petrology and geochronology of paleoproterozoic garnet bearing amphibolites from the dunhuang block eastern tarim craton abstract the dunhuang block located in the eastern margin of the tarim craton possesses a wealth of information about the geological evolution of the tarim craton in this study we conducted detailed petrological geochemical and geochronological investigations on garnet bearing amphibolites as well as granites in the dunhuang block which record important metamorphic and magmatic events that are poorly constrained based on field occurrence geochronological and geochemical characteristics the garnet bearing amphibolites can be divided into two groups group i garnet bearing amphibolite displays typical features of island arc basalts which may formed at 1 83 ga in a collisional setting during the assembly of the columbia supercontinent whereas group ii has geochemical characteristics of ocean island basalt which may formed at 1 61 ga in an ocean island setting the ca 1 83 ga amphibolite has positive hf t values 4 4 to 8 9 with tdm1 model ages of 2 02 1 83 ga indicating an important addition of juvenile mantle derived materials during the formation of its protolith the ca 1 61 ga amphibolite has hf t values ranging from 0 6 to 5 6 and corresponding tdm1 model ages varying from 2 03 ga to 1 84 ga which suggest that juvenile mantle derived materials were dominantly contributed to the formation of its protolith but were later contaminated by crustal materials the u pb ages obtained from the rims of the zircon grains in amphibolites and granites recorded a tectonothermal event at ca 400 ma which is coeval with the widespread magmatism in the beishan orogen we thus propose that these two groups of garnet bearing amphibolites were possibly not related to each other during paleoproterozoic time and a late paleozoic orogenic event brought them together during which both of them underwent metamorphism

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

the 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

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

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

reassessment of continental growth during the accretionary history of the central asian orogenic belt we argue that the production of mantle derived or juvenile continental crust during the accretionary history of the central asian orogenic belt caob has been grossly overestimated this is because previous assessments only considered the palaeozoic evolution of the belt whereas its accretionary history already began in the latest mesoproterozoic furthermore much of the juvenile growth in central asia occurred in late permian and mesozoic times after completion of caob evolution and perhaps related to major plume activity we demonstrate from zircon ages and nd hf isotopic systematics from selected terranes within the caob that many neoproterozoic to palaeozoic granitoids in the accreted terranes of the belt are derived from melting of heterogeneous precambrian crust or through mixing of old continental crust with juvenile or short lived material most likely in continental arc settings at the same time juvenile growth in the caob occurred during the latest neoproterozoic to palaeozoic in oceanic island arc settings and during accretion of oceanic island arc and precambrian terranes however taking together our data do not support unusually high crust production rates during evolution of the caob significant variations in zircon ehf values at a given magmatic age suggest that granitoid magmas were assembled from small batches of melt that seem to mirror the isotopic characteristics of compositionally and chronologically heterogeneous crustal sources we reiterate that the chemical characteristics of crustally derived granitoids are inherited from their source s and cannot be used to reconstruct tectonic settings and thus many tectonic models solely based on chemical data may need re evaluation crustal evolution in the caob involved both juvenile material and abundant reworking of older crust with varying proportions throughout its accretionary history and we see many similarities with the evolution of the sw pacific and the tasmanides of eastern australia

hf isotopes in zircon reveal contrasting sources and crystallization histories for alkaline to peralkaline granites of temora southeastern australia peralkaline granites exhibit the hallmark features of a type igneous rocks but strongly differentiated chemistry and intense hydrothermal alteration camouflage their ultimate origins we present the first in situ hf isotope data from zircons of peralkaline granites aimed at clarifying the protoliths of these plutons and their genetic relationship to associated metaluminous weakly peraluminous granites this study used rocks of the devonian narraburra complex in southeastern australia and found that correlations between hf isotopes and trace element ratios reveal fundamentally different origins for the nonperalkaline and peralkaline granites the latter have a depleted mantle like ancestry whereas a weakly peraluminous rock formed from melts of older arc crust that were modified by interaction with juvenile probably alkaline magmas juxtaposition of crust and mantle derived magmas reflects the high heat flow and lithosphere scale faults associated with continental extension and explains the diversity of a type granites

