reply to comment by qi and wang on similar crust beneath disrupted and intact cratons arguments against lower crust delamination as a decratonization trigger

tectonic evolution of the qinling orogenic belt central china new evidence from geochemical zircon u pb geochronology and hf isotopes abstract the qinling orogenic belt qob located between the north china craton ncc and the yangtze craton yzc is composed of the northern qinling belt nqb and the southern qinling belt sqb this study presents new geochemical data zircon u pb ages and hf isotopes from a suite of metamorphic rocks from the kuanping qinling danfeng douling and wuguan groups in the nqb and the sqb our results suggest that the qinling group in the nqb formed in an active continental margin related to the subduction of the songshugou ocean under the southern ncc during grenvillian period the kuanping group in the northern nqb was deposited in the within plate extension basin during late neoproterozoic to early paleozoic the detritus of sedimentary rocks of the kuanping group were mainly sourced from the nqb and subordinately from the basement rocks of the ncc the danfeng group in the southern nqb formed later than 827 ma with the sediments derived mainly from the qinling terrane the nqb witnessed multiphase tectono thermal events from mesoproterozoic to early paleozoic including several magmatic events and the early paleozoic metamorphic event the original douling group in the sqb represents the basement complex of the sqb which is composed of rocks with different formation ages the oldest rock in the complex is represented by 3 0 ga granitic gneiss which underwent at least two periods of metamorphism at 2 5 ga and 0 97 0 91 ga in this study the douling group is redefined as a suite of neoproterozoic 860 750 ma rocks formed in an extensional setting after the subduction of the songshugou ocean the wuguan group in the sqb formed in the middle late neoproterozoic later than 740 ma and was subjected to early paleozoic metamorphism our data indicate that the sqb experienced early neoproterozoic 970 865 ma and early paleozoic 444 ma metamorphism and strong middle neoproterozoic 870 730 ma magmatism the age spectra of zircons from the nqb trace major tectono thermal events at 1462 ma 927 ma and 412 ma which are different from those recorded in the southern ncc and northern yzc suggesting that the nqb had a distinct evolution during mesoproterozoic to early neoproterozoic grenvillian the hf isotopic compositions of zircons from the various rocks suggest that the nqb probably developed on the basement of the southern ncc the rocks in the sqb show zircon age spectra and hf isotope compositions similar to those in the northern yzc suggesting a close affinity we therefore interpret the sqb to have developed on the basement of the northern yzc integrating the new data in this study with those from previous works we propose a new tectonic model for the formation and evolution of the qob during late mesoproterozoic to early paleozoic involving the following major events 1 late mesoproterozoic to early neoproterozoic grenvillian northward subduction of the songshugou ocean 2 early middle neoproterozoic 870 800 ma bidirectional subduction and collision 3 middle neoproterozoic 800 710 ma post collision extension 4 middle late neoproterozoic 710 600 ma within plate extension 5 late neoproterozoic early paleozoic 600 520 ma opening of the shangdan ocean and 6 early paleozoic 520 420 ma subduction collision we thus trace at least two distinct wilson cycles in the qob

subduction collision and exhumation in the ultrahigh pressure qinling dabie orogen c abstract high pressure metamorphism and ophiolite emplacement songshugou ophiolite attended suturing of the yangtze craton to rodinia during the c l o ga grenvillian oro geny the qinling microcontinent then rifted from the yangtze craton at c 750ma the erlangping intraoceanic arc formed in the early ordovician was emplaced onto the qinling microcontinent in the ordovician silurian and then both units were accreted to the sino

palaeomagnetic constraints on the collision and rotation of north and south china the eastern half of china is dominated by the north china block ncb and south china block scb separated by a complex fold system that trends roughly east west interpretation of the geology of this fold belt has led to a variety of estimates of the age of amalgamation of these two blocks1 7 palaeomagnetic results are few and have also been subject to conflicting interpretations8 11 here we present our own palaeomagnetic results for late permian rocks for both the scb and ncb and summarize other relevant palaeomagnetic data by reinterpreting the polarity given in some previous studies we are able to synthesize the results for the widespread emeishan basalts of the scb and to suggest a new model for the collision and relative rotation of the two blocks

granulites in the tongbai area qinling belt china geochemistry petrology single zircon geochronology and implications for the tectonic evolution of eastern asia the tongbai area of the eastern qinling belt in china includes granulite grade metamorphic assemblages qinling complex which were previously regarded as archean to early proterozoic in age and belonging to the southern margin of the north china plate craton our petrological and geochemical data characterize these rocks as two pyroxene granulites and garnet granulites which formed at temperatures of 757 840 c and pressures of about 9 5 kbar and are now found as xenoliths in granodioritic gneisses the protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro the 207pb 206pb ratios derived from evaporation of single zircons yield ages of 470 20 and 470 14 ma respectively for the basic granulites which we interpret to reflect the time of protolith emplacement these are intruded by a 435 14 ma granodioritic gneiss post dating granulite formation a metaquartzite sample contains detrital zircons as old as 2555 8 ma two samples of granitoid gneiss from the tongbai complex s of the qinling granulites have single zircon 207pb 206pb evaporation ages of 776 8 and 746 10 ma respectively and document late proterozoic igneous activity we suggest that the qinling granulites document an important and hitherto unknown phase of early silurian crustal thickening following subduction and continental collision and that both the qinling and tongbai complexes were part of the southern margin of the north china craton prior to this event and record late proterozoic igneous activity

