structural and geochronological constraints on the early mesozoic north longmen shan thrust belt foreland fold thrust propagation of the sw qinling orogenic belt northeastern tibetan plateau

zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

seismic structure and rheology of the crust under mainland china abstract the crust and upper mantle in mainland china were relatively densely probed with wide angle seismic profiling since 1958 and the data have provided constraints on the amalgamation and lithosphere deformation of the continent based on the collection and digitization of crustal p wave velocity models along related wide angle seismic profiles we construct several crustal transects across major tectonic units in mainland china in our study we analyzed the seismic activity and seismic energy releases during 1970 and 2010 along them we present seismogenic layer distribution and calculate the yield stress envelopes of the lithosphere along the transects yielding a better understanding of the lithosphere rheology strength beneath mainland china our results demonstrate that the crustal thicknesses of different tectonic provinces are distinctively different in mainland china the average crustal thickness is greater than 65 km beneath the tibetan plateau about 35 km beneath south china and about 36 38 km beneath north china and northeastern china for the basins the thickness is 55 km beneath qaidam 50 km beneath tarim 40 km beneath sichuan and 35 km beneath songliao our study also shows that the average seismic p wave velocity is usually slower than the global average equivalent with a more felsic composition of crust beneath the four tectonic blocks of mainland china resulting from the complex process of lithospheric evolution during triassic and cenozoic continent continent and mesozoic ocean continent collisions we identify characteristically different patterns of seismic activity distribution in different tectonic blocks with bi or even tri peak distribution of seismic concentration in south tibet which may suggest that crustal architecture and composition exert important control role in lithosphere deformation the calculated yield stress envelopes of lithosphere in mainland china can be divided into three groups the results indicate that the lithosphere rheology structure can be described by jelly sandwich model in eastern china and creme brulee models with weak and strong lower crust corresponding to lithosphere beneath the western china and kunlun orogenic belts respectively the spatial distribution of lithospheric rheology structure may provide important constraints on understanding of intra or inter plate deformation mechanism and more studies are needed to further understand the tectonic process es accompanying different lithosphere rheology structures

the cenomanian hiatus in the southern benue trough nigeria basal turonian ezeaku sandstone ridges along the cenomanian hiatus in afikpo area contain sandstone pebbles cobbles and boulders randomly distributed in nearly all the coarsening upward subfacies of the host units contacts with the host sandbody are sharp and in places lined with solution grooves generally the angular to subrounded clasts have their 2 mm rims ferruginized are fine to coarse grained and very strongly bioturbated they are compositionally and texturally more similar to the adjacent albian asu river group sandunits these characteristics suggest an extrabasinal origin for these marine sandstone clasts albian asu river group sediments uplifted by the cenomanian deformation are the most likely source the general absence of similar clasts towards the eastern palaeostrandline where the primary cretaceous source rocks lay is consistent with the interpretation the remarkable parallelism between the trend of the clast outcrops and the axis of the santonian anticlinorium ne sw suggests the same axial direction for both tectonic phases

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

provenance of the beihuaiyang lower grade metamorphic zone of the dabie ultrahigh pressure collisional orogen china evidence from zircon ages the beihuaiyang zone of the dabie ultrahigh pressure collisional belt which was formed by the early mesozoic collision of the north and south china yangtze blocks is diversely interpreted as an active or a passive sedimentary sequence it comprises the luzhenguan and foziling complexes six granitoid rocks of the luzhenguan complex are dated at 770 ma to 720 ma and one schist sample also contains detrital zircons of 760 ma to 720 ma these data indicate that this complex was of the south china affinity and probably originated from the northern yangtze block two quartzite samples of the foziling complex contain abundant detrital zircons of archean to paleozoic ages clustering around 2 5 ga 1 9 1 8 ga 1 0 0 7 ga and 0 5 0 4 ga this age spectrum demonstrates a mixed source of these two blocks the youngest zircons suggest a paleozoic deposition along an active continental margin south of the north china block while these late proterozoic zircons are characterized for the yangtze block subsequently it is suggested that a terrain drifted from the northern yangtze block in early paleozoic must have been situated to the southern margin of the north china block to provide material source prior to the final collision in early mesozoic q 2003 elsevier ltd all rights reserved

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

metasomatic mantle source and crustal contamination for the formation of the neoproterozoic mafic dike swarm in the northern yangtze block south china abstract in the huangling region of the northern yangtze block south china the 820 ma huangling felsic pluton was intruded by 800 ma mafic dikes these dikes are composed of fine to medium grained diabase porphyritic diabase and olivine gabbro quartz xenocrysts occur locally in the olivine gabbro the rocks are alkaline to sub alkaline in composition and have sio2 ranging from 43 3 to 57 2 wt mgo from 3 7 to 9 2 wt and total alkali from 3 0 to 7 3 wt they exhibit lree enrichment la yb n 3 3 28 5 with slightly negative eu anomalies eu eu 0 6 1 0 their primitive mantle normalized trace element patterns are characterized by enrichment of large ion lithophile elements rb ba th and u depletion of high field strength elements nb ta zr and hf and positive pb and negative ti anomalies they have relatively high initial 87sr 86sr ratios ranging from 0 7052 to 0 7080 and variably negative end t values from 4 2 to 10 9 the rocks have a large range of whole rock ehf values 0 35 to 11 34 and a relatively narrow range of pb isotope ratios 206pb 204pb 16 96 to 17 97 207pb 204pb 15 41 to 15 58 and 208pb 204pb 38 08 to 39 66 initial 87sr 86sr ratios correlate negatively with 206pb 204pb ratios whereas 143nd 144nd and 208pb 204pb ratios and ehf values correlate positively with 206pb 204pb ratios these features are indicative of an em2 source region suggesting a lithospheric mantle source modified by subducted components during the neoproterozoic the quartz xenocrysts and linear isotopic variations indicate strong crustal contamination during magma emplacement the energy constrained assimilation and fractional crystallization ec afc modeling calculations reveal that the parental magmas were contaminated by continental crust including archaean amphibolites granitic gneisses metasedimentary rocks and granitic rocks the mafic dikes are 20 myrs younger than the huangling felsic pluton which was produced by partial melting of a thickened continental crust suggesting that the mafic dikes were a consequence of post orogenic extension the huangling region may have originally been located at the centre of the yangtze block the possible heat source for the generation of the huangling felsic pluton and mafic dikes was asthenospheric wedging resulting from oceanic slab subduction beneath the northern margin of the yangtze block this conclusion rules out the possibility that the yangtze block was located in the central part of the rodinia supercontinent the block was more likely situated along the margin of rodinia

delamination and detachment of a lithospheric root abstract orogens can be formed by shortening of the lithosphere on time scales much shorter than that of thermal diffusion for time scales of a few ma the major part of the uplift of an orogen which may occur during the transition from crustal shortening to extension may be due to delamination of the mantle lithosphere rather than to convective thinning the negative buoyancy forces of a lithospheric root may cause lateral shortening while the lithospheric root delaminates peels off from the crust detaches and is replaced by hot upwelling asthenosphere this provides a large input of heat into the crust geological observations show that the late orogenic phase may be characterized by volcanic activity high temperature low pressure metamorphism extension high heat flow and a rapid within a few ma increase of the topography we present dynamically and thermally consistent numerical models of the evolution of an orogenic root mantle rheology is based on laboratory data of olivine while models with the rheology of olivine do not allow for delamination due to its very high viscosity the incorporation of a quasi brittle rheology is necessary for modelling delamination of the mantle lithosphere successful delamination models are always associated with a rapid change in topography we examined the parameter range of our models and found different regimes of behaviour of the orogenic root of the mantle lithosphere no delamination delamination delamination and full detachment for a fixed viscosity of the upper crust of 1023 pa s delamination occurs for a range of viscosities of 1023 pa s to 1020 pa s for the lower crust depending on the thickness of the lithospheric root but full detachment of the delaminated lithospheric slab only occurs if the viscosity of the lower crust is greater than approximately 1021 pa s hot asthenosphere is welling up into the gap opening during delamination further implications of our models are lithospheric roots or unsupported slabs of at least 100 170 km depth extent are needed to provide sufficient negative buoyancy to allow delamination and detachment in all delamination models significant amounts of lower crustal material are subducted into the mantle in applying our models to selected orogens we argue that mantle delamination may have played an important role in the variscan and the himalayan orogenies