hafnium isotopic heterogeneity in zircons from granitic rocks geochemical evaluation and modeling of zircon effect in crustal anatexis abstract we carried out a geochemical evaluation and modeled the mechanism responsible for varied hf isotopic ratios in magmatic zircons of a single granitic rock specimen five representative granitic samples were selected from southern china based on preliminary hf isotopic data our new dataset of zircon hf isotopes confirmed significant hf isotope variations 5 9 epsilon units for each sample and these zircons show roughly positive th u versus t crystallizing temperature correlations while the lu hf variation is independent from t in addition some zircons show significantly higher ti concentrations in the rims compared to the interiors implying reverse thermal zonation based on the ti in zircon thermometry these geochemical features in zircons suggest open system processes which may have resulted from frequent replenishment we modeled zircon dissolution during crustal anatexis to reveal the hf isotopic evolution in the extracted melts the model suggests that the extracted melts may have extremely variable hf isotope compositions 20 epsilon units if the bulk zr concentration in the source is initially above 100 ppm and the melting is rapid 10 4 yr 1 the decoupled release of zircon hf and non zircon hf from a single crust derived magma source can lead to significant hf isotope variations in a solidified granitic body this work provides an alternative explanation for the hf isotopic heterogeneity in magmatic zircons of granitic rocks which is in contrast to the common interpretation by mixing with mantle derived magmas

origin of a mesozoic granite with a type characteristics from the north china craton highly fractionated from i type magmas we report geochronological geochemical and isotopic data for the mesozoic shangshuiquan granite from the northern margin of the north china craton the granite is highly fractionated with sio2 74 occurrence of annitic biotite high contents of alkalis k2o na2o rb y nb and heavy rare earth elements high feot mgo low contents of cao al2o3 ba and sr and large negative eu anomalies makes it indistinguishable from typical a type granites a mantle derived origin for the rocks of the granite is not favored because their high initial 87sr 86sr 0 706 and low end t 15 are completely different from either those of the lithospheric or asthenospheric mantle in fact their sr nd isotopes fall within the range of sr nd isotopic compositions of the archean granulite terrains and are comparable to those of mesozoic crustal derived i type granitoids in the region therefore the shangshuiquan granite is considered to be dominantly derived from partial melting of the ancient lower crust its parental magmas prove to be similar to i type magmas and to have undergone extensive fractionation during its ascent this is supported by the fact that some of the nearby hannuoba feldspar rich granulite xenoliths can be indeed regarded as the early cumulates in terms of their mineralogy chemistry sr nd isotopes and zircon u pb ages and hf isotopes it is furthermore argued that some of highly fractionated granites worldwide especially those with a type characteristics and lacking close relationship with unfractionated rocks may in fact be fractionated i type granites this suggestion can explain their close temporal and spatial associations as well as similar sr nd isotopes with i type granites our study also sheds new light on the petrogenesis of deep crustal xenoliths

oxygen and asteroids hundreds of thousands of asteroids have been discovered in the asteroid belt and in near earth space oxygen is an abundant element in meteorites and presumably in most asteroids spectral reflectance measurements of asteroids in the visible and near infrared can identify oxygen bearing minerals such as those found in the olivine pyroxene and serpentine groups due to their distinctive absorption features interpretation of the mineralogy of asteroids is complicated by the effects of space weathering which tends to redden and darken the surfaces of asteroids asteroids are primarily classified into a number of taxonomic classes and subclasses according to their spectral properties in the visible wavelength region however asteroids with similar spectral properties in the visible may have different spectral properties in the near infrared and therefore different interpreted mineralogies definite trends in the abundances of different taxonomic classes versus heliocentric distance are apparent these trends appear to be a function of both composition differences and degree of heating however dynamical processes have significantly affected these trends close observation of asteroids by spacecraft such as near shoemaker to 433 eros and hayabusa to 25143 itokawa are currently the best way to discern the mineralogies of individual asteroids

