mafic and felsic magma interaction in granites the hercynian karkonosze pluton sudetes bohemian massif the hercynian post collisional karkonosze pluton contains several lithologies equigranular and porphyritic granites hybrid quartz diorites and granodiorites microgranular magmatic enclaves and composite and lamprophyre dykes field relationships mineralogy and majorand trace element geochemistry show that 1 the equigranular granite is differentiated and evolved by small degrees of fractional crystallization and that it is free of contamination by mafic magma 2 all other components are affected by mixing the end members of the mixing process were a porphyritic granite and a mafic lamprophyre the degree of mixing varied widely depending on both place and time all of the processes involved are assessed quantitatively with the following conclusions most of the pluton was affected by mixing implying that huge volumes 475 km of mafic magma were available this mafic magma probably supplied the additional heat necessary to initiate crustal melting part of this heat could have also been released as latent heat of crystallization only a very small part of the karkonosze granite escaped interaction with mafic magma specifically the equigranular granite and a subordinate part of the porphyritic granite minerals from these facies are compositionally homogeneous and or normally zoned which together with geochemical modelling indicates that they evolved by small degrees of fractional crystallization 520 accessory minerals played an important role during magmatic differentiation and thus the fractional crystallization history is better recorded by trace rather than by major elements the interactions between mafic and felsic magmas reflect their viscosity contrast with increasing viscosity contrast the magmatic relationships change from homogeneous hybrid quartz diorites granodiorites to rounded magmatic enclaves to composite dykes and finally to dykes with chilled margins these relationships indicate that injection of mafic magma into the granite took place over the whole crystallization history consequently a long lived mafic source coexisted together with the granite magma mafic magmas were derived either directly from the mantle or via one or more crustal storage reservoirs compatible element abundances e g ni show that the mafic magmas that interacted with the granite were progressively poorer in ni in the order hybrid quartz dioritescgranodioritescenclavesccomposite dykes this indicates that the felsic and mafic magmas evolved independently which in the case of the karkonosze granite favours a deep seated magma chamber rather than a continuous flux from mantle two magma sources mantle and crust coexisted and melted almost contemporaneously the two reservoirs evolved independently by fractional crystallization however mafic magma was continuously being intruded into the crystallizing granite with more or less complete mixing several lines of evidence e g magmatic flux structures incorporation of granite feldspars into mafic magma feldspar zoning with fluctuating trace element patterns reflecting rapid changes in magma composition indicate that during its emplacement and crystallization the granite body was affected by strong internal movements these would favour more complete and efficient mixing the systematic spatial temporal association of lamprophyres with crustal magmas is interpreted as indicating that their mantle source is a fertile peridotite possibly enriched metasomatized by earlier subduction processes

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

the os and nd isotopic systematics of c 2 44 ga akanvaara and koitelainen mafic layered intrusions in northern finland abstract we have determined os and nd isotopic compositions of various rock types in and adjacent to the c 2 44 ga akanvaara and koitelainen layered intrusions which are part of the extensive paleoproterozoic ultramafic to felsic mantle plume related magmatism in the fennoscandian shield the initial e nd in the layered sequences of both intrusions fall within a limited range around 2 0 being compatible with results from other coeval layered intrusion in the shield the initial os isotopic ratios in chromitites are near chondritic the total range of os t is from 1 to 6 with the highest values reported for low os chromites no systematic differences are found in the osmium isotopic compositions of the chromites between the intrusions or relative to the stratigraphic position of the sample the studied intrusions share similar isotopic and geochemical features with rocks from younger flood basalt provinces and also the same problem of the origin of the lithospheric source component viz crustal assimilation versus melt generation within lithospheric mantle the observed nd os isotopic systematics of the intrusions require that the magmas had negative e nd values already before emplacement and can best be accounted for by 15 25 deep crustal contamination of a komatiitic melt initially having a near chondritic os isotopic composition coupled with additional minor in situ assimilation of country rock material this hypothesis is supported by the existence of coeval crustally contaminated komatiitic volcanism on the fennoscandian shield

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

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

early paleozoic high mg diorite granodiorite in the eastern kunlun orogen western china response to continental collision and slab break off abstract phanerozoic high mg diorites and granodiorites are known to be related to diverse subduction styles at convergent margin settings but have not been linked to collision processes the santonggou complex in the eastern kunlun orogen is composed of a dominant granodiorite intruded by small stocks and dykes of porphyritic granodiorites and diorites which are all characterized by high mgo and ni but low yb and y contents mafic microgranular enclaves mmes occur in the granodiorite and the porphyritic granodiorites la icpms u pb analyses of zircon from the granodiorite porphyritic granodiorite and porphyritic diorite yield similar ages of 432 2 ma 432 2 ma and 427 3 ma respectively plagioclase in the granodiorite exhibits complex zonations with resorbed and patchy cores plagioclase and hornblende phenocrysts in the porphyritic granodiorites and diorites also show textural and compositional zonations the highest e nd t value of 0 6 in mmes suggests asthenosphere derived mafic magma these lines of mineralogical and geochemical evidence together with results from geochemical modeling suggest that the porphyritic diorites were derived from mantle metasomatized slab melts the porphyritic granodiorites and the granodiorite were generated by simultaneous assimilation and fractional crystallization of the magma approximated by the porphyritic diorites plus mixing of asthenosphere derived mafic and evolved granodioritic magmas pressure estimates from the al in hornblende geobarometer suggest that the granodiorite had been uplifted to near the surface before the emplacement of the porphyritic granodiorites partial melting of the slab upwelling of the asthenosphere and rapid orogenic uplift at 430 ma suggest that the cessation of subduction and the initiation of collision as well as slab break off occurred almost simultaneously in the eastern kunlun orogen

geochemical and numerical constraints on neoarchean plate tectonics this paper discusses early neoarchean 2 8 2 7 ga plate tectonics by integrating knowledge from new geochemical observations and numerical models based on a geochemical dataset of 295 granitoid samples from the karelian and kola cratons of the fennoscandian shield we divide neoarchean juvenile extracted from oceanic crust or mantle granitoids into three groups 1 low hree heavy rare earth elements ttgs tonalite trondhjemite granodiorite high sio2 low mg low hree higher sr lower ybn and higher nb ta 2 high hree ttgs slightly lower range of sio2 larger range of mgo contents and higher cr and ni contents high hree lower sr higher ybn and lower nb ta and 3 high ba sr sanukitoids medium hree high mg and high k ba sr the main difference between the low and high hree groups lies in their pressure sensitive element contents which indicates high pressure melting conditions for the low hree group and low pressure conditions for the high hree group a possible tectonic scenario for the genesis of the two groups is an incipient hot subduction underneath a thick oceanic plateau protocrust melting in the lower part of thick basaltic oceanic crust could produce ttgs of the low hree type whereas low pressure melting of subducting slab and possible interactions with the mantle wedge at shallow depths would be capable of generating high hree ttgs the third group of archean high ba sr sanukitoids was formed after the ttgs their low sio2 and high mg k ba sr contents suggest an origin by melting in an enriched metasomatized mantle source probably as a result of a slab breakoff following a continental collision or attempted subduction of a thick oceanic plateau ttg protocontinent such hypothesis is supported by numerical modeling results that suggest an increased occurrence of slab breakoff in the archean which locally increased temperatures within the mantle wedge more frequent breakoff resulted because subducting plates were weaker due to rheologically thinner lithosphere and a thicker basaltic crust and tensile stresses within the subducting plate were larger due to a thick crust that transforms from buoyant basalt to dense eclogite

