Research On The Petrogenesis,Tectonic And Metallogeny For Mafic Rocks In The Ziyang-langao Area, Shaanxi Province

Many Early paleozoic mafic rocks and some magma mineral deposit related to mafic rocks located in Ziyang-Langao area, Shaanxi. Five section and two magma mineral deposit related to mafic rocks in Ziyang-Langao area were selected to be systematic studied in this thesis. Basic on the field geologic investigation, petrographic examination, electron microprobe analysis, zircon U-Pb chronology, geochemical analysis and Sr-Nd isotopic analysis, the petrogenesis and petrogenic tectonic setting for these Early paleozoic mafic rocks were studied in this paper, and discussed the mineralization for mineral deposit related to mafic rocks.The main rock association of mafic rock in Ziyang-Langao area are deep-semi-deep-shallow sequence, which include diabase, diabase-porphyrite, gabbro-diabase, gabbro and pyroxene diorite. The minerals mostly contain pyroxene, plagioclase, hornblende and include accessory minerals like zircon, sphene, apatite, magnetite. The pyroxene is clinopyroxene, plagioclase include bytownite, and the albitite is the result of albitization for plagioclase. The barometer of pyroxene and hornblend indicate the mafic rocks in Ziyang-Langao area are deep-derived characteristics.The chronology of mafic rock in Ziyang-Langao area is Sillurian, and the geochemistry of mafic rock shows high alkaline and high-Ti characteristic and the parental magma of mafic rocks, which is plotted in the alkaline area for the Si-alkaline diagram, is an alkaline series. Mg#ranging from20to60(most between33-49), indicate a highly evolved magma. Regional mafic rock have primitive mantle-normalized multi-element and Chondrite-normalized REE patterns similar to that of OIB, LREE/HREE from5.45to8.94and (Gd/Lu)N from1.84to5.43, besides obvious Eu abnormality, the basic rocks enriched in large ion lithophile elements (such as Rb, Ba), slightly enriched in Ti, and pyroxene diorite depleted in high field strength elements(such as Sr, Ti and P).Magmatic origin of magnetite, mineral chemical composition and O isotope for the magnetite are all reveal that the regional iron deposit are typical magmatic deposits for Zhamugou and Zhuxihe iron mineral deposit related to mafic rocks. Location of ore body, texture and structure of ore, mineral for different stage and the temperature of Fe-Ti oxide which is lower than that of mafic silicate mineral all indicate that the large amount of Fe-Ti oxides crystallized after the deuteric injection of Fe-rich fluid, and eventually completed the mineralization of regional mineral deposit.Mafic rocks in Ziyang-Langao area are without crustal contamination and have partial melt in low degree (5%-10%), plotted in garnet and spinel transition facies with75-80km depth, indicating the depth of partial melt is high and have deep-derived characteristics. The initial87Sr/S6Sr,143Nd/144Nd and εNd(t) of pyroxene diorite and published mafic rocks in Daba mountains and OIB-like geochemistry of mafic rocks are consistent with enrichment characteristic of mafic rocks,which further indicate that the mantle source for mafic rocks in Daba mountains is a mixture source of HIMU, EMⅡ and EMI. Mantle geodynamic research indicate that the enrichment of mantle maybe related to the subduction.Rock association, mineral characteristic and geochemistry characteristic indicate that the mafic rocks formed in subduction related to the back-arc basin and the production of extention for back-arc basin. The asthenospheric upwelling mixed with the above enriched lithospheric mantle which is modified by the mantle mesomatism. The lithospheric thinning cause the decompressional melting and form large mafic dyke/bed in Daba Mountains.