differentiation of tholeiitic basalt to a type granite in the sept iles layered intrusion canada the undeformed 564 ma sept iles layered intrusion quebec canada is a large igneous body of c 20 000km3 from the base to the top it consists of a layered series dominated by troctolite and gabbro an anorthositic upper border series and a dominantly granitic upper series the parent magma of the layered series is inferred to be an iron rich tholeiitic basalt 48 wt sio2 15wt feot whole rock compositions from the chilled margin dykes cross cutting the layered series and silicic rocks from the upper series display continuous major and trace element geochemical trends ranging from basalts to ferroan metaluminous a type granites 77 wt sio2 initial 143nd 144nd 0 51201 0 51207 and 87sr 86sr 0 70353 0 70548 indicate a juvenile mantle source and minimal contamination by old crust 1 2 during crystallization geochemical modeling using the melts thermodynamic calculator combined with equations predicting mineral melt equilibria from experiments on tholeiitic basalts indicate that basaltic to monzonitic melt compositions are in equilibrium with the troctolites and gabbros of the layered series fe ti oxides saturate early in the layered series after 14 fractionation of plagioclase olivine cumulates further fractionation of fe ti oxide bearing gabbros drives the residual liquids toward silica enrichment and iron depletion major and trace element modeling indicates that the a type granites from the upper series were produced by protracted fractional crystallization of an iron rich basaltic parent magma at a fraction of residual liquid of only 8 the observed relative volumes of mafic cumulates and silicic rocks in the intrusion are in agreement with the calculations most of the intermediate compositions correspond to magmatic mafic enclave bearing granitoids and display geochemical evidence of hybridization intermediate compositions produced by fractional crystallization are scarce and a daly gap occurs from 57 to 67 wt sio2 this gap could result either from the fractional crystallization process or from silicate liquid immiscibility during that compositional interval

stratigraphic evidence for late quaternary dune activity near hudson on the piedmont of northern colorado stabilized dune fields are common features near hudson on the piedmont of northern colorado exposures in dune and interdune sites expose a sequence of eolian sediments and paleosols that record a complex history of eolian activity during the late quaternary radiocarbon and thermoluminescence age estimates on a horizons buried by eolian sand indicate that dunes were reactivated sometime between 7 and 9 ka on the basis of morphology of surface soils the dunes were most recently stabilized 3 ka at present there are no data to indicate if there were multiple periods of dune movement and stabilization during the holocene the penultimate pre holocene dune forming episode probably terminated ca 13 ka and may be coincident with the pinedale glaciation the stratigraphy at interdune sites shows at least two eolian depositional events prior to the penultimate event they were separated by periods of pedogenesis one of which may have exceeded 40 ka this analysis indicates that dunes in northern colorado were active during both cold arid and warm arid periods in the late quaternary

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

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

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

the latest neoarchean paleoproterozoic evolution of the dunhuang block eastern tarim craton northwestern china evidence from zircon u pb dating and hf isotopic analyses abstract the dunhuang block in the easternmost segment of the tarim craton exposes precambrian mafic granulite and felsic gneiss to better understand the poorly known formation and evolution of the tarim craton we carried out detailed field based petrological and geochronological investigations on the major lithologies of the dunhuang block u pb dating and hf isotopic analyses on the mafic granulite and felsic gneiss ttg gneiss of the dunhuang block reveal two distinct age populations the latest neoarchaean 2 5 ga and the late palaeoproterozoic 1 82 1 85 ga the zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of 2 5 ga supporting the existence of archean rocks in the dunhuang block the short time interval between latest neoarchean magmatism and high grade metamorphism suggests that they were related to the same latest neoarchean tectonothermal event the 2 5 ga zircons show hf depleted mantle model ages between 2 55 ga and 2 8 ga with a peak at 2 7 ga suggesting that 2 7 ga juvenile crust of the dunhuang block was reworked during a 200 300 m y period at the end of the neoarchean the age of 1 85 ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high pressure granulite facies metamorphism combined with the hf isotopic data these data suggest that the dunhuang block experienced 2 7 ga crustal growth a 2 5 ga magmatic metamorphic event and a 1 82 1 85 ga hp granulite facies metamorphic event this sequence of events is very similar to that of the north china craton implying that the evolutionary history of the dunhuang block was consistent with that of the north china craton from the neoarchean to paleoproterozoic