u pb and hf isotope analysis of zircons in mafic xenoliths from fuxian kimberlites evolution of the lower crust beneath the north china craton mafic xenoliths from the paleozoic fuxian kimberlites in the north china craton include garnet granulite and minor pyroxene amphibolite metagabbro anorthosite and pyroxenite the formation conditions of the amphibolites are estimated at 745 820 c and 7 6 8 8 kb 25 30 km the granulites probably are derived from greater depths in the lower crust lam icpms u pb dating of zircons from four granulites reveals multiple age populations recording episodes of magmatic intrusion and metamorphic recrystallisation concordant ages and upper intercept ages interpreted as minimum estimates for the time of magmatic crystallisation range from 2 620 to 2 430 ma in three granulites two amphibolites and two metagabbros lower intercept ages represented by near concordant zircons are interpreted as reflecting metamorphic recrystallisation and range from 1 927 to 1 852 ma one granulite contains two metamorphic zircon populations dated at 1 927 55 ma and 600 700 ma separated minerals from one granulite and one amphibolite yield sm nd isochron ages of 1 619 48 ma 143nd 144nd i 0 51078 and 1 716 120 ma 143nd 144nd i 0 51006 respectively these ages are interpreted as recording cooling following metamorphic resetting model ages for both samples are in the range 2 40 2 66 ga lam mc icpms analyses of zircon show a range in 176hf 177hf from 0 28116 to 0 28214 corresponding to a range of ehf from 34 to 12 the relationships between 207pb 206pb age and ehf show that 1 the granulites amphibolites and metagabbro were derived from a depleted mantle source at 2 6 2 75 ga 2 zircons in most samples underwent recrystallisation and pb loss for 100 200 ma after magmatic crystallisation consistent with a residence in the lower crust 3 metamorphic zircons in several samples represent new zircon growth incorporating hf liberated from breakdown of silicates with high lu hf 4 in other samples metamorphic and magmatic zircons have identical 176hf 177hf and the younger ages reflect complete resetting of u pb systems in older zircons the fuxian mafic xenoliths are interpreted as the products of basaltic underplating derived from a depleted mantle source in neoarchean time an important period of continental growth in the north china craton paleoproterozoic metamorphic ages indicate an important tectonic thermal event in the lower crust at 1 8 1 9 ga corresponding to the timing of collision between the eastern and western blocks that led to the final assembly of the north china craton the growth of metamorphic zircon at 600 700 ma may record an asthenospheric upwelling in neoproterozoic time related to uplift and a regional disconformity in the north china craton

re os dating of molybdenites from au mo deposits in xiaoqinling gold ore district and its geological significance molybdenum mineralization with economic value was recently discovered in the dahu gold deposit and its nearby quanjiayu gold bearing quartz veins in the xiaoqinling gold ore district henan province for the purpose of studying the chronology of the molybdenum mineralization in this district the authors purified molybdenite from the dahu gold deposit and the quanjiayu gold bearing quartz veins re os dating of these molybdenite minerals yielded modal ages of 223 0 2 8 ma 223 7 2 6 ma and 232 9 2 7 ma for the three samples from dahu and 129 1 1 6 ma and 130 8 1 5 ma for the two samples from quanjiayu these results indicate that au mo mineralization in the xiaoqinling gold ore district took place not only at the yanshanian stage but also at the indo chinese stage different from things of jinduicheng huanglongpu and other mo deposits the molybdenites from the dahu and quanjiayu deposits have a very low content of re showing a crustal source for ore forming materials

mid paleozoic collision in the north qinling sm nd rb sr and 40 ar 39 ar ages and their tectonic implications abstract different isotopic geochronometers have been used to study the nature of the mid paleozoic tectonothermal event in the north qinling arc system at the southern margin of the north china block 40ar 39ar age of amphibole from sample qh 3 426 2 ma rb sr mineral isochron age 411 5 ma of sample qsp 10 of the danfeng group and rb sr whole rock isochron age 406 22 ma of a greenschist facies pillow lava sample of the erlangping group indicate a widespread late silurian tectonothermal event in the north qinling area the data also suggest a limited uplift in the north qinling during the late silurian tectonothermal event this event is restricted to the north qinling area and the southern margin of north china block and not observed in the south qinling belt and the northern margin of the south china block consequently it is related to the collision between the north china block and a north qinling arc system which resulted in the closure of the north qinling back arc basin s

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

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

phanerozoic magma underplating and crustal growth beneath the north china craton evidence for post archaean crustal growth via magma underplating is largely based on u pb dating of zircons from granulite facies xenoliths however whether the young zircons from such xenoliths are genetically related to magma underplating or to anatexis remains controversial the lower crustal xenoliths carried by igneous rocks in the chifeng and ningcheng north china craton have low sio2 and high mgo indicating that parental melts of their protoliths were of unambiguous mantle origin the xenoliths contain abundant magmatic zircons with late palaeozoic ages and have more radiogenic zircon hf isotope compositions and hence younger model ages than ancient crustal magmas and the reworking array of the basement rocks our data suggest that the granulites represent episodic magmatic underplating to the lower crust of this craton in phanerozoic time considering the observation that regional lowermost crust 5 km is mafic and characterized by phanerozoic zircons this work reports an example of post archaean crustal growth via magma underplating

the grenvillian songshugou ophiolite in the qinling mountains central china implications for the tectonic evolution of the qinling orogenic belt abstract the qinling mountains in central china mark a gigantic composite orogenic belt with a complex tectonic evolution involving multiple phases of rifting and convergence this belt separates the north china and south china blocks and consists of the south and north qinling terranes separated by the shangdan suture the suture is marked by the grenvillian songshugou ophiolite along the southern margin of the north qinling terrane which is key to understanding the proterozoic tectonic evolution of the belt the ophiolite consists of highly metamorphosed ultramafic and mafic rocks three groups of meta basalts are present group 1 rocks are lree depleted and have a morb compositional affinity their low ta yb ratios 0 12 and display slight enrichment of lree suggesting that the original magmas were derived from a depleted mantle source mixed with some enriched material samples from group 3 are enriched in lree and other incompatible elements ti zr ta nb suggesting derivation from an enriched mantle source possibly a plume all the basalts have high end t 4 2 to 6 9 variable esr t and high 207pb 204pb and 208pb 204pb ratios for given 206pb 204pb ratios these characteristics are compatible with formation at a mid ocean ridge system above an anomalous dupal mantle region the mafic rocks have a sm nd whole rock isochron age of 1030 46 ma the songshugou ophiolite was emplaced onto the southern margin of the north qinling terrane an active continental margin from the meso proterozoic to neo proterozoic