lithospheric thinning during the evolution of continental large igneous provinces a case study from the north atlantic tertiary province lithospheric thickness can act as an important physical control on the composition of asthenosphere derived melts by restricting the minimum depth at which decompression melting can occur a case study of plume related tertiary lavas from the isle of mull scotland has revealed the presence of three successive magma types which become progressively more depleted in the incompatible trace elements geochemical modeling suggests that melting initially took place within the garnet iherzolite stability field but subsequently involved more extensive melting 10 of spinel iherzolite at shallower mantle depths these results can be explained in terms of a progressively thinning lithosphere beneath the region during the evolution of the volcanic center the observed degree of lithospheric thinning cannot be explained by extension alone and it appears that some relatively rapid lower lithospheric erosion by the plume head is also required theoretical modeling suggests that lithospheric erosion can be a comparatively rapid process and this has important implications for the various starting plume models

early cretaceous adakitic granites in the northern dabie complex central china implications for partial melting and delamination of thickened lower crust abstract to date few adakitic rocks have been reported in direct association with contemporary intra continental extensional structures which has cast doubt on genetic models involving partial melting of the lower crust this study presents early cretaceous 143 129 ma new sensitive high resolution ion microprobe shrimp zircon u pb ages adakitic granites which are directly associated with a contemporary metamorphic core complex i e the northern dabie complex in the dabie area these granites exhibit relatively high sr contents negligible to positive eu and sr anomalies high la yb and sr y ratios but very low yb and y contents similar to subducted oceanic crust derived adakites they are also characterized however by very low mgo or mg and ni values and nd sr isotope compositions end t 14 6 to 19 4 and 87sr 86sr i 0 7067 0 7087 similar to triassic continent derived eclogites subducted in the dabie sulu orogen additionally late granitic dikes in the adakitic intrusions exhibit low sr contents clearly negative eu and sr anomalies low la yb and sr y ratios but relatively high yb and y contents similar to 118 105 ma granites in the northern dabie complex based on composition and geochronology data of neoproterozoic amphibolites and orthogneisses triassic high to ultra high pressure metamorphic rocks and early cretaceous mafic ultramafic intrusive rocks and the constraints provided by experimental melt data for tonalites metabasaltic rocks and eclogites we suggest that the adakitic granites were most probably generated by partial melting of thickened amphibole or rutile bearing eclogitic lower crust as a consequence of triassic middle jurassic subduction and thrusting the late dikes probably originated from plagioclase bearing intermediate granulites moreover we suggest that late mesozoic delamination or foundering of thickened eclogitic lower crust is also a more plausible mechanism for the petrogenesis of early cretaceous mafic ultramafic intrusive rocks in the dabie area and probably involved partial melting of a mixed source comprising eclogitic lower crust that had delaminated or foundered into upper lithospheric or asthenospheric mantle peridotite asthenospheric upwelling in response to post collisional delamination of lithospheric mantle was likely to have provided the heat source for the cretaceous magmatism

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

composition provenance and tectonic setting of eze aku sandstone facies in the afikpo synclinorium southern benue trough nigeria field study petrographic and major element geochemical composition of the eze aku sandstone facies outcropping in the afikpo synclinorium southeastern nigeria were utilized in this study the aim is to interpret the provenance and tectonic setting for sandstones in the sedimentary basin field observation shows alternating sequence of parallel sandstone ridges trending ne sw and shales which represent the low lying plains in the area four major sandstone facies were identified massive conglomeritic sandstone facies laminated sandstone facies bioturbated sandstone facies and cross bedded sandstone facies ternary plots for framework elements quartz feldspar and rock fragments from petrography of the sandstones indicate the sandstones as arkosic and subarkosic arenites tectonic setting discrimination diagrams plotted based on major element oxides revealed continental block provenance dominantly in a passive continental margin petrographic and geochemical results indicate large amounts of detritus were derived principally from plutonic igneous rocks with minor contribution from metamorphic source region probably from obudu plateau oban and cameroon massifs

how sediment promotes narrow rifting application to the gulf of california 1 if the gravitational stress changes related to crustal thinning are large enough to affect the mode of extension then sedimentation should also affect that mode we propose that the weight of sediments reduces the difference in crustal buoyancy forces caused by local crustal thinning allowing a rift to extend more easily in a narrow rift mode we examine the effect of deposition of large amounts of nonlocally derived sediment on extensional style using two dimensional regional scale numerical experiments of extending thick continental crust with varied initial thermal conditions geometries rheological parameters and sedimentation properties depending on initial model conditions the onset of rifting in thick continental crust occurs in the narrow rifting wide rifting or core complex mode with continued extension all cases eventually transition to a narrow rifting mode for a system in wide rift mode moderate to fast sedimentation shortens the time needed to transition to a narrow rift in the gulf of california changes in extensional style correlate with sediment thickness with an earlier transition to narrow rifting in the north versus the south we compare our model results to the extensional history of the gulf of california where sediments may have caused the northern and north central domain of the gulf delfin tiburon and guaymas segments to transition to narrow rifting before the south central domains alarcon segment

petrogenesis and tectonic implications of a type granites in the dabie orogenic belt china geochronological and geochemical constraints the dabie orogenic belt is characterized by the presence of large volumes of intrusive and volcanic rocks that formed in late mesozoic times most of the intrusive bodies are i type granites but it is still unclear whether there are contemporary a type granites here we report the first unambiguous discovery of a type granite from baiyashan in the north dabie tectonic belt the crystallization age of the body has been fixed as 120 4 1 2 ma using u pb analysis of zircons by la icpms the baiyashan granite is enriched in si k na rb and ree has elevated feo tot feo tot mgo and ga al ratios and is depleted in mg ca mn ba sr p and ti the ree composition shows highly fractionated patterns with la yb n 6 95 16 68 and eu eu 0 33 0 59 its crystallization age field relationships petrographic and geochemical data show beyond doubt that the baiyashan granite is an early cretaceous a type granite sr nd isotope systematics are characterized by a high i sr of 0 708 0 714 and a low nd of 7 5 to 19 4 with t dm2 1 5 2 5 ga and these data indicate that the magmas were dominantly sourced from partial melting of middle to lower crustal intermediate felsic igneous rocks and mingling with mafic to intermediate magmas during rift related magmatism associated with subduction of the palaeo pacific plate beneath eastern china in early cretaceous times

