Geochemistry, Geochronology And Its Tectonic Significances Of The Boyang Quartz Syenite-Porphyry

A set of volcanic rocks consisting mainly of rhyolite (namely Xiaohezi volcanic rocks, XVR) develop in the Boyang area of Tianshui city in the western Qinling orogenic belt, and a set of subvolcanic rocks consisting mainly of quartz syenite-porphyry (namely Xiaohezi subvolcanic rocks, XSR) near the south of the Xiaohezi volcanic rocks. Although all of the researchers consider that both of the XVR and XSR are the synchronous and comagmatic igneous rocks, this viewpoint has never been confirmed directly by the geochronological and geochemical data. In recent rears, the formation age of the XVR has been changed from the original Eogene to Late Triassic, but the formation age of the XSR hasn’t been determined, and the geochemical features of the XSR has also been studied poorly.The thesis researches into the petrology, geochemistry and geochronology of the quartz syenite-porphyry (part of the XSR) outcropping in the western Banpo village of Boyang town, and gains the following main conclusions.(1)The quartz sysenite-porphyry and XVR are the products of the synchronous and comagmatic volcanism of Late Triassic.Both of Boyang quartz syenite-porphyry and XVR are characterized by the similar geochemical features, such suggests that they are resulted from the comagmatic volcanism. Zircon U-Pb LA-ICP-MS dating on the magmatic genetic zircon yields and age of209.3±5.8Ma (MSWD=16), representing the formation age of the Boyang quartz syenite-porphyry, and such formation age is basically consistent with the formation ages (211±2.6Ma,214.6±3.8Ma) of the rhyolites of XVR which have been determined respectively by Xu Xueyi et al.(2007), Su Xiaohong (2008) and Qiu Xinwei (2011).So SVR (rhyolite) and XSR represented by the Boyan quartz syenite-porphyry are really the products of the synchronous and comagmatic volcanism of Late Triassic.(2) The post-collisional extensional setting is dominant during Triassic in the northern part of the Western Qinling orogenic belt. The granitic igneous rocks being consistent with XVR and XSR develop widely in the western Qinling orogenic belt. The geochemical features of the Boyang quartz syenite-porphyry, combined with geochemical features of the XVR, suggest that the magmatic activity of the Late Triassic should arise under the post-collisional extensional setting.(3) It is seems to exist the perfect sedimentary response in the Ordos basin to the regional extension of Late Triassic in the northern part of the western Qinling orogenic belt.A depocenter of Yangchang Formation of Upper Triassic occurs in the south-western margin where is the most close to the northern part of the western Qinling orogenic belt, and it is possibly that Xiaohezi volcanic activity of Late Triassic provide the Ordos basin with the intermediate-acidic tuffaceous material because the Xiaohezi volcanic activity is the only intermediate-acidic volcanic activity of Late Triassic around the Ordos basin up to now.