北山造山带白云山蛇绿混杂岩的地球化学特征、时代及地质意义

白云山蛇绿混杂岩是北山造山带中红柳河-牛圈子-洗肠井蛇绿岩带的一部分,由蛇纹石化二辉橄榄岩、蛇纹岩、辉长岩(堆晶辉长岩、块状辉长岩)、基性枕状熔岩(变玄武岩)及深海-次深海远洋、半远洋相沉积物(硅质岩、板岩)、白云岩等组成。其中辉长岩LA-ICP-MS锆石U-Pb测年结果为496.4±2.2 Ma,指示蛇绿岩形成于晚寒武世;混杂带中玄武岩地球化学特征为洋底玄武岩,兼有洋岛碱性玄武岩(OIB)和洋脊玄武岩(MORB)的地球化学性质,枕状熔岩与硅质岩交互产出,指示其形成环境为深水洋盆。结合区域地质资料,推测该蛇绿混杂岩带为寒武纪洋中脊存在的遗迹。

Geochemical characteristics and U-Pb age of Baiyunshan ophiolite mélange in the Beishan orogenic belt and their geological implications

The Baiyunshan ophiolite mélange is a part of the Hongliuhe-Niujuanzi-Xichangjing ophiolite belt in the Beishan orogenic belt and has a complete sequence composed of serpentinized lherzolite, serpentinite, gabbro (cumulate gabbro, massive gabbro), mafic pillow lavas (metabasalt) and bathyal-abyssal or pelagite-hemipelagite facies sediments (silicalite, slate), dolomite and some other kinds of rocks. Zircon LA-ICP-MS U-Pb dating results indicate that the gabbro has the age of 496.4±2.2 Ma, indicating that this ophiolite was formed in Late Cambrian. Geochemical characteristics of pillow basalts in the ophiolite mélange show dual nature of ocean floor alkaline basalt (OIB) and middle ocean ridge basalt (MORB); in addition, pillow basalts appear alternately with silicalites. All the evidence suggests that they formed in the deepwater ocean basin. Regional geological analyses indicate that the Baiyunshan ophiolite mèlange represents a relic of early Paleozoic middle ocean ridge.