Reworking And Recycling Of The Subucted Crustal Materials

The North Qaidam orogen is a typical continental collision orogen that has experienced an entire process from oceanic subduction through continental subdcution/collision to post-collisional collapse.Both oceanic and continental eclogites have been found in this orogen,and the timing of continental subduction and ultrahigh pressure(UHP)metamorphism has been constrained to be about 440-423 Ma.Magmatic rocks formed at different stages are exposed in this orogen,making it an ideal place for us to investigate the evolution of the orogen and study the recycling of subducted crutal materials and crust-mantle interaction from oceanic subduction to continental collision.An integrated study of geochronology and geochemistry has been carried out for the Paleozoic magmatic rocks in the North Qaidam orogen.Our study characterizes the geochemical features and identifies the origin of the magmatic rocks formed at different stages,broadens our knowledge of reworking of subducted crustal materials and crust-mantle interaction in both fossil oceanic subduction and continental subduction channels,and has great significance for understanding the recycling of subducted crustal materials and tectonic evolution of the orogen.Subduction zones are critical places where different layers of Earth interact.Yet it is still poorly understood how subduction initiates and evolves in the early stage.Here we address these issues by investigating the Cambrian mafic igneous rocks from the North Qaidam ultrahigh-pressure metamorphic belt,northern Tibet.Combining data from previous studies and this study,we found that these rocks were formed during a period of 542-513 Ma as revealed by in situ zircon U-Pb dating.They exhibit a change in geochemical features with temporal-spatial variations,including whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as zircon Lu-Hf isotopes.In particular,the ca.542 Ma mafic rocks(group I)are only located in the Xitieshan area.They are calc-alkaline with island-arc type trace element distribution patterns.Their isotopic compositions are less depleted than N-MORB,with whole-rock(87Sr/86Sr)i ratios of 0.7039-0.7044,εNd(t)values of 0 to+2.6,εHf(t)values of+3.7 to+6.6 and zircon in situ εHf(t)values of+2.0 to+7.2.The ca.535 Ma basalts(group II)are located in the Lvliangshan ophiolite;they have slightly increased ratios of large ion lithophile elements(LILE)to high field strength elements(HFSE)and are depleted in light rare earth elements(LREE).They originated from a back-arc basin as suggested by trace element distribution patterns and confirmed by our modelling results.The ca.520 Ma mafic igneous rocks(groupⅢ)exhibit oceanic island basalt(OIB)-like,enriched mid-ocean ridge basalt(E-MORB)-like,and CAB-like trace element features and are distributed in the Xitieshan area.Our modelling results suggest that these geochemical compositions were primarily caused by the addition of different proportions of slab-derived materials(both fluids and melts)and the upwelling of asthenospheric mantle.The ca.513 Ma Lvliangshan ophiolite consists of a rock assemblage of fore-arc basalt(group Ⅳ),boninite,and andesite.The presence of contemporaneous high-Nb basalts(group Ⅳ)and adakitic rocks in adjacent areas suggests that the subducting slab was hot at that time.Our modelling results indicate that the temperature of the molten mantle was as high as 1380℃.Therefore,the ca.513 Ma ophiolite and associated magmatic rocks were formed by the subduction of a back-arc spreading ridge.This paper highlights the identification of ridge subduction in the back-arc basin during the early evolution period of a fossil subduction zone by a combined petrological,geochemical and modelling study.Partial melting of the deeply subducted continental crust in the collisional orogen commonly occurs in the syn-exhumation period.The resultant felsic melts can not only form granitoids,but also modify the composition of the mantle wedge above continental subduction zones.This study investigates crustal anatexis and its role in granitoid formation and crust-mantle interaction during continental collision using new whole-rock major-element element,Sr-Nd-Hf isotope,and zircon U-Pb and Lu-Hf isotope data for the early Paleozoic granitoids from the North Qaidam orogen.Zircon U-Pb dating of the granitoids yields ages between 416±6 Ma and 393±2 Ma,coeval with the exhumation of the deeply subducted continental crust.This indicates that these granitoids formed as a result of syn-exhumation magmatism during the continental collision.Relict zircons in the granitoids have Neoproterozoic and early Paleozoic U-Pb ages that are consistent with the protolith and metamorphic ages of UHP metaigneous rocks in the orogen,respectively.The granitoids have calc-alkaline to high-K calc-alkaline compositions,with total alkali contents of 4.07-8.09 wt%.They are metaluminous to weakly peraluminous with A/CNK values of 0.86-1.14.They are enriched in large ion lithophile elements(LILE)and depleted in high field strength elements(HFSE),yielding arc-like trace element patterns.They have high(87Sr/86Sr)i ratios of 0.7058-0.7195 and negativeεNd(t)values of-7.1 to-1.4 and have whole-rock and zircon εHf(t)values of-2.8 to+2.7 and-8.6 to+4.7,respectively.These data indicate that the granitoids are compositionally similar to the UHP metaigneous rocks in the collisional orogen,suggesting a genetic link between them.This in turn indicates that the granitoids