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

paleo to mesoarchean ttg accretion and continental growth in the western dharwar craton southern india constraints from shrimp u pb zircon geochronology whole rock geochemistry and nd sr isotopes a multidisciplinary study involving field petrographic shrimp u pb zircon titanite ages whole rock geochemical and nd sr isotope data is presented for the peninsular gneisses and associated plutons forming core of the dharwar craton two major periods 3350 3280 ma and 3230 3200 ma of crustal growth through ttg accretion sub contemporaneous with greenstone volcanism are documented elemental and nd sr isotope data suggest that the ttgs originated by low and high pressure melting of heterogeneous mafic sources thickened island arc or oceanic plateau crust at different depths among the early accreted ttgs magmatic protoliths of low al gneisses formed by low pressure 10 12 kbar melting of a depleted mafic source at shallow levels island arc type crust with plagioclase amphibole ilmenite in the residue whilst high al gneisses were probably produced by high pressure melting 14 18 kbar of a chondritic or a less depleted mafic source with garnet amphibole pyroxene ilmenite in the residue base of island arc crust or thickened oceanic plateau the 3230 3200 ma trondhjemitic plutons marking the second accretion event derived by higher pressure 14 16 kbar melting of a mafic source island arc type crust with garnet plagiocalse hornblende ilmenite with a minor involvement of previously accreted ttgs this plutonism was coeval with a major phase of crustal scale diapiric overturn that had a major imprint on crustal reworking a later magmatic episode at ca 3100 ma produced granitic plutons derived from crustal melting and leucocratic gneisses derived from a depleted mafic source in the lower crust documentation of depleted shallow and deep mantle reservoirs during 3350 3280 ma magmatism implies massive differentiation of the mantle during an earlier episode of crust formation around 3800 600 ma as substantiated by nd model ages and published pb isotope data as well as u pb detrital zircon ages with in situ hf isotope data

neoarchean crustal growth by accretionary processes evidence from combined zircon titanite u pb isotope studies on granitoid rocks around the hutti greenstone belt eastern dharwar craton india abstract the neoarchean hutti greenstone belt hosts mesothermal gold deposits and is surrounded by granitoid rocks on all sides combined u pb dating of zircon and titanite from the granitoid rocks constrains their emplacement history and subsequent geologic evolution the golapalli and yelagatti granodiorites occurring to the north of the hutti greenstone belt were emplaced at 2569 17 ma the yelagatti granodiorite yielded a younger titanite age of 2530 6 ma which indicates that it was affected by a post crystallization thermal event that exceeded the titanite closure temperature the western granodiorites from kardikal have identical titanite and zircon ages of 2557 6 ma and 2559 19 ma respectively the eastern kavital granodiorites yielded titanite ages of 2547 6 ma and 2544 24 ma which are identical to the published u pb zircon shrimp ages these ages imply that the granitoid rocks surrounding the hutti greenstone belt were formed as discrete batholiths within a short span of ca 40 ma between 2570 ma and 2530 ma ago they were juxtaposed by horizontal tectonic forces against the supracrustal rocks that had formed in oceanic settings at the end of the archean the first phase of gold mineralization coincided with the last phase of granodiorite intrusion in the hutti area a metamorphic overprint occurred at ca 2300 ma ago that reset the rb sr isotope system in biotites and possibly caused hydrothermal activity and enrichment of au in the ore lodes the eastern dharwar craton consists of quartz monzodiorite granodiorite granite suites of rocks that are younger than the greenstone belts that are older than 2650 ma reported from earlier studies the granitoid magmatism took place between 2650 and 2510 ma ago indicating accretionary growth of the eastern dharwar craton

rb sr ages of late archean metavolcanics and granites dharwar craton south india and evidence for early proterozoic thermotectonic event s abstract the archean greenstone sequence of the dharwar craton is intruded by late and post kinematic granites like the chitradurga and toranagallu closepet granites with whole rock rb sr isochron ages of 2603 28 ma 2 and 2452 50 ma respectively the whole rock rb sr ages of their host metavolcanic rocks show a wide variation bababudan 2728 102 ma shimoga 2520 62 ma ingaldhal 2240 50 ma mardihalli 2160 174 ma gadag 2456 76 ma and sandur 2377 646 ma while the ages of the bababudan and the shimoga volcanic suites can be related to the time of the pervasive regional folding and metamorphic events 2 7 2 6 ga those of the other volcanic suites from the chitradurga and sandur supracrustal belts indicate effects of post archean thermal event s the internal isochron ages of 2 0 0 1 ga for the chitradurga and toranagallu granites suggest isotopic resetting on a mineral scale about 500 ma after their emplacement and support the evidence from the metavolcanic suites for an early proterozoic thermal event at 2 0 ga in the dharwar greenstone granite gneiss terrain

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

basement terrane correlations and crustal recycling in the western superior province nd isotopic character of granitoid and felsic volcanic rocks in the wabigoon subprovince n ontario canada abstract the western superior province represents a collage of mesoarchean continental 3 2 2 8 ga and neoarchean continental arc and oceanic 2 8 2 66 ga crustal fragments nd isotope data presented here for 89 felsic volcanic and plutonic rocks have been combined with compiled data to generate a data set of over 240 analyses from the wabigoon and winnipeg river subprovinces these samples from both oceanic and continental environments have been used as crustal probes their nd isotopic character used to investigate the extent of basement terranes the ancestry of crustal fragments their interaction with each other and the degree of sialic crustal reworking the wabigoon and winnipeg river subprovinces include three isotopically distinct terranes 1 the winnipeg river terrane comprises the winnipeg river subprovince the north central wabigoon and the north eastern wabigoon subprovince this region is dominated by neoarchean granitoid rocks with mesoarchean vestiges of previous workers have suggested that the boundary between the western wabigoon and winnipeg river terranes represents a cryptic suture resulting from a collision at circa 2 71 ga the boundary between the western wabigoon and marmion terranes although obscured by late intrusions may be a continuation of this suture zone the boundary between the winnipeg river and marmion terranes in the central wabigoon is also cryptic isotopic data suggest that the marmion terrane is allochthonous with respect to the winnipeg river terrane common magmatism in these two terranes at 2 93 2 87 ga suggests that they may have collided during this time in the eastern wabigoon subprovince this collision zone is speculated to occur at the humboldt bay high strain zone a complex history of crustal recycling throughout the mesoarchean and neoarchean is revealed which demonstrates the importance of autochthonous development of granite greenstone terranes in a continental environment the data also support models of tectonic accretion during the archean indicated by the presence of granite greenstone terranes of allochthonous origin

evolution of archaean crust in the dharwar craton the nd isotope record abstract a synthesis of the available nd isotope data from the dharwar craton puts important constrains on antiquity of crust extent of basement terranes and events marking juvenile crustal addition and crustal recycling the craton is divided into two blocks namely eastern dharwar craton edc and western dharwar craton wdc nd model ages trace crust extraction as far as 3 5 ga back in both the blocks although rock record of such antiquity is yet to be found in the craton in wdc 3 35 3 0 ga is the most significant period of juvenile crustal addition as well as crustal recycling significantly depleted mantle 1 5 to 6 4 existed below the wdc as early as 3 35 ga which was probably refertilized in a later event from the resultant chondritic to slightly enriched mantle 0 3 to 0 4 juvenile mafic crust was added to the wdc during 2 9 2 6 ga a widespread crustal recycling event at 2 6 ga marks the last major event in this block in edc 3 3 3 0 ga granitoids occur as vestiges and show nd model ages higher than crystallizations ages suggesting recycling of palaeoarchaean crust during 2 7 2 5 ga extensive juvenile magmatism took place in edc from variably depleted mantle 1 4 to 5 6 which is later than the global 2 7 ga peak of crustal growth attendant crustal recycling destroyed most of the earlier crust distribution of nd model ages suggests in contrast to general believe that palaeoarchaean crust was not only abundant in the wdc but also widespread in an area extending from the wdc edc boundary to further 60 150 km east up to the hungund ramagiri kolar hrk belt this belt appears to be a major crustal boundary along which terranes of different nd isotope signatures were amalgamated during the late neoarchaean by horizontal accretion major neoarchaean crustal recycling events documented in the dharwar craton are missing from nd isotope record of the eastern block eb of the north china craton this fact does not support correlation between the two cratons as suggested by some authors

typhoon haiyan overwash sediments from leyte gulf coastlines show local spatial variations with hybrid storm and tsunami signatures abstract marine inundation associated with the 5 to 8 m storm surge of typhoon haiyan in 2013 left overwash sediments inland on the coastal plains of the northwestern shores of leyte gulf philippines the haiyan overwash deposit provides a modern sedimentary record of storm surge deposition from a category 5 landfalling typhoon we studied overwash sediments at two locations that experienced similar storm surge conditions but represent contrasting sedimentological regimes namely a siliciclastic coast and a mixed siliciclastic carbonate coast the contrasting local geology is significantly reflected in the differences in sediment grain size composition and sorting at the two sites the haiyan overwash sediments are predominantly sand and silt and can be traced up to 1 6 km inland extending farther beyond the previously reported

palaeoproterozoic 1 9 ga extension and breakup along the eastern margin of the eastern dharwar craton se india new sm nd isochron age constraints from anorogenic mafic magmatism in the neoarchean nellore greenstone belt abstract the neoarchean nellore greenstone belt located along the eastern margin of the eastern dharwar craton was the site for younger igneous activity the timing of this magmatism and the host metabasalt critical for constraining extensional events along the deformed southeast margin of the eastern dharwar craton was conducted by the sm nd method samples from the large differentiated gabbroic kandra igneous complex a subset kandra group a define an isochron age of 1926 110 ma nd i 0 51006 12 mswd 0 98 n 5 e nd 1 7 and the kandra group b subset yielded a model 3 isochron age of 1882 120 ma nd i 0 50994 14 mswd 1 8 n 4 e nd 5 1 whereas the three samples from north pamuru gabbro anorthosite yielded an isochron age of 1882 160 nd i 0 51012 18 mswd 0 064 e nd 1 7 the kandra group a and north pamuru samples regressed together yielded a preferred isochron age of 1911 88 ma nd i 0 51008 10 mswd 0 62 n 8 e nd 1 6 for the emplacement of gabbros in contrast host greenstone belt metabasalt from chundi yielded a sm nd isochron age of 2693 94 ma nd i 0 50920 8 mswd 1 4 n 4 e nd 1 2 whereas regressing the chaganam and chundi samples yielded an isochron age of 2654 100 ma nd i 0 50923 9 mswd 1 5 n 11 e nd 0 7 basal metapelite underlying the metabasalt yields old nd model ages between 2888 and 3054 ma the results of this study show that the 2 7 ga nellore greenstone belt was intruded by enriched morb type gabbros at 1911 ma it is proposed that this palaeoproterozoic magmatism formed in response to a major extensional event along the eastern dharwar craton margin that likely initiated breakup from the southeastern margin of the north china craton this exposed craton margin later evolved as the site for collisional events during the late palaeoproterozoic to early mesoproterozoic

