amphibolites of the ashe and alligator back formations north carolina samples of late proterozoic early paleozoic oceanic crust the amphibolites of the late proterozoic early paleozoic ashe and alligator back formations in western north carolina represent metamorphosed tholeiitic basalts unrelated to the mafic dikes of continental tholeiitic affiliation that intrude the grenville age basement rocks on the basis of their geochemistry the amphibolites are divisible into three compositional groups i ii and iii the protoliths of which appear to have evolved from different parental magmas at a spreading center group ii intermediate ti amphibolite the predominant variety in the study area constitutes a highly fractionated suite zr 40 200 ppm tio2 0 7 2 3 wt with geochemical characteristics similar to those of n type morb formed from a heterogeneous mantle source group iii high ti amphibolite is enriched in high field strength incompatible elements zr nb ti y ree s especially ti tio2 2 8 3 4 wt and is interpreted as t type morb on the basis of its zr nb and y nb ratios group i low ti amphibolite restricted to the ashe formation is characterized by extremely low abundances of incompatible elements including ti tio2 0 45 wt and u shaped ree profiles requiring the mixing of a depleted morb type mantle source with a fluid phase enriched in lree s the juxtaposition of depleted low ti basalt group i amphibolite and morb like basalts group ii and group iii amphibolites is suggestive of a back arc basin setting as has been postulated for many ophiolites but it can also be accounted for by multi stage melting of an upper mantle source adjacent to a mantle plume in a mid oceanic ridge environment in either case the ashe and alligator back amphibolites represent metamorphosed oceanic crust material generated at a spreading center

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

late holocene erosion in upland britain climatic deterioration or human influence the evidence for enhanced erosion in upland britain during the late holocene is evaluated though there is widespread evidence for recent soil erosion on high plateaux this has not been precisely dated radiocarbon dating confirms the occurrence of late holocene solifluction on high ground but at only one site has evidence been found for recent accelerated activity several areas however have yielded evidence of late holocene debris cone accumulation and parts of the scottish highlands experienced a drastic increase in debris flow activity over the past few centuries climatic deterioration and vegetation degradation resulting from overgrazing or burning are frequently cited as the causes of enhanced erosion but cause and effect are often related only by coincidence of timing in consequence only circumstantial evidence links erosion with climatic deterioration though anthropogenic influences have been more firmly established at a few sites the role of extreme events in initiating phases of uplan

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

collapse of archean orogens and the generation of late to postkinematic granitoids intrusion of late stage granitoids into archean granite greenstone terranes may be spatially and temporally related to late orogenic upper crustal extensional collapse and formation of metamorphic core complexes the late to postkinematic granitoid magmas are derived through partial melting of upper mantle and lower crustal sources and are locally associated with late stage extensional structures thick roots of depleted lithospheric mantle of archean age tectosphere beneath archean cratons makes melting of the upper mantle difficult it is proposed that the archean tectosphere is made of imbricated slabs of buoyant archean oceanic crust and lithosphere together with trapped wedges of fertile mantle this would explain several seemingly contradictory phenomena decompression from upper crustal extension generates significant amounts of basaltic melts from the trapped wedges of fertile mantle these basaltic magmas intrude and partially melt the lower crust becoming more silicic in composition they migrate upward to solidify in the middle and upper crust forming the late to postkinematic granitoid suite the tectosphere in this model is less dense and colder than surrounding asthenosphere and is consistent with eclogitic xenoliths from subducted oceanic lithosphere found entrained in kimberlites the direction and magnitude of shear wave anisotropy in the upper mantle is also explained in this model through the orientation of olivine crystals in the underplated slabs of oceanic lithosphere

uhp metamorphism and exhumation of the dabie orogen china evidence from shrimp dating of zircon and monazite from a uhp granitic gneiss cobble from the hefei basin a shrimp u pb study of zircons separated from an ultrahigh pressure uhp granitic gneiss cobble sample ds12 from the upper jurassic fenghuangtai formation of the hefei basin north of the dabie orogen has identified three different domains 1 cores some of which show straight boundaries and strong oscillatory zoning with th u ratios of 0 12 0 70 and an imprecise upper intercept age of 777 220 ma mswd 2 4 2 mantles variable in shape and cathodoluminescence cl intensity and containing many uhp mineral inclusions including coesite and omphacite and with th u ratios of 0 02 0 26 and a weighted mean 206pb 238u age of 244 5 ma mswd 4 7 and 3 rims which are more homogeneous and luminescent in cl than the mantles and contain fewer uhp mineral inclusions with th u ratios of 0 01 0 05 and a weighted mean 206pb 238u age of 226 2 ma mswd 0 65 monazites from the same sample do not contain coesite and omphacite inclusions but they do show sector zones and are composed of two contrasting domains the light domains in backscattered electron bse images have a th u ratio of 7 3 18 9 and a weighted mean 206pb 238u age of 221 2 ma mswd 0 94 the dark domains in bse have a th u ratio of 2 8 7 2 and have a weighted mean 206pb 238u age of 218 2 ma mswd 0 96 based on these data and a review of the literature it can be concluded that 1 the protolith of gneiss sample ds12 is a granitoid of neoproterozoic age 2 the ages of 244 and 226 ma for the mantle and rim domains of the zircons are time records of two discrete stages in the uhp evolution of dabieshan 3 the ages of 221 and 218 ma for the light and dark gray domains respectively of the monazites record the times of two stages of retrogressive metamorphism during exhumation of the uhp granitic gneiss to a higher crustal level 4 the exhumation rate of the gneiss is estimated to be 6 km ma and 5 the uhp metamorphic blocks of the dabie orogen provided significant detritus to the hefei basin in the jurassic

tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

high precision isotopic characterization of usgs reference materials by tims and mc icp ms the pacific centre for isotopic and geochemical research pcigr at the university of british columbia has undertaken a systematic analysis of the isotopic sr nd and pb compositions and concentrations of a broad compositional range of u s geological survey usgs reference materials including basalt bcr 1 2 bhvo 1 2 andesite agv 1 2 rhyolite rgm 1 2 syenite stm 1 2 granodiorite gsp 2 and granite g 2 3 usgs rock reference materials are geochemically well characterized but there is neither a systematic methodology nor a database for radiogenic isotopic compositions even for the widely used bcr 1 this investigation represents the first comprehensive systematic analysis of the isotopic composition and concentration of usgs reference materials and provides an important database for the isotopic community in addition the range of equipment at the pcigr including a nu instruments plasma mc icp ms a thermo finnigan triton tims and a thermo finnigan element2 hr icp ms permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the tims and mc icp ms methods e g nd isotopic compositions for each of the reference materials 5 to 10 complete replicate analyses provide coherent isotopic results all with external precision below 30 ppm 2 sd for sr and nd isotopic compositions 27 and 24 ppm for tims and mc icp ms respectively our results also show that the first and second generation usgs reference materials have homogeneous sr and nd isotopic compositions nd isotopic compositions by mc icp ms and tims agree to within 15 ppm for all reference materials interlaboratory mc icp ms comparisons show excellent agreement for pb isotopic compositions however the reproducibility is not as good as for sr and nd a careful sequential leaching experiment of three first and second generation reference materials bcr bhvo agv indicates that the heterogeneity in pb isotopic compositions and concentrations could be directly related to contamination by the steel mortar pestle used to process the materials contamination also accounts for the high concentrations of certain other trace elements e g li mo cd sn sb w in various usgs reference materials

unexposed archean components and complex post archean accretion reworking processes beneath the southern yangtze block revealed by zircon xenocrysts from the paleozoic lamproites south china abstract integrated u pb dating and hf isotope analysis of zircon xenocrysts from the baifen lamproites of the zhenyuan area in the southeastern guizhou province offer a unique glimpse into the evolution of the deep continental crust beneath the southern part of the yangtze block south china a total of 148 zircons with 90 110 concordance yield u pb ages varying from 3458 9 ma to 252 2 ma they generally have high th u ratios 0 3 indicating a magmatic origin the two youngest magmatic zircons give a weighted mean 206pb 238u age of 253 0 2 8 ma which is interpreted as the emplacement age of the host baifen lamproites most zircons with pre eruption ages are xenocrysts derived from the deeper crust of the southern yangtze block they define two major paleoproterozoic age populations at 2 17 ga and 1 8 ga with similar hf model ages tcrust of 2 4 2 8 ga peak at 2 64 ga the oldest zircon has a 207pb 206pb age of 3458 9 ma with ehf t 1 47 and tcrust 3 7 ga this demonstrates the presence of archean crustal components or remnants as old as 3 7 ga beneath the southern yangtze block magmatic events involving significant addition of juvenile mantle derived materials mainly occurred at 2 2 2 1 ga and 2 0 1 8 ga but the latter event also involved reworking of the neoarchean crust and might represent the cratonization of the yangtze continent during the middle paleoproterozoic which could be correlated to the assembly of the columbia supercontinent this segment of the yangtze block became tectonically quiescent until 1 2 ga implying that the crustal stabilization lasted for ca 600 million years a few late mesoproterozoic zircon xenocrysts within the age range 1 3 1 0 ga suggest the occurrence of grenville aged events these neoproterozoic zircon xenocrysts 992 547 ma have a wide range of ehf t values 8 47 to 22 07 consistent with the zircon hf isotopic compositions of widespread coeval igneous rocks in the yangtze block the data show that the yangtze block has undergone important crustal growth and reworking during the neoproterozoic in response to the breakup of the rodinia supercontinent the early middle paleozoic zircon xenocrysts 513 357 ma have negative ehf t values 2 97 to 12 69 and tcrust hf model ages of 1 59 2 27 ga indicating reworking of the precambrian crustal basement without juvenile contributions during the caledonian orogeny the positive ehf t values 13 79 to 0 22 of twelve late paleozoic zircons 284 264 ma with a major age peak at ca 275 ma suggest that the precambrian basement beneath the southern yangtze block experienced an episode of strong rejuvenation during the permian we propose a possible linkage between the baifen lamproite magmatism 284 252 ma and the emeishan flood basalt volcanism

