Geochronology and Tectonic Evolution of the West Section of the Jiangnan Orogenic Belt

As an important part of South China Old Land, the Jiangnan Orogenic Belt plays a significant role in explaining the assembly and the evolution of the Upper Yangtze Block and Cathaysia, as well as the structure and growth mechanism of continental lithosphere in South China.The Lengjiaxi and the Banxi groups are the base strata of the west section of the Jiangnan Orogenic Belt.Thus, the research of geochronology and tectonic evolution of the Lengjiaxi and the Banxi groups is significant.The maximum sedimentary age of the Lengjiaxi Group is ca.862 Ma, and the minimum is ca.822 Ma.The Zhangjiawan Formation, which is situated in the upper part of the Banxi Group is ca.802 Ma.The Lengjiaxi Group and equivalent strata should thus belong to the Neoproterozoic in age.The Jiangnan Orogenic Belt consisting of the Lengjiaxi and the Banxi groups as important constituents is not a Greenville Orogen Belt（1.3 Ga–1.0 Ga）.The Jiangnan Orogenic Belt is a recyclic orogenic belt, and the prototype basin is a foreland basin with materials derived from the southwest and the sediments belong to the active continental sedimentation.By combining large amounts of dating data of the Lengjiaxi and the Banxi groups as well as equivalent strata, the evolutionary model of the western section of the Jiangnan Orogenic Belt is established as follows： Before 862 Ma, the South China Ocean was subducted beneath the Upper Yangtze Block, while a continental island arc was formed on the side near the Upper Yangtze Block.The South China Ocean was not closed in this period.From 862 Ma to 822 Ma, the Upper Yangtze Block was collided with Cathaysia; and sediments began to be deposited in the foreland basin between the two blocks.The Lengjiaxi Group and equivalent strata were thus formed and the materials might be derived from the recyclic orogenic belt.From 822 Ma to 802 Ma, Cathaysia continued pushing to the Upper Yangtze Block, experienced the Jinning-Sibao Movement（Wuling Movement）; as result, the folded basement of the Jiangnan Orogenic Belt was formed.After 802 Ma, Cathaysia and the Upper Yangtze Block were separated from each other, the Nanhua rift basin was formed and began to receive the sediments of the Banxi Group and equivalent strata.These large amounts of dating data and research results also indicate that before the collision of the Upper Yangtze Block with Cathaysia, materials of the continental crust became less and less from the southwest to the east in the Jiangnan Orogeneic Belt; only island arc and neomagmatic arc were developed in the eastern section.Ocean-continent subduction or continent-continent subduction took place in the western and southern sections, while intra-oceanic subduction occurred in the eastern section.Comprehensive analyses on U-Pb ages and Hf model ages of zircons, the main provenance of the Lengjiaxi Group is Cathaysia.

贵州梵净山群沉积时代——来自原位锆石U-Pb测年证据

贵州梵净山地区地层出露完整，构造关系清楚，是研究江南造山带西南段构造演化的理想地区。该区区域角度不整合面之下地层称为梵净山群，分为7个岩性组，中部出现大量火山岩，形成一个完整的盆地沉积记录。对该区梵净山群地层中的火山碎屑岩夹层及细碎屑岩中的锆石进行了LA-ICP—MSU-Pb测年工作，结果显示，两件火山碎屑岩锆石平均U-Pb年龄分别为832Ma（铜厂组）和851Ma（余家沟组）；另外两件余家沟组细碎屑岩中锆石原位U-Pb测年结果比较接近，平均年龄分别为845Ma和849Ma。结合前人数据可以比较精确限定梵净山群的沉积时限在855～815Ma之间。该时段的上限精确限定了武陵／四堡运动界面的时限，为江南造山带不同部位地质事件的合理对比、了解江南造山带地质事件的时空迁移规律提供了基础资料。

闽赣粤交界新元古代侵入岩的发现及地质意义

闽赣粤交界寻乌县罗珊-武平县民主地区新发现4个新元古代岩体,岩性为花岗质、闪长质或云英闪长质片麻岩,具有过铝质S型花岗岩特征。LA-ICP-MS锆石U-Pb年代学特征显示,其中羊子杰岩体的成岩结晶年龄为(994±23)Ma,结合区域地质研究认为,该岩体应与同构造期的岩浆活动有关,区域上可能存在四堡运动,是闽赣粤交界的新发现。