formation age and tectonic setting of the shirengou neoarchean banded iron deposit in eastern hebei province constraints from geochemistry and sims zircon u pb dating abstract the shirengou iron deposit a typical bif deposit located in eastern hebei province in the north china craton is hosted in neoarchean metamorphic rocks the metamorphic rocks include hornblende plagiogneiss plagioclase amphibolite and magnetite quartzite we use contents and ratios of major elements for protolith reconstruction showing that protoliths of the hornblende plagiogneiss and plagioclase amphibolite should be dacite and basaltic andesite respectively based on geology and geochemistry of the hosting rocks we infer that the shirengou bif is an algoma type deposit formed in an arc related basin in the neoarchean constrains from sims u pb dating and oxygen isotopic analysis of zircon from wall rocks and interlayer of the shirengou iron bodies can be summarized as follow 1 igneous zircons from two samples of amphibolite and gneiss yield u pb ages of 2541 21 ma and 2553 31 ma respectively providing the age of the neoarchean protolith because the shirengou bif is an algoma type deposit determined u pb age 2541 2553 ma for zircons from hosting volcanic rocks can constrain the bif forming time 2 metamorphic overgrowth zircons from amphibolite and gneiss give two u pb ages of 2512 13 ma and 2510 21 ma respectively corresponding to the neoarchean matemorphism 3 the zircons with 18o values of 5 9 7 6 also indicate magmatic origin the zircons have 18o values of 6 8 9 9 suggesting that exotic high 18o fluids were involved during metamorphism based on the geology and geochemistry of bif and combing with zircon age and oxygen isotopic systematics we suggest that the shirengou iron deposit was formed in a submarine volcanic setting related to subduction of an oceanic slab

evidence against subduction related magmatism for the jiaoziyan gabbro northern dabie shan china the largest gabbroic intrusion jiaoziyan in the northern orthogneiss of the dabie shan was investigated to test whether such intrusions represent arc related or collision related magma as postulated in recent regional tectonic models the jiaoziyan intrusions comprise a gabbro monzogabbro suite and are locally characterized by cumulus textured anhydrous silicates petrologic indications against arc magmatism include 1 coexisting olivine plagioclase compositions beyond the documented range of arc gabbros and 2 a compositional trend of al ti ratios in the gabbroic clinopyroxenes that is significantly different from subduction related gabbros worldwide in contrast the jiaoziyan rocks are mineralogically similar to rift gabbros geochronologic and geometric arguments against collision related magmatism i e as a consequence of slab breakoff include the following 1 crystallization of the jiaoziyan mafic intrusion postdated the triassic 245 240 ma subduction and peak orogenic metamorphism by 100 m y and 2 the rocks of the dabie cretaceous magmatic suite including jiaoziyan are on the footwall plate rather than on the hanging wall of the triassic subduction zone framework and do not show a linear distribution these observations argue that the voluminous early cretaceous magmatism in the northern orthogneiss is geodynamically unrelated to the qinling dabie orogeny but was caused by extensional tectonism related to regional magmatic underplating beneath the thinned dabie crust

neoproterozoic tonalite and trondhjemite in the huangling complex south china crustal growth and reworking in a continental arc environment the huangling igneous complex in the central yangtze block of south china is composed of 850 ma tonalite and trondhjemite intruded by 820 ma granite tonalites mainly consist of plagioclase and quartz with abundant hornblende and biotite and contain fine to coarse grained hornblende rich igneous enclaves they have variably low sio2 56 4 67 5 wt and sr 230 517 ppm and high mgo 1 47 4 02 wt and high y 8 87 15 7 ppm with low sr y 17 43 unlike archean tonalites their whole rock end values range from 8 3 to 10 3 and zircon ehf values from 3 9 to 12 6 similar to those of spatially associated neoproterozoic mafic rocks the tonalites are thus proposed to have been produced by three stages involving partial melting of subduction modified lithospheric mantle 5 to 20 percent crustal contamination of hydrous basaltic magmas in the middle to lower crust and fractional crystallization of amphibole plagioclase biotite magnetite trondhjemites are composed of plagioclase quartz and alkali feldspar with minor biotite and hornblende they have high sio2 67 3 75 6 wt and sr 340 840 ppm and low mgo 0 09 0 78 wt and y 4 8 11 2 ppm with high sr y 32 162 similar to archean trondhjemites and high pressure partial melts of mafic rocks these rocks have extremely negative end 18 7 to 19 9 and zircon ehf values 18 9 to 49 2 with two stage hf isotope model ages of 2 46 to 3 90 ga indicating their formation in the lower crust by melting of archean amphibolites and granulites which were probably widely distributed beneath the yangtze block their highly variable chemical compositions also resulted from fractional crystallization of plagioclase hornblende biotite thus the association of the tonalite and trondhjemite is different from the archean ttg series this difference may reflect different geothermal gradients however modeling using tonalitic and trondhjemitic samples of the huangling complex suggests that mixing of the two magmas can generate a typical ttg affinity series suggesting that the origin of some ttg series may have involved both crustally derived and mantle derived melts the generation of the huangling igneous complex was linked to slab subduction beneath the northern margin of the yangtze block during the neoproterozoic similar to the cenozoic arc magmatism in the andes therefore melting of metasomatized hydrous mantle in arc settings can generate mafic magmas that not only under plated and contributed to continental crustal growth but also caused melting of thickened ancient crust and newly formed lower mafic crust

petrogenesis of early cretaceous post collisional granitoids at shapinggou dabie orogen implications for crustal architecture and porphyry mo mineralization abstract the shapinggou molybdenum deposit is located in the qinling tongbai hong an dabie orogen which accommodates the largest molybdenum ore belt in the world new laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating geochemical and sr nd hf isotope data for the granitoids at shapinggou are presented in this paper two stages of early cretaceous magmatism 138 128 ma and 116 114 ma took place at shapinggou in a post collisional setting these granitoids can be divided into four groups including the early stage granites 134 129 ma sio2 62 33 to 76 27 and intermediate rocks 129 128 ma sio2 52 41 to 60 72 and the late stage quartz syenite 116 ma sio2 64 09 to 65 12 and mo bearing granite porphyry 114 ma sio2 74 95 to 77 26 the early stage granites have high sr contents sr y and lan ybn ratios but low y yb and mgo contents showing low mg adakitic chemical features they were possibly generated by partial melting of the thickened mafic lower continental crust the early stage intermediate rocks have relatively high mgo contents 2 71 to 4 84 and mg values 46 to 60 but low sr y and lan ybn ratios which were likely derived from a hybrid mantle metasomatized by melts from foundered eclogitic lower continental crust and subsequent fractional crystallization the late stage quartz syenite shows chemical features of metaluminous a type granites including high abundances of alkalis hfse zr nb and trivalent ree the late stage mo bearing granite porphyry displays high sio2 alkalis zr ce contents and a cnk ratios but low ba sr p and ti showing affinity to highly fractionated i type granites the late stage quartz syenite and granite porphyry were possibly produced by partial melting of intermediate compositions of middle crust our results support the conclusion that there were three layer continental crusts in the dabie orogen prior to early cretaceous magmatism and suggest that post collisional granitoids in the dabie orogen were derived from not only lower crust but also middle crust the giant shapinggou porphyry mo deposit was most likely formed by effective transport of volatiles and mo in a convecting granitic magma column rather than by direct melting of mo enriched sources the mo bearing rocks at shapinggou are characterized by a high degree of magma differentiation close associations with a type granite and formation at the end of regional magmatism