the late archean history of the wyoming province as recorded by granitic magmatism in the wind river range wyoming abstract the wyoming province a small ca 500 000 km 2 archean craton is the most southwestern of the archean provinces in north america it is composed primarily of late archean potassium rich granitic rocks in contrast to many other archean provinces rocks of tonalite trondhjemite affinity are rare over most of the province and are restricted to rocks older than 2 8 ga field petrologic geochemical and isotopic study of the late archean granites exposed in the wind river range have allowed us to identify at least four periods of potassic calc alkalic magmatism at 2 8 2 67 2 63 and 2 55 ga granitic rocks of these ages appear to be widespread across the wyoming province the oldest calc alkalic granites of the wind river range emplaced at ca 2 8 ga appear to be derived predominantly from pre existing crust however nd isotopic data suggest that these granites cannot be the product solely of partial melting of older tonalitic gray gneisses during at least two other periods of plutonism at 2 67 and 2 63 ga generation of the wind river range batholiths involved the incorporation of substantial amounts of isotopically juvenile material either from depleted mantle or young continental crust the information presented below as well as data available from elsewhere in the wyoming province is interpreted to suggest that the wyoming province unlike other archean cratons is not composed of a tectonic amalgamation of smaller exotic terranes although the wyoming province did experience crustal addition in archean time it was not by lateral accretion but by incorporation of mantle derived melts into large granitic batholiths

plutonism in three orogenic pulses eastern blue ridge province southern appalachians department of geological sciences university of north carolina mitchell hall chapel hill nc 27599 3315

electrical conductivity structure of southeastern north america implications for lithospheric architecture and appalachian topographic rejuvenation abstract we present the first three dimensional view of the lithospheric electrical conductivity structure beneath southeastern north america by inverting earthscope long period magnetotelluric mt data we obtain an electrical conductivity image that provides new insights into both the architecture of the appalachian orogen and the cryptic post rifting geodynamic history of the southeastern united states our inverse solutions reveal several elongate electrically conductive features that we interpret as major terrane sutures within the appalachian orogen most significantly we resolve a highly electrically resistive layer that extends to mantle depths beneath the modern piedmont and coastal plain physiographic provinces as high resistivity values in mantle minerals require cold mantle temperatures the mt data indicate that the sub piedmont thermal lithosphere must extend to greater than 200 km depth this firm bound conflicts with conclusions from seismic results the boundary between the anomalously thick resistive sub piedmont lithosphere and the relatively thin moderately conductive sub appalachian lithosphere corresponds within resolution to the modern appalachian topographic escarpment this newly recognized contrast in lithospheric properties likely has important implications for appalachian topographic rejuvenation

metamorphic conditions in the ashe metamorphic suite north carolina blue ridge taconian metamorphism of mafic rocks in the ashe metamorphic suite can be characterized by reference to an isograd corresponding to the reaction bio epi hbl gar which separates rocks into two zones of low variance assemblages temperatures and pressures estimated from mineral exchange geothermometers and a barometer suggest that this reaction occurred at approximately 600 650 c and 7 5 kbar phase equilibria between biotite and hornblende as well as the sharpness of the mapped isograd indicate that the reaction is discontinuous inferred differences in metamorphic grade between ashe amphibolites and mafic dikes in the underlying basement suggest that these units are in faulted contact isograd patterns in pelitic rocks suggest an elongated domal uplift that developed after metamorphism and thrusting the core of which is exposed in the adjacent grandfather mountain window

petrogenesis and structure of the buck creek mafic ultramafic suite southern appalachians constraints on ophiolite evolution and emplacement in collisional orogens integration of detailed field petrological structural and geochemical observations of the buck creek mafic ultramafic suite which is among the larger southern appalachian ultramafic exposures points to an igneous origin as a mid ocean ridge cumulate massif with subsequent emplacement in a deep subduction zone setting these results help to clarify the unresolved role of alpine type ultramafic bodies in the southern appalachians a problem complicated by their generally small and fragmented nature and by significant tectonic overprinting as an ophiolite fragment the buck creek rocks are unusual in their preservation of relatively high pressure anhydrous conditions and thus they provide constraints on mechanisms of ophiolite emplacement field support for a cumulate origin for the buck creek suite includes centimeter to meter scale interlayering of dunite troctolite anorthosite and spinel rich layers and local gradation between buck creek troctolitic rocks and surrounding amphibolites relict anorthitic plagioclase forsteritic olivine and augitic clinopyroxene define a cumulate triangle on an mgo versus al 2 o 3 diagram meta troctolite and meta dunite compositions define a linear array between olivine and plagioclase compositions and many amphibolites fall within this triangle reflecting gabbroic protoliths the relatively abundant troctolite cumulates scarce pyroxene in the ultramafic rocks and the high al 2 o 3 tio 2 and low cr numbers in spinel best match lot or l type lherzolite ophiolites consistent with crystallization in a slow spreading mid ocean ridge setting sapphirine bearing spinel symplectites in the metatroctolites suggest that buck creek rocks remained anhydrous to 800 c and 0 9 1 1 gpa representing conditions atypical of ophiolite emplacement comparisons to the zermatt saas ophiolite and bergen eclogite suggest rapid subduction of buck creek rocks to depths of 30 km where partial hydration perhaps facilitated by pro grade dehydration reactions in surrounding rocks caused strain softening that permitted ductile disaggregation and aided in their emplacement into the overlying accretionary complex alternatively emplacement might have resulted from a switch in subduction polarity with hydration following emplacement uplift shortening and hydration within the accretionary sequences are responsible for the dominant structural features

zircon u pb age trace element and hf isotope composition of kongling terrane in the yangtze craton refining the timing of palaeoproterozoic high grade metamorphism u pb age trace element and hf isotope compositions of zircon were analysed for a metasedimentary rock and two amphibolites from the kongling terrane in the northern part of the yangtze craton the zircon shows distinct morphological and chemical characteristics most zircon in an amphibolite shows oscillatory zoning high th u and 176lu 177hf ratios high formation temperature high trace element contents clear negative eu anomaly as well as hree enriched patterns suggesting that it is igneous the zircon yields a weighted mean 207pb 206pb age of 2857 8 ma representing the age of the magmatic protolith the zircon in the other two samples is metamorphic it has low th u ratios low trace element concentrations variable hree contents 33 8 lun 2213 14 7 lun smn 354 and 176lu 177hf ratios 0 000030 0 001168 the data indicate that the zircon formed in the presence of garnet and under upper amphibolite facies conditions the metamorphic zircon yields a weighted mean 207pb 206pb age of 2010 13 ma these results combined with previously obtained palaeoproterozoic metamorphic ages suggest a c 2 0 ga palaeoproterozoic collisional event in the yangtze craton which may result from the assembly of the supercontinent columbia the zircon in two samples yields weighted mean two stage hf model tdm2 ages of 3217 110 and 2943 50 ma respectively indicating that their protoliths were mainly derived from archean crust

chronology of paleozoic metamorphism and deformation in the blue ridge thrust complex north carolina and tennessee the blue ridge province in northwestern north carolina and northeastern tennessee records a multiphase collisional and accretionary history from the mesoproterozoic through the paleozoic to constrain the tectonothermal evolution in this region radiometric ages have been determined for 23 regionally metamorphosed amphibolites granitic gneisses and pelitic schists and from mylonites along shear zones that bound thrust sheets and within an internal shear zone the garnet ages from the pumpkin patch a thrust sheet 458 455 and 451 ma are similar to those from the structurally overlying spruce pine thrust sheet 460 456 455 and 450 ma both thrust sheets exhibit similar upper amphibolite facies conditions because of the high closure temperature for garnet the garnet ages are interpreted to date growth at or near the peak of taconic metamorphism devonian metamorphic ages are recognized in the spruce pine thrust sheet where sm nd and rb sr garnet ages of 386 and 393 ma and mineral isochron ages of 397 14 and 375 27 ma are preserved hornblendes record similar sup 40 ar sup 39 ar sm nd and rb sr ages of 398 to 379 ma devonian sup 40 ar sup 39 ar hornblende ages are also recorded in the structurally lower pumpkin patch thrust sheet the devonian mineralmore ages are interpreted to date a discrete tectonothermal event as opposed to uplift and slow cooling from an ordovician metamorphic event the mississippian mylonitization is interpreted to represent thrusting and initial assembly of crystalline sheets associated with the alleghanian orogeny the composite thrust stack of the blue ridge complex was subsequently thrust northwestward along the linville falls fault during middle alleghanian orogeny about 300 ma less

