Study Of Mafic Microgranular Enclaves

The mafic microgranular enclaves (MME) general distribution in granitoid canprovide plenty of important information, such as composition of materials in the deep earth,crustal-mantle substances and their energy exchanging, and the relation between magmamixing activities and origin of granite; it’s not only related to how to understand thegeological structure, metamorphism and mineralization in crustal rocks, but also a vitalwindow to know magma activities and processes in the deep earth, and is one of thefrontier tasks in current geological research. And the most critical to research MME is thediscussion about its causes. In recent years, the Theory about analysis of enclave mainlyinclude “magma mixing and residual melting”. This paper take the Yuegelu granite inEastern Kunlun belt for exemple, comprehensive research the MME in granitoid.To study the mafic microgranular enclaves in system, we often through survey theform, size, composition, type, distribution, the relationship with host rocks and othermacroscopic characteristics of MME in granite in the wild, summarize the characteristicsand related parameters in system. Then application of advanced test technology to researchthe mineral composition, microstructure, geochemistry, isotope chronology characteristicsand so on, using geochemical methods for tracing the source of the package body, thus toexplore the genetic types of inclusions and tectonic evolution environment. Through thesystem study of petrographic, mineralogical and geochemical in MME of Yuegelu granite,the main conclusions are as follows:(1) The MME is various shapes, size disparity and uneven distribution, thecomposition and structure is also change along with different place; most of MMEcomposition in diorite, and main distribution in granodiorite, monzogranite,amphibole-gabbro and quartz diorite.(2) The MME is mainly composed of plagioclase、hornblende, biotite and so on, alsocontains a small amount of quartz, potash feldspar and some accessory minerals；there aremore dark mineral in MME relative to host granitoid, but on the contrary there are morequartz and potash feldspar; in addition, the MME has a large mineral composition anduneven distribution of content. (3) Most of the enclaves have boundaries in the host rock, and condensation edgevisible; most of enclaves have typical igneous texture, the mineral usual arrange well; itscommonly seen the coquartz ocelli xenocryst surrouned by hornblende and biotite,plagioclases wrapped by hornblende, biotite and other minerals, and acicular apatites occurwidely; in addition, plagioclases with core-rim zoning texture and in the MME, zoningtexture are often found in plagioclases, and contains the idiomorphic regular inner andirregular overgrowths structure.(4) From the electron microprobe analysis data and calculation results of plagioclase,hornblende, biotite: most of the plagioclase are andesine, the date of An is more changethan host rock; addition, with a certain rhythm in core-rim zoning of plagioclases,indicating that the turbulent changes of the crystalline environment. The amphibole mainlybelongs to Mg-Hb and Fe-Tsch-Hb in calc-hornblende; most of them poor Ti, but Mg is thesi similar with Fe, the hornblende belong to slightly rich of magnesium, suggesting thatthere are mantle source. The biotite are rich of Mg, it can inferred that the MMEs has thecharacteristics of crust-mantle mixing from the content of MgO.(5) From the characteristics of the geochemical composition of the MMEs, most areneutral rocks, they belongs to the calc-alkaline and weak aluminum series; it contain onlower REE, benlongs to LREE-rich and HREE-poor type; All enclaves are anomaly of Eunegative, but rich in LILE (e.g., Rb, Th, La), and poor in HFSE (e.g., Ba, Nb, Sr, Ti),shows the the geochemical characteristics of mainly in crust source.(6) The U-Pb dating of MMEs and host rocks are242±6Ma and241±5Marespectively, suggest that MMEs and the host rocks were coeval, declare that MMEs arenot xenoliths or unmelt residual.In summary, the MMEs in Yuegelu granitoids are xenoliths, and they are not partialmelting residual body perceived from igneous texture, petrographical characteristics and soon, but formed by magma mixing.