a note on the iugs reclassification of the high mg and picritic volcanic rocks the purpose of this short note is to offer some comments and observations on the new iugs classification of high mg rocks le bas 2000 particularly its use of the terms komatiite and picrite with no reference to texture or the original usage of these terms at the outset of this discussion we would like to place on record that we are in agreement with the need to redefine the nomenclature of high mg rocks there are however important issues relating to both the use of the term komatiite for all extrusive volcanic rocks with 18 wt mgo and the definition and use of the term picrite neither of which are discussed by le bas

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

hafnium neodymium isotope systematics of the 2 7 ga gadwal greenstone terrane eastern dharwar craton india implications for the evolution of the archean depleted mantle abstract the neoarchean gadwal greenstone belt in the eastern dharwar craton india hosts a well preserved metavolcanic sequence that is dominated by tholeiitic and calc alkaline basalt andesite dacite rhyolite series which includes boninitic geochemical varieties bulk rock lu hf and sm nd isotope systematics of these apparently arc related volcanic rocks yield indistinguishable ages of 2 701 0 024 ga and 2 702 0 026 ga respectively on the basis of the close spatial association and identical ages of the different rock types we suggest 2 70 0 03 ga as the age of crystallization of the different rock types within the gadwal metavolcanic sequence in contrast bulk rock pb pb isotope systematics of the same samples yield a significantly younger and less precise age of 2 466 ga 0 068 0 110 ga we tentatively interpret this younger age to represent a metallogenic and crustal reworking event in the dharwar craton which disturbed the u pb system but not the lu hf or sm nd systems the gadwal metavolcanic rocks have positive initial ehf 2 70ga 1 6 to 8 7 and slightly negative to positive end 2 70ga 0 1 to 3 0 values consistent with an origin from a long term depleted source relative to a chondritic reservoir at 2 7 ga lack of correlation between initial isotopic compositions and major or trace element indices of fractionation and alteration suggest that the observed isotope variability probably reflects compositional variation in the gadwal source similar to that observed in modern day island arcs two boninitic samples of the gadwal sequence have ehf 8 3 and 8 7 and are more radiogenic than average depleted mantle for the time period 3 2 to 2 5 ga ehf 4 to 6 early perhaps hadean differentiation events that led to a depleted and heterogeneous mantle are apparent in the nd and hf isotope systematics of 3 7 3 8 ga isua supracrustal rocks the radiogenic hf isotopes of the gadwal boninites and the hf nd isotope systematics of rocks from other locations in the 3 4 to 2 5 ga time period are consistent with the survival of fragments of an early depleted mantle later in the archean from 2 0 ga to present the time integrated 176lu 177hf and 147sm 144nd of the depleted mantle appears nearly constant and similar to the present day average morb source these data indicate that progressive elimination of early 4 5 ga formed heterogeneities in the depleted mantle dominated the history of the archean mantle and that portions of early depleted reservoirs survived through the mesoarchean these results have implications for the mixing scales for the early terrestrial mantle and the timing of the initiation of present day plate tectonics

the genesis of refractory melts in the formation of oceanic crust refractory primary liquids arising in various oceanic plate tectonic settings are characterized by high mgo sio2 ca na low tio2 and generally low incompatible element abundances relative to primary liquids parental to morb we propose that the former melts segregate from upper mantle peridotite which has earlier been depleted by extraction of picritic melts which were parental to morb a compositional range in the second stage melts is expected depending on the extent of previous depletion of the peridotite the temperature and pressure of melt segregation and the possible influence of volatile phases c h o present during melting

on the initiation of metamorphic sulfide anatexis mineral assemblages in common sulfide ore deposits are examined together with phase relations to 1 investigate the pressure temperature conditions required for the onset of metamorphically induced partial melting involving economic minerals and 2 place constraints on the amount of melt produced deposits that contain sulfosalt or telluride minerals may start to melt at conditions ranging from lowest greenschist facies to amphibolite facies deposits lacking sulfosalt and or telluride minerals may begin to melt once p t conditions reach the upper amphibolite facies if galena is present or well into the granulite facies if galena is absent the result is two broad melting domains a lowto medium temperature low melt volume domain involving melting of volumetrically minor sulfosalt and or telluride minerals and a high temperature potentially higher melt volume domain involving partial melting of the major sulfide minerals epithermal gold deposits which are especially rich in sulfosalt minerals are predicted to commence melting at the lowest temperatures of all sulfide deposit types massive pb zn cu deposits may start to melt in the lower to middle amphibolite facies if pyrite and arsenopyrite coexist at these conditions and in the upper amphibolite facies if they do not excepting sulfosalt bearing occurrences massive ni cu pge platinum group element deposits will show little to no melting under common crustal metamorphic conditions whereas disseminated cu deposits are typically incapable of generating melt until the granulite facies is reached when partial melting commences in bornite bearing rocks the volume of polymetallic melt that can be generated in most deposit types is therefore largely a function of the abundance of sulfosalt minerals even at granulite facies conditions this volume is usually less than 0 5 the exception is massive pb zn deposits where melt volumes significantly exceeding 0 5 vol may be segregated into sulfide magma dykes allowing mobilization over large distances