geochronology and geochemistry of igneous rocks from the kongling terrane implications for mesoarchean to paleoproterozoic crustal evolution of the yangtze block abstract the kongling terrane is a high grade metamorphic complex exposed in the northern part of the yangtze block it mainly consists of ttg gneisses amphibolites migmatites and metasedimentary rocks the amphibolites occur as enclaves or layers in the ttg gneisses and both the amphibolites and the ttg gneisses are intruded by granitic and diabasic dykes u pb ages of zircons from the amphibolite migmatite granite and diabase are determined by la icp ms igneous zircons from an amphibolite have an upper intercept age of 2945 13 ma mswd 1 2 while metamorphic zircons yield an upper intercept age of 2870 13 ma mswd 0 9 igneous zircons from a migmatite and a granite have weighted average 207pb 206pb age and upper intercept age of 1987 5 ma mswd 1 7 and 1985 6 ma mswd 1 3 respectively igneous zircons from a diabase dyke yield an upper intercept age of 1783 23 ma mswd 0 86 the protoliths of the amphibolites are tholeiitic basalts and exhibit slightly depleted to variably enriched lree patterns with moderate enrichment of rb and ba and depletion in nb and zr similar to oceanic island arc volcanics they have high cr co ni mgo and feot content with negative nd t values indicating that the tholeiitic basalts resulted from high degree batch melting of enriched mantle wedge the granites are strongly peraluminous with a cnk 1 1 and they have relatively low hree and y and negative nd t values detailed elemental and isotopic data suggest that they formed by partial melting of recycled products of the ttg gneisses amphibolites and older crustal material in a continental collisional setting the diabases are characterized by relatively high al2o3 feot and tio2 contents and low zr ba ratios their high initial 87sr 86sr ratios and negative nd t values indicate that they were derived from enriched lithospheric mantle in extensional setting therefore the kongling terrane occurred as an oceanic island arc at ca 2 95 ga and experienced earliest metamorphism at ca 2 87 ga which probably contributed to the formation of the high grade metamorphic terrane this terrane experienced a strong collisional event at 2 0 ga which may have led to cratonization over all of the yangtze block and also experienced lithospheric extension during 1 85 1 78 ga the global 2 0 ga paleoproterozoic collisional events were related to the assembly of supercontinent columbia

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

early permian a type granites from central inner mongolia north china magmatic tracer of post collisional tectonics and oceanic crustal recycling abstract a type granitoids constitute volumetrically minor but genetically important building blocks of most post archean continental landmasses on earth and their diversified genesis encapsulates key information for ascertaining critical geodynamic controls on continental crustal formation and differentiation this zircon u pb dating and geochemical study documents the early permian peralkaline granites from central inner mongolia of china eastern central asian orogenic belt caob these granites show an a type magmatic affinity with typical enrichment in alkalis ga zr nb and y depletion in sr and p and fractionated ree patterns showing strong negative eu anomalies they have meaningful initial 87 sr 86 sr ratio of 0 70594 e nd t of 4 25 to 6 05 zircon e hf t from 9 8 to 16 0 and 18 o values from 2 41 to 4 72 these elemental and isotopic characters argue for parental magmas from partial melting of a newly trapped charnockitic lower crustal protolith and their subsequent fractional crystallization with an inherent hydrothermally altered oceanic crustal heritage and possible tectonic linkage to post collisional slab breakoff this kind of a type granites could not only serve as an insightful temporal milestone for monitoring the amalgamation between various constituent terranes in the caob but also present a spatial magmatic proxy for marking the maturation stage of juvenile crustal reworking during the final phase of a large orogenic event

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

early paleoproterozoic magmatism in the yangtze block evidence from zircon u pb ages sr nd hf isotopes and geochemistry of ca 2 3 ga and 2 1 ga granitic rocks in the phan si pan complex north vietnam abstract our understanding of the early evolution of the yangtze block is limited by the sparsely dispersed nature of pre neoproterozoic exposures new integrated petrographic zircon u pb age and hf nd isotope analyses and whole rock geochemical data for early paleoproterozoic granites in the phan si pan complex provides new insights into the evolution of the yangtze block as well as its role in the pre nuna supercontinent la icp ms zircon u pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207pb 206pb ages of 2306 12 ma and 2096 15 ma respectively zircons from the quartz monzonite have ehf t values ranging from 4 1 to 2 1 corresponding to tdm2 model ages of 3002 2890 ma whereas zircons in the gneissic granite have ehf t values between 0 95 and 1 72 and corresponding tdm2 model ages of 2660 2516 ma which are consistent with their whole rock nd isotope values geochemically the quartz monzonites are i type granites combined with their relatively high sr y ratios and low y concentrations as well as fractionated ree patterns with relatively high lree but low hree concentrations they were probably generated by partial melting of the thickened middle lower crust under elevated temperature geochemical and isotopic signatures suggest that the ca 2 1 ga gneissic granites are high k calc alkaline ferroan a type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material the phan si pan complex has a distinct early paleoproterozoic crustal evolution history compared with the other crustal provinces of the yangtze block suggesting independent histories that were not unified until the late paleoproterozoic during the assembly of nuna moreover the magmatism and tectonic evolution of the north vietnam region is broadly similar to that of the arrowsmith orogen of the rae craton in laurentia suggesting a potential spatial linkage the geologic record of the yangtze block does not support an early paleoproterozoic shutdown of plate tectonics

petrogenesis of neoarchean ttg rocks in the yangtze craton and its implication for the formation of archean ttgs abstract the yangtze craton is one of the most important cratonic blocks in eastern asia however its early evolution has not been well constrained yet because of the poor exposure of archean basement rocks in this contribution a combined study of zircon u pb age hf o isotope compositions as well as whole rock element compositions was carried out for gray gneiss rocks from the douling complex in the northwestern part of the yangtze craton zircon u pb dating for two gneiss samples yielded weighted mean ages of 2509 21 and 2496 15 ma the gray gneisses have high sio2 63 64 73 74 na2o 3 66 6 21 but low k2o 0 76 2 93 contents and belong to the trondhjemite tonalite and granodiorite series rocks they are characterized by enrichments of liles and lrees but notable depletions in nb ta and ti as well as heavy rees and y all the samples plot into the fields of typical archean ttg rocks in the la yb n vs yb n and sr y vs y diagrams therefore the gray gneisses in the douling complex are neoarchean ttg rocks the relatively high nb ta and zr sm ratios of 19 6 46 0 and 14 5 143 4 suggest their formation under eclogite facies conditions they have low hf t values of 3 21 and 3 39 and hf model ages of 3194 26 and 3205 24 ma indicating their derivation from ca 3 2 ga juvenile mafic rocks their 18o values of 6 41 0 12 and 5 44 0 12 suggest that surface rocks were differently recycled into their sources collectively the ttg rocks in the douling complex were generated by partial melting of thickened mafic crust rather than direct partial melting of oceanic crust in a convergent plate boundary a compilation of available zircon u pb and hf isotope compositions of the yangtze craton reveals that there are quite differences of hf isotope compositions in archean the ca 3 8 3 4 ga zircons exhibit hf t values of 5 8 to 0 0 these chondritic to slightly enriched hafnium isotope compositions imply that their host rocks were formed by protracted intra crustal reworking of ancient protoliths without juvenile input by contrast zircons with ages younger than ca 3 2 ga register both negative and positive hf t values up to the depleted mantle providing clear evidence for the addition of juvenile materials these imply that the subduction driven plate tectonics might have been operated since ca 3 2 ga in the yangtze craton

