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

building the wutai arc insights into the archean paleoproterozoic crustal evolution of the north china craton abstract the wutai complex is located within the trans north china orogen tnco which marks the collision zone between the eastern and western blocks of the north china craton ncc here we investigate the timings and mechanisms of precambrian arc building in wutai from a suite of newly identified greenstone felsic tuff banded iron formation bif phyllite sequence zircon grains from the felsic tuff layer intercalated within the greenstones provide robust constraints on the timing of bif deposition geochemical features of the greenstones and felsic tuff suggest e morb and volcanic arc affinity whereas the bifs are of submarine hydrothermal origin zircon grains in the greenstones define an upper intercept age of 2951 41 ma and a lower intercept age of 1845 190 ma those from the felsic tuff show 207pb 206pb weighted mean age of 2557 10 ma whereas those from the bif intercalated with felsic tuff shows an age of 2585 14 ma metamorphic zircon grains in the bif define two age populations of ca 2471 and 1906 ma zircon grains in the phyllite yield an age of 2594 14 ma kernel density plots define age peaks at 2873 and 2578 ma marking the major phase of arc building in wutai zircon ehf t values are mostly positive in the range of 3 15 7 72 suggesting new crustal growth from juvenile depleted mantle components our study provides insights into the mesoarchean to neoarchean crust building through subduction related arc magmatism followed by paleoproterozoic metamorphism in the north china craton

an exotic mesoarchean microcontinent the coorg block southern india abstract sandwiched between the dharwar craton in the north and the neoarchean proterozoic crustal blocks to the south the coorg block in southern india is composed dominantly of a suite of arc magmatic rocks including charnockites ttg tonalite trondhjemite granodiorite related granitoid suite and felsic volcanic tuffs together with minor accreted oceanic remnants along the periphery of the block coeval mafic and felsic magmatism with magma mixing and mingling in an arc setting is well represented in the block here we present the petrology geochemistry zircon u pb geochronology and lu hf isotopes of all the major lithologies from this block computation of metamorphic p t conditions from mineral chemical data shows consistent granulite facies p t conditions of 820 870 c and up to 6 kbar our geochemical data from major trace and ree on representative samples of the dominant rock types from the coorg block corroborate an arc related signature with magma generation in a convergent margin setting the zircon data yield weighted mean 207pb 206pb ages of 3153 4 9 to 3184 0 5 5 ma for syenogranites 3170 3 6 8 ma for biotite granite 3275 5 1 ma for trondhjemite 3133 12 to 3163 8 6 9 ma for charnockites 3156 10 to 3158 3 8 2 for mafic enclaves 3161 16 ma for diorite and 3173 16 ma for felsic volcanic tuff an upper intercept age of 3363 59 ma and a lower intercept age of 2896 130 ma on zircons from a charnockite as well as an evaluation of the th u values of the zircon domains against respective 207pb 206pb ages suggest that the mesoarchean magma emplacement which probably ranged from 3 3 to 3 1 ga was immediately followed by metamorphism at ca 3 0 to 2 9 ga the ages of magmatic zircons from the charnockites and their mafic granulite enclaves as well as those from the volcanic tuff and biotite granite are all remarkably consistent and concordant marking ca 3 1 ga as the peak of subduction related crust building in this block within the tectonic milieu of an active convergent margin the majority of zircons from the coorg rocks show hf isotope features typical of crystallization from magmas derived from juvenile sources their hf crustal model ages suggest that the crust building might have also involved partial recycling of basement rocks as old as ca 3 8 ga the crustal blocks in the southern granulite terrane in india preserve strong imprints of major tectonothermal events at 2 5 ga 2 0 ga 0 8 ga and 0 55 ga associated with various subduction accretion collision or rifting events however the coorg block is exceptional with our data suggesting that none of the above events affected this block importantly there is also no record in the coorg block for the 2 5 ga pervasive regional metamorphism that affected all the other blocks in this region the geochronological data raise the intriguing possibility that this block is an exotic entity within the dominantly neoarchean collage in the northern domain of the southern granulite terrane of india the mesoarchean arc related rocks in the coorg block suggest that the magma factories and their tectonic architecture in the early earth were not markedly different from those associated with the modern style plate tectonics

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

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

coupled u pb and rb sr laser ablation geochronology trace archean to proterozoic crustal evolution in the dharwar craton india abstract the western dharwar craton wdc and the karwar block kb in southern peninsular india provide important windows to earth s early crustal evolution and continental growth as these domains likely represent fragments of the first landmass or supercontinent on our planet here we employ for the first time a coupled laser ablation zircon u pb dating and in situ rb sr dating of micas biotite muscovite and feldspars to evaluate the meso to neoarchean crustal evolution and proterozoic magmatism metamorphism in the karwar block this novel approach based on integrated geochronological results show that the wdc and kb experienced i a major mesoarchean continental growth event c 3 2 to 2 9 ga ii younger neoarchean and paleoproterozoic magmatism migmatization syn and post tectonic granite formation events at c 2 8 2 6 ga and 2 2 ga respectively and iii mesoproterozoic metamorphism thermal imprint as young as 1 2 ga the zircon ehf t values exhibit a range from 0 4 to 9 8 in amphibolite hornblende gneiss and trondhjemite 3 28 3 08 ga as well as the 6 1 to 9 7 range for 2 96 ga old zircons in porphyritic granite which suggests juvenile source for the magmatism that constructed this crustal block overall this study illustrates the benefits of using coupled zircon u pb and in situ rb sr mica and feldspar dating approaches as the former u pb provides robust constraints or a baseline age for initial crystallization of crustal sequences whereas the latter rb sr is sensitive to younger magmatic and metamorphic events thus the integration of these two laser ablation icp ms based geochronometers applied to crustal evolution studies has the potential to provide more comprehensive insights into continental growth in the early earth and later magmatic and metamorphic history of the region

