late archean sequences the investigation of sanukitoids is of particular importance to a better understanding of crust forming processes and continental growth in this paper we report zircon u pb and ar ar ages and major element trace element and sr nd hf isotope data for the bieluagaxi dioritic pluton and dikes from the southern part of the west junggar nw china these rocks formed in the early permian and show a remarkable geochemical affinity with cenozoic sanukitoids of the setouchi volcanic belt of sw japan with high mg 48 73 values cr 54 539 ppm ni 21 197 ppm contents and th la 0 15 0 37 ratios and low sr y ratios 16 27 and sr 263 442 ppm contents they may be generated by the partial melting of subducting sediments and subsequent melt mantle interaction additionally the sanukitoids are also widespread in the karamay baogutu area of west junggar accompanied by high sr average 713 ppm contents and sr y 50 130 ratios and low y 6 9 12 6 ppm contents the difference in petrochemical characteristics between the baogutu karamay and bieluagaxi sanukitoids can be explained by the difference in depth of initial melting origin composition and fractional crystallization the baogutu karamay sanukitoids were probably formed under eclogitic conditions while the bieluagaxi sanukitoids were at a shallower depth moreover the compositional similarity between continental crust forming rocks and the bieluagaxi sanukitoids suggests that the sanukitoids genesis could be closely related to the process of continental crust formation the late carboniferous early permian sanukitoids in the west junggar may be an indicator of anomalous thermal activity ridge subduction may play a crucial role in the evolution and growth of the continental crust in central asia

mantle derived archaean monozodiorites and trachyandesites the 2 680 2 750 myr old1 5 intrusive granite greenstone terranes6 8 in the rainy lake region ontario and in and adjacent to the vermilion district minnesota of the superior province in north america include a wide variety of volcanic and intrusive rocks with initial pb sr and nd isotope ratios close to those for the mantle4 5 9 12 the isotopic constraints require the production of large volumes of silica oversaturated magmas from silica undersaturated mantle within 100 200 myr we present here geochemical data on monzodiorites and trachyandesites from the rainy lake area which are strongly enriched in large ion lithophile elements lile we conclude that they are derived by direct melting of the mantle at depths of 50km in either anhydrous or hydrous conditions their rare earth element ree abundances and initial 143nd 144nd ratios suggest that their mantle sources were enriched in lile shortly before melting the monzodiorites trachyandesites and their granodioritic derivatives may comprise up to 20 of the exposed igneous rocks in the rainy lake region and vermilion district if these rocks are as abundant in other archaean terranes a significant part of the early continental crust could have formed by direct melting of lile enriched mantle

late archaean crust mantle interactions geochemistry of lree enriched mantle derived magmas example of the closepet batholith southern india the closepet batholith in south india is generally considered as a typical crustal granite emplaced 2 5 ga ago and derived through partial melting of the surrounding peninsular gneisses 3 3 to 3 0 ga in the field it appears as a composite batholith made up of at least two groups of intrusions a an early sio2 poor group clinopyroxene quartz monzonite and porphyritic phyritic monzogranite is located in the central part of the batholith these rocks display a narrow range in both initial 87sr 86sr 0 7017 0 7035 and nd 0 9to 4 1 b a later sio2 rich group equigranular grey and pink granites is located along the interface between the sio2 poor group and the peninsular gneisses they progressively grade into migmatised peninsular gneisses thus indicating their anatectic derivation their isotopic characteristics vary over a wide range 87sr 86sr ratios 0 7028 0 7336 and nd values from 2 7 to 8 3 at 2 52 ga field and geochronological evidence shows that the two groups are broadly contemporaneous 2 518 2 513 ga and mechanically mixed this observation is supported by the chemical data that display well defined mixing trends in the sr vs nd and elemental variation diagrams the continuous chemical variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end members derived from different source regions enriched peridotitic mantle and peninsular gneisses it is proposed that the intrusion of mantle derived magmas into mid crustal levels occurred along a transcurrent shear zone these magmas supplied additional heat and fluids that initiated anatexis of the surrounding crust during this event large scale mixing occurred between mantle and crustal melts thus generating the composite closepet batholith the mantle derived magmatism is clearly associated with granulite facies metamorphism 2 51 0 01 ga ago both are interpreted as resulting from a major crustal accretion event possibly related to mantle plume activity