petrogenesis and tectonic implications of 2 45 ga potassic a type granite in the daqingshan area yinshan block north china craton abstract an archean paleoproterozoic a p a type granite 15 km wide and 25 km long has been identified in the daqingshan area south of the yinshan block yb north china craton ncc known as the jinpen pluton this body is the largest individual granite pluton in the daqingshan area its crystallization ages of 2460 2450 ma has been determined precisely using the secondary ion mass spectrometer sims zircon u pb method the jinpen pluton is lithologically homogeneous consisting mainly of gneissic monzogranites with metaluminous affinity and high k2o na2o 1 08 2 06 ga al and zr nb ce y values with high zircon saturation temperatures tzr of 889 837 c these features are typical of a type granite most samples contain enriched large ion lithophile elements liles and light rare earth elements lrees high y and yb abundances with pronounced eu negative anomalies low sr y and la yb n ratios and low mgo cr and ni contents implying a shallow crustal source for the pluton its whole rock nd isotope composition end t 2 5 0 6 and zircon hf isotope composition ehf t 5 2 1 7 tdmc 2 88 2 65 ga indicate that it mainly originated from pre existing crustal materials the zircon 18o values of 6 58 5 71 suggest a crustal source of orthogneiss with little metasedimentary material previous studies have demonstrated that the yb experienced significant crustal growth at 2 8 2 7 ga and further crustal reworking at about 2 5 ga this study reveals that the jinpen potassic a type granite was generated at 2 46 2 45 ga by partial melting of pre existing crustal materials at relatively low pressure p and high temperature t conditions symbolizing the first and essential cratonization of the lithospheric stabilization of the ncc

constraints on the magnitude and rate of co2 dissolution at bravo dome natural gas field the injection of carbon dioxide co2 captured at large point sources into deep saline aquifers can significantly reduce anthropogenic co2 emissions from fossil fuels dissolution of the injected co2 into the formation brine is a trapping mechanism that helps to ensure the long term security of geological co2 storage we use thermochronology to estimate the timing of co2 emplacement at bravo dome a large natural co2 field at a depth of 700 m in new mexico together with estimates of the total mass loss from the field we present to our knowledge the first constraints on the magnitude mechanisms and rates of co2 dissolution on millennial timescales apatite u th he thermochronology records heating of the bravo dome reservoir due to the emplacement of hot volcanic gases 1 2 1 5 ma the co2 accumulation is therefore significantly older than previous estimates of 10 ka which demonstrates that safe long term geological co2 storage is possible integrating geophysical and geochemical data we estimate that 1 3 gt co2 are currently stored at bravo dome but that only 22 of the emplaced co2 has dissolved into the brine over 1 2 my roughly 40 of the dissolution occurred during the emplacement the co2 dissolved after emplacement exceeds the amount expected from diffusion and provides field evidence for convective dissolution with a rate of 0 1 g m2y the similarity between bravo dome and major us saline aquifers suggests that significant amounts of co2 are likely to dissolve during injection at us storage sites but that convective dissolution is unlikely to trap all injected co2 on the 10 ky timescale typically considered for storage projects

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

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

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

calibration review and geomorphic implications of postglacial radiocarbon ages in southeastern alberta canada abstract calibration of radiocarbon dates of postglacial geomorphic events in southeastern alberta canada reveals a strong association between type of geomorphic event and seasonality of insolation the late glacial and postglacial periods can be divided into four major episodes glaciation ca 20 000 cal yr b p deglaciation ca 20 000 12 000 cal yr b p landscape stability ca 12 000 10 000 cal yr b p and landscape instability ca 10 000 0 cal yr b p this indicates that there was significantly more time than previously thought 12 000 13 000 yr for postglacial processes glaciation deglaciation maximum landscape stability indicating maximum aridity and landscape instability did not lag behind minimum increasing maximum and decreasing seasonality of insolation respectively

deformation induced pb isotope exchange between k feldspar and whole rock in neoarchean granitoids implications for assessing proterozoic imprints abstract we present pb isotope and microstructural studies of deformed k feldspar megacrystic granodiorites in the westernmost part of the karelian craton near the archean proterozoic boundary zone in east central finland our studies place constraints on the timing and the temperature and strain rate conditions of the deformation the pb isotope evidence indicates that u was decoupled from th approximately at the time of crystallization which generated high whole rock th u ratios with time high 208 pb 204 pb ratios were developed at high th u domains of the rock observed correlation between 208 pb 204 pb ratios and deformation microstructures of k feldspar megacrysts suggests a later deformation induced pb isotope redistribution between k feldspars and whole rock the coupled 238 u 206 pb and 235 u 207 pb systematics show that this pb isotope exchange occurred at ca 1 9 ga concurrently with the peak of the svecofennian orogeny our th pb and u pb model age calculations for k feldspar whole rock pairs indicate different degrees of incomplete equilibration that correlates with the degree of deformation the microstructures of k feldspars indicate low to medium t conditions 400 500 c and variable strain rates for deformation we conclude that pb isotope compositions and microstructures of k feldspars provide valuable constraints on assessing proterozoic imprints in archean rocks

