桂东北恭城盆地的物源变迁:华南中生代构造体制转换的产物？

Sedimentary Provenance of the Mesozoic Gongcheng Basin in Northeastern Guangxi, South China: Product of Changing Tectonic Configuration?

桂东北中生代恭城盆地位于南岭成矿带西段,是研究华南印支期-燕山早期构造事件动力学过程中浅表地质响应的重要窗口。盆地内上二叠统乐平组被上三叠统-下侏罗统天堂组不整合覆盖,天堂组与下侏罗统大岭组、中侏罗统石梯组为整合接触。沉积相分析表明,乐平组为海陆交互相碎屑岩,天堂组底部砾岩、含砾粗砂岩代表盆地早期山前快速堆积的冲积扇相沉积,天堂组上部、大岭组和石梯组砾岩、砂岩和泥质岩则代表河流相沉积。碎屑锆石U-Pb年龄结果显示,乐平组和天堂组底部岩屑石英砂岩均以980 Ma和~2500 Ma锆石为主,暗示晚二叠世以及晚三叠世-早侏罗世早期盆地的碎屑物主要来自南部云开地区。然而天堂组上部、大岭组和石梯组长石岩屑砂岩则以1790~1875 Ma和2370~2490 Ma锆石为主,并伴随有燕山早期(189~174Ma)锆石,指示早-中侏罗世时盆地的碎屑物主要来源于北东部武夷山-南岭地区,与古水流方向分析的结果一致。恭城盆地早-中侏罗世时沉积物源的变迁规律很好地记录了华南东南部从印支期挤压应力状态向燕山早期伸展拉张作用的转变以及区域构造机制的转换。

The Mesozoic Gongcheng Basin of the northeastern Guangxi province,in the western segment of the Nanling Metallogenic Belt,is the key area to reveal the link between sedimentation and geodynamics during the Indosinian-Early Yanshanian in the southeastern South China.The Late Permian Leping Formation is overlain angular unconformably by the Late Triassic-Early Jurassic Tiantang Formation,which inturn is conformably overlain by the Early Jurassic Daling Formation and the Middle Jurassic Shiti Formation.Sedimentary facies analyses indicate that the Leping Formation comprises shallow-marine to deltaic siliciclastic rocks.The lower part of the Tiantang Formation comprises alluvial-fan conglomerate,pebbly sandstone and mudstone,which record initiation of the Gongcheng Basin.The upper part of the Tiantang Formation,and the Daling and Shiti formations are composed of fluvial conglomerate,sandstone and mudstone.LA-ICP-MS U-Pb zircon dating results show that the quartz-lithic sandstones of the Leping Formation and the lower part of the Tiantang Formation are dominated by detrital zircon with modal ages of 980 Ma and ca.2500 Ma,implying that the Late Permian and Late Triassic sediments were derived mainly from the Yunkai Terrane to the south.However,sandstones with higher feldspathic compositions in the upper part of the Tiantang Formation,and in the Daling and the Shiti formations are characterized by detrital zircon with modal ages of 2370–2490 Ma and 1790–1875 Ma,as well as 189–174 Ma of the Early Yanshanian period.The implication is that the Early Jurassic to Middle Jurassic sediments were sourced mainly from the Wuyishan and the eastern Nanling areas to the northeast.This significant change in zircon provenance is consistent with the Middle Jurassic paleoflow directions recorded in the Gongcheng basin.The change in sedimentary provenance(sandstone compositions and detrital zircon)of the Gongcheng Basin is related to the switch from the compressive stress during the Indosinian stage to the extension during the Early Yanshanian stage,corresponding to changes in the tectonic configuration of the southeastern South China during the Mesozoic.