The Petrological Characteristics And The Crust

The study area is located between the north China plate and the Yangtze plate,near the ShangDan suture zone in the Qinling orogenic belt.The Shahewan granite pluton outcroppes in the southwest of Shangluo area.The host rocks with the porphyritic quartz monzonite has a mass of mafic microgranular enclaves in the center and edge.The mafic microgranular enclave is the most important symbol of crust-mantle mixing.The mafic microgranular enclave is an important object for studying magma mixing origin,adding its important tectonic position,therefore,the research on the characteristics of the mafic microgranular enclave of Shahewan granite pluton has a great significance on exploring its formation mechanism,structural evolution of Qinling orogenic belt.This paper has done detailed exploration and discussion from the aspects of mineralogy,petrology geochemistry about the Shahewan granite pluton,which based on the study of geology,chronology from predecessors.The main points we get as follows:1.The mafic microgranular enclave,with SiO₂ content of 51.78%-60.15%,belongs to quasi-aluminous and high potassium Ca-alkaline series-Shoshonite series,which has the characteristics of similar potassium content,higher iron,magnesium and calcium,lower differentiation degree compared with the host.The mafic microgranular enclaves are enriched in LILE?K? Rb?,LREE,depleted in HFSE Nb? Ta? P?Ti.The rare earth elements present the right distribution curve,which is roughly paralleled with the host.2.There are a lot of mafic microgranular enclaves in Shehewan granite pluton,which are rich in melanocratic minerals and have the typical structure of magmatic rock.There developed acicular apatite and feldspar phenocrysts,quartz phenocryst included by microgranular enclaves,these are the products of magma mixing.The characteristics of some trace elements also show that there are both source magma and mantle derived magma during the formation of rock mass.3.The main constituents of the clinopyroxenes show the approximate band characteristics in the mafic microgranular enclave.We get the rough formation depth of 38.89-62.8km of the mafic microgranular enclave calculated by the geobarometer,which is near the boundary of crust and mantle.Then it can infer the early crystallization and differentiation process of the mafic microgranular enclaves.The first stage is generating early products by crystal differentiation of pyroxene and other minerals: the mixture of high temperature basic magma and lower temperature acidic magma caused rapid crystallization differentiation of high temperature basic magma,and some basic mineral assemblage like monoclinic pyroxene started to crystallize in this stage.The second stage is the process of mafic microgranular enclaves crystallizing : Afterward the magma experienced crystallization differentiation,mafic microgranular enclaves invaded upward with residual partial basic and acidic magma.The process of pressure gradually reducing in the formation of the monoclinic pyroxene,which infers that the monoclinic pyroxene continue to invade after the beginning crystallization4.Based on the construction of the geological background and the related tectonic environment discrimination diagrams,we can infer that the Shahewan pluton should be formed under the post-collisional transformation tectonic environment from compression to extension,caused by crust-mantle mixing action that mantle magmas mix with crustal magma in the boundary of mantle and crust after the former invaded into the lower crust.