四川江油马角坝