compositional variations in the mesoarchean chromites of the nuggihalli schist belt western dharwar craton india potential parental melts and implications for tectonic setting the chromite deposits in the archean nuggihalli schist belt are part of a layered ultramafic mafic sequence within the western dharwar craton of the indian shield the 3 1 ga ultramafic mafic units occur as sill like intrusions within the volcano sedimentary sequences of the nuggihalli greenstone belt that are surrounded by the tonalite trondhjemite granodiorite ttg suite of rocks the entire succession is exposed in the tagdur mining district the succession has been divided into the lower and the upper ultramafic units separated by a middle gabbro unit the ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite bearing serpentinites the chromitite bodies occur in the form of pods and elongated lenses 60 500 m by 15 m detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate rich chromitite 50 modal chromite and serpentinite 2 modal chromite throughout the entire ultramafic sequence however the primary composition of chromite is preserved in the massive chromitites 60 75 modal chromite from the byrapur and the bhaktarhalli mining district of the nuggihalli schist belt these are characterized by high cr ratios cr cr al 0 78 0 86 and moderate mg ratios mg mg fe2 0 38 0 58 the compositional variability occurs due to sub solidus re equilibration in the accessory chromite in the serpentinite mg ratio 0 01 0 38 cr ratio 0 02 0 99 and in silicate rich chromitite mg ratio 0 06 0 48 cr ratio 0 60 0 99 in the massive chromitites the sub solidus re equilibration for chromite is less or absent however the re equilibration is prominent in the co existing interstitial and included olivine fo96 98 and pyroxene grains mg numbers 97 99 compositional variability on the scale of a single chromite grain occurs in the form of zoning and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite in the zoned grains the composition of the core is modified and the rim is ferritchromit in general ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains in this case ferritchromit formation is not very extensive this indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization in some occurrences the ferritchromit rim is very well developed and only a small relict core appears to remain in the chromite grain however complete alteration of the chromite grains to ferritchromit without any remnant core is also present the regular well developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the nuggihalli schist belt to be intense pervasive and affected by later low grade metamorphism the primary composition of chromite has been used to compute the nature of the parental melt the parental melt calculations indicate derivation from a high mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the western dharwar craton tectonic discrimination diagrams using the primary composition of chromites indicate a supra subduction zone setting ssz for the archean chromitites of nuggihalli and derivation from a boninitic magma the composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back arc rift settings formation of the massive chromitites in nuggihalli may be due to magma mixing process involving hydrous high mg magmas or may be related to intrusions of chromite crystal laden magma however there is little scope to test these models because the host rocks are highly altered serpentinized and deformed the present configurations of the chromitite bodies are related to the multistage deformation processes that are common in archean greenstone belts

crustal configuration of the dharwar craton india based on joint modeling of regional gravity and magnetic data abstract regional gravity and magnetic investigations were conducted along a 600 km transect across the northern part of the dharwar craton in the precambrian indian peninsular shield from vasco da gama latitude 15 23 40 n and longitude 73 48 55 e to jadcharla latitude 16 41 40 n and longitude 78 08 18 e joint modeling of the two data sets was carried out utilizing gm sys geosoft software assuming a four layer model a tectonic re classification of the craton is proposed prompted by the location of four inferred deep seated faults into the western and eastern dharwar blocks with an intervening upthrown block this classification is at variance with the current perceptions of the configuration of the dharwar craton some of which hold that the craton consists of only two sub cratons an eastern and a western divided by the chitradurga thrust fault however in the present study this thrust fault is found to extend no deeper than the basement peninsular gneisses furthermore according to the proposed classification the thrust fault falls within the intervening block that forms the divide between the sub units of the craton rather than representing a tectonic boundary in addition twenty six near surface geological members in the region supracrustals consisting mainly of schists and younger granites were identified along the transect and modeled for the representative near surface crustal configuration of the craton

geoelectric structure of the dharwar craton from magnetotelluric studies archean suture identified along the chitradurga gadag schist belt summary broad band magnetotelluric data were collected at 50 stations over a 400 km long approximately east west profile over the granite greenstone terrain of dharwar southern india the tensor decomposed data were interpreted using a 2 d inversion scheme the geoelectric model is suggestive of a suture along the chitradurga gadag schist belt formed by the thrusting of the west dharwar craton beneath its eastern counterpart with an easterly dip of 20 30 the thrust proposed here pre dates the formation of these schists which occurred during the late archean 2600 ma the accretionary wedge of the thrust and the depressed part of the west dharwar craton may have controlled the emplacement of the chitradurga gadag and shimoga dharwar schists the subsequent weathering the several episodes of tectonic activity witnessed during the precambrian and the vertical block movements caused during the passage of the indian plate over the reunion hotspot may have modified the crust leading to the present day geological configuration despite its age and several tectonothermal episodes the signature of this thrust is adequately preserved in the dharwar craton several similarities with younger sutures as is evident from the observed relics of the oceanic rocks present along the chitradurga schist belt suggest that the tectonic processes leading to this archean event may have had a close resemblance to those witnessed in recent times magnetotelluric studies also image a zone of low resistivity at a depth of 40 km beneath the west dharwar craton this seems to be a regional feature extending to the north over a distance of at least 250 km beneath the deccan volcanics the low heat flow values and the high density associated with this feature make partial melting an unlikely explanation for the low resistivity the grain boundary graphites and the sulphides deposited in the form of pyrites may have caused the low resistivity in the lithospheric mantle of the west dharwar craton although the fluids generated and trapped in the mantle during the passage of the indian plate over the reunion hotspot in the waning phase of its outburst could also be a possibility the asthenosphere is delineated at a depth of about 100 km beneath the east dharwar craton

periodic black auroral patches at the dawnside dipolarization front during a substorm 1 periodic black auroral patches were observed in the canadian postmidnight sector by ground based all sky cameras during an auroral substorm the aurora consisted of several black patches of 50 km 1 in longitude scales that appeared along the equatorward edges of the proton auroras over a 3 h local time segment from gillam to sanikiluaq ground stations 15 min after the auroral intensification occurred in alaska to the best of our knowledge this type of black auroras has never been reported before although the surrounding luminous auroras evolve into the well known torch structures later in situ observations by the themis satellites three of which were close to being magnetically conjugate in the plasma sheet as well as ground based proton auroral observations show that a dipolarization front and associated dawnward ion flows propagated eastward at poleward of the black patches coinciding with their appearance the flow motions of auroras observed poleward of the black patches are possibly generated by decelerations of dawnward ion flow because of the consistency of both durations and positions we speculate based on manifestations of high pressure regions in dipolarized flux tubes the pressure driven interchange instability formed a beading shape periodic black patches of field aligned currents the black auroral patches developed into torch structures after the flow disappearance indicating the dawnward flows might stabilize and limit the instability within a narrow l shell where the pressure gradient is directed earthward and flows cannot enter we conclude that the periodic black patches arise as a consequence of a relaxation process of enhanced plasma pressure near the inner boundary of the plasma sheet during a substorm

1891 1883 ma southern bastar cuddapah mafic igneous events india a newly recognized large igneous province abstract a newly recognized remnant of a paleoproterozoic large igneous province has been identified in the southern bastar craton and nearby cuddapah basin from the adjacent dharwar craton india high precision u pb dates of 1891 1 0 9 ma baddeleyite and 1883 0 1 4 ma baddeleyite and zircon for two se trending mafic dykes from the bd2 dyke swarm southern bastar craton and 1885 4 3 1 ma baddeleyite for a mafic sill from the cuddapah basin indicate the existence of 1891 1883 ma mafic magmatism that spans an area of at least 90 000 km2 in the south indian shield this record of 1 9 ga mafic ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south eastern margin of the south indian shield is a widespread geologic phenomenon on earth similar periods of intraplate mafic ultramafic magmatism occur along the margin of the superior craton in north america 1 88 ga molson large igneous province and in southern africa along the northern margin of the kaapvaal craton 1 88 1 87 ga dolerite sills intruding the waterberg group existing paleomagnetic data for the molson and waterberg 1 88 ga large igneous provinces indicate that the superior and kalahari cratons were at similar paleolatitudes at 1 88 ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a limpopo like 2 0 ga deformation zone in the superior province the widespread occurrence of 1 88 ga intraplate and plate margin mafic magmatism and basin development in numerous archean cratons worldwide likely reflects a period of global scale mantle upwelling or enhanced mantle plume activity at this time

lead isotopic compositions of the western dharwar craton southern india evidence for distinct middle archean terranes in a late archean craton abstract lead and sr isotopic compositions of whole rocks and feldspar separates from several fielddefined suites of rocks from the western dharwar craton of southern india elucidate a complex history of the area the evolution of the craton earlier than 3 0 ga is difficult to determine because of thermal and metasomatic overprinting at 3 0 ga many whole rocks were completely reset then but feldspar separates preserve some of the earlier history apparently a series of terranes evolved from the mantle over the period 3 4 to 3 0 ga to form the western dharwar craton in each case metamorphism and anatexis of older terranes accompanied extraction of juvenile material from the mantle the oldest gneisses 3 4 ga had radiogenic pb and sr isotopic compositions when they formed and were derived from older crustal protoliths that were plausibly accreted to the crust at or even before 3 8 ga the old gneisses were greatly depleted in rb probably at 3 4 ga when the protoliths of younger magmatic rocks may have separated from the mantle younger gneisses apparently formed from these mafic intermediate protoliths in the age range of 3 2 to 3 0ga the 3 0 ga event in the central part of the craton was accompanied by intrusion of small diapiric trondhjemites formed by melting of older crust and penecontemporaneous metasomatism of country rocks by hydrous fluids carrying u and other lithophilic elements the resultant deep crust was greatly depleted in heat producing elements and the craton has been inactive since 3 0 ga except for production of minor granites from depleted source regions at 2 5 2 6 ga the lack of activity at 2 5 ga contrasts strongly with extensive magmatism and metamorphism in surrounding areas that had not undergone depletion at 3 0 ga lower crustal depletion also has caused present day reduced heat flow to be very low

a review of the inferred geodynamic evolution of the dharwar craton over the ca 3 5 2 5 ga period and possible implications for global tectonics the geological history and evolution of the dharwar craton from ca 3 5 2 5 ga is reviewed and briefly compared with a second craton kaapvaal to allow some speculation on the nature of global tec

strain patterns in archaean dome and basin structures the dharwar craton karnataka south india abstract models simulating the role of gravity in the earth s crust are the main source of criteria used for the field identification of diapirs after a short review of the distribution of finite strain in experimental diapir models we describe an example where differential erosion of a tilted craton has provided 3d geometrical information on patterns in a diapiric system at various depths from greenschist to granulitic facies conditions within the archaean continental crust of the dharwar craton south india diapirs and anti diapirs sagducted greenstones are observed at different levels structures and strain are consistent with all the field identification criteria inferred from experimental simulation but some peculiar structural features are also observed which are not predicted by the models these features such as strain trajectory triple junctions result from i the progressive deformation of diapiric domes or sagducted basins ii interferences between diapiric structures body forces and iii the superimposition of diapiric strain fields on regional horizontal shortening boundary forces

correlations between the eastern block of the north china craton and the south indian block of the indian shield an archaean to palaeoproterozoic link abstract recent tectonometamorphic data reveal that the north china craton formed by amalgamation of an eastern block eb and a western block wb along the trans north china orogen tnco and the indian shield formed by amalgamation of the south indian block sib and the north indian block nib along the central indian tectonic zone citz the eb of the north china craton and the sib of the indian shield share many similar magmatic and sedimentary features the early archaean supracrustal sequences in both the eb and sib consist of 3 6 3 4 ga fuchsite quartzites pelitic gneisses calc silicates marbles banded magnetic quartzites and amphibolites called the caozhuang group in the eb and the older metamorphic group omg in the sib the middle archaean assemblages in both the blocks can be divided into an early 3 4 3 3 ga phase represented by the chentaigou supracrustal rocks and orthogneisses in the eb and the holenarsipur supracrustal rocks and the gorur orthogneisses in the sib and a late 3 1 2 9 ga phase represented by the qianan supracrustal rocks and the yangyashan orthogneiss in the eb and the sargur group and the peninsular gneiss in sib the late archaean taishan group and its equivalents in the eb are stratigraphically similar to the dharwar supergroup in the sib moreover both the eb and sib are characterised by near contemporaneity of late archaean granitoid intrusive and metamorphic events in addition dome and basin structures are dominant in both the eb and sib and anticlockwise p t paths charaterise the metamorphic evolution of the late archaean granulites in the two blocks the palaeoproterozoic sedimentary volcanic rocks in both blocks comprise lower clastic rich middle volcanic rich and upper clastic carbonate sequences the strong resemblance of the archaean to palaeoproterozoic magmatic and sedimentary records suggests that it is possible that the eb and sib are dispersed remnants of what was once a single continent a possible fit is proposed for the reconstruction of the eb and sib where the northern margin of the eb is placed adjacent to the western margin of the sib with the tnco and wb representing the continuations respectively of the citz and nib this fit suggests that the eb sib and the wb nib represent two discrete continental blocks that joined along the tnco citz orogen during a palaeoproterozoic collisional event fragmentation of the eb and sib may have occurred at the beginning of the mesoproterozoic and by 1 4 ga the eb and sib may have separated into two different continental blocks as evidenced by striking differences in the mesoproterozoic sedimentary successions and associated bimodal volcanics in the two blocks