geochemical characteristics of granulite facies rocks in the archean varpaisjarvi area central fennoscandian shield abstract archean lower crustal granulites are exposed as fault bounded blocks in the varpaisjarvi area central finland the prevailing rock types in these blocks are igneous enderbites tonalitic trondhjemitic granodioritic migmatites and mafic granulites although metamorphosed at moderate pressures 8 10 kbar the varpaisjarvi granulites are not in a geochemical sense good representatives of depleted restitic lower crust having invariably negative or negligible eu anomalies only the enderbites have k rb ratios showing depleted granulite trends the enderbites are orthopyroxene bearing igneous rocks ranging in composition from diorite to tonalite they have distinctive flat lree patterns with la sm from 2 03 2 86 which may be due to the removal of a lree rich phase during fractionation if not the result of infiltration by a co 2 rich fluid mafic granulites occur as intercalations within the tonalitic trondhjemitic granodioritic gneisses and while all are tholeiitic in composition they fall into two groups on the basis of major and trace element abundances the group 1 tholeiites are mostly basaltic with steeply increasing alkali contents over a short range in sio 2 ti zr ratios are close to the chondritic value of 110 the group 2 tholeiites vary from picrobasaltic to andesitic in composition with the latter being dominant ti zr ratios lie below the chondritic value the group 1 rocks have higher ni and cr and lower zr y ratios than their group 2 counterparts the varpaisjarvi area represents at least two accreted blocks or terranes that exhibit marked lithological and geochemical differences with the jonsa block being the most distinctive the main differences are the two contrasting groups of tholeiitic mafic granulites the jonsa block having only the group 2 metatholeiites and the presence of quartz cordierite and cordierite orthoamphibole orthopyroxene rocks only in the jonsa block where they are found as interlayers amongst the group 2 tholeiites the ti zr and zr y ratios of these latter lithologies are closely comparable with those of the group 2 tholeiites the jonsa block cordierite orthoamphibole orthopyroxene rocks have high mgo mgo feo tot ratios compared to most cordierite orthoamphibole rocks produced by seawater basalt interaction their major and trace element geochemistry nevertheless indicates that they were derived from a mafic igneous precursor

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

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

archean crustal evolution of the suomussalmi district as part of the kianta complex karelia constraints from geochemistry and isotopes of granitoids abstract this study presents new geochemical and isotopic data from granitoids belonging to the kianta complex of the western karelian terrane collected from the suomussalmi district in eastern finland geochemically the archean granitoids can be divided into five groups ttgs tonalite trondhjemite granodiorite sensu stricto transitional ttgs sanukitoids high mg granitoids quartz diorites and anatectic leucogranitoids this study includes chemical data for the first four and isotopic data for all the suites the majority 78 out of 80 samples of ttgs sensu stricto k2o na2o 0 5 display higher lile concentrations but otherwise closely resemble the ttgs sensu stricto sanukitoids and quartz diorites have moderate la yb n 6 50 and higher hree cr and ni than the ttgs sanukitoids can be differentiated from quartz diorites based on higher ba averages 1453 vs 626 ppm k2o averages 3 02 vs 1 94 and lree lan averages 141 vs 92 contents although overlap between these two suites exists ttg magmatism in suomussalmi and the surrounding parts of kianta complex can be divided into three major phases ca 2 95 ga 2 83 2 78 ga and 2 76 2 74 ga sanukitoid magmatism 2 73 2 71 ga partly overlaps in error limits with quartz diorites aged 2 70 ga and is coeval with one transitional ttg sample 2 72 ga migmatization of older rocks and intrusion of leucogranitoids took place at 2 70 2 69 ga geochemistry inherited zircons

paleomagnetic constraints on an archean paleoproterozoic superior karelia connection new evidence from archean karelia abstract charno enderbitic granitoids in the karelia craton of the fennoscandian shield have been studied paleomagnetically the characteristic remanence component has a steep negative inclination and is interpreted to record magnetization at a maximum age of 2684 2 ma consistently stable results were obtained from 12 sites in the koitere area corresponding to regions with high positive magnetic anomalies and high remanence intensities petrographic studies coupled with rock magnetic investigations indicate that the remanence resides in fine sd psd magnetite grains formed during neoarchean clinopyroxene alteration cross cutting vertical subvertical paleoproterozoic dolerite dykes suggest that the koitere granitoids are in their original orientations and were not affected by svecofennian deformation at ca 1 9 1 8 ga the koitere granitoids have an opposite polarity compared to the steep positive inclination remanence direction of the previously studied ca 2 63 ga varpaisjarvi enderbites and granulites the data from koitere and varpaisjarvi imply that at ca 2 7 2 6 ga the karelia craton was located at high latitudes of 80 60 whereas previous paleomagnetic data from ca 2 5 ga formations in the vodlozero terrane in nw russia indicate a near equatorial position comparison of paleomagnetic data from the koitere and varpaisjarvi granulite grade rocks with rocks of similar age in the superior craton shows that at ca 2 7 2 6 ga the superior and karelia cratons were located at high latitudes and in close proximity although the present data cannot demonstrate that the cratons were amalgamated however during the archean paleoproterozoic transition at ca 2 50 ga both cratons experienced significant rotation and drifting to near equatorial paleolatitudes suggesting that the superior and karelia cratons may have been attached at that time

geochemical and u pb zircon geochronological constraints on the development of a late archean greenstone belt at birch lake superior province canada abstract this paper presents new high precision u pb zircon ages that place constraints on the timing of magmatic activity volcanic and plutonic deformation and gold mineralization within the late archean birch confederation lakes greenstone belt within the uchi subprovince of the superior province geochemical data including sm nd isotopic analyses place additional constraints on the nature of the magmatic activity three volcanic rocks from the birch lake area have similar ages 2742 2 1 8 1 7 2739 1 2 8 and 2741 7 2 3 2 2 ma and together with previously published data suggest that a major proportion of the volcanic rocks of the greenstone belt were erupted in a narrow time interval from 2742 to 2735 ma older volcanic rocks recognized to the south of the birch lake area are either absent or of limited extent plutonic rocks include the pre to syn tectonic horseshoe island stock 2725 0 2 5 ma the syn tectonic mainprize lake batholith 2724 3 1 8 1 5 ma and the late to post tectonic shabumeni lake stock 2710 8 1 9 1 3 ma and okanse lake pluton 2700 8 1 9 1 4 ma these ages and structural relationships indicate that the development of a d2 regional penetrative fabric at least in part post dates the emplacement of the horseshoe island stock this is consistent with the interpretation that d2 deformation developed synchronously with and perhaps in response to the emplacement of the mainprize lake batholith an earlier d1 penetrative deformational event as well as possible large scale subhorizontal structures d0 developed subsequent to the deposition of the youngest volcanic rocks and prior to d2 plutons are younger than and unrelated to volcanism in contrast to volcanic rocks plutons show no indication of having interacted with older felsic crustal rocks and are interpreted to be derived from partial melting of metasomatized mantle and subducted tholeiitic basalt the okanse lake pluton has affinities with the sanukitoid suite and is closely comparable in age to similar plutons over a wide area it is proposed that a 2700 ma sanukitoid magmatic event may reflect melting of previously metasomatized mantle in response to thermal re equilibration associated with the transition from an earlier subduction dominated regime to a later post collisional stage culminating in the stabilization of the northern superior province