application of vitrinite reflectance in reconstruction of tectonic features in anambra basin nigeria implication for petroleum potential quantitative analysis of vitrinite reflectance data from the anambra basin nigeria has revealed for the first time tectonism and unconformities corresponding to the campanian maastrichtian and paleocene eocene boundaries this analysis also afforded definite subsurface evidence for the cenomanian as well as the santonian deformations the estimated thickness of each rock section that was removed by erosion to give the cenomanian santonian and upper campanian unconformities is approximately 373 m 512 m and 217 m respectively the estimates for the cenomanian and santonian erosion also represent the height of the corresponding uplifts in the lower benue trough the thicknesses of the rock sections eroded upon cessation of subsidence in the anambra basin in the early ertiary to produce the present topography vary systematically from 73 to 3063 m therefore the reconstructed depth corresponding to the top of the oil window in the early tertiary ranges from 1521 to 5127 m a temperature range of 81 to 191 c because the top of the oil window was located at that time within the nkporo shale or younger units the eze aku shale awgu shale nkalagu formation and in many cases the nkporo shale were originally placed at temperatures 150 c that favored mainly generation of gaseous hydrocarbons in many parts of the basin this could have played at least a supportive role in producing the predominance of gas plays in the anambra basin hence reconstruction of the maximum burial depth and paleotemperature of viable source beds is recommended as necessary input into a successful exploration strategy for the basin

neoarchean algoma type banded iron formations from eastern hebei north china craton shrimp u pb age origin and tectonic setting abstract algoma type bif hosted iron deposits occur in the shirengou xingshan erma lingshan and sijiaying areas of eastern hebei the deposits are hosted in archean rocks and their formation was related to volcanic and sedimentary processes in small volcano sedimentary basins supracrustal rocks of the qianxi and dantazi groups were metamorphosed from granulite facies to amphibolites facies available data indicate that these bif deposits mainly formed during paleoarchean and neoarchean times the paleoarchean bif hosted iron deposits are represented by the xingshan deposit that yielded a shrimp u pb zircon age of 3389 5 7 6 ma the other bif hosted iron deposits formed in the late neoarchean represented by the shirengou shuichang lingshan erma and sijiaying deposits that formed between 2605 ma and 2503 ma the late neoarchean bif iron deposits are considered to have formed under a mantle plume environment the eu anomalies of the ore samples from eastern hebei show that the bif ores formed in a hydrothermal related volcanic submarine environment where submarine hydrothermal effluents associated with volcanic activities may have supplied both the iron and silica for the bifs from eastern hebei

unraveling the record of successive high grade events in the central zone of the limpopo belt using pb single phase dating of metamorphic minerals abstract dating of relic metamorphic assemblages can provide important information about the timing and character metamorphic grade and or pt evolution of early high grade episodes in polymetamorphic provinces using pb stepwise leaching of metamorphic silicates we have dated multiple granulite facies metamorphic episodes in the central zone cz of the limpopo belt ages of 2 52 ga were obtained from sillimanite and cogenetic garnet and ages of about 2 01 ga from titanite garnet and clinopyroxene together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data these results lead us to a reinterpretation of the tectono metamorphic history of the cz three distinct high grade events at about 3 2 3 1 ga 2 65 2 52 ga and 2 0 0 05 ga are recognized each of these is suggested to correspond to a tectonic episode of distinct character a for the 3 2 ga event magmatic activity can mainly be identified best represented for example by the sand river gneisses or the messina layered intrusion the field relationships concerning the tectonometamorphic history of this early archean event are largely erased by at least two high grade metamorphic overprints b late archean 2 6 2 52 ga low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism the anticlockwise p t evolution of these granulites probably reflects deep crustal processes associated with magmatic underplating or in plating contemporaneous with vertical crustal growth of the zimbabwe craton around 2 6 ga c during the proterozoic event 2 05 1 95 ga tectonic thickening was caused by the collision of the kaapvaal and zimbabwe cratons the cz was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise p t evolution the structural metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny

crustal scale rheological transitions during late orogenic collapse abstract orogeny involves crustal thickening followed by thermal relaxation and radiogenic heat production in the thickened crust culminating in crustal melting and magma intrusion which decrease the crustal viscosity by several orders of magnitude and cause late orogenic collapse collapse of the canadian cordillera is expressed in the early tertiary shuswap metamorphic core complex british columbia which displays a three layer crustal section separated by two fundamental rheological discontinuities 1 the brittle ductile transition across which high angle normal faults in the upper crust control basin formation merge into a low angle detachment zone where leucogranite laccoliths ponded and deformed progressively under submagmatic to low temperature conditions and 2 the metatexite diatexite transition across which the rocks lose their solid framework and behave like a viscous magma this transition has the potential to mechanically decouple the upper crust from the rest of the lithosphere during a late orogenic gravitational collapse

ultrahigh pressure metamorphism and multistage exhumation of eclogite of the luotian dome north dabie complex zone central china evidence from mineral inclusions and decompression textures abstract the studied eclogite is located in the luotian dome in the southwestern part of the north dabie complex zone ndz central china and is a portion of deeply subducted mafic lower continental crust of the south china block petrologic analysis suggests that the eclogite underwent ultrahigh pressure uhp and high pressure eclogite facies metamorphism and subsequent hp granulite facies overprinting and amphibolite facies retrogression during continental subduction and exhumation as a result multiple decompression textures were produced suggesting a multistage exhumation history from eclogite to granulite to amphibolite facies conditions a striking feature of this eclogite is the widespread exsolution of different phases oriented needles of rutile clinopyroxene amphibole apatite in garnet quartz amphibole hypersthene sodic plagioclase in clinopyroxene and pyrrhotite in apatite most importantly we provide for the first time conclusive evidence of quartz pseudomorphs after coesite in garnet and relic coesite confirmed by in situ raman spectroscopy in zircon in addition there are two groups of apatite one is fluor f apatite that occurs as inclusions in garnet or in matrix which is rich in f 3 wt lakes exsolution features and formed at uhp metamorphic conditions the second is associated with the oriented pyrrhotite exsolution and formed during decompression microtextural and petrologic analysis suggests that the eclogites from the southwestern segment of the ndz are similar to eclogites from the northern segment of the ndz e g huangweihe and baizhangyan and underwent uhp metamorphism with a peak pressure 5 7 gpa these results combined with published geochronological data imply that the ndz experienced triassic uhp metamorphism as a coherent unit

application of ti in zircon and zr in rutile thermometers to constrain high temperature metamorphism in eclogites from the dabie orogen central china abstract ultrahigh pressure uhp eclogites from the northern dabie orogen central china underwent a complex metamorphic evolution during triassic continental deep subduction and subsequent exhumation the eclogites were strongly affected by multiple decompression and re crystallization processes during exhumation thus making the determination of peak metamorphic conditions particularly challenging the recently calibrated ti in zircon and zr in rutile thermometers provide new tools to estimate the peak and post peak temperatures rutiles within uhp mineral bearing domains of zircon have zr contents of 1030 4310 ppm and record a temperature range of 880 1080 c with a peak at 970 c rutiles within high pressure hp mineral bearing domains contain zr contents of 800 5800 ppm and define a temperature range of 780 1030 c with an average value of 873 c rutiles in granulite facies mineral bearing rims of zircon yield a temperature of 845 c in contrast rutile inclusions within garnet have lower zr contents of 100 800 ppm yielding lower temperatures of 600 850 c the uhp and hp mineral bearing mantle domains of zircon have ti contents of 2 15 97 8 ppm yielding ti in zircon temperatures of 620 1020 c ti contents in the rim domains of zircon are low 900 c for the uhp and hp domains of zircon therefore this study combined with the previously published data suggests that the eclogites in the region experienced a protracted high temperature ht metamorphic evolution from uhp eclogite facies to hp granulite facies metamorphic stages this long lived ht evolution coupled with slow cooling and partial melting may explain the rare preservation of uhp traces in the area hp uhp mineral inclusions in garnet and zircon experienced variable degrees of decompression breakdown or retrogression therefore also ti concentrations in zircons and zr concentrations in rutiles within garnet and zircon may have been strongly modified by re crystallization or partial melting as a result only few zircons record their actual crystallization temperatures in comparison the rutile inclusions in zircon generally define higher temperatures we therefore suggest that rutile inclusions in zircon are the most suitable candidates for ht estimates especially in strongly retrogressed eclogites