pb sr and nd isotope systematics of metavolcanic rocks of the hutti greenstone belt eastern dharwar craton constraints on age duration of volcanism and evolution of mantle sources during late archean abstract the hutti greenstone belt hosting major gold mines is made up of tholeiitic basalts felsic volcanic rocks and sediments metamorphosed to amphibolite facies and surrounded by granitoid rocks predominantly of granodiorite in composition the granitoid rocks show intrusive contact relationship at places along the eastern and northern margins of the belt while such relationships could not be observed along the western margin as it is covered by soil the tholeiitic basalts have flat to moderately lree enriched chondrite normalized ree patterns and show depletion in nb relative to morb the hutti metabasalts define a collinear array in the rb sr isotope evolution diagram with a slope corresponding to an age of 2706 130 ma the higher uncertainty is due to scatter caused by mobility of rb and sr during metamorphism and low temperature hydrothermal alteration in the 206 pb 204 pb vs 207 pb 204 pb diagram the metabasalts define a collinear array corresponding to an age of 2637 150 ma with significant scatter indicating that pb was also mobilized by the post crystallization alteration processes the sm nd isotope system in the hutti metabasalts has been least disturbed giving an isochron age of 2662 81 ma which is suggested as the age of formation of their precursors combined with the published u pb shrimp ages on zircons from the felsic volcanic rocks it is inferred that the hutti greenstone belt formed over a period of 86 ma the major greenstone belts of the eastern dharwar craton have evolved coevally at ca 2700 ma ago and are older than the surrounding granitoid rocks the metabasalts have e nd values of 1 4 to 3 0 and e sr values of 23 4 to 2 0 and plot close to the mantle array they were derived from long term lree depleted mantle source the felsic volcanic rocks have e nd values of 1 3 and 1 4 and e sr values of 30 6 and 89 3 respectively their magmas were derived from older continental crustal sources

nd isotopic evidence for early to late archean 3 4 2 7 ga crustal growth in the western superior province ontario canada abstract new sm nd analyses of archean metavolcanic plutonic gneissic and metasedimentary rocks from sachigo and berens river subprovinces north caribou terrane and winnipeg river and wabigoon subprovinces in conjunction with previously published data are used to trace the crustal evolution of the western superior province wsp from ca 3 4 to 2 7 ga the extensive wsp geochronological database which contains 3 17 2 55 ga plutonic rocks allows the calculation of precise e t nd values and the identification of the main periods of new crustal addition at 3 4 0 1 3 02 2 92 and 2 76 2 69 ga mass balance calculations indicate that the western superior province represents a collage of archean crusts comprising 7 44 and 48 wt of crust created at 3 4 3 0 and 2 7 ga respectively although these calculations depend on the isotopic signatures of the mantle and those of the average crust at different times several important observations can be made 1 the importance of pre 2 7 ga terranes recycled in the 2 7 ga orogen 2 the existence of time intervals of 150 300 ma dominated by intra crustal processes resulting in a large range of crystallization ages u pb zircon ages that do not reflect new additions from the mantle 3 much of the new crustal additions at 2 7 ga were contaminated by crustal material subducted into the source region ttg suites or were contaminated during intrusion into the crust sanukitoids

sources of continental magmatism adjacent to the late archean kolar suture zone south india distinct isotopic and elemental signatures of two late archean magmatic series conspicuous nd sr and pb isotopic differences exist between the archean gneiss terranes adjoining the suture at the kolar schist belt south india these gneisses which are the deformed equivalents of plutonic and volcanic rocks have known or inferred igneous ages of 2630 to 2530 ma initial isotopic ratios of nd sr and pb suggest that metaplutonic gneisses west of the kolar schist belt were emplaced into and variably contaminated by an evolved continental crust that formed prior to 3200 ma felsic metaigneous gneisses that occur as slivers on the western margin of the schist belt have an isotopic character similar to that of the metaplutonic rocks on the same side of the kolar schist belt on the east side of the kolar schist belt the isotopic evidence suggests that the 2530 ma granitic gneisses were not derived from or contaminated by an older continental crust their source probably evolved with a nd isotopic composition similar to that of typical archean mantle but became light rare earth element enriched after 2900 to 2700 ma the inferred tectonic setting for the west side of the kolar schist belt is an andean continental magmatic arc for the east side of the kolar schist belt a possible phanerozoic analog is an evolved island arc such as japan

formation of archean 3600 2500 ma continental crust in the dharwar craton southern india abstract the generation preservation and destruction of continental crust on earth is of wide interest in understanding the formation of continents cratons and supercontinents as well as related mineral deposits in this contribution we integrate the available field petrographic geochronologic elemental nd hf pb isotope data for greenstones ttg gneisses sanukitoids and anatectic granites from the dharwar craton southern india this review allows us to evaluate the accretionary processes of juvenile crust mechanisms of continental growth and secular evolution of geodynamic processes through the 3600 2500 ma window hence providing important insights into building of continents in the early earth the dharwar craton formed by assembly of micro blocks with independent thermal records and accretionary histories the craton can be divided into three crustal blocks western central and eastern separated by major shear zones the western block contains some of the oldest basement rocks with two generations of volcano sedimentary greenstone sequences and discrete potassic plutons whereas the central block consist of older migmatitic ttgs abundant younger transitional ttgs remnants of ancient high grade supracrustal rocks linear volcanic dominated greenstone belts voluminous calc alkaline granitoids of sanukitoid affinity and anatectic granites in contrast the eastern block comprises younger transitional ttgs abundant diatexites thin volcanic sedimentary greenstone belts and calc alkaline plutons published geochronologic data show five major periods of felsic crust formation at ca 3450 3300 ma 3230 3150 ma 3000 2960 ma 2700 2600 ma and 2560 2520 ma which are sub contemporaneous with the episodes of greenstone volcanism u pb ages of inherited zircons in ttgs as well as detrital zircons together with nd pb hf isotope data reveal continental records of 3800 3600 ma the u pb zircon data suggest at least four major reworking events during ca 3200 ma 3000 ma 2620 2600 ma and 2530 2500 ma corresponding to lower crustal melting and spatially linked high grade metamorphic events the ttgs are sub divided into the older 3450 3000 ma ttgs and the younger 2700 2600 ma transitional ttgs the older ttgs can be further sub divided into low al and high al groups elemental and isotopic data suggest that the low al type formed by melting of oceanic island arc crust within plagioclase stability field in contrast the elemental and isotopic features for the high al group suggest derivation of their magmatic precursor by melting of oceanic arc crust at deeper levels 55 65 km with variable garnet and ilmenite in residue the transitional ttgs likely formed by melting of composite sources involving both enriched oceanic arc crust and sub arc mantle with minor contamination of ancient crustal components the geochemical and isotopic compositions of granitoids with sanukitoid affinity suggest derivation from enriched mantle reservoirs finally anatectic granites were produced by reworking of crustal sources with different histories in the light of the data reviewed in this contribution we propose the following scenario for the tectonic evolution of the dharwar craton during 3450 3000 ma ttgs sources oceanic arc crust formed by melting of down going slabs and subsequent melting of such newly formed crust at different depths resulted in ttg magmas on the contrary by 2700 ma the depth of slab melting increased melting of slab at greater depth alongside the detritus results in enriched melts partly modified the overlying mantle wedge subsequent melting of such newly formed enriched oceanic arc crust and surrounding arc mantle generated the magmatic precursor to transitional ttgs finally at ca 2600 2560 ma eventual breakoff of down going slab caused mantle upwelling which induced low degree 10 15 melting of overlying enriched mantle at different depths thereby generating the sanukitoid magmas which upon emplacement into the crust caused high temperature metamorphism reworking and final cratonization the crustal accretion patterns in the dharwar craton share similarities with those in other archean cratons such as the bundelkhand craton in central india pilbara yilgarn craton in western australia southern africa swaziland and limpopo belt north china craton tanzania craton antongil craton ne madagascar

smnd pbpb and rbsr geochronology and petrogenesis of the mafic dyke swarm of mahbubnagar south india implications for paleoproterozoic crustal evolution of the eastern dharwar craton abstract the sm nd rb sr and pb pb isotopic systematics of whole rock and separated mineral fractions from four tholeiitic mafic dykes of the mahbubnagar swarm have been investigated this dyke swarm intrudes the granitic basement of the cuddapah basin in the eastern dharwar craton edc the sm nd data on whole rock samples of four gabbros and one dolerite and 15 mineral fractions yielded a linear array with an isochron age of 2173 64 ma and an initial nd value of 0 4 0 3 the sm nd data of a metapyroxenite and its three mineral fractions plot parallel to this array yielding an isochron age of 2189 123 ma with a negative initial nd value of 1 8 1 4 rb sr and pb pb internal isochron ages of three gabbros and a pb pb internal isochron age of a metapyroxenite dyke range between 2031 and 2284 ma and within the limits of experimental error broadly conform to the sm nd isochron age by contrast rb sr internal isochron ages of three other samples range between 1474 and 1952 ma and one sample yields a pb pb internal isochron age of 3446 ma showing the effects of partial isotopic resetting possibly due to hydrothermal alteration these data suggest that mafic dykes of the mahbubnagar swarm were emplaced at c 2170 ma marking an episode of the crustal extension and fracturing which ultimately resulted in the formation of the cuddapah basin different petrological and geochemical variants of mafic dykes seen in the area most probably represent generation and or crystallization of the progressively evolved batches of magma during a relatively short time interval the initial nd values calculated at 2170 ma range from 0 6 to 2 0 and can be explained as reflecting contamination of depleted mantle derived magma by crustal rocks of edc the initial 87 sr 86 sr ratios 0 7027 0 7064 and model 1 values 8 3 8 5 of apparently undisturbed samples are also consistent with this suggestion

nd isotopic constraints on crustal formation in the north china craton abstract recent tectonic analysis suggests that the north china craton consists of two archean continental blocks called the eastern and western blocks separated by the paleoproterozoic trans north china orogen although the published geochronological data are not sufficient to constrain the detailed tectonothermal evolution of the craton the available nd isotopic data show some important differences in nd model ages between the tectonic units the eastern block shows two main nd model age peaks one between 3 6 and 3 2 ga and the other between 3 0 and 2 6 ga limited nd isotopic data from the western block show a large range of model ages between 3 2 and 2 4 ga these differences are consistent with the recently proposed model the nd isotopic data from mantle derived mafic rocks indicate that the mantle beneath the north china craton was depleted in the archean consistent with major crustal growth during this period in the paleoproterozoic however the mantle derived mafic rocks show negative end t values implying crustal contamination this may have resulted from subduction and collision between the eastern and western block implying that the mechanisms of crustal formation and evolution may have been different between the archean and paleoproterozoic the north china craton was re activated by addition of mantle derived magma into the lower crust in the late mesozoic resulting in rejuvenation of the lower crust this indicates that underplating is also an important mechanism for continental addition although in this case it may not equate to crustal growth since it was preceded by removal of lithospheric mantle and possible some lower crust

