Sedimentary Evolution Of The Cretaceous Coqen Basin(Southern Tibet)and Implications For Early Topographic Growth On The Lhasa Terrane

The Lhasa terrane was the last terrane accreted onto the Tibetan Plateau before the India-Asia collision.The tectonic evolution of the Lhasa terrane plays an important role in the formation of the Tibetan Plateau.During Cretaceous time,there were extensive magmatisms and sedimentary basins developed on the Lhasa terrane,because of the subduction of the Neo-tethyan oceanic crust in the south and Lhasa-Qiangtang terrane collision in the north.However,the above issues are still in debate.Recently,with the development of the single-mineral analysis(e.g.,in-situ zircon U-Pb dating and Hf isotopic analysis),it is more effective to investigate the crustal evolution,crust-mantle interaction and provenance analysis in the sedimentary basins.In this study,stratigraphy,sedimentology,petrology,geochemistry,zircon U-Pb dating and Hf isotopic analysis and low temperature thermochronologic study were conducted on the Cretaceous Coqen basin,which provides a new insight into the sedimentary evolution,basin tectonic type,Cretaceous magmatism of the Coqen basin,and implications for the early topographic growth on the Lhasa terrane.The Coqen basin is located in the central Lhasa terrane(Southern Tibet),and bounded by the Gangdese magmatic arc to the south and the Gaize-Selin Co thrust to the north.The Cretaceous strata are well exposed in the Coqen basin,including Zelong volcanic rocks,Duoni clastic rocks,Langshan limestone and Daxiong conglomerate from the bottom to the top.In the Guolong section,Lower Cretaceous Duoni Formation lies conformably upon the Zelong volcanic rocks,and contains fine-conglomerate and sandstone interbedded with siltstone,shale and mudstone,which could represent the deltic deposits.In the Zhukang and Xialong sections,the Duoni Formation is mainly composed of the well-sorted and matured quartz sandstone interbedded with siltstone and shale,which indicate they are deposited in the coastal marine and continental shelf environments.Sandstone modal compositions in the Guolong section reveal that they are mainly composed of abundant volcanic fragments(?80%in average),and minor sedimentary fragments,indicating a magmatic source;while in the Xialong and Zhukang sections,the sandstone have a large amount of the quartz(some samples are above 90%),reflecting the recycled orogen source.About 52%(317 out of 608)of the detrital zircons from the Duoni Formation yield a main age range of 103-155 Ma with the peak at?130 Ma.The relative zircon Hf isotopes have the low 176Hf/177Hf initial ratios and negative ?Hf(t)values.These zircon characteristics are consistent with the Early Cretaceous Zelong volcanic rocks on the central-northern Lhasa terrane,which could be the possible source.In addition,some pre-Mesozoic zircon ages distribute the age ranges of 500-600 Ma,900-1000 Ma,1050-1250,1700-2000 Ma and?2500 Ma,which are similar to the detrital zircons from the Paleozoic strata on the Lhasa terrane,indicating they are recycled from the orogen source.Conformably on the Duoni Formation,the Lower Cretaceous Langshan Formation is an orbitolinid-bearing wackstone and packstone,with the thickness of-800 m.The Langshan Formation can be divided into 7 biozones based on their larger benthic foraminifera analysis,indicating the age ranging from 119 to 98 Ma.Based on the characteristics of the foraminifera and limestone,we interpret the Langshan Formation has been deposited in the high/low energy reefal environment and low energy lagoon.Upper Cretaceous Daxiong Formation distributes along the Dawa Co and can be divided into 5 members.The members 1,3 and 5 are composed of the coarse-medium sandstones,and member 2 is the thick(above 300 m)conglomerate beds.Member 4 is the massive siltstone and shale.Sedimentology indicates the Daxiong Formation has been deposited in the braided rivers and alluvial fans.Clast counts and sandstone modal compositions indicate the magmatic source.Detrital zircon U-Pb ages yield about 80%zircons distribute 92-150 Ma(?110 Ma)and the Hf isotopic results low 176Hf/177Hf ratios and negative ?Hf(t)values.The upper part of the Daxiong Formation(members 3,4 and 5)has a lot of old zircon ages,which distribute the range of 500-600 Ma and 1000-1300 Ma.Combined with the above data,the provenance of the Daxiong Formation is similar to the Duoni Formation,which was mostly derived from the Early Cretaceous volcanic rocks and pre-Mesozoic strata in the central-northern Lhasa terrane.This provenance is consistent with the Paleocurrent data.There are a lot of debates on the tectonic type of the Cretaceous basins on the Lhasa terrane.Based on the above studies in the Coqen basin,we interpret there are two possible models for the Early Cretaceous Coqen basin.Model 1,Lhasa terrane was influenced by the subduction of the Neo-tethyan oceanic crust in the south,which triggered the eruption of the Zelong volcanic rocks.A retro-arc foreland basin was formed behind the Zelong magmatic arc and received the deposits during the Early Cretaceous.Under this tectonic model,the Duoni-Langshan Formations were deposited in the foredeep-forebulge area;Model 2,the southward subducted Bangong-Nujiang oceanic crust beneath the Lhasa terrane can trigger the Zelong volcanic rocks.The Duoni-Langshan Formation recorded the inter-arc basin deposits.During the Late Cretaceous,based on the regional paleogeographic context,we conclude that the Daxiong Formation in the Coqen basin records local crustal shortening and flexure resulting in foreland basin development on the central Lhasa terraneUpper Cretaceous Daxiong Formation lies conformably on the Langshan limestone.During the Late Cretaceous time,two thrust systems with opposite vergence were responsible for transforming the central-northern Lhasa terrane into an elevated mountain range.This process resulted in the evolution from a shallow marine environment(Langshan Formation)into a terrestrial depositional environment(Daxiong Formation)on the southern margin of the central-northern Lhasa terrane,which implies early topographic growth of the central-northern Lhasa sterrane in southern Tibet during?98-91 Ma.Zircon U-Pb ages and Hf isotopes and the whole-rock geochemistry indicate that the Azhang adakitic rocks were derived from the juvenile thickened lower crust.Therefore,the presence of the?90 Ma Azhang adakitic rocks in the north of the Coqen basin could reveal that the crust could experience the thickening and shortening before?90 Ma,which is consistent with the Late Cretaceous Coqen basin evolution.In this study,we believe the Lhasa-Qiangtang collision and the Neo-tethyan subduction could combine to exert the important role in this crust thickening and uplift.In this study,we also analyze the apatite fission track and U-Th/He ages from the Early Cretaceous intrusive rocks in the Coqen basin.The results reveal the fission track ages are 61-40 Ma and the He ages are 60-37 Ma,which is similar to the published data in the Baingoin area.The time-temperature modeling indicates that these intrusive rocks experienced rapid cooling history during Paleocene to early Eocene times.Considering the regional tectonic settings,we suppose this process could be resulted from the India-Asia collision and the Lhasa-Qiangtang post-collision.However,because of the low closure temperature of the apatite fission track(?120-80?)and U-Th/He systems(?80-60?),the apatites from the Early Cretaceous intrusive rocks reached the closure temperature till the Paleocene.