shrimp zircon age constraints from the larsemann hills region prydz bay for a late mesoproterozoic to early neoproterozoic tectono thermal event in east antarctica this paper reports a geochronological study of granulite facies rocks from the larsemann hills of prydz bay east antarctica shrimp zircon ages were obtained for a total of 14 samples that cover various rock types including felsic gneiss mafic gneiss paragneiss enderbite granitic gneiss and leucogneiss these age results combined with zircon geochemical data and cathodoluminescence images enable us to explore the morphological complexity in zircons that record polytectonic events features of multiple age zoning for example were observed in zircon separates from three felsic gneiss samples studied which contain magmatic cores of ca 1 13 ga and metamorphic mantles and rims of ca 1 0 and 0 53 ga respectively this suggests a period of magmatism and crust formation in the late mesoproterozoic and two subsequent phases of high grade metamorphism during the early neoproterozoic and early paleozoic respectively similar phenomena of zircon overgrowth were also observed in other rock types although early paleozoic tectonic activity has been considered as most significant in antarctic crustal evolution our study provides the first convincing age evidence for the existence of a prydz bay late mesoproterozoic mobile belt the ca 1 0 ga metamorphic record preserved in diverse rocks from the study area furthermore supports the proposal of a continuous circum east antarctica late mesoproterozoic grenville age orogenic belt within this framework the prydz bay orogen is proposed to have been located in the central part of the rodinia assembly that brought the eastern ghats of india together with antarctica at ca 1 0 ga

monazite dating of granitic gneisses and leucogranites from the kerala khondalite belt southern india implications for late proterozoic crustal evolution in east gondwana two stages of granitic magmatism occurred during the pan african evolution of the kerala khondalite belt kkb in southern india granitic gneisses were derived from porphyritic granites which intruded prior to the main stage of deformation and peak metamorphism subsequently leucogranites and leucotonalites formed during fluid absent melting and intruded the gneiss sequences monazites from granitic gneisses leucogranites and a leucotonalite were investigated by conventional u pb and electron microprobe dating in order to distinguish the different stages of magma emplacement u pb monazite dating yielded a wide range of ages between 590 520 ma which are interpreted to date high grade metamorphism rather than magma emplacement the results of this study indicate that the kkb experienced protracted heating 50 ma at temperatures above 750 800 c during the pan african orogeny the tectonometamorphic evolution of the study area is comparable to southern madagascar which underwent a similar sequence of events earlier than the kkb the results of this study further substantiate previous assertions that the timing of high grade metamorphism in east gondwana shifted from west to east during the late proterozoic

indo antarctic correlation a perspective from the eastern ghats granulite belt india available lithological petrological and geochronological data on the rocks of the eastern ghats belt egb which has a key role to play in any model of indo antarctic correlation in precambrian and cambrian times have been synthesized and interpreted longitudinal lithological subdivisions of the belt are mostly not supported by recent geochronological data which rather suggest a fourfold division based on protolith ages of the ortho and paragneisses mineral ages show prominent tectonothermal events at mesoproterozoic grenvillian and pan african times however not all of the imprints of all the events are preserved in all the geochronological domains in the most well studied domain ii three phases of metamorphism are recorded an early ultra high temperature metamorphism with an anticlockwise p t path a second granulite facies event at c 1000 ma with a retrograde trajectory of near isothermal decompression and a third of amphibolite facies the available information however is not conclusive as to whether or not the entire egb experienced polyphase uht metamorphism the propagation of the grenvillian orogenic front in the egb is identified

the growth reworking and metamorphism of early precambrian crust in the jiaobei terrane the north china craton constraints from u th pb and lu hf isotopic systematics and ree concentrations of zircon from archean granitoid gneisses abstract archean granitoid tonalite trondhjemite granodiorite ttg gneisses are widely distributed in the early precambrian metamorphic basement of the jiaobei terrane the north china craton ncc and record the formation and evolution of the continental crust in the early precambrian in this study zircon cathodoluminescence cl images have assisted in obtaining u th pb and lu hf isotopic systematics and rare earth element ree concentrations from distinct zircon domains of complex zircons and the results have been integrated in order to unravel the growth reworking and metamorphism of the early precambrian crust in the jiaobei terrane zircons from seven representative archean granitoid samples collected from the jiaobei terrane record three groups of magmatic ages of 2 9 2 7 and 2 5 ga and two groups of metamorphic ages of 2 5 and 1 86 ga the 2 5 ga metamorphic event is thought to be linked to underplating of large amounts of mantle derived magma and the 1 86 ga metamorphic event to arc or continent continent collision magmatic zircons have positive hf t values 0 7 to 12 58 average 5 5 and two stage hf model ages cluster at 2 7 2 8 and 2 9 3 0 ga this indicates that the archean granitoid rocks were derived from partial melting of juvenile crust that was formed mainly during 2 7 2 8 and 2 9 3 0 ga which represent two periods of growth of juvenile crust the most significant period of crustal growth is 2 7 2 8 ga whereas 2 5 ga represents a period of reworking of the 2 7 2 8 ga juvenile crust in the jiaobei terrane as well as other areas in the ncc

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

pan african extension and near isothermal exhumation of a granulite facies terrain dronning maud land antarctica t he m uhlig hofmann and filchnerfjella in central dronning maud land antarctica consist of series of granitoid igneous rocks emplaced in granulite and upper amphibolite facies meta morphic rocks the area has experienced high temperature metamorphism followed by near isothermal decompression partial crustal melting voluminous magmatism and extensional exhumation during the later phase of the late neoproterozoic to cambrian pan african event remnants of kyanite garnet ferritschermakite rutile assemblages indicate an early higher pressure metamorphism and crustal overthickening the gneisses experienced peak granulite facies temperatures of 800 900 ca t intermediate pressures breakdown of garnet sillimanite spinel bearing assemblages to cordierite shows subsequent re equilibration to lower pressures an e w foliation dominating the gneisses illustrates transposition of migmatites and leucocratic melts which evolved during the near isothermal decompression occurrence of extensional shear bands and shear zones evolving from the ductile partial melting stage through semiductile towards brittle conditions shows that the uplift persisted towards brittle crustal conditions under tectonic w sw vergent extension late orogenic pan african quartz syenites intruded after formation of the main gneiss fabric contain narrow semiductile to brittle shear zones illustrating that the extensional exhumation continued also after their emplacement the latest record of the pan african event is late magmatic fluid infiltration around 350 400 c and 2 kbar at this stage the pan african crust had undergone 15 20 km exhumation from the peak granulite facies conditions we conclude that the later phase of the pan african event in central dronning maud land is characterized by a near isothermal decompression p t path and extensional structures indicating tectonic exhumation which is most likely related to a late orogenic collapsing phase of the pan african orogen

zircon u pb ages ree concentrations and hf isotope compositions of granitic leucosome and pegmatite from the north sulu uhp terrane in china constraints on the timing and nature of partial melting granitic leucosome and pegmatite are widely distributed within biotite bearing orthogneiss in the northern part of the sulu ultrahigh pressure uhp metamorphic terrane eastern china a combined study of mineral inclusions cathodoluminescence cl images u pb shrimp dates and in situ trace element and lu hf isotope analyses of zircons provided insight into the nature and timing of partial melting in these rocks zircon grains separated from biotite bearing orthogneiss typically have three distinct domains 1 pre metamorphic magmatic cores with qtz kfs pl ap inclusions which record a neoproterozoic protolith age of 790 ma 2 mantles with coe phe ap inclusions that record triassic uhp age at 227 3 ma and 3 narrow rims with quartz inclusions that record hp granulite facies retrograde metamorphism at 210 3 ma in contrast zircons separated from granitic leucosome have only two distinct domains 1 the central uhp areas with coe phe ap inclusions record triassic uhp age of 227 3 ma and 2 outer magmatic areas with qtz kfs ab ap inclusions that record partial melting time of 212 2 ma zircons separated from pegmatite contain mineral inclusions of qtz kfs ap and show regular magmatic zoning from centre to edge the centres record partial melting time of 212 2 ma in line with the outer domains of granitic leucosome whereas the edges give a younger age of 201 2 ma related to pb loss and partial recrystallization during late triassic regional amphibolite facies retrogression these data indicate that partial melting in the north sulu uhp gneissic rocks took place during post uhp retrograde hp granulite facies metamorphism pre metamorphic magmatic zircon cores from biotite bearing orthogneiss give uniform 176hf 177hf of 0 28187 0 00003 2 sd standard deviation corresponding to ehf 790 and hf model ages tdm2 of about 16 3 and 2 41 ga respectively this is consistent with the generation of its protolith by reworking of paleoproterozoic to late archean crust in contrast uhp zircon domains from biotite bearing orthogneiss and granitic leucosome are characterized by distinct trace element composition with low lu hf 0 006 low th u 0 1 and considerably higher 176hf 177hf 0 28233 0 00002 2 sd than the pre metamorphic cores the uniform but significantly different hf isotope composition between the uhp ehf 227 14 6 0 8 2 sd and pre metamorphic ehf 227 27 7 domains indicates equilibration of the lu hf isotope system only within the uhp metamorphic mineral assemblage the disequilibrium between whole rock and uhp zircon suggests that about two thirds of the whole rock hf retained in the pre metamorphic zircon domains zircon domains crystallized during partial melting at 212 ma in granitic leucosome and pegmatites have a hf isotope composition indistinguishable from that of the uhp zircon domains this suggests that only hf and zr equilibrated during uhp metamorphism was remobilized during partial melting while pre metamorphic zircon remained stable or was not accessible in contrast the magmatic zircon edges from pegmatite have somewhat lower 176hf 177hf 0 28216 and ehf t 17 6 1 2 2 sd indicating some release of less radiogenic hf for instance by dissolution of pre metamorphic zircon during late regional amphibolite facies retrogression

