Geological Characteristics Of Cenozoic Volcanic Rocks And Its Geodynamic Implication In Shiquanhe-Gerze Area On Qinghai-Tibet Pleatue

Cenozoic magmatism in the Qinghai-Tibet Plateau can reveal important information about tectonic evolution of the Plateau.In this paper,the Cenozoic volcanic rocks developed in the Gerze Basin in the south of Qiangtang Block,Xiongba Basin in the west of Lhasa Block and Shiquanhe-Zuozuo area of Western Gangdise Magmatic Belt in Qinghai-Tibet Plateau are taken as the main research objects.On the basis of detailed field investigation and previous work,the magmatic petrology,mineralogy,zircon U-Pb chronology,petrogeochemistry and Sr-Nd-Pb-Hf isotope tracing are used.The geological characteristics of Cenozoic volcanic rocks in these area have been identified.The relationship between magmatism,tectonic evolution and uplift of the Qinghai-Tibet Plateau has been discussed,which provides a scientific basis for the Cenozoic syn-to post-collisional process of the Qinghai-Tibet Plateau.After research,the present thesis gets the following innovative achievements and new understandings.?1?The time series of Eocene to Miocene volcanic activity in the southwestern Qinghai Tibet Plateau has been determined.The upper age limit of Meishu Formation has been controversial for a long time.Zircon U-Pb dating of rhyolite and trachyandesite at the top of Meishu Formation shows that the upper age limit of Meishu Formation is 37.4±0.4 Ma.Zircon U-Pb dating results of trachyandesite samples in Gegyal-Xiongba area are 24.9±0.3 Ma and22.9±0.6 Ma,representing that these volcanic rocks were formed from the end of Oligocene to the beginning of Miocene.The volcanic rocks of the Langjiu Formation distributed in Shiquanhe-Zuozuo area are considered to be magmatic products of the Early Cretaceous.Zircon U-Pb dating of rhyolite developed in the Langjiu Formation is carried out.The dating results are quite different from those of predecessors.The zircon crystallization age of rhyolite is 22.3±0.2Ma.It is speculated that the Langjiu Formation may have formed in the early Miocene.?2?Late Eocene volcanic rocks in the southwestern Qiangtang area contain 59.11%to72.73%SiO₂ and 0.98 to 1.47 ASI indices.They are enriched in LREE.They show negative Eu anomalies in varying degrees.They are obviously enriched in Rb,Th and U,and are depleted in HFSE such as Nb,Ta and Ti.The?Hf?t?is+4.6 to+7.6,the average+6.2,and the crustal model age varies from 877 to 1148 Ma,the value of?Nd?t??+0.4⁺2.4?suggests that volcanic rocks of late Eocene were formed in the primary crust and that mantle-derived materials played an important role in the formation of volcanic rocks.Oligocene-Miocene volcanic rocks are potassium basalt series.The SiO₂ content is 55.59%to 76.85%.They are enriched with LREE,showing negative Eu anomalies.They are obviously enriched with Rb and U,depleted with LILE Ba,depleted with HFSE such as Nb,Ta,and Ti.The whole is more neutral than the post collisional potassic volcanic rocks discovered in the Lhasa block.With epsilon?Hf?t?of-27.9^-9.6,and the crustal model ages ranging from 2423 to 4007 Ma.All of the samples placed below the 1.1 Ga crustal evolution line.The?Nd?t?value?-14.2^-10.2?suggests that the volcanic rocks may mainly come from partial melting of the thickened lower crust of the Qinghai-Tibet Plateau,with less contribution from mantle-derived materials.?3?The quartz monzonite porphyry developed in Shiquanhe-Zuozuo area overthrusts the Oligocene red-bed sandy conglomerate,representing a strong tectonic nappe event in the early Miocene.Based on geochronology,geochemistry,Sr-Nd-Pb-Hf isotope tracing and tectonic geology,it is believed that the Qiangtang area in the central part of the Qinghai-Tibet Plateau have already uplifted during the Eocene,and that the Lhasa block in the south reached its present elevation in the Miocene?about 25 Ma?.