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

the case for archaean boninites rare archaean light rare earth element lree enriched mafic rocks derived from a strongly refractory mantle source show a range of features in common with modern boninites these archaean second stage melts are divided into at least two distinct groups whundo type and whitney type whundo type rocks are most like modern boninites in terms of their composition and association with tholeiitic to calc alkaline mafic to intermediate volcanics small compositional differences compared to modern boninites including higher al2o3 and heavy ree hree probably reflect secular changes in mantle temperatures and a more garnet rich residual source whundo type rocks are known from 3 12 and 2 8 ga assemblages and are true archaean analogues of modern boninites whitney type rocks occur throughout the archaean as far back as ca 3 8 ga and are closely associated with ultramafic magmatism including komatiites in an affiliation unlike that of modern subduction zones they are characterised by very high al2o3 and hree concentrations and their extremely depleted compositions require a source which at some stage was more garnet rich than the source for either modern boninites or whundo type second stage melts low la yb and la gd ratios compared to whundo type rocks and modern boninites either reflect very weak subduction related metasomatism of the mantle source or very limited crustal assimilation by a refractory mantle derived melt regardless the petrogenesis of the whitney type rocks appears either directly or indirectly related to plume magmatism if whitney type rocks have a boninitic petrogenesis then a plume related model similar to that proposed for the modern tongan high ca boninites might apply but with uniquely archaean source compositions and source enrichment processes second stage melts from barberton s africa 3 5 ga and ca 3 0 ga rocks from the central pilbara australia have features in common with both whundo and whitney types but appear more closely related to the whitney type subduction zone processes essentially the same as those that produce modern boninites have operated since at least 3 12 ga while a uniquely archaean boninite forming process involving more buoyant oceanic plates and very inefficient mantle source enrichment may have occurred before then

paleoproterozoic 1 90 1 86 ga arc volcanism in the flin flon belt trans hudson orogen canada geochemical and isotopic nd sr data are reported on paleoproterozoic 1904 1864 ma maficintermediate 1870 ma collision and subsequent intra arc deformation

the ashe metamorphic suite northwest north carolina metamorphism and observations on geologic history the ashe metamorphic suite ams consists of amphibolite schist and gneiss metabasalts pelitic schist and gneiss metasediments and ultramafic rocks evidence for four metamorphic events m1 m2 m3 m4 exists m1 affected only the ultramafic rocks in the suite the peak metamorphism m2 was an upper amphibolite facies event that affected all lithologies in the ams during the ordovician taconic orogeny three m2 zones of the amphibolite facies are recognizable in the pelitic rocks conditions during m2 are estimated to have been p 7 kb and t 700 degrees c the effects of m3 and m4 are minor in the pelitic and mafic rocks and identified with the growth of chlorite upon partial hydration of these rocks there are at least four possible plate tectonic sites for the origin of the ams 1 on a passive ensialic margin 2 on a passive ensimatic or transitional margin 3 on an active ensimatic margin and 4 in an ensialic or ensimatic back arc basin modified journal abstract