new zircon u pb and hf nd isotopic constraints on the timing of magmatism sedimentation and metamorphism in the northern prince charles mountains east antarctica abstract the northern prince charles mountains pcm in east antarctica represent the largest continuously exposed section of the rayner complex and may provide important insights into the tectonic evolution of the rayner orogen we present new u pb and hf isotopic data for zircons from felsic orthogneisses mafic granulites paragneisses and charnockites and additional nd isotopic data for the former two rock types from the beaver lake area in the northern pcm zircons from the felsic orthogneisses document protolith ages of ca 1170 1070 ma with hf and nd model ages of 1 99 1 74 ga suggesting the generation of the felsic magmas by partial melting of crustal rocks that were extracted from the mantle during the paleoproterozoic detrital zircons from one paragneiss sample yield a major age population at ca 1480 1140 ma and three subordinate populations at ca 2130 1850 1780 1620 and 1010 860 ma whereas those from another paragneiss sample produce a major age population at ca 1180 830 ma and a subordinate age population at ca 1370 1230 ma discounting the effects of zircon recrystallization during post depositional metamorphism we infer that the sedimentary precursors to the aforementioned paragneiss samples were deposited after ca 1200 and 1020 ma respectively in intra or back arc basins of the rayner continental arc charnockites were either emplaced at ca 980 ma or episodically at ca 1050 and ca 950 ma with hf model ages of 1 97 1 90 ga they were derived from partial melting of a paleoproterozoic source region similar to the surrounding felsic orthogneisses at deeper levels zircon overgrowth domains from all of the studied rock types indicate that high grade metamorphism took place at ca 945 915 ma only one paragneiss sample from the else platform preserves evidence of cambrian metamorphic reworking based on published data from the rayner complex and the eastern ghats belt of india we speculate that long lived convergent processes between the indian craton and east antarctica lasted from ca 1500 to 900 ma therefore the rayner complex may represent the exposed orogenic root of a large meso neoproterozoic accretionary orogen

low temperature replacement of monazite in the ireteba granite southern nevada geochronological implications the ireteba pluton is a relatively homogeneous 64 ma zircon ion probe age two mica granite that was intruded by two 16 ma miocene plutons at depths ranging from 5 to 13 km deeper levels of the ireteba and miocene plutons were ductilely deformed at 15 16 ma at shallow levels remote from the miocene plutons the ireteba granite appears to have experienced little miocene heating and deformation monazites from different portions of the pluton reflect the different histories experienced by the host rock irregularly shaped patchy zones with high huttonite component thsio4 are widespread in monazite at deep levels adjacent to miocene plutons but less common in shallow level rock monazite grains with extensive replacement generally have irregular embayed surfaces in undeformed rocks distant from the miocene plutons monazites are less modified and more nearly euhedral though fine networks of replacement veins are common and irregular rims are evident in some grains secondary monazite from these samples is poorer in huttonite ion probe th pb dating yields 60 65 ma ages for magmatic and some replacement zones in monazite from the shallow samples and veins yield apparent ages as young as mid tertiary monazites from deep samples yield a few 55 65 ma ages for remnant magmatic zones and abundant miocene ages for replacement zones 14 18 ma these data demonstrate extensive miocene replacement of magmatic monazite especially at deep levels near miocene plutons and they suggest an early replacement episode as well both events were probably related to influxes of fluid the first may have been associated with initial solidification of the ireteba pluton and the second with the miocene plutons and or extensional deformation ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at t as low as 400 c or less

zircon u pb geochronology and hf isotopes of major lithologies from the jiaodong terrane implications for the crustal evolution of the eastern block of the north china craton abstract as an important component of the western shandong complex in the eastern block of the north china craton the yishui terrane consists of neoarchean high grade supracrustal rocks granitoid gneisses and charnockites la icp ms zircon u pb dating and hf isotopic analyses on these lithologies have been carried out and the results provide new insights into the neoarchean crustal evolution of the eastern block of the north china craton new zircon dating results reveal that the magmatic precursors of the supracrustal metapelites and granitoid gneisses were generated at 2 54 2 53 ga and 2 57 2 55 ga respectively and the charnockites were emplaced contemporaneously around 2 56 2 53 ga single metamorphic zircon grains and overgrowth rims from these rocks document consistent metamorphic ages at 2 50 ga suggesting that the yishui terrane experienced a high grade metamorphic event at the end of the neoarchean detrital zircons from pelitic gneisses yield 207pb 206pb ages of 2 89 2 65 ga with a major age peak at 2 53 ga and a subordinate age peak at 2 70 ga implying that the sedimentary protoliths of the pelitic gneisses may have been sourced from 2 53 ga rocks in the yishui terrane and 2 70 ga rocks in the adjacent luxi granite greenstone terrane hf isotopic compositions show that the neoarchean magmatic zircons have positive ehf t values ranging from 1 4 to 7 8 and depleted mantle model ages of 2 92 2 60 ga with a prominent peak at 2 8 2 7 ga suggesting that the neoarchean crust was derived mainly from juvenile sources and partly from the recycling of old continental crust in the yishui area combined with previous data from the adjacent luxi granite greenstone terrane and other complexes in the eastern block it is concluded that the neoarchean was an important period of crust accretion in the eastern block of the north china craton

evolution of the western margin of australia during the rodinian and gondwanan supercontinent cycles abstract the proto darling fault zone and its successor the darling fault extend for 1 000 km along the western continental margin of australia and appear to have been active at several periods during the geological past deformation commenced at 2 570 ma and affected late archaean granitoids along the western margin of the yilgarn craton much of the later activity reflects events related to the accretion and breakup associated with the rodinia and gondwanaland supercontinent cycles in the north rocks of the northampton and mullingarra complexes form part of a high grade grenvillian orogenic belt lying to the west of the darling fault referred to as the pinjarra orogen they underwent granulite facies metamorphism 1080 ma ago and form part of the global collisional event that resulted in the amalgamation of rodinia these rocks extend southward beneath phanerozoic sedimentary cover the perth basin where they are constrained to the east by the darling fault and to the west by the dunsborough fault the latter marking the eastern boundary of the leeuwin complex the leeuwin complex is a fragment of pan african crust that has traditionally been considered part of the pinjarra orogen it is composed predominantly of upper amphibolite to granulite facies felsic orthogneisses derived from a type anorogenic granitoids conventional and shrimp u pb zircon geochronology has established that the granitoids evolved between 780 ma and 520 ma and were metamorphosed at 615 ma these events are equated with rifting associated with the breakup of rodinia sm nd whole rock data support the juvenile nature of the crust and provide no evidence for the involvement of pre existing archaean continental material during the phanerozoic the dunsborough and darling faults were reactivated as normal faults defining the inner arm of a major rift system within eastern gondwanaland and controlling sedimentation in the perth basin that now overlies the grenvillian terrane major normal movement on the darling fault ceased by the late jurassic and it appears that continental breakup in the early cretaceous occurred along fractures closely related to the western boundary of the leeuwin complex that defined the eastern margin of the outer arm of the rift system breakup between australia and greater india commenced at 132 ma and was followed by eruption of the bunbury basalt at 130 ma and 123 ma this possibly resulted from hot spot activity beneath eastern gondwanaland and may have been a reflection of the kerguelen plume though the evidence is equivocal it is argued from the petrographic geochemical and isotopic characteristics together with the likely contiguity of the eastern gondwanaland continents since the assembly of rodinia that the leeuwin complex evolved within an intracrustal rift and is not an exotic terrane it is distinct from adjacent portions of the pinjarra orogen and should be considered a separate terrane it is recommended that use of the term pinjarra orogen be confined to rocks recording the grenvillian events thereby excluding those rocks the leeuwin complex that evolved during the later pan african orogeny

the age isotopic signature and significance of the youngest mesozoic granitoids in the jiaodong terrane shandong province north china craton abstract most cretaceous granitoids in the jiaodong terrane of shandong province north china craton were intruded between 130 and 122 ma however four granitoid batholiths are significantly younger with an average zircon u pb shrimp emplacement age of 116 2 ma these are the sanfoshan batholith 118 1 ma the aishan batholith 116 1 ma the yashan pluton 113 2 ma and the laoshan batholith 115 2 ma these granitoids post date proposed delamination that occurred prior to 125 ma in the eastern ncc and their geochemistry thus provides information about the nature of the continental crust at this time the rocks are mostly monzogranites and syenogranites of i type affinity with local alkali feldspar granite of a type affinity in addition a monzogranite sample from the aishan batholith records an age of 125 3 ma indicating a direct association with the slightly older granitoids in the area indeed they all contain inherited zircons with ages of 125 120 ma indicating interaction with these earlier granitoids most samples are metaluminous enriched in light rare earth elements and depleted in high field strength elements paleo to neoarchean t dm2 model ages ranging from 3900 ma to 2491 ma and negative e hf t values imply the involvement of archean basement and older recycled continental crustal material in the magma source region they were likely derived from partial melting of the lower or middle crust due to mafic magma underplating followed by various degrees of interaction between mantle derived mafic magma and felsic crustal magma accompanied by fractional crystallization we propose that lithospheric thinning and extension in the early cretaceous due to roll back of the pacific plate beneath the eastern north china craton led to asthenospheric upwelling and basaltic magmatism this underplating in turn led to crustal melting and evolution of the granitic intrusions described here

