SHRIMP U-Pb zircon age of tuff from the Kunyang Group in central Yunnan: Evidence for Grenvillian orogeny in South China
SHRIMP U-Pb zircon age of tuff from the Kunyang Group in central Yunnan: Evidence for Grenvillian orogeny in South China
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DOI：
10.1007/s11434-007-0225-x
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发表时间：
2007-06
影响因子：
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通讯作者：
Chuan-heng Zhang;Linzhi Gao;Zhenjie Wu;Xiaoying Shi;Q. Yan;Dajian Li
中科院分区：
文献类型：
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作者：
Chuan-heng Zhang;Linzhi Gao;Zhenjie Wu;Xiaoying Shi;Q. Yan;Dajian Li
由于缺乏高质量的、有区别的资料，华南地区是否发生过格伦维利亚造山运动仍然存在很大的争议，因此，解决这一问题的关键是找到与格伦维利亚造山运动有关的可测定的火山岩和/或沉积岩。滇中昆阳群中下段富良棚段明显存在此类岩石；该单元厚度超过100米，由安山岩质火成岩、凝灰岩、陆相碎屑岩和碳酸盐岩组成。这些火山岩发育于四堡褶皱冲断带以南，代表了滇中早前寒武纪晚期钙碱性火山活动，与碎屑岩向碳酸盐岩的沉积相转变和其他地区震积岩发育的开始是同一时期的。从该火山单元收集了两个样品进行虾分析。富良棚段底部G3-29-2样品为火成岩，阴极发光（CL）成像显示约100颗锆石晶体呈自面体状，扇形分带较为简单，表明其岩浆成因。对25颗锆石进行了分析，得到了加权平均U-Pb年龄（1032±9 Ma）。富良棚段上部样品G3-29-3为凝灰岩，恢复了许多圆润、明显的碎屑锆石。对其中9颗锆石进行了分析，最古老的单粒U-Pb锆石年龄为1938±26 Ma，表明华夏地区发育古元古代基底。结合富良棚段的大地构造研究，认为华南西部晚中元古代有造山带火山活动，扬子克拉通与华夏克拉通的碰撞是罗迪尼亚组合的组成部分。
Whether or not Grenvillian orogeny occurred in South China still remains highly controversial because high-quality, discriminating data are lacking, and therefore, the key to resolve this matter is to find datable volcanic and/or sedimentary rocks related to Grenvillian orogeny. Such rocks are apparently present in the Fuliangpeng Member from the lower-middle part of Kunyang Group in central Yunnan; here the unit is more than 100 m thick and consists of andesitic ignimbrite, tuffite, terrigeous clastic rocks and carbonates. These volcanic rocks, developed south of the Sibao fold-thrust belts, represent the earliest calc-alkaline volcanic activity in late Precambrian time from central Yunnan and are coeval with both a change in sedimentary facies from detritus to carbonates and the beginning of seismite development elsewhere. Two samples for SHRIMP analysis were collected from this volcanic unit. Sample G3-29-2, from the bottom of Fuliangpeng Member, is an ignimbrite, and about 100 zircon crystals recovered from it have euhedral shapes and display relatively simple sector zonation under cathodoluminescent (CL) imaging, suggesting a magmatogenic origin. Twenty-five of the zircons were analyzed and a weighed-mean U-Pb age of 1032±9 Ma was obtained. Sample G3-29-3 from uppermost part of Fuliangpeng Member is a tuffite, and many rounded, evidently detrital zircons were recovered. Nine of these zircons were analyzed, and the oldest single-grain U-Pb zircon age is 1938±26 Ma, implying that Paleoproterozoic basement developed in Cathaysia. The dating result, combined with the geotectonic research on the Fuliangpeng Member, leads us to conclude that late Mesoproterozoic orogenic volcanic activity occurred in the western part of South China, and that the related collision of Yangtze and Cathaysian cratons was an integral part of the assembly of Rodinia.