Geochronological Constraints On The Exhumation History Of The Late Jurassic Intermediate-felsic Magmatic Body In The Rena Co Of The Qiangtang

The Qiangtang terrane,located in the central Tibetan Plateau,is an ideal area for studying the formation and growth history of the Tibetan Plateau.The Rena Co area is located in the southern margin of the central and western part of the Qiangtang terrane.This area is characterized by the occurrence of intermediate-felsic intrusive rocks.In this paper,three magmatic bodies were selected for detailed field geological investigations and related samples were collected.The petrology,zircon U-Pb geochronology,hornblende major element,and apatite(U-Th)/He age analysis were conducted.The thermal evolution history and tectonic process of these Mesozoic magmatic bodies are revealed by the above data.The exhumation rates of the magmatic bodies at different stages are calculated assuming the geothermal gradient.The magmatic bodies sampled in Rena Co area are mainly granodiorite and diorite.The zircon U-Pb ages are 153.2±1.4 Ma,149.09±0.99 Ma,and 150.5±3.3 Ma,respectively,suggesting that they were formed in the Late Jurassic.Based on the zircon trace elements characteristics,all the zircons are continental origin.The zircons of granodiorite are similar to those of I-type granite,while zircons of diorite are similar to those of S-type granite.The zircon Ti thermometers results revealed an average temperature of <800 ℃.The crystallization pressure of two diorite samples obtained by the all-aluminum pressure gauge are 2.80 kb and 2.38 kb,respectively.The corresponding depths were10.58 km and 9.01 km,respectively.The average exhumation rates since the Late Jurassic were 0.071 km/Ma and 0.060 km/Ma,calculated by the emplacement depth and zircon U-Pb ages.Weighted mean ages for apatite(U-Th)/He ages of the two diorites are 52.63±2.87 Ma and 54.02±2.94 Ma,respectively.According to the modelled thermal history and the emplacement depth of the magmatic bodies,two diorite samples have experienced three stages of thermal evolution history.The first stage ranged from ~150 Ma to 58-50 Ma,and the exhumation amount is up to 6-7.5 km,which is related to the crustal uplift in the central part of the Tibetan Plateau during the Cretaceous,which might be induced by the subduction of the Bangong-Nujiang oceanic slab,the collision and post-collision extension of the Lhasa-Qiantang terrane.The relatively fast cooling stage started at58-50 Ma and ended at 45 Ma,with an exhumation rate ranging from 0.107 km/Ma to0.280 km/Ma,which may be related with the remote effect of the India-Eurasia collision.The third stage had an extremely low exhumation rate of 0.002 km/Ma after 45 Ma,which is consistent with the low exhumation rate after mid-Eocene in central part of the Tibetan Plateau,indicating that the Proto-Tibetan Plateau was extended into the west by45 Ma.