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

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

timing of collision of the north and south china blocks controversy and reconciliation the qinling orogen was formed by the joining of the north and south china blocks but the timing of their integration has been debated for more than a decade the controversies obviously stem from different approaches to reconstruction of the integration history two contrasting lines of evidence yield two different ages for collision of the north and south china blocks middle paleozoic and late triassic the shangdan suture within the qinling was regarded in previous studies as the trace along which the north and south china blocks collided our studies however demonstrate that there are two sutures within the qinling the well documented shangdan suture and the newly discovered mianlue suture we show in this paper that the late proterozoic to early mesozoic evolution of the qinling involved interactions between the north china block the north and south qinling orogens and the south china block the middle paleozoic collision along the shangdan suture as constrained by some evidence accreted only the south qinling orogen to the southern part i e the north qinling of the north china block contemporaneous rifting of the south china block and subsequent drifting separated the south qinling from the south china block during the middle to late paleozoic the separation of the south from the north china blocks is supported by other evidence in particular geomagnetic data evidently it was the late triassic collision of the south china block with the south qinling orogen along the mianlue suture that led to final integration of the north and south china blocks

highly fractionated late triassic i type granites and related molybdenum mineralization in the qinling orogenic belt geochemical and u pb hf and re os isotope constraints abstract we present new data on the highly fractionated late triassic i type liyuantang granite which is located in the middle segment of the south qinling subzone of central china and is associated with molybdenum mineralization zircon u pb dating indicates that the granite was emplaced at 210 1 1 9 ma with a single zircon containing an inherited core that yielded an age of 449 8 7 1 ma magmatic zircons from the granite have e hf t values of 4 0 to 1 5 whereas the inherited zircon core has a e hf t value of 5 3 calculated hf model ages of crust formation are indicative of substantial contributions from melting of proterozoic crust that ranges in age from 1501 to 1155 ma the granite contains high concentrations of si al na and k is enriched in rb th and u has elevated rb sr and ga al ratios and is depleted in ti fe mn mg ca and p with significantly negative eu anomalies eu 0 33 0 50 similar to other highly fractionated i type granites these data indicate that the magmas that formed the liyuantang pluton were produced during partial melting of proterozoic garnet absent quartz amphibolites the magmas then fractionated apatite feldspar ti bearing phases biotite and hornblende prior to emplacement re os isotope analysis of molybdenite from the study area yields a mineralization age of 200 9 6 2 ma suggesting that the liyuantang molybdenum deposit formed during a previously unrecognized mineralization event the present results together with previous data demonstrate that highly fractionated i type granites associated with the second pulse of magmatism in the south qinling subzone should be considered highly prospective for mineral exploration focusing on triassic early jurassic granitoids

triassic diorites and granitoids in the foping area constraints on the conversion from subduction to collision in the qinling orogen china abstract the western qinling is characterized by triassic granitic plutons which are keys to constrain the timing of subduction and collisional processes between the north china and south china blocks this paper presents a set of new la icpms u pb zircon geochronology major and trace element geochemistry for several representative diorite and granitoid plutons in the foping area the diorites are characterized by high mgo al2o3 y and yb contents but low sio2 sr y and la yb ratios as well as depletion of nb ta p and ti suggesting derived from a subduction related setting la icpms zircon u pb age of 216 9 1 5 ma mswd 0 69 from the diorite constrains the time of the subduction event the medium and fine grained monzogranites as well as the biotite granites exhibit relative high sio2 mgo and rb low y and yb contents and low sr y and la yb ratios as well as various depletions of nb ta p and ti these features indicate a derivation from lower continental crust but had been influenced by slab released fluids in a syn collisional setting the medium grained monzogranite and biotite granite yield la icpms zircon u pb ages of 207 3 2 1 ma mswd 1 8 and 201 6 1 2 ma mswd 1 04 respectively proposing the syn collision during ca 207 201 ma geochemical and geological evidences suggest that the granodiorites intruding in the syn collisional monzogranites were related to the post orogenic collapse after the collision between the north china and south china blocks the la icpms u pb zircon age of 189 3 2 9 ma mswd 5 6 for the granodiorite constrains the time of the post orogenic collapse our geochronological and geochemical data provide important constraints on the triassic evolution from subduction to collision then up to post collisional collapse along the qinling orogen

seismic crustal structure of the north china craton and surrounding area synthesis and analysis we present a new digital model nccrust of the seismic crustal structure of the neoarchean north china craton ncc and its surrounding paleozoic mesozoic orogenic belts 30 45 n 100 130 e all available seismic profiles complemented by receiver function interpretations of crustal thickness are used to constrain a new comprehensive crustal model nccrust the model presented on a 0 25 0 25 grid includes the moho depth and the internal structure thickness and velocity of the crust specified for four layers the sedimentary cover upper middle and lower crust and the pn velocity in the uppermost mantle the crust is thin 30 32 km in the east while the moho depth in the western part of the ncc is 38 44 km the moho depth of the sulu dabie qinling qilian orogenic belt ranges from 31 km to 51 km with a general westward increase in crustal thickness the sedimentary cover is 2 5 km thick in most of the region and typical thicknesses of the upper crust middle crust and lower crust are 16 24 km 6 24 km and 0 6 km respectively we document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence the depth of the moho there is no systematic regional pattern in the average crustal vp velocity and the pn velocity we examine correlation between the moho depth and topography for seven tectonic provinces in the north china craton and speculate on mechanisms of isostatic compensation