neoarchean continental growth through arc magmatism in the nilgiri block southern india the formation and growth of continental crust in the archean have been evaluated through models of subduction accretion and mantle plume the nilgiri block in southern india exposes exhumed neoarchean lower crust uplifted to heights of 2500 m above sea level along the north western margin of the peninsula major lithologies in this block include charnockite with or without garnet anorthosite gabbro suite pyroxenite amphibolite and hornblende biotite gneiss ttg all these rock types are closely associated as an arc magmatic suite with diffuse boundaries and coeval nature the charnockite and hornblende biotite gneisses ttg show sio2 content varying from 64 to 73 wt the hornblende biotite gneisses ttg are high al type with al2o3 15 wt whereas the charnockites show al2o3 15 wt the composition of charnockite is mainly magnesian and calcic to calc alkaline the mafic ultramafic rocks show composition close to that of tholeiitic series the low values of k 2 o 3 wt k rb k2o 500 zr ti and trace element ratios like la yb n sr y y nb y nb rb y ta rb yb ta indicate a volcanic arc signature for these rocks the geochemical signature is consistent with arc magmatic rocks generated through oceanic plate subduction the primitive mantle normalized trace element patterns of these rocks display enrichment in large ion lithophile elements lile and comparable high field strength elements hfse in charnockite and hornblende biotite gneisses ttg consistent with subduction related origin primitive mantle normalized ree pattern displays an enrichment in lree in the chamockite and hornblende biotite gneisses ttg as compared to a flat pattern for the mafic rocks the chondrite normalized ree patterns of zircons of all the rock types reveal cores with high hree formed at ca 2700 ma and rims with low hree formed at 2500 2450 ma log transformed la th nb th sm th yb th discrimination diagram for the mafic and ultramafic rocks from nilgiri displays a transition from mid oceanic ridge basalt morb to island arc basalt iab suggesting a morb source the u pb zircon data from the charnockites mafic granulites and hornblende biotite gneisses ttg presented in our study show that the magma generation during subduction and accretion events in this block occurred at 2700 2500 ma together with the recent report on neoarchean supra subduction zone ophiolite suite at its southern margin the nilgiri block provides one of the best examples for continental growth through vertical stacking and lateral accretion in a subduction environment during the neoarchean c 2014 elsevier b v all rights reserved

melt inclusions in pegmatite quartz complete miscibility between silicate melts and hydrous fluids at low pressure fluorine boron and phosphorus rich pegmatites of the variscan ehrenfriedersdorf complex crystallized over a temperature range from about 700 to 500 c at a pressure of about 1 kbar pegmatite quartz crystals continuously trapped two different types of melt inclusions during cooling and growth a silicate rich h2o poor melt and a silicate poor h2o rich melt both melts were simultaneously trapped on the solvus boundaries of the silicate fluorine boron phosphorus water system the partially crystallized melt inclusions were rehomogenized at 1 kbar between 500 and 712 c in steps of 50 c by conventional rapid quench hydrothermal experiments glasses of completely rehomogenized inclusions were analyzed for h2o by raman spectroscopy and for major and some trace elements by emp electron microprobe both types of melt inclusions define a solvus boundary in an xh2o t pseudobinary system at 500 c the silicate rich melt contains about 2 5 wt h2o and the conjugate water rich melt about 47 wt h2o the solvus closes rapidly with increasing temperature at 650 c the water contents are about 10 and 32 wt respectively complete miscibility is attained at the critical point 712 c and 21 5 wt h2o many pegmatites show high concentrations of f b and p this is particularly true for those pegmatites associated with highly evolved peraluminous granites the presence of these elements dramatically reduces the critical pressure for fluid melt systems at shallow intrusion levels at t 720 c water is infinitely soluble in a f b and p rich melt simple cooling induces a separation into two coexisting melts accompanied with strong element fractionation on the water rich side of the solvus very volatile rich melts are produced that have vastly different physical properties as compared to normal silicate melts the density viscosity diffusivity and mobility of such hyper aqueous melts under these conditions are more comparable to an aqueous fluid

boiling colloid nucleation and aggregation and the genesis of bonanza au ag ores of the sleeper deposit nevada a deep parent composition for bonanza oreforming fluids at the sleeper deposit was calculated by the computer program solveq using fluid inclusion microthermometric and gas data and by assuming equilibrium with the following minerals present in vein samples below the bonanza zones gold chalcedony adularia pyrite chalcopyrite and acanthite the calculated dissolved gold content of 295 ppb is approximately 2 orders of magnitude higher than that assumed for typical geothermal systems thus a gold enriched fluid appears to have been a principal factor in the genesis of bonanza au ag ores at the sleeper deposit geochemical modelling of possible ore forming processes using the computer program chiller with the reconstructed ore forming solution as a starting composition indicates that boiling most closely reproduces observed minerals and their relative abundances in bonanza ores the constraint imposed by the association of amorphous silica with gold precludes all of the mixing scenarios modelled such as mixing with cold and steam heated groundwaters acid sulfate co2 rich modelling indicates that boiling of a gold rich deep solution leads to rapid gold precipitation and that the amount of gold precipitated initially is large relative to other minerals these factors apparently led to nucleation of colloidal gold particles instead of in situ gold deposition or coprecipitation with other phases gold colloids apparently were entrained in the upward flowing hydrothermal solutions and grew as they travelled upon reaching a critical size 10 100 nm they were deposited due to orthokinetic aggregation at an elevation and temperature at which amorphous silica was nucleating and aggregating

paleoproterozoic basement beneath the eastern cathaysia block revealed by zircon xenocrysts from late mesozoic volcanics abstract the eastern cathaysia block located to the western side of the pacific plate was mainly covered by late mesozoic intrusive and volcanic rocks and the age and composition of its basement remain enigmatic here we firstly report u pb and hf isotopes in zircon xenocrysts from late mesozoic mafic sub volcanic rocks in pingtan island the east coastal area of the eastern cathaysia block with surprising paleoproterozoic to archean u pb isotopic ages neither ca 1 8 ga nor 1 0 0 7 ga age peaks that are very typical in the outcropping rocks including the neoproterozoic sedimentary rocks in pingtan island in the south china block is identified the two main precambrian age peaks of 3 4 3 0 ga and 2 6 2 2 ga of these zircon xenocrysts are explained by contamination from the sediments in the unexposed basement the youngest two paleoproterozoic zircon xenocrysts may indicate the deposition of the basement close to ca 1 8 ga the identification provides clues to the existence of the paleoproterozoic basement in the eastern cathaysia block it may further suggest that the late mesozoic continental arc magmatism in the eastern cathaysia block was most likely developed on and contaminated by the paleoproterozoic basement