ttg suite from the bundelkhand craton central india geochemistry petrogenesis and implications for archean crustal evolution abstract the bundelkhand craton is one of the important archean protocontinental nuclei in peninsular india for which not much information on tectonic setting evolutionary history and petrogenetic processes is available here we provide comprehensive geochemical data on the tonalite trondhjemite granodiorite ttg rocks which form the basement rocks for the craton compositionally these rocks are variably evolved between diorite and granites and display both high and low al ttg types with geochemical features typical of convergent margin tectonic setting the evolved granitoids are essentially low pressure type further grouped into sodic true ttg and potassic transitional ttg types the low pressure and moderate shallow depths of melting conditions and low mg ni and cr contents do not support a direct slab melting scenario alternately these features favor the derivation of the bngc granitoids through the melting of a sub arc basaltic source possibly at the lower part of thickened arc crust in a suprasubduction regime the occurrence of identical lithological assemblages in other cratons in peninsular india and similar petrogenetic conditions for the emplacement of ttgs suggest the involvement of subduction accretion processes in building the ancient cratonic nuclei of the indian shield

experimental constraints on the thermal peak of a granulite from mcintyre island enderby land east antarctica abstract high pressure experiments have been carried out at 11 22 kbar and 900 1200 c using a piston cylinder apparatus to constrain the thermal peak condition of a granulite characterized by the mineral assemblage of orthopyroxene sillimanite quartz from mcintyre island enderby land east antarctica the bulk composition of the starting material is 85 wt mcintyre granulite 15 wt sillimanite at 11 kbar orthopyroxene sillimanite and quartz are stable below 1000 c at 1050 c sillimanite does not appear and sapphirine coexists with orthopyroxene and quartz these experimental results indicate that the mcintyre granulite has undergone the ultra high temperature metamorphism at 1000 1050 c represented by the diagnostic mineral assemblage of orthopyroxene sillimanite and quartz

provenance of proterozoic basal aravalli mafic volcanic rocks from rajasthan northwestern india nd isotopes evidence for enriched mantle reservoirs abstract komatiites komatiitic basalts and tholeiites from nathdwara rajasthan have been investigated for their nd isotopic characteristics 143nd 144nd vary broadly in the komatiites 0 51131 0 51252 and show a restricted range in komatiitic basalts and tholeiites 0 51120 0 51179 and 0 51141 0 51202 respectively the corresponding epsilon nd nd values indicate an enriched source for these rocks the model ages of komatiitic basalts and tholeiites cluster between 2 2 and 3 0 ga the komatiites show older model ages between 3 5 and 3 8 ga if the correlations found between nd and sm isotopic ratios can be interpreted as isochrones then the komatiites point to the age of 2300 ma and the komatiitic basalts and tholeiites constrain the age of 1800 2000 ma the age of 2300 ma for the komatiite samples is similar to the range estimated for the komatiitic basalt and tholeiite samples 1800 2000 ma considering the errors involved collectively the data suggest an age of 2300 1800 ma for the basal aravalli volcanism the variation of nd isotopic ratios between the komatiites and the associated basalts is too large to reflect analytical bias or open system behavior for sm and nd it may be due to significant mantle heterogeneity in the source region isotopically distinct sources for the komatiites and komatiitic basalts are possible but simple mixing between a depleted mantle component and a crustal component in the manner of magmatic assimilation of wall rock material is inconsistent with the observed isotopic ratios the inference of an enriched mantle component appears inevitable as the alternative model of mixing would have generated more siliceous high magnesian boninitic rocks with much lower fe and ti than are observed this enriched material may have been part of the sub continental lithospheric mantle sclm under the aravalli craton which could have generated the basal aravalli magmatic rocks we suggest that during early middle proterozoic the sclm under cratonic regions consisted of relatively fertile mantle capable of generating komatiites tholeiites and alkalic magmas