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

formation and stabilization of the dunhuang block nw china constraints from the late paleoproterozoic a type granites of the dunhuang complex abstract the geochemical composition and temporal evolution of a type granites may hold key information regarding critical geodynamic processes that occurred during the evolution of the continental crust moreover these aspects may provide an indication of when orogenic processes ended and craton formation stabilization began in early precambrian blocks in this study we report the results of zircon u pb ages hf isotope data and whole rock geochemistry for the proterozoic huoyanshan potassic granites in the dunhuang block northwest china zircon u pb dating reveals that the huoyanshan granites were emplaced at ca 1791 1786 ma representing a late paleoproterozoic magmatism event the granites have high sio2 70 71 72 20 wt total alkali contents k2o na2o 8 81 8 96 wt k2o na2o 1 57 1 66 tio2 mgo ratios 2 19 2 57 low cao 0 77 1 27 wt mgo contents 0 12 0 16 wt and mg values 7 72 8 67 with a cnk ratios of 0 96 1 04 pointing to alkaline and metaluminous to weakly peraluminous affinities these granites are characterized by high total rare earth element concentrations ree 419 755 ppm and variable la yb n values 8 24 with significantly negative eu anomalies eu eu 0 31 0 42 the primitive mantle normalized trace element patterns suggest that the huoyanshan plutons are depleted in ba nb ta sr p and ti but enriched in rb th and pb additionally the granites have a high zirconium saturation temperature of 855 897 c with an average of 878 c these geochemical characteristics indicate that huoyanshan potassic granites are an aluminous a type granite and further classified as a2 type together with the zircon hf isotope data ehf t 11 21 to 2 76 indicates that these granites were generated from a crustal source the heterogeneous hf isotope compositions in zircons are principally caused by the disequilibrium melting of zr bearing sources in addition two stage hf model ages of zircons from the granites are ca 2 6 3 1 ga and plot on the average crustal evolution trend of archean tonalite trondhjemite granodiorite ttg gneiss 2 5 to 3 0 ga in the dunhuang block thus we propose that a type granites in the study area were generated from the disequilibrium partial melting of archean tonalitic to granodioritic rocks under high temperature and low pressure conditions according to their low al2o3 cao na2o k2o molar values and high 10000 ga al values 2 9 subsequently the magma experienced prominent fractional crystallization of k feldspar plagioclase orthopyroxene and or magnetite our results with available data suggest that the huoyanshan granites formed in a tectonic transform regime within a collisional to extensional setting and serve as a precursor for the formation and stabilization of the dunhuang block during the late paleoproterozoic furthermore it is probable that the post collisional magmatism event at ca 1 79 ga in the dunhuang block was closely related to the assembly of the columbia supercontinent

formation of high δ18o fayalite bearing a type granite by high temperature melting of granulitic metasedimentary rocks southern china the genesis of a type granites has been controversial fayalite granite is a member of the most reduced a type granites commonly thought to have been primarily sourced from tholeiitic rocks in this paper we report petrography whole rock geochemistry sr nd isotope and in situ zircon hf o isotope results for a fayalite bearing a type granite suite at jiuyishan in southern china high zircon 18 o 8 0 9 8 negative zircon ehf t 6 2 to 2 3 and evolved whole rock sr nd isotopes i sr 0 7151 0 7181 end t 7 4 to 6 6 indicate the reworking of old supracrustal rocks isotopic and geochemical results particularly downward inflections of zr and ba at 70 wt sio 2 point to fractional crystallization rather than magma mixing as the controlling process for the evolution of the igneous suite integrated petrological geochemical and isotopic studies present the first convincing case that reduced a type granites can also be generated by high temperature 960 c melting of granulitic metasedimentary rocks likely related to the upwelling of the asthenosphere and or underplating and intrusion of basaltic melts we emphasize that key factors for the genesis of this unique rock type are low oxygen fugacity f o 2 low f h 2 o and high temperature