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

petrogenesis of basalt high mg andesite adakite in the neoarchean veligallu greenstone terrane geochemical evidence for a rifted back arc crust in the eastern dharwar craton india abstract the 2 7 ga veligallu greenstone terrane in the eastern dharwar craton is one of the least studied neoarchean greenstone belts in india to our knowledge there is no published dataset that provides a detailed account of the petrogenetic processes which culminated into the evolution of this greenstone belt this paper presents a comprehensive account of field petrography and geochemistry of the basalt high mg andesite adakitic volcanic rock association in the veligallu greenstone belt the basalts are tholeiitic in nature and are characterized by primitive geochemical compositions with 49 wt sio2 9 wt mgo mg 54 cr 197 ppm ni 122 ppm y 27 ppm yb 2 7 ppm zr y 8 display systematically flat to light ree depleted chondrite normalized ree patterns lan ybn 0 76 1 0 in contrast to the samples with nb th the veligallu high mg andesites are orthopyroxene bearing calc alkaline andesites and are characterized by moderate sio2 56 wt high mgo 8 wt mg 61 67 cr 174 330 ppm and ni 173 242 ppm contents low y 13 ppm and yb 1 2 ppm concentrations relative to typical island arc andesites they exhibit light ree enriched lan smn 1 7 and heavy ree depleted gdn ybn 1 9 fractionated ree patterns on a primitive mantle normalized trace element variation diagram they exhibit negative nb and ti and contrasting positive zr anomalies relative to the neighboring ree the la yb systematics suggest that these high mg andesites were generated by hydrous partial melting of a slab melt metasomatized mantle wedge in the sub arc mantle the felsic volcanic rocks are characterized by high sio2 73 wt na2o 4 5 wt and significantly low concentrations of mgo 0 72 wt cr 19 ppm ni 23 ppm y 5 9 and yb 0 44 ppm strongly fractionated chondrite normalized ree patterns lan ybn 30 with occasionally minor positive eu anomaly and primitive mantle normalized negative nb and ti and contrasting positive zr anomaly the geochemical attributes of the veligallu felsic volcanic rocks are analogous to the cenozoic high silica type adakites that are produced by partial melting in the sub arc mantle and presumably without any significant interaction of the slab melts with the overlying mantle wedge the juxtaposed occurrence of basalts with the banded iron formations and their close spatial association with the high mg andesites in the veligallu greenstone belt indicates that the petrogenesis of these volcanic rocks is consistent with their eruption in an intraoceanic setting the close proximal association of the back arc type basalts and the high mg andesites and adakites within the veligallu greenstone belt potentially suggests that this greenstone belt represents a rifted back arc crust this event appears to have significantly contributed to the peak of crustal growth activity in the eastern dharwar craton at 2 7 ga

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

boninites from the neoarchaean gadwal greenstone belt eastern dharwar craton india implications for archaean subduction processes abstract boninites are rare in archaean arcs all over the world whilst there are some reports from the phanerozoic convergent margins in this paper boninites are reported for the first time from an archaean greenstone belt of india which have not been reported from any other indian greenstone belt as yet the greenstone belts of western dharwar craton are characterized by high mg basalts and komatiites whereas in the greenstone belts of eastern dharwar craton boninites represent the high mg lavas these boninites are characterized by 1 high mgo 12 24 wt 2 enriched in transition elements such as ni 423 1184 ppm cr 943 2765 ppm 3 low tio2 0 22 0 36 wt 4 high al2o3 tio2 ratio 29 50 5 gd ybn less than one 6 slight enrichment in lree depletion in mree compared to lree and hree resulting in u shaped ree patterns 7 negative nb ta zr hf ti and v anomalies despite the depletion in mree these characteristics differentiate them from deep mantle plume related magmas these boninites are compositionally similar to those found in modern and fossil island arcs from intraoceanic subduction settings and provide evidence to propose that two stage melting of the archaean mantle wedge generated this type of high mg magma the other high mg type magma komatiitic suite found in the greenstone belts of western dharwar craton has an enriched deep mantle plume source and its dry melting whereas boninites are generated by wet melting of the refractory depleted mantle wedge under high temperature and fluid pressure generated by dehydration of the slab the geological setting hfse and ree data show that possibly the 2 7 ga peak of geological activity had transitional characteristics between those of paleoarchaean and phanerozoic high thermal and fluid flux is inferred to generate these mg enriched and mree hfse lfse depleted suite this discovery of boninites from gadwal greenstone belt indicate that even in archaean subduction of small young and hot oceanic plates produced magmatic assemblage resembling to those of some of the cenozoic subduction complexes