oxidized magnetite series rapakivi type granites of carajas brazil implications for classification and petrogenesis of a type granites abstract the varying geochemical and petrogenetic nature of a type granites is a controversial issue the oxidized magnetite series a type granites defined by anderson and bender anderson j l bender e e 1989 nature and origin of proterozoic a type granitic magmatism in the southwestern united states of america lithos 23 19 52 are the most problematic as they do not strictly follow the original definition of a type granites and approach calc alkaline and i type granites in some aspects the oxidized jamon suite a type granites of the carajas province of the amazonian craton are compared with the magnetite series granites of laurentia and other representative a type granites including finnish rapakivi and lachlan fold belt a type granites as well as with calc alkaline i type orogenic granites the geochemistry and petrogenesis of different groups of a types granites are discussed with an emphasis on oxidized a type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis oxidized a type granites are clearly distinguished from calc alkaline cordilleran granites not only regarding trace element composition as previously demonstrated but also in their major element geochemistry oxidized a type granites have high whole rock feo t feo t mgo tio 2 mgo and k 2 o na 2 o and low al 2 o 3 and cao compared to calc alkaline granites the contrast of al 2 o 3 contents in these two granite groups is remarkable the cao feo t mgo tio 2 vs cao al 2 o 3 and cao feo t mgo tio 2 vs al 2 o 3 diagrams are proposed to distinguish a type and calc alkaline granites whole rock feo t feo t mgo and the feo t feo t mgo vs al 2 o 3 and feo t feo t mgo vs al 2 o 3 k 2 o na 2 o diagrams are suggested for discrimination of oxidized and reduced a type granites experimental data indicate that besides pressure the nature of a type granites is dependent of o 2 conditions and the water content of magma sources oxidized a type magmas are considered to be derived from melts with appreciable water contents 4 wt originating from lower crustal quartz feldspathic igneous sources under oxidizing conditions and which had clinopyroxene as an important residual phase reduced a type granites may be derived from quartz feldspathic igneous sources with a metasedimentary component or alternatively from differentiated tholeiitic sources the imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct a type granite groups assuming conditions near the nno buffer as a minimum for oxidized granites magnetite bearing granites formed near fmq buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced a type granites and respectively magnetite series and ilmenite series granites is not always observed

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 and nd isotopic composition of potassic magmatism in the neoarchaean musoma mara greenstone belt northern tanzania abstract the neoarchaean musoma mara greenstone belt mmgb of northern tanzania is underlain in part by the 2649 ma post orogenic potassic rich granites which are the most abundant intrusive rocks in the belt the rocks are composed of plagioclase k feldspar quartz biotite sphene zircon hornblende chlorite they are characterized by high contents of sio 2 68 90 77 76 wt k 2 o 3 71 5 44 wt and low na 2 o 3 27 5 70 wt leading to low ratios of na 2 o k 2 o 0 63 1 02 the rocks are depleted in cao 0 22 2 41 wt as well as in sr 15 412 ppm cr 16 ppm and ni 5 ppm and their major element composition are similar to those of experimental melts derived from partial melting of tonalite on chondrite normalized ree patterns these rocks show fractionated patterns la yb cn 1 22 41 32 that are characterised by moderate to strongly negative eu anomalies eu eu 0 04 0 86 on primitive mantle normalized spidergrams these rocks are generally enriched in th u k and pb and depleted in ba sr nb ta and ti relative to adjacent elements the k rich granites have nd o at 2 649 ga values of 0 55 to 1 70 that compare well with those of associated volcanic rocks and ttg nd 0 44 to 2 66 which predate the emplacement of the k rich granitoids their mean crustal residence ages are 170 to 450 ma older than their emplacement ages the overall geochemical features of this suite of rocks together with evidence from experimental results are consistent with their generation by partial melting of relatively juvenile igneous rocks within the continental crust at pressures corresponding to depths