sinking of the dharwar basin south india implications for archaean tectonics abstract archaean tectonic regimes can be constrained by structural analysis of greenstone belts and adjacent granite gneiss masses the late archaean bababudan greenstone belt dharwar craton south india contains a low grade volcanosedimentary basin sequence the dharwar supergroup which unconformably overlies a previously eroded basement made up mainly of granite and gneiss fabric trajectories kinematic indicators and superimposed deformation patterns show that greenstones have undergone a radial converging downward flow with respect to underlying basement rocks as well as strike slip shearing this is attributed to the progressive sagduction gravity sinking of the greenstone belt into its basement along a decollement horizon and to late transcurrent shearing we propose a deformation model for the bababudan belt that may explain common features of many greenstone belts such as decollements and superimposed upright fold patterns which are attributed to the progressive sinking of the supracrustals into their basement the results of our study and other field investigations in the crust of the dharwar craton as well as structural evidence from other cratons zimbabwe and pilbara raise the possibility that a unique continental tectonic regime existed during the archaean controlled by the emplacement of dense volcanic traps mantle underplating and thermal blanketing of the continental crust this tectonic regime differs from that observed within the modern lithosphere as it is characterized by the pervasive development of gravity i e rayleigh taylor instabilities between the volcanic traps and the rest of the continental crust it allowed heat and material transfer through the crust and has to be seen as an efficient recycling mechanism for the young continental crust during periods of mantle underplating the diapiric phenomenon is independent of the boundary forces acting at the limits of the protocontinents and is associated in time and space with large scale accretion events of the continental crust perhaps in hot spot environments

multi element geochemical modelling of crust mantle interactions during late archaean crustal growth the closepet granite south india the closepet granite in the dharwar craton of south india is a large late archaean magmatic body its composition can be explained as a result of interactions between mantle derived melts and pre existing continental crust ttg gneisses using geochemical modelling based upon major and trace element compositions the following petrogenetic model is proposed for the formation of the batholith i an enriched mantle garnet and amphibole bearing lherzolite melts to produce a basaltic liquid ii the basaltic liquid undergoes limited fractional crystallization of biotite and amphibole f 0 9 iii the differentiated liquid rises into the old continental crust and induces water saturated anatexis of the ttg gneisses k feldspar qtz plagioclase melt iv both mantle derived and anatectic liquid mix and give rise to magma compositions ranging from quartz monzonite to granite the origin of the mantle enrichment is discussed in the context of the regional geological setting the most likely possibility seems to be metasomatism by slab melts the metasomatism of a mantle wedge by slab melts this suggests a two stage evolution for the dharwar craton during the late archaean i a subduction related event with the formation of ttgs sanukitoids and with associated mantle metasomatism and ii re melting of the metasomatized mantle this evolutionary history implies that at least some of the k rich late archaean granites are juvenile rather than products of intracrustal reworking as frequently assumed

three dimensional field perspective on deformation flow and growth of the lower continental crust dharwar craton india 1 the study of fabric development and juvenile batholith emplacement across the tilted crustal section of the eastern dharwar craton shows that horizontal constrictional deformation affected large volumes of the midcrust and lower crust at the time of regional partial melting and magmatic accretion constriction is achieved by a combination of coeval shallow and steep planar fabrics sharing a common horizontal elongation direction two sets of conjugate strike slip shears and extensional shear zones the eastern dharwar craton illustrates an end member deformation mode by which a particularly weakened lithosphere responds to shortening by developing distributed horizontal plane strain on a crustal scale resulting from the combination of crustal shortening and lateral gravity driven flow thinning accompanying constrictional deformation is interpreted as compensating for juvenile magmatic accretion and thickening of greenstone belts and as acting to maintain a stable crustal thickness such a midcrustal to lower crustal deformation process may provide a resolution of the batholithic room problem in a softened crust submitted to lateral shortening and may explain nearly isobaric retrograde pressure temperature time paths of high temperature low pressure high grade terrains

the advent of the proterozoic a trigger for extensive intracrustal processes in the south indian shield abstract geochemical sr isotopic and geochronological data on the major granitic activity in the south indian shield suggest a significant event of intracrustal melting and differentiation processes particularly at middle and lower crustal levels resulting in irreversible geochemical differentiation of the crustal column into a large ion lithophile lil element depleted anhydrous lower crust and an upper crust characterised by copious granitic activity marking the archean proterozoic boundary a perceptible link between the formation of granulites in the lower crust and the granitic activity in the upper crust is recognised in this region major element geochemistry of the granites suggests an anatectic origin for these granites the geochemical signatures of the granites in the closepet hampi karnataka lepakshi kadiri tanakallu and hyderabad regions andhra pradesh suggest that the source regions for the granitic melts were mainly the juvenile crustal additions of the tonalilte trondhjemite granodiorite ttg types at lower crustal levels sources which had little crustal residence time are indicated by relatively low initial strontium isotopic ratios this granitic activity was mostly of the granodiorite quartz monzonite and granite types which exhibit i type character these granites usually have highly fractionated ree patterns mostly with negative eu anomalies implying significant fractionation of palgioclase into the residue and the strong effects of intracrustal differentiation processes the heat required for melting might have been provided by crustal thickening and radioactive decay processes with possible involvement of minor amounts of mantle input at times co2 streaming which reduces considerably the melting temperatures and helps in the removal of lile into the melt may also have played an important role most of the granites have geochemical characteristics of origins in volcanic arc or syn collisional settings suggesting the operation of plate tectonic processes and crustal accretion mechanisms during the archaean proterozoic transition in the south indian shield proterozoic crustal accretion and the granitic crustal differentiation contributed to relative stability of the continental crust and resulted in cratonisation process

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

palaeoproterozoic assembly of the napier complex southern india and western australia implications for the evolution of the cuddapah basin abstract a detailed examination of lithostratigraphic tectonothermal geochronologic and palaeomagnetic database of the dharwar bastar cratons of south india the yilgarn craton of western australia and the east antarctic shield has shown broad similarity two palaeomagnetic key poles of 2400 ma age from the dharwar and yilgarn cratons suggest near neighbor positions matching patterns of craton outlines orientation of mafic dyke swarms and palaeo north direction for 2400 ma have established a continental assembly of siwa an acronym for south india si and western australia wa at 2400 ma in this assembly the yilgarn craton fits against the bastar craton and the singhbhum craton the available geological data from the napier complex of the eastern antarctica and the dharwar craton of south india were used to prepare the barcode style reconstruction of archaean tectonostratigraphic events of both the cratons the method of matching continental outlines the internal features of two continental blocks and the palaeomagnetic data demonstrate that the napier complex was also a part of siwa during the period 2200 1900 ma and was located at the position of the cuddapah basin of india adjacent to the dharwar craton and the yilgarn craton the separation of the napier complex from the south indian block led to the development of the cuddapah basin at 1950 ma

pb and nd isotope constraints on the origin of high mg and tholeiitic amphibolites kolar schist belt south india the late archean north south trending kolar schist belt in south india 4 km wide by 80 km long is thought to be a suture between two gneiss terranes krogstad et al 1989 within this volcanics dominated belt are recognized both tholeiitic and high mg komatiitic and picritic amphibolites which make up some 70 and 5 respectively of the exposed outcrops a massive tholeiitic amphibolite separates the belt into western and eastern parts a pb pb whole rock age of 2732 155 ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock samples of this rock have nd values at 2700 ma that range between 3 8 and 6 8 1 initial 238u 204pb of 7 5 and k1 initial 232th 238u of about 4 two different types of high mg amphibolites are recognized from the western part of the belt a picritic or p type and a komatiitic or k type the p type have highly variable ce al ratios all greater than chondritic nd yb ratios greater than chondritic nd at 2700 ma of 1 5 to 8 and pb isotope compositions variable in 207pb 204pb with 1 of about 8 0 and k1 of about 4 the trace element data suggest that the light ree enrichment is a character of the mantle source and is not due to residual garnet the k type amphibolites have near chondritic ce al and nd yb ratios nd at 2700 ma of 1 5 to 8 and 1 of about 8 and k1 of about 4 although the p type is light ree enriched compared to the k type both types have similar ce nd ratios as well as initial pb and nd isotopes if the 2696 136 ma age for the sm nd isochron which includes both types of high mg amphibolite has any significance it dates the time of light ree enrichment of the mantle source for the p type komatiitic amphibolites the high mg amphibolites in the eastern part of the belt are light ree enriched have pb isotopic compositions that are variable in 207pb 204pb with a 1 about 8 5 and nd at 2700 ma of 1 8 to 4 5 hydrothermal fluids associated with metamorphism and shearing prior to about 2400 ma ago were responsible for the introduction of gold quartz carbonate veins into the kolar schist belt the pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous pb from adjacent older granitoid gneisses into the amphibolites which could be responsible for the variability in the 207pb 204pb ratios of the samples this extraneous pb probably is not responsible for the distinct pb isotope character of each type of amphibolite

archean granite greenstone tectonics at kolar south india interplay of diapirism and bulk inhomogeneous contraction during juvenile magmatic accretion 1 the structural study of the kolar greenstone belt and surrounding granite gneiss terrains combined with u pb dating reveals that the middle and lower crustal tectonoplutonic pattern of the eastern dharwar craton developed during a major magmatic accretion event between 2550 and 2530 ma the granite greenstone pattern resulted from the interference of gravity driven sagging of the greenstones i e diapirism e w bulk inhomogeneous shortening combined with horizontal n s stretching and syntectonic juvenile pluton emplacement bulk inhomogeneous contraction is accommodated by the synchronous development of a pervasive n s trending vertical foliation shallow stretching lineation and conjugate strike slip shear zone pattern within and outside the greenstone belt resulting in regional horizontal pure shear deformation the plutons around the greenstone belt record regional contraction by developing one set of strike slip c s fabrics of the shear zone pattern the development of the granite greenstone pattern was coeval and compatible with deformation during juvenile magmatic accretion melting and granulite metamorphism in the lower crust the kolar example points to a specific crustal rheology that allowed sagduction of the greenstones and regional distributed bulk inhomogeneous strain due to mechanical homogeneity and low viscosity provided by large scale melting during the accretion event this example further suggests specific boundary conditions to the craton that allowed e w inhomogeneous shortening to be accommodated by n s stretching and spreading of the crust without significant tectonic thickening such tectonoplutonic pattern is specific to the archean and may develop as a consequence of mantle plume activity in intracontinental settings

paleoproterozoic crustal evolution of the sao luis craton brazil evidence from zircon geochronology and sm nd isotopes abstract the sao luis craton northern brazil is composed of a few granitoid suites and a metavolcano sedimentary succession new single zircon pb evaporation ages and nd isotope data combined with other available information show that the metavolcano sedimentary succession developed from 2240 ma to approximately 2200 2180 ma from juvenile protoliths the subduction related calc alkaline suites of granitoids spatially associated with the metavolcano sedimentary sequence formed in an oceanic island arc setting between 2168 2147 ma most of these granitoids are tonalitic and formed from juvenile mantle or oceanic plate derived protoliths whereas minor true granites are the product of the reworking of the juvenile island arc material these arc related successions represent an accretionary event around 2 20 0 05 ga which is coincident with one of the main periods of crustal growth in the south american platform this accretionary orogen has subsequently been involved in a collision episode at ca 2100 2080 ma which is mainly recorded in the nearby gurupi belt the rock associations inferred geological settings and the crustal evolution detected in the sao luis craton are similar to what is described in paleoproterozoic domains of major geotectonic units of the south american platform such as part of the sao francisco craton southeastern guyana shield and of the west african craton

continental lithospheric evolution constraints from the geochemistry of felsic volcanic rocks in the dharwar craton india abstract felsic magmatism associated with ocean ocean and ocean continent subduction processes provide important evidence for distinct episodes of crust generation and continental lithospheric evolution rhyolites constitute an integral component of the tholeiitic to calc alkaline basalt andesite dacite rhyolite badr association and contribute to crustal growth processes at convergent plate margins the evolution of the dharwar craton of southern peninsular india during meso to neoarchean times was marked by extensive development of greenstone belts these granite greenstone terranes have distinct volcano sedimentary associations consistent with their geodynamic setting the present study deals with geochemistry of rhyolites from the chitradurga shimoga greenstone belts of western wdc and the gadwal kadiri greenstone belts of eastern edc sectors of dharwar craton to compare and evaluate their petrogenesis and geodynamic setting and their control on the continental lithospheric evolution of the dharwar craton at a similar range of sio2 al2o3 fe2o3 the rhyolites of wdc are more potassic whereas the edc rhyolites are more sodic and less magnesian with slight increase in tio2 minor increase in mgo content of wdc rhyolites reflects their ferromagnesian trace elements which are comparatively lower in the rhyolites of edc the relative enrichment in lile k rb and depletion in hfse nb ta zr hf marked by negative nb ta zr hf and ti anomalies endorse the convergent margin processes for the generation of rhyolites of both the sectors of dharwar craton the high silica potassic rhyolites of shimoga and chitradurga greenstone belts of wdc showing prominent negative eu and ti anomalies flat hree patterns correspond to type 3 rhyolites and clearly point towards their generation and emplacement in an active continental margin environment the geochemical characteristics of gadwal and kadiri rhyolites from eastern dharwar craton marked by aluminous compositions with low and fractionated hree patterns and minor negative eu anomalies are in conformity with type 1 rhyolites and suggest that they were erupted in an intraoceanic island arc system the overall geochemical systematics of the rhyolites from both the sectors of dharwar craton suggest a change in the geodynamic conditions from intraoceanic island arc of eastern dharwar craton and an active continental margin of western dharwar marked by ocean ocean subduction and migration of oceanic arc towards a continent followed by arc continent collision that contributed for the evolution of continental lithosphere in the dharwar craton