crystallized from magmas generated by the partial melting of the deeply subducted continental crust during its exhumation.The resultant granitic melts not only generated the syn-exhumation granitoids,but also caused metasomatism of the overlying mantle wedge.The continental collision orogen is characterized by the presence of syn-exhumation and post-collisional magmatic rocks,but absence of syn-subduction magmatic rocks.Orogenic peridotite that records the crust-mantle interaction is frequently identified in the UHP metamorphic belt.However,whether the sources of syn-exhumation and post-collisional mafic igneous rocks have changed and whether there is a genetic relationship between these mafic igneous rocks and the orogenic peridotite is mysterious.To address these issues,a combined study involving petrology,whole-rock geochemistry and zircon in situ Hf-O isotopic geochemistry has been performed on the syn-exhumation and post-collisional mafic igneous rocks in the North Qaidam orogen.These mafic rocks were formed at 420 ± 8 Ma to 368± 3 Ma,corresponding to the stages across from syn-exhumation to post-collision.Relict zircons with the Neoproterozoic and Paleozoic U-Pb ages were identified,which are consistent with the protolith and metamorphic ages of the metaigneous rock in this orogen,respectively.They are calc-alkaline to high-K calc-alkaline,enriched in LILEs and depleted in HFSEs,and thus exhibit an arc-type trace element distribution pattern.They have enriched to depleted whole-rock Sr-Nd isotopic composition,with the post-collisional mafic rocks more depleted than the syn-exhumation mafic rocks.Besides,they exhibit similar whole-rock Sr-Nd isotopic composition to that of the orthogneiss and continental eclogite in this orogen,indicating that their mantle sources have been metasomatized by the melts from these metaigneous rocks.Furthermore,the syn-exhumation mafic rocks have similar zircon Hf-O isotopic composition(εHf(t)=-3.17-3.16,δ18O=6.5-7.7‰)to the syn-exhumation granites.Based on the Hf-O isotopic composition,the post-collision maifc rocks can be classified into two groups,with one group having εHf(t)and δ18O values of 8.2-11.4 and 5.9-6.9 ‰,and the other group of 3.3-6.8 and 6.5-7.7 ‰.These compositions are similar to those of the continental eclogite.Given that the lower crust has the same isotopic compositions as the continental eclogite,our results suggest that the mantle sources of the syn-exhumation and post-collision mafic rocks were metasomatized by the melts originated from the upper and lower continental crust,respectively.We also back calculated the composition of the primitive melts and the mantle source of these mafic rocks and compared them with the orogenic peridotite in this orogen.Our results indicate that most of these mafic rocks could be produced by partial melting of the garnet lherzolite and that a few samples could be mainly produced by partial melting of the garnet pyroxenite and peridotite.This is also supported by the indentical zircon in situ Hf-O isotopic composition between the post-collisional mafic rocks and the garnet pyroxenite.Post-collisional granites are abundant in continental collisional orogens.Their source and forming conditions are critical for us to understand the evolution of the collisional orogeny and the differentiation of the continental crust.In this study,we integrate our new results of the post-collisional granites and the data from previous studies in this orogen to address their genesis.These post-collisional granites were formed at about 384 ± 5 Ma to 356±3 Ma,postdating the time of continental collision.Relict zircons yield U-Pb ages varying from 405 to 1414 Ma,most of which are consistent with the age of syn-exhumation magmatic rocks and the protolith and metamorphic ages of the metaigneous rocks in this orogen.They have calc-alkaline to high-K calc-alkaline compositions,with total alkali contents of 4.25-9.39 wt%.Most of them have A/CNK ratios less than 1.1 with a few higher than 1.1,indicating that most of them are metaluminous to weakly peraluminous.Some samples have high Sr/Y and(La/Yb)N ratios and fall into the adakitic rock area.They have low MgO and Ni contents as well as low Mg number,indicating the they were produced by partial melting of the thickened lower crust rather than the delaminated mafic lower crust.They are enriched in LILE and depleted in HFSE,yielding arc-like trace element patterns.They also have high(87Sr/86Sr)i ratios of 0.7060-0.7129 and negative to positive εNd(t)values of-9.2 to+0.6.The majority of these samples have zircon εHf(t)values of-7.56 to+8.38 and a few have values as low as-15.These data indicate that most of the granites have similar isotopic compositions to the UHP metaigneous rocks in the collisional orogen,suggesting a genetic link between them.Some samples with more enriched whole-rock Sr-Nd isotopic compositions than the orthogneiss may have additional contributions from partial melting of metasedimentary rocks.This suggests that the granites were formed from magmas generated by the partial melting of different types of rocks at different pressures during the post-collisional stage when the upwelling of the asthenosphere mantle was caused by the collapse of the subduction-thickened orogenic root.The extraction of low-density felsic melts would not only produce the vast of post-collisional granites but also leave more mafic restites of high density in the orogenic root which caused intra-crustal chemical differentiation.