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

geochemistry and zircon u pb hf isotopic systematics of the ningshan granitoid batholith middle segment of the south qinling belt central china constraints on petrogenesis and geodynamic processes abstract the ningshan granitoid batholith ngb is located in the middle segment of the south qinling belt sqb section of the qinling orogenic belt in shaanxi province china and consists of the laocheng yanzhiba and lanbandeng intrusions the laocheng intrusion is dominated by quartz diorites granodiorites and monzogranites with the yanzhiba intrusion being granodiorite dominated and the lanbandeng intrusion predominantly consisting of two mica monzogranites la icp ms zircon u pb isotopic dating indicates that the ngb formed during two distinct episodes of magmatism with an earlier episode at 222 216 ma and a later episode at 210 ma whole rock geochemistry and geochronology data have allowed us to divide the ngb into three groups with group 1 rocks consisting of laocheng intrusion quartz diorites and granodiorites and yanzhiba intrusion granodiorites that were produced in the early magmatic episode group 1 samples have high k calc alkaline compositions and are characterized by high mg values high sr cr and ni concentrations and high la yb n and sr y ratios but low yb and y concentrations implying that these rocks were formed from a mixed magma derived from melting of neoproterozoic crustal basaltic rocks and magmas sourced from a region of depleted mantle group 2 rocks are composed of the mid k calc alkaline monzogranites in the laocheng intrusion that were formed in the later magmatic episode and are characterized by lower mg values and lower cr ni and co concentrations than the group 1 samples but with high sr concentrations and la yb n and sr y ratios low y and yb concentrations negligible eu anomalies and a significant range in zircon ehf t values the highest zircon ehf t values of the group 2 samples plot close to the depleted mantle evolutionary line indicating that these rocks were derived from a mixed magma with both crustal and mantle derived components group 3 samples consist of high k calc alkaline two mica monzogranites of the lanbandeng intrusion that were formed during the later magmatic episode these rocks are characterized by high sio2 al2o3 and k2o concentrations with low mgo and sr concentrations low la yb n and sr y ratios nb ta p and ti depletions and significantly negative eu anomalies implying that the magmas from which these rocks formed were derived from partial melting of sedimentary rocks and minor amount of basalt the early magmatic episode associated with formation of the ngb occurred in a continental arc environment at 222 216 ma with 210 ma magmatism during a geodynamic transition from syn collisional to post collisional tectonics across the sqb

geochemical evidence for origin of magma mixing for the triassic monzonitic granite and its enclaves at mishuling in the qinling orogen central china abstract whole rock major trace elements and sr nd isotopes mineral chemistry and zircon u pb and lu hf isotope compositions are reported for i type monzogranite and its mafic microgranular enclaves from the triassic mishuling granite in the qinling orogen central china zircon u pb dating yields an age of 212 5 ma for the mafic enclaves and 213 3 ma for the host monzogranite whole rock major trace elements sr nd isotopes and zircon hf isotopes preclude simple crystal fractionation or restite unmixing for a genetic link between the enclaves and the monzogranite mixing between mafic and felsic magmas is compatible with the geochemical data the monzogranite displays an enriched lile and lree pattern high 87sr 86sr i ratios of 0 7068 to 0 7071 and low end t values of 9 2 to 5 7 suggesting a crustal origin its high mg 47 6 to 50 7 cr and ni contents clearly indicate its derivation from mafic refractory lower crust most of the zircons in the monzogranite have negative ehf t values of 12 5 to 0 1 and two stage hf model ages of 1049 to 1683 ma and some zircons in the host monzogranite have positive ehf t values of 0 3 to 4 8 and single stage hf model ages of 665 to 845 ma this indicates that the monzogranite is most likely to be formed by partial melting of a proterozoic lower continental crust the mafic enclaves have fine grained igneous textures and contain abundant acicular apatite that indicates rapid cooling they also have a high 87sr 86sr i ratio of 0 7071 and a low end t value of 6 5 suggesting a derivation from ancient subcontinental lithosphere mantle sclm zircons in the mafic microgranular enclaves have ehf t values of 1 6 to 5 1 and single stage hf model ages of 700 to 910 ma suggesting a possible origin from partial melting of the neoproterozoic sclm that may form in association with contemporaneously supercontinental rifting in the south china block therefore the mishuling i type granite may form by anatexis of meso neoproterozoic lower crust with mixing between the mafic and felsic magmas in particular we interpret the positive ehf t values as being derived from reworking of neoproterozoic juvenile lithosphere in response to tectonic extension in the late stage of the triassic continental collision between the south china block and the north china block

early mesozoic mo mineralization in the qinling orogen an overview abstract as one of the most important mo provinces in the world the qinling orogen is host to mainly late mesozoic late jurassic early cretaceous porphyry mo systems recently additional early mesozoic mainly triassic plus minor early jurassic mo deposits have been identified which occur as fault controlled quartz veins or as carbonatite veins and as such they are fundamentally different from the commonly observed porphyry systems furthermore early mesozoic porphyry systems are also present but they are different from those of late mesozoic age thus early mesozoic mo systems not only offer a new and comprehensive view of the mo metallogenesis but also provide unique insight into the tectonic evolution of the qinling orogen taking into consideration the role of mineral deposits as ideal probes of the geodynamic evolution the known early mesozoic mo deposits in qinling can be classified into three types based on their geological and geochemical features porphyry carbonatite and orogenic systems porphyry mo systems including porphyry porphyry skarn and intrusion related veins are spatially and genetically associated with granitic porphyries they display typical porphyry type alterations and are formed by magmatic water carbonatites are considered as mantle derived igneous rocks and mo mineralization is associated with magmatic fluids orogenic mo systems occur as fault controlled quartz lodes and the ore forming fluids are medium high temperature co2 rich metamorphic fluids spatially early mesozoic porphyry mo systems are concentrated in the western and middle part of the qinling orogen whereas carbonatite veins and orogenic mo systems are located in the northeastern part temporally orogenic mo deposits were the earliest mainly between 220 and 250 ma followed by carbonatite veins mainly between 205 and 225 ma and porphyry mo systems mainly between 190 and 205 ma with some exceptions there is generally a trend of decreasing deposit ages toward the western part of the orogen detailed comparison reveals that the early mesozoic porphyry mo systems in the qinling orogen are different from the late mesozoic ones in terms of their economic importance spatial distribution ore causative intrusions hydrothermal alteration and the ore forming fluids to explain the above geological and geochemical features the triassic qinling is considered as an active continental margin caused by the northward subduction of the mian lue oceanic plate a transition from continental arc to back arc basin is suggested for the formation of triassic orogenic and carbonatite vein mo deposits whereas continental arc is host to the early mesozoic porphyry mo systems