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

neoarchean paleoproterozoic crustal growth and tectonic evolution of the trans north china orogen north china craton evidence from granite greenstone successions in the dengfeng complex granite greenstone successions and associated tonalite trondhjemite granodiorite ttg suites constitute dominant components of precambrian cratons here we present petrology geochemistry zircon u pb geochronology and lu hf isotopes on monzonite granodiorite granite and amphibolite from the dengfeng complex in the southern part of the north china craton ncc zircon u pb data yield weighted mean 207pb 206pb ages of 2498 6 ma for monzonite 2481 16 ma for granodiorite and 2475 21 ma for granite lu hf isotope systematics show ehf t values ranging from 1 3 to 6 7 and crustal model ages t text dm text c in the range of 2584 2934 ma for the rocks the monzonite granodiorite and granite show moderate total rare earth element ree contents enrichment in light rees lree and depletion in heavy rees hree negative nb ta and ti anomalies high al2o3 sr y and la yb n ratios with low cr and ni indicating that the magma was possibly sourced from partial melting of the lower crust the geochemical features of amphibolites are characterized by low sio2 na2o k2o sr y la yb n and high feo low total rees values limited lree hree fractionation and absence of obvious ce anomaly showing similarity to island arc magmatic rocks our results are consistent with the model of subduction and arc continent collision in the trans north china orogen during the neoarchean to paleoproterozoic

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

implications from inclusions in topaz for greisenisation and mineralisation in the hensbarrow topaz granite cornwall england textural and geochemical studies of inclusions in topaz from greisens in the hensbarrow topaz granite stock st austell cornwall are used to constrain the composition of fluids responsible for late stage greisening and mineralisation the topaz contains an abundant and varied suite of inclusions including aqueous liquid vapour l v quartz zinnwaldite albite k feldspar muscovite ilmenorutile apatite columbite zircon varlamoffite sn fe o oh 2 and qitianlingite fe 2 mn 2 2 nb ta 2w 6o10 primary l v inclusions in topaz show relatively high th mainly 300 to 500 c and a narrow range of salinities 23 30 wt nacl equivalent compared with those in greisen quartz 150 450 c 0 50 wt nacl equivalent textures indicate that topaz formed earlier than quartz and the fluid inclusion data are interpreted as indicating a cooling of the hydrothermal fluids during greisenisation mixing with meteoric waters and a decrease in pressure causing intermittent boiling the presence of early formed albite and k feldspar as inclusions in the topaz is likely to indicate that the greisen forming fluid became progressively more acid during greisenisation the most distinctive inclusions in the topaz are wisp and bleb shaped quartz 50 m in size which show textural characteristics indicating former high degrees of plasticity they often have multiple shrinkage bubbles at their margins rich in sn fe mn s and cl and more rarely contain euhedral albite k feldspar stannite or pyrrhotite crystals up to 40 m in size the quartz inclusions show similar morphologies to inclusions in topaz from quartz topaz rocks elsewhere which have been interpreted as trapped silicate melt their compositions are however very different to those expected for late stage topaz normative granitic melts from their textural and chemical characteristics they are interpreted as representing crystallised silica colloid probably trapped as a hydro gel during greisenisation there is also evidence for the colloidal origin of inclusions of varlamoffite in the topaz these occurrences offer the first reported evidence in natural systems for the formation of colloids in high temperature hydrothermal fluids their high ore carrying potential is suggested by the presence of varlamoffite and the occurrence of stannite pyrrhotite and sncl within the quartz inclusions

bimodal magmatism in the eastern dharwar craton southern india implications for neoarchean crustal evolution abstract archean cratons provide windows to the crustal evolution history in the early earth the dharwar craton is one of the major cratonic blocks in peninsular india and is composed of several ancient microcontinents where multiple continental growth and recycling occurred from 3 8 to 2 5 ga here we investigate a suite of magmatic rocks including granitoids mafic magmatic enclaves ultramafic inclusions and synplutonic mafic dyke from the western margin of the eastern dharwar craton which provide insights on bimodal magmatism in a subduction related arc setting and crustal growth during neoarchean field evidence and geochemical features suggest mixing and mingling of mafic and felsic magmas geochemical features including incompatible and high field strength element features of the enclaves indicate heterogeneous sources involving dominantly enriched mantle reservoirs with minor depleted mantle whereas the host granitoids display depleted to enriched source we present zircon u pb age data from the various rock types and the combined results display unimodal distribution with a single peak of 207pb 206pb age at 2531 ma n 159 suggesting a major neoarchean bimodal magmatic pulse the zircon u pb ages of the mafic enclaves indicate either similar ages or slightly younger ages as compared to their host granitoids implying that enclaves represent late stage injections into crystallizing host granitoid magmas the trace and rare earth element data on zircon from these rocks are consistent with magmatic crystallization zircons from most of the rocks in our study display uniformly positive ehf t 0 3 8 6 suggesting depleted mantle juvenile source for the neoarchean arc building and formation of new crust some of the mafic magmatic and ultramafic enclaves as well as the mafic dyke and charnockite display mixed positive and negative ehf t values 7 0 to 3 8 implying mixed depleted mantle and reworked crustal components of neoarchean to mesoarchean age most of the hf depleted mantle model ages tdm 2520 3115 ma and crustal residence ages tdmc 2507 ma to 3452 ma fall within the region bounding chur and depleted mantle lines suggesting that the dominant magma source involved juvenile depleted mantle components thus latest neoarchean marks a major phase of continent building in the eastern dharwar craton and the results presented in our study suggest that bimodal magmatism in a subduction related setting identical to those in modern convergent margins contributed to crustal growth

