Late Cretaceous-early Paleocene Magmatism In The Middle And Eastern Parts Of The Southern Margin Of The Lhasa Block And Its Geological Significance

During the Late Cretaceous-Early Paleocene,there have been large area distribution of magmatic rocks in the southern of Lhasa block,and the magmatic rocks distributed in the southern margin of the Lhasa block were considered to be the products of the disappearance of the Neo Tethys Ocean to the collision process between India and Eurasia.The auther studied systematic on the rocks of the Dianzhong Formation?E₁d?volcanic rocks and the middle section of the Mainong area's intrusive rocks in the eastern section of the southern margin of the Lhasa block,to exploring the source of the magma,the petrogenesis of the magma and the tectonic setting of the Late Cretaceous-Early Paleocene in the southern margin provide a basis for the magma evolution and tectonic evolution of the southern margin of the Lhasa block.The data of U-Pb chronology shows the Dianzhong Formation?E₁d?volcanic rock formed in?69.97±0.45?Ma and?66.80±1.10?Ma in the Changguo area in the eastern margin of the Lhasa block.The data of U-Pb chronology shows the Dianzhong Formation volcanic rocks?E₁d?volcanic rock formed in?69.3±4.1?Ma in the Gnnong area in the middle section of the Lhasa block.Combined with the research results of predecessors,the Dianzhong Formation?E₁d?volcanic rocks were formed during the Late Cretaceous-Early Paleocene in the southern margin of the Lhasa block.the Dianzhong formation is mainly neutral andesite and trachyandesite,with a small amount of acidic rhyolite,is a calc-alkalic and high-potassic calc-alkalic series;relatively enriched large-ion lithophile elements,relative loss of high field strength elements,with negative Nd,Ta and Ti anomalyes,showing characteristics of typical arc volcanic rocks.Consistent with the geochemical characteristics of the Dianzhong Formation volcanic rocks in the Linzhou Basin.The volcanic rocks are collected from the Danshiting Formation in the Changguo area in the eastern part of the Lhasa block,but those volcanic rocks in this study belong to the Dianzhong Formation volcanic rocks and have similar geochemical characteristics with the Dianzhong Formation volcanic rocks,so the Danshiting Formation may be is the formation of Dianzhong in the Changguo area.The Mainong mass?small intrusion?located in the middle section of the Lhasa block,and invadeing the Dianzhong Formation?E₁d?.the LA-ICP-MS zircon U-Pb age of the Mainong mass is?63.4±1.3?Ma,the A/CNK=0.87⁰.96 which means they are metaluminous,with low MgO,enriched of large-ion lithophile elements,depleted in high field strength elements,the LREE and HREE are not obvious,weak negative Eu anomaly,Mg#?43.3-55.9?is higher,and the low Sr/Y value,that shows a typical island arc rock characteristics.During the Late Cretaceous-Early Paleocene,the Dianzhong Formation?E₁d?volcanic rocks formed subduction of the Neo Tethys oceanic lithosphere,which induceing by slab roll-back,causing partial melting of the overlying mantle wedge in the eastern margin of the Lhasa block.After the intrusive rock mass also undergoes the above process,and then the partially molten magma is contaminated by the shell source material during the ascending process.