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

archean crustal evolution in china the taishan complex and evidence for juvenile crustal addition from long term depleted mantle abstract the main framework for crustal evolution of the late archean taishan complex in the shandong province of china is constructed using data from field investigation petrographic examination geochemical analysis and isotopic age determination the history of the complex began with emplacement of basic and tonalitic magmas about 2 7 2 75 ga ago which were metamorphosed designated as metamorphism i to amphibolites and grey gneisses shortly after their emplacement these rocks constitute a metamorphic basement termed the wangfushan gneisses the basement gneisses were later intruded by a plutonic association including 1 alkali and light rare earth element lree enriched dioritic rocks and their differentiates 2 6 ga ago 2 granites and trondhjemites collectively termed the hushan granites also 2 6 ga ago and 3 the aolaishan type granites sensu stricto 2 4 2 5 ga ago the plutonic association in conjunction with the older basement gneisses was later subjected to a second period of deformation and metamorphism metamorphism ii 2 ga ago as inferred from published k ar biotite dates the chronological order of all the above events has been determined by a multichronometric approach and the age results in ma with 2 uncertainties are summarized as follows the relatively low i sr initial 87 sr 86 sr ratio and high positive nd t values for the wangfushan gneisses and the diorite group provides strong evidence for continental growth by addition of juvenile material ultimately separated from long term depleted mantle sources by contrast the sr and nd isotopic data for the aolaishan type granites coherently suggest that they are likely to be derived by anatexis of the basement gneisses at shallow depths crustal evolution of the taishan complex offers an additional example supporting the concept of a continental accretion differentiation superevent that during such a worldwide tectonothermal episode particularly in the archean important differentiation of the upper mantle has contributed to the growth of the continental crust by addition of juvenile material

paleoproterozoic accretionary orogenesis in the north china craton a shrimp zircon study abstract the inner mongolia suture zone imsz and the trans north china orogen tnco incorporate the major paleoproterozoic accretionary orogens in the north china craton ncc with the jiao liao ji belt jljb representing the third one here we investigate the paleoproterozoic tectonics of the imsz and tnco through zircon shrimp geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks shrimp analysis of zircons with textures indicating extreme recrystallization under ultrahigh temperature uht conditions from the metapelites at heling er in the southern domain of the imsz reveals a single population with a weighted mean 207 pb 206 pb age of 1913 17 ma the zircons in another uht granulite from this locality yield a weighted mean 207 pb 206 pb age of 1910 18 ma these data correlate with the ca 1 92 ga age reported from zircons in sapphirine bearing uht granulites further north and confirm the regional extent of the paleoproterozoic uht metamorphism within the imsz zircons in a charnockite from the southern margin of the khondalite belt fringing the uht granulites in the imsz show two distinct age groups an older population with a magmatic paragenesis and a weighted mean 207 pb 206 pb age of 1932 24 ma and a younger group of metamorphic zircons with an age of 1858 26 ma we also report zircon ages from charnockites in two localities around xing he in the huangtuyao belt belonging to the huai an complex within the westernmost domain of the tnco at the junction with the imsz the charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean 207 pb 206 pb age of 2477 2 ma and the younger population showing an age of 1807 38 ma the internal structure as revealed from cl images and the overall high th u values up to 2 42 of the older zircons suggest their magmatic affinity whereas the younger group with extremely low th u 0 02 0 09 is of metamorphic origin zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older magmatic group having a weighted mean 207 pb 206 pb age of 2147 11 ma and a minor younger group with an age of 1958 25 ma the range of ages from 2477 to 2147 ma from magmatic zircons in the charnockites from the eastern periphery of the imsz within the western margin of the tnco in combination with similar ages reported in recent studies from zircons in magmatic complexes within the imsz suggest a prolonged history of subduction related arc magmatism and accretionary tectonics analogous to those in some of the phanerozoic belts such as the central asian orogenic belt and the western pacific subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high pressure hp and uht metamorphic rocks the available data from the imsz and tnco suggest long lived convergent margins associated with the southward subduction of the yinshan block and westward subduction of the eastern block in a double sided subduction realm prior to the final amalgamation of the ncc and its incorporation within the columbia supercontinent in the late paleoproterozoic

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

thermobarometric modelling of zircon and monazite growth in melt bearing systems examples using model metapelitic and metapsammitic granulites u pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the earth despite the emphasis placed on zircon and monazite ages the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy in addition few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner as per this study the data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock here experimental data for zircon and monazite saturation in melt bearing rocks are integrated with phase diagram calculations the calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure temperature and silicate mineral assemblages in high grade melt bearing metasedimentary rocks several key results emerge from this modelling first that in aluminous metapelitic rocks i e garnet cordierite sillimanite assemblages zircon ages are older than monazite ages in the same rock second that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures and third that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature s at which zircon and monazite grow rather than differences in closure temperature systematics using our methodology the cooling rate of granulites from the reynolds range central australia have been constrained at 4 c myr 1 this study serves as a first pass template on which further research in applying the technique to a field study can be based

the behaviour of monazite from greenschist facies phyllites to anatectic gneisses an example from the chugach metamorphic complex southern alaska monazite is a common accessory mineral in various metamorphic and magmatic rocks and is widely used for u pb geochronology however linking monazite u pb ages with the pt evolution of the rock is not always straightforward we investigated the behaviour of monazite in a metasedimentary sequence ranging from greenschist facies phyllites into upper amphibolites facies anatectic gneisses which is exposed in the eocene chugach metamorphic complex of southern alaska we investigated textures chemical compositions and u pb dates of monazite grains in samples of differing bulk rock composition and metamorphic grade with particular focus on the relationship between monazite and other ree bearing minerals such as allanite and xenotime in the greenschist facies phyllites detrital and metamorphic allanite is present whereas monazite is absent in lower amphibolites facies schists 550 650 c and 3 4 kbar small medium y monazite is wide spread mnz1 indicating monazite growth prior and or simultaneous with growth of garnet and andalusite in anatectic gneisses new low y high th monazite mnz2 crystallised from partial melts and a third high y low th monazite generation mnz3 formed during initial cooling and garnet resorption u pb shrimp analysis of the second and third monazite generations yields ages of 55 50 ma monazite became unstable and was overgrown by allanite and or allanite epidote apatite coronas within retrograde muscovite and or chlorite bearing shear zones this study documents polyphase complex monazite growth and dissolution during a single relatively short lived metamorphic cycle

discovery of a neoproterozoic basin in the prydz belt in east antarctica and its implications for gondwana assembly and ultrahigh temperature metamorphism abstract detrital zircon u pb age data from ultrahigh temperature metasedimentary rocks 950 c and 10 12 kbar in the rauer group and a granulite grade metasedimentary rock 850 c and 7 8 kbar in the bolingen islands in prydz bay east antarctica indicate the existence of a neoproterozoic ediacaran cover sequence that was deposited on basement ranging in age between approximately 2800 ma and 940 ma the neoproterozoic succession has a maximum depositional age of approximately 600 ma with derivation from igneous and metamorphic sources of archaean palaeoproterozoic and neoproterozoic age the minimum depositional age is constrained to be approximately 575 ma based on existing th u pb monazite metamorphic ages from the uht metasedimentary rocks the identification of late neoproterozoic protoliths to the uht granulites in the rauer group demonstrate that ultrahigh temperature metamorphism in prydz bay must be ediacaran to cambrian in age the timing of basin development ca 600 575 ma is coincident with the amalgamation of eastern gondwana the palaeogeography of the prydz bay region during the amalgamation of gondwana is uncertain but in our proposed reconstruction we suggest that inherited zircons were derived from indo antarctic crust ocean closure is inferred to have occurred along a suture zone to the southeast of prydz bay and the prince charles mountains our palaeogeographic model invites further work on the possibility of a shared history between india and a large portion of east antarctica prior to the final amalgamation of eastern gondwana during the ediacaran cambrian

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

where is the north china south china block boundary in eastern china the qinling dabieshan belt results from the collision of the north china and south china blocks the eastern extension of the belt the sulu area consists of several stacked units from top to bottom these are 1 a weakly metamorphosed slate sandstone unit of neoproterozoic paleozoic age 2 a gneiss quartzite unit metamorphosed under high pressure p conditions 3 a marble amphibolite unit 4 an ultrahigh p unit that was very deeply subducted and then exhumed and 5 a migmatitic dome resulting from the melting of ultrahigh p rocks all these units underwent the same deformation which was characterized by a top to the northwest extensional ductile shearing during their exhumation granulite facies restites in the migmatite are not significantly different from granulitized eclogites derived from the ultrahigh p rocks the migmatite exhibits the same structural and petrologic evolution as that determined for the other units the resemblance of petrologic and structural features and the lack of ocean basin rock shows that the boundary between the north china block and south china block must be placed north of the sulu area

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

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

new constraints on metamorphism in the rauer group prydz bay east antarctica granulite facies metapelites of the mather and filla paragneisses within the rauer group east antarctica possess markedly different compositions the metamorphic evolution of the two metapelite types has been interpreted as temporally distinct with the rauer group preserving at least two distinct granulite facies tectonothermal episodes calculated p t pseudosections and orthopyroxene al content indicate the revised maximum preserved p t conditions within the mather paragneiss to lie in the vicinity of 950 975 c and 10 10 6 kbar less extreme than previous estimates the range of possible p t paths for the mather paragneiss consistent with mineral textural relationships and pseudosections contoured for mineral proportion are significantly shallower dp dt than previous estimates a near isothermal decompression p t path and extreme peak metamorphic conditions are not necessary to explain the development of preserved mineral reaction textures the filla paragneiss contains pelitic assemblages less amenable to rigorous quantitative analysis nevertheless possibilities for the shared or otherwise metamorphic evolution of the mather and filla paragneisses may be postulated on the basis of calculated pseudosections in the context of existing geochronology for the rauer group and preserved microstructures a shared evolution most likely during pan african granulite facies metamorphism is plausible and consistent with mineral assemblage development geochronology and microstructures a revised interpretation of the rauer group s preserved metamorphic evolution may warrant the revision of existing tectonic models applicable also to the remainder of prydz bay more generally the employed approach may incite a revision of peak p t and p t paths in other granulite facies terranes

texturally controlled u th pb monazite geochronology reveals paleoproterozoic uht metamorphic evolution in the khondalite belt north china craton