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

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

magma mixing controlling the origin of the early cretaceous fangshan granitic pluton north china craton in situ u pb age and sr nd hf and o isotope evidence abstract in situ u pb age and sr nd hf and o isotope analysis of accessory minerals zircon apatite and titanite have been obtained for mafic microgranular enclaves and their host granites from the early cretaceous fangshan pluton in the north china craton in order to constrain their sources and petrogenesis the fangshan pluton is composed of quartz monzodiorite at the margins and granodiorite with abundant mafic microgranular enclaves in the core zircons from the marginal quartz monzodiorite record u pb ages of 130 to 133 ma identical to those of granodiorite in the core the enclaves have zircon u pb ages of 133 to 134 ma identical within error to those of the host granodiorite establishing that the mafic and felsic magmas were coeval furthermore apatite and titanite from the enclaves and host granodiorite have the same u pb ages as the zircons indicating rapid cooling of the fangshan pluton the granodiorites have relatively high sio 2 and low mgo concentrations they have variable negative e hf t values of 25 1 to 18 2 and 18 o values for zircon of 6 2 to 8 3 the negative e nd t values of 19 7 to 14 0 for titanite and apatite with initial 87 sr 86 sr ratios of 0 7052 0 7058 for apatite indicate that they were derived from an ancient lower crustal source with addition of materials with higher e hf t and e nd t values and low 18 o values the enclaves have relatively high mgo contents up to 15 5 wt at intermediate silica concentrations indicating that their parental magmas were derived from a mantle source their zircons have strongly negative hf isotopic compositions e hf t 22 9 to 14 9 the nd isotopic composition of titanite and apatite is also strongly negative e nd t 17 6 to 12 1 all these features combined with the 18 o zircon values of 4 8 8 2 for zircons and relatively low initial sr isotopic compositions of apatite initial 87 sr 86 sr 0 7051 0 7058 indicate that they were derived from an enriched and or depleted mantle source but with involvement of crustal materials the marginal quartz monzodiorites have transitional geochemical features between those of the enclaves and host granodiorites in situ sr nd hf and o isotopic data of accessory minerals coupled with field observations and whole rock geochemical data rule out simple crystal liquid fractionation or wall rock contamination as the major genetic link between the enclaves and host rocks instead mixing of mantle derived mafic magmas with crustal derived felsic magmas coupled with assimilation of wall rocks is compatible with the data our study shows that in situ hf o nd and sr isotopic analyses of accessory minerals are powerful geochemical tracers that provide unique petrogenetic information regarding the origin of granitoids

origin of 2 5 ga potassic granite from the nellore schist belt se india textural cathodoluminescence and shrimp u pb data in a geochemical and geochronological investigation of archean and proterozoic magmatism in the nellore schist belt we conducted shrimp u pb analyses of zircons from two cospatial granitic bodies at guramkonda and vendodu the former is a ba and sr rich hornblende bearing tonalite whereas the latter is a rb zr pb th u and ree rich biotite bearing leucogranite the guramkonda tonalite displays a restitic texture with remnants of trapped granitic melt whereas the vendodu leucogranite contains residual partially melted plagioclase grains both rock types contain two generations of zircon tonalite contains a group of euhedral zoned zircons enclosed within plagioclase and a group of subhedral patchy zircons associated with trapped melt quartz feldspar matrix and leucogranite also contains a group of doubly terminated euhedral zircons included within orthoclase as well as a group of zircons with visible cores mantled by later rim growth cathodoluminescence images also clearly document two distinctly textured varieties of zircon the tonalite contains a population characterized by narrowly spaced uninterrupted oscillatory zoning and a second population lacking zoning but exhibiting a random distribution of dark u rich and light u poor regions the leucogranite contains u rich zoned zircons and u poor zircon cores mantled by u rich rims the ree chemistry of zircon cores from the vendodu leucogranite is very similar to the ree of zoned zircons from the guramkonda tonalite zircon ages from both plutons exhibit bimodal distributions in u pb concordia diagrams the tonalite defines an age of 2 521 ma 5 ma for zoned magmatic zircons and 2 485 ma 5 ma for unzoned newlyprecipitated zircons whereas the leucogranite has an age of 2 518 ma 5 ma for u poor zircon cores relics of the tonalite pluton and 2 483 ma 3 ma for u rich zoned magmatic zircons the trace element geochemistry of the 2 520 ma zircons is distinctly different from the 2 485 ma zircons irrespective of the host rock our textural cl image and shrimp u pb analyses document the origin of the leucogranite by partial melting of the tonalite high alkalis na2o k2o rb nb hree feot mgo and low ca al ba sr and large negative eu anomalies characterize the leucogranite as a thermal minimum melt whereas the very low k and rb of the tonalite attests to its residual nature we suggest that the leucogranite formed by high t 900 950 c moderate pressure 10 kbar dehydration partial melting of the tonalite under reducing conditions the calculated source compositions of the leucogranite melt and the tonalite residue show strong similarities to melts that are considered to have been produced in a subduction zone environment the leucogranite probably formed in a post collisional realm immediately after accretion of the tonalitic crust

geochemical and isotopic nd o evidence bearing on the origin of late to post orogenic high k granitoid rocks in the western superior province implications for late archean tectonomagmatic processes abstract the granitoid rock dominated central wabigoon subprovince of the superior province records low k trondhjemite tonalite granodiorite ttg type magmatic episodes at

