江南中段超基性-基性-中性火山岩组合及华南新元古代汇聚

It is generally accepted that the Jiangnan orogenism created the South China Block (SCB) via the amalgamation of the Yangtze with Cathaysia Blocks during the Neoproterozoic period. The Neoproterozoic amalgamation process of the SCB is one of the hotspots in Precambrian research. It has been a matter of debate as to the tectonic nature (plume vs. subduction/collision vs. plate-rift) during ~860 -830 Ma. Previous studies focused on the igneous and sedimentary rocks of eastern and western segments of the Jiangnan Orogen. However, less attention has been paid to the geochronology and geochemistry of the igneous rocks in the Central Jiangnan Orogen, especially those of Ultramafic-mafic-intermediate rocks association. Our field investigation identified the rocks-association of ultramafic-mafic-intermediate volcanics in boundary of Jiangxi and Hunan provinces involving the ultramafic volcanics, normal andesites and boninites, Nb-enriched and komatiitic-like basalts. In this proposal, we aim to understand the petrogenesis and tectonic settings of the above-mentioned rock association and constrain its location to the margin of Rodinia on basis of a set of field, petrological, geochronological, geochemical, Sr-Nd-Pb-Os and zircon in-situ Hf-O isotopic data.

江南造山带是华南前寒武纪地质研究的热点与前沿。江南造山带新元古代(~860 - 830 Ma)构造属性受到了广泛关注，目前有“地幔柱”、“岛弧岩浆作用”和“俯冲汇聚+裂谷作用”三种观点。现有研究主要集中于江南造山带东段和西段，对中段的湘东-赣西地区的工作相对较少，少有区内岩石组合的发育及相关年代学和地球化学研究的报道。申请者前期在浏阳文家地区识别出有超基性-基性-中性火山岩组合，表现为超基性岩、富铌玄武岩、具“鬣刺结构”玄武岩、正常安山岩与玻安岩共存，该岩石组合对约束江南造山带中段新元古代构造属性具有重要意义。本项目拟通过野外地质、岩石(相)学、岩石/矿物化学、同位素年代学、元素、Sr-Nd-Pb-Os同位素及锆石原位Hf-O研究，剖析该组合内不同类型岩石的精细时代、岩石成因及相互之间的内在联系，厘定其大地构造意义，从而为确证江南造山带中段弧岩浆作用的存在，以及华南新元古代汇聚及其与罗迪尼亚超大陆关系的理解提供关键证据。

江南造山带被普遍接受为扬子陆块与华夏陆块在新元古代的拼合带，是解析华南前寒武纪汇聚作用的关键地区。其新元古代(~860 - 830 Ma)构造属性近几十年受到了广泛关注，目前有“地幔柱”、“岛弧岩浆作用”和“俯冲汇聚+裂谷作用”三种代表性观点。但现有研究主要集中于江南造山带东段和西段，对中段的湘东-赣西地区的工作相对较少，本项目在野外地质考察、岩相学观察及矿物学分析的基础上，通过详细的元素地球化学、Sr–Nd同位素地球化学、锆石U–Pb年代学等综合研究，初步厘定了江南造山带中段新元古代弧-盆体系背景。本项目在浏阳文家市清风亭-涧溪冲-南棚一带识别出一套变质沉积-火山岩系，其中的变基性岩具有“鬣刺结构”，对于该套岩石的精细研究确立了江南造山带中段~840Ma的弧构造背景；对湘赣交界慈化地区共生的高镁安山岩和高镁英安岩系统研究构建了江南造山带中段在~832 Ma的俯冲作用背景；通过冷家溪群和板溪群地层中的沉积岩样品研究得出冷家溪群的沉积年龄在865-835 Ma，板溪群的沉积时代在~810-720 Ma；对夹于冷家溪群和板溪群地层中的沧水铺群的林家湾组砾岩和银珠坝组集块岩的年代学和地球化学研究得到扬子与华夏陆块拼贴开始和结束的时间分别在~830 Ma和~810 Ma，明显晚于约1.0 Ga全球格林威尔造山事件形成Rodinia大陆的时间，推断华南板块位于Rodinia边缘。