giant mesozoic gold provinces related to the destruction of the north china craton abstract lode gold deposits in precambrian cratons represent the world s major gold source and were mostly generated during formation and stabilization of the cratons however there is an extraordinary exception in the north china craton ncc where lode gold deposits formed after prolonged stabilization of the craton molybdenite re os and hydrothermal sericite and biotite 40 ar 39 ar dating of major gold deposits from the xiaoqinling district southern ncc bracket their emplacement in the range of 154 1 1 1 to 118 9 1 2 ma n 23 postdating formation of the craton by more than 1 7 billion years fluid inclusions extracted from gold bearing pyrite have elevated 3 he 4 he ratios 1 52 0 22 ra and mantle like ne isotopes 20 ne 22 ne 10 02 9 22 and 21 ne 22 ne 0 033 0 027 indicating presence of mantle derived fluids in the ore system measured 34 s of pyrite and d and 18 o of hydrothermal micas and fluid inclusion waters in auriferous quartz further confirm a magmatic mantle source for sulfur and ore fluids gold deposits of similar ages also widely occur in the eastern and northern margins of the ncc which together with those in the xiaoqinling district have a total reserve of 2500 t gold forming the only known giant late mesozoic gold province in the world s precambrian cratons these deposits formed coevally with extensive felsic to mafic magmatism development of intracontinental rift basins and exhumation of metamorphic core complexes across the eastern ncc events interpreted as indicating thinning and destruction of the lithosphere beneath the craton rising of asthenosphere coupled with destruction of the lithosphere has generated voluminous mafic and felsic magmas that provided sufficient fluids sulfur and by inference other ore components to form the giant gold provinces

geochronology and geochemistry of the neoarchean lulong complex in the eastern hebei province north china craton implications on regional crustal evolution abstract the lulong complex exposed in the southeastern part of the eastern hebei province in the north china craton ncc is mainly composed of meta basalt biotite plagioclase gneiss migmatite amphibolite and granitoids mineral assemblages indicate the rocks in this complex underwent upper amphibolite facies metamorphism zircon u pb and lu hf data constrain that both protolith of the biotite plagioclase gneiss and the homogeneous domain of the migmatite formed at 2 53 ga and were mainly derived from the reworking of 2 8 2 7 ga crustal source the similar mineral association zircon u th pb systematics and geochemical features suggest that their protolith should be dominated by felsic volcanic sequences with limited contributions from older crust the protolith of the amphibolite gabbro or diabase was metamorphosed at 2477 20 ma geochemistry and zircon lu hf systems indicate that these meta mafic rocks originated from an enriched subcontinental lithospheric mantle source the granitoids exposed in the lulong area yield zircon u pb ages of 2507 21 ma to 2473 38 ma and record 2 5 ga regional metamorphic event the initial zircon 176hf 177hf ratios of the granite are in the range of 0 280640 0 281678 and suggest a complicated source where as those from the granodiorite indicate melting of juvenile crustal materials geochemically the felsic meta volcanics of the lulong complex are comparable to rhyodacite dacite with calc alkaline affinities the granitoids suite comprises 2 47 ga peraluminous granite felsic dikes and 2 51 ga metaluminous granodiorite all these felsic intrusions are characterized by moderate differentiation of ree patterns and low sr y ratios

geochronology and geochemistry of leucosomes in the north dabie terrane east china implication for post uhpm crustal melting during exhumation migmatites are widespread in the north dabie ultrahigh pressure metamorphic terrane ndt of dabie orogen east china idiomorphic and poikilitic amphibole grains in both leucosome and melanosome contain inclusions of plagioclase quartz and biotite suggesting formation of leucosome by fluid present melting of biotite plagioclase quartz bearing protoliths at p 5 7 kbar t 700 800 c precise sims zircon u pb dating indicates that migmatization of dabie orogen initiated at 140 ma and lasted for 10 ma coeval with the formation of low mg adakitic intrusions in dabie orogen based on mineralogical petrographic and geochemical data leucosomes in ndt can be subdivided into three groups 1 high la yb n medium sr y group group i whose high dy yb n but medium sr y ratios are caused by amphibole and plagioclase residual during partial melting of dioritic to granodioritic gneisses 2 low la yb n low sr y group group ii whose flat hree patterns are produced by entrainment of peritectic amphiboles into melts derived from partial melting of dioritic gneiss 3 high la yb n high sr y and eu group group iii whose extremely high sr and eu but low other ree concentrations are caused by accumulation of plagioclase and quartz although group i and iii fall in the adakitic fields on la yb n yb n and sr y y diagrams they are chemically distinct from contemporary high pressure adakitic intrusions in dabie orogen in a series of geochemical indexes for example lower dy yb n and or sr y ratios at given la yb n ratio lower sr cao ratios lower rb concentration but higher k rb ratios therefore leucosomes are produced by anatexis of the exhumed ultrahigh pressure metamorphic rocks at middle crustal level instead of partial melting of thickened lower crust with garnet rich and plagioclase poor residual the coeval occurrence of migmatites and high pressure adakitic intrusions in dabie orogen indicates large scale partial melting of middle to thickened lower crustal column in the early cretaceous the required heat source may be the mantle heat conducting through the lithospheric mantle whose lower parts have been convectively removed

isotopic evidence for lithospheric thinning during extension southeastern great basin mafic rocks erupted during late cenozoic extension near las vegas nevada show temporal patterns of variation in nd and sr isotope ratios and in bulk chemistry the patterns indicate that depths of magma generation were varying through time and that lithospheric mantle was partially replaced with asthenospheric mantle as extension proceeded in alkalic rocks epsilon sub nd changed through time from minus 9 1 typical of lithospheric mantle in this area before the onset of major 16 ma extension to 6 4 typical of asthenospheric mantle after extension 4 6 ma near the end of the period of major extension 10 6 ma tholeiitic rocks erupted the epsilon sub nd of which ranged from minus 10 1 to minus 7 9 this indicates that the lithosphere had not thinned sufficiently by that time to bring asthenospheric mantle into the depth range of tholeiitic magma generation 33 50 km the lithosphere in the las vegas area appears to have thinned by about 50 less than would be predicted by the magnitude of upper crustal extension a factor of 3 or 4 and indicative of a nonuniform lithospheric response to extension

early cretaceous granitoids and their implications for the collapse of the dabie orogen eastern china shrimp zircon u pb dating and geochemistry abstract two stages of early cretaceous post orogenic granitoids are recognized in the dabie orogen eastern china which recorded processes of extensional collapse of the orogen the early stage granitoids 132 ma are foliated hornblende quartz monzonites and porphyritic monzogranites they are of high k calc alkaline series and metaluminous to weakly peraluminous with high k 2 o and low mgo contents mg values 32 0 46 0 they contain high sr low y and heavy rare earth elements hree and have high sr y and la yb n ratios without clear negative eu sr and ti anomalies the early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite rich plagioclase and thus indicate the existence of an over thickened 50 km crustal root beneath the orogen at 132 ma the later stage granitoids 128 ma are undeformed fine grained monzogranites fine grained k feldspar granites and coarse grained k feldspar granite porphyry they belong to a peraluminous and high k calc alkaline to shoshonite series and display a flat hree pattern and have strong negative eu sr and ti anomalies with low sr y and la yb n ratios the late stage granitoids are equilibrated with residues rich in anorthite rich plagioclase hornblende ilmenite titanite and poor in garnet indicating that the crust of the dabie orogen became thinner