origin of high magnesian andesites in the setouchi volcanic belt southwest japan i petrographical and chemical characteristics high magnesian andesites of middle miocene age occur in southwest japan forming an obvious volcanic belt these andesites have low feo mgo ratios 0 546 0 931 and are rich in ni 101 312 ppm co 30 0 45 1 ppm and cr 208 756 ppm they are relatively aphyric phenocrysts 10 vol and the phenocrysts of magnesian olivine fo88 are in equilibrium with the host high magnesian andesite magmas on the basis of the fe mg exchange partitioning these features suggest that the high magnesian andesites are not differentiated or accumulative they appear to represent primary andesites generated in the upper mantle these southwest japanese high magnesian andesites are rich in incompatible elements and show light rare earth enrichment relative to boninites suggesting that the former is derived from a less depleted mantle source than the latter

experimental evidence for the origin of lead enrichment in convergent margin magmas it has been proposed1 5 that the low ce pb ratio of subduction related basalts relative to their oceanic counterparts arises by the preferential transfer of lead to the mantle wedge overlying the subducting slab by non magmatic processes fluxing of the mantle wedge by low ce pb fluids generated by the dehydration of sub ducted oceanic crust is one mechanism favoured for this process see for example ref 5 here we report the results of a series of high pressure experiments which confirm that low ce pb fluids coexist with the dominant mineral phases garnet and clinopyroxene produced during high pressure dehydration of altered basalt our results show that the production of subduction zone magmas from mantle sources fluxed by basalt derived fluid is a mechanism by which relatively lead rich cerium poor mantle derived material is added to the continents the lead enrichment of the earth s continental crust is thus a continuing process occurring at conver gent margins

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

single grain zircon u pb age constraints of the western and eastern sanukitoid zones in the finnish part of the karelian province abstract a single grain zircon u pb secondary ion mass spectrometer sims study of nine granitoid intrusions with sanukitoid geochemistry from the finnish part of the karelian province shows their time limited emplacement the age results indicate emplacement at 2715 and 2745 ma we observed also a significant number of relict cores in the zircons with 207 pb 206 pb ages up to 3 2 ga emphasizing the importance of inheritance processes in the petrogenesis of the sanukitoids compiled single grain age data of the finnish and russian bibikova et al 2005a parts of the karelian province confirm the occurrence of two temporally diverging sanukitoid zones throughout the province the western zone shows a younger average age 2718 ma and the eastern zone an older average age 2740 ma with an apparent age difference of 20 ma between the zones most of the sanukitoid intrusions in finland belong to the western sanukitoid zone but the southernmost intrusions can be regarded as a part of the eastern sanukitoid zone the boundary between the sanukitoid zones specifies the boundary of the western and central karelian terranes worldwide sanukitoid magmatism at 2 95 2 55 ga as reported from several archean blocks followed sharply after prolonged tonalite trondhjemite granodiorite ttg magmatism indicating a short lived tectonomagmatic phenomenon at the end of convergent accretion

zircon u pb ages and geochemistry of the huai an ttg gneisses terrane petrogenesis and implications for 2 5ga crustal growth in the north china craton abstract the huai an gneiss terrane is a typical archean terrane within the trans north china orogen of the north china craton ncc and is composed of tonalite trondhjemite granodiorite ttg suite and dioritic granulite facies gneisses precise shrimpii and cameca ims1280 zircon u pb ages demonstrate that the huai an gneisses formed around 2 55 2 50 ga and underwent granulite facies metamorphism at 1 85 1 80 ga the ttg gneisses are compositionally equivalent to high sio2 adakites with high sio2 and al2o3 concentrations strongly fractionated rare earth element ree patterns la yb n 10 54 with positive or negligible eu anomalies eu eu 0 9 3 4 low heavy ree hree and y concentrations high sr 435 938 ppm concentrations and high sr y ratios 48 261 high mg numbers 33 60 and high compatible element concentrations e g cr 11 183 ppm and ni 11 82 ppm indicate that the magmas that formed these gneisses were derived from interaction between adakitic melts and mantle material the dioritic gneisses have compositions similar to high mg diorites or sanukitoids with high al2o3 14 46 17 61 wt mgo 3 0 6 4 wt cr 100 304 ppm and ni 57 109 ppm concentrations and fractionated ree patterns with weak negative or no eu anomalies eu eu 0 8 1 0 all the ttg and dioritic gneiss samples analysed during this study have negative nb ta and ti anomalies compared to neighbouring elements in spider diagrams with the majority having high positive nd t and hf t values close to values of the depleted mantle these geochemical characteristics strongly suggest that protoliths of the huai an ttg and dioritic gneisses originated in a juvenile crust forming event at 2 55 2 50 ga through partial melting of subducted oceanic crustal material these magmas also assimilated mantle peridotite material during their ascent this indicates that 2 5 ga mantle derived magmatism led to significant crustal growth in the north china craton and was probably associated with subduction

growth of archaean crust in the kuhmo district eastern finland u pb and sm nd isotope constraints on plutonic rocks abstract neoarchaean plutonism in the kuhmo district in eastern finland karelia includes tonalites sanukitoids leucocratic granodiorites and leucogranites a u pb sims and tims study of zircons and titanites places age constraints on this plutonic activity and whole rock sm nd isotopes are used to characterise source variation u pb results indicate that the tonalites were emplaced during at least three separate periods 2 81 2 78 and 2 76 2 74 ga the initial nd values vary from 0 3 to 2 2 and suggest no major involvement of significantly older crust in the genesis of tonalites the tonalites were followed by 2 74 2 70 ga possibly also 2 68 ga sanukitoids with initial nd values of 0 7 to 1 2 close temporal relations between the 2 74 ga tonalites and sanukitoids indicate that their genesis may be linked subsequent leucocratic granodiorites and leucogranites were emplaced at 2 70 2 68 ga and commonly contain inherited zircons the initial nd values of these granodiorites and granites vary from 1 1 to 3 4 indicative of multiple magma sources the general decrease in initial nd values with decreasing age demonstrate progressive recycling of pre existing crust thus stressing the importance of crustal recycling as a process in forming the leucocratic granitoid rocks overall the archaean plutonic rocks of the kuhmo district are temporally similar to those in the adjacent russian karelia suggesting that 2 83 2 68 ga was one of the most extensive periods of crustal growth throughout the karelia craton

the effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis a model abstract the initial pb isotopic composition of anatectic granites is modelled as a mixture of pb derived from major minerals that have characteristically low 238 u 204 pb and 232 th 204 pb ratios i e feldspars biopyroboles etc with pb contributed from dissolved accessory minerals with characteristically high 238 u 204 pb and 232 th 204 pb ratios i e zircon monazite etc the initial pb isotopic composition of anatectic granites is a function of a the age type modal distribution and heterogeneity in the initial u and th content of the accessory minerals b variation in melt composition and temperature during anatexis c the fraction of the source melted and d the extent to which the melt was homogenized prior to crystallization the differential behavior of accessory minerals during anatexis can explain the common yet enigmatic decoupling of the pb isotopes from other chemical and isotopic systems in granite rocks granitic plutons derived by anatexis of the same source material are not required to have the same initial pb isotopic composition low temperature peraluminous granites are more likely to exhibit homogeneous initial pb isotopic compositions dominated by the pb contained within the major mineral assemblage of the source material in contrast the initial pb isotopic composition of higher temperature metaluminous or peralkaline anatectic granites is more likely to be heterogeneous unless the melts are homogenized during emplacement and will record a larger fraction of the radiogenic pb component contained in the accessory mineral assemblage of the source material heterogeneity in the initial pb isotopic composition of individual granites or granitic suites can preserve the weighted average age of the radiogenic pb present in the accessory mineral assemblage of the source region even if these minerals are entirely dissolved during anatexis the use of isotopic tracer studies in terrane analysis will be optimized by the isotopic characterization of granitic suites

petrological constraints on crystallization conditions of mesoarchean sanukitoid rocks southeastern amazonian craton brazil we report petrological and geochemical data for the 2 87 ga rio maria sanukitoid granodiorite and associated rocks from mesoarchean granite greenstone terranes of the eastern amazonian craton brazil the dominant rocks have granodiorite to subordinate monzogranitic compositions with minor proportions of intermediate quartz diorites or quartz monzodiorites in addition to mafic endmembers occurring as layered rocks or as enclaves the mineral assemblage is dominated by amphibole plagioclase biotite and epidote minerals all of inferred magmatic origin pyroxenes being notably absent textural and compositional criteria indicate that amphibole is a principal mineral on the liquidus of all the rio maria rocks crystallization conditions have been derived from a comparison between natural phase assemblages proportions and compositions and experimental studies carried out on similar magma compositions the comparison shows that the parental magmas were water rich with more than 7 wt dissolved h2o with crystallization temperatures in the range 950 680 c the mg mg fe ratios of both amphibole and biotite indicate fo2 conditions in the range nno 0 5 to nno 2 5 where nno is nickel nickel oxide buffer therefore pointing to both water rich and oxidizing conditions for sanukitoid magmas amphibole compositions indicate emplacement at around 200 mpa and record a high pressure stage of magma crystallization around 600 900 mpa sanukitoid magmas share two of the principal characteristics of modern arc magmas elevated redox state and volatile contents which suggest that they may have formed in a geodynamic environment broadly similar to present day subduction zones

analog modeling of one way gravitational spreading of hot orogens a case study from the svecofennian orogen fennoscandian shield abstract geodynamic models offer insight into deformation styles of syn and post collisional extensional processes such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries this paper presents a set of crustal scale analog experiments of one way post collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks pre existing cuts with varying dips and three layers with contrasting rheologies two blocks represent strong mature and weak juvenile crustal parts both blocks have three horizontal crustal layers brittle upper weak partially molten middle and dry strong lower layer cuts represent pre existing crustal scale shear zones and tectonic boundaries the experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain when the boundaries dip opposite to the flow direction the tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures shortening elongation or rotation the tectonic boundaries rotated along vertical axes toward the flow direction the modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units in both lateral and vertical directions to validate the modeling the results are compared to geophysical and geological data from the paleoproterozoic svecofennian orogen in the fennoscandian shield the comparison suggests that the orogen underwent 50 extension and was thinned by maximum of 20 via westward gravitational spreading this spreading rotated the crustal scale blocks shear zones and tectonic contacts it also brought exhumed high grade middle crustal blocks into contact with subsided low grade upper crustal blocks we suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens which have undergone post convergence continental mid crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions

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

petrogenesis and geodynamic implications of late carboniferous sanukitic dikes from the bieluagaxi area of west junggar nw china abstract sanukitic dikes mainly consisting of pyroxene diorite porphyrite diorite porphyry quartz diorite porphyrite and quartz diorite were widely distributed in the bieluagaxi area southern west junggar in this contribution we present new la icp ms zircon u pb ages major and trace elements and sr nd hf isotopes for these dioritic dikes these rocks formed in the early late carboniferous ca 319 315 ma coeval with abundant magmatic rocks in the region and exhibit high mgo cr and ni concentrations and k na ratios and also show strong enrichment in large ion lithophile elements relative to high field strength elements with pronounced negative nb ta and ti anomalies analogues to those of cenozoic sanukitoids of the setouchi volcanic belt compositionally the sanukitic dikes are divided into two groups group 1 have mgo contents greater than 6 wt 6 18 9 34 wt with mg values ranging from 58 2 to 73 7 they also have relatively low sio2 51 7 55 7 wt and sr 356 ppm in average contents and low sr y 15 1 25 4 ratios but high concentrations of cr 6 2 490 ppm and ni 12 8 163 ppm compared to the group 1 the group 2 samples have slightly lower mgo 2 12 5 58 wt mg 45 8 68 5 cr 10 1 329 ppm and ni 2 84 136 ppm but relatively higher sio2 58 6 65 7 wt and sr 304 521 ppm concentrations and sr y 19 2 41 0 ratios making them akin to adakitic rocks elsewhere these dikes also exhibit depleted sr nd hf isotopic signatures with 87sr 86sr i 0 70353 0 70480 end t 5 03 to 7 61 ehf t 10 9 to 15 3 geochemical and sr nd hf isotopic data suggest that the group 1 rocks are similar to the typical sanukitoids of the setouchi volcanic belt and were likely derived from partial melting of a depleted mantle source with subducted sediment melts while the group 2 rocks show a remarkable geochemical affinity with the northwestern karamay sanukitoids and were derived from partial melting of a depleted mantle source metasomatized by slab derived adakitic melts and fluids combined with the identified and reported subduction related magmatism i e adakites sanukitoids nb enriched basalts lagged arc volcanic rocks and i type granitoids in the western junggar region and taking into consideration the previous achievements therefore we consider a multi stage subduction accretionary orogeny model may account for the difference of subduction duration between the northern segment and southern segment of western junggar

pb isotopes a multi function tool for assessing tectonothermal events and crust mantle recycling at late archaean convergent margins abstract lead isotopes are a powerful tool in studying archaean granitoids and continental growth because 207pb is a sensitive indicator of involvement of lead from old archaean crustal sources this paper aims to demonstrate the significance and promote the use of lead isotopes by reviewing the theory concepts and traditions for interpretations of pb isotope data as well as by introducing a global model based on crust mantle recycling for discriminating late archaean convergent margin settings of granitoids case plots of k feldspar and whole rock data of ca 2 7 ga granitoids from several archaean cratons show that lead isotope systematics when interpreted in the correct geological context can be used for resolving many questions on crustal evolution from magmatic and tectonothermal history of individual intrusions to large scale tectonics and supercontinent correlations

