阿拉善地块东缘早前寒武纪地壳演化及对华北克拉通西部边界的制约

In recent years, numerous studies revealed that the eastern part of the Alxa Block has different evolution from the North China Craton (NCC) during the Paleoproterozoic-Neoproterozoic, indicating that the Alxa Block is probably not a part of the NCC. The basement rocks exposed in the eastern Alxa Block were mainly formed and metamorphosed during the Paleoproterozoic time, which suggests that these rocks are likely part of a Late Paleoproterozoic mobile belt striking NE along the eastern margin of the Alxa Block. This Paleoproterozoic mobile belt is likely the boundary between the Alxa Block and the NCC. However, the nature, evolution history and tectonic significances of this belt remain poorly understood, and this has hampered ascertaining the tectonic affinity of the Alxa Block. In this proposal, comprehensive studies of geology and petrology, whole-rock major and trace element as well as Sm-Nd isotope, and in situ zircon U-Pb, Hf and O isotopic analyses were carried out for the Diebusige and Boluositanmiao complexes in the eastern margin of the Alxa Block and the adjacent Ulashan Group in Langshan area. These results provide important constraints on the petrogenesis and tectonic setting of these rocks. Furthermore, it can be used to constrain the Early Precambrian geological evolution of the eastern margin of the Alxa Block, the west boundary of the NCC, the tectonic affinity of the Alxa Block and the position of the NCC in the Columbia supercontinent.

最近几年的研究发现，阿拉善地块东部在古元古代-新元古代具有与华北克拉通不同的演化历史，暗示它可能不是华北克拉通的组成部分。阿拉善地块东缘的基底岩石主要形成于古元古代并经历古元古代变质事件，大致构成一条北东向展布的古元古代活动带，可能是华北克拉通的西部边界。但是，该活动带的性质、演化历史和构造意义还不明确，这导致阿拉善地块的构造属性仍不能有效确定。本项目拟对阿拉善地块东缘的叠布斯格杂岩、波罗斯坦庙杂岩和狼山地区的乌拉山岩群进行系统的地质及岩石学研究和全岩主微量元素、Sm-Nd同位素以及锆石U-Pb定年、Hf-O同位素分析，揭示各期岩浆事件的成因和形成的构造背景，探讨阿拉善地块东缘早前寒武纪的演化历史，约束华北克拉通的西部边界和阿拉善地块的构造属性，为检验华北克拉通在Columbia超大陆中的相对位置提供新的证据。

阿拉善地块的构造属性以及华北克拉通的西部边界一直是悬而未决的问题。通过对阿拉善地块东部的叠布斯格杂岩、波罗斯坦庙杂岩和狼山地区的乌拉山岩群进行系统的地质年代学和岩石地球化学的研究，可有效地解决这一问题。对波罗斯坦庙杂岩及邻区的研究表明，其主体形成于280-260 Ma，不是古元古代基底岩石；结合阿拉善地块广泛分布的这一时期岩石，提出存在早二叠世（280 Ma）的酸性火成岩省，这个火成岩省是塔里木地幔柱下的产物。对乌拉山岩群的研究表明，其主体岩石为混合岩，形成于280-240 Ma，不是前人所认为的晚太古代或古元古代基底。只有叠布斯格杂岩形成于古元古代，对基性岩和变沉积岩的进一步研究表明，其主体形成于2.3-2.0 Ga，经历了1.9 Ga和1.8 Ga的变质作用。结合这一区域有古元古代高压麻粒岩的发现，表明阿拉善东缘存在古元古代的造山带。通过对阿拉善地块东部及邻区主要火成岩的进一步分析，发现在阿拉善东缘存在NE向的古生代花岗岩带。结合巴彥乌拉山-叠布斯格杂岩带西缘断裂两侧的古元古代-二叠纪火成岩的全岩Nd模式年龄和Pb同位素存在明显不同，表明阿拉善地块不是华北克拉通的组成分部，华北克拉通的西部边界很可能位于此断裂附近。因此，在阿拉善地块和华北克拉通间可能存在隐伏的早古生代造山带，其记录了再者间的拼合。