petrology and geochemistry of boninite series volcanic rocks from the mariana trench boninite series volcanic rocks have been recovered from three dredge hauls on the inner slope of the mariana trench these hauls included olivine boninites boninites boninitic andesites and boninitic dacites as well as island arc tholeiitic basalts and andesites the boninite series volcanics range from 52 to 68 sio2 and are characterized by very low abundances of high field strength cations and heavy rare earth elements boninites and olivine boninites have phenocrysts of olivine and orthopyroxene the andesites phenocrysts of orthopyroxene and clinopyroxene and the dacites orthopyroxene clinopyroxene and plagioclase most of the major and trace element variation in the series from boninite to boninitic dacite can be modelled by fractionation of olivine orthopyroxene clinopyroxene and plagioclase in the proportions 2 5 4 1 2 leaving 47 residual liquid the fractionation must be in part open system reverse zoned phenocrysts resorbed olivine and plagioclase xenocrysts and bulk rock compositions which cannot be fit by simple closed system crystallization indicate some magma mixing and phenocryst accumulation two boninitic magma stems can be identified with similar high field strength element abundances but different amounts of ca na al and light rare earth elements there is also evidence for a magma stem transitional in chemistry from the boninites to arc tholeiites the compositions of these boninites are consistent with hypotheses for boninite formation by partial melting of a depleted mantle mixed with an incompatible element enriched fluid the mariana forearc boninite series lacks a strong iron enrichment but produces andesites with lower ti al and y zr and higher mg ni and cr than typical calcalkaline arc andesites and dacites boninites in the mariana system were erupted only in the earliest phases of subduction zone activity

archean metavolcanics from the rouyn noranda district abitibi greenstone belt quebec 2 mobility of trace elements and petrogenetic constraints qualitative evaluation of altered mafic pillows from the rouyn noranda district of the abitibi metavolcanic belt reveals that for low grade metamorphism the high field strength elements hfs zr y ti and the rare earth elements ree are immobile however where alteration is extreme the ree elements are leached and redeposited by carbonate rich and k rich metasomatic fluids whilst zr y and zr ti remain unchanged rubidium ba and to a lesser extent sr are mobile at all levels of alteration however provided certain filters are applied the original geochemical values of the lavas for the alkaline elements the hfs elements and the ree can be defined the geochemical variations thus identified for the volcanic series of the rouyn noranda district are ascribed to a combination of contamination and fractional crystallization of a primitive tholeiitic magma

contrasting geochemical patterns in the 3 7 3 8 ga pillow basalt cores and rims isua greenstone belt southwest greenland implications for postmagmatic alteration processes abstract pillow basalts from the early archean 3 7 to 3 8 ga isua greenstone belt west greenland are characterized by well preserved rims and concentric core structures the pillow rims and cores have different mineral assemblages and chemical and isotopic compositions the rims have systematically higher contents of fe2o3 mgo mno k2o rb ba ga y and transition metals than the cores in contrast the cores possess higher concentrations of sio2 na2o p2o5 sr pb u nb and the light rare earth elements rees than the rims these compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration variations of many major and trace element concentrations between the rims and cores of the isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration al2o3 tio2 th zr and the heavy rees display similar values in both rims and cores suggesting that these elements were relatively immobile during postemplacement alteration in addition the rims and cores have distinctive sm nd and rb sr isotopic compositions in that the rims are characterized by higher 143nd 144nd and 87sr 86sr ratios than the cores the pillow basalts yield 2569 170 ma and 1604 170 ma errorchron ages on 143nd 144nd vs 147sm 144nd and 87sr 86sr vs 87rb 86sr diagrams respectively the sm nd errorchron age may correspond within errors to a late archean tectonothermal metamorphic event recorded in the region the sm nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of nd relative to sm during late archean metamorphism although the rb sr errorchron age overlaps with the timing of an early to mid proterozoic tectonothermal metamorphic event recorded in the region because of a considerably large mean square of weighted deviates value and scatter in 86sr 87sr and 87rb 86sr ratios this age may not have a precise geological significance the 1 6 ga rb sr errorchron is likely to have resulted from the loss of radiogenic 87sr collectively the sm nd and rb sr data obtained from the 3 7 3 8 ga isua pillow basalt rims and cores are consistent with disturbances of the sm nd and rb sr systems by tectonothermal metamorphic events long after their eruption in contrast to the sm nd and rb sr systems the lu hf system appears to be largely undisturbed by metamorphism five core samples and three rim samples yield a 3935 350 ma age within error of the approximate age of eruption 3 7 to 3 8 ga two rim samples that have gained lu give an age of 1707 140 ma within error of the rb sr errorchron age initial 176hf 177hf ratios of the undisturbed samples at 3 75 ga lie within 1 e unit of the chondritic value suggesting no long term depletion in the mantle source of the basalts

