The Analysis Of Geochemical Characteristics And The Genesis Of Intrusive Rocks In The Middle Part Of Longmen Mountains

The study area is located in the middle part of the Longmen Mountains thrust nappe structural belt between the Yangtze block and the Songpan-Ganzi block.which the geological structure is developed and magmatic activity are frequent.on the basis of extensive collection of previous research data,section surveys and route investigation of intrusive rocks are made in the northern part of the Longmen Mountains to understand the lithology and combinations,its assemblage,spatial distribution,structure of the intrusive rocks and the contact relationship with the surrounding rocks.More samples were collected systematically for rock and ore identification,LA-ICP-MS zircon U-Pb dating and main trace element analysis.the spatial distribution map of magmatic rocks in the study area was plotted,the rock mineral composition,age and geochemical properties of intrusive rocks were analyzed.Therefore,the genesis,material source and tectonic environment of the intrusive rocks in this area are clearly judged.the main understanding can be achieved as follows:?1?According to the difference of lithology and mineral composition,the intrusive rocks in Penguan complex are divided into three kinds of rock series:1.peridotite-gabbro;2.tonalites-trondhjemite;3.monzongranite-syenite.?2?According to the zircon U-Pb chronology of intrusive rocks,the diagenesis age of quartz diorite is 788�40Ma,the diagenesis age of the granodiorite is753�35Ma,which are the product of the Jinning-Chengjiang magmatic activity.The formation of intrusive rocks in the middle part of Longmen Mountains has gone through two stages with the combination of previous studies:one is Jinning-Chengjiang period and the other is Chengjiang period.large scale magmatic activity began in the early Nesoproterozoic era,reaching its peak in mid Neoproterozoic period,There was intermittent magmatic activity in the later period.?3?During the magmatic diagenesis,tonalites-trondhjemite and monzonitic granite-syenogranite are mainly partial melting,At the same time,they have undergone fractional crystallization.granites belong to calcium alkaline quasi-aluminum peraluminous-weak I type feldsic granites,which have similar characteristics and change laws of main and trace elements and the same magma source or evolution characteristics.?4?The evolution of magmatism is closely related to the deep crust mantle process and plate tectonics.The graphic of the lg[w?CaO?/?w?K₂O?+w?Na₂O??]-w?Si-O₂?implies that magma is formed in the post orogenic stage of the transition from the compressive environment to the extensional environment,the Ta/Yb-Th/Yb diagram shows that the granite body is near the oceanic island arc on the active continental margin,indicating that the granite is related to the subduction of the oceanic plate to the Yangtze plate.?5?During the Jinning-Chengjiang period,the horizontal tectonic movement began,the oceanic plates subducted to the Yangtze block,the continental margin arc continental crust was thickened,the continental crust was remelted to form magma,and is mixed with mantle derived magma.it occurs separation and crystallization in the hybridized magma,a large-scale series of calcium alkaline trondhjemite granite was finally formed,which composed of gabbro?diorite?�tonalite.at the same time,the delamination has occurred.The magma density increased with the enrichment of elements such as iron and magnesium and is recirculated into the mantle,a small amount of source area residual was carried to the surface.The convergence of this plate basically formed the Yangtze continent,which is a manifestation of the global Rodinia supercontinent event in the study area.