The Study Of Magma Mixing Of Xiaerpu Granite In West Junggar, Xinjiang

The post-collision granites zone along with the Dalabute fault, of which the Xiaerpu rocks is the most important one, is the most concentrated zone of the West Junggar granites, where develops quite quantity of the Late Paleozoic granites. This dissertation is object on the Xiaerpu granites in the West Junggar, researching systematically from the field outcrop surveying, rock observing under the microscope,petrochemistry, tectonic setting,mather magmas traits, magmas evolvement, forming mechanisms and the contribution to crust vertical accretion. Below show the progresses:l.The Xiaerpu granite composed of the host rocks, mafic microgranular enclaves and basic dykes, intruded into the Carboniferous sediments. The host rocks are composed of (quartz) diorite granodiorite and monzodiorite, gushingly intruding each other; the mafic microgranular enclaves mainly consist of quartzdiorite and diorite porphyrite, very common in all of the type of the host rocks, clear boundary or transitional boundary with host rocks; basic dykes are mainly comprised by diabase and minor diorite porphyrite, clear boundary with host rocks.2. The Xiaerpu rocks develops abundant and typieal petrographical evidence of magma mixing. In field outcrop, dark mineral distribut asymmetrically, and dark mineral aggregation, Weenie mafic enclave, cingulum of ununiformity mixing are developed; mafic microgranular enclave shows the character of strong plastic distortion, have clear boundary or.transitional relationships with host rocks, develope back-vein and the capture feldspar phenocrysts (capture crystal); basic dyke swarm and mafic micogranular enclave accrete closely, distribute in the same direction, and basic dyke swarm develop the capture feldspar phenocrysts of the host rock, also are traversed by the back-vein of host rock. Under the microscope, the abnormal zoning of plagioclase and mang types of the disequilibrium mineral symbiosis are developed in enclave and cingulum of ununiformity mixing, the acicular apatite is developed in enclave.3. The petrochemistry study shows, the host rocks rich in aluminum, calcium and magnesium belong to calc-alkali sub-aluminum or over-aluminum I-style granite, petrochemistricly similar with the mafic microgranular enclaves,but apparently different with the host rocks. The host rocks are rich in calcium and magnesium, whereas the mafic microgranular enclaves are rich in aluminum and ferrous ions. In dioxide-SiO2, FeOT-MgO and R1-R2 figure show a linear relationship in Hack and the mafic microgranular enclaves are above this evolvement line, whereas basic dykes owns.4. The geochemistry research indicates, the host rocks and basic dykes are comparatively deficit in Nb and Ta. The host rocks are similar with I-style post-collision granite, whereas the mafic microgranular enclaves are in chaos with big component varies, which seems due to the different blending degree between these and the host rocks. The host rocks show lowΣRee, little riched LRee, low heavy rear earth element distribution degree, unapparently low Eu and 8Eu consistent with crust-mantle granites; the mafic microgranular enclaves show low ERee, nomal Eu or unapparently low Eu, high rear earth element distribution degree. Figures of denominator as Sr, the mafic microgranular enclaves have a linear relationship with the host rocks otherwise quite different with basic dykes. Moreover, in compatible element and inconsistent element figure, the host rocks and mafic microgranular enclaves show strong double diffusion.5. It was indicated by the study that the age of the host rock, mafic microgranular enclave and basic dyke swam are 292.8±2.0Ma,295.7±8.0Ma and 294.0±5.0Ma, which is consistent with the arrangement of the error.they belong to the Early Permian. These illustrate that they are the production of the same event, which give the most important evidence of the magma mixing.6. Combined with the geological characters of the nearby regions,the petrographic features of the magma mixing and the geochemistrical and isotopical geochemistrical characters. Xiaerpu granite belongs to the Post-collision Granite, which was related with the under-plating of the depleted upper mantle magma, the mantle-derived magma then moved throughout the faults, and convected with acid magma. The different degrees of magma mixing happened in the acid magma, from magma mixed rock to mafic microgranular enclaves, at last, the numerous basic dyke swams. Xiaerpu granite recorded the process of the mgma mixing, which is the rare and typical terrane originated from magma mixing. Besides, that show a mass of mantle magma go into the crust, standing for a important events of growth of the continental crust in West Junggar.