adakite ttg connection and fate of mesoarchaean basaltic crust of holenarsipur nucleus dharwar craton india abstract holenarsipur nucleus hn is one of the oldest part 3 5 3 1 ga of the continental crust of india it is made up of mafic ultramafic komatitic 3 35 ga greenstone belts migmatitic ttg tonalite trondhjemite granodiorite 3 3 ga and diapiric trondhjemites 3 1 ga new major trace and ree data on these rocks presented here provide constraints on adakite ttg connection model and petrogenesis of these rocks of hn in addition to recycling of the early mesoarchaean basaltic crust and evolution of mesoarchaean continental crust although of two different ages the chemical characteristics of the migmatitic ttgs and diapiric trondhjemites of the hn resemble with the archaean and cenozoic high silica adakites hsa and the adakites from two neoarchaean greenstone belts of dharwar craton they also display similarities with the early mesoarchaean ttgs from different cratons based on the compositional variations observed in ttg and diapiric trondhjemites of hn we propose that plume fed basaltic and or basaltic komatiitic oceanic crust slab flatly shallow angle subducted and melted under high archaean geothermal regime to produce ttgs identification of hsa in 2 7 ga old greenstone belt subduction complexes that are compositionally similar to ttg suggests the possibility of continuation of this process into neoarchaean and supports a genetic connection between ttg and adakites

high precision pb sr nd hf isotopic characterization of usgs bhvo 1 and bhvo 2 reference materials the recent development of multiple collector inductively coupled plasma mass spectrometry mc icp ms and increasing use of the technique have created the need for well characterized rock standards especially for isotopic systems where no internal fractionation correction can be applied this paper presents a careful leaching experiment on the u s geological survey usgs reference materials bhvo 1 and bhvo 2 hawaiian basalts and documents the evidence for contamination of the rock powders during processing this contamination accounts for the difference in pb isotopic ratios of bhvo 1 and bhvo 2 as well as for their lack of homogeneity both in pb isotopic compositions and in some trace element contents copyright 2005 by the american geophysical union

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

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

petrogenesis of ultramafics in the neoarchean veligallu greenstone terrane eastern dharwar craton india constraints from bulk rock geochemistry and lu hf isotopes abstract we report on the bulk rock geochemistry and lu hf isotope systematics and infer the petrogenesis of the ultramafic rocks in the neoarchean veligallu greenstone terrane eastern dharwar craton india the ultramafics along with the basalts and adakites yield a bulk rock lu hf isochron age of 2 696 0 054 ga consistent with the available sims zircon u pb age of 2 697 5 ma reported for the felsic volcanics in the literature the rocks have positive initial ehf 2 696 ga 3 0 to 6 5 consistent with an origin from a long term depleted mantle source relative to a chondritic reservoir at 2 7 ga geochemically the ultramafic rocks are characterized by high mgo 24 34 wt mg 89 cr 2290 3855 ppm and ni 604 966 ppm contents moderate to low sio2 52 49 wt al2o3 9 2 4 7 wt and tio2 0 30 0 14 wt to first order these chemical compositions broadly resemble komatiites the rocks exhibit higher than chondrite al2o3 tio2 21 37 ratio depletions in their middle ree relative to the heavy ree i e gdn ybn

the petrogenesis of gorgona komatiites picrites and basalts new field petrographic and geochemical constraints gorgona island colombia is remarkable not only because it contains the only phanerozoic komatiites but also because it has mafic to ultramafic lavas with a wide range of compositions from moderately enriched to extremely depleted relative to bulk earth the komatiite flows are in many respects similar to archaean komatiites they formed from mgo rich 18 liquids and have upper spinifex zones and lower cumulate zones the cumulate zones of archaean komatiites contain many solid grains in contrast more than 90 of the olivine in the gorgona cumulates is highly skeletal this combined with the fact that the gorgona cumulate zones are thinner than those in archaean komatiites suggests that the komatiite magma became strongly superheated en route to the surface the komatiites have trace element contents intermediate between those of the basalts and the ultramafic tuffs some basalts have isotope compositions indicative of long term enrichment in incompatible elements whereas other basalts and ultramafic volcanics have isotopic signatures that imply corresponding depletion it is apparent that the plume source region of the gorgona magmas was markedly heterogeneous with at least two source components contributing to the observed variation in composition this heterogeneity may have resulted from the incorporation of different components into the plume source or it may be the result of complex melting and melt extraction processes during the ascent of a heterogeneous plume despite earlier suggestions that there may have been a significant age gap between depleted komatiite and basalt flows and the enriched basal new 4oar 39ar dating of basalts and gabbros are more consistent with all being generated at 87 ma during formation of the caribbean colombian plateau possibly at the galapagos hotspot