藏北尼玛县阿索乡上白垩统马莫勒组的建立及其意义

在青藏高原中部尼玛县阿索乡马莫勒地区识别出一套紫红色辫状河三角洲平原亚相地层,岩性主要为紫红色砂岩中砾岩、紫红色复成分中砾岩、紫红色砂岩细砾岩,局部夹含砾粗砂岩或岩屑杂砂岩。在剖面测制和区域对比的基础上建立了上白垩统马莫勒组。对马莫勒组岩屑杂砂岩层进行碎屑锆石采样和U-Pb定年,得到最年轻的碎屑锆石年龄为99Ma。碎屑锆石主要存在3个年龄峰值,其中主峰值在125Ma左右,代表了班公湖-怒江中特提斯洋演化的岩浆活动,其他2个弱峰值分别为550Ma和980Ma,且分别代表了泛非期和格林威尔期—晋宁期的岩浆活动热事件。研究认为,马莫勒组的沉积时代应介于99~90Ma之间,马莫勒组的出现标志着尼玛地区在晚白垩世初期完成了由洋到陆的转换,进入陆内环境。

The establishment of Upper Cretaceous Mamole Formation in Asuo Town of Nima County, northern Tibet, and its significance

In this study, a set of purple red braided river delta plain subfacies sediments were firstly discovered in Asuo Town of Nima County, northern Tibet. Rock association is mainly composed of medium conglomerate in sandstone, medium conglomerate in complex composition and fine conglomerate in sandstone, intercalated with gravel and lithic sandstones. We establish the Mamole Formation based on the section measurement and regional correlation. The authors collected zircon samples from lithic sandstones in Mamole Formation, and the analytical results show that the youngest detrital zircon age is 89Ma and there exist three peak ages for the samples, i.e., 125Ma, 550Ma and 980Ma. The ages 550Ma and 980Ma represent respectively the Pan-African and Green Weir-Jinning period magma thermal events. The principal peak of 125Ma represents the magmatic activity related to the evolution of Bangong Co-Nujiang Tethyan Ocean in Cretaceous. The deposition time of Mamole Formation was 99~90Ma, and the appearance of Mamole Formation indicates that in the Nima area the ocean-continent transition ended and evolved into intracontinental environment in the early Late Cretaceous.