geochemical and u pb age constraints on the occurrence of polygenetic titanites in uhp metagranite in the dabie orogen abstract relict magmatic titanite was identified in the core of a few titanite grains with the overgrown rim of metamorphic titanite in uhp metagranite in the dabie orogen la icpms u pb dating gave neoproterozoic ages for the magmatic titanite but triassic ages for the metamorphic titanite the magmatic and metamorphic titanites are clearly distinct by differences in petrological and geochemical compositions the magmatic titanite occurs as residual cores that show bright bse the presence of allanite and quartz inclusions low contents of cao al2o3 and tio2 but high contents of fe2o3 and mgo in trace elements the magmatic titanite exhibits high ree and hfse contents distinctly negative eu anomalies with flat mree hree patterns and high th u ratios in contrast the metamorphic titanite occurs as rims and grains of homogeneously dark bse that contain inclusions of epidote quartz k feldspar rutile biotite and phengite and have relatively high contents of cao al2o3 and tio2 but low contents of fe2o3 and mgo and relatively low ree and hfse contents slightly negative eu anomalies with hree depletion relative to mree and low th u ratios the zr in titanite thermometry yields 727 to 877 c at 0 5 to 1 0 gpa for the magmatic titanite and 729 to 870 c at 1 5 to 2 0 gpa for the metamorphic titanite the neoproterozoic u pb chronometric system of magmatic titanite survived the triassic continental subduction zone hp uhp metamorphism this suggests a relatively high closure temperature of 800 c for the titanite u pb system the metamorphic titanite is principally a product of retrograde metamorphism during decompression exhumation at the transition from hp eclogite facies to amphibolite facies therefore titanite holds a great potential to geochronological and petrogenetic studies of continental subduction zone metamorphic rocks

zircon u pb hf isotopes and geochemistry of two contrasting neoarchean charnockitic rock series in eastern hebei north china craton implications for petrogenesis and tectonic setting abstract the zunhua qinglong microblock in eastern hebei north china craton exposes two contrasting charnockitic rock series in the yuhuzhai taipingzhai and cuizhangzi areas charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks and charnockite series showing tonalitic and granodioritic components petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks medium fine grained anhedral orthopyroxenes are preserved in both series and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series in situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene whereas the medium fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements ree and no obvious eu anomalies resulting in left inclined ree patterns combined with petrographic features these new data indicate that the medium fine grained anhedral orthopyroxenes were formed by granulite facies metamorphism caused by dehydration reactions whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530 17 and 2523 12 ma and those of the charnockite series were emplaced between 2527 28 and 2515 22 ma we conclude that these magmatic precursors are contemporaneous emplacements of two neoarchean granitoid magmatic events which were followed by regional granulite facies metamorphism at 2 45 ga the charnockitic plagioclase gneiss series and charnockite series exhibit similar hf t values of 2 4 to 5 3 and 3 2 to 5 5 indicating either depleted mantle or juvenile crust affinity and involvement of ancient crustal components the charnockitic plagioclase gneiss series exhibit relatively low sio2 contents 54 1 65 2 with various mgo contents 1 93 5 36 mg 0 36 0 55 and la yb n values 3 49 56 00 the charnockite series exhibit higher sio2 contents 60 1 69 2 with similar la yb n values 14 61 42 69 mgo contents 2 10 3 99 and mg 0 29 0 52 these geochemical and zircon lu hf isotopic features together with chemical modeling suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab derived melts fluids and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases the aforementioned features further suggest that the charnockite series originated from a partial melting of the subducted slab and it was strongly contaminated by mantle peridotites these new data combined with the previous studies of neoarchean basement rocks in the zunhua qinglong microblock in eastern hebei suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back arc related tectonic environment at a convergent plate margin

deep and near surface consequences of root removal by asymmetric continental delamination abstract the occurrence of continental delamination has been proposed for a number of areas characterized by highly variable geodynamic settings in this study we present results of numerical simulations considering different initial setups representative for geodynamic scenarios where delamination could potentially develop to mimic a post collisional orogenic scenario we have designed an initial state characterized by the presence of an area of orogenic lithosphere with both crustal and lithospheric roots in a second setup we have considered a lithospheric root representative of a remnant slab with a flat overlying crust we focus on predicted evolution of surface and near surface observables namely the crustal structure surface heat flow and isostatic and dynamic topography evolution our results show that a high density orogenic lower crust likely related to the presence of eclogite significantly accelerates the sinking of the lithospheric mantle the pattern of local isostatic elevation is characterized by laterally migrating surface uplift subsidence this pattern is shown to be little sensitive to lower crust density variations in contrast predicted dynamic topography is more sensitive to these changes and shows surface subsidence adjacent to the delaminating lithospheric mantle for the model with a high density lower crust and surface uplift above the slab for a model with a less dense lower crust the reason for uplift in this second model is that the effect of the positive buoyancy of the thickened crust overwhelms the effect of negative buoyancy of the slowly sinking lithospheric mantle we infer from our modeling that there is not a specific characteristic pattern of topography changes associated with delamination but it depends on the interplay between highly variable factors as slab sinking velocity asthenospheric upwelling and changes in crustal thickness

petrogenesis of early cretaceous alkaline dolerite dykes in the dabie orogen china constraints on the timing of lithospheric thinning abstract the dabie orogen in central china has long been proposed to transform from triassic compression related to northward deep subduction to late jurassic to early cretaceous extension involving marked crustal thinning and doming processes zircon u pb ages mineral chemistry and whole rock geochemistry of alkaline dykes intruding early cretaceous intermediate mafic ultramafic complexes are presented herein to provide evidence for the timing of lithospheric thinning and extension these dykes of dominantly dolerites with minor monzodiorites have zircon u pb ages of 128 3 129 7 ma and are characterized by sr nd isotopic ratios around the enriched mantle i the dolerites can be divided into two groups with the first group having lower incompatible elements but more enriched sr nd isotopes than the second group equilibrium partial melting and isotopic mixing modeling show that the dolerites were first generated from partial melting of a heterogeneous enriched mantle at different degrees and then experienced fractional crystallization and crustal assimilation systematic normative minerals trace elements and isotopic contrasts of early cretaceous gabbros 125 130 ma dolerite dykes of this study and early cretaceous mafic dykes 111 115 ma indicate that the source regions for the dolerites are deeper than those for the host gabbros therefore the lithospheric mantle thickening probably continued until the formation of the dolerites and the lithospheric mantle thinning occurred significantly later than previously proposed the thickness of the lithosphere was more than the depth range for the source region of alkaline basalts at 125 130 ma but changed to less than that for the source region of tholeiitic basalts before 115 ma suggesting a minimum decrease of 20 km within 10 m y this rate of thinning can be explained only by delamination of voluminous crustal root with the underlying lithospheric mantle between 125 and 115 ma