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

oldest rocks from peninsular india evidence for hadean to neoarchean crustal evolution abstract the evolution of continental crust during hadean and archean and related geodynamic processes provides important clues to understand the early earth history here we report evidence for hadean and eoarchean crust from the fringe of the coorg block one of the oldest crustal blocks in peninsular india we present geological petrological and geochemical data together with zircon u pb ages and lu hf isotopes from a suite of metaigneous granitoids diorite charnockite metavolcanics and metasedimentary quartz mica schist calcareous schist ferruginous quartzite and bif rocks petrological and geochemical studies indicate that the igneous suite formed from subduction related arc magmatism and that the sedimentary suite represents an imbricated accretionary package of continental shelf sequence and pelagic components mineral thermometry suggests metamorphism under temperatures of 710 730 c and pressures up to 8 kbar magmatic zircons in the metaigneous suite show oscillatory zoning and high th u contents up to 3 72 and record multiple pulses of magmatism at ca 3 5 3 2 2 7 and 2 5 2 4 ga the metasedimentary rocks accreted along the margins of the coorg block show multiple zircon population with mean 207pb 206pb ages at 3 4 3 2 3 1 2 9 2 7 2 6 2 5 2 2 2 0 and 1 3 ga and overprinted by younger thermal event at ca 0 8 0 7 ga zircons in the 3 5 ga dioritic gneiss show positive ehf t values ranging from 0 0 to 4 2 and hf crustal model ages tdmc of 3517 to 3658 ma suggesting that the parent magma was derived from eoarchean juvenile sources the zircons in the 3 2 ga charnockite display ehf t values in the range of 3 0 to 2 9 and hf crustal model ages tdmc of 3345 to 3699 ma the neoarchean metagranites diorites and felsic tuff show both positive and negative ehf t values and a range of tdmc values from 2904 to 3609 ma suggesting magma derivation from meso to eoarchean juvenile and reworked components the tdmc values of the dominant zircon population in the metasedimentary suite range from 3126 to 3786 ma with the oldest value 4031 ma recorded by zircon grain in a ferruginous quartzite the ehf t values of detrital zircons also show both positive and negative values with a dominant crustal source our zircon u pb and lu hf data suggest vestiges of neohadean primordial continental crust in peninsular india with episodic crustal growth during eoarchean mesoarchean and neoarchean building the continental nuclei the results contribute to the understanding of crustal evolution in the early history of the earth

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

colloidal gold and silica in mesothermal vein systems some of the textural features of mesothermal gold quartz veins may be best explained by the initial precipitation of amorphous silica gel colloid which subsequently crystallizes to quartz this can occur in brittle ductile shear zones where a significant fluid pressure drop occurs during stick slip failure such a process rapidly supersaturates the hydrothermal fluid with respect to amorphous silica which precipitates instead of quartz owing to favorable kinetics depressurization also commonly leads to fluid unmixing and destabilization of soluble gold complexes however the presence of colloidal silica can stabilize gold colloid allowing further transport of particulate gold in suspension in the hydrothermal fluid silica gel would be highly unstable under mesothermal conditions and would undergo rapid syneresis and crystallization to form quartz solid impurities would tend to be expelled toward grain boundaries this model can account for the primary anhedral aggregate textures typical of mesothermal quartz veins the concentration of gold along grain boundaries and the formation of discrete gold nuggets and the rare occurrence of low order silica polymorphs and relict spheroidal structures the transport of gold in colloidal form may be one reason for the frequently consistent bulk grade distribution in gold quartz vein systems over many hundreds of metres in some cases kilometres of depth in addition the formation of charged colloidal particles may help to explain the attraction of gold grains to specific mineral surfaces

preserved and modified mid archean crustal blocks in dharwar craton seismological evidence abstract we report significant lateral variability in shear wave velocity and moho depth in the archean crust beneath the dharwar craton using earthquake waveform data recorded over 50 broadband seismographs the craton is a continuously exposed archean continental fragment divided into the west dharwar craton wdc of age 2 7 3 36 ga and the east dharwar craton edc of age dominantly 2 5 ga the craton progressively transitions into the southern granulite terrain sgt with age of metamorphism around 2 6 ga the inversion and modeling of receiver function data reveal significant variation of moho depth viz 38 54 km in the wdc and 40 46 km in the sgt and 32 38 km in the edc the average shear wave velocity vs of crust beneath the wdc is 3 85 km s as compared to 3 6 km s in the edc we infer highly variable thickness 16 30 km of mafic cumulate vs 4 0 km s and vp 7 0 km s beneath the wdc in contrast with a thin one 50 km as compared to felsic to intermediate composition for the edc crust with almost flat moho down to 38 km these results suggest preserved mafic crustal root beneath the middle archean terrain in the wdc that has remained inert since then on the other hand felsic to intermediate composition of crust with a nearly flat moho beneath the late archean edc could be a consequence of regional delamination of lower crust the preserved distinct moho topography across the archean terrains suggests it as compositional boundary considering the surface exposure of 15 20 km crust based on p t condition in the granulite segment of the wdc we speculate a himalaya like crustal thickness 50 70 km beneath the middle archean crust pointing toward a plate tectonic like scenario at 3 0 ga

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

spherulites and lithophysae 200 years of investigation on high temperature crystallization domains in silica rich volcanic rocks high temperature crystallization domains htcds including spherulites and lithophysae form during cooling of silica rich lava and welded ignimbrites spherulites grow in silicate melts or hot glass and they display a radiating or microcrystalline texture typically consisting of cristobalite tridymite and sanidine lithophysae are htcds comprising one or more cavities this contribution reviews the research and discussions on htcds carried out over the last 200 years the emphasis here is on lithophysae and summarizes current knowledge of their formation a number of parameters influence the initiation and growth of lithophysae as well as their shapes and internal textures the most likely cause of cavity formation is transient tensional stress that produces a mechanical opening and widening at the interface between the crystallization front and the host melt e g where t tg cavity growth and expansion forced by rising vapor pressure is considered less important in some cases further growth of htcd cavities results from vapor phase corrosion and brecciation