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

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

p t t development of archaean granulites in varpaisjarvi central finland ii dating of high grade metamorphism with the u pb and sm nd methods exposed blocks of lower crustal archaean granulites separated by palaeoproterozoic faults occur near the western boundary of the karelian craton in central finland the southwesternmost granulite block in the study area the jonsa block differs from other granulites by having younger zircon u pb and sm nd tdm model ages the u pb ages on zircons and monazites from leucosomes and mesosomes of mafic and pelitic granulites in the jonsa block are ca 2 63 ga which is interpreted as the age of granulite metamorphism igneous enderbites which make up a considerable part of the bedrock in granulite blocks are ca 50 ma older outside the jonsa block both conventional and ion probe zircon ages of mesosomes of migmatitic mafic granulites are 3 2 3 1 ga which are the protolith ages sm nd tdm model ages of the mafic granulites are 2 7 2 9 ga in the jonsa block but ca 3 2 ga in the other granulites the age differences are interpreted to represent terrane accretion that juxtaposed 3 2 ga rocks next to younger rocks all granulites were metamorphosed in similar pt conditions at 2 63 ga granulite metamorphism affected the whole area but was not able to reset zircons in older rocks in all blocks the sm nd garnet whole rock ages are younger ranging from 2 48 2 59 ga this reflects relatively low closure temperature of the sm nd system in garnet in these rocks the u pb age of a palaeoproterozoic dolerite which cuts the granulites is 2 3 ga close to the k ar age of amphibole in a retrogressed fracture

three tier transition of neoarchean ttg sanukitoid magmatism in the beit bridge complex southern africa abstract neoarchean ttg sanukitoid associations of contrasting scales occur within the beit bridge complex terrane of the limpopo complex in southern africa these include the smaller 2 65 2 63 ga avoca granitoid and the voluminous 2 73 2 64 ga alldays granitoid this study characterizes the wide compositional spectrum preserved in these two granitoids the elliptical avoca pluton consists of a biotite amphibole orthopyroxene clinopyroxene bearing core that is dominantly trondhjemite with less dominant tonalite and granodiorite variants and a thin amphibole biotite bearing granite rim with local occurrence of two pyroxene bearing metabasite boudins while both the core and rim rocks exhibit a linear fabric the granite in addition preserves a penetrative foliation field relations of granite enclaves in the core rocks together with available ages indicate that the core rocks intruded the granite the foliated biotite amphibole bearing alldays granitoid contains inclusions of older supracrustals and rocks of the messina layered intrusion and is widely distributed compositionally it include tonalites and granodiorites and to a lesser extent trondhjemites both the avoca core and rim rocks are characterized by difference in mineral chemistry with the mafic minerals mg rich in the ttg core while they are fe rich in the granite and metabasite in comparison biotite is mg rich and amphibole is fe rich in the alldays granitoid two groups of alldays ttg can be delineated in terms of whole rock geochemical characteristics and are comparable to the low to medium pressure ttg groups delineated by moyen 2011 while the avoca ttg is similar to the high pressure ttg group the lowest silica samples from each group of granitoid have geochemical characteristics comparable to archean sanukitoids with those from the avoca granitoid similar to low ti sanukitoids and those from the alldays granitoid similar to low ti and high ti sanukitoids separate petrogenetic models are suggested for different phases of the avoca core with the trondhjemite tonalites considered as high pressure melts of metabasalt while the granodiorite with lower sio2 content higher k2o and mgo contents and higher incompatible element contents than the trondhjemite tonalites is a product of hybridization of earlier ttg melts and peridodite granite from the avoca rim are low pressure melts of pre existing crustal lithologies the two groups of alldays ttg with lower sr y ratios than the avoca ttg are considered as low to medium pressure melts of metabasalt whose progressive interaction with peridotitic mantle at shallower angles account for the unique composition of alldays low ti and high ti sanukitoids taken together with their spatial and temporal transition from southeastern 2 73 2 72 ga low pressure ttg low ti sanukitoid to central 2 65 2 64 ga medium pressure ttg high ti sanukitoid to northwestern 2 63 ga high pressure ttg low ti sanukitoid parts of the beit bridge complex the three tier transition of ttg sanukitoid magmatism argues for the southern margin of the beit bridge complex to represent an active arc in the neoarchean

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

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

neoarchean sanukitoids and associated rocks from the tengzhou pingyi intrusive complex north china craton insights into petrogenesis and crust mantle interactions abstract quartz monzodiorites granodiorites and monzogranites constitute the dominant lithological assemblage in the neoarchean tengzhou pingyi intrusive complex tpic in the southwestern part of western shandong province wsp in the eastern part of the north china craton ncc and mafic magmatic enclaves mmes are widely developed only in granodiorites of the tpic la icp ms zircon u pb isotopic dating reveals that the tpic was emplaced at 2 54 2 52 ga the 2 54 ga monzogranites display geochemical affinities with high k granites and they were likely derived from the partial melting of metamorphic greywackes at lower crustal levels the 2 54 2 52 ga quartz monzodiorites and granodiorites show significant geochemical similarities to the archean sanukitoids of the western karelian province of finland with higher mg 43 57 values cr 24 88 ppm sr 547 1154 ppm and ba 662 1961 ppm contents and k2o na2o 0 42 1 00 ratios therefore we define the quartz monzodiorites and granodiorites as neoarchean sanukitoids based on their lithological assemblages and geochemical characteristics these sanukitoids can be subdivided into sanukitoid quartz monzodiorites sqms and porphyritic sanukitoid granodiorites psgs the sqms exhibit higher positive hf t2 values from 1 9 to 3 6 with tdm values of 2 70 to 2 77 ga and are strongly enriched in light rare earth elements lrees and large ion lithophile elements liles implying a mantle source that was enriched by either fluids or melts the psgs have a wide range of hf t2 values from 3 4 to 3 8 suggesting that they were formed by crust mantle magma mixing this interpretation is also supported by the petrological textural and compositional disequilibrium of plagioclase phenocrysts from the psgs moreover the mmes and their host psgs have identical crystallization ages 2 54 ga similar mineral chemistries and zircon lu hf isotopic compositions indicating that the mmes are cumulates from the earlier host psgs magma petrogenetic studies and zircon lu hf isotopic features of these lithological assemblages indicate that the interval of 2 55 2 52 ga in the tpic represents an important period of crustal growth and crust mantle interactions considering the spatial temporal relationships and the petrogenesis of the diverse lithological assemblages we propose that the neoarchean tpic was most likely formed in an active continental margin setting