mineralogy geochemistry and petrology of neoarchean ferroan to magnesian granites of carajas province amazonian craton the origin of hydrated granites associated with charnockites abstract 2 75 to 2 73 ga old granitic intrusions associated with coeval charnockitic rocks were emplaced in the northern domains of the carajas province the vila jussara suite was recognized recently and its geologic mineralogical and geochemical characteristics are presented in this paper and compared with the data available in the literature on the estrela complex serra do rabo and igarape gelado granites and planalto suite monzogranites and syenogranites are dominant in most units but in the igarape gelado granite and vila jussara suite granodiorites and tonalites are also relatively abundant the main mafic phases in all these granitoids are amphibole potassian hastingsite with subordinate magnesiohastingsite 0 95 fe fe mg 0 47 and biotite 0 88 fe fe mg 0 52 with rare occurrence of clinopyroxene relict crystals magmatic epidote of pistacite component from 26 to 31 occurs only in the vila jussara suite the rocks of estrela serra do rabo and planalto units contain only ilmenite generally with coronas of titanite or ilmenite associated with magnetite and are essentially reduced ferroan granites whole rock feot feot mgo 0 89 that evolved at low f o 2 fmq 0 5 the igarape gelado granite needs additional studies but it is formed apparently by reduced ferroan and oxidized ferroan granites the vila jussara suite is also composed of reduced ferroan granites that are similar to those of the other granite units however it has additionally oxidized ferroan and magnesian granitoids which contain magnetite ilmenite as the main fe ti oxide phase and evolved respectively at moderate f o 2 nno 0 5 and at comparatively higher f o 2 nno to nno 1 the magmas of these granites were formed at temperatures 900 c and derived from a heterogeneous dominantly reduced and tholeiitic granulitic crust locally varying to oxidized and calc alkaline in composition plagioclase orthopyroxene clinopyroxene and fe ti oxides were the main phases in the melt residues and the liquids were enriched in ba zr y and ree except for eu but not in sr compared to their sources the magmas should have relatively high water content 4 wt the magmatic evolution was strongly influenced by partial melting processes the different varieties of the vila jussara suite are not linked by fractional crystallization and derived from distinct magmas the neoarchean granites of the carajas province were emplaced in a collisional tectonic setting they are syntectonic granites that were deformed and sometimes intensely recrystallized during their emplacement and subsequent subsolidus cooling the magmas were generated in the crust at pressures of 800 to 1000 mpa and emplaced at pressures of 200 to 500 mpa the studied granitoids are similar geochemically to the ferroan and magnesian groups of charnockites the magnesian variety of the vila jussara suite is similar to the pyroxene free granitoids of the neoarchean matok pluton of the collisional limpopo belt the neoarchean granites of carajas are hydrated granites emplaced at comparatively higher crustal levels when compared with the matok pluton

coexisting clinopyroxene spinel and amphibole spinel symplectites in metatroctolites from the buck creek ultramafic body north carolina blue ridge buck creek metatroctolites preserve evidence of three stages in the geologic history of the region a relict igneous stage a high pressure granulite facies metamorphic stage and a hydrous alteration stage during high pressure granulite facies metamorphism adjacent olivine and plagioclase reacted to produce complex coronitic textures relict olivine is surrounded by radial columnar orthopyroxene and plagioclase is rimmed by fine grained symplectite composed of separate grains of clinopyroxene spinel tschermakitic amphibole spinel symplectite and minor clinopyroxene sapphirine symplectite examination of textural relationships in back scattered electron images and x ray element maps of the symplectites demonstrates that amphibole spinel and clinopyroxene spinel symplectites grew simultaneously at high temperature to replace plagioclase in these rocks clinopyroxene sapphirine symplectite grew in the latest stages of plagioclase replacement in some samples a successful model for the main corona forming reaction in sample bc7 1 00 pl 1 72 ol 0 17 h 2 o 0 59 di 0 17 amp 0 68 spl 0 79 opx results only if amphibole is included the model predicts relative volumes of product clinopyroxene amphibole and spinel that are generally consistent with the proportions in imaged symplectites proportions of clinopyroxene and amphibole hosted symplectite appear to have been governed by local mass balance of na and al in plagioclase and or availability of h 2 o the most successful model for formation of the inner clinopyroxene sapphirine corona 1 00 pl 1 47 ol 0 17 h 2 o 0 59 di 0 17 amp 0 30 spr 10 ox 0 57 opx also includes amphibole which is present in association with clinopyroxene sapphirine symplectite sufficient h 2 o for formation of symplectic amphibole must have been present in the intergranular fluid of relict igneous troctolites or have infiltrated during metamorphism at high p t conditions

the oldest known rocks in south western china shrimp u pb magmatic crystallisation age and detrital provenance analysis of the paleoproterozoic dahongshan group abstract the paleoproterozoic dahongshan group outcrops along the present day southwest margin of the yangtze craton it is the oldest rock found in the region and forms the basement for the late mesoproterozoic sedimentary successions the dahongshan group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist lower amphibolite facies some of the original sedimentary features such as unimodal cross bedding in quartzite and rounded conglomerate clasts are still preserved suggesting deposition in a fluvial environment abundant carbonate rocks also found within the sedimentary sequence indicate periods of shallow marine or lagoonal conditions a weighted mean shrimp 206pb 207pb age of 1675 8 ma for a tuffaceous schist unit confirms the paleoproterozoic formation age of the group shrimp and lam icp ms u pb analysis of detrital zircon grains from metasedimentary units indicate a late archean to paleoproterozoic ca 2780 1860 ma source region the age spectrum of the detrital zircons is similar to that of the late archean and early paleoproterozoic basement rocks previously identified in the yangtze craton but includes a population of paleoproterozoic grains ca 2400 2100 ma which have no known source region in the exposed yangtze craton

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

quartz exsolution in clinopyroxene is not proof of ultrahigh pressures evidence from eclogites from the eastern blue ridge southern appalachians u s a oriented quartz needles in clinopyroxene have become one of the diagnostic indicators of ultrahigh pressure uhp metamorphism the presence of apparently exsolved quartz is taken as evidence of decompression of a non stochiometric ca eskola component ca0 5 0 5alsi2o6 caes that is presumed to be stable only at uhp conditions eclogite from the eastern blue ridge north carolina contains clinopyroxene jd20cats5ac5caesdi65hd5 with oriented needles of quartz and calcic amphibole that appear to have exsolved together the quartz amphibole intergrowths are surrounded by 1 5 m haloes of neoformed pyroxene jd10cats10ac5caesdi70hd5 the modes of quartz amphibole and clinopyroxene haloes were determined using bse images and reintegrated with the host clinopyroxene viewing the quartz and amphibole needles down the c axis of the pyroxene host provides a better estimate of their proportions than in prismatic sections reintegrated pyroxene compositions were nearly identical to the analyzed host pyroxene with no caes component clinopyroxene with caes solid solution has been repeatedly synthesized at uhp conditions however examination of the phase equilibria usually cited as evidence for caes stability at conditions of 25 kbar shows that clinopyroxene with 10 mol caes is stable well within the quartz field and provides a pressure minimum similar to the albite jadeite quartz barometer exsolution of quartz and associated amphibole is commonplace in clinopyroxene from the blue ridge eclogite that lacks coesite or other evidence for uhp metamorphism the presence of a diluted 5 10 caes component in clinopyroxene does not require uhp conditions

nature of the archean midcrust in the core of the vredefort dome central kaapvaal craton south africa extreme uplift associated with the formation of the 2 02 ga vredefort dome has exposed a substantial cross section through the crystalline early archean basement complex rocks of the kaapvaal craton the rocks comprise polydeformed high grade tonalite trondhjemite granodiorite ttg gneisses migmatites and late tectonic intrusive granitoids that straddle the upper amphibolite to granulite facies transition field petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal scale layering as has been previously proposed no evidence has been found to support exposure of either a melt depleted refractory lower crust or an upper crustal batholithic granite layer however the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones granitic leucosomes in the upper amphibolite facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks rather than of in situ derivation leucosome compositions in the granulite facies migmatites are more variable ranging from granitic and charnockitic to enderbitic probably reflecting at least some local derivation leucosomes and small granitoid bodies show local scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration and fractionation of minerals such as k feldspar within the melts

shrimp u pb zircon ages of the fuping complex implications for late archean to paleoproterozoic accretion and assembly of the north china craton the fuping complex is situated in the central part of the north china craton and consists of four major lithological assemblages fuping tonalitic trondhjemitic granodioritic gneisses longquanguan augen gneisses wanzi supracrustal assemblage and nanying granitic gneisses shrimp u pb geochronology combined with u th and cathodoluminescence cl imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the fuping complex cl images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the fuping complex the metamorphic zircons occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores and are all characterized by nebulous zoning or being structureless with extremely high luminescence and very low th contents these features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning comparatively low luminescence and high th and u contents shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic plutons of the fuping gneisses were emplaced at 2523 14 ma 2499 10 ma and 2486 8 ma respectively whereas the monzogranitic and granitic plutons of the longquanguan augen gneisses were intruded respectively at 2510 22 ma and 2507 11 ma prismatic and oscillatory zoned zircons dominate in the pelitic rocks of the wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks the concordant and discordant u pb ages of 2502 5 ma and 2507 14 ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than 2507 ma ago in addition a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 5 1 ma which may provide a maximum depositional age for the wanzi supracrustal rocks shrimp results also reveal that granitic magmatism assigned to the nanying granitic gneisses occurred over a protracted interval from 2077 13 ma to 2024 21 ma the nebulously zoned zircon grains and overgrowth recrystallization zircon rims from different rocks yielded similar concordant 207pb 206pb ages in the range 1875 to 1802 ma interpreted as approximating the age of regional metamorphism of the fuping complex timing of primary zircon crystallization and regional metamorphism of the fuping complex is in general agreement with recent u pb zircon ion probe results for the wutai and hengshan complexes that bound the fuping complex to the northwest these areas are characterized by the emplacement of major granitoid bodies at around 2 50 ga to 2 48 ga ago deposition of supracrustal rocks in the paleoproterozoic intrusion of paleoproterozoic granitic bodies at 2 1 to 2 0 ga and regional metamorphism at 1 875 to 1 802 ga these data indicate that the fuping and hengshan complexes do not represent an older crystalline basement to the wutai complex as suggested in previous tectonic models but together with the wutai complex represent elements of a single late archean to paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western north china blocks at 1 85 ga which resulted in the final assembly of the north china craton