an overview of the palaeoproterozoic geology of peninsular india and key stratigraphic and tectonic issues abstract the archaean proterozoic rock successions in india have the potential to enrich the global database on precambrian stratigraphic development and to offer valuable clues to global tectonic reconstructions built over four distinct archaean cratonic nuclei the major palaeoproterozoic supracrustal belts cover sequences include the dhanjori group the singhbhum group including the dalma volcanics and the chandil formation in eastern india the cuddapah supergroup and the adjoining nellore schist belt bordering the eastern dharwar craton the aravalli supergroup in the aravalli delhi fold belt in nw india the lower vindhyan semri group and the mahakosal sausar betul belts close to the central indian tectonic zone citz and possibly the dongargarh supergroup in bastar a major erosional unconformity separates the archaean tonalite trondjhemite gneiss basement in these cratons from the overlying volcano sedimentary successions an overview of palaeoproterozoic stratigraphic development in these cratonic blocks is presented to bring out the salient features for global comparison and to highlight issues requiring further attention multiple metamorphic magmatic and deformation events are recorded in the fold belts at the join of the cratons or their margins inviting application of the plate tectonic paradigm however a comprehensive tectonic model for the amalgamation of the indian archaean nuclei is yet to emerge and is crucial for our understanding of palaeoproterozoic supercontinent development

highly differentiated juvenile crust derived magmas linked with the xilekuduke porphyry mo cu deposit in east junggar nw china abstract the east junggar region links the kazakhstan junggar plate with the siberian craton along the western part of the central asian orogenic belt among the major ore deposits in this region the xilekuduke molybdenite deposit is a unique example of mo dominated porphyry molybdenum and copper mineralization here occur as disseminated sulfides or veinlets mainly in granite porphyry and monzogranite with a prominent alteration from an inner potassic through sericitic to an outer propylitic zone zircon la icp ms u pb ages indicate that the granite porphyry and monzogranite were emplaced at 326 4 2 ma and 328 2 1 8 ma respectively both rocks show enrichment in light rare earth elements lree and rb k sr hf zr but depletion in nb ta ti and p suggesting magma generation in a post collisional setting a relatively high oxidation state is inferred from zircon ce values and oxygen fugacity of parental magma the ore hosted intrusions also show high differentiation index di indicating a highly oxidized parental magma and prolonged differentiation process the range of zircon ehf t values 11 5 to 17 young zircon hf model ages and consistent but slightly low zircon 18o values 4 84 to 5 20 together with the range of mg 35 53 suggest derivation of the parental magma through partial melting of juvenile crust with minor contribution from mantle components and sediments we propose that melting of juvenile oceanic components at depth with high degree of differentiation played a significant role in generating the mo mineralization in xilekuduke deposit within a post collisional setting in the east junggar domain

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

multi stage crustal growth and neoarchean geodynamics in the eastern dharwar craton southern india abstract the dharwar craton is a composite archean cratonic collage that preserves important records of crustal evolution on the early earth here we present results from a multidisciplinary study involving field investigations petrology zircon shrimp u pb geochronology with in situ hf isotope analyses and whole rock geochemistry including nd isotope data on migmatitic ttg tonalite trondhjemite granodiorite gneisses dark grey banded gneisses calc alkaline and anatectic granitoids together with synplutonic mafic dykes along a wide northwest southeast corridor forming a wide time window in the central and eastern blocks of the dharwar craton the dark grey banded gneisses are transitional between ttgs and calc alkaline granitoids and are referred to as transitional ttgs whereas the calc alkaline granitoids show sanukitoid affinity our zircon u pb data together with published results reveal four major periods of crustal growth ca 3360 3200 ma 3000 2960 ma 2700 2600 ma and 2570 2520 ma in this region the first two periods correspond to ttg generation and accretion that is confined to the western part of the corridor whereas widespread 2670 2600 ma transitional ttg together with a major outburst of 2570 2520 ma juvenile calc alkaline magmatism of sanukitoid affinity contributed to peak continental growth the transitional ttgs were preceded by greenstone volcanism between 2746 ma and 2700 ma whereas the calc alkaline magmatism was contemporaneous with 2570 2545 ma felsic volcanism the terminal stage of all four major accretion events was marked by thermal events reflected by amphibolite to granulite facies metamorphism at ca 3200 ma 2960 ma 2620 ma and 2520 ma elemental ratios la yb n sr y nb ta hf sm and hf nd isotope data suggest that the magmatic protoliths of the ttgs emplaced at different time periods formed by melting of thickened oceanic arc crust at different depths with plagioclase amphibole garnet titanite ilmenite in the source residue whereas the elemental ba sr la yb n sr y nb ta hf sm and hf nd isotope data ehf t 0 67 to 5 61 end t 0 52 to 4 23 of the transitional ttgs suggest that their protoliths formed by melting of composite sources involving mantle and overlying arc crust with amphibole garnet clinopyroxene plagioclase ilmenite in the residue the highly incompatible and compatible element contents ree k ba sr mg ni cr together with hf and nd isotope data ehf t 4 5 to 3 2 end t 1 93 to 1 26 of the sanukitoids and synplutonic dykes suggest their derivation from enriched mantle reservoirs with minor crustal contamination field elemental and isotope data ehf t 4 3 to 15 0 end t 0 5 to 7 0 of the anatectic granites suggest their derivation through reworking of ancient as well as newly formed juvenile crust secular increase in incompatible as well as compatible element contents in the transitional ttgs to sanukitoids imply progressive enrichment of neoarchean mantle reservoirs possibly through melting of continent derived detritus in a subduction zone setting resulting in the establishment of a sizable continental mass by 2700 ma which in turn is linked to the evolving earth the neoarchean geodynamic evolution is attributed to westward convergence of hot oceanic lithosphere with continued convergence resulted in the assembly of micro blocks with eventual slab break off leading to asthenosphere upwelling caused extensive mantle melting and hot juvenile magma additions to the crust this led to lateral flow of hot ductile crust and 3d mass distribution and formation of an orogenic plateaux with subdued topography as indicated by strain fabric data and strong seismic reflectivity along an e w crustal profile in the central and eastern blocks of the dharwar craton

silica and gold textures in bonanza ores of the sleeper deposit humboldt county nevada evidence for colloids and implications for epithermal ore forming processes

eoarchean to mesoarchean crustal evolution in the dharwar craton india evidence from detrital zircon u pb and hf isotopes abstract the formation and evolution of continental crust in the early earth are of fundamental importance in understanding the emergence of continents their assembly into supercontinents and evolution of life and environment the dharwar craton in southern india is among the major archean cratons of the world where recent studies have shown that the craton formation involved the assembly of several micro continents during meso to neoarchean through subduction accretion collision processes here we report u pb hf isotope data from detrital zircons in a suite of metasediments including quartz mica schist fuchsite quartzite and metapelite from the southern domain of the chitradurga suture zone that marks the boundary between the western and central dharwar craton morphology and internal structure of the zircon grains suggest that the dominant population was derived from proximal granitic felsic sources zircon u pb data are grouped into paleo mesoarchean and neoarchean to paleoproterozoic with peaks at 3227 ma and 2575 ma the age spectra of detrital zircon grains in combination with the lu hf isotopic analyses indicate sediment provenance from magmatic sources with model ages in the range of ca 3 67 to 2 75 ga a transition from dominantly juvenile to a mixture of juvenile and recycled crustal components indicate progressive crustal maturity the results from this study suggest major crustal growth events during ca 3 2 ga and 2 6 ga in dharwar our study provides insights into continental emergence weathering and detrital input through river drainage systems into the trench during eoarchean to mesoarchean

