青海刚察泉吉地区中酸性侵入岩年代学特征及其意义

青海刚察泉吉地区中酸性侵入岩发育,为了进一步查明侵入岩的特征、侵入时代和形成环境,对其进行了地质学、岩石地球化学及年代学研究。LA-ICP-MS锆石U-Pb定年结果表明,研究区中酸性侵入岩侵位年龄主要集中在430~455 Ma,指示岩体为晚奥陶世—早志留世,指示两个期次岩浆产物,侵位于巴龙贡噶尔组的花岗闪长岩测年结果显示锆石206Pb/238U值为444±11 Ma,指示该花岗闪长岩体的侵位时代属晚奥陶世,因此限定了巴龙贡噶尔组(Sb)时代上限应该早于晚奥陶世。从岩石地球化学特征来看,花岗岩类表现为富钾、准铝—过铝质,富集大离子亲石元素Rb、K和Ba,亏损高场强元素Nb、Ti和P,具弱铕负异常;闪长岩类表现为富钾、准铝质、铕负异常特征不明显。从形成环境来看,研究区中酸性侵入岩均具有大陆碰撞环境的特征。结合区域地质构造环境及前人研究成果,研究区侵入岩是俯冲—碰撞环境下晚奥陶世-早志留世的加里东期产物。

Geochronology and Significance of Intermediate-acid Intrusive Rocks in Quanji Area,Gangcha, Qinghai

There are many intermediate-acid intrusive rocks in Quanji area. In order to better understand of the characteristics, intrusion age, and formation environment, we have studied the geology, geochemistry, and geochronology of intermediate-acid intrusive rocks in Quanji area. LA-ICP-MS zircon U-Pb results show that the intrusion age of granitoid rocks is 430~455 Ma, indicating that magmatic intrusion took place in different phases during Late Ordovician to Early Silurian in the study area. U-Pb age data show that the granodiorite was formed at 444±11 Ma, i.e., late Ordovician. Thus, the Balonggonggaer Formation should be earlier than the Late Ordovician. Geochemical characteristics show that granitoid is rich in K and excessive in Al, enriched in LILE(Rb, K and Ba) , depleted in HFSE (Nb,Ti and P) with a weak negative europium anomaly. Diorites are rich in K, and quasi-aluminous and have a negative europium anomaly that is not obvious. Granite and granodiorite have a weak europium negative anomaly. Quartz diorite is not obvious. Intermediate-acid intrusive rocks have characteristics of continental collision in terms of formation environment. Based on regional tectonic setting and previous research results, we infer that intermediate-acid intrusive rocks were produced in a subduction or collision setting during the Caledonian orogeny from Late Ordovician to Early Silurian.