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

petrology of the regional sillimanite zone west central new hampshire u s a with implications for the development of inverted isograds abstracr sillimanite from the regional sillimanite zone in west central new hampshire is fibrolitic and overprints f2 folds nappe stage of earlier mica foliation regional sillimanite zone samples show no evidence for earlier staurolite parageneses despite the fact that staurolite is abundant at lower grade because sillimanite was produced directly from garnet chlorite by the prograde heating reaction garnet chlorite muscovite quartz sillimanite biotite h20 the pressure at which this reaction occurs is sensitive to the mno and cao contents of garnet and phase equilibrium arguments reveal that at the regional pressures of west central new hampshire 2 4 kbar staurolite parageneses are only possible in rocks with low mno cao the inferred p t path is counterclockwise with nearly isobaric initial heating at 2 kbar followed by loading t heating to a peak metamorphic temperature of 600 t 25 c at 4 kbar followed by nearly isobaric cooling garnets were compositionally homogenized near the metamorphic peak but subsequent cooling was rapid 100 c m y these data argue against the folded isotherm model of chamberlain 1986 instead the present distribution of metamorphic grades is interpreted to be the result of regional stacking of high grade thrust sheets on lower grade rocks followed by depression of high grade rocks to lower structural levels

post orogenic extension in the eastern part of the jiangnan orogen evidence from ca 800 760ma volcanic rocks we present a systematic geochronological and geochemical study on ca 800 760 ma volcanic rocks in the eastern part of the jiangnan orogen the xucun composite dykes are dated at ca 805 ma the mafic components have oib like trace element patterns and positive anomalies in zr and hf the least contaminated sample has relatively depleted nd isotopic features suggesting the xucun mafic dykes may have been generated from the partial melting of oib like asthenosphere with later crustal contamination the xucun felsic dykes have decoupled nd hf isotopes and the hf isotope compositions of zircons indicate that the dykes may be derived from the partial melting of the early neoproterozoic juvenile crustal materials with minor incorporation of paleoproterozoic crustal components the ca 800 790 ma shangshu volcanics include two compositional series calc alkaline and tholeiitic the shangshu calc alkaline volcanics in the minjiawu area have low abundances of lile hfse and high na2o contents and sr y ratios similar to adakitic rocks the evident arc like geochemical features and radiogenic nd isotopes nd t values of 3 7 to 4 8 suggest that these rocks may have been generated from the partial melting of juvenile lithospheric mantle metasomatized by na rich melts released from the subducted slab the tholeiitic mafic rocks from the shangshu bimodal volcanics represent two different magma sources the partial melting of metasomatized lithospheric mantle led to the formation of arc like basalts with low tio2 contents negative anomalies in zr and hf and high values of mg and nd t 6 2 whereas the partial melting of asthenospheric mantle generated volcanic rocks with high tio2 contents and low positive nd t 1 4 to 2 7 without negative anomalies of nb ta zr and hf the shangshu felsic rocks were formed by the reworking of early neoproterozoic juvenile arc crustal materials the ca 760 ma mafic rocks from the puling bimodal volcanics generally have low tio2 contents 0 9 wt nearly flat ree distributions and arc like trace element patterns they may have been generated from the high degree partial melting of metasomatized lithospheric mantle one sample has a high tio2 content 2 41 wt and high nd t 6 2 with overall oib like trace element patterns implying the local partial melting of asthenospheric mantle the occurrence of significant volumes of bimodal volcanics in the eastern part of the jiangnan orogen suggests an extensional setting in the period ca 800 760 ma the evident partial melting of newly metasomatized lithospheric mantle and subordinate partial melting of asthenosphere suggest that post orogenic extension shortly after the neoproterozoic orogenesis may be a better explanation for the genesis of the mid neoproterozoic magmatic rocks in the eastern part of the jiangnan orogen post orogenic extension may be diachronous along the whole orogenic belt and probably has no direct relationship with the rodinia rifting event a more detailed model is presented to illustrate the evolution of the eastern part of the jiangnan orogen

genesis of baddeleyite and high δ18o zircon in impure marble from the tongbai orogen central china insights from petrochronology and hf o isotope compositions coexistence of baddeleyite and zircon is rarely documented in metacarbonate rocks and poorly understood in this study metamorphic baddeleyite and zircon were recognized in marbles from the tongbai orogen central china we carried out high spatial resolution analysis of trace element geochemistry and u pb lu hf o isotope systematics for the zircon and baddeleyite sims secondary ion mass spectroscopy u pb analysis yields weighted average 207pb 206pb ages of 1848 0 3 3 ma msdw 1 2 n 12 for baddeleyite and 1840 13 ma mswd 1 7 n 13 for zircon the baddeleyite grains have relatively low ree rare earth element contents ree 1 30 47 3 ppm with flat chondrite normalized patterns and moderate contents of hfse high field strength element which are ascribed to the nature of its host metacarbonate rock and mineral assemblage the zircon grains have low ti contents 1 63 3 56 ppm positive ce anomalies ce ce 1 91 6 96 and mineral inclusions of hematite calcite dolomite lizardite and h2o these characteristics indicate zircon formation in a relatively low temperature setting under oxidizing and water rich conditions the baddeleyite and zircon have similar ranges of ehf t values of 7 7 to 10 5 and 7 8 to 11 6 respectively which suggest their derivation from a relatively uniform hf source the high 18o values of these zircon grains 20 0 21 0 preclude any percolation of external meteoric fluids during their crystallization therefore a detrital zircon dissolution reaction is preferred to provide the zr for the metamorphic zircon and baddeleyite in this study the o isotope disequilibrium between whole rock 18o 16 2 16 3 and metamorphic zircon is most likely caused by metamorphic devolatilization or interaction with an h2o dominant fluid during later metamorphism on the other hand the baddeleyite grains yield 18o values ranging from 4 5 to 8 7 which is in disequilibrium with metamorphic zircon at the temperature of detrital zircon dissolution reaction ca 660 710 c zircon dolomite baddeleyite forsterite calcite co2 the genesis of metamorphic zircon can be interpreted by a baddeleyite to zircon reaction of zro2 sio2 zrsio4 at a retrograde stage consistent with the low ti in zircon temperatures the results highlight the importance of integrating trace element and multiple isotopic compositions in studying the genesis of metamorphic baddeleyite and zircon additionally the u pb geochronology data also indicate that the protoliths of the marble were deposited in the early paleoproterozoic and experienced a metamorphism at 1 85 ga the ca 1 85 ga high grade metamorphic event might have been induced by the coeval magmatism in an extensional setting

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

archean geodynamics in the central zone north china craton constraints from geochemistry of two contrasting series of granitoids in the fuping and wutai complexes abstract the archean to paleoproterozoic central zone of the north china craton is situated between the eastern and western archean continental blocks and contains two contrasting series of neoarchean granitoids the 2523 2486 ma tonalite trondhjemite granodiorite ttg gneisses in the fuping complex and the 2555 2525 ma calc alkaline granitoids tonalite granodiorite granite and monzogranite in the wutai complex the fuping ttg gneisses most likely formed from partial melting of 2 7 ga basalts at 50 km with an involvement of 3 0 ga crustal material the wutai granitoids have higher k 2 o lile and rb sr but lower sr y and la n yb n than the fuping ttg gneisses are characterized by nd t dm from 2 5 to 2 8 ga and e nd t from 0 49 to 3 34 and are derived from partial melting of a juvenile source at the geochemistry of these two contrasting series of neoarchean granitoids provides further evidence that the wutai complex originated and evolved separately from the fuping complex the wutai complex most likely formed as an oceanic island arc with volcanism and synvolcanic granitoid intrusions at 2555 2525 ma the wutai complex was subsequently accreted onto the eastern archean continental block and was probably responsible for crustal thickening and ttg magmatism at 2523 2486 ma in the fuping complex as part of the taihangshan hengshan block at the western margin of the eastern archean continental block

