Geochronology And Geochemistry Of Late Cretaceous Dikesin Southern Nanmulin Area,Tibet

In this paper,a systematic study of petrology,geochronology and geochemistry has been carried out to study the late Cretaceous magmatic rocks in the southern part of Lhasa block.The time and space distribution of Tethys evolution before Mesozoic NeoTethys Ocean subduction and continental collision provide more factual evidence.On the basis of zircon U-Pb dating results,the magmatic rocks in the southern part of South Wood Forest occur in the late Cretaceous,with a age of about 91Ma,according to the zircon U-Pb dating results.The geochronological characteristics and tectonic environment are similar to those in the areas near the study area,such as Kequ Kun 94.9-96.2Ma,Azagi 90.5-98.7 Ma-Li,Songkatun 90.3-92.4Ma-Maand Sangri 90-97Ma.and so on,which have been reported in Nanmulin area?92.3⁹8.1Ma?and the areas near the study area,such as Kectun 94.9-96.2Ma.The characteristics of magmatic zircon rare earth elements are consistent,and the general characteristics of zircon rare earth elements of Nanmulin syenite vein show the characteristics of enrichment of heavy rare earth elements?HREEs?,depleted light rare earth elements?LREEs?,obvious positive anomalies of cerium and negative anomalies of europium.In line with the overall high REE content in the whole rock,the zircon REE content of the diorite and diabase veins is higher than that of the syenite vein.The magmatic rocks in Nanmulin area all show positive zircon?_Hf?t??8.7¹4.8?,and have a relatively new crustal model age of 181-569 Ma.The southern part of Nanmulin shows the characteristics of LREEs enrichment and HREEs relative depletion.The diorite host rocks are light and heavy rare earth elements have no obvious difference from dike rocks and the EU anomaly is very weak.The enrichment of large ion lithophile element Lil Esn and the depletion of high field strength element HFSEs have high Pb content.The syenite vein is composed of high potassium calc-alkaline series to potassium metamorphic series,peraluminous rocks,which may be the product of partial melting of aluminum-rich sediments in the crust,and diabase veins are mostly high-potassium calc-alkaline series.A small part is medium potassium calcium alkaline series.In addition,part of the acid syenite is also related to adakite.According to previous studies,the early late Cretaceous magmatic activity in the southern Lhasa block is related to the subduction process of the new Tethys ocean plate.The magmatic rocks in Nanmulin area are characterized by island arc magma,which is a subduction related petrogenetic feature,which accords with the existing data characteristics of the late Cretaceous in the Lhasa block in the south of China.Both diabase and granite form"bimodal"rock assemblage,and the distribution of vein size in the field may represent the product of magmatism in the back-arc extensional setting.This paper holds that the late Cretaceous magmatic rocks of the southern Lhasa block represented by Nanmulin rock mass were controlled by the new Tethys oceanic crust northward subduction and formed in the background of the back-arc extension caused by the subduction.