magma mixing origin for the post collisional adakitic monzogranite of the triassic yangba pluton northwestern margin of the south china block geochemistry sr nd isotopic zircon u pb dating and hf isotopic evidences petrogenesis of high mg adakitic rocks in intracontinental settings is still a matter of debate this paper reports major and trace element whole rock sr nd isotope zircon u pb and hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves mmes at yangba in the northwestern margin of the south china block these geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle sclm may account for the origin of high mg adakitic rocks in the intracontinental setting the host monzogranite and mmes from the yangba pluton have zircon u pb ages of 207 2 and 208 2 ma respectively the mmes show igneous textures and contain abundant acicular apatite that suggests quenching process their trace element and evolved sr nd isotopic compositions 87sr 86sr i 0 707069 0 707138 and end t 6 5 indicate an origin from sclm some zircon grains from the mmes have positive ehf t values of 2 3 8 2 with single stage hf model ages of 531 764 ma thus the mmes would be derived from partial melts of the neoproterozoic sclm that formed during rift magmatism in response to breakup of supercontinent rodinia and experience subsequent fractional crystallization and magma mixing process the host monzogranite exhibits typical geochemical characteristics of adakite i e high la yb and sr y ratios low contents of y 9 5 14 5 ppm and yb no significant eu anomalies eu eu 0 81 0 90 suggesting that garnet was stable in their source during partial melting its evolved sr nd isotopic compositions 87sr 86sr i 0 7041 0 7061 and end t 3 1 to 4 3 and high contents of k2o 3 22 3 84 and th 13 7 19 0 ppm clearly indicate an origin from the continental crust in addition its high mg 51 55 cr and ni contents may result from mixing with the sclm derived mafic magma most of the zircon grains from the adakitic monzogranite show negative ehf t values of 9 4 to 0 1 with two stage hf model ages of 1 043 1 517 ma some zircon grains display positive ehf t of 0 1 3 9 with single stage hf ages of 704 856 ma these indicate that the source region of adakitic monzogranite contains the neoproterozoic juvenile crust that has the positive ehf t values in the triassic thus the high mg adakitic granites in the intracontinental setting would form by mixing between the crustal derived adakitic magma and the sclm derived mafic magma the mafic and adakitic magmas were generated coevally at late triassic temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the south china block this bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the south china and north china blocks

palaeozoic pre and post collision calc alkaline magmatism in the qinling orogenic belt central china as documented by zircon ages on granitoid rocks based on large scale reconnaissance mapping we identified two calc alkaline plutonic assemblages from the northern qinling orogenic belt central china the older assemblage of intrusions closely associated and deformed coevally with their host volcanic arc sequences seems to represent the fractionation product of basaltic arc magma it therefore predates the collision of the north china block with the central qinling island arc system that developed in a sw pacific type oceanic domain south of the north china block single zircon 207pb 206pb evaporation dating yielded early to middle ordovician ages for this assemblage with a relatively small range from 487 2 1 1 to 470 2 1 3 ma intrusions of the younger assemblage are largely undeformed and truncate structures shown in rocks of the older assemblage they are interpreted as post collisional calc alkaline granitoids single zircon dating provided an age of 401 8 0 8 ma for the younger assemblage consistent with earlier work that defines an age range from c 420 to 395 ma our data favour a tectonic model involving formation and amalgamation of island arc and microcontinent terranes between ca 490 and 470 ma ago to create the central qinling zone which subsequently collided with the north china block prior to c 400 ma ago a late precambrian age of 762 0 0 7 ma for a granitoid gneiss at the northern margin of the yangtze block supports a gondwana affinity for this large continental block

regional variation in moho depth and poisson s ratio beneath eastern china and its tectonic implications abstract eastern china comprises a complex amalgamation of geotectonic blocks of different ages and undergone significant modification of lithosphere during the meso cenozoic time to better characterize its deep structure we conducted h stacking of receiver functions using teleseismic data collected from 1143 broadband stations and produced a unified and detailed map of moho depth and average poisson s ratio of eastern china a coexistence of modified and preserved crust with generally in airy type isostatic equilibrium was revealed in eastern china which correlates well with regional geological and tectonic features crust is obviously thicker to the west of the north south gravity lineament but exhibits complex variations in with an overall felsic to intermediate bulk crustal composition moho depth and values show striking differences as compared to the surrounding areas in the rifts and tectonic boundary zones where earthquakes usually occur systematic comparison of moho depth and values demonstrated that there are both similarities and differences in the crustal structure among the northeast china north china craton south china and the qinling dabie orogen as well as different areas within these blocks which may result from their different evolutionary histories and strong tectonic magma events since the mesozoic using new data from dense temporary arrays we observed a change of moho depth by 3 km and of by 0 04 beneath the tanlu fault zone and an alteration of moho depth by 5 km and of by 0 05 beneath the xuefeng mountains in addition striking e w difference in crustal structure occur across the xuefeng mountains to the east the moho depth is overall has values of 40 km and shows complex and large range variation with values between 0 22 and 0 32 these together with waveform inversion of receiver functions and sks shear wave splitting measurements suggest that the tanlu fault zone is at least a crustal scale fault and the xuefeng mountains may be a deep tectonic boundary between the eastern yangtze craton and western cathaysia block