ultrahigh temperature granulite metamorphism 1050 c 12 kbar and decompression in garnet mg70 orthopyroxene sillimanite gneisses from the rauer group east antarctica highly magnesian and aluminous migmatitic gneisses from mather peninsula in the rauer group eastern antarctica preserve ultrahigh temperature uht metamorphic assemblages that include orthopyroxene sillimanite quartz garnet sillimanite quartz and garnet orthopyroxene sillimanite garnet that ranges up to xmg of 71 5 coexists with aluminous orthopyroxene that shows zoning from cores with 7 5 8 5 wt al2o3 to rims with up to 10 6 wt al2o3 adjacent to garnet peak p t conditions of 1050 c and 12 kbar are retrieved from fe mg al thermobarometry involving garnet and orthopyroxene in very good agreement with independent constraints from petrogenetic grids in feo mgo al2o3 sio2 and related chemical systems sapphirine orthopyroxene and cordierite form extensive symplectites and coronas on the early phases the specific reaction textures and assemblages involving these secondary phases correlate with initial garnet xmg with apparent higher pressure reaction products occurring on the more magnesian garnet and are interpreted to result from an initial phase of ultrahigh temperature near isothermal decompression uht itd from 12 to 8 kbar at temperatures in excess of 950 c later textures that involved biotite formation and then partial breakdown along with garnet relics to symplectites of orthopyroxene cordierite or cordierite spinel may reflect hydration through back reaction with crystallizing melts on cooling below 900 850 c followed by itd from 7 to 8 kbar to c 5 kbar at temperatures of 750 850 c the tectonic significance of this p t history is ambiguous as the rauer group records the effects of archean tectonothermal events as well as high grade events at 1000 and 530 ma late stage biotite formation and subsequent itd can be correlated with the p t history preserved in the proterozoic components of the rauer group and hence with either 1000 or 530 ma collisional orogenesis however whether the preceding uht itd history reflects a temporally unrelated event e g archean or is simply an early stage of either the late proterozoic or pan african tectonism as recently deduced for similar uht rocks from other areas of the east antarctica remains uncertain

crustal growth in the north china craton at 2 5ga evidence from in situ zircon u pb ages hf isotopes and whole rock geochemistry of the dengfeng complex abstract the dengfeng complex is located on the southern margin of the north china craton ncc and belongs to the southern portion of the trans north china orogen this terrane is important to understand the formation and evolution of ncc during late neoarchean 2 5 ga the dengfeng complex is well exposed in the junzhao region and comprises two distinct lithologic units supracrustal assemblage and plutonic rocks la icpms magmatic zircon u pb age data shows that the rocks formed within the range of 2547 2504 ma the available hf isotope data indicate that the majority of ca 2 5 ga zircons from the dengfeng complex have high e hf t values close to the initial hf isotope ratios of the contemporaneous depleted mantle these data indicate that the rocks in the dengfeng represent juvenile crust the ttg gneisses in the dengfeng complex display low mg 41 48 mgo 59 69 mgo 3 5 6 6 wt ni 82 130 ppm and cr 148 237 ppm abundances elevated sr 1759 1927 ppm and ba 1742 2289 ppm concentrations and high lree la n 38 487 such geochemical features are similar to archean sanukitoids a two stage model is applied here to explain the genesis of metadiorites of dengfeng complex 1 firstly the mantle is metasomatized either by melts or by aqueous fluids from a subducted slab 2 subsequently sanukitoid magmas were produced by partial melting of the hybridized mantle furthermore the amphibolites of supracrustal rocks have a mixture of morb and arc like geochemical affinities suggesting the development of a back arc in the southern ncc at ca 2 5 ga the contemporary late neoarchean ttgs sanukitoids and morb back arc association may represent a late neoarchean tectonic melange implying a neoarchean subduction accretion process which would suggest that modern style plate tectonics processes was probably initiated in the southern ncc by 2 5 ga

poly phase metamorphism of garnet bearing mafic granulite from the larsemann hills east antarctica p t path u pb ages and tectonic implications abstract a rare garnet bearing mafic granulite boulder has recently been identified from the larsemann hills east antarctica with the development of two stage decompression textures detailed petrographic study and p t estimates suggest three distinct metamorphic stages 1 peak ultrahigh temperature uht stage m1 represented by coarse grained garnet orthopyroxene bearing porphyroblastic assemblage with p t conditions of 9 3 kbar and 900 950 c 2 post peak decompression stage i m2 represented by relatively coarse grained orthopyroxene plagioclase ilmenite bearing corona assemblage with p t conditions of 7 2 7 6 kbar and 810 830 c and 3 post peak decompression stage ii m3 represented by fine grained orthopyroxene plagioclase ilmenite bearing symplectite assemblage on garnets and along garnet cracks with p t conditions of 6 2 6 9 kbar and 730 770 c combined with p t pseudosection calculation in the nckfmashto system these p t data define a clockwise p t path with post peak two stage decompression accompanied by important cooling la icp ms zircon u pb age dating results show two weighted mean ages of 927 13 ma and 547 30 ma suggesting that m1 uht stage occurred in the early neoproterozoic rayner orogeny 990 900 ma whereas m2 m3 stages occurred during the early palaeozoic pan african tectonic event 550 500 ma the peak p t conditions post peak two stage decompression p t path and age data resemble those of the early neoproterozoic garnet bearing mafic granulite on sostrene island it is inferred that the mafic granulite boulder may have been derived from the bedrock under inland ice sheet of the larsemann hills thus the early neoproterozoic metamorphism m1 may reflect crustal thickening followed by post orogenic extension likely associated with the assembly of rodinia continent while the overprinting of early palaeozoic metamorphism m2 m3 may reflect an extensive intra continental reworking

archaean cambrian crustal development of east antarctica metamorphic characteristics and tectonic implications abstract the east antarctic shield consists of a variety of archaean and proterozoic cambrian high grade terranes that have distinct crustal histories and were amalgamated at various times in the precambrian cambrian high grade pan african tectonism at 600 500ma is recognized from four distinct belts the dronning maud land lutzow holm bay prydz bay and denman glacier belts these high grade belts juxtapose distinct mesoproterozoic and neoproterozoic crustal provinces maud rayner and wilkes the rauer terrane and have also marginally affected archaean cratonic remnants in the napier complex and southern prince charles mountains the wilkes province experienced its principal tectonothermal events prior to 1130ma and was not affected by the younger events that characterize the maud province 1150 and 1030 990ma the rayner province 990 920ma and the rauer terrane 1030 990ma these differences between the isotopic event records of the basement provinces now separated by the pan african belts require that the older provinces were not formerly parts of a continuous grenville belt as proposed in the sw us east antartic model east antarctica was not a single unified crustal block within either east gondwana or rodinia until the cambrian which is now demonstrated to be the key phase of high grade and ultrahigh temperature uht metamorphism associated with supercontinent assembly the high grade pan african tectonism is characterized by extensive infracrustal melting clock wise p t paths rapid post peak exhumation along isothermal decompression paths to shallow or mid crustal levels by 500 ma and the generation at least locally of uht conditions a significant flux of heat from the mantle into the deep and initially overthickened crust is required to produce these observed metamorphic effects whilst the thermal evolution can be explained by models that invoke the removal of most of the lithospheric mantle following crustal thickening and prior to rapid extension of the remaining crust these one dimensional models are inconsistent with present crustal thicknesses of 25 35km in the pan african domains of the east antarctic shield

two stages of zircon and monazite growth in anatectic leucogneiss shrimp constraints on the duration and intensity of pan african metamorphism in prydz bay east antarctica

ultramafic to mafic granulites from the larsemann hills east antarctica geochemistry and tectonic implications the larsemann hills area is part of a reworked early neoproterozoic metamorphic terrain in southwestern prydz bay east antarctica ultramafic and mafic granulites whose origins remain controversial occur as lenses boudins or layered bodies within the para and ortho gneiss in the region the ultramafic and mafic granulites show spinel olivine bearing and two pyroxene bearing mineral assemblages recrystallized at 860 900 c their bulk rock analyses indicate an origin as igneous cumulates with high mg molar mgo mgo feo from 0 73 to 0 84 for ultramafic granulite and from 0 46 to 0 78 for mostly mafic granulite as well as high cr and ni contents cr and ni up to 1826 ppm and 1400 ppm respectively for ultramafic granulite and cr of 1460 ppm for mafic granulite trace element patterns show pronounced negative nb anomalies suggesting a subduction related tectonic setting for their precursors consistent with derivation from arc basalts also suggested by low tio2 and k enrichment in mafic granulites the ultramafic to mafic granulites may have been formed in a subduction related continental back arc basin environment simultaneously with peak metamorphism associated with arc continent collision during the early neoproterozoic 990 900 ma rayner orogeny

different age response of zircon and monazite during the tectono metamorphic evolution of a high grade paragneiss from the ruhla crystalline complex central germany results of tims sims and sem analyses show that zircon and monazite in a high grade paragneiss of the ruhla crystalline complex central germany were formed and or altered during different stages of a tectono metamorphic history between early devonian and permian times detrital zircon cores of 460 ma place an older limit on the age of anatexis and show that the paragneiss sequence contains rocks at least as young as early cambrian metamorphic zircon growth commenced at 365 ma peaking at 360 355 ma at the same time that granite dykes were emplaced in contrast monazite in the paragneiss preserves little record of the metamorphic peak most monazite grains grew or were recrystallised in the lower carboniferous at 339 ma contemporaneous with the emplacement of voluminous diorite and granite bodies these intrusions and related tectonics caused some of the high u zircon overgrowths to undergo moderate to severe pb loss a second pb loss event between 300 and 280 ma can be related to late carboniferous early permian large scale block faulting

grand sarcoui volcano chaine des puys massif central france a case study for monogenetic trachytic lava domes abstract the grand sarcoui is a prominent trachytic volcano of the intraplate quaternary volcanic field of chaine des puys massif central france which fulfills basic requirements for being qualified as monogenetic grand sarcoui looks like a simple axisymmetric lava dome but close observation reveals a complex and dissymmetric structure and composition the construction of the dome about 12 5 ka ago combined both endogenous and exogenous growth which resulted in variable modes of emplacement and textures of the lava one of its most interesting features is a large 0 29 106 m2 fan of deposits bearing hummocks and secondary hydro eruption craters cross sections of these deposits demonstrate that they originated from a sector collapse accompanied by a blast like event the dome is covered by a thin layer of lapilli and ash attributed to a delayed summit eruption which occurred about 10 6 ka ago surprisingly late after its construction so this volcano has at a reduced scale features that are more usually observed in large composite volcanoes however some of these features differ slightly from those that have been documented to date and they remain partly unexplained this shows that monogenetic well preserved trachytic lava domes are uncommon and poorly known unlike rhyolitic andesitic and dacitic domes