geochemistry and zircon u pb geochronology of paleoproterozoic arc related granitoid in the northwestern yangtze block and its geological implications abstract the early history of the yangtze block has not been well constrained yet due to the scarce outcrops of archean to paleoproterozoic rocks in this study we report an integrated study of zircon u pb age major and trace element data and sr nd hf isotope compositions for gray gneisses from the houhe complex in the northwestern part of the yangtze block zircon u pb dating yields a weighted mean 207 pb 206 pb age of 2081 9 ma for a gneiss this age is interpreted as the formation age of the houhe complex and thus the houhe complex represents the oldest rocks found in the western part of the yangtze block the gray gneisses from the houhe complex range in sio 2 contents from 58 63 to 68 59 and na 2 o from 3 88 to 5 28 and have relatively high fe 2 o 3 contents of 2 86 6 69 al 2 o 3 of 16 01 18 88 and mgo of 0 97 2 65 these rocks show low sr 149 390 ppm cr 9 07 45 1 ppm and ni 4 97 21 3 ppm contents but high y 12 9 32 7 ppm and yb 0 95 2 25 ppm they are characterized by a relative enrichment in liles and lrees but a depletion in hfses these features are similar to those of calc alkaline granitoids suggesting that their formation might be related to a subduction related process or remelting of preexisted arc rocks compiled age spectra of archean to paleoproterozoic zircon grains reveal that the western and eastern parts of the yangtze block have similar archean and early paleoproterozoic age ranges indicative of the occurrence of an old continental nucleus in both parts on the other hand there are large amounts of ca 2050 2400 ma zircon grains revealed in the western part of the yangtze block implying that there might be a microcontinent with an active type continental margin during the paleoproterozoic times in the western part of the yangtze block because of the poor outcrops of archean to paleoproterozoic rocks in the yangtze block it is no possible to establish the exact nature of such an active margin yet the occurrence of ca 2 0 ga khondalitic and metasedimentary rocks has been documented in the eastern part of the yangtze block implying that there might be a passive type continental margin in the eastern part of the yangtze block at ca 2 0 ga the collision between the western microcontinent and the eastern part may have resulted in the final arc continental collision and the amalgamation of the yangtze block

formation of high pressure metabasites in the southern appalachian blue ridge via taconic continental subduction beneath the laurentian margin 1 tectonic models that account for the occurrence of eclogite retrograde eclogite peridotite and migmatitic basement gneisses in the southern blue ridge invoke closure of a small ocean basin between the eastern blue ridge ebr and western blue ridge wbr in this obducted ophiolite model franciscan type eastward subduction of oceanic crust beneath the piedmont arc occurred prior to obduction of the arc onto the laurentian margin the discovery of kilometer scale eclogite bodies near the ebr wbr boundary are cited as evidence for obducted ophiolite accreted island arc models however thermobarometric estimates based on mineral chemistry from hp metabasites of the western central and eastern blue ridge indicate that eclogites and granulites are medium temperature type 550 900 c inconsistent with the obducted ophiolite model mid ocean ridge basalt subducted beneath continental margins and island arcs rarely reaches temperatures in excess of 550 c thermobarometric comparisons of the present study and a reexamination of deep seismic profiles from the blue ridge allow for a model of westward subduction of continental material beneath laurentia during the taconic orogeny that produced a megamelange now preserved in the central and eastern blue ridge tectonic provinces p t paths determined from blue ridge hp metabasites are similar to p t paths determined previously from caledonian and variscan eclogites thermobarometric similarities imply a common tectonic history of continental collision subduction of continental crust to depths of 50 65 km followed by rapid eduction

the soapstone ridge complex southern appalachians petrographic mineral compositional and oxygen isotope investigation samples of the soapstone ridge complex ssr in the southern appalachians examined for this study are composed of megacrystic orthopyroxenites amphibole schists and talc chlorite amphibole schists the mineralogy of the ssr samples studied consists of varying proportions of amphibole chlorite talc orthopyroxene and fe ti oxides amphibole is the domi nant mineral in almost all ssr samples studied the amphiboles display zoning whereby cores that are composed of magne siohornblende and edenite consistent with amphibolite facies conditions are rimmed by tremolite actinolite consistent with greenschist facies conditions oxygen isotope compositions of ssr amphiboles investigated show a wide range in 18o values from a low value of 1 8 to a high value of 6 1 such wide ranges in 18oamphibole values occur in ophiolite complexes and oceanic crustal rocks relict orthopyroxene from the ssr has a 18o value of 3 4 a value lower than those from layered intrusions but comparable to some pyroxene 18o values from ophiolites equilibration temperatures calculated for actinolite chlorite yield a temperature of 227 c consistent with greenschist facies conditions it is proposed that the ssr amphibole cores were metamorphosed probably during terrane terrane collision under amphibolite facies conditions with the actinolite rims possibly being formed during thrusting of the ssr rocks onto the continental margin

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

3800 to 2500 ma crustal evolution in the anshan area of liaoning province northeastern china shrimp u pb zircon geochronology has revealed several generations of granitoids and gneisses from the anshan area of northeastern china two samples of the baijiafen granite sensu lato 3804 5 and 3812 4 ma chentaigou granite 3306 13 ma lishan granite 3142 7 ma east and west anshan granites 3001 4 and 2994 8 ma tiejiashan granite 2962 4 and 2964 6 ma and qidashan granite 2475 ma the 3800 ma baijiafen granite consists of banded migmatitic trondhjemitic gneisses and forms only a small part of the area nonetheless they are the oldest known rocks in asia and are one of very few locations in the world where rocks of this antiquity have been found the younger suites are predominantly granite sensu stricto in composition and although foliated they have not been transformed into banded gneisses nd isotopic data suggest that the 3800 ma granites were produced at least in part by remelting of very ancient lree enriched sources perhaps like the exposed remnants of the 3800 ma rocks the chentaigou supracrustals contain layered mafic and ultramafic rocks banded iron formation bif and siliceous layered rocks of probable volcanogenic origin a sample of the latter has yielded a concordant u pb zircon age of 3362 5 ma interpreted as the age of deposition a trondhjemitic sheet cutting these rocks has yielded a u pb zircon age of 3342 6 ma the chentaigou supracrustals are the oldest recognised in the area younger supracrustal sequences collectively known as the anshan group contains the main economic bif units in the area bif is intruded by the 2475 ma qidashan granite indicating that the anshan group is archaean or earliest proterozoic in age

newly discovered eclogite in the southern appalachian orogen northwestern north carolina abstract recent detailed geologic mapping in the blue ridge belt of northwestern north carolina has revealed the first reported eclogite sensu stricto in the southern appalachian orogen the eclogite occurs at the base of the ashe metamorphic suite within a shear zone that separates the spruce pine thrust sheet from the underlying grenville age basement rocks this contact juxtaposes rocks of apparent oceanic affinities predominantly metapelite metabasite and meta ultramafic rocks with rocks of the ancient north american craton the primary metamorphic assemblage in the eclogite is garnet omphacite quartz rutile hornblende and plagioclase occur only as retrograde amphibolite facies phases in the eclogite pressure temperature estimates for the primary eclogite assemblage are consistent with minimum pressures of 13 17 kbar at 625 790 c while the retrograde amphibolite facies assemblage suggests conditions of 8 5 12 kbar and 650 730 c ultramafic blocks also occur within the shear zone containing the eclogite the presence of high pressure metamorphic rocks associated with ultramafic rocks and pelitic schists within a shear zone separating continental rocks from oceanic rocks is strong evidence for an ancient subduction event and indicates that at least part of the ashe metamorphic suite is a subduction related accretionary melange

mantle derivation of archean amphibole bearing granitoid and associated mafic rocks evidence from the southern superior province canada amphibole bearing late archean 2 73 2 68 ga granitoids of the southern superior province are examined to constrain processes of crustal development the investigated plutons which range from tonalite and diorite to monzodiorite monzonite and syenite share textural mineralogical and geochemical attributes suggesting a common origin as juvenile magmas despite variation in modal mineralogy the plutons are geochemically characterized by normative quartz high al2o3 15 wt na rich fractionation trends mol na2o k2o 2 low to moderate rb generally 150 ppm are comagmatic with the evolved granitoids and indicate that the suites are mantle derived isotopic studies of archean monzodioritic rocks have shown lree enrichment and initial 143nd 144nd ratios indicating derivation from mantle sources enriched in large ion lithophile elements lile shortly before melting mineral assemblages record lower ph2o with increased alkali contents of the suites this evidence in conjunction with experimental studies suggests that increased alkali contents may reflect decreased ph2o during mantle melting these features indicate that 2 73 ga tonalitic rocks are derived from more hydrous mantle sources than 2 68 ga syenitic rocks and that the spectrum of late archean juvenile granitoid rocks is broader than previously recognized comparison with phanerozoic and recent plutonic suites suggests that these archean suites are subduction related

