The Study On The Dongjiangkou Granitic Pluton And Its Mafic Microgranular Enclaves In The South Qinling Terrain

Various types of mafic microgranular enclaves(MME)in granitic rocks,through the study of the mafic microgranular enclaves(MME)in the granitic rocks,we can reveal the information of the deep crust material composition,crust-mantle material and energy exchange.And it can provides new constraints for petrogenesis,mineralization and tectonic setting.Therefore,the research on MME has been paid much attention by the scholars in the field of Geology.At present,the main causes of MME in the academic field are residual rock fragments of granitic host magma metasomatism;early condensation of rock mass;in the case of magma,the relatively concentrated or dispersed in the dark minerals;the refractory material residue produced by the partial melting of the rock in the source area;mixing by felsic magma which produced by partial melting of the crust and mantle derived magma.In the research of this subject,the study is carried out to research the mafic microgranular enclaves in the South Qinling terrain.In this paper,through the systematic sampling,combined with laboratory systematic experimental analysis of geochemistry and mineralogy,to explore the genetic types and tectonic background of xenoliths,provide new constraints for the host granite tectonic background,origin rocks and regional tectonic magmatic evolution model.Through the research,we get the main points as follows:(1)The morphology and size of the MME contained in the granite of the South Qinling terrain orogenic belt are quite different,and the density is very different,there are significant differences in composition and microstructure.Most of the MME are diorite,and mainly concentrated in the granodiorite,monzonitic granite,hornblende gabbro and quartz diorite.(2)MME is rich in plagioclase,amphibole,biotite and other minerals,while there are some quartz,potassium feldspar,etc.Compared with the host rock,the MME contains more dark minerals,host rock quartz,potassium feldspar content was significantly higher.In addition,there are significant differences in the composition of the enclaves,and uneven distribution in the host rock.(3)Most of the enclaves and host rocks adjacent to the site to see the obvious condensation side,the two have a clear boundary.Most of the enclaves have a representative magmatic structure,and the minerals in the enclaves are predominantly oriented.The celestial quartz captive crystal contained in the enclaves is in the middle of the amphibole and biotite,and the two are wrapped in plagioclase,and also contain a lot of needle-like apatite.In addition,the oblique feldspars in the enclaves are particularly developed to see more regular kernels and irregular accretionary tectonic.(4)Based on the analysis of the mineral compositions of plagioclase,hornblende and biotite by electron probe,it is found that most of the plagioclase in the enclaves belongs to the middle feldspar,and the An value is higher than that of the host rock,and the variation range is larger.In addition,the central ring shows obvious rhythmicity,indicating that the crystalline environment is changing.The amphibole is almost all of the amphibole and amphibolite,Ti content is very low,Mg and Fe content is basically the same,is a certain magnesium element hornblende,indicating that it may have mantlem characteristics.The biotite in the MME is a magnesium-rich biotite,according to the content of which MgO analysis shows that the body may be crust-mantle mixed cause.(5)From the perspective of geochemical composition,most of the enclaves are neutral rocks,belonging to the calc-alkaline weak aluminum series.REE content is not high,are light rare earth enrichment,heavy rare earth loss type;all enclaves were negative Eu anomalies,but the rock inside the Rb,Th,La and other large ion lithophile elements are enriched,and so on showed a loss State,fully proved that the enclaves in this area is characterized by shell-based.(6)The zircon U-Pb crystallization age of the enclaves and host rocks is 222±6Ma and 223±5Ma,the difference between the two data is not large,that is to say,the two are almost produced in the same age.This evidence denies that the dark matter Xenoliths or refractory residues.Through the above analysis,it can be seen that mafic microgranular enclaves in the granite are not the xenoliths of the surrounding rock,the magmatic rock structure and the enclaves are also proved that it is not a melting residual body,which is caused by magma mixing.