tectonic evolution of the east qinling mountains china in the palaeozoic a review and new tectonic model east asia is a composite assemblage built up from various plates and the qinling mountains are prominent among the orogenic belts dividing these plates because of its complex and prolonged history interpretation of the geology of the qinling mountains has led to a variety of tectonic models we review and summarize recently published data and previous work and develop a new tectonic model for the eastern part of this mountain range four tectonic zones are recognized from north to south the lesser qinling the north qinling the central qinling and the south qinling zones the lesser and north qinling zones are interpreted as a late proterozoic to early palaeozoic passive continental margin bordering the north china block the central qinling separated from the lesser north qinling zone by a suture zone mainly consists of volcanic arcs and a high grade metamorphic complex the south qinling zone forms part of the northern margin of the yangtze block and has a long latest proterozoic to middle triassic depositional history as revealed by its sedimentary cover available data do not support the hypothesized existence of a major suture divide between the central and the south qinling zones these two zones seem to have collectively formed an early palaeozoic destructive continental margin on the northern edge of the yangtze block with the central qinling as an island arc system and the south qinling as a wide highly attentuated marginal basin late ordovician to early silurian collision between the north china and the yangtze block built up the palaeozoic orogenic belt in the northern part of the qinling and gave rise to the embryonic east asian continent yet the south qinling was transformed to a remnant basin and remained undeformed until the late triassic when the palaeo tethys collision to the south of the yangtze block drove the yangtze block to move northeastwards deformation in a transpressive regime during this late event largely reconstructed the qinling belt and hence further complicated its tectonics

archean granulite gneisses from eastern hebei province china rare earth geochemistry and tectonic implications the granulite gneisses and their retrograded products of the qianxi group from eastern hebei province china have been investigated for their isotope and trace element geochemistry a consistent age of about 2 5 ae has been obtained by the rb sr and sm nd whole rock isochron methods in agreement with the zircon u pb data pidgeon 1980 d y liu unpubl geochemical arguments from initial isotopic ratios isr and ind and elemental distribution patterns have led us to conclude that this age of about 2 5 ae represents the time of granulite facies metamorphism which must have followed closely the primary emplacement of their protoliths previous claims for early archean ages 3 5 ae of these granulites are not substantiated the mineral isotope systematics register an important thermal event at about 1 7 ae roughly corresponding to the time of the widespread luliang orogeny ma and wu 1981 or chungtiao movement huang 1978

tectonics of the qinling central china tectonostratigraphy geochronology and deformation history abstract the qinling orogen preserves a record of late mid proterozoic to cenozoic tectonism in central china high pressure metamorphism and ophiolite emplacement songshugou ophiolite assembled the yangtze craton including the lower qinling unit into rodinia during the 1 0 ga grenvillian orogeny the lower qinling unit then rifted from the yangtze craton at 0 7 ga subsequent intra oceanic arc formation at 470 490 ma was followed by accretion of the lower qinling unit first to the intra oceanic arc and then to the sino korea craton subduction then imprinted a 400 ma andean type magmatic arc onto all units north of the northern liuling unit oblique subduction created silurian devonian wnw trending sinistral transpressive wrench zones e g lo nan shang dan and late permian early triassic subduction reactivated them in dextral transpression lo nan shang xiang shang dan and subducted the northern edge of the yangtze craton exhumation of the cratonal edge formed the wudang metamorphic core complex during dominantly pure shear crustal extension at 230 235 ma post collisional south directed shortening continued through the early jurassic cretaceous reactivation of the qinling orogen started with nw se sinistral transtension coeval with large scale early cretaceous crustal extension and sinistral transtension in the northern dabie shan it presumably resulted from the combined effects of the siberia mongolia sino korean and lhasa west burma qiangtang indochina collisions and pacific subduction regional dextral wrenching was active within a ne sw extensional regime between 60 and 100 ma an early cretaceous andean type continental magmatic arc with widespread early cretaceous magmatism and back arc extension was overprinted by shortening related to the collision of yangtze indochina block with the west philippines block strike slip and normal faults associated with eocene half graben basins record paleogene nne ssw contraction and wnw ese extension the neogene is characterized by normal faults and nne trending sub horizontal extension pleistocene quaternary nw se extension and ne sw contraction comprises sinistral strike slip faults and is part of the nw se extension imposed across eastern asia by the india asia collision

archaeovenator hamiltonensis a new varanopid synapsida eupelycosauria from the upper carboniferous of kansas study of a nearly complete skeleton of a small amniote preserved as part and counterpart in upper carboniferous limestone slabs of the hamilton quarry kansas indicates that it is a new genus and species of varanopid synapsid archaeovenator hamiltonensis gen and sp nov is the oldest known varanopid phylogenetic analysis indicates that it is the sister taxon to all other known varanopids discovery of this new varanopid is important because it improves our knowledge of the terrestrial fauna of the hamilton quarry expands the known diversity of late carboniferous basal synapsids and extends the fossil record of varanopidae deeper within the stephanian varanopidae has the longest fossil record late carboniferous upper middle permian of all known clades of basal synapsids

episodic widespread magma underplating beneath the north china craton in the phanerozoic implications for craton destruction abstract a comprehensive synthesis of u pb geochronology and hf isotopes of zircons from granulite pyroxenite xenoliths entrained in phanerozoic magmatic rocks and inherited xenocrysts from the associated lower crust rocks from various domains of the north china craton ncc provides new insights into understanding the phanerozoic evolution of the lower crust in this craton episodic widespread magma underplating into the ancient lower crust during phanerozoic has been identified throughout the ncc from early paleozoic to cenozoic broadly corresponding to the caledonian hercynian indosinian yanshanian and himalayan orogenies on the circum craton mobile belts the early paleozoic 410 490 ma ages come from xenoliths in the northern and southern margins as well as the central domain of the eastern block of the craton which mark the first phase of phanerozoic magma underplating since the final cratonization of the ncc in the paleoproterozoic the magmatism coincided with the northward subduction of the paleotethysian ocean in the south and the southward subduction of the paleoasian ocean in the north the subduction not only triggered magma underplating but also led to the emplacement of the diamondiferous kimberlites on the craton marking the initiation of decratonization the late paleozoic event as represented by the 315 ma garnet pyroxenite and or lherzolite xenoliths in hannuoba was restricted to the northern and southern margins of the craton correlating with the arc magmatism continuous associated with the subduction of the paleotethysian and paleoasian oceans and resulting in the interaction between the melts from subducted slabs and the lithospheric mantle lower crust the early mesozoic event also dominantly occurred in the northern and southern margins and was related with the final closure of the paleotethysian and paleoasian oceans as well as the collisional orogeny between the ncc and the yangtze craton the late mesozoic ca 120 ma was a major and widespread magmatic event which manifested throughout the ncc associated with the geothermal overturn due to the giant south pacific mantle plume the cenozoic magmatism identified only in the dark clinopyroxenite xenoliths in the hannuoba was probably induced by the himalayan movement in eastern asia and might also have been influenced by the subduction of the pacific ocean to some extent these widespread and episodic magma underplating or rejuvenation of the ancient lower crust beneath the ncc revealed by u pb and hf isotope data resulted from the corresponding addition of juvenile materials from mantle to lower crust with a mixing of the old crust with melts the process inevitably resulted in the compositional modification of the ancient lower crust similar to the compositional transformation from the refractory lithospheric mantle to a fertile one through the refractory peridotite infiltrated melt reaction as revealed in the lithospheric mantle beneath the craton