qingyuan high grade granite greenstone terrain in the eastern north china craton root of a neoarchaean arc abstract the qingyuan high grade granite greenstone terrain is one of the first recognized granite greenstone terrains in the eastern north china craton similar to other archaean terrains its tectonic environment is debated was it an intra continental rift or a modern analogy of arc occurrence and petrogenesis of major igneous series the ultramafic mafic and felsic volcanic rocks 2510 ma and the plutons of the quartz diorite 2570 2510 ma ttg 2570 2510 ma and quartz monzodiorite 2510 2490 ma series in xinbin area are investigated the mafic intrusives and volcanic rocks have high mgo content 5 4 7 5 wt and mg number 48 61 they show slightly depleted to flat trace element patterns the ultramafic rocks serpentinite could be genetically related to the mafic rocks the meta dacite rhyolite is adakitic with enriched light and middle rees and liles but significantly depleted hfses the quartz diorite has high mg number 60 64 moderately enriched light and middle rees and liles the ttg shows distinct light but moderate middle ree enrichment prominent nb ta depletion but zr hf enrichment zr sm 100 the quartz monzodiorite has moderate light and middle ree enrichment significant nb ta depletion and negative eu sr anomalies the ttg has more depleted sr nd isotopes endt 2 6 87sr 86srt 0 700 than all the others endt 0 2 87sr 86srt 0 701 703 their petrogenesis can be explained by an archaean style subduction defined as a mantle wedge absent flat hot subduction with significant vertical tectonism in the overriding slab the ultramafic mafic rocks were originated from primitive mantle the meta dacite rhyolite was originated from the eclogite facies overriding crust the quartz diorite was a mixture of melts from mantle and the overriding crust the ttg was from the subducting slab under amphibolite to amphibole bearing eclogite facies and the quartz monzodiorite was from the subducting slab after the derivation of the ttg the qingyuan terrain represents remnants of the root of a neoarchaean arc its anticlockwise p t path recorded the initiation of the arc to its cession and exhumation cratonization at 2480 ma

how much magma is required to rift a continent article i nfo article history many continental rifts are associated with and preceded by the formation of large igneous provinces partial melting of a large region of anomalously hot upwelling mantle isrequired to explain these huge volumes of pre and syn rift basaltic magma the uplift associated with a shallow region of hot mantle is often invoked as a cause of the rift push force that may drive the observed extension the rift push force is likely to be an order of magnitude toosmall todrive tectonicstretching of normal continentallithosphere but may besufficient to drive opening of lithosphere cutting dikes intrusion of a sequence of such dikes along a narrow zone may provide enough heat to significantly weaken the lithosphere here we use a newly developed 2d numerical approach in which magma filled dikes open in continental lithosphere in response to an evolving model stress field and the advection and diffusion of heat is tracked to investigate the relation between the volume of magma and the amount of lithospheric weakening the available rifting force lithospheric structure and parameters controlling magma flux are varied in the numerical simulations for continental lithosphere 60 km thick hl h1200 c heat diffusion is unimportant and only a small amount of magma is required 10 20 m y similar to magmatic rifts like the red sea

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

structural and geochronological evidence for early cretaceous orogen parallel extension of the ductile lithosphere in the northern dabie orogenic belt east china abstract the wnw trending north dabie dome ndd in the northern dabie orogenic belt is bounded by the xiaotian mozitan ductile shear zone xmsz in the north and the wuhe shuihou shear zone wssz in the south fabric geometries and kinematics of both the xmsz and the wssz are similar they both show a top to nw or wnw sense of shear and display similar microstructures which indicate that the deformation temperature was about 600 650 c 40 ar 39 ar dating of hornblende and biotite from the mylonites of the xmsz suggests that the shear zone was developed prior to 142 ma u pb zircon la icpms dating of undeformed granite dikes that cut the xmsz suggests that the ductile shearing occurred before 130 ma the wssz was also developed in the early cretaceous the interior of the ndd is dominated by top to nw or wnw sense of shear in the migmatitic gneiss and deformed plutons of the earliest early cretaceous ages we suggest that the xmsz the wssz and the interior of the ndd were involved in the same early cretaceous deformation the xmsz and the wssz were originally a single connected more flat lying zone which we call the east dabie detachment zone eddz it developed at a crustal level of over 18 km in depth kinematics of the eddz suggest that the ductile crust and possibly part of the lithospheric mantle in the eastern dabie orogen underwent pervasive orogen parallel and ese ward extension at the beginning of the early cretaceous large scale magmatic intrusions following this deformation and the resultant increase in buoyancy led to the exhumation of the ndd and the warping of the eddz erosion separated the eddz into two zones the xmsz and the wssz as observed today many metamorphic core complexes of early cretaceous were developed in the eastern north china craton most showing top to nw or wnw sense of shear we suggest that such a widespread uniform shear sense reflects a uniform flow direction of the ductile lithosphere in the east china that has also been documented in the liaonan yiwulushan and yumenshan metamorphic core complexes in the northeastern part of the north china craton

tectonic setting of the low grade metamorphic rocks of the dabie orogen central eastern china abstract the tectonic setting on both the northern and southern sides of the dabie mountains reveals that low grade metamorphic rocks are important constituents produced by the subduction of the oceanic crust prior to collision between the sino korean and yangtze cratons the zhangbaling group mulanshan schist is a pre ordovician oceanic crust the sujiahe and xinyang foziling groups are trench sediments of the ordovician devonian age and constitute an accretionary prism associated with subduction the yangshan coal measures meishan group was a forearc basin sediment of carboniferous age and was overthrust by the accretionary prism during collision the susong group is composed of passive continental margin sediments of the yangtze craton backarc basin sediments are postulated to be concealed by mesozoic cenozoic sediments to the north of the dabie mountains high ultrahigh pressure terrains are exotic tectonic slices exhumed from depths located between low grade metamorphic rocks and disturb the integrity of the earlier subduction orogen subduction occurred during the ordovician to devonian periods and collision initiated at the beginning of the permian

early cretaceous low mg adakitic granites from the dabie orogen eastern china petrogenesis and implications for destruction of the over thickened lower continental crust it is generally accepted that the low mg adakitic rocks were derived from the partial melting of metabasalts eclogites in this study we demonstrate that the early cretaceous low mg adakitic granites in the north dabie complex ndc were generated by the partial melting of the ndc orthogneisses here we present in situ u pb and lu hf isotopes in zircon with whole rock geochemical and sr nd isotopic compositions were carried out for the tiantangzhai porphyritic monzogranites from the dabie orogen eastern china the monzogranites are characterized by high sr 576 988 ppm low y 7 3 19 0 ppm and depletion in hree yb 0 50 1 78 ppm thus resulting in high sr y 34 3 135 2 and la yb n 17 0 105 2 ratios without a negative eu anomaly they also exhibit high sio2 66 5 73 5 wt and k2o 2 7 4 7 wt and low mgo 0 4 1 6 wt or mg 28 2 45 3 mostly 40 values whole rock geochemical compositions suggest that the monzogranites represent low mg adakitic rock with high si and rich k features equilibrated with residues rich in garnet sr nd isotopic compositions end t 16 2 to 20 3 87sr 86sr i 0 707798 0 708804 tdm2 nd 2 3 2 6 ga of the monzogranites are distinct from that of the eclogites and amphibolites in the dabie orogen but similar to that of the neoproterozoic 700 800 ma gneisses in the ndc u pb dating of zircons gives a consistent age of 130 0 3 4 ma with discordia upper intercept age of 716 34 ma for inherited cores identified by cl imaging correspondingly in situ lu hf analyses of early cretaceous young age spots from zircons yield initial 176hf 177hf ratios from 0 281898 to 0 282361 ehf t values from 28 1 to 17 6 and two stage crust hf model ages tdm2 from 2293 89 to 2949 108 ma which are generally in agreement with values of 0 281891 to 0 282218 28 2 to 11 7 and 1927 87 to 2963 92 ma for the pre mesozoic inherited cores respectively as for individual core rim pairs in zircon th u ratios increase from the inherited cores to the young growth rims possibly due to variable degrees of partial melting whereas 176lu 177hf ratios greatly decrease because of the garnet effect in residues thus we suggest that the early cretaceous low mg adakitic granites were derived from the partial melting of the ndc neoproterozoic 700 800 ma gneisses and the foundering of the garnet bearing residues could have caused the destruction of the over thickened lower continental crust

