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The Geological And Geochemical Characteristics Of Xumai Ultramafic Rock

Posted on:2011-02-16Degree:MasterType:Thesis
Country:ChinaCandidate:J Q LiFull Text:PDF
GTID:2120360308959237Subject:Institute of Geochemistry
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The ophiolitic melanges of the Jinshajiang zone exposed in Deqin county in the Northwest Yunnan province and Derong county in the Southwestern Sichuan province, and distributed from north to south in turn in Tongduo, Dongzhulin, Shusong, Baimangxueshan, Xumai, Gajinxueshan and Rongjiao area, extending more than 300 kilometers long and usually a few kilometers wide. Its lenticular plane intermittent north-south distribution, form the main body of the Jinshajiang suture zone. Xumai ultramafic rock is one of several large-scale and the better surface ore in the Jinshajiang suture zone, and intrudes into the Gajinxueshan group of the Permain (Pg).Through comprehensive analysis of the system by geology, petrology and geochemistry of Xumai ultramafic rock, and study of the rocks'major and trace element geochemical data and tectonic setting an initial understanding of the following is mainly obtained:(1) Xu Mai irregularly shaped rock is of long flat strip, with the section being of oxbow-shaped asymmetry. Rocks are in frequent contact position with the outer alteration zone, and the rock are in intrusive contact, was the fault of local contacts. Xu Mai rock suffered a degree of serpentinization and carbonation.(2) Analysis results of major elements show that SiO2 content ranges between 35.27%~42.25%,TFeO content between 5.072%~9.083%, and MgO content between 34.85%~38.16%. The overall content features are similar to the Troodos of harzburgite composition of major elements geochemistry and having the features of low TiO2 (in TiO2 <2.5% for the standard) and low P2O5 features. Mg# is between 80.00~87.95, its rock consolidation index (SI) is 79.52~87.92, rittmann indexδis less than 0.001, Na2O + K2O is between 0.0022%~0.0373%, and alkalinity ratio AR is between 1.03~1.12, being a calcium base of rocks. In the (K2O+Na2O)-SiO2 (TAS) diagram, SiO2-Nb/Y diagram, Zr/TiO2-Nb/Y diagram and SiO2-AR(alkalinity ratio) diagram, Xumai ultramafic rock mostly behave as the calc-alkaline rock series. Rock m/f is between 7.06~13.4, being a magnesium ultramafic rock. In the FMC diagram, all samples of Xumai ultramafic rock fall in the mafic poor area of calcium.(3) The transition elements Ti and V of Xumai ultramafic rock are significantly lower than the corresponding value of the primitive mantle, while Cr, Co, Ni are similar to the original mantle, the primitive mantle normalized element distribution patterns is the asymmetric "W" type, Ti and Cu form two distinct negative anomaly "valley", the partition characteristics of the residual mantle. The large ion lithophile elements (LILE) Rb, Sr, Ba, U are obviously enriched and high field-strength elments(HFSE) Nb, Ta, Zr are relative loss. The appearance of enrichment, Sr, Rb and other of elements, in particular the loss of Nb, shows some differences with a typical mid-ocean ridge N-MORB, which reflects the geochemical characteristics of island arc enviroment figure.(4) The REE contents of Xumai ultramafic rock are low, compared with the heavy losses of mantle. Chondrite standard curve of rare earth elements reveals LREE enrichment, HREE heavy losses, and slightly right-wing type. LREE has obvious internal fractionation, HREE has relatively weak fractionation and significant positive Eu anomalies. The LREE of Xumai ultramafic rock is enrichment, probably mantle peridotite metasomatism which has been late and alteration. Mantle peridotite REE abundance is very low, subduction of oceanic crust during the release of LREE and LILE-rich plasma in the water flow by adding a large extent changed the nature of rare earth elements of rock, resulting in LREE enrichment.(5)Through the use of abundance ratios of trace elements and major elements, trace elements and more pattern synthesis method, the formation of igneous rock Xumai island are arc calc-alkaline magma, resulting from the reduction onto the edge of the plate closed.
Keywords/Search Tags:Ultramafic rocks, Geochemistry, Tectonic environment, Xumai, Ophiolitic melanges of the Jinshajiang zone
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