paleoproterozoic porphyries and coarse grained granites manifesting a vertical hierarchical structure of archean continental crust beneath the yangtze craton abstract granites generated by anatexis are ideal crustal probes to decipher the differentiation processes within deep continental crust in this study the successive emplacement of granites including the porphyries and the coarse grained granites was found in the nucleus the kongling complex of the yangtze craton the porphyries intruding at 1840 17 ma shrimp ages occur as veins and contain phenocrysts of k feldspar 15 quartz 5 plagioclase 4 and garnet 1 setting in a fine grained groundmass 75 of k feldspar plagioclase quartz and biotite the coarse grained granites emplacing at 1821 8 ma shrimp ages are outcropped as pluton and comprise k feldspar 60 quartz 30 biotite 5 plagioclase 5 they are similar to a type granites with relatively high sio2 69 2 71 5 wt 72 6 74 5 wt high fe2o3t mgo 21 5 27 7 5 9 19 5 104 ga al ratios 3 6 4 3 2 7 3 5 respectively and enriched hfses th u pb ga the porphyries contain higher total trace element contents and zirconium saturation temperature mean 892 c than those of the coarse grained granites mean 803 c magmatic zircons from the porphyries and coarse grained granites have ehf t values of 17 6 to 11 7 and 21 0 to 14 8 which yield two stage hf model ages of 3 6 3 3 ga and 3 8 3 4 ga respectively both falling into the evolutionary trend of the 2 9 2 8 ga kongling ttgs it implies both granites are anatectic crustal reworking derivatives from partial melting of the archean ttgs under sustaining extension in paleoproterozoic applying trace element modelling we further identify a unique tonalitic source for both granites which is analogous to the exposed natural kongling tonalite and find the porphyries slightly affected by the juvenile crust based on the intrusion of porphyry veins into granitic gneisses we envisage the crustal structure beneath the craton is layered from older 2 94 ga tonalites upward to younger 2 87 ga granitic gneisses such hierarchical structure could shed new light on the imperceptible vertical differentiation process as cratonization

u pb zircon and sm nd isotopic study of the huangtuling granulite dabie sulu belt china implication for the paleoproterozoic tectonic history of the yangtze craton sims data demonstrate archean 2 9 to 2 7 ga and paleoproterozoic 1991 43 ma ages for rounded detrital zircon cores and unzoned rims respectively in a graulite sample collected from huangtuling dabie sulu orogenic belt china the granulite rocks in this locality have pelitic compositions and represent an exposure of the northern margin of the yangtze craton the rounded zircon cores show oscillatory zoning and have th u ratios 0 17 0 78 systematically higher than those of the unzoned luminescent rims 0 005 0 03 the age of the detrital zircon cores and whole rock nd isotope model age suggest that the huangtuling granulite was possibly derived from a provenance dominated by archean rocks similar to the kongling complex exposed 200 km in the southwest multiple phases of metamorphism have been identified for the granulites with the dominant granulite facies mineral assemblage defining an early clock wise metamorphic p t path orthopyroxene plagioclase and whole rock material define a linear trend in a 147sm 144nd 143nd 144nd diagram corresponding to an age of 1926 146 ma this result and the u pb data for the zircon rims suggest that a collisional event may have occurred along the northern margin of the yangtze craton at 2 0 ga inviting further study on whether the yangtze craton was involved in the assembly of the recently proposed paleo mesoproterozoic supercontinent columbia

the 2 65 ga a type granite in the northeastern yangtze craton petrogenesis and geological implications abstract the presence of old residual continental crustal rocks in cratons is crucial for investigation of their early evolution due to the poor exposure of archean rocks the early evolution of the yangtze craton has not been well constrained yet in this study a combined study of zircon u pb and hf o isotopes as well as whole rock major and trace elements and nd isotopes have been conducted for a granite pluton at huji town in the northern part of the yangtze craton u pb dating for magmatic zircons yielded a weighted mean age of 2656 6 ma revealing another archean outcrop in the yangtze craton besides the kongling terrane the granite samples show relatively evaluated k 2 o na 2 o cao tfeo mgo ratios and zr nb ce ga and y contents sharing the same features as the a type granite they have pronounced eu sr and ba negative anomalies on the primitive mantle normalized trace element spidergram implying the residual of plagioclase in the magma sources their high zr contents of 440 529 ppm correspond to initial magma temperatures of 930 962 c the zircons have negative hf t values of 2 7 to 0 1 and hf model ages of ca 3 2 ga respectively the 18 o values of the zircons range between 4 94 and 5 72 and give an average of 5 26 0 14 suggesting no contribution of supracrustal rock into the host magma therefore the hf model ages indicate the generation of the huji granite by reworking of ca 3 2 ga juvenile crustal rocks combined with previous studies the finding of the neoarchean huji granites with ca 3 2 ga hf model ages suggests the widespread of archean basement in the yangtze craton the occurrence of the ca 2 65 ga huji granites likely records the thermally stable lithosphere and thus heralds the craton stabilization of the eastern part of the yangtze block

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

retrograde metamorphism of eclogite in the southern appalachian mountains u s a a case involving seamount subduction this work supports a growing body of evidence that the ashe metamorphic suite ams of the eastern blue ridge province in north carolina has an ensimatic origin and is part of a subduction related accretionary melange marking the taconic suture between the north american craton and the inner piedmont in a palinspastic reconstruction the thrust fault at the base of the ams appears to have intercepted the greatest depths i e highest p metamorphic rocks beneath parts of the ams now exposed adjacent to the grandfather mountain window the greatest volume of mafic rock is found in these same areas we suggest that the nascent subduction related basal thrust fault was deflected downward by an obstacle in the form of an isolated mafic volcanic edifice on the oceanic crust a sea mount pelitic and mafic rocks dominate the ams north of the grandfather mountain window retrograded eclogite occurs in the amphibolite near the base of the ams textures and mineralogy indicate that an original eclogite assemblage was subjected to the following sequence of parageneses a eclogite i facies omphacite garnet quartz b eclogite ii facies omphacite garnet epidote quartz c symplectic diopside plagioclase garnet epidote quartz d amphibolite facies diopside plagioclase garnet epidote hornblende quartz e amphibolite facies plagioclase garnet epidote hornblende quartz p t conditions estimated from geothermobarometry applied to relevant mineral compositions are c 720 c and c 16 kbar for b eclogite ii facies c 655 c and c 8 5 kbar for e amphibolite facies

episodic growth of the gondwana supercontinent from hafnium and oxygen isotopes in zircon it is thought that continental crust existed as early as 150 million years after planetary accretion but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records for example the striking global peaks in juvenile igneous activity 2 7 1 9 and 1 2 gyr ago imply rapid crustal generation in response to the emplacement of mantle super plumes rather than by the continuous process of subduction yet uncertainties persist over whether these age peaks are artefacts of selective preservation and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle these model ages are only meaningful if the host magma lacked a mixed or sedimentary source component but the latter can be diagnosed by oxygen isotopes which are strongly fractionated by rock hydrosphere interactions here we report the first study that integrates hafnium and oxygen isotopes all measured in situ on the same precisely dated detrital zircon grains the data reveal that crust generation in part of gondwana was limited to major pulses at 1 9 and 3 3 gyr ago and that the zircons crystallized during repeated reworking of crust formed at these times the implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts

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

vapor absent melting at 10 kbar of a biotite and amphibole bearing tonalitic gneiss implications for the generation of a type granites vapor absent melting experiments on a biotite and amphibole bearing archean tonalitic gneiss agc150 at 10 kbar and 875 to 1050 c show that amphibole breaks down from 900 to 950 c producing garnet orthopyroxene and granitic melt biotite dehydration melting produces 10 wt melt up to 950 c via incongruent melting reactions that produce garnet orthopyroxene and titanomagnetite widespread biotite dehydration melting occurs between 950 and 975 c and produces orthopyroxene magnetite titanomagnetite and 20 wt fluorine rich melt up to 0 31 wt f minor f rich 2 7 wt biotite is present even at 1000 c our experiments show that under vapor absent conditions intrusion of hot mantle derived magmas into the lower crust is necessary to initiate widespread biotite dehydration melting in rocks with compositions like agc150 we propose that the high thermal stability of biotite in agc150 suggests that this rock is residual after a previous episode of partial dehydroxylation that left behind somewhat f enriched biotite we show that dehydration melting of such f enriched biotite produces f rich granitic liquids with compositions within the range of a type granite and leaves behind a granulitic residue consisting of orthopyroxene plagioclase quartz titanomagnetite and magnetite

