华北中部造山带左权变质杂岩ca. 2.5Ga和ca. 1.9Ga变质年龄记录及其地质意义

华北克拉通早前寒武纪基底由多个微陆块组成,其主期拼合时代是困扰地质学家的一个重大课题。华北中部造山带作为新太古代-古元古代一条重要的碰撞型造山带已得到广泛共识。大量高精度年代学资料显示,华北中部造山带至少记录了大约1. 85Ga、1. 95Ga和2. 5Ga的三组变质年龄信息。但目前,2. 5Ga左右的变质年龄仅在华北中部造山带中部的阜平和赞皇等少数杂岩区有零星报道。左权变质杂岩位于华北中部造山带中南段东侧、阜平杂岩以南,向东紧邻赞皇杂岩,是洞悉早前寒武纪时期华北克拉通基底形成及演化过程的一个重要窗口。杂岩区出露多种早寒武纪变质岩石,其中长英质黑云斜长片麻岩分布范围最广,局部暗色矿物富集;斜长角闪岩或角闪片麻岩多以透镜状或似层状方式产出于长英质片麻岩中;杂岩区南部发育多个小型磁铁矿矿床。本文对研究区多种类型岩石样品进行了细致的岩相学、锆石U-Pb年代学和锆石稀土元素研究,发现多数样品中发育变质成因锆石,记录至少两组变质年龄信息。第一组年龄(1903Ma)仅被个别角闪片麻岩样品保存,反映了杂岩区所经历的变质峰期或近峰期阶段的时代;第二组年龄(2483～2507Ma)分布广泛,所代表的变质事件发生在区域片麻理之前,与华北克拉通约25亿年时发生的大规模构造热事件有关。

Records and its geological implication of metamorphic ages of ca. 2.5Ga and ca. 1.9Ga from the Zuoquan metamorphic complex in the Trans-North China Orogen

A coherent Precambrian basement of the North China Craton (NCC) was composed of several micro-blocks, and the timing of their main amalgamation is still a scientific challenge to geologists. The Trans-North China Orogen (TNCO) has been well known as a Neoarchean-Palaeoproterozoic orogenic belt. A large number of metamorphic ages have been obtained for metamorphic complexes in the TNCO, which can be divided into three groups at least, including ca. 1.85Ga, ca. 1.95Ga and ca. 2.5Ga. Metamorphic age of ca. 2.5Ga has only been recorded by rare rocks from the Fuping and Zanhuang complexes in the TNCO. The Zuoquan metamorphic complex is located at the central-southern segment of the TNCO and exposed to the southwest of the Zanhuang complex, and the south of the Fuping complex. It can provide notable insight into the formation and evolution of the Early Precambrian basement of the NCC. The Zuoquan complex consists of many types of Precambrian rocks, in which various felsic gneisses occupy the most area and some magnetite ores expose in the south. Amphibolite and hornblend gneiss occur as boudinaged lenses or bed-like forms in the felsic gneiss, with mafic minerals rich locally. A combined study of petrography, zircon U-Pb geochronology and the rare-earth elements (REE) of zircons was carried out on types of rocks here, and dating of metamorphic zircons reveal two discrete, meaningful age groups. A smaller, younger age group was obtained by individual hornblend gneiss sample, exhibiting peak/pre-peak metamorphism at 1903Ma. Whereas most types of rocks exhibit a metamorphic event between 2483Ma and 2507Ma, which is probably related to the extensive tectono-thermal event occurred at ca. 2.5Ga in the NCC.