The Geochemical Constraints Of The Late Mesozoic Tectonic Evolution Of The Mid-Western Bangong-Nujiang Suture Zone

The Late Mesozoic tectonic evolution of the Bangong-Nujiang Suture Zone(BNSZ)located between the Qiangtang terrane to the North and the Lhasa terrane to the south is still controversial.Given that the great significance of the BNSZ on plate tectonics and orogenic research and economic value of the Bangong-Nujaing metallogenic belt,the dispute hinders our research and development of national economy.This paper try to provide more reasonable explanation for the dispute by studying the Late Mesozoic tectonic evolution of the BNSZ.The subduction polarity and related arc–magmatic history of the Bangong–Nujiang Ocean,which separated the South Qiangtang terrane to the north from the North Lhasa terrane to the south during the Mesozoic,remain debated.This study reconstructs the subduction and evolution of the Bangong–Nujiang Ocean based on U–Pb and Hf isotopic analyses of detrital zircons in samples from sedimentary rocks of the middle–western section of the Bangong–Nujiang suture zone in Gaize County,central Tibet.The Jurassic Muggargangri Group in the Bangong–Nujiang suture zone was deposited in a deep-sea basin setting on an active continental margin.Later Jurassic strata,such as the Sewa Formation,are also widely distributed in the South Qiangtang terrane and represent deposition on a shelf.The Early Cretaceous Shamuluo Formation unconformably overlies the Muggargangri Group and was probably deposited in a residual marine basin setting.The age spectra of the Muggargangri Group contain seven age populations: 2.6–2.4Ga,1.95–1.75 Ga,950–900Ma,850–800Ma,650–550Ma,480–420Ma,and 350–250Ma,which is similar to the age spectra of sedimentary rocks in the South Qiangtang terrane.In addition,the age spectra of the Shamuluo Formation are similar to those of the Muggargangri Group,indicating that both had a northern terrane provenance.This provenance indicates northward subduction of the Bangong–Nujiang oceanic crust.In contrast,two samples from the Sewa Formation yield variable age spectra: the lower sample has age spectra similar to those of the South Qiangtang terrane,whereas the upper possesses only one age cluster with a peak at ca.156 Ma.We conclude that these Jurassic–Early Cretaceous strata record significant information about the northward subduction and related arc–magmatic history of the Bangong–Nujiang oceanic crust.The findings,together with information from the record of magmatism,indicate that the earliest prevalent arc magmatism occurred during the Early Jurassic(ca.185Ma)and that the principal arc–magmatic stage occurred during the Middle–Late Jurassic(ca.170–150Ma).The magmatic gap and scarcity of detrital zircons at ca.140–130Ma may indicate collision between the Qiangtang and Lhasa terranes.The late Early Cretaceous(ca.125–100Ma)magmatism on both sides of the Bangong–Nujiang suture zone was probably related to slab break-off or lithospheric delamination after closure of the Bangong–Nujiang Ocean.The closure time of the Bangong-Nujiang Ocean is still the hot topic of the tectonic research of the suture zone.There are many methods to constrain the closure time.However,one of the most efficient way is the time of change of the sediments from two terranes.Detrital zircon research of the Early cretaceous Duoren Formation,Risong Formation and Duoni Formation shows the evident age groups of the Early Cretacoeus sediments,ca.400–450Ma?ca.800–900Ma and ca.900–950Ma,derive from the southern Qiangtang terrane.This result suggests Lhasa terrane had collided with Qiangtang terrane by this time.The most youngest age of the Early Cretaceous sediments is ca.115 Ma.In other words,Lhasa and Qiangtang terranes had collided at ca.115 Ma.The collision of the Lhasa–Qiangtang terranes was one of the most important tectonic events to affect the Tibetan Plateau area prior to the India–Asia collision.Late Cretaceous molasse sediments,including the Jingzhushan and Abushan formations,are widely distributed along the paleo-Lhasa–Qiangtang collision zone,which is termed the Bangong–Nujiang Suture Zone(BNSZ).The maximum depositional age obtained from detrital zircon dating of both the Jingzhushan and Abushan formations is ca.100 Ma,which indicates that terrane collision had finished by this time.Therefore,the collision between the Lhasa and Qiangtang terranes took place prior to ca.100 Ma,most likely in the Late Jurassic–Early Cretaceous.Based on the present results and previous research,we divide the collision into initial collision(150–140Ma),syn-collision(140–130Ma),and post-collision(130–100Ma)stages.Detrital zircons from both the Jingzhushan and Abushan formations are mainly late Mesozoic in age,along with a minor component of more ancient zircons.Based on the zircon age spectra and Hf isotopic analysis,it is shown that these molasse sediments were derived mainly from late Mesozoic magmatic rocks located around the BNSZ.Consequently,these molasse sediments record post-collision processes.Early Cretaceous alkaline volcanic rocks in the Shemalagou area within the BNSZ,crustal adakites in the Dazhecuo area,and S-type granites in the Darucuo area of the northern Lhasa terrane are magmatic records of post-collision processes of the Lhasa–Qiangtang terranes.Geochemical features of the Early Cretaceous magmatic rocks around the BNSZ are different from those of the Jurassic arc magmatism,which suggests that the Early Cretaceous magmatism was not the result of subduction of the Bangong–Nujiang Ocean,but more likely occurred during the post-collision stage.Given this result and the distribution of Early Cretaceous magmatism,lithospheric delamination is the best model to explain the Early Cretaceous magmatism,which likely took place at 120–115Ma.