| Alxa is located in the junction of Central Asian orogenic belt, Tarim plate and North China plate. The geotectonics belongs to the south of the west part of North China plate and connects to the south of North Qilian orogenic belt. Within the long history of geological development in this area, the tectonic features are complicated, magmatic activities are frequent, the metallogenic conditions are superior and the mineral resources are rich. Based on the field research and references in this region, distribution characteristics and tectonic background of genesis of Mesozoic granitic plutons have been studied by using comprehensive analysis of the petrology, geochemistry, isotope geochronology and structural geology etc., in order to provide a reliable petrologic and chronological evidence for magmatism and crust evolution of the early Mesozoic granites in Alxa.Granites are distributed extensively in this area, LA-ICP-MS U-Pb age of zircon of Yaoquan gneissic granites ranges from 1180±16Ma to 1396±18Ma with the weighted average of 1269±90Ma, which indicates the gneissic granites were formed in the late Mesoproterozoic. Yaoquan gneissic granites are showing poor aluminum, magnesium, calcium, titanium, and high silicon andalkali. SiO2 is greater than 70% n all the samples and has an average value of 75.40%, which could be assigned to silicon supersaturated series; K2O is also greater that 4% n all the samples which has an average value of 7.96%, and K2O/Na2O values varies from 1.08 to 1.88; Poor aluminum, calcium and magnesium that Al2O3 content has average value of 12.84%, CaO content average is 1.04%, MgO content is much less than 3% ith average value of 0.18%; Poor titanium that TiO2 average content is 0.17%. A/CNK varies from 0.84 to 0.98, while A/NK value is between 1.07 and 1.81, which indicate high potassium but quasi aluminum. From the rock original mantle trace element standard spider diagram, Ba?Nb?Ta?Sr?P?Ti are showing strongly loss while Th?U?K?Rb are showing enrichment. The total amount of rare earth elements in granite are between 147.0×10-6 and 219.0×10-6 with average value of 187.6×10-6. LREE/HREE is between 1.01 and 2.26 with average content of 1.49, (La/Yb)N is between 2.17-6.33 with average value of 3.75, which means light rare elements enrichment. Europium is strongly negative anomal with ?Eu=0.35-0.44. HFSE is enriched, total Fe2O3 content is greater than 1% and Rb content is high with the maximum value of 203×10-6. Also 10000Ga/Al varies from 2.39 to 2.719. So based on the data analysis above, the granites could be assigned to A type. By combining the regional geological background study, it could be concluded that, the late Mesoproterozoic gneissic granites are the products of crust partial melt, and presence of fractional crystallization of plagioclase during magmatism or residual plagioclase in the source. The granite in this area could be determined as A2 type from the Ai, A2 discrimination diagram which means a post-collisional tectonic evolution. |
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