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

the source of granitic gneisses and migmatites in the antarctic peninsula a combined u pb shrimp and laser ablation hf isotope study of complex zircons zircons gneisses and migmatites collected from the antarctic peninsula have different core rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent two samples contain inherited zircon that is partially dissolved in these samples the 176hf 177hf rations of the inherited zircon and new magmatic zircon rims are on average indistinguishable and consistent with in situ melting in such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source variation in 176hf 177hf within the magmatic zircon rims from grain to grain suggests that hf isotopes were only partially homogenized during melt migration alternatively zircon growth may have taken place within small volumes of partial melt other samples do not preserve textural evidence for zircon dissolution during melt generation in these samples the 176hf 177hf values of the inherited zircon and new magmatic zircon rims are different the zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon whether this relates to a separate juvenile source or alternatively is derived from minerals other than zircon at the source cannot be resolved inherited zircon irrespective of age has been strongly influenced by the reworking of a juvenile late mesoproterozoic source suggesting that such crust underlies the antarctic peninsula our results therefore suggest that hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts

a petrogenetic study of anorogenic felsic magmatism in the cretaceous paresis ring complex namibia evidence for mixing of crust and mantle derived components abstract paresis is one of a group of cretaceous ring complexes extending from the coast some 350 km ne across the damara belt it consists of over 90 rhyolites and comendites with subordinate intrusions of quartz syenite and alkali feldspar syenite these felsic units are accompanied by very minor amounts of silica undersaturated basalt phonolite and lamprophyre located near the edge of the congo craton paresis is the farthest inland of any felsic complex in the cretaceous ring complex group the other ring complexes in the vicinity consist of carbonatites and undersaturated alkaline rocks e g okorusu kalkfeld etaneno ondurakorume geochemical and isotopic data provide evidence for both mantle and crustal components in the sources of the paresis magmas the alkaline basalts phonolite and lamprophyre have overlapping sr and nd isotopic initial ratios which plot within the mantle array close to bulk earth values e nd 0 9 to 2 8 and 87 sr 86 sr i 0 7042 0 7054 and may indicate a mantle plume component the rhyolite units comprise metaluminous feldspar rhyolites and peraluminous more differentiated quartz feldspar rhyolites both units show prominent negative nb and ta anomalies on mantle normalized multielement diagrams and have extremely nonradiogenic nd ratios e nd 21 and sr initial ratios of 0 7117 0 7138 these isotopic values suggest a crustal origin from pre damara early precambrian gneisses and granitoids which are exposed in the congo craton and related inliers in northern namibia the comendites are peralkaline highly differentiated rocks in contrast to the rhyolites comendites lack mantle normalized nb and ta anomalies have higher hfse and lree contents extreme negative eu anomalies and e nd values of 11 like the comendites alkali feldspar syenites are peralkaline lack nb and ta anomalies have negative eu anomalies and high hfse concentrations their e nd values are 6 5 to 8 the quartz syenites overlap with the peralkaline units in isotopic composition but they are chemically very different they are metaluminous have a low degree of differentiation and have prominent negative mantle normalized nb and ta anomalies like the rhyolites the sr nd isotopic composition and incompatible element ratios of the comendites quartz syenites and alkali feldspar syenites are intermediate between those of the rhyolites and basalts precluding a simple origin by basalt fractionation or crustal melting instead the data indicate that the comendite and syenites formed from hybrid magmas with both crustal and mantle derived components the metaluminous quartz syenites can be modelled by assimilation of pre damara crust by basaltic magma the peralkaline nature and trace element characteristics of comendite and alkali feldspar syenites on the other hand require an alkaline and hfse enriched basic endmember whose fractionated equivalent may be the paresis phonolite

