The Causes Of East The Qinling Erlangping Region West Of Zhuanghe Granite Pluton And Formation Mechanism

The Xizhuanghe granite locates in Erlangping area of Xixia County, western of Henan Provinces. It has been considered as mantle-derived granite which was related to the Erlangping Ophiolite. And zircon Pb-Pb evaporation dating of this granite had been employed to constrain the formation age of the Erlangping Ophiolite. Based on detailed field investigation, the thesis focuses its studies on petrography, geochemistry, geochronology and zircon Hf isotope of the Xizhuanghe granite and mafic enclaves within it. The main achieves of the thesis can sum up in the following aspects:1. Petrographic studies found abundant microstructure of magmatic mixing within the mafic enclaves, for instance:(1) the enclaves show porphyritic-porphyaceous texture or diabasic texture while the host granite shows coarse-grained granitic texture; (2) acicular prismatic apatites occurring in the enclaves reflecting rapid cooling of magma; (3) plagioclase phenocrysts from the enclaves display composition zoning; (4) quartz phenocrysts within the enclaves are surrounded by dark colored minerals like biotite and amphibole; (5) plagioclase phenocrysts lie on the boundary of granite and the mafic enclaves.2. Geochemical studies indicate that the host granite has high SiO2 (66.84%～71.57), alkali (K2O+Na2O=6.08～7.41%), Sr (327.04×10-6～613.97×10-6) contents, low Y (12.30×10-6～17.17×10-6) and Yb (1.36×10-6～1.88×10-6) contents, with high Sr/Y ratios (24～38). LREE and HREE of the granite are markedly fractionated.δEu shows slightly negative-obviously positive anomaly (8Eu=0.97-1.34). Primitive mantle normalized trace element spider pattern shows obvious enrichment in LILE and LREE such as Rb, Ba, Th, U, etc. and relative depletion in HSFE and HREE such as Nb, Ta, Ti, etc., displaying geochemical features of O-type adakites which are related to subduction of oceanic slab.Mafic enclaves have relative lower SiO2 (53.38%-56.39%) but high Al2O3 (14.11%～16.78%), MgO(MgO=3.24%～3.9% and Mg number is (45.1～51.0) and alkali (K2O+Na2O=5.92%-7.43%) contents, belong to the shoshonite-high-K calc-alkalic series. The enclaves have high∑REE (120.91×10-6～220.87×10-6) abundance and LREE-enriched Chondrite-normalized REE patterns with moderate-slightly negative anomaly (δEu=0.65-0.86). Primitive mantle normalized trace element spider pattern shows obvious enrichment in LILE and LREE such as Rb, Ba, Th, U, etc. and relative depletion in HSFE and HREE such as Nb, Ta, Ti, etc., displaying geochemical features of island-arc volcanics.Comparing with the host granite, the mafic enclaves show obviously high REE contents but lower LREE-HREE fractionating, excluding the possibility of mafic enclaves resulted from crystal fractionation. Meanwhile, the granite and the mafic enclaves show similar variation trend of trace elemental compositions on the primitive mantle normalized spider patterns and linear correlation on the covariogram of Harker and other major and trace elements, indicating the magmatic mixing.3. CL images and trace element analysis show that zircons from the host granite and mafic enclaves show the characteristics of magma crystallization. In situ LA-ICP-MS zircon U-Pb dating yield the crystallization ages of 460±0.9 Ma for the host granite and 461±1.4 Ma for the enclaves.4. Zircon Hf isotopic compositions of the host granite are different from those of the mafic enclaves. ZirconεHf(t) values range from 8.37-11.13 for host granite and 11.65-15.16 for enclaves.5. Studies of petrography, geochemical, geochronology and zircon Hf isotopes show that, the Xizhuanghe granite which located in Erlangping area of the western Henan Province was formed in island arc setting. Mixing of the medium-acid magma produced by partial melting of the early Paleozoic subducting oceanic slab with the basaltic magma derived from partial melting of the mantle wedge resulted in the formation of the Xizhuanghe granite. The mafic enclaves within the granitic pluton may be the enclosed basaltic magma during the magmatic mixing process.