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

the bastar craton central india a window to archean paleoproterozoic crustal evolution abstract the bastar craton in central india surrounded by cratonic blocks and paleoproterozoic to neoproterozoic orogenic belts is a window to investigate the archean paleoproterozoic crustal evolution and tectonic processes here we propose a new tectonic classification of the craton into the western bastar craton wbc eastern bastar craton ebc and the intervening central bastar orogen cbo we present petrologic geochemical and zircon u pb ree and lu hf data from a suite of rocks from the cbo and along the eastern margin of the wbc including 1 volcanic successions comprising meta andesite and fine grained amphibolite representing arc related volcanics along a convergent margin 2 ferruginous sandstone in association with rhyolite representing a volcano sedimentary succession deposited in an active trench and 3 metamorphosed mafic ultramafic suite including gabbro pyroxenite and dunite invaded by trondhjemite representing the section of sub arc mantle and arc root adjacent to a long lasting subduction system petrologic studies indicate that the mafic ultramafic suite crystallized from an island arc tholeiitic parental magma in a suprasubduction zone environment the chondrite normalized and primitive mantle normalized diagrams of the mafic and ultramafic rocks suggest derivation from morb magma the mixed characters from n morb to e morb of the studied samples are consistent with subduction modification of a morb related magma involving partial melting of the metasomatized mantle wedge our zircon u pb age data suggest that the cratonic nuclei was constructed as early as paleoarchean we present evidence for active subduction and arc magmatism through mesoarchean to neoarchean and early paleoproterozoic with the trench remaining open until at least 2 3 ga two major crust building events are recognized in the bastar craton during mesoarchean recycled paleoarchean subduction related as well as juvenile depleted mantle components and neoarchean accretion of juvenile oceanic crust arc magmatism including granite batholiths and related porphyry mineralization the final cratonization occurred during latest paleoproterozoic followed by collisional assembly of the craton and its incorporation within the peninsular indian mosaic during mesoproterozoic in the global supercontinent context the craton preserves the history of ur the earliest supercontinent followed by the paleo mesoproterozoic columbia as well as minor thermal imprints of the neoproterozoic rodinia and associated grenvillian orogeny

preliminary report on the paleomagnetism of 1 88ga dykes from the bastar and dharwar cratons peninsular india abstract we report the results of a preliminary paleomagnetic study on well dated 1 9 ga dykes from the bastar craton india this suite of nw se trending dykes was linked to similarly aged magmatic activity in the dharwar craton and cuddapah basin in india as part of a large igneous province this igneous activity may have extended across many cratons in the columbia supercontinent including the north china craton laurentia baltica australia siberia and the kaapvaal and zimbabwe cratons southern africa the bastar dykes along with the cuddapah traps and dyke yield a dual polarity magnetization with d 126 i 15 2 k 27 95 11 9 and a corresponding paleomagnetic pole at 31 n 330 e dp 6 3 dm 12 2 the relatively robust paleomagnetic and geochronologic database at 1 85 1 90 ga allow us to test one of the configurations of the supercontinent columbia there are some critical differences between our paleomagnetically based reconstruction and the archetypal and geologically based columbia configuration of zhao et al 2004 most notably siberia india and australia cannot be linked to laurentia in the archetypal columbia fit proposed links between western australia and the kaapvaal and zimbabwe cratons are possible as is the fit between baltica and laurentia the robust paleomagnetic data reported in this paper require that either the columbia supercontinent did not exist at 1 9 ga or requires major modification given that our data provide only a snapshot on the paleoproterozoic we conclude that the columbia supercontinent remains a viable possibility although relationships between individual elements should be re evaluated as more data become available

a late cretaceous felsic magmatic suite from the tengchong block western yunnan integrated geochemical and isotopic investigation and implications for sn mineralization the tengchong block within the sanjiang tethys belt in the southeastern part of the tibetan plateau experienced a widespread intrusion of a felsic magmatic suite of granites in its central domain during late cretaceous times here we investigate the guyong and xiaolonghe plutons from this suite in terms of their petrological geochemical and sr nd zircon u pb and lu hf o isotopic features to gain insights into the evolution of the neo tethys the guyong pluton 76 ma is composed of metaluminous monzogranites and the xiaolonghe pluton 76 ma is composed of metaluminous to peraluminous medium and fine grained syenogranite a systematic decrease in eu ba sr p and ti concentrations a decrease in zr hf and lree hree ratios and an increase in the rb ba and ta nb ratios from the guyong to xiaolonghe plutons suggest fractional crystallization of biotite plagioclase k feldspar apatite ilmenite and titanite they also show the characteristics of i type granites the negative zircon ehf t isotopic values 10 04 to 5 22 and high 18o values 6 69 to 8 58 and the negative whole rock end t isotopic values 9 7 to 10 1 and high initial 87sr 86sr ratios 0 7098 0 7099 of the guyong monzogranite suggest that these rocks were generated by partial melting of the precambrian basement without mantle input the zircon ehf t isotopic values 10 63 to 3 04 and 18o values 6 54 to 8 69 of the xiaolonghe syenogranite are similar to the features of the guyong monzogranite and this similarity suggests a cogenetic nature and magma derivation from the lower crust that is composed of both metasedimentary and meta igneous rocks the xiaolonghe fine grained syenogranite shows an obvious rare earth element tetrad effect and lower nb ta ratios which indicate its productive nature with respect to ore formation in fact we discuss that the sn mineralization in the region was possible due to sn being scavenged from these rocks by exsolved hydrothermal fluids we correlate the late cretaceous magmatism in the central tengchong block with the northward subduction of the neo tethys beneath the burma tengchong block

