中国东南部晚中生代火成岩的基底探讨

A Study on the Tectonic Basement of Late Mesozoic Igneous Rocks in Southeastern China

根据浙闽赣粤地区出露的中深变质岩石构造组合、构造形迹和近年报道的同位素测年数据认为 ,中国东南部晚中生代火成岩之下存在一个前泥盆纪的变质基底。其中 ,前震旦纪的变质基底以呈面形分布的片岩、片麻岩及混合岩为特征 ,年龄值大于 8亿年 ,是构成华南地区最古老的陆壳基底之一 ,俗称华夏古陆。 9亿年左右 ,该陆块和扬子陆块碰撞 ,其缝合带为绍兴—江山—东乡—萍乡断裂带。稍后 ,受区域拉张构造事件的影响 ,它被裂解为三个走向各异、分布在浙东南—闽西北、赣中—赣南和云开三个地区的块体 ,分别呈NEE、NNE和NE方向展布。三者之间为巨厚的震旦纪—早古生代沉积物质所充填 ,并有复式岩浆岩流出现。研究表明 ,中国东南部晚中生代火成岩的基底构造至少经历过三期构造—热事件的演化。这种古构造格局 ,制约了中国东南部中生代大规模构造 -岩浆活动、岩浆成分差异及其空间展布。

According to middle high grade metamorphic rock assemblages, structural deformational styles and newer data of isotopic ages, this paper suggests that a Precambrian continental block existed in the southern Zhejiang-northern Fujian-central southern Jiangxi-eastern Guangdong region, South China. This oldland, with the oldest isotopic age of 2100 Ma and named the Cathaysian Oldland, is characterized by large scale development of schist, gneiss and migmatic rocks with the interval from 1800Ma to 1200 Ma, which comprise the continental crustal basement. About 1100～900Ma, the Cathaysian Oldland collided with the Yangzi continental plate to form the Shaoxing-Jiangshan-Dongxiang-Pingxiang Fault Zone. Not long time later, about 800 Ma, the Cathaysian Oldland was dispersed into three blocks with different strikes and distinct outline, that is, the NEE striking Southeastern Zhejiang-Northwestern Fujian Block, the sub N S striking Central Jiangxi-Southern Jiangxi Block and the NE striking Yunkaidashan Block, respectively. The Sinian-Early Paleozoic sedimentary materials with a large thickness filled among these three blocks. Study proposes that this oldland subjected to three phase tectonic thermal events at least, the Luliang event (1800 1600Mha), the Jinning event (1100 900Ma) and the Caledonian event (420 400Ma) since the Neo Proterozoic to become the tectonic basement of the Late Mesozoic (mainly Cretaceous) volcanic intrusive complexes. The basement feature of the Southeastern Coastal Region is not clear due to lack of Precambrian outcropped rocks and isotopic age data. But according to the geochronological age of 1806Ma by the whole rock Rb Sr method on the core of gneissic rock from the Linfeng hole N0.1 in East China Sea (East of Wenzhou), we guess that a Proterozoic metamorphic block should exist under the Meso-Cenozoic sedimentary rocks of the East China Sea. During the Late Mesozoic-Paleogene, a large scale intra continental rift basin was formed along the Shaoxing-Jiangshan-Dongxiang-Pingxiang Fault Zone, in which red bed was filled. This ancient tectonic framework constrains large scale Mesozoic tectonics magmatism and difference in magmatic compositions and distributrion.