关键词: 流纹岩, 锆石U-Pb测年, 地球化学, Hf同位素, 岩石成因, 大兴安岭西坡, 得耳布尔地区

Abstract:  The Middle Jurassic acidic volcanic-subvolcanic rocks are widely distributed in  Derbur area of the northern part of the western slope of the Great Xing’an Range. The rock assemblages are mainly composed of rhyolite, rhyolitic lithic tuff and quartz porphyry, and no systematic chronological and geochemical studies have been carried out on their diagenetic age, petrogenesis and diagenetic dynamic mechanism. In this paper, petrography, LA-ICP-MS zircon U-Pb dating, geochemistry and zircon Hf isotope studies were carried out on the rhyolites in  Biliya leal-zine polymetallic mining area. LA-ICP-MS single grain zircon U-Pb concordance age is (164.2±1.6) Ma (MSWD=7.7, n=14), belongs to Middle Jurassic. They have relatively high contents of major elements Si, K and Al, and belong to high-K, calc-alkaline and peraluminous rock series. Trace elements show that they are enriched in large ion lithophile elements, weakly enriched in high field strength elements, and obviously depleted in Ba, Sr, Ti and other elements.The ratios of Nb/Ta, Zr/Hf and Th/U are 13.05-22.36, 39.57-43.57 and 3.37-3.51, respectively. There are negative Eu anomalies in REE (the values of  δEu are 0.69-0.76), the ratios of LaN/YbN, La/Ce and La/Yb are  9.95-10.12, 0.48-0.50 and 14.13-15.05, respectively. In addition, they also have low 10 000Ga/Al (2.33-2.34) and (K2O+Na2O)/Cao (9.54-9.96) ratios. According to the calculation of Ti element in zircon, the saturation temperatures of zircon are between 683 and 756 ℃. These characteristics indicate that the Middle Jurassic rhyolite has the property of I-type rhyolite, and the diagenetic magma has the characteristics of volcanic arc or active continental margin magma. The 176Hf/177Hf ratios in zircons are 0.282 853-0.282 990, the corresponding εHf(t) values range from 6.22 to 10.83, and the two-stage Hf model ages range from 734.2 to 474.0 Ma, which are similar to the one-stage Hf model ages (592.6-410.1 Ma), indicating that their diagenetic magma is mainly derived from the partial melting of the juvenile lower crust. The magma was contaminated by crustal materials during the process of emplacement or diagenesis, and constituted the characteristics of “bimodal volcanic rocks” with the contemporaneous intermediate-basic volcanic rocks in this area. Combined with the temporal and spatial distribution of regional magmatic rocks and the evidence of geophysical data, it is further believed that the diagenetic geodynamic background of the Middle Jurassic acid lavas (rhyolite, rhyolitic lithic tuff and quartz porphyry) in this area is closely related to the passive continental margin which influenced by the subduction of the paleo-Pacific plate into Eurasia.

Key words: rhyolite, zircon U-Pb dating, geochemistry, Hf isotope, petrogenesis, western slope of Great Xing’an Range, Derbur area