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

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

smnd ages of archaean metavolcanics of the dharwar craton south india abstract well constrained sm nd arrays of whole rocks are reported for the first time from three important stratigraphic units of the dharwar volcano sedimentary sequence of south india if interpreted as isochrons these arrays will correspond to the following ages and initial nd ratios relative to chur 2 911 0 049 ga and 0 20 0 40 for the kalasapura mafic basal metavolcanics of the bababudan greenstone belt 2 848 0 070 ga and 0 88 0 82 for the santaveri formation comprising mafic to felsic metavolcanics overlying the kalasapura rocks and 2 747 0 015 ga and 1 72 0 10 for the ingaldhal mafic felsic metavolcanic suite of the chitradurga greenstone belt the possibility that these sm nd arrays are mixing lines between unrelated components depleted and enriched respectively in light rare earth elements has been discussed leading to the conclusion that the ages closely correspond to their time of eruption the older suites from the bababudan belt from a mantle source of chur like composition the younger suite from the chitradurga belt from one with a prior history of lree depletion

an outline of juvenile crust formation and recycling history in the archaean western dharwar craton from zircon in situ u pb dating and hf isotopic compositions

sm nd isotopic data from archean metavolcanic rocks at holenarsipur south india results of a sm nd isotopic analysis of archean metavolcanics in the holenarsipur greenstone belt karnataka south india give a whole rock isochron age of 2 62 ga for lightly deformed metabasaltic amphibolites in the northern part of the belt this is within error of the age of high grade metamorphism and crustal thickening which affected areas further to the south during the late archean together with the geochemical affinities of these and other metavolcanics in karnataka and results of regional structural analysis this unexpected age supports a model relating volcanism and crustal thickening to northward subduction and crustal accretion during the late archean data from basic and ultrabasic metavolcanics from the more strongly deformed and higher grade southern arm of the holenarsipur belt do not permit an age greater than 3 0 ga previously these rocks were regarded as part of an older supracrustal sequence that predated the local 3 0 to 3 3 ga gneissic complex the new dates therefore consider

formation and tectonic evolution of southeastern china and taiwan isotopic and geochemical constraints abstract the southern part of china consists of the proterozoic yangtze craton and the phanerozoic south china foldbelts including the himalayan foldbelt of taiwan models for continental growth have been many and controversial isotopic and geochemical data from mesozoic and younger granitoids and sediments are used here to place constraints on the tectonic evolution of southeastern china and to evaluate whether the young phanerozoic foldbelts are representative of old rejuvenated landmass whether they characterize crustal accretion through successive subduction processes hence suggesting a net growth of continental mass or whether they represent some intermediate situation available sm nd isotopic data for phanerozoic granitoids and metasediments from the south china foldbelts and taiwan invariably show proterozoic model ages t dm ranging from 1 to 2 5 ga with a mean of 1 54 0 30 1 ga all rocks have negative nd t values 2 to 15 suggesting variable but important contributions of old continental materials to the sources of the phanerozoic rocks obviously they do not represent wholesale growth of continental mass the granitoids of the south china foldbelts have multiple origins as viewed from their chemical and isotopic characteristics secular geochemical variation has been established for inland granitoids of proterozoic to mesozoic age their high i sr values 0 710 0 737 suggest that most granitoids are essentially anatectic products of ancient continental crust this is compatible with remelting via microcontinental collision or terrane accretion repeated intracrustal reprocessing by partial fusion and differentiation has undoutedly contributed to important mineralization mesozoic granitoids excluding a type alkaline granites in the coastal region and taiwan show relatively low i sr 0 705 0 710 and high nd values implying that greater amounts of mantle components have been added to ancient continental material in the generation of these rocks this means that mesozoic magmatism in the coastal region resulted from subduction of a paleo pacific plate kula beneath the asian continent the overall geochemical evidence seems to be consistent with a model of serial collisions between microcontinental blocks for the bulk of the south china foldbelts the latest arc continent collision began about 4 ma ago in eastern taiwan

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

geodynamics of the eastern yilgarn craton abstract over the last decade there have been significant advances in our understanding of the stratigraphy magmatism deformation metamorphism and timing of mineralisation in the eastern yilgarn craton eyc of western australia the integration of these disciplines has enabled a holistic review of the tectonic history of the eyc which favours a paraautochthonous tectonic model a significant advance has been the recognition of a 2810 ma rifting event off the eastern margin of the youanmi terrane which set up the north northwest trending architecture of the eyc rifting was followed by a 100 myr long phase of tectonic activity initiated by the establishment of a convergent margin at 2715 ma characterised by a west dipping subduction zone to the east of the eyc subduction was associated with the deposition of 2715 2670 ma volcanic stratigraphy and the emplacement of voluminous tonalite trondhjemite granodiorite magmatism which resulted in magmatic thickening of the crust volcanism was terminated by a 5 myr pulse of east northeast contraction followed by mid orogenic extension possibly linked to lithospheric and lower crustal delamination the lack of ultra high pressure metamorphism and the presence of high geothermal gradients preclude this contractional event from recording continent continent collision mid orogenic extension at 2665 ma resulted in the introduction of metasomatised mantle melts mafic granites and syenites deposition of siliciclastic basins which record metamorphic anticlockwise ptt paths and the start of significant economic gold mineralisation in the eyc high stretching factors and or delamination associated with this extension event resulted in significant heat input into the base of the crust which eventually led to the emplacement of low ca crustal melt granites and cratonisation of the eyc major gold mineralisation postdates mid orogenic extension and was associated with renewed compression transpression 2650 ma and the development of steep predominantly north northwest striking sinistral and later north to north northeast striking dextral strike slip faults during low ca granite emplacement

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

c o sr and nd isotope systematics of carbonates of papaghni sub basin andhra pradesh india implications for genesis of carbonate hosted stratiform uranium mineralisation and geodynamic evolution of the cuddapah basin abstract the cuddapah basin cb is one of a series of proterozoic basins that overlie the archaean cratons of india and contains a unique stratiform carbonate hosted uranium mineralisation in the present work we discuss stable c o and radiogenic nd sr isotope systematics of carbonates of the papaghni sub basin in order to understand uranium ore forming processes and geodynamic evolution of the cb uranium mineralised dolomites umds of the basal vempalle formation show a significantly lighter 1 5 c isotope signature compared to that of open marine stromatolitic sub tidal facies suggesting input of isotopically lighter carbon through in situ remineralisation of organic matter om this implies deposition in a hydrologically restricted redox stratified lagoonal basin wherein exchange with open oceanic dissolved inorganic carbon dic was limited persistent bottom water anoxia was created and maintained through consumption of dissolved oxygen do by decaying om produced in oxidised surface water zone significantly more radiogenic end t of umd 6 31 0 54 compared to that of dharwar upper crust 8 64 3 11 indicates that dissolved constituents did not originate from the dharwar craton rather were derived from more juvenile exotic sources possibly from a continental arc dissolved uranyl ions u 6 were introduced to the basin through fluvial run off and were reduced to immobile uranous ions u 4 at the redox interface resulting in precipitation of pitchblende and coffinite carbonate horizons of upper vempalle formation and tadpatri formation show progressively more radiogenic nd isotope compositions signifying increased juvenile arc contribution to the papaghni sub basin through time which is also corroborated by the presence of younger zircons 1923 22 ma in pulivendla quartzites we propose that the papaghni sub basin opened as a back arc extensional basin at 2 ga as a result of westerly directed subduction of oceanic crust beneath the eastern indian continental margin the papaghni back arc basin eventually evolved to upper cuddapah foreland basin with collision of the dharwar craton and napier block of antarctica at 1 6 ga possibly during the final stage of amalgamation of the columbia supercontinent

lithosphere of the dharwar craton by joint inversion of p and s receiver functions summary the archean dharwar craton in south india is known for long time to be different from most other cratons specifically at station hyderabad hyb the ps converted phases from the 410and 660 km mantle discontinuities arrive up to 2 s later than in other cratons of comparable age which implies lower upper mantle velocities to resolve the unique lithosphere asthenosphere system of the dharwar craton we inverted jointly p and s receiver functions and teleseismic p and s traveltime residuals at 10 seismograph stations this method operates in the same depth range as long period surface waves but differs by much higher lateral and radial resolution we observe striking differences in crustal structures between the eastern and western dharwar craton edc and wdc respectively crustal thickness is of around 31 km with predominantly felsic velocities in the edc and of around 55 km with predominantly mafic velocities in the wdc in the mantle we observe significant variations in the p velocity with depth practically without accompanying variations in the s velocity in the mantle s velocity there are azimuthdependent indications of the hales discontinuity at a depth of 100 km the most conspicuous feature of our models is the lack of the high velocity mantle keel with the s velocity of 4 7 km s 1 typical of other archean cratons the s velocity in our models is close to 4 5 km s 1 from the moho to a depth of 250 km there are indications of a similar upper mantle structure in the northeast of the indian craton and of a partial recovery of the normal shield structure in the northwest a division between the high s velocity western tibet and low s velocity eastern tibet may be related to a similar division between the northeastern and northwestern indian craton

geochemical and nd isotope constraints on petrogenesis of granitoids from nw part of the eastern dharwar craton possible implications for late archaean crustal accretion abstract geochemical and nd isotope data on granitoids of the nw part of the late archaean eastern dharwar craton are presented to elucidate their petrogenesis and role in crust formation the granitoids are divided into three suites viz trondhjemite granodiorite gneisses biotite monzogranites and porphyritic biotite granodiorites the gneisses are pre to syn kinematic with respect to deformation in the adjacent hungund kushtagi schist belt which show variable sio2 and al2o3 enriched lree and depleted hree with slightly negative to no eu anomalies they display unusual chemistry in having higher feo t k2o ba cr and ni compared to the typical archaean tonalite trondhjemite granodiorite ttg the biotite monzogranites are mostly syn to late kinematic and exhibit evolved calc alkaline compositions with high sio2 k2o lile and lree depleted to undepleted hree and strongly negative to no eu anomalies the porphyritic granodiorites show syn to late kinematic calc alkaline sanukitoid like character with a wide range of sio2 higher tio2 p2o5 sr ba cr and ni and lower rb they however uniquely display higher k2o ree and th than typical sanukitoids the trondhjemite granodiorite gneisses are interpreted as product of melting of a subducted basaltic slab followed by slight contamination from the overlying metasomatized mantle wedge subsequent melting of the extremely metasomatized mantle wedge resulted in formation of the parental magma of the porphyritic granodiorites intrusion of the latter triggered melting of the ttg crust accreted earlier and generated the evolved monzogranites the monzogranites occurring east of the hungund kushtagi schist belt show higher end but lower tdm ages than those occurring to the west indicating that terranes with different histories were juxtaposed by lateral accretion