high magnesian granitoids in the precambrian continental crust implication for the continuum between ferro potassic and magnesio potassic rock suites abstract ferro potassic fe k to magnesio potassic mg k hornblende biotite granitoids hbgs with a wide range of sio2 from 52 to 77 wt are typical of precambrian continental crust and are generally related to late and post collisional orogenic events despite their broadly uniform mineralogy and tectonic environment the chemical compositions of these granites suggest different sources from lower crust for the fe k suites to enriched mantle for the mg k counterparts here we propose a classification of the magnesian rocks into moderate mg high mg and ultra mg with corresponding examples from various parts of the world such as the closepet type sanukitoids and their high mg counterparts of the elan type the elan type granitoids are characterized by the highest mgo cr ni end t and lowest tio2 feot feot mgo contents among all the precambrian hbgs and are hence close to a true end member the precambrian hbgs show common features such as a wide range of sio2 contents enrichment in lile k ba sr lree nb ti negative primitive mantle normalized anomalies and weak negative or no eu anomalies in intermediate suites the mg of magmatic and cumulus relict amphiboles displays a compositional array of ultra mg to fe granites and shows positive correlation with the whole rock mg continuum between the compositions of mg k and fe k granitoids is reflected in a gradual decrease in mg cr ni of the rocks and mg of the mafic silicates increase in ti p zr y zn u total ree and decrease of the nd initial isotopic composition together with a change in the paragenesis of fe ti oxides we interpret these features to result from mixing of metasomatized mantle and crustal magmas and their homogenization at the lower crust level the lower crustal source component increases from the mg k to fe k series during their passage to higher crustal levels the magmas underwent fractional crystallization and crustal contamination leading to the wide range in silica content

origin and evolution of granitoids associated with the kadiri greenstone belt eastern dharwar craton a history of orogenic to anorogenic magmatism abstract different tectonic models have been proposed to explain the neoarchaean evolution of the eastern dharwar craton edc here we present whole rock elemental and nd isotopic compositions coupled with zircon u pb age and trace elemental data on granitoids associated with the neoarchaean kadiri greenstone belt edc to track the tectonic evolutionary history of the terrain earlier works on volcanic rocks of the kadiri belt tholeiitic oceanic plateau basalt high mg andesite and potassic adakite envisaged continental margin arc oceanic plateau collision in this work the oldest recognized granitoid is a 2 56 ga syntectonic sanukitoid like hornblende granodiorite enriched in large ion lithophile as well as ferromagnesian elements it was probably derived from a metasomatized mantle wedge above a subduction zone this is followed by syntectonic emplacement of 2 54 ga transitional ttgs and potassic adakitic volcanic rocks intrusion of 2 51 2 52 ga crustally derived granites mostly k rich nd 0 9 to 3 7 marks the end of orogenesis in the area a convergent orogenic setting characterized by assembly of diverse terranes repeated slab break off and crustal reworking is proposed to interpret the association of various granitoids and volcanic rocks finally intrusion of crustally derived highly silicic k rich a type granite nd 0 6 to 4 8 and mantle derived syenite occurred in a post subduction non compressive environment during palaeoproterozoic lithospheric delamination and attendant mantle melting and crustal reworking are suggested as possible mechanism for generation of these rocks the negative nd values and mesoarchaean nd depleted mantle model ages of some neoarchaean to palaeoproterozoic kadiri granitoids attest to the presence of older crust in the eastern part of the edc the final cratonization of the edc 2 52 2 50 ga occurred later than in most of the other cratons

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

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

2 7ga juvenile crust formation in the north china craton taishan xintai area western shandong province further evidence of an understated event from u pb dating and hf isotopic composition of zircon abstract the evolution of the north china craton ncc is well known for a marked 2 55 2 50 ga tectonothermal event however supracrustal and intrusive rocks of 2 75 2 70 ga are in fact widely distributed in the western shandong province the most important area of archaean basement in the eastern part of the ncc this paper reports shrimp u pb dating and la icpms hf isotopic composition of zircons from 2 75 2 70 ga supracrustal and trondhjemite tonalite granodiorite ttg rocks in that area three fine grained hornblende biotite gneiss samples known locally as leptynite with meta volcanic or volcanosedimentary rock protoliths and five ttg samples have shrimp zircon u pb ages varying from 2 75 to 2 70 ga and 2 74 to 2 71 ga respectively zircons from most of the samples have high positive hf t values 4 7 to 10 0 and t dm hf ages 2 85 2 60 ga similar to their zircon u pb ages this indicates that the rocks represent largely juvenile crustal additions derived from depleted mantle only a short time before however some granitoids show hf t zircon values of 13 6 to 5 1 and t dm hf of 3 51 2 80 ga therefore the strong 2 75 2 70 ga tectonothermal event in the western shandong province involved not only juvenile addition to the continental crust but also intracrustal recycling of older components combined with craton wide data it is shown that the ncc is similar to many other cratons around the world where tectonothermal events of 2 7 ga are well developed however the main difference is that in the ncc superimposed 2 5 ga tectonothermal events were much stronger than in most other cratons

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

middle neoarchean magmatism in western shandong north china craton shrimp zircon dating and la icp ms hf isotope analysis abstract western shandong province is a typical area of the north china craton ncc where neoarchean plutonic and supracrustal rocks are widely distributed early studies documented that 2 7 ga and 2 5 ga magmato tectono thermal events are well developed in the area here we report shrimp u pb ages and hf in zircon isotopic data from ten samples of different magmatic rock types including hornblendite gneissic tonalite gneissic trondhjemite and gneissic granite magmatic zircon grains have 207 pb 206 pb ages ranging from 2667 to 2598 ma some rocks contain 2 5 ga metamorphic rims and 2 7 ga zircon cores the magmatic zircon grains have hf t values and hf crustal model ages of 1 1 to 11 3 and 3 02 2 4 ga respectively combined with an earlier study our main conclusions are that the middle neoarchean rocks mainly occur together with early neoarchean rocks in the northeastern portion of the central belt juvenile additions to continental crust and crustal recycling played important roles in the middle neoarchean of western shandong province and the entire neoarchean tectonic evolution can be divided into middle to early neoarchean 2 75 2 6 ga and late neoarchean 2 6 2 5 ga events

petrogenesis of mantle derived lile enriched archean monzodiorites and trachyandesites sanukitoids in southwestern superior province reply in southwestern superior province diorite monzodiorite and trachyandesite sanukitoids occurring within syn to post tectonic intrusive complexes and within greenstone belts have the following chemical characteristics 55 60 wt sio2 mgo 6 wt mg 0 60 ni and cr both 100 ppm na2o k2o 6 wt sr and ba both 600 1800 ppm and rare earth element ree patterns that are strongly light rare earth element lree enriched cen 80 250 ybn 4 10 and show no eu anomalies sanukitoids and their granodioritic derivatives constitute at least 5 of the exposed crust in the study area the sanukitoids cannot be derived by melting fractionation or crustal contamination of basalts or lamprophyres that are coeval with the sanukitoids crustal contamination of komatiites fails to explain the high large ion lithophile element lile contents of the sanukitoids rather we suggest that the sanukitoids were derived by hydrous melting of lile enriched mantle peridotite at pressures between 10 and