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

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

colloidal transport of gold and silica in epithermal precious metal systems evidence from the sleeper deposit nevada bonanza vein gold ore from the sleeper deposit nevada was deposited at a depth of less than 500 m and exhibits a characteristic texture of alternating colloform bands of fine grained quartz and bands of opaline silica containing abundant electrum textural evidence indicates that the quartz bands recrystallized from a gel like amorphous precursor whereas the gold rich bands were deposited originally as amorphous precipitate formed by the coagulation of colloidal silica and gold particles gold and silica precipitated initially as colloidal particles at deeper levels in the hydrothermal system and were mechanically transported upward by the hydraulic action of the ore forming fluids coagulation of the colloidal particles may have been caused by solution cooling or boiling and catalyzed by hydrated polycationic al 3 aqueous species transport of gold as colloidal particles in epithermal systems indicates that at least in some instances higher gold content is possible than in true solution a gold enriched ore fluid appears necessary to explain the origin of the bonanza grade ore at the sleeper mine and the colloidal gold transport process locally might have played a role in the genesis of the more common disseminated epithermal precious metal deposits

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

first direct evidence for natural occurrence of colloidal silica in chalcedony hosted vacuoles and implications for ore forming processes different forms of abundant silica e g quartz and chalcedony are closely associated with many types of ore deposits in igneous metamorphic and sedimentary environments occurrence of quartz and metal bearing minerals together strongly indicates that silica is an important component in hydrothermal fluids transporting and concentrating economic metals to the ore grade however chemical and physical characteristics of such ore forming media remain debated understanding of the environment in which chalcedony forms is largely hampered by the lack of chalcedony hosted fluid inclusions our study reports for the first time fortuitously preserved large sized up to 150 m fluid inclusions in chalcedony from the gonchak deposit of optical calcite in the early triassic basalts belonging to the siberian large igneous province the application of microthermometric methods scanning electron microscopy with cathodoluminescence and laser raman spectroscopy to the fluid inclusions and their host chalcedony recognized the formation of chalcedony from a colloidal suspension the fluid inclusions represent a gel like saline aqueous fluid that is residual after precipitation of spherulitic chalcedony aggregates with numerous h2o bearing and h2o poor layers we propose that the colloidal nature of fluids forming chalcedony lends strong support to the natural existence of experimentally predicted silicothermal fluids such fluids can be instrumental in mobilizing and transporting large quantities of both silica and nano and micro particles of ore minerals followed by efficient separation of the latter from coagulating silica gel into ore rich zones and bodies

silicothermal fluid a novel medium for mass transport in the lithosphere new experimental data from synthetic fluid inclusion studies in the system k 2 o co 2 sio 2 h 2 o kcsh show that a potassic silica rich 90 wt sio 2 fluid can coexist immiscibly with a supercritical alkaline aqueo carbonic fluid and quartz from temperatures as low as 300 c to more than 750 c at relatively low geologic pressures 2 rich fluids if they form in the lithosphere are likely to be important in the mobilization and transport of silica and large ion lithophile elements e g k cs ba and metals of economic significance e g au ag u

geochemical characteristics and ore controlling factors of hutouya polymetallic deposit qinghai province mineralization of the hutouya deposit is very complicated and the unclear mineralization and ore controlling factors constitute the bottleneck which restricts the breakthrough of the ore prospecting work detailed studies of characteristics of the ore deposit and geochemistry geochronology and geochemistry of intrusive rocks were conducted by the authors with some achievements obtained zircon la mc icp ms u pb isotopic age of monzogranite in no ore belt is 217 5 1 1 ma mswd 1 4 monzogranite of mineral belt and mineral belt have such features of post collision rocks as high content of k2o and na2o higher k2o than na2o higher differentiation gently right oblique shape in chondrite normalized ree patterns enrichment of lile moderate enrichment of hfse and depletion of ti and ba mineralization is controlled by such factors as magma sources of intrusive rocks evolution mode of intrusive rocks strata and structure the monzogranite intrusions of mineral belt whose material was partly contributed by the mantle had features of strong fractional crystallization and relatively strong ore forming capability they mainly formed skarn type cu pb zn polymetallic deposits mineral belt monzogranite which originated only from crustal sources had inferior mineralization capacity and they were mainly related to mineralization of such skarn type ore bodies as tin molybdenum and iron

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

crustal melts below 400 c we propose that the internally zoned li bearing tin mountain pegmatite in the harney peak granite pegmatite system of the black hills south dakota crystallized from fluid rich compositionally complex melts at 400 350 c the low crystallization temperatures resulted from the combined fluxing effects of li b p h 2 o and carbonate anions the presence of hydrous silicate melts at temperatures of 350 c is revealed by microthermometric data on primary fluid inclusions cogenetic with crystallized melt inclusions mineral assemblages of the crystallized melt inclusions and chemistry of bulk fluid leachates indicate that the melts and fluids contained significant amounts of li and na as carbonates and or borates that acted as powerful fluxes the low temperatures give a new perspective on the stability of silicate melts in the crust

immiscibility between silicate magmas and aqueous fluids a melt inclusion pursuit into the magmatic hydrothermal transition in the omsukchan granite ne russia abstract exsolution unmixing of the volatile element rich phases from cooling and crystallising silicate magmas is critical for element transport from the earth s interior into the atmosphere hydrosphere crustal hydrothermal systems and the formation of orthomagmatic ore deposits unmixing is an inherently fugitive phenomenon and melt inclusions droplets of melt trapped by minerals provide robust evidence of this process in this study melt inclusions in phenocrystic and miarolitic quartz were studied to better understand immiscibility in the final stages of cooling of and volatile exsolution from granitic magmas using the tin bearing omsukchan granite ne russia as an example primary magmatic inclusions in quartz phenocrysts demonstrate the coexistence of silicate melt and magma derived cl rich fluids brine and vapour and emulsions of these during crystallisation of the granite magma microthermometric experiments in conjunction with pixe and other analytical techniques disclose extreme heterogeneity in the composition of the non silicate phases even in fluid globules within the same silicate melt inclusion we suggest that the observed variability is a consequence of strong chemical heterogeneity in the residual silicate melt brine vapour system on a local scale owing to crystallisation immiscibility and failure of individual phases to re equilibrate the possible evolution of non silicate volatile magmatic phases into more typical hydrothermal chloride solutions was examined using inclusions in quartz from associated miarolitic cavities