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

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

geochemistry and geochronology of high grade rocks from the grove mountains east antarctica evidence for an early neoproterozoic basement metamorphosed during a single late neoproterozoic cambrian tectonic cycle abstract the grove mountains of east antarctica are an inland continuation of the prydz belt the high grade metamorphic complex in this area is composed of felsic orthogneisses and mafic granulites with minor paragneisses and calc silicate rocks u pb zircon analyses using shrimp and la icp ms techniques reveal that the protoliths of mafic granulites and felsic orthogneisses were emplaced during a short time interval of ca 920 910 ma mafic granulites can be divided into low and high ti groups they have initial nd values nd t ranging from 0 8 to 1 9 tio 2 is positively correlated with feo t mgo and la nb ratios whereas it shows a negative correlation with nd t values indicating that the petrogenesis of their protoliths involved partial melting of a weakly enriched subcontinental lithospheric mantle and fractional crystallization of the magma accompanied by minor crustal contamination felsic orthogneisses have an affinity of a 2 type granites characterized by enrichment in ree y zr th and ga and high ga al and y nb ratios most of them have nd t values of 0 7 to 3 5 and nd depleted mantle model ages t dm of 1 76 1 65 ga and a few have low nd t values of 10 4 to 10 6 and old t dm of 2 46 2 27 ga reflecting a heterogeneity in their source region their protoliths were probably produced by high temperature partial melting of tonalitic granodioritic rocks triggered by the underplating of mantle derived mafic magma during post orogenic extension u pb analyses also reveal a metamorphic age of ca 2050 ma from detrital zircons in a paragneiss suggesting that the sedimentary materials of the paragneiss may have come from an as yet undiscovered early paleoproterozoic orogen of unknown provenance voluminous mafic felsic intrusives and a small amount of sedimentary rocks constitute an early neoproterozoic basement of the grove mountains subsequently this basement experienced only a single late neoproterozoic cambrian metamorphic cycle at ca 550 535 ma the available data indicate that the prydz belt is a collage of multiple basement terranes and each of them has a distinct tectonic evolution this supports the suggestion that the prydz belt may represent a late neoproterozoic cambrian collisional orogen that resulted in the final phase of the gondwana assembly

on the application of in situ monazite chemical geochronology to constraining p t t histories in high temperature 850 c polymetamorphic granulites from prydz bay east antarctica we present electron microprobe based in situ th u pb monazite chemical age data from granulite facies metapelites in prydz bay east antarctica the monazite age data define two distinct age populations late neoproterozoic to early palaeozoic c 570 520 ma and neoproterozoic c 950 820 ma that confirm the polytectonic nature of prydz bay our data suggest that similarity in lineation orientation along the prydz bay coast is not sufficient to necessarily indicate time equivalance the minimum duration of early palaeozoic tectonism spanning at least 60 ma is constrained from an mg al rich metapelite monazite hosted by coarse grained orthopyroxene defines the oldest early palaeozoic population whereas cordierite orthopyroxene symplectites define the youngest early palaeozoic population the spatial distribution of monazites and their ages is correlatable with the inferred mineral assemblage evolution we are able to characterize the evolution of early palaeozoic tectonism in quantitative p t t mineral assemblage space demonstrating that ultrahigh temperature tectono metamorphism in prydz bay is of early palaeozoic age the survival of neoproterozoic inheritance in fe al rich metapelites has implications for high pb retentivity in monazite when chemical and kinetic conditions are favourable the approach and logic applied herein are entirely and directly transferable to the interrogation of any other metamorphic terrane

age trends in garnet hosted monazite inclusions from upper amphibolite facies schist in the northern grouse creek mountains utah we performed in situ th pb dating of monazite in upper amphibolite facies pelitic schist from the grouse creek mountains in northwest utah sixty six ages from inclusions in four garnet grains range from 37 to 72 ma and decrease with radial distance from garnet cores the age range of 30 matrix monazite grains overlaps and extends to younger ages than inclusions 25 58 ma the monazite grains are not intersected by cracks in the garnets through which dissolution reprecipitation or pb loss might occur and are generally too small 20 lm to allow for more than one age determination on any one grain processes that might explain inclusion ages that decrease with radial distance from garnet cores include 1 pb diffusion in monazite 2 dissolution and reprecipitation of monazite and 3 co crystallization of monazite and garnet after consideration of these possibilities it is concluded that the co crystallization of monazite and garnet is the most plausible with monazite neoblasts deriving rees from the breakdown of muscovite garnet ages derived by regression of the inclusion ages and assuming a constant rate of volume increase during garnet growth yield model ages with a maximum difference between core and rim of 22 m y

textural development of monazite during high grade metamorphism hydrothermal growth kinetics with implications for u th pb geochronology monazite has become an important tool for geochronology but it commonly exhibits complex internal zoning of composition and age experiments were conducted to characterize the textural development and the rate and mechanism of growth of finely powdered 2 o at 1 0 gpa and 1000 degrees c coarsely crushed quartz crystals 500 mu m in diameter grew rapidly and progressively engulfed monazite crystals to form arrays of monazite inclusions the mean diameter of all monazite crystals decreased in the first 24 h then increased at a constant rate consistent with growth by grain boundary diffusion controlled ostwald ripening with a minimum rate constant k1 4 4 41x10 super 2 mu m s 4 using small quartz crystals of uniform diameter approximately 0 5 mu m in the starting material reduced quartz grain boundary mobility and limited the development of inclusions monazite grew by matrix volume diffusion controlled ostwald ripening with k1 3 1 02x10 super 2 mu m s 3 in all run products matrix coarsening produced linear crystal size distributions that reflect continuous recrystallization and nucleation textural evidence suggests that matrix coarsening induced coalescence was also an important growth mechanism during annealing of fluid filled rock growing host crystals may occlude small monazite crystals preserving their isotopic composition large monazite crystals may pin grain boundaries while smaller crystals may move with grain boundaries by recrystallizing a process that resets isotopic systems monazite crystals on grain boundaries may grow by ostwald ripening to form rims and by coalescence accurate interpretations of monazite ages therefore require knowledge of the texture growth history of the rock and its dated grains

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

shrimp u pb zircon geochronology of palaeoproterozoic metasedimentary rocks in the north china craton evidence for a major late palaeoproterozoic tectonothermal event abstract although metasedimentary successions are widely distributed across the north china craton those of palaeoproterozoic age are restricted to three orogenic belts namely the trans north china orogen the khondalite belt and the jiao liao ji belt prior to this work some of those successions were considered to be archaean in age the palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 0 to 2 1 ga with the majority recording an age between 2 3 and 2 0 ga this latter group define a series of magmatic events that occurred between formation of the archaean basement and deposition of the metasedimentary rock protoliths the metasedimentary rocks occur as two associations older volcano sedimentary units that formed between 2 37 and 2 0 ma and younger sedimentary units that formed mainly between 2 0 and 1 88 ga all these rock associations were then subjected to metamorphism during the late palaeoproterozoic 1 88 and 1 85 ga tectonothermal event named the luliang movement in early chinese literature which was a continent continent collision that led to the amalgamation of the north china craton

th u pb monazite ages from al mg rich metapelites rauer group east antarctica the age of high temperature granulite facies metamorphism 800 850 c in the rauer group prydz bay east antarctica is relevant for establishing the metamorphic and temporal architecture of the prydz bay mobile belt monazites within al mg rich granulite facies metapelites give an overall tanh estimated pan african age of 511 4 ma 2r using in situ electron microprobe based th u pb chronology consistent with existing u pb zircon geochronology from the rauer group and prydz bay monazite occurs primarily within cordierite bearing coronae and symplectic min eral reaction textures and also within biotite pan african granulite facies metamorphism is preferred as responsible for the development of the cordierite bearing microstructures and probably peak coarse grained assemblages constrained using an integrated geologic geochronologic and metamorphic framework thus pan african granulite facies metamorphism affected the rauer group within the prydz bay mobile belt moreover integrated monazite geochronology may be used to decipher the temporal metamorphic histories of potentially complex high temperature terrains

temperature and bulk composition control on the growth of monazite and zircon during low pressure anatexis mount stafford central australia the formation age and trace element composition of zircon and monazite were investigated across the prograde low pressure metamorphic sequence at mount stafford central australia three pairs of inter layered metapelites and metapsammites were sampled in migmatites from amphibolite facies t 600 c to granulite facies conditions t 800 c sensitive high resolution ion microprobe u pb dating on metamorphic zircon rims and on monazite indicates that granulite facies metamorphism occurred between 1795 and 1805ma the intrusion of an associated granite was coeval with metamorphism at 1802 3ma and is unlikely to be the heat source for the prograde metamorphism metamorphic growth of zircon started at t 750 c well above the pelite solidus zircon is more abundant in the metapelites which experienced higher degrees of partial melting compared with the associated metapsammites in contrast monazite growth initiated under sub solidus prograde conditions at granulite facies conditions two distinct metamorphic domains were observed in monazite textural observations petrology and the trace element composition of monazite and garnet provide evidence that the first metamorphic monazite domain grew prior to garnet during prograde conditions and the second in equilibrium with garnet and zircon close to the metamorphic peak ages from sub solidus prograde and peak metamorphic monazite and zircon are not distinguishable within error indicating that heating took place in less than 20 myr