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

nature and evolution of the lower crust in the eastern north china craton a review abstract in this paper published data for granulite terrain rocks exposed at the surface lower crustal xenoliths and mesozoic intermediate felsic igneous rocks from the eastern north china craton ncc are integrated to constrain the nature and evolution of the lower crust in this area u pb zircon dating shows that the protolith ages for most of the granulite terrain rocks are 2500 to 2600 ma and that many of them experienced 1800 1900 ma metamorphism lower crustal xenoliths entrained in volcanic rocks with ages varying from 460 to 10 ma suggest that the lower crust is dominated by neoarchean rocks although there may be minor rocks with ages of meso to paleoarchean 3000 ma 45 ma and possibly 1900 ma locally the mesozoic intrusive rocks although varying from diorite to granite and spanning from triassic to cretaceous contain 2500 ma inherited zircons and have magmatic zircons with hf crust model ages t dm hf c ages of 2500 2700 ma and whole rock sr nd isotopic compositions falling within the field of the granulite terrain rocks pointing to their derivation by the melting of neoarchean lower crust the combined data for the granulite terrain rocks lower crustal xenoliths and mesozoic intermediate felsic igneous rocks indicate that the present lower crust is dominated by rocks with neoarchean ages and is intermediate to mafic in composition i e sio 2 87 sr 86 sr i e nd t and e hf t of the lower crust at 130 ma are considered to be 0 705 to 0 716 10 to 28 and 13 to 28 respectively the e nd t range is very different from that proposed previously 32 to 44 the large range of e hf t for the lower crust implies that significant e hf t variations for magmatic zircons from the mesozoic intermediate felsic igneous rocks do not necessarily reflect mixing of mantle and crustal magmas as commonly thought instead they may reflect heterogeneity in the ancient lower crust given that the voluminous mesozoic intermediate to felsic igneous rocks in the eastern ncc are derived dominantly by partial melting of the archean lower crust it requires that a large amount of archean lower crust be restitic a restite origin can explain some of the hannuoba granulite xenoliths having higher mg than the granulite terrains it may be applicable to other parts of the world

crustal structure in the junction of qinling orogen yangtze craton and tibetan plateau implications for the formation of the dabashan orocline and the growth of tibetan plateau

timing of synorogenic granitoids in the south qinling central china constraints on the evolution of the qinling dabie orogenic belt abstract the ca 400 km long granitoid belt in the south qinling is believed to be a synorogenic product of the collision between the north and south china blocks along the qinling dabie orogenic belt in central china single and multigrain zircon u pb dating of six of these granitoid bodies indicate that the granitoids were formed between documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 220 pm 1 end document and documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage m

remnants of a neoproterozoic collisional orogenic belt in the core of the phanerozoic qinling orogenic belt china abstract nw se trending paleotectonics are recognized from the wnw ese trending qinling complex in the core of the qinling belt they consist of trending stratum boundary shear zones deformed linear plutons and ophiolite the field relationships and ages of the deformation magmatism and metamorphism suggest that the paleotectonics formed in neoproterozoic the ages of deformed and undeformed plutons approximately constrain the peak deformation during 958 889 ma both the two trending tectonics constitute an orogen scale tectonic replacement by s2 phanerozoic wnw ese tectonics of s1 nnw sse trending neoproterozoic similar to structural replacement in outcrop the strong contractional deformation high pressure metamorphism and granitic magmatism that evolved from syn collisional s type to post collisional i type and to post collisional a type as well as regional extension at 885 700 ma in qinling display a cycle of a collisional orogeny thus the nw se trending paleotectonics could be remnants of a collisional orogenic belt the belt originally could trend nnw sse if reworking and modification of paleozoic and or mesozoic nne ssw contractional deformation are eliminated accordingly assemblage and breakup of some continental blocks occurred in neoproterozoic in qinling which seems to correspond to assemblage and breakup of rodinia supercontinent in the world this study provides new insight into the evolution of the qinling orogenic belt and the reconstruction of proterozoic supercontinent of china and also presents a case study of recognition of an old orogen from a young orogen

crustal structure and tectonic study of north china craton from a long deep seismic sounding profile abstract in this paper we present the results of modeling refracted and reflected waves employing 2d ray tracing for a 1600 km long refraction reflection profile that extends e w across the entire north china craton ncc the resulting p wave velocity models reveal substantial structural variations among different geological units of the ncc the taihangshan mountains are the geographical and geological demarcation lines of the ncc in the eastern part of the ncc bohai bay basin the craton appears to be destructed as evidenced from the widespread of crust thinning 30 km development of large sedimentary basins and weaken of crust mantle boundary coupling these features indicated that cenozoic tectonic reactivation succeeded mesozoic destruction in the eastern part of ncc in the western part of ncc the craton remains high crustal p wave velocity stable cold and strongly crust mantle boundary coupling crust the tanlu faults and the taihangshan faults are the tectonic boundaries in the ncc and play important roles in the process of cratonic destruction the structural feature of the east secondary tectonic differential settlement in the process of basin extension is that the most faults in bohai bay basin are the boundaries between the depression basin and uplift area comprehensive study found an apparent difference between the north and south part in eastern ncc destruction in that the large scale cenozoic stretching mainly occurred in the bohai bay basin and the weakly existed only in the local area and in hehuai basin we discussed the crust and upper mantle tectonic evolution of ncc under the action of the convergence of regional tectonic stress field in different geological periods since the mesozoic era the plate movement around ncc affects the lithospheric and crustal modification of the eastern north china and controls the deep structural background of seismicity and metallogenesis