extreme alkali bicarbonate and carbonate rich fluid inclusions in granite pegmatite from the precambrian ronne granite bornholm island denmark our study of fluid and melt inclusions in quartz and feldspar from granite pegmatite from the precambrian ronne granite bornholm island denmark revealed extremely alkali bicarbonate and carbonate rich inclusions the solid phases daughter crystals are mainly nahcolite nahco3 zabuyelite li2co3 and in rare cases potash k2co3 in addition to the volatile phases co2 and aqueous carbonate bicarbonate solution rare melt inclusions contain nahcolite dawsonite naal co3 oh 2 and muscovite in addition to fluid and melt inclusions there are primary co2 rich vapor inclusions which mostly contain small nahcolite crystals the identification of potash as a naturally occurring mineral would appear to be the first recorded instance from the appearance of high concentrations of these carbonates and bicarbonates we suggest that the mineral forming media were water and alkali carbonate rich silicate melts or highly concentrated fluids the coexistence of silicate melt inclusions with carbonate rich fluid and nahcolite rich vapor inclusions indicates a melt melt vapor equilibrium during the crystallization of the pegmatite these results are supported by the results of hydrothermal diamond anvil cell experiments in the pseudoternary system h2o nahco3 sio2 additionally we show that boundary layer effects were insignificant in the bornholm pegmatites and are not required for the origin of primary textures in compositionally simple pegmatites at least

tectonostratigraphy of the late archean dharwar supergroup dharwar craton india defining a tectonic history from spatially linked but temporally distinct intracontinental and arc related basins abstract the dharwar supergroup comprises the unconformity bound bababudan and chitradurga groups the bababudan group which is best preserved in the western dharwar craton records a divergent margin comprising a basalt dominant intracontinental rift sequence a shale bif drift sequence and a sandstone shale thermal subsidence sequence the rift stage evolved from 2765 ma to 2720 ma whereas the succession was folded uplifted and eroded during development of a convergent continental margin from 2680 ma ocean opening was to the east or southeast the chitradurga group records a two stage back arc basin behind an east facing continental arc stage i evolved between 2609 ma and 2582 ma and stage ii between 2582 ma and 2540 ma the two stages are separated by a subaerial unconformity stage i comprises siliciclastic fluvial shallow marine and deep marine sedimentary rocks and bimodal volcanic rocks stage i is best preserved in the western dharwar craton but there are equivalents in the eastern dharwar craton stage ii in the western dharwar craton comprises deep marine turbidites of siliciclastic and volcaniclastic provenance and basaltic and felsic volcanic rocks volcanic stage ii sequences in the eastern dharwar craton define an arc adjacent position tectonically driven cyclic uplift and erosion were responsible for the mismatch between preserved stratigraphic thickness and time particularly for stage ii sequences a remnant of a

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

neoarchean greenstone volcanism and continental growth dharwar craton southern india constraints from sims u pb zircon geochronology and nd isotopes abstract we present sims u pb zircon ages and nd isotope data for the felsic volcanic rocks from seven neoarchean greenstone belts of the eastern dharwar craton edc and from the chitradurga greenstone belt in the western dharwar craton wdc zircon ages show bimodal age distribution of felsic volcanism the ca 2 70 2 65 ga felsic volcanic event is contemporaneous with 2 7 ga mafic greenstone volcanism and emplacement of juvenile tonalitic to granodioritic crust while 2 58 2 54 ga felsic volcanics are coeval and spatially and probably genetically in some cases linked to the major episode calc alkaline magmatic accretion in the edc the chitradurga and veligallu greenstone belts host felsic volcanics of the first generation the latter showing inheritance at ca 2 95 ga four of the five greenstone belts hosting the second generation of felsic volcanics chitradurga kolar kadiri hutti show crustal inheritance at ca 2 6 2 7 2 9 3 0 3 1 and 3 3 ga ndt indicate derivation of the felsic volcanics from juvenile sources or short lived crustal sources with minor influence of older crust the new and existing data are consistent with two stage growth of the dharwar craton in the neoarchean first stage accretion led to the growth of a 2 7 2 6 ga juvenile crustal province of mafic volcanics and felsic plutons along the eastern margin of the wdc second stage accretion 2 58 2 52 ga led to the emplacement of ttg and calc alkaline plutons and felsic volcanics throughout the eastern dharwar craton an active margin context could apply for the two stage accretion scenario considering a west dipping subducting slab beneath the craton in the framework of long lived ultra hot accretionary orogen but the great width of influence of magmatic accretion and or reworking particularly during the second stage would suggest large scale mantle flow reorganization that would have generated large plume s

first insights on the metallogenic signature of magmatic fluids exsolved from the active magma chamber of vesuvius ad 79 pompei eruption abstract fluid inclusions hosted in sanidine from xenoliths representing the crystallization front of the ad 79 magma chamber of vesuvius offer a rare opportunity to track the composition of magmatic fluids exsolving at the peripheral part of a growing k alkaline magma chamber disrupted by an explosive eruption this work demonstrates that the exsolving magmatic fluids have a complex composition taking in evidence the great capability of metal transport by means of these solutions inclusions are particularly enriched in zn mo w sb bi tl and pb metals display two different positive correlations 1 pb zn tl bi 2 mo w the different trends may reflect the different complexing agent for these elements probably chloride for pb zn tl bi and probably hydroxy for mo w the occurrence of a silicothermal component in these fluids may furthermore favor the metal transport of these solutions this is probably a major previously underestimated process in the enrichment of some metals in magmatic hydrothermal systems this hypothesis may challenge conventional ideas on fluid phase mass transport of components by means of magmatic hydrothermal fluids and should be taken into account in future works on these topics