华南前泥盆纪构造演化:从华夏地块到加里东期造山带

Predevonian Tectonic Evolution of South China: from Cathaysian Block to Caledonian Period Folded Orogenic Belt

在全球Rodinia超大陆的构造框架中，华夏地块占有突出的地位。然而，华夏地块在国内一直存在不同认识，其核心一是年龄，二是范围。根据出露在研究区的中一高级变质岩、韧滑流变形迹和近年大批高质量测年数据，认为华南曾经存在过一个前成冰纪的古老基底，由元古代片岩、片麻岩、混合岩等组成，原岩为碎屑岩、火山岩和深成侵入岩，最老年龄达2Ga，习称华夏地块，但范围比Grabau描述的要小。在8～9亿年间，伴随古华南洋的闭合，华夏地块与扬子陆块碰撞聚合，成为Rodinia超大陆的一部分。聚合不久，受成冰纪Rodinia超大陆裂解事件的影响，原华夏地块被肢解成浙南-闽北、赣中-赣南和云开大山三个古陆残块，中间是裂谷或海槽。其裂解残块集中分布在绍兴-江山-萍乡断裂和政和-大埔断裂之间的地带内，结束了其完整古陆块的历史。震旦纪-早古生代，这些海槽被进一步扩张变宽，其内被巨厚的碎屑岩（含灰岩）、浊积岩层所充填，但缺少同期蛇绿岩和火山岩，暗示拉张强度没有深达上地幔，为一被动陆缘环境。最新年代学结果表明，原定早古生代的蛇绿岩和火山岩均为前震旦纪的年龄，8～9亿年居多，原先的早古生代构造格架需要再研究。到志留纪，华南发生了强烈的构造-热事件，导致震旦纪-早古生代海槽关闭，巨厚沉积物褶皱隆升，在元古代变质基底上形成了加里东期褶皱造山带。其造山的驱动力目前尚未明确。此期褶皱变形、韧滑流变非常普遍，有推覆与走滑两种，变形峰期在420～400Ma。同时，还发生了强烈的花岗岩浆活动：岩浆峰期为430～400Ma，但绝大多数是过铝质的S型花岗岩，Ⅰ型花岗岩少见。之后，晚泥盆世砂砾岩层呈角度不整合大规模地覆盖在整个华南前泥盆纪岩层之上；至此，研究区和江南等邻区的沉积环境与古地理才得以真正统一。

The interpretation of Predevonian tectonics of South China is controversial for a long time both in its age and distribution. Based on the middle-high grade metamorphic rocks, ductile slipping rheologic structures exposed in the Zhejiang-Fujian-Jiangxi-Guangdong domain and high quality dating data published recently, the author believes that an ancient continental block certainly existed in South China, which is composed of Proterozoic schist, gneiss and migmatite with an oldest age of 2 Ga. Their protolithes were clastic rocks, volcanic rocks and plutons. This old land is temporally called as Cathaysian Continental Block which is limited in the domain between Shaoxing- Jiangshan-Pingxiang fault and Zhenghe-Dapu fault, and its scope is less than that defined by Grabau. During 800 - 900 Ma, following the closure of paleo-South China ocean, Cathaysian block collided with Yangzi block and then became a part of Rodinia supercontinent. Not long time after converging, by affection of breakup of Rodinia supercontinent, the proto-Cathaysian continental block was split into three sub-blocks, namely the southeastern Zhejiang- northwestern Fujian, the central-southern Jiangxi and the Yunkaidashan, and several rifts or sea channels occurred among them. From Early Sinian to Late Ordovician, these sea channels were expanded and were filled by 10000 - 20000 m thick clastic rocks （containing limestone） and turbidites. However, the coeval ophiolite and volcanic rocks are absent, implying that extended fault did not reach to upper mantle. New geochronological results indicate that the ophiolite and volcanic rocks in the study areas, which were described as Early Paleozoic by previous researchers, yielded pre-Sinian ages, concentrating mainly between 800 Ma and 900 Ma. Thus, the previous Early Paleozoic tectonic framework needs to be re-constructed. In the Silurian, a strong tectono-thermal event took place in South China, causing closure of Sinian-Early Paleozoic sea channels and folding-uplifting of mega-thick sediments. The South China Caledonian fold and orogenic belt was formed on the Proterozoic metamorphic basement. In the study area, folding deformation and ductile slipping rheology were very common, including thrust and strike-slip deformation. The peak period of deformation was 420 - 400 Ma. At the same time, a violent granitic magmatism started, forming numerous strongly peraluminous S-type granites with A/CNK （molar Al2O3/[ CaO ＋ Na2O ＋ K2O] ） 〉 1.1, and the Ⅰ - type granitoids were rare. Peak period of granitic magmatism took place in the 430 - 400 Ma. Then, the whole South China Caledonian folded geological bodies were overlain unconformably by the Late Devonian conglomerate and coarse sandstone, indicating termination of folding and orogeny. From the Late Devonian, a united paleogeographic and sedimentary environment occurred really in the study domain and its neighboring areas.