early cretaceous high mg diorites in the yanji area northeastern china petrogenesis and tectonic implications abstract mesozoic granitic rocks are widely distributed in northeast ne china however high mg dioritic rocks are considerably rare here we report a newly recognized high mg diorite the xintun diorite in the yanji area ne china to constrain its origin and implications for the tectonic evolution of eastern asian continental margin zircon u pb dating yields a crystallization age of 128 1 ma for the xintun diorite the diorites are characterized by high mgo 4 4 6 6 wt cr 119 239 ppm ba 419 514 ppm and sr 649 747 ppm contents and mg values 59 64 but low feototal mgo ratios 1 2 1 4 with geochemical features similar to those of sanukitic high mg andesites hmas they show moderate radiogenic sr isr 0 7047 0 7050 and nd end 0 3 1 1 with high la sm ratios which are indicative of contributions from sediment components the mineral assemblage of euhedral hornblende magnetite and titanite implies a water rich and oxidized signature for their primitive magmas these features suggest that the xintun high mg diorites were probably formed via partial melting of the subducting sediments and subsequent interaction of mantle peridotites with both melts and aqueous fluids geochemical modeling reveals that hornblende dominated fractional crystallization under water sufficient conditions enabled the evolved magmas to acquire adakitic signatures we believe that the paleo pacific subduction beneath eastern asian continental margin caused large scale back arc extension of ne china in the early cretaceous and consequently induced the asthenospheric flow toward the mantle wedge reheating subducting sediments enough to cause melting therefore the occurrence of the xintun high mg diorites signifies the onset of extensive back arc extension of eastern asian continental margin at ca 128 ma

petrogenesis of the devonian high mg rock association and its tectonic implication for the chinese altai orogenic belt nw china abstract the chinese altai is a key region to decipher the accretionary history of the gigantic caob one of the largest accretionary orogenic belts on the earth however tectonic implication of the widely distributed paleozoic magmatic rocks remain debated in this study a suite of igneous rocks including high mg dacite of the kangbutiebao formation and the associated gneissic granodiorites have been studied for their petrogenesis and tectonic implication the high mg dacite is dated at 400 2 ma which broadly resembles the emplacement age of the high mg gneissic granodiorites of 406 9 ma possibly suggesting that they were generated in the same tectono magmatic event both rocks have predominant zircon inheritances of 500 ma which is consistent with the age of predominant zircon population for the widespread habahe sedimentary sequence indicating that their precursor magmas were probably mainly derived from the habahe sediments the high mg rock association has similar ree and trace element patterns to those of the habahe sediments and they plot close to the field of the habahe sediments in th ta versus la ta diagram their high a cnk ratios 1 33 2 05 imply magma source dominated by sedimentary rocks similarly their high la sm ratios also suggest a greater contribution of sediments rather than altered oceanic crust in the magma generation however the high mg values 51 of the studied rocks argue against derivation purely from remelting of the habahe sediments in order to explain the high mg values of these rocks the participation of mantle derived melt into the precursor magma is considered zircons from this rock association mostly yield e hf t values from 0 85 to 9 71 supporting the involvement of juvenile materials in the magma generation samples from the high mg rock association fall in the same field as the sanukitoids in tio 2 versus mg and sr y versus y diagrams and they plot in the fractional crystallization trend of differentiated sanukitoids in harker diagram possibly suggesting a petrogenetic process like the typical sanukitoid magma elsewhere this study shows that similar to the sanukitoids in the setouchi area our high mg rock association also involved melting of subducted sediments in a hot subduction regime combined with previous work a ridge subduction regime is considered to interpret the early devonian tectonic evolution of the chinese altai

zircon geochronology geochemistry and origin of the ttg suites of the rio maria granite greenstone terrane implications for the growth of the archean crust of the carajas province brazil abstract ttg suites are the most voluminous granitoid rocks exposed in the mesoarchean rio maria granite greenstone terrane rmggt southeastern amazonian craton extensive field work in key areas of the rmggt integrated with petrographic geochemical and geochronological studies the latter employing the pb pb evaporation and u pb la mc icpms on zircon techniques indicates that the ttg magmatism in the rmggt can be divided into three episodes 1 at 2 96 0 02 ga the older rocks of the arco verde tonalite and the mogno trondhjemite 2 at 2 93 0 02 ga caracol tonalitic complex mariazinha tonalite and the younger rocks of the arco verde tonalite and 3 at 2 86 0 01 ga agua fria trondhjemite the new data demonstrate that the mogno trondhjemite is significantly older than thus far assumed reveal the existence of a new ttg suite mariazinha tonalite and indicate that the volume of ttg suites formed during the 2 86 event was relatively small the arco verde tonalite yielded significant age variations 2 98 2 93 ga but domains with different ages could not be discerned the tonalitic trondhjemitic suites of the rmggt derived from sources geochemically similar to the metabasalts of the andorinhas supergroup that were extracted from the mantle during the mesoarchean 3 0 2 9 ga three groups of ttg granitoids were distinguished in rio maria 1 high la yb group with high sr y and nb ta ratios derived from magmas generated at relatively high pressure 1 5 gpa from sources with residual garnet and amphibole 2 medium la yb group with magmas formed at intermediate pressure 1 0 1 5 gpa but still in the garnet stability field and 3 low la yb group with low sr y and nb ta ratios crystallized from magmas generated at lower pressures 1 0 gpa from an amphibolitic source with residual plagioclase these three geochemical groups do not have a direct correspondence with the three episodes of ttg generation and a single ttg unit can be composed of rocks of different groups a model involving hot subduction underneath a thick oceanic plateau was envisaged to explain the tectonic evolution of the rmggt in this context the low la yb group was formed from magmas originated by the melting of the base of a thickened basaltic oceanic crust at comparatively lower pressures ca 1 0 gpa whereas the medium and high la yb groups were derived from slab melting at differing pressures 1 0 1 5 and 1 5 gpa respectively part of the ttg magmas formed between 2 98 and 2 92 ga reacted during their ascent with the mantle wedge and were consumed in the process leaving behind a metasomatized mantle 50 m y later at ca 2870 ma thermal events possibly related to slab break off and resultant asthenosphere mantle upwelling or to a mantle plume induced melting of the metassomatized mantle to form sanukitoid magmas these magmas may have heated the base of the archean continental crust and could have led to local melting of basaltic crust forming the agua fria trondhjemite magma