湘东南中泥盆统石英砂砾岩物源分析及其大地构造意义

Provenance Analysis and Tectonic Implications of Middle Devonian Quartzose Conglomerate and Sandstone in Southeastern Hunan Province,South China

早古生代江南隆起的形成与剥蚀作用可以用来制约华南广西期造山作用及其动力学机制.通过对江南隆起南侧湘东南南湾地区中泥盆统跳马涧组石英砂砾岩的碎屑锆石U-Pb年代学、锆石微量元素地球化学以及全岩地球化学的分析,讨论了其物源及其大地构造意义.4件样品的270颗有效碎屑锆石U-Pb年龄谱由430~440 Ma主峰与800~1 100 Ma、1 700~2 000 Ma和2 400~2 600 Ma次峰组成.锆石CL图像、Th/U比值以及稀土元素配分图指示碎屑锆石以岩浆锆石为主,仅有少量变质锆石和热液锆石.矿物形态、组成以及成熟度指示其源区为近源,沉积于滨海环境.综合源区分析表明,湘东南中泥盆统跳马涧组石英砂砾岩的物源来自其北侧的江南隆起.江南隆起形成于广西期陆内造山作用.早古生代岩浆锆石的微量元素地球化学特征表明其为大陆型锆石,形成于大陆弧构造环境.结合区域地质特征,华南广西期造山作用是其南侧大洋俯冲作用的产物,与冈瓦纳大陆的聚合有关.

The formation and denudation of the Early Paleozoic Jiangnan uplift can be used to restrict the Kwangsian orogeny and its dynamic mechanism in South China. Based on detrital zircon U-Pb geochronology and trace element geochemistry of the quartzose conglomerate of the Middle Devonian Tiaomajian Formation in Nanwan, south of the Jiangnan Uplift, the provenance and tectonic significance of the conglomerate are discussed. U-Pb ages of 270 available detrital zircons from the four samples consist of main peaks of 430-440 Ma and sub-peaks of 800-1 100 Ma, 1 700-2 000 Ma and 2 400-2 600 Ma. Zircon CL images, Th/U ratios and REE partition patterns indicate that detrital zircons are dominated by magmatic zircons, with only a small amount of metamorphic and hydrothermal zircons. The mineral morphology, composition and maturity indicate that the source area is near source and deposited in coastal environment. The comprehensive source analysis shows that the provenance of the quartzite conglomerate of the Middle Devonian Tiaomajian Formation in southeastern Hunan is from the Jiangnan Uplift on the north side. The Jiangnan uplift was formed during the Kwangsian period of intracontinental orogeny. The trace element geochemical characteristics of the Early Paleozoic magmatic zircons indicate that they are of continental type and formed in a continental arc tectonic setting. Combined with the regional geological characteristics, the Kwangsian orogeny in South China is the product of oceanic subduction on its southern side, which is related to the aggregation of Gondwana continent.