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

the evolution of a layered metaigneous complex in the rauer group east antarctica evidence for a distinct archaean terrane abstract the rauer group in prydz bay east antarctica contains both archaean and proterozoic crustal components archaean geological relationships have been defined from the scherbinina layered complex slc a polydeformed and metamorphosed fe tholeiite intrusive cut by several generations of deformed metabasite dykes the slc and three of these dyke generations are intruded by tonalitic and trondhjemitic sheets representative of archaean felsic orthogneisses present outside the complex u pb shrimp dating of zircons precisely defines the magmatic age of the slc as 2844 6 ma a tonalite sheet that cuts the slc preserves a complex zircon isotopic array dominated by inherited grains derived from source rocks up to 3470 30 ma old but from field evidence was emplaced at ca 2800 2550 ma the presence of 3470 ma source rocks is unique in the east antarctic shield and this feature as well as the prominence of 2840 2800 ma orthogneisses establishes the rauer group as a distinct archaean terrane the archaean of the rauer group is not comparable with the nearby vestfold hills where ca 2520 2480 ma orthogneisses form the dominant rock units it is likely that the vestfold hills and rauer group terranes were amalgamated at either 1000 or 500 ma rather than at the end of the archaean a mid proterozoic tectonothermal event documented in the rauer group from syn metamorphic 1030 1000 ma intrusives kinny p d black l p and sheraton j w 1993 zircon ages and the distribution of archaean and proterozoic rocks in the rauer islands antarctic sci 5 193 206 has not been detected in the zircon u pb data obtained on the archaean gneisses examined here instead the slc and intrusive tonalite show resetting of zircon u pb systematics at ca 520 ma this resetting reflects pb loss within irregular compositional zones and healed cracks in older zircon and is attributed to the effects of fluid infiltration under greenschist to amphibolite facies conditions these effects correlate in time with ca 530 510 ma higher grade deformation and metamorphism evident in proterozoic paragneisses exposed in areas to the south west of the rauer group and may record amalgamation of the rauer group with these areas during the east gondwanian pan african orogeny

superimposed tectonic events at 2450 ma 2100 ma 900 ma and 500 ma in the north mawson escarpment antarctic prince charles mountains abstract the north mawson escarpment in antarctica s prince charles mountains forms part of a palaeoproterozoic crustal complex which separates archaean cratonic material to the south from early neoproterozoic and early palaeozoic metamorphic belts to the north it consists of nappe like masses of grey gneiss and metasupracrustal rocks that were subjected to repeated ductile deformation under upper amphibolite to lower granulite facies metamorphic conditions in this paper we report zircon u pb dating results that constrain the principal tectonothermal events to the periods 2490 2420 ma 2180 2080 ma 940 880 ma and 530 490 ma the magmatic precursors of volumetrically important grey gneiss were produced and reworked at least partly during the 2490 2420 ma and 2180 2080 ma events the superimposition of these events is clearly documented by u pb ages from several composite zircons the formation of associated metasupracrustals possibly in a passive margin setting occurred between the 2490 2420 ma and 940 880 ma events both grey gneiss and metasupracrustals were subsequently reworked during the 940 880 ma and 530 490 ma events corresponding to the superposition of both the rayner and prydz belts onto palaeoproterozoic lithosphere of the lambert complex transposition and peak metamorphism preceded the intrusion of leucogranite dykes at 905 ma and based on metamorphic zircon growth at around 930 ma in metapelitic schist are related to the rayner orogeny it is suggested that the north mawson escarpment was assembled to most coastal regions of the antarctic sector between 45 and 80 e by at least 900 m y ago

bulk chemical control on metamorphic monazite growth in pelitic schists and implications for u pb age data several petrographic studies have linked accessory monazite growth in pelitic schist to metamorphic reactions involving major rock forming minerals but little attention has been paid to the control that bulk composition might have on these reactions in this study we use chemographic projections and pseudosections to argue that discrepant monazite ages from the mount barren group of the albany fraser orogen western australia reflect differing bulk compositions a new sensitive high mass resolution ion microprobe shrimp u pb monazite age of 1027 8 ma for pelitic schist from the mount barren group contrasts markedly with previously published shrimp u pb monazite and xenotime ages of c 1200 ma for the same area all dated samples experienced identical metamorphic conditions but preserve different mineral assemblages due to variable bulk composition monazite grains dated at c 1200 ma are from relatively magnesian rocks dominated by biotite kyanite and or staurolite whilst c 1027 ma grains are from a ferroan rock dominated by garnet and staurolite the latter monazite population is likely to have grown when staurolite was produced at the expense of garnet and chlorite but this reaction was not intersected by more magnesian compositions which are instead dominated by monazite that grew during an earlier greenschist facies metamorphic event these results imply that monazite ages from pelitic schist can vary depending on the bulk composition of the host rock samples containing both garnet and staurolite are the most likely to yield monazite ages that approximate the timing of peak metamorphism in amphibolite facies terranes samples too magnesian to ever grow garnet or too iron rich to undergo garnet breakdown are likely to yield older monazite and the age difference can be significant in terranes with a polymetamorphic history

timing of metamorphism in the paleoproterozoic jiao liao ji belt new shrimp u pb zircon dating of granulites gneisses and marbles of the jiaobei massif in the north china craton abstract the paleoproterozoic jiao liao ji belt lies in the eastern block of the north china craton with its southern segment extending across the bohai sea into the jiaobei massif high pressure pelitic and mafic granulites have been recently recognized in the paleoproterozoic jingshan group jiaobei massif new shrimp u th pb geochronology combined with cathodoluminescence cl imaging of zircon has been applied to the determination of the timing of the metamorphism of the high temperature and high pressure granulites and associated gneisses and marbles metamorphic zircons in these high pressure granulites gneisses and marbles occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores metamorphic zircons are all characterized by nebulous zoning or being structureless with high luminescence and relatively low th u values metamorphic zircons from two high pressure mafic granulites yielded 207pb 206pb ages of 1956 41 ma and 1884 24 ma one metamorphic zircon from a garnet sillimanite gneiss also gave an apparent 207pb 206pb age of 1939 15 ma these results are consistent with interval of ages of c 1 93 1 90 ga already obtained by previous studies for the north and south liaohe groups and the laoling group in the northern segment of the jiao liao ji belt metamorphic zircons from a high pressure pelitic granulite and two pelitic gneisses yielded weighted mean 207pb 206pb ages of 1837 8 ma 1821 8 ma and 1836 8 ma respectively two diopside olivine phlogopite marbles yielded weighted mean 207pb 206pb ages of 1817 9 ma and 1790 6 ma these paleoproterozoic metamorphic ages are largely in accordance with metamorphic ages of c 1 85 ga produced from the ji an group in the northern segment of the jiao liao ji belt and c 1 86 1 80 ga obtained for the high pressure pelitic granulites from the jingshan group in the southern segment as this metamorphic event was coeval with the emplacement of a type granites in the jiao liao ji belt and its adjacent areas it is interpreted as having resulted from a post orogenic or anorogenic extensional event

experimental constraints on rutile saturation during partial melting of metabasalt at the amphibolite to eclogite transition with applications to ttg genesis tio2 solubility in rutile saturated felsic melts and coexisting minerals was determined at 1 5 3 5 gpa 750 1250 c and 5 30 wt h2o tio2 solubility in the melt primarily increases with temperature and melt basicity it increases slightly with water content in the melt and it decreases with pressure a general tio2 solubility model was obtained and is expressed as ln tio2 melt ln tio2 rutile 1 701 9041 t 0 173 p 0 348fm 0 016h2o where tio2 and h2o are in wt t is in kelvin p in gpa and fm is the melt composition parameter given by fm 1 si na k 2 ca fe mn mg al in which the chemical symbols represent cation fractions tio2 solubility in amphibole garnet and clinopyroxene also increases with temperature and empirical equations describing this temperature dependence were derived these data were used to assess the protolith tio2 content required for rutile saturation during partial melting of hydrous metabasalt at the amphibolite to eclogite transition the results show that only 0 8 1 0 wt tio2 is required for rutile saturation during low degree 20 melting rutile is stable up to 1150 c with 1 6 wt tio2 in the protolith and 30 40 melting for dehydration melting and up to 1050 c and 50 60 melting for fluid present melting the data also show that 0 7 0 8 wt tio2 in the protolith is needed for rutile saturation during subsolidus dehydration therefore nearly all basaltic protoliths in deep crustal settings and subduction zones will be saturated with rutile during subsolidus dehydration and low degree melting at hydrous conditions archean tonalites trondhjemites granites ttg are widely accepted to be the products of low degree melting of metabasalts at the amphibolite to eclogite transition with rutile being present in the residue comparison of natural ttg compositions with our experimental rutile solubility data indicates that the dominant ttg magmas were produced at temperatures of 750 950 c which requires that the partial melting occurred at hydrous conditions models involving melting at the base of oceanic plateaus are inadequate to explain ttg genesis because the plateau root zones are likely dominated by anhydrous cumulates a slab melting model satisfies the requirement of a hydrous metabasalt which during subduction would melt to produce voluminous ttg melts under high archean geothermal gradients the geothermal gradients responsible are estimated to be between 10 and 19 c km based on a pressure range of 1 5 2 5 gpa for the amphibolite to eclogite transition