evaluating the origin of garnet cordierite and biotite in granitic rocks a case study from the south mountain batholith nova scotia evaluating the origin of garnet cordierite and biotite in granites provides important insight into closed and open system magma evolution we present field textural major and trace element mineral chemical and sr nd whole rock isotopic data on garnet cordierite and biotite rich zones from the peraluminous south mountain batholith we infer that 1 garnet rich zones of decimeter to meter size with 30 vol large subhedral garnet with abundant inclusions of detrital country rock monazite represent partially assimilated metapelitic country rocks where garnet is the incongruent product of biotite dehydration melting 2 cordieriterich zones tens of meters to kilometers in dimension with 5v ol large subhedral to euhedral zoned cordierite formed by crystallization from relatively evolved magmas and subsequent crystal accumulation 3 biotite rich zones with large subhedral to euhedral biotite with abundant euhedral apatite inclusions making up 80 vol centimeter scale or 25 vol kilometer scale of the rocks formed dominantly by fractional crystallization throughout the chemical evolution of the batholith our results suggest that for garnet and cordierite a combination of textural and mineral chemical characterization is probably sufficient to determine their origin in granites however for biotite and other readily equilibrated minerals evaluating both mineral and rock textures and majorelement trace element and isotopic compositions is essential

juvenile magmatism and crustal recycling at the end of the neoarchean in western shandong province north china craton evidence from shrimp zircon dating western shangdong province experienced major crustal growth as a result of 2 75 to 2 50 ga tectonothermal events different from other archean areas in the north china craton besides early neoarchean tonalite trondhjemite granodiorite ttg and supracrustal assemblages there are large volumes of late neoarchean gabbroic dioritic and granitic rocks in western shandong shrimp zircon dating of thirty one samples of different rock types from late neoarchean rocks yielded a narrow age range from 2560 to 2490 ma based on these data and previously published results the following conclusions can be drawn 1 the archean basement can be divided into three belts a late neoarchean 2525 2490 ma crustally derived granite belt in the northeast an early neoarchean 2 75 2 60 ga rock belt in the center and a late neoarchean 2550 2500 ma belt of juvenile rocks in the southwest 2 the tectonic regime in western shandong province probably changed from compressional to extensional at around 2525 ma between 2530 and 2520 ma 3 crustal recycling with addition from the mantle occurred at the end of the neoarchean 4 the late archean magmatic rocks were probably formed in an arc environment

geochemistry of the neoarchaean high mg basalts boninites and adakites from the kushtagi hungund greenstone belt of the eastern dharwar craton edc implications for the tectonic setting abstract volcanic rocks of the kushtagi hungund greenstone belt from the eastern dharwar craton edc show extreme diversity due to the presence of high mg high fe high mg fe basalts high mg dacites and andesites boninitic affinities and sodic plagioclase quartz rich felsic rocks adakites with na2o 4 6 wt the adakites drummond and defant 1990 martin 1999 and high mg dacites andesites are being reported for the first time from greenstone belts of the dharwar craton trace elements in these different types of volcanic rocks have distinct characteristics for example the basalts have 1 al2o3 tio2 16 pm value of 17 2 la yb n 1 3 3 low order negative anomalies of nb nb zr zr hf hf and ti ti 4 zr nb 4 10 and 5 ce nb 3 4 two samples of andesites having mg enrichment and al2o3 tio2 ratios of 31 35 are interpreted to have boninitic affinities other dacites and andesites are characterized by 1 high mg 8 10 wt 2 high al2o3 tio2 22 27 3 zr nb 8 14 4 la yb n 2 7 5 negative nb nb zr zr hf hf and ti ti anomalies and 6 sc lu 67 172 the adakites are characterized by 1 a narrow range of sio2 67 70 wt 2 enriched na2o 4 6 wt with very high na2o k2o 2 5 7 7 3 low mg 0 38 0 48 4 high la yb n 13 31 with fractionated patterns no eu anomaly and gd yb n 2 4 5 very high al2o3 tio2 50 80 and 6 enriched ni cr and co total 50 ppm these adakites resemble 3 0 ga tonalite trondhjemite gneisses of the dharwar craton which are interpreted as products of partial melting of the basaltic slab the available data suggests that partial melting of the different mantle and slab sources generated this collage of compositionally different rocks within an island arc setting

facies distributions and structure of a dharwar volcanosedimentary basin evidence for late archaean transpression in southern india the bababudan nallur basin c 60 km 125 km is part of a series of late archaean basins accommodating volcanic and sedimentary rocks of the dharwar supergroup within the low grade terrain of the province of karnataka initial development of the bababudan nallur basin was characterized by rapid subsidence with eruption of mainly basaltic lavas breaks in eruption were marked in some instances by thin but persistent deposits of shallow marine quartz sands and muds subsidence in the south continued with accumulation of banded ferruginous cherts and muds whilst uplift of the volcanic pile took place in the north this uplift promoted erosion of the volcanic cover and its basement gneisses and granodiorites with deposition of alluvial and shallow marine fans and debris flows adjacent to the uplifted blocks the fans passed laterally into intertidal and offshore quartz sands greywackes and muds with interbedded dolomitic limestones accumulation of rhyolites banded ferruginous cherts and greywackes took place further north the structure of the basin is dominated by mainly upright folds and faults with the trend of the folds broadly parallel to the boundaries of exposed blocks of basement the basement deformed cataclastically on myriad fractures in contrast with the ductile behaviour of the cover many cover basement contacts are faults with mylonites although unconformities are preserved especially in the south the facies distributions and structure of bababudan nallur and other dharwar belts have characteristics in common with mixed mode basins developed as a consequence of strike or oblique slip on long lived fault zones these relations suggest that transcurrent plate interactions comparable with those of the phanerozoic were significant in the late archaean evolution of southern india

age of younger tonalitic magmatism and granulitic metamorphism in the south indian transition zone krishnagiri area comparison with older peninsular gneisses from the gorur hassan area abstract a major episode of continental crust formation associated with granulite facies metamorphism occurred at 2 55 2 51 ga and was related to accretional processes of juvenile crust dating of tonalitic trondhjemitic granitic gneisses and charnockites from the krishnagiri area of south india indicates that magmatic protoliths are 2550 2530 5 ma as shown by both u pb and 207pb 206pb single zircon methods monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 10 ma these data provide precise time constraints and sr nd isotopes confirm the existence of late tonalitic granodioritic juvenile gneisses at 2550 ma pb single zircon ages from the older peninsular gneisses gorur hassan area are in agreement with some previous sr ages and range between 3200 20 and 3328 10 ma these gneisses were derived from a 3 3 3 5 ga mantle source as indicated from nd isotopes they did not participate significantly in the genesis of the 2 55 ga juvenile magmas all these data together with previous work suggest that the 2 51 ga granulite facies metamorphism occurred near the contact of the ancient peninsular gneisses and the 2 55 2 52 ga juvenile tonalitic trondhjemitic terranes during synaccretional processes subduction mantle plume rb sr biotite ages between 2060 and 2340 ma indicate late cooling probably related to the dextral major east west shearing which displaced the 2 5 ga juvenile terranes toward the west

zircon ages and geochemistry of late neoarchean syenogranites in the north china craton a review abstract at the end of the neoarchean continental evolution voluminous syenogranites were emplaced in the north china craton together with other magmatic rocks trondhjemite tonalite granodiorite ttg monzogranite diorite gabbro syenogranites are widely distributed in anshan benxi qinhuangdao and western shandong and also occur in southern jilin northern liaoning northwestern hebei and central henan based on geological relationships degree of metamorphism deformation and magmatic zircon ages two phases of syenogranite magmatism are recognized rocks produced during the first phase show a gneissic texture and were formed between 2 53 and 2 52 ga and locally comprise abundant ttg rocks of the second phase cut late neoarchean ttg and supracrustal rocks display a massive structure and mainly formed between 2 52 and 2 50 ga all syenogranites share the same features in major element compositions being high in sio2 and low in cao total feo mgo tio2 and p2o5 however they are different in trace and ree compositions and can be subdivided into three types 1 type 1 shows a large variation in total ree contents low la yb n ratios strong negative eu eu anomalies and ba depletion 2 type 2 is similar to type 1 but has higher la yb n ratios 3 type 3 shows a large variation in total ree and la yb n ratios and significantly do not show strongly negative eu eu anomalies and ba depletion whole rock sm nd isotopic compositions show large variations in nd t values and tdm nd modal ages ranging from 9 49 to 4 72 and 3 70 to 3 25 ga type 1 0 55 1 03 and 2 77 2 71 ga type 2 and 2 35 to 1 23 and 2 93 2 66 ga type 3 respectively hf isotopic compositions of zircons from three samples have hf t values and tdm1 hf ages of 0 7 7 2 and 2 84 2 56 ga type 1 2 6 7 4 and 2 74 2 56 ga type 2 and 2 1 6 3 and 2 76 2 60 ga type 3 it is concluded that syenogranites were generated by melting of continental crust with different mean crustal residence ages and most of them were emplaced during the second phase 2 52 2 50 ga in an extensional tectonic regime the formation of these voluminous syenogranites marks a tectono magmatic event resulting in stabilization of the north china craton at the end of the neoarchean

precambrian continental strain and shear zone patterns south indian case 1 this study addresses the tectonic and mechanical significance of crustal scale strain and shear zone patterns in the south indian precambrian lithosphere it is based on a tectonic map of the dharwar craton derived from landsat imagery existing documentation and our own field observations we document the contrasted responses of the two main parts of the craton to late archean 2 56 2 51 ga shortening the old 3 ga lithosphere of the western dharwar craton stabilized at 2 61 ga underwent moderate shortening and strain localization along spaced shear zones the eastern dharwar craton rejuvenated by late archean juvenile magmatic accretion responded to shortening by flowing laterally against the western dharwar craton shortening operated without significant thickening because of the high buoyancy of the juvenile crust and the very low strength of its mantle lithosphere the late archean crustal scale shear zone network did not accommodate large displacements and only contributed to smoothing out of strain heterogeneities during the latest stages of flow the shear zone pattern only reflects the symmetry of the regional finite strain field of a wide hot orogen it does not result from terrain amalgamation but accompanies crustal flow and does not compare with those of modern active margins which accommodate unidirectional transpression our analysis further suggests that the shear zones bounding the craton did not record large horizontal displacements during final assembly of gondwana but rather transverse shortening and vertical extrusion

petrography chemistry and isotopic ages of peninsular gneiss dharwar acid volcanic rocks and the chitradurga granite with special reference to the late archean evolution of the karnataka craton southern india abstract new petrographic major and trace element and isotopic age data are presented for four suites of rocks from the low to medium grade part of the south indian archaean craton in central karnataka two suites are from the peninsular gneiss basement to the late archaean dharwar supergroup the third is a suite of dharwar potassic rhyolites and the fourth is from the chitradurga granite which has intrusive contacts with the dharwar supergroup the chikmagalur granite granodiorite which with its host gneisses forms part of the basement peninsular gneiss to the dharwar supergroup in the bababudan basin yields rb sr and pb pb whole rock isochron dates of 3080 110 ma and 3175 45 ma respectively an initial 87 sr 86 sr ratio of 0 7013 0 0009 and a 1 value 238 u 204 pb of 7 98 host tonalitic gneisses to the chikmagalur granite give an rb sr whole rock isochron date of 3060 160 ma with an initial 87 sr 86 sr ratio of 0 7015 0 0004 and a pb pb whole rock isochron date of 3190 100 ma with a model 1 value of 8 00 a suite of granites and granodioritic tonalitic gneisses chitradurga granitic rocks and gneisses forming part of the peninsular gneiss basement to the dharwar supergroup west of the chitradurga belt yields rb sr and pb pb whole rock isochron dates of 2970 100 ma and 3028 28 ma respectively with an initial 87 sr 86 sr ratio of 0 7035 0 0013 and a model 1 value of 7 62 the suite of dharwar acid volcanic rocks from north of honnali defines a poorly fitted 207 pb 206 pb linear array mswd 15 8 which gives an apparent age of 2565 28 ma and a model 1 value of 7 46 the poor fit of this isochron shows that a closed system was not maintained a conclusion supported by high k abundances which are attributed to a syn or post magmatic ion exchange process the chitradurga granite which intrudes the older rocks of the dharwar supergroup in the chitradurga belt yields a well fitted pb pb whole rock isochron date of 2605 18 ma with a model 1 value of 7 68 the u pb relations in these suites from karnataka are in marked contrast with the commonly severe u depletion and consequently unradiogenic pb isotopic compositions observed in deeply eroded high grade archaean gneiss terranes such as those bordering the north atlantic this contrast indicates that the cratonic rocks in central karnataka represent a relatively high level in the original archaean continental crust