tectonics of the qinling belt build up and evolution of eastern asia the structure and history of the qinling range which separates the north and south china cratons are a key to the tectonic evolution of eastern asia the mountain range is composed of a northern palaeozoic belt and a southern mesozoic belt the palaeozoic orogeny probably involved the closure of an oceanic basin along a north dipping subduction zone followed by south vergent obduction of ophiolites and intracontinental thrusting during the devonian the mesozoic belt appears to be the product of purely intracontinental shallow crustal deformation over a large decollement with at least 100 km of horizontal transport since the devonian both belts have been sliced by huge east west sinistral strike slip faults

mount st helens potential example of the partial melting of the subducted lithosphere in a volcanic arc mount st helens 50 km to the west of mount adams and the main cascade volcanic chain is only 80 km above the subducting oceanic lithosphere the elevated temperatures off the subducting slab because of the close proximity of the juan de fuca ridge to the trench may induce slab melting at a depth of 80 km dacites from mount st helens have geochemical compositions off magmas that are derived by direct partial melting of metamorphosed basalts at high pressure i e relatively high ai al2o3 15 at 70 sio2 low y and yb because of garnet and amphibole stability in the source low sc and high sr and eu trace element modeling of the partial melting of mid oceanic ridge basalt morb from the juan de fuca ridge that yields a hornblende eclogite residue can reproduce the mount st helens data results off the model are quite distinct from data derived from the mount adams volcanic rocks in contrast mount adams is 135 km above the subducting slab and is associated with normal arc magmatism believed to be derived from the mantle above the subducting plate the cascade are has been active in its present locality because of oblique subduction for the past 7 m y the major volcanoes along the arc have existed for at least 500 ka but mount st helens has existed for 40 ka we suggest that the subducting plate may have reached elevated temperatures because of the approach of north america to the juan de fuca ridge at 40 ka which initiated melting of the slab

paleozoic metamorphism in the qinling orogen tongbai mountains central china the collision of the sino korean and yangtze blocks to form a significant part of china is recorded in the qinling tongbai and dabie mountains radiometric ages of the ultrahigh pressure metamorphic rocks in the south qinling orogenic belt suggest that subduction and collision took place during the triassic period our new 40 ar 39 ar geochronology of units in the north qinling orogenic belt confirms that high grade metamorphism and deformation took place also during the silurian devonian and carboniferous periods these results imply that the amalgamation of eastern china was a multistage process extending over at least 200 m y

extrusion tectonics inferred from fabric study of the guanzizhen ophiolitic melange belt in the west qinling orogen central china abstract the recently identified guanzizhen ophiolitic melange belt gomb in the west qinling orogen is regarded by many geochemical and geochronological works as the westward extension of the shangdan belt in the east qinling that separated the north and south china blocks the gomb trends generally nw se and comprises ophiolites and forearc volcanic sedimentary complex the liziyuan group ductile shear zones with various widths are a striking structural feature of the gomb and studying them could contribute to a better understanding of the tectonic processes of the collisional orogeny in this area here we characterize the deformation style of these shear zones by combining the low field anisotropy of magnetic susceptibility ams data microstructural and field observations the new dataset demonstrates that the deformation of these ductile shear zones is dominated by dextral transpressional shearing under low amphibolite grade condition combining the dextral transpression of the gomb and the coeval sinistral transpression of the shangdan belt in the east qinling we propose a late triassic bilateral extrusion tectonics of the qinling orogen in response to the collision between the ordos and sichuan blocks

silurian devonian provenance changes of south qinling basins implications for accretion of the yangtze south china to the north china cratons abstract the north china and yangtze south china cratons are the most important tectonic units of china the two cratons finally coalesced through collision with the formation of the e w trending qinling mountains that are considered to be the eastern extreme of the paleotethys the north qinling and south qinling belts constitute the southern active margin of the north china craton and the northern passive margin of the yangtze craton respectively we have studied chemical compositions of 573 fine grained clastic sedimentary rocks of all the early proterozoic to neogene ages from the north china craton the yangtze craton and the north and south qinling orogenic belts as well as their ten tectonostratigraphic subunits the purpose of this study is to investigate possible provenance changes that are expected to have been recorded in the phanerozoic clastic sediments of the south qinling basins if north and south china had approached and accreted because the north china craton north qinling belt and yangtze craton have distinct upper crustal and provenance compositions both the north china and yangtze cratons and the north qinling belt are characterized by a relatively uniform non evolutionary composition of clastic sediments in la th sc co systematics during post archean times they can be however distinguished from each other in provenance in terms of the la th sc th th co as well as k2o na2o ratios in contrast there is a significant provenance change in the silurian devonian clastic sediments from the baiyun zhashui liuba yunxian and western xichuan subunits of the south qinling belt the late sinian 700 600 ma cambrian and ordovician pelites show characteristics that are consistent with the yangtze craton being high in la th 3 5 10 and sc th 1 4 2 4 and low in th co 0 2 1 0 and lacking la th ratios 3 5 these are accompanied by the increasing immaturity of the silurian devonian clastic sediments illustrated by the decreasing k2o na2o ratio which is attributed to the near source of the north qinling belt dominated by low k2o island arc igneous suites and immature metagraywackes we propose that the south qinling basins except the eastern xichuan subunit had been in a close proximity and accreted to the north qinling belt in silurian devonian times the result agrees well with recent studies of benthic faunas granite distribution age of granulite and clastic provenance directions of the south qinling devonian sediments but it is apparently inconsistent with the age of coesite and diamond bearing ecologite in the dabie mountains to the east and with paleomagnetic data both suggesting an important triassic jurassic collision event between north and south china it is likely that the silurian devonian and the mesozoic represent two different episodes phases of accretion and collision between north and south china new models regarding the phanerozoic tectonic evolution of east china are needed to reconcile the contrasting evidence