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

contrasted tectonic styles for the paleoproterozoic evolution of the north china craton evidence for a 2 1 ga thermal and tectonic event in the fuping massif structural analysis along with 40ar 39ar and u pb datings in the fuping massif provide new insight into the evolution of the eastern part of the trans north china belt north china craton from 2 7 ga to 1 8 ga d1 is responsible for the development of a dome and basin structure coeval with crustal melting giving rise to migmatite and nanying gneissic granites at 2 1 ga this dome and basin architecture resulted from the interference between a n s compression of a weak ductile crust and gravity driven vertical flow in a high thermal regime the next events involved flat lying ductile thrusting d2 and normal faulting d3 dated at around 1880 ma and 1830 ma respectively the d2 and d3 events belong to the trans north china orogeny that results in the final amalgamation of the north china craton the d1 deformation is considered as evidence for an earlier orogen developed around 2 1 ga prior to the trans north china orogeny the change in the deformation style between the 2 1 ga and 1 8 ga could be viewed as a consequence of the cooling of the continental crust in the north china craton

fluid mediated partial alteration in monazite the role of coupled dissolution reprecipitation in element redistribution and mass transfer monazite ce lree th u ca p si o4 with complex zoning in th and other elements is commonly observed in metamorphic and igneous rocks the hypothesis that this alteration is a product of fluid mediated element mass transfer has been tested in the piston cylinder press caf2 assembly cylindrical graphite oven at 1 000 mpa and 900 c and in cold seal autoclaves on a hydrothermal line at 500 mpa and 600 c experiments included a relatively homogeneous monazite ce 7 8 wt tho2 from a heavy mineral sand plus a series of alkali bearing fluids including 2n naoh 2n koh and na2si2o5 h2o experiments were conducted using bse imaging emp analysis and both tem and hrtem a subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in th si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior these thsio4 enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks in the na2si2o5 h2o experiments scarce inclusions of britholite formed in the altered monazite the altered monazite is also characterised by strong depletion in pb ca and y thorium and si mobility coupled with the formation of britholite inclusions during partial alteration in the monazite grain is considered to be the product of fluid aided coupled dissolution reprecipitation as opposed to solid state diffusion since other fluids including nacl and kcl brines do not result in the formation of these textures the experimental replication of thsio4 enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced specifically by alkali bearing fluids if true complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event presuming that all the original radiogenic pb has been removed

paleoproterozoic ultrahigh temperature granulites in the north china craton implications for tectonic models on extreme crustal metamorphism abstract ultrahigh temperature uht metamorphic rocks associated with the khondalite belt within the inner mongolia suture zone imsz provide robust evidence for extreme thermal metamorphism in the north china craton ncc the imsz marks the collisional suture between the yinshan block to the north and the ordos block to the south as the ncc was incorporated within the columbia supercontinent amalgam during paleoproterozoic here we present a synthesis of the salient features of the uht rocks from the ncc including petrologic indicators fluid characteristics and monazite and zircon chronometry on the extreme crustal metamorphism the granulites carry diagnostic uht mineral assemblages including sapphirine quartz low zn fe3 spinel quartz high alumina orthopyroxene sillimanite quartz and high temperature mesoperthite the stability fields of the typical mineral assemblages conventional geothermobarometry and phase equilibria modeling using pseudosections as reported in a number of recent studies converge to indicate that these uht rocks experienced metamorphic temperatures up to or in excess of 1000 c at ca 10 kbar followed by an isobaric cooling segment the rocks were exhumed along a near isothermal decompression path microstructures mineral reactions and phase equilibria modeling suggest an anti clockwise p t path similar to those displayed by metamorphic orogens developed in subduction collision settings the dominant category of fossil fluids preserved within the major uht minerals is co2 consistent with the stability of the broadly anhydrous mineral assemblage in these rocks both chemical and radiogenic isotopic ages from monazite and zircon chronometry suggest the timing of the uht event as around 1 92 ga the paleoproterozoic high grade metamorphism younging from 1 95 ga in the western domain to 1 92 ga in the eastern domain of the khondalite belt might suggest a scissor like closure of oblique collision between the yinshan and the ordos blocks the salient features of the uht metamorphism in the ncc include 1 extreme metamorphic temperatures at moderate pressures 2 dominantly anhydrous nature of the mineral assemblages typically the stability of orthopyroxene 3 common presence of co2 rich fluid inclusions as the trace of the ambient fluid 4 regional extent of the uht granulites and 5 the association of the uht orogen with an accretionary belt in a continental collisional suture we evaluate the diverse models on the generation of uht orogens including their formation in thickened and inverted back arcs orogen self heating through heat producing elements heat and co2 input by plume impingement below a carbonated tectosphere and asthenospheric upwelling through ridge subduction and slab window process or during post collisional slab break off the ultra hot and dry uht rocks in the ncc provide one of the well preserved examples from the paleoproterozoic globe for investigating extreme metamorphism and related tectonic processes within the plate tectonic paradigm

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

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

geochronology of the karadere basement nw turkey and implications for the geological evolution of the istanbul zone earlier geological work in the istanbul zone western pontide tectonic belt has revealed the presence of extensive basement outcrops exposed underneath palaeozoic and mesozoic to tertiary cover sequences the basement of suspected neoproterozoic age plays an important role in understanding the crustal accretion process in nw turkey we report the first results of a detailed pb pb and u pb zircon study complemented by nd sr whole rock and mineral data from basement rocks exposed in the karadere valley safranbolu area five samples were selected for this study comprising three metagranitoids and two metasediments zircon geochronology indicates that the metagranitoids were formed during late proterozoic pan african magmatic events between 590 and 560 ma the rocks are of tonalitic and granitic composition and have low nb y ratios and ti contents consistent with those of arc rocks a continental arc setting is supported by their sr and nd isotope data that indicate a contribution of a mantle source as well as crustal assimilation during magma genesis the metasediments can clearly be distinguished from the metagranitoids by their higher 87sr 86sr ratios and lower end values at 580 ma which supports the suggestion that the arc was underlain by mature continental crust zircons from the metasediments yield a range of pb pb ages between 1 860 and 710 ma thirty per cent of them fall between 890 and 710 ma possibly suggesting a derivation from gondwana afro arabian regions a sm nd garnet whole rock analysis obtained on a metagranite gives an age of 559 8 ma which either reflects pre metamorphic magmatic growth of garnet in a felsic melt or a syntectonic high temperature metamorphic event uplift and cooling of the basement is further constrained by rb sr biotite ages of 548 545 ma these lower cambrian mineral ages demonstrate that the istanbul zone was not thermally reactivated during the hercynian cimmerian or alpine orogeny in contrast to its neighbouring tectonic zones confirming its role as a suspect terrane in the modern western pontide tectonic belt

pan african tectonism in the western maud belt p t t path for high grade gneisses in the h u sverdrupfjella east antarctica extensive high grade polydeformed metamorphic provinces surrounding archaean cratonic nuclei in the east antarctic shield record two tectono thermal episodes in late mesoproterozoic and late neoproterozoic cambrian times in western dronning maud land the high grade mesoproterozoic maud belt is juxtaposed against the archaean grunehogna province and has traditionally been interpreted as a grenvillian mobile belt that was thermally overprinted during the early palaeozoic integration of new u pb sensitive high resolution ion microprobe and conventional single zircon and monazite age data and ar ar data on hornblende and biotite with thermobarometric calculations on rocks from the h u sverdrupfjella northern maud belt resulted in a more complex p t t evolution than previously assumed a c 540ma monazite hosted by an upper ampibolite facies mineral assemblage defining a regionally dominant top to nw shear fabric provides strong evidence for the penetrative deformation in the area being of panafrican age and not of grenvillian age as previously reported relics of an eclogite facies garnet omphacite assemblage within strainprotected mafic boudins indicate that the peak metamorphic conditions recorded by most rocks in the area t 687 758 c p 9 4 11 3kbar were attained subsequent to decompression from p 12 9kbar by analogy with limited u pb single zircon age data and on circumstantial textural grounds this earlier eclogitefacies metamorphism is ascribed to subduction and accretion around 565ma post peak metamorphic k metasomatism under amphibolite facies conditions is ascribed to the intrusion of postorogenic granite at c 480ma the recognition of extensive panafrican tectonism in the maud belt casts doubts on previous rodinia reconstructions in which this belt takes a pivotal position between east antarctica the kalahari craton and laurentia evidence of late mesoproterozoic high grade metamorphism during the formation of the maud belt exists in the form of c 1035ma zircon overgrowths that are probably related to relics of granulite facies metamorphism recorded from other parts of the maud belt the polymetamorphic rocks are largely derived from a c 1140ma volcanic arc and 1072 10ma granite

40ar 39ar geochronolgy and neoproterozoic tectonics along the northern margin of the eastern ghats belt in north orissa india abstract the post 1100 950 ma cooling history of the northern margin of the high grade eastern ghats belt is determined from 40ar 39ar data collected from schist and gneiss samples the data define two groups of dates with distinct cooling histories between ca 700 and 420 ma the oldest group of ages define a slow apparent cooling path whereas a younger group of 40ar 39ar dates from the west and north margin of the eastern ghats belt and along the southern margin of the rengali province define a more rapid cooling path the younger 40ar 39ar dates are interpreted to reflect a variable and locally intense thermal event at ca 550 500 ma associated with reactivation of major shear zones final exhumation to current exposed levels occurred by ca 300 ma as constrained by unconformable permo triassic coal bearing sediments overlying the northern eastern ghats belt and northwestern rengali province dextral reverse reactivation after ca 950 ma and possibly prior to ca 700 ma along the kerajang and barakot fault zones was associated with regional shortening within the eastern ghats belt and rengali province further reactivation of major shear zones within the same shortening regime at ca 550 500 ma is indicated by the 40ar 39ar data resetting of the k ar system was locally more intense along the eastern ghats boundary fault and kerajang fault zones and was associated with ingress of high temperature fluids the resulting post grenvillian tectonic framework for northern orissa provides insight into the character of intra cratonic pan african tectonism within east gondwanaland