geochemistry and petrogenesis of granitoids around the kolar schist belt south india constraints for the evolution of the crust in the kolar area the peninsular gneiss surrounding the n s trending kolar schist belt in the dharwar craton includes several components of granitic rocks that range in composition from tonalite to granite on the west the different phases are referred to as the dod the dosa and the banded gneiss and the patna granite the gneisses on the east are dominantly of one type and are referred to as the kambha gneiss the champion gneiss occurring within the eastern part of the belt is compositionally similar to the plutonic dod gneiss and probably represents a near surface intrusion late aplites are present on both sides of the belt the banded gneiss was formed by partial melting of tonalitic rocks and the aplites were formed by the action of aqueous fluids during amphibolite grade metamorphism excepting these two rocks all other granitoids seem to have been formed from silica oversaturated parent magmas derived from lile enriched mantle sources by fractionation processes including liquid immiscibility and fractional cry

nd isotope disequilibrium during crustal anatexis a record from the goat ranch migmatite complex southern sierra nevada batholith california geological and geochemical studies of a pelitic migmatite complex within the isabella pendant of the southern sierra nevada batholith california suggest that the leucosomes represent the products of partial melting of the metapelite host driven by the emplacement of the adjacent goat ranch pluton ca 100 ma the leucosomes preserve a record of largemagnitude nd isotope disequilibrium with respect to their pelitic source the leucosomes have a wide range of nd 100 ma from 26 0 to 211 0 as compared to 28 7 to 211 3 for the source they can be subdivided into two groups based on their major elements and sr and nd isotope geochemistry group i leucosomes have higher p2o5 contents and nd 100 ma values than those of group ii the nd 100 ma values of group i leucosomes are significantly higher than those of metapelites and migmatites by two to four epsilon units suggesting that group i leucosomes are in nd isotope disequilibrium with their sources correlations among p2o5 contents nd 100 ma values and sm nd ratios in the leucosomes suggest that apatite or monazite has played a dominant role in fractionating sm from nd and generating nd isotope disequilibrium dissolution of apatite or monazite might play a critical role in regulating the behavior of the sm nd isotope systems and thus the nd isotope compositions of melts generated during crustal anatexis especially in metasedimentary protoliths

postcrystallization disturbance in the neodymium and lead isotope systems of metabasalts from the ramagiri schist belt southern india archean metabasaltic rocks from three separate blocks that constitute the north south trending ramagiri schist belt of the eastern dharwar craton have large variations in nd2750 even samples of the same rock type from one outcrop that are less than 50 cm apart from each other show a range of up to 5 5 nd2750 units the trends of the fields defined by the three suites in smnd vs nd2750 space intersect at smnd 0 1 to 0 2 and nd2750 8 to 12 suggesting that these rocks have lost lrees and have interacted with and exchanged at least nd with a common extraneous phase the isotopic composition of this phase suggest derivation of its neodymium from sources which had a long term history of lree enrichment this phase may have derived its isotopic composition from older continental crust one of the unaltered dikes cutting the three blocks in the belt and the surrounding gneisses gives a 147sm144nd 143nd144nd age of 2454 100 ma a minimum for the time of block assembly and alteration of rees including nd while a set of samples from the central block lie along an isochron with a pb pb age of 2746 64 ma the samples from the eastern and western blocks show large uncorrelated variations in 207pb204pb with 206pb204pb residues after acid leaching of sample powders have lead isotope ratios that are distinct for samples from each of the block and are compatible with their derivation from archean mantle the lead isotope composition of the acid leaches suggest disturbance of the metabasalts by an isotopically extraneous phase that had variable and high 207pb204pb and 208pb204pb ratios for a given 206ph204pb relative to expected archean mantle values both lead and neodymium isotope disturbances are best explained as a result of alteration of these rocks by an extraneous phase which had derived its isotopic composition from ca 3 5 ga continental crust

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

u pb ages of zircon and sphene for two gneiss terranes adjacent to the kolar schist belt south india evidence for separate crustal evolution histories orthogneisses adjacent to the kolar schist belt were emplaced as tonalitic to granodioritic magmas between 2630 and 2530 ma an orthogneiss unit east of the kolar schist belt apparently was a homogeneous granodioritic intrusion at about 2532 ma cooling to the blocking temperature of the u pb system in sphene by 2521 2517 ma gneisses on the west side of the kolar schist belt were emplaced as at least three separate intrusions dated at 2631 2610 and 2552 ma evidence from an older basement into which these rocks were intruded comes from inherited zircons with an age of 2800 ma rocks with zircons of the appropriate age to be part of this basement occur as tectonic fragments and inclusions in the area gneisses west of the kolar schist belt were last cooled through the blocking temperature of the u pb system in sphene at about 2552 ma synchronous with the last major intrusive event late felsic dikes and plutons apparently postdate deformation and yield sphene pb pb ages of 2480 ma a sample of muscov

late neoarchean magmatic and subsequent metamorphic events in the northern north china craton shrimp zircon dating and hf isotopes of archean rocks from yunmengshan geopark miyun beijing abstract the miyun geopark is located in the northern part of the north china craton and includes a variety of archean granitoids and metamorphic rocks shrimp zircon dating documents their emplacement ages at 2567 2508 ma metamorphic events at ca 2 50 ga 2 44 ga and 1 82 ga late neoarchean basement rocks elsewhere in the north china craton document a similar evolutionary history with intrusion of granitoid rocks commencing at ca 2590 ma and reaching three peaks at about 2550 2520 and 2500 ma these rocks then underwent regional metamorphisms at ca 2 50 ga 2 44 ga 1 92 ga and 1 82 ga positive e hf t values of 1 7 to 7 6 with model ages younger than 3 0 ga for magmatic zircons from these late neoarchean intrusive rocks indicate that they are predominantly derived from juvenile crustal sources and suggest that significant crustal growth occurred in the northern north china craton during the neoarchean the intrusive rocks in the miyun area are part of an extensive province of late neoarchean magmatism and appear to be related to mantle plume activity at this time

growth and reworking of the early precambrian continental crust in the north china craton constraints from zircon hf isotopes abstract we synthesize more than 2600 hf isotope data on the archean paleoproterozoic zircons from the north china craton ncc recalculation of the data based on single stage and two stage hf model ages of the eastern block of the ncc shows peak ages of 3902 13 ma and 3978 18 ma respectively and also small peaks at 3 5 4 0 ga the majority of zircon ehf t values are positive suggesting the possibility of the crust and the mantle differentiation at ca 3 9 4 0 ga in the eastern block of the ncc most magmatic zircons from the whole of ncc have their hf model age range of 2 4 2 9 ga and the single stage model ages is cluster at 2698 4 ma whereas the two stage model ages concentrate at 2714 5 ma implying that the protoliths were juvenile crustal rocks the most prominent peak at 2 7 ga indicates that this period marks the most important stage of the crust mantle differentiation and crust formation of the ncc the widespread 2 5 ga rocks in the ncc and the absence of the 2 5 ga peaks in hf model ages are consistent with the partial melting and reworking of the juvenile rocks at 2 5 ga furthermore the 2 5 1 7 ga zircon hf isotope features are also related to the reworking of the crustal rocks our results from the integration of a large database suggest that the eastern block and the trans north china orogen have undergone similar crust mantle differentiation and magmatism leading to the conclusion that the essential cratonization of the north china took place at the end of neoarchean

configuration of the late paleoproterozoic supercontinent columbia insights from radiating mafic dyke swarms abstract remnants of 2 1 1 8 ga orogens can be recognized in nearly every craton assembled within reconstructions of the rodinia supercontinent although no particular pattern of laterally extensive orogenic belts emerges these cratons may be the fragments of an older supercontinent formed in response to the collision and accretion of even older cratons during the early earth history paleomagnetic constraints have played a limited role in many previous reconstructions of the early precambrian supercontinent mainly because of the poor age control and a large scatter of the paleomagnetic poles the geometry of giant radiating dyke swarms and orogenic belts provide important constraints for the reconstruction of the pre rodinian supercontinent some early unmetamorphosed and undeformed mafic dyke swarms in north china southern peninsular india and north america share coeval age of 1 85 ga if these continents are assembled the overall pattern exhibited by the mafic dyke swarms appears to constitute a giant radiating swarm with a piercing point between the cuddapah rift in south india and the xiong er aulacogen in north china this suggests that the north china craton indian craton and canadian shield may be fragmented from the same landmass the reconstruction of 1 85 ga giant radiating dyke swarm attempted in this paper suggests that the north china craton india craton and canadian shield were united together to form a landmass within the columbia supercontinent before its extension and break up the 1 90 1 85 ga andean style north orogenic belt suggests that a subduction zone existed on the northern margin of the north china craton wopmay orogenic belt developed in a 1 88 1 84 ga continental volcano plutonic arc which is interpreted as the product of eastward subduction of oceanic lithosphere the transantarctica orogenic belt in east antarctica may be another candidate to be linked with the subduction zone the north orogenic wopmay orogenic and transantarctica orogenic belts could be connected to form a continuous subduction zone the orogenic comparison strengthens the configuration of the columbia supercontinent proposed by the reconstruction of the giant radiating dyke swarm the laurentia west australia and east antarctica were relatively stable from 1 85 ga to 1 20 ga as inferred from the data on dyke swarms magmatism and the paleomagnetism these continents constituted the core of the columbia supercontinent in the late paleoproterozoic time the north china craton and indian craton were however separated from the columbia supercontinent during the mesoproterozoic time

the dharwar craton southern india interpreted as the result of late archaean oblique convergence abstract the dharwar craton comprises two distinct parts separated by a steep n s sinistral shear zone in the western part a pre 2900 ma complex of orthogneisses granodiorites and narrow tracts of supracrustal rocks sargur group forms the basement to volcanic and sedimentary basins of the dharwar supergroup ca 2800 2550 ma late archaean deformation is characterised by ne sw crustal shortening and steep n s sinistral shear zones the eastern part is underlain by parallel steep n s or nw se linear belts of calc alkaline anatectic and juvenile granites and granodiorites dharwar batholith ca 2750 2510 ma with intervening volcanic and sedimentary schist belts ca 2800 2550 ma the plutonic belts are 15 25 km wide up to 150 km long and bounded by steep nw se high strain zones up to 2 km wide with sinistral shear sense except one which is dextral magmatic state fabrics and structures in the plutonic rocks are parallel to solid state sinistral shear fabrics in the high strain zones but diffuse magmatic banding is commonly oblique to these zones and coincides with the plane of instantaneous shortening in sinistral shear magmatic state structures swarms of vertical nw se dykes of granite and the vertical wedge shape of the linear belts are consistent with emplacement of the batholith during sinistral shear when magma pressure exceeded regional horizontal compressive stress upright folds and schistosity steep reverse faults and effects of regional ht lp metamorphism show that deformation was partitioned into ne sw shortening in the schist belts during emplacement of the plutonic belts in the sinistral shear regime the western part of the craton is interpreted as the foreland to an accretionary arc represented by the batholith and schist belts intra arc basins in the east ne sw shortening and sinistral transcurrent displacements in the foreland and arc are consistent with arc normal and arc parallel displacements during oblique convergence analogous to mesozoic cenozoic convergent settings

