Mid-Permian Magmatic Activities And Dynamic Background For Basin Forming In Santanghu Area Of Xinjiang

Santanghu Basin, located in the northeastern margin of the Junggar Basin, constitutes the Junggar Region together with Junggar Basin and its surrouding orogenic belts. Santanghu Area developed three sets of volcanic rocks, which belong to Upper Carboniferous Haerjiawu Formation, Lower Permian Kalagang Formation and Middle Permian Tiaohu Formation. The volcanic rocks in Tiaohu Formation are mostly basic, thick and associate with picritic basalts. Regarding Tiaohu Formation volcanic rocks as the research subject of the dissertation, we use many kinds of methods such as mineral petrology, rock geochemistry, isotope geochemistry, isotope chronology and platinum group element (PGE) geochemistry to discuss the the dynamic background of the magmatism during this period. We also studied effects of Tiaohu formation volcanic rocks on Mid-Permian Lucaogou and Tiaohu Formation hydrocarbon rocks. The main achievements are as follows:1) We got the andesite sample of Tiaohu Formation in Santanghu Area for zircon U-Pb dating and its concordant age is (267.1±5.3)Ma (MSWD=1.13), belonging to Middle Permian. Confirmed by seismic sections and compound logging data, the layer has normal sedimentary sequence, so the age of andesite can represent the age of Tiaohu Formation, that is, Tiaohu Formation formed in Middle Permian.2) The volcanic rocks in Tiaohu formation mainly include picritic basalts and basalts, and the main rock-forming minerals are chrysolite, augite, labradorite, feldspar and alkali-feldspar. The Fo values of olivine range from73to78, and the MgO content of melt which is balanced with the highest-Fo-value olivine is nearly8percent. Picritic basalts are characterized by high Mg#value (68～77), and basalts own relatively low Mg#value (41～54). Both of them are like in-plate basalts, being enriched in light REEs but depleted in heavy REEs, but showing obvious negative abnormality of Nb, Ta elements on primitive-mantle-normalized multi-element patterns. The isotopic compositions of Sr, Nd, Hf of Tiaohu Formation volcanic rocks are as follows:εNd(t)=+6.29～+7.01,εSr(t)=-11.2～-2.67,εHf(t)=+14.0～+14.4. We conclude that excess olivine joined in the picritic basalts, and Tiaohu Formation volcanic rocks are formed by partial melting of depleted lithospheric mantle which experienced transformation by subduction fluid earlier, with addition of OIB-type magmas, which may be related to mantle plume. PGE analysis shows that primitive magmas of Tiaohu Formation has experienced sulfide segregation in the source.3) The isotopic compositions of Sr, Nd, Hf for Tiaohu Formation volcanic rocks are similar to ocean island basalts’and some mantle plume’s. In addition, Tiaohu Formation volcanic rocks are enriched in LREEs and LILEs, and falling into intra-plate setting on discrimination diagrams (such as2Nb-Zr/4-Y and Zr/Y-Zr). These characteristics reflect that the forming of Tiaohu Formation volcanic rocks may be related to mantle plume. Besides, Tiaohu Formation volcanic rocks are thick (about1000meters), showing a planar distribution in the basin, which are formed by the interaction between mantle plume and lithospheric mantle. It is more suitable for mantle dynamics mechanism to explain the magmatism and rifting in Mid-Permian Santanghu Area.4) The geochemical characteristics of Tiaohu Formation volcanic rocks (267Ma) show that they had no matter with the melting of upwelling asthenosphere caused by the delamination of lithospheric mantle in post-collisional stage, but related with mantle plume. Statistical analysis of the age for Late Carboniferous-Permian granite and basic intrusion in Junggar Region, shows that the ages of post-collisonal granite are almost older than290Ma and a part of them can extended to280Ma, while the ages of basic intrusion are younger than290Ma on the whole. It concludes for Junggar Region that the post-collisional stage was finished in290Ma, conversion of post-collisional and intra-plate evolution stage occurred in280～290Ma, and the intra-plate evolution stage started in280Ma. That is, in Junggar Region, the extension from Late Carboniferous to Early Permian was mainly caused by post-collisional collapse, while the extension in Early Permian-Middle Permian may be connected with the upwelling of deep mantle on its own.5) The ages of ancient zircons in the diabase and andesite sample in Santanghu Area range from Paleoproterozoic to Neoproterozoic, indicating that the magmatic rocks may pass though the ancient Precambrian crystalline basement. Comprehensive study on the range ofd(t) of granite arround Junggar Basin shows that theird(t) values are more than+5in marjority, some range from+3to+5for granite in Northeast and South Junggar Region, and some are ranging from+2to-4for granite in Northeast Junggar Region. The latter two are obviously contaminated by ancient continental material. The ancient Precambrian crystalline basement exists in Junggar Region.6) Volcanic activities (including mantle hydrothermal) of Tiaohu Formation have not only promoted the thermal evolution of the Lucaogou and Tiaohu Formation hydrocarbon rocks, increasing the amount of hydrocarbon generation and conversion rates, may also brought inorganic oil and gas. Crack development section of Tiaohu Formation volcanic rocks formed fractured oil and gas reservoirs.