geochemical evolution and metamorphic development of the early precambrian in eastern hebei china abstract within the archaean qianxi group in eastern hebei china two roughly coeval tectonic units can be recognized the qian an granulite complex and the zunhua high grade greenstone belt both belong to the high grade terranes and have recorded almost the same history of metamorphism the granulite facies metamorphism was generally followed by a retrograde metamorphism of amphibolite facies and multistage anatexis despite the similar metamorphic histories the geochemical features of the two units are different 1 1 the protoliths of the qian an granulite complex consist of a calc alkaline volcanic series dominated by andesitic composition and a shallow water sedimentary series whereas that of the zunhua greenstone belt clearly show the character of a bimodal magmatic series 2 2 most metabasalts of the qian an complex belong to the continental basalt type and grade into andesite whereas in the zunhua belt they are characterized by a suite of high magnesian series low k2o tholeiites and a large amount of ultramafic mafic layered complexes 3 3 in terms of regional geochemistry the qian an complex is rich in potassium and potassium charnockites of autochthonous anatectic and intrusive origin are widespread in contrast the zunhua belt is rich in sodium and tonalitic gneisses of deep seated mixed origin are widespread finally a model for archaean crustal evolution of high grade terranes is suggested

dynamic modeling of the transition from passive to active rifting application to the pannonian basin we examine a number of first order features of pannonian basin evolution in terms of the feedback relation between passive far field induced extension and active raleigh taylor instable upwelling of the astheno sphere we show that active mantle upwelling following a phase of passive extension are viable mechanisms ex plaining the pannonian basin formation the dynamic interplay between far field driven passive extension and active thinning of the mantle lithosphere by convective upwelling beneath the rift zone is modeled using ther momechanical finite element methods our modeling results predict a first phase of passive lithospheric thin ning which is followed by a second phase of late synrift to postrift active mantle lithosphere thinning due to buoyancy induced flow beneath the rift zone we argue that the pattern of coeval extension in the thinning re gion and compression in the flanking regions may be explained by the buoyancy forces due to lithosphere thinning it is demonstrated that timescales of and stresses generated by both processes are comparable the model appears also to explain the occurrence of late shallow mantle related decompression melts in the pannonian region and late regional doming

a crustal model of the ultrahigh pressure dabie shan orogenic belt china derived from deep seismic refraction profiling we present a new crustal cross section through the east west trending ultrahigh pressure uhp dabie shan orogenic belt east central china based on a 400 km long seismic refraction profile data from our profile reveal that the cratonal blocks north and south of the orogen are composed of 35 km thick crust consisting of three layers upper middle and lower crust with average seismic velocities of 6 0 6 0 2 km s 6 5 6 0 1 km s and 6 8 6 0 1 km s the crust reaches a maximum thickness of 41 5 km beneath the northern margin of the orogen and thus the present day root beneath the orogen is only 6 5 km thick the upper mantle velocity is 8 0 6 0 1 km s modeling of shear wave data indicate that poisson s ratio increases from 0 24 6 0 02 in the upper crust to 0 27 6 0 03 in the lower crust this result is consistent with a dominantly felsic upper crustal composition and a mafic lower crustal composition within the amphibolite or granulite metamorphic facies our seismic model indicates that eclogite which is abundant in surface exposures within the orogen is not a volumetrically significant component in the middle or lower crust much of the triassic structure associated with the formation of the uhp rocks of the dabie shan has been obscured by post triassic igneous activity extension and large offset strike slip faulting nevertheless we can identify a high velocity 6 3 km s zone in the upper 5 km depth crustal core of the orogen which we interpret as a zone of ultrahigh pressure rocks a north dipping suture and an apparent moho offset that marks a likely active strike slip fault

on bimodal differentiation by solidification front instability in basaltic magmas part 1 basic mechanics extensive coarse grained silicic lenses are common in the upper portions of large diabase sills lava lakes and gabbroic intrusions these lenses which often contain up to 10 wt more silica than their host rocks record a clear process of bimodal differentiation that may play a fundamental role in generating significant quantities of silicic magma in predominantly basaltic systems the spatial compositional textural and contact relations suggest that these silicic segregations form by infilling of tears formed during gravitational instability of the upper solidification front this process is investigated in some detail by exploring the thermal and mechanical conditions leading to internal failure of the front and segregation formation as the upper solidification front grows inward and thickens the local strength of the developing crystalline network must increase upward through the front sufficiently fast to offset the increasing weight of the front itself this necessary local variation in strength dictates how crystallinity must vary to prevent failure but the actual variation in crystallinity is determined by phase equilibria which in general varies in a fashion that is noncovariant with that required for mechanical stability it is this mismatch that apparently leads to failure and formation of silicic segregations the rate of filling of the tears by porous flow of interstitial melt from below regulates the rate of tear opening and the overall size of the tears is regulated by the local rate of advance of the solidification front segregations therefore thicken downward as the growth of the front slows and then die out altogether with approach of the lower solidification front this process irreversibly introduces siliceous noise into basaltic systems the silicic signal can be enhanced through subsequent reprocessing of the host rock to form volumetrically important masses of silicic magma this process may be fundamental to achieving the strong compositional diversity of the igneous rocks to yielding continental masses and to the differentiation of earth itself

moho mapping in the dabie ultrahigh pressure collisional orogen central china eight wide angle reflection refraction seismic profiles with a total length of 1700 km and near vertical reflection seismic profiles with a total length of 340 km across the dabie orogen eastern china have been completed in recent years and allow the moho depth and uppermost mantle velocity to be mapped across 40 000 km 2 these data reveal the detailed structure of the crust and upper mantle of this orogen that resulted from collision of the north china craton ncc with the yangtze craton yc the dabie orogen is an asymmetric orogen with a thin crustal root 6 km preserved in its northern part highlighted by a north dipping moho compared to the nearly seismically transparent lower crust the moho crust mantle transition zone under the dabie orogen is a prominent north dipping strongly layered reflection which is inferred to reflect the remnants of subduction of yc under the edge of the ncc or post collisional exhumation of ultrahigh pressure uhp rocks and indicates the northward polarity of subduction of the yc the moho of the yc and ncc meet beneath the northern part of the dabie mountains and the ncc moho north is uplifted by 4 to 5 km offset 1 over the yc moho south marking the ncc yc collision zone along the southern margin of the dabie mountains the moho below the yangtze foreland is underthrust beneath the moho of the dabie mountains forming another moho overlap offset 2 with a depth difference of 5 to 6 km the two moho overlaps offsets at both flanks of the dabie mountains formed two crustal scale boundaries at depth between the ncc and the dabie orogen and the dabie orogen and the yc respectively offset 1 is considered to be the triassic suture between the yc and ncc as supported by the metamorphic ages of uhp rocks this displays a wedge shaped offset zone opening towards the east that acted as a channel for exhumation of uhp rocks offset 2 under the southern margin of the dabie orogen is connected to the xiangfan guangji fault that gave rise to a large scale thrust detachment that propagated towards the foreland of the dabie orogen during the jurassic thus our moho mapping confirms that there are structural remnants of the triassic deep continental subduction preserved despite the superimposed jurassic deformation