the lower crust of the dharwar craton southern india patchwork of archean granulitic domains u pb geochronological major and trace element and isotope geochemical data identify crustal domains of contrasted petrological origins and histories in the lower crust of the dharwar craton all affected by granulite facies metamorphism and deformation in the neoarchean four lower crustal domains have been identified and investigated across the neoarchean amphibolite to granulite facies transition zone 1 in the southern part of the western dharwar craton wdc high grade metamorphism affects rocks formed before 3 0 ga high grade metamorphism is recorded at ca 2 51 ga followed by possible complex cooling and resetting until 2 2 ga 2 the br hills mm hills domain represents the lower crust of the central dharwar province cdp which is here defined as a transitional crust between the old 3 ga wdc and the eastern dharwar province edp not older than 2 7 ga this granulitic domain contains the oldest rocks of the dharwar craton with ttg suites formed at ca 3 4 ga and ca 2 6 ga suites of probable crustal recycling origin both suites are involved in 2513 5ma granulite facies metamorphism 3 the nilgiri granulitic domain is tectonically juxtaposed with the deepest level of the wdc as a consequence of neoarchean dextral slip along the moyar shear zone the massif consists of rocks of orogenic affinities formed between 2 70 and 2 56 ga and overprinted by granulite facies metamorphism at 2518 10 ma 4 the krishnagiri domain exposes the deepest levels of the eastern dharwar craton it is composed of ttg and calc alkaline igneous protoliths formed between 2 55 and 2 53 ga migmatization in the upper part of the domain took place at 2507 6ma and granulite facies overprint in the deepest parts of the domain is constrained at 2516 5 ma the krishnagiri domain was magmatically accreted to the lower crust of both the cdp and edp by interaction with mafic magmatism inducing reworking of the 2 7 2 6 ga or older overlying crust the nilgiri granulites share several temporal and petrological properties with the madras granulites that make the easternmost outcrop of the dharwar craton we tentatively suggest that the nilgiri and madras granulites evolved as parts of a common madras domain in the lower crust of the edp before activation of the moyar shear zone in this sense the nilgiri massif may be considered as an allochtonous unit in a context of neoarchean ultra hot orogeny in south india

the nature of the crust in southern india implications for precambrian crustal evolution 1 we present crustal thickness and poisson s ratio determinations from receiver function analyzes at 32 sites on the archaean and proterozoic terrains of south india the crustal thickness in the late archaean 2 5 ga eastern dharwar craton varies from 34 39 km similar crustal thickness is observed beneath the deccan volcanic province and the cuddapah basin the most unexpected result is the anomalous present day crustal thickness of 42 51 km beneath the mid archaean 3 4 3 0 ga segment of the western dharwar craton since the amphibolite grade metamorphic mineral assemblages 5 7 kbar paleopressures in this part of western dharwar craton equilibrated at the depths of 15 20 km our observations suggest the existence of an exceptionally thick 57 70 km crust 3 0 ga ago beneath the exhumed granulite terrain in southernmost india the crustal thickness varies between 42 60 km the poisson s ratio ranges between 0 24 0 28 beneath the precambrian terrains indicating the presence of intermediate rock type in the lower crust these observations of thickened crust suggest significant crustal shortening in south india during the archaean

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

petrogenesis and geodynamics of late archean magmatism in eastern hebei eastern north china craton geochronological geochemical and nd hf isotopic evidence abstract the eastern part of the north china craton contains large volumes of late archean hornblenditic tonalitic dioritic granodioritic and granitic rocks la icpms zircon u pb dating of three plutons from eastern hebei eastern north china craton record emplacement ages of 2526 2515 ma and metamorphic ages of 2500 2490 ma for the diorite granodiorite and monzogranite and 2440 ma for the k feldspar granite hornblendites have high mgo 8 6 10 6 wt cr 674 1126 ppm ni 125 159 ppm and v 224 315 ppm at low basic silica contents sio2 45 7 49 0 wt they are enriched in light rare earth elements lrees and large ion lithophile elements liles and depleted in high field strength elements hfses with positive nd t values 1 2 to 2 0 indicating they were derived from an enriched mantle source the monzogranites have high sr and ba contents and sr y and la yb ratios low y and hrees abundances and positive nd t and zircon hf t values with nd and hf model ages less than 3 0 ga reflecting that they were derived from partial melting of a juvenile lower crust at high pressure however the k feldspar granites with low sr and ba concentrations and high hree and y contents are considered to have originated from partial melting of juvenile crustal materials at shallow crustal levels the diorites and granodiorites have variable major and trace element concentrations magmatic zircons from these rocks have heterogeneous hf t values but record uniform homogeneous u pb ages indicating they resulted from mixing of mafic and felsic magmas coupled with fractional crystallization because of the widespread distribution of late archean magmatism and evidence for magma mixing possibly related to mafic magma underplating it appears likely that this event in the eastern north china craton may relfect mantle plume activity that resulted in significant growth of continental crust during the late archean

3 35ga komatiite volcanism in the western dharwar craton southern india constraints from nd isotopes and whole rock geochemistry abstract we present field petrographic sm nd whole rock isochron and whole rock geochemical data for komatiites from sargur group greenstone belts of the western dharwar craton field evidence such as pillow structure indicates their eruption in a marine environment petrographic data reveal that the igneous mineralogy has been altered during post magmatic hydrothermal alteration processes corresponding to greenschist to lower amphibolite facies conditions with rarely preserved primary olivine and orthopyroxene a 16 point sm nd whole rock isochron gives an age of 3352 110 ma for the timing of eruption of komatiite lavas about 60 of the studied komatiite samples show al depletion whilst the remaining are al undepleted the al depleted komatiites are characterised by high cao al2o3 ratios 1 01 1 34 and low al2o3 tio2 5 16 whereas al undepleted komatiites show lower cao al2o3 ratios 0 59 0 99 and higher al2o3 tio2 17 26 trace element distribution patterns of komatiites suggest that most of the primary geochemical and nd isotopic compositions are preserved with only minor influence of post magmatic alteration processes and negligible crustal contamination the chemical characteristics of al depleted komatiites such as high gd yb n together with lower hree y zr and hf imply their derivation from deeper upper mantle with garnet majorite involvement whereas lower gd yb n slightly higher hree y zr and hf suggest derivation from shallower upper mantle without garnet involvement the observed chemical characteristics cao al2o3 al2o3 tio2 mgo ni cr nb zr y hf ree indicate derivation of the komatiite magmas from different depths in a plume setting whereas sub contemporaneous felsic volcanism and ttg accretion can be attributed to an arc setting in order to explain the spatial association of komatiite volcanism with contemporaneous mafic felsic volcanism and ttg accretion we propose a combined plume arc setting nd isotope data of the studied komatiites indicate depleted mantle reservoirs which may have evolved by early 4 53 ga global differentiation of the silicate earth as suggested by boyet and carlson boyet m carlson r w 2005 142nd evidence for early 4 53 ga global differentiation of silicate earth science 309 577 581 or extraction of continental crust during the early archaean

continental growth and secular evolution constraints from u pb ages and hf isotope of detrital zircons in proterozoic jixian sedimentary section 1 8 0 8 ga north china craton a b s t r a c t in order to understand the precambrian crustal growth and reworking history of the north china craton in situ u pb and lu hf isotope analyses were carried out on the detrital zircons from a well preserved nearly undeformed proterozoic sedimentary section with time interval of 1 8 0 8 ga totally 806 zircons separated from 7 samples collected from each representative stratigraphic unit were measured for upb and lu hf isotopic compositions of which 430 grains yielded concordant u pb ages the concordant zircons show three prominent age groups of 1 6 1 8 ga 1 9 2 2 ga and 2 3 2 6 ga both positive and negative ehf t values were exhibited by zircons from all groups suggesting both juvenile crust addition and crustal reworking for all age groups the hf crustal model ages of zircons reveal that the juvenile materials were continuously added to the crust between 4 0 and 2 0 ga with a prominent peak at 2 7 3 0 ga and a less significant peak at 3 1 3 4 ga the juvenile contributions to the newly formed crust decrease stepwise through time and crustal reworking has dominated over juvenile inputs to the continental crust since 2 6 ga compared with the detrital zircons from modern rivers running within the north china craton the sedimentary rocks contain more older zircons implying the relatively poor preservation of archean crustal rocks in modern river sand and accordingly the weights of archean crystallizations ages and crustal growth inferred from detrital zircons in modern river are significantly underestimated the integrated studies of detrital zircons from both modern river sands and ancient sedimentary rocks are essential to decipher the early crustal growth history more faithfully

geochronology and geochemistry of precambrian granitic rocks of goa sw india abstract the precambrian rock succession in goa representing the nw extremity of the south indian precambrian craton includes 1 a low grade supracrustal sequence known as the goa group and 2 several acid granitic and gneissic bodies occurring in close spatial association with the supracrustals the anmod ghat trondhjemitic gneisses of ne goa which form the basement to the goa group yield a rb sr whole rock isochron age of 3400 140 ma the pervasive late archaean acid plutonism post dates evolution of the goa group supracrustal sequence and spans syn to post tectonic regimes with respect to the last major penetrative folding in the segment the rb sr whole rock ages of the granitic plutons and migmatites chandranath granodiorite granite 2650 100 ma dudhsagar granite 2565 95 ma londa migmatites 2650 130 ma canacona graniteporphyry 2395 390 ma suggest a minimum age of the goa group of c 2 6 ga and confirm the correlation of the goa group with the chitradurga group which occurs to the south of the craton geochemical data presented bring out essential differences between the c 3 4 ga old basement trondhjemitic gneisses and the younger group of granites c 2 6 2 4 ga the early archaean trondhjemitic gneisses show low rb sr 0 20 th u 1 45 1 93 highsr ba 0 5 low ree 24 14 52 81 with a slightly fractionated ree pattern moderately depleted in hree cen ybn 2 1 6 5 compared with the early archaean gneisses the younger granites are distinctly potassic with higher rb sr 0 50 0 87 th u 4 05 14 91 lower sr ba 0 22 0 43 a lree enriched pattern cen ybn 15 1 26 6 and much higher ree 132 06 180 84

sm nd isotopic mapping of lithospheric growth and stabilization in the eastern kaapvaal craton whole rock sm nd isotope systematics of 79 archean granitoids from the eastern kaapvaal craton southern africa are used to delineate lithospheric boundaries and to constrain the timescale of crustal growth assembly and geochemical differentiation c 3 66 2 70 ga offsets in end values for 3 2 3 3 ga granitoids across the barberton greenstone belt bgb are consistent with existing models for c 3 23 ga accretion of newly formed lithosphere north of the bgb onto pre existing c 3 66 ga lithosphere south of the bgb along a doubly verging subduction margin the nd isotopic signature of c 3 3 3 2 ga magmatic rocks show that significant crustal growth occurred during subduction accretion after c 3 2 ga however the nd signature of intrusive rocks c 3 1 and 2 7 ga is dominated by intracrustal recycling rather than by new additions from the mantle signalling cratonic stability

u pb ages for zircon and titanite from the ramagiri area southern india evidence for accretionary origin of the eastern dharwar craton during the late archean abstract the north south trending ramagiri schist belt is trident shaped with three prongs separating three granitic terranes whereas the western and eastern prongs contain metatholeiites and banded ferruginous quartzite the central prong includes felsic metavolcanics and volcanogenic metasediments the u pb zircon age for the pyroclastics in the central prong of the ramagiri schist belt is documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 2707 pm 18 end document ma considered as the time of emplacement of the felsic volcan

sedimentology of quartz pebble conglomerates and quartzites of the archean bababudan group dharwar craton south india evidence for early crustal stability the 3200 to 3000 ma bababudan group locally as thick as 1800 m consists of a variety of rocks indicating deposition on a stable platform u and au bearing pyritiferous quartz pebble conglomerate a few meters thick overlies a regolith at the basal unconformity with a gneissic basement the bulk of the column consists of mafic volcanics including subaerial flows lahars and sills ultramafic and felsic rocks iron formation and pelites are minor lithologies in most of the sequence but iron formation forms a major unit at the top of the group quartzite units intercalated with the volcanics are generally less than 40 m thick and make up 25 to 50 of the column they are mineralogically mature and texturally submature recrystallization generally obscures original textures but units that were originally clayey quartz arenites have survived total recrystallization rounded zircons dominate the nonopaque detrital heavy mineral suite verifying a history of extensive abrasion paleocurrent patterns bas