paleoproterozoic crustal evolution of the tarim craton constrained by zircon u pb and hf isotopes of meta igneous rocks from korla and dunhuang abstract the tarim craton is one of three large cratons in china presently there is only scant information concerning its crustal evolutionary history because most of the existing geochronological studies have lacked a combined isotopic analysis especially an in situ lu hf isotope analysis of zircon in this study precambrian basement rocks from the kuluketage and dunhuang blocks in the northeastern portion of the tarim craton have been analyzed for combined in situ laser ablation icp mc ms zircon u pb and lu hf isotopic analyses as well as whole rock elements to constrain their protoliths forming ages and magma sources two magmatic events from the kuluketage block at 2 4 ga and 1 85 ga are revealed and three stages of magmatic events are detected in the dunhuang block i e 2 0 ga 1 85 ga and 1 75 ga the 1 85 ga magmatic rocks from both areas were derived from an isotopically similar crustal source under the same tectonic settings suggesting that the kuluketage and dunhuang blocks are part of the uniform precambrian basement of the tarim craton zircon hf model ages of the 2 4 ga magmatism indicate that the crust of the tarim craton may have been formed as early as the paleoarchean period the 2 0 ga mafic rock from the dunhuang block was formed in an active continental margin setting representing an important crustal growth event of the tarim craton in the mid paleoproterozoic that coincides with the global episode of crust formation during the assembly of the columbia supercontinent the 1 85 ga event in the kuluketage and dunhuang blocks primarily involved the reworking of the old crust and most likely related to the collisional event associated with the assembly of the columbia supercontinent while the 1 75 ga magmatism in the dunhuang block resulted from a mixture of the reworked archean crust with juvenile magmas and was most likely related to a post collisional episode

reduced rapakivi type granites the tholeiite connection reduced rapakivi type granites are the most iron enriched and reduced i e least oxidized of the anorogenic granite association the low oxygen fugacity and chemical composition of these granites severely limit their sources in this paper we argue that reduced rapakivi type granites and their eruptive equivalents high potassium fayalite rhyolites are derived from mafic sources because tholeiitic magmas and their derivatives have the required low oxygen fugacity reduced rapakivi type granites are produced either by extreme differentiation of basaltic melts or by partial melting of underplated basalts and their differentiated equivalents they form in extensional environments where the asthenosphere is present at shallow depths we envision three stages in the origin of these rocks 1 tholeiitic melts are emplaced at the base of the crust 2 continued introduction of heat partially remelts these tholeiitic rocks and 3 the hot dry melts so produced migrate into the middle crust to produce rapakivi batholiths or erupt as rhyolites partial melting of felsic continental crust may accompany the intrusion of rapakivi type magmas thereby producing the other metaluminous and peraluminous granite compositions of the anorogenic suite

alkali calcic and alkaline post orogenic po granite magmatism petrologic constraints and geodynamic settings abstract the end of an orogenic wilson cycle corresponds to amalgamation of terranes into a pangaea and is marked by widespread magmatism dominated by granitoids the post collision event starts with magmatic processes still influenced by subducted crustal materials the dominantly calc alkaline suites show a shift from normal to high k to very high k associations source regions are composed of depleted and later enriched orogenic subcontinental lithospheric mantle affected by dehydration melting and generating more and more k and lile rich magmas in the vicinity of intra crustal magma chambers anatexis by incongruent melting of hydrous minerals may generate peraluminous granitoids bearing mafic enclaves the post collision event ends with emplacement of bimodal post orogenic po suites along transcurrent fault zones two suites are defined i the alkali calcic monzonite monzogranite syenogranite alkali feldspar granite association characterised by biotite plagioclase fractionation and moderate lile hfse enrichments and ii the alkaline monzonite syenite alkali feldspar granite association characterised by amphibole alkali feldspar fractionation and displaying two evolutionary trends one peralkaline with sodic mafic mineralogy and higher enrichments in hfse than in lile and the other aluminous biotite bearing marked by hfse depletion relative to lile due to accessory mineral precipitation alkali calcic and alkaline suites differ essentially in the amounts of water present within intra crustal magma chambers promoting crystallisation of various mineral assemblages the ultimate enriched and not depleted mantle source is identical for the two po suites the more primitive lile and hfse rich source rapidly replaces the older orogenic mantle source during lithosphere delamination and becomes progressively the thermal boundary layer of the new lithosphere present rock compositions are a mixture of major mantle contribution and various crustal components carried by f rich aqueous fluids circulating within convective cells created around magma chambers in favourable areas po suites pre date a new orogenic wilson cycle

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

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