侵入梵净山群白岗岩锆石U-Pb年龄及白岗岩底砾岩对下江群沉积的制约

下江群代表了新元古代末期的新裂谷沉积的起点,确定下江群的时代对新元古代构造运动有着重要的地质意义。贵州印江县地区有一套新元古代白岗岩侵入到梵净山群并被下江群覆盖,在下江群底砾岩中发现大量未经搬运的白岗岩底砾岩砾石。因此,笔者对梵净山地区的白岗岩进行锆石U-Pb年代测定,获得锆石U-Pb年龄(835±5)Ma。该年龄对下江群沉积启动时间和下覆梵净山群的沉积上限均有时代的制约。结合下江群甲路组斑脱岩锆石U-Pb年龄(814±6 Ma)和考虑到武陵运动的年限,下江群沉积年代不应老于白岗岩底砾岩的年代和武陵运动的界面。

湘东北文家市蛇绿混杂岩带的发现及意义

在湘东北文家市首次识别出蛇绿岩套残片 ,它由变沉积岩、变玄武岩、变辉绿岩、变辉长岩、变辉石岩类及变硅质岩组成。综合其地质学、岩石学、地球化学和同位素年代学的资料 ,提出文家市蛇绿岩可能形成于有限小洋盆的构造环境。文家市蛇绿混杂岩带、前陆盆地、磨拉石建造和“S”型花岗岩等构造岩石组合记录了碰撞造山的全过程 ,证实了江南雪峰古陆属元古宙造山带。这一发现为华南前寒武纪的大地构造格架和地壳演化提供了依据 ,同时该地区也将成为研究早期华南大陆地壳演化———武陵运动的关键地带。

论板溪群的时代归属和层位对比

根据多种地层划分的原则,板溪群应定义为雪峰—武陵山区的一个岩石地层单位,它包括马底驿组和五强溪组。从有关地质事件、微古植物组合、同位素年龄及有争议的地层剖面的研究,板溪群和长江峡东震旦系经典剖面的莲沱组之间不存在上、下层位关系。它们同是晋宁(武陵)造山运动之后,南华大冰期之前这一地质演化阶段的沉积,具有可对比性,时代上应同归于早震旦世。

扬子地块周缘中元古代末-新元古代主要构造运动梳理与探讨

限于地层年代及地层划分对比的差异,对扬子地块周缘中元古代末-新元古代晋宁与武陵造山运动、甚或雪峰运动的界定,一直存在争议。近年来我国前寒武纪地层年代研究取得众多新的进展与成果,为地层序列建立、划分对比和相关构造运动的研究与限定提供了新的客观依据。本文以扬子地块周缘中-新元古代地层年代研究及地层划分对比成果为基础,对晋宁运动、武陵运动及雪峰运动进行了重新梳理与探讨界定,对深入研究扬子地块周缘中-新元古代地层划分对比、盆地演化及其成矿资源响应具有重要意义。

贵州梵净山地区新元古代拉伸纪岩浆演化时序

为获得江南造山带西段梵净山地区新元古代拉伸纪的岩浆演化时序,为区域综合对比提供可靠的典型地区岩浆岩年代学研究基础资料,以及为梵净山申报地质公园提供详实的科普素材,对研究区新发现的辉绿岩体进行了年代学研究,获得该岩体U-Pb年龄为805.3±4.5Ma。通过对已有年龄数据的系统梳理与总结,认为梵净山群回香坪组内玄武岩的喷发结束时间为840Ma,存在831Ma、814～805Ma两次基性-超基性岩浆活动和855Ma、834Ma两次酸性岩浆活动。梵净山地区的新元古代拉伸纪岩浆活动时序可与黔东南—桂北地区对比。结合岩石类型及其地球化学特征,认为江南造山带西段拉伸纪早期岩浆岩的成因可能与扬子和华夏板块的俯冲碰撞有关,拉伸纪晚期岩浆岩的形成可能与华南大陆伸展裂陷有关。

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。

湘东北隆起区前中生代抬升剥蚀过程的地层记录

湘东北地区为一主要出露新元古界冷家溪群的构造隆起带,其前中生代的抬升剥蚀过程模糊不清且尚未得到研究。本文通过新元古代的冷家溪群—上古生界地层序列、地层发育和缺失情况及其反映的不整合特征,尝试性重塑了湘东北前中生代抬升剥蚀过程,形成以下主要认识:湘东北地区自北往南可划分为临湘、岳阳、金井、长沙、醴陵等5个特征有别的抬升-剥蚀区,造成抬升-剥蚀的构造事件包括冷家溪期末的武陵运动、板溪期末的雪峰运动和早古生代后期的加里东运动,其中加里东运动包括奥陶纪末的北流运动和志留纪中后期的广西运动;武陵运动自北而南在各抬升-剥蚀区造成的抬升剥蚀量分别为>0~5200、800~6900、600~3200、500~3200、500~8000 m;雪峰运动造成的抬升剥蚀量在临湘抬升-剥蚀区不明,在岳阳抬升-剥蚀区约300~800 m,在金井抬升-剥蚀区约1800 m,在长沙和醴陵抬升-剥蚀区不明显;加里东运动自北而南在各抬升-剥蚀区造成的抬升剥蚀量分别约为500、3300~>4400、2400~7900、300~7100、3100~9300 m。上述抬升剥蚀过程为武陵、雪峰和加里东等构造运动特征提供了一定约束。

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。