The Study Of Acidic Volcanic Rock In Shetang-boyang, Tianshui Area, Western Qinlin

Previous studies have shown that there are Cenozoic terrestrial acidic volcanic rocksdeveloping along Weihe fault in Tianshui area, western Qinlin, China. The volcanic rocks are asign of the activity of Weihe fault at western Qinlin. However, the exact formation age of thisvolcanic rocks is still unknown, which constrains the onset time research of Weihe fault. Based ondetailed field investigation, and petrography, geochronology and geochemistry study, this paperhas done systematic research of the volcanic rocks and has finally drawn conclusions of the typeof the volcanic rock, formation age, petrogenesis and tectonic significances, which provideimportant fundamental data for the tectonic evolution of western Qinlin, China.According to the research, we get the main points as follows:1. The zircon U-Pb Shrimp dating of the Shetang-Boyang acidic volcanic rocks in Tianshui area,western Qinlin yields the zircon ages of the four samples are212.4±2.4Ma、210.3±2.9Ma、210.8±3.1Ma、213.4±2.1Ma respectively. Therefore, the formation age of the volcanic rocks is lateTriassic, and the age is not Cenozoic which proposed by the former researchers.2. The late Triassic volcanic rocks are kinds of weakly peraluminous A-type granitoid. Thevolcanic rocks’ types are rhyolite and hypabyssal granite porphyry, developing along Weihe fault.Microscopic thin section analysis suggests that the phenocrysts mainly consist of plagioclase,orthoclase and quartz, and the content proportion of the three is nearly the same. The matrixes arequartz-feldspathic, with rhyolitic structure and felsitic texture, while accessory mineral includescorundum. Geochemistry researches show that the volcanic rocks are rich in Si, alkali, Al andpoor of Mg, Ca and Ti. The contents of HFSE such as Zr, Hf, and LILE such as Rb, U, Th are high,however, the contents of Ba, Sr show the depleted state. The primitive mantle-normalized traceelement spiderdiagram shows right deviation and negative slope characteristics, and indicate thevalley of Sr, Ba, Nb, Ti, P. The front end show the enrichment and the back end show the negative.It shows strong LREE enrichment and fractionation opposite to HREE, and the strong negativestate of Eu, Yb in the chondrite normalized REE partition graph.3. The acidic volcanic rock magma balanced with high-pressure granulite facies originate in thelower crust after orogenic thickening with0.8GPa-1.5GPa pressure and40km-50km depth. with the characteristics of A2type granite which match orogen，the acidic volcanic rock formed bydelamination of the lithosphere of West Qinling orogen at the end of the Indosinian period, soWeihe fault as the migration pathway of magma formed in Late Triassic.