petrogenesis of archean ttgs and potassic granites in the southern yangtze block constraints on the early formation of the yangtze block abstract the southern yangtze is underlain by mesoarchean igneous rocks that were emplaced in two pulses including trondhjemites at ca 3092 3071 ma followed by potassic granites at 2920 ma these represent the oldest ttg suites identified in the southern yangtze and the oldest potassic granites in the entire yangtze block hf isotopic analysis reveals ehf t values of 2 49 7 49 and hf crustal model ages tdmc of 3601 3000 ma for zircons from the trondhjemites whereas those from the potassic granites show ehf t values of 1 79 4 18 and tdmc ages of 3453 3043 ma the trondhjemites have high sio2 mg and low al2o3 and k2o na2o and are highly evolved with low ni cr and co the trondhjemites are characterized by low sr y eu gd and la yb n and high y and hree typical of high hree and low pressure ttgs their geochemical and isotopic compositions indicate that they are derived from melting of juvenile mafic crust with little contribution from older crustal materials consistent with a plagioclase bearing basaltic source the multielement patterns of potassic granites are similar to that of the trondhjemites albeit with lower hree and y contents their high k2o values strongly fractionated ree patterns and low y contents indicate the presence of garnet and or amphibole as residual phases in the source region which indicates they formed at higher pressure than the trondhjemites the isotopic and geochemical data imply that the formation of the potassic granites could be related to reworking of older ttgs the increasing k2o na2o of the 3092 3071 ma trondhjemites to potassic 2920 ma granites corresponds with stabilization maturation and thickening of the archean continental crust in the southern yangtze the geochemical data of the ttgs in the kunming region show important differences from that of contemporaneous ttgs from the kongling complex suggesting that southern yangtze is composed by an independent old nucleus which is exotic to the northern yangtze a synthesis of petrogenesis geochronology geochemistry and isotope data of archean granites of the yangtze block provides important insights into the architecture of its archean crust we propose that the yangtze block can be subdivided into discrete northwestern northeastern and southern domains but which are all affected by late paleoproterozoic tectonothermal events suggesting that they had been assembled together by then to form the united yangtze block

did the character of subduction change at the end of the archean constraints from convergent margin granitoids large ion lithophile and high field strength element distributions in juvenile upper continental crust are controlled chiefly by the abundance of tonalite trondhjemite granodiorite ttg in the archean shifting to a combination of ttg calc alkaline granitoid and graywacke control thereafter geochemical differences between ttg and high silica adakites do not require production of most ttg magmas in descending slabs changes in the ratio of ttg to calc alkaline granitoids after 2 5 ga indicate that archean subduction zones must have differed from younger subduction zones in two very important ways 1 a deep mafic crust served as a ttg magma source either as thickened crust or in descending slabs and 2 they did not give rise to significant volumes of calc alkaline magma thickened mafic crust in the late archean may have resulted from plate jams in subduction zones caused by thicker oceanic crust and oceanic plateaus produced during late archean mantle thermal events

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

archean crustal structure of the kaapvaal craton south africa evidence from the vredefort dome abstract crystalline archean basement rocks in the core of the vredefort dome present a profile through a substantial part of the middle and lower crust of the kaapvaal craton previously this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal scale late archean brittle ductile thrust zone lithological and structural mapping across the dome indicates however that the basement lithologies share a common polyphase tectonic history culminating in high grade metamorphism and melting at 3 1 ga no evidence was found of the postulated tectonic terrane boundary but the alleged boundary does coincide with a 1 2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses the implications of these results for archean regional tectonic models for the kaapvaal craton are discussed

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

conductive incubation and the origin of dome and keel structure in archean granite greenstone terrains a model based on the eastern pilbara craton western australia 1 the archean east pilbara granite greenstone terrane epggt of the pilbara craton in western australia preserves a prolonged record of voluminous mafic dominated volcanism and felsic plutonism commencing at circa 3515 ma and ultimately resulting in the development of spectacular dome and keel structures early crustal growth was dominated by basaltic magmatism and the regional development of a 12 18 km thick greenstone sequence by circa 3335 ma overlying widespread 3470 3430 ma tonalite trondhjemite granodiorite ttg suites emplaced into the middle crust stratigraphic and structural relationships imply that voluminous granitoid plutonism and crustal scale dome and keel formation in the southeastern epggt was not initiated until circa 3325 ma but proceeded rapidly over the interval 3325 3308 ma this scenario is consistent with a conductive incubation period of 10 100 myr duration following the burial of radiogenic granitic crust beneath the accumulated greenstone pile for heat production parameters appropriate to the 3515 3308 ma epggt we show that the burial of 3470 3430 ma granitoid rich crust beneath the 3335 ma euro basalt and its precursors led to long term crustal temperature increases of 170 234 c at 35 km depth with a corresponding reduction in effective viscosity of 2 5 orders of magnitude in combination with the greenstone over granitoid density inversion the changes in effective viscosity of the mid to deep crust due to the burial of heat sources may have provided the crucial trigger for the initiation of dome and keel formation similarly cooling and strengthening of the crust during syndoming exhumation of the heat production potentially terminated large scale dome and keel amplification

stabilization of the southern portion of the sao francisco craton se brazil through a long lived period of potassic magmatism abstract although the southern portion of the sao francisco craton in southeast brazil encompasses one of the largest segments of meso to neoarchaean crust in the south american plate there is little understanding of the processes leading to cratonization of this region our combined field and u pb age data show that the crust experienced massive intrusions of potassic k rich granitoids between 2750 and 2600 ma and that this magmatic event marked the beginning of a fundamental change in the tectonics of the terrain with the ensuing billion years being dominated by the deposition of a series of thick clastic and chemical sedimentary successions thus deep crustal partial melting and subsequent transport and intrusion of these granites in the upper crust is a likely mechanism by which this segment of the sao francisco crust attained buoyancy to survive recycling regional mapping indicates that the potassic granitoids are distributed over an area of 25 000 km 2 and are related to one of the most prolific periods of potassic magmatism in the craton la icp ms and shrimp u pb data from 16 samples of potassic granitoids together with published tims ages suggest a long lived period of magmatism from 2750 to 2700 ma and a small pulse of granite crystallisation at ca 2612 ma the u pb shrimp and la icp ms data also indicate that the archaean granitoids do not record evidence of u pb zircon resetting our interpretation is that partial melting and transport of granitoid melts rich in heat producing elements to the upper crust gave rise to a thermally stable lower crust and that this lower crustal segment in the craton area became sufficiently refractory and resistant to further partial melting

complex growth and reworking processes in the yangtze cratonic nucleus abstract the kongling complex kc including the northern nkc and southern parts skc represents a rare relic of archean crust in the yangtze craton and an ideal target to investigate the precambrian evolution of the continental crust in the skc history of archean paleoproterozoic growth and reworking has not yet been clearly understood new geochemical and geochronological data on the basement rocks i e gneiss migmatite amphibolites and their overlying metasediments in the skc combined with published results from the nkc allow a further subdivision into western middle and eastern zones which has proven useful in understanding the precambrian history of the yangtze cratonic nucleus u pb dating and lu hf isotopes of zircons in biotite gneisses with or without amplibole and amphibolites suggest that the skc experienced a crustal re melting reworking event at 2 9 ga an accretion reworking event at 2 7 ga and later reworking at 1 73 ga and 0 8 ga these records are partly consistent with what is known about the nkc the main differences between them are that rocks with ages 3 0 ga and 2 1 2 0 ga are only found in the nkc but not in the skc hence the skc is not a copy of the nkc according to the consistent rock assemblages and ages the skc may have affinity to the western zone of the nkc that affinity is also supported by geophysical results that suggest the presence of a fossil huangling magma chamber beneath the skc and the western nkc at 10 20 km depth mei et al 2013 therefore our data provide a new understanding of the kongling complex the skc stayed together with the western part of the nkc from mesoarchean to neoproterozoic 2 9 0 8 ga time and they were separated by the intrusion of the huangling magmas at 0 8 ga

zircon u pb age and trace element evidence for paleoproterozoic granulite facies metamorphism and archean crustal rocks in the dabie orogen abstract zircon u pb ages and trace elements were determined for granulites and gneiss at huangtuling which are hosted by ultrahigh pressure metamorphic rocks in the dabie orogen east central china cl images reveal core rim structure for most zircons in the granulites the cores show oscillatory zoning relatively high th u ratios and hree enriched patterns consistent with a magmatic origin they gave a weighted mean 207pb 206pb age of 2766 9 ma interpreted as dating magma emplacement of the protolith the rims are characterized by sector or planar zoning low th u ratios negative eu anomalies and flat hree patterns consistent with their formation under granulite facies metamorphic conditions zircon u pb dating yields a weighted mean 207pb 206pb age of 2029 13 ma which is interpreted to record a metamorphic event possibly during assembly of the supercontinent columbia the gneiss has a protolith age of 1982 14 ma which is younger than the zircon age of the granulite facies metamorphism suggesting a generally delay between ht metamorphism and the intrusion of post collisional granites a few inherited cores with igneous characteristics have 207pb 206pb ages of 2 90 3 28 and 3 53 ga suggesting the presence of mesoarchean to paleoarchean crustal remnants in the yangtze craton a few cretaceous metamorphic ages were also obtained suggesting the influence of post collisional collapse in response to cretaceous extension of the dabie orogen it is inferred that the recently discovered archean basement of the yangtze craton occurs as far north as the dabie orogen