geophysical and geological tests of tectonic models of the north china craton abstract the geometry and timing of amalgamation of the north china craton have been controversial with three main models offering significantly different interpretations of regional structure geochronology and geological relationships one model suggests that the eastern and western blocks of the ncc formed separately in the archean and an active margin was developed on the eastern block between 2 5 and 1 85 ga when the two blocks collided above an east dipping subduction zone a second presumes the eastern block rifted from an unknown larger continent at circa 2 7 ga and experienced a collision with an arc perhaps attached to the western block above a west dipping subduction zone at 2 5 ga and the 1 85 ga metamorphism is related to a collision along the northern margin of the craton when the ncc joined the columbia supercontinent a third model suggests two collisions in the central orogenic belt at 2 1 and 1 88 ga but recognizes an early undated deformation event recent seismic results reveal details of the deep crustal and lithospheric structure that support both the second and third models showing that subduction beneath the central orogenic belt was west directed and that there is a second west dipping paleosubduction zone located to the east of the cob dipping beneath the western block ordos craton the boundaries identified through geophysics do not correlate with the boundaries of the trans north china orogen suggested in the first model and the subduction polarity is opposite that predicted by that model high pressure granulite facies metamorphism at 1 85 ga is not restricted to the tnco as suggested by the first model but is documented across the ncc as predicted by the second model suggesting a major continent continent collision along the north margin of the craton at 1 85 ga further it has recently been shown that in the southern tnco there is no record of metamorphism at circa 1 85 ga but only at 2 7 2 5 ga showing that the tnco as defined as a circa 1 85 ga orogen does not exist this is further confirmed by recent re os isotopic studies which show that the subcontinental lithospheric mantle beneath the southern cob is late archean in age and that a province in the northern ncc is circa 1 8 ga correlating with the proposed collision belt of the ncc with the columbia supercontinent across the entire ncc the cob is an archean convergent belt re worked in the paleoproterozoic and the paleoproterozoic tectonism is widespread across the ncc as predicted by the model whereby the previously amalgamated eastern and western blocks experienced a continental collision with columbia at circa 1 85 ga but uplift exhumation rates are slow necessitating a re evaluation of the tectonic models of the ncc

u pb th pb and ar ar geochronology from the southern sierras pampeanas argentina implications for the palaeozoic tectonic evolution of the western gondwana margin

zircon u pb ages geochemistry and nd hf isotopes of the ttg gneisses from the jiaobei terrane implications for neoarchean crustal evolution in the north china craton abstract the precambrian basement in the jiaobei terrane is largely composed of tonalite trondhjemite granodiorite ttg suite of rocks and offers important insights into the crustal evolution history of the north china craton ncc the la icp ms zircon u pb age data presented in this study show that the magmatic protoliths of the ttg gneisses formed during 2508 2547 ma and recorded the paleoproterozoic metamorphism 1905 ma the rocks are enriched in lile rb ba and sr and depleted in hfse nb ta zr and hf they are characterized by high sr contents 406 2906 ppm sr y ratios 31 3 355 and subchondritic nb ta ratios 18 5 68 9 the ttgs show relatively high ree contents 72 0 266 ppm with strongly enriched lree la yb n 11 5 121 and positive or negligible negative eu anomalies eu eu 0 84 1 89 these geochemical features suggest that the magma source might have been rutile bearing amphibole eclogite their high mg numbers 42 56 and high cr 153 285 ppm and ni contents 22 2 74 5 ppm indicate interaction with the mantle wedge during magma ascent the whole rock end t values 2 6 to 3 8 and most of the magmatic zircon ehf t values 1 3 to 7 6 suggest juvenile to evolved isotopic signatures all these lines of evidence suggest that the ttg rocks in this study formed through partial melting of subducted oceanic slab in a continental arc environment the drill holes in the jiaobei terrane are dominated by 2 5 ga ttg gneisses suggesting that the ttg magma at 2 5 ga is more widely distributed deep underground than that of 2 7 2 9 ga at least within the approachable depth range of our research some zircon grains from jiaobei ttgs give high ehf t values plotting above the curve of 0 75 ehf of dm and their tcdm ages are very close to the time of the zircon crystallization however the majority of the ehf t values fall below the curve of 0 75 ehf of dm and their tcdm ages are concentrated between 2 7 2 9 ga therefore combining with other related studies we propose that the 2 5 2 6 ga magmatism in the ncc probably represents a coherent event of crustal growth and major reworking remelting

doming in compressional orogenic settings new geochronological constraints from the nw himalaya 1 in the central and southeastern parts of the himalayas the high himalayan crystalline hhc high grade rocks are mainly exhumed in the frontal part of the range as a consequence of a tectonic exhumation controlled by combined thrusting along the main central thrust mct and extension along the south tibetan detachment system stds in the nw himalaya however the hanging wall of the mct in the frontal part of the range consists mainly of lowto medium grade metasediments chamba zone whereas most of the amphibolite facies to migmatitic gneisses of the hhc of zanskar are exposed in a more internal part of the orogen as a large scale dome structure referred to as the gianbul dome this gianbul dome is cored by migmatitic paragneisses formed at peak conditions of 800 c and 8 kbar this migmatitic core is symmetrically surrounded by rocks of the sillimanite kyanite staurolite garnet biotite and chlorite mineral zones the structural data from the miyar gianbul valley section reveal that the gianbul dome is bounded by two major converging thrust zones the miyar thrust zone and the zanskar thrust zone which were reactivated as ductile zones of extension referred to as the khanjar shear zone ksz and the zanskar shear zone zsz respectively geochronological dating of monazites from various migmatites and leucogranite in the core of the gianbul dome indicates ages between 26 6 and 19 8 ma these results likely reflect a high temperature stage of the exhumation history of the hhc of zanskar and consequently constrain the onset of extension along both the zsz and the ksz to start shortly before 26 6 ma several recent models interpret that ductile extrusion of the high grade low viscosity migmatites of hhc reflects combined extension along the zsz and thrusting along the mct hence our new data constrain the onset of the thrusting along the mct to start shortly before 26 6 ma citation robyr m b r hacker and j m mattinson 2006 doming in compressional orogenic settings new geochronological constraints from the

u pb geochronological constraints on neoarchean tectonism multiple compressional events in the northwestern hearne domain western churchill province canada in the yathkyed lake area the tyrrell shear zone tsz marks the boundary between the northwestern and central hearne subdomains of the western churchill province the tsz is dominated by proterozoic dextral strain but in areas of low dextral strain older shear fabrics are consistent with an earlier thrusting event four syntectonic granite sheets from within low dextral strain zones in the tsz and contiguous shear zones at the base of the yathkyed belt have ages of 2636 2629 2652 8 2665 2639 and 2644 3 ma the yathkyed belt occurs in the hanging wall of these shear zones and comprises a greenschist to amphibolite grade overturned panel that is overlain by an upper amphibolite grade polydeformed panel the geometry is consistent with a thrust imbricated stack two episodes of deformation in the hanging wall are bracket between ca 2660 and 2616 6 4 ma and between 2616 6 4 ma and ca 2 60 ga the yathkyed belt is interpreted to have initially been emplaced as a thick skinned thrust nappe alon

apatite monazite and xenotime in metamorphic rocks this chapter focuses on phosphates that are significant in metamorphic rocks a quick survey of phosphate mineral descriptions at a commercial mineral web site revealed over 500 phosphate mineral names remarkably only three are common in metamorphic rocks apatite monazite and xenotime and this chapter is restricted to discussion of these minerals apatite monazite and to a lesser extent xenotime have enjoyed intensive study during the previous half century following world war ii considerable study was made of minerals that contain fissionable materials and the sometimes large concentrations of u and th in monazite culminated in a number of seminal papers on the occurrence of that mineral e g overstreet 1967 during the 1970s and early 1980s the study of apatite and monazite turned towards their role as a sink for rees and other trace elements in rocks e g watson 1980 watson and capobianco 1981 throughout this period monazite and to some extent xenotime enjoyed attention as a geochronometer e g parrish 1990 and through today this attention has continued to grow accessory minerals such as monazite xenotime and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons first it has recently been recognized that trace elements in metamorphic rocks and especially trace element zoning in garnet and other major phases contain considerable detailed information about the reaction history a rock has experienced coupled with the fact that diffusion of many trace elements in metamorphic minerals is in general considerably slower than diffusion of major elements in those same minerals the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements attention has naturally turned to their paragenesis it has also recently been discovered that

the relative stability of monazite and huttonite at 300 900 c and 200 1000 mpa metasomatism and the propagation of metastable mineral phases monazite is both partially replaced and overgrown by a thsio4 phase along grain rims in a series of experiments from 300 to 900 c and 200 to 1000 mpa all experiments consisted of 10 mg of 100 500 m size euhedral to subhedral crystals of a natural th free monazite ce 5 mg of th no3 4 5h2o 2 5 mg of sio2 and 5 mg of h2o loaded into 3 mm wide 1 or 1 3 cm long platinum capsules that were arc welded shut experimental conditions were 300 c at 200 and 500 mpa 300 400 500 600 and 700 c at 500 mpa cold seal hydrothermal autoclave and 900 c at 1000 mpa catz assembly piston cylinder press back scattered electron bse imaging electron back scattered diffraction ebsd analysis and transmission electron microscopy tem indicates that in the experiments from 500 to 900 c the thsio4 phase took the form of monoclinic huttonite implying that huttonite associated with monazite could exist metastably over a much greater p t range than previously thought tem analysis of a foil cut perpendicular to the monazite huttonite interface from the 600 c 500 mpa experiment using a focused ion beam fib indicates that the huttonite as well as the interface between the huttonite and monazite is characterized by fluid inclusions high resolution tem analysis indicates that the huttonite monazite interface is coherent in the case of replacement of monazite by huttonite fluid aided dissolution reprecipitation is proposed as the most likely mechanism responsible

exhumation tectonics of the ultrahigh pressure metamorphic rocks in the qinling orogen in east china new petrological structural radiometric insights from the shandong peninsula 1 in eastern china the sulu area is recognized as the eastern extension of the qinling dabie belt which is famous for its ultrahigh pressure uhp metamorphism although numerous petrologic and geochemical works are available structural data are still rare this paper provides the first extensive study of bulk geometry and kinematic analysis of the shandong peninsula the study area is divided into three tectonic areas by cretaceous faults namely a southern uhp belt or sulu area a northern migmatite area and an eastern eclogite and migmatite area or weihai area conversely to the deeply entrenched idea that the later area belongs to the north china belt and the two others to the south china block scb we argue that all three areas are parts of the scb structural petrologic 40ar 39ar and u pb data comply with this new interpretation in the north shandong area mafic granulites enclosed as blocks within gneissic migmatites do not significantly differ from the sulu and weihai eclogites which also experienced a granulite facies overprint before migmatization the circa 210 200 ma age of the main ductile deformation is related to an extensional event during the triassic or indosinian orogeny this date corresponds to the temperature climax but the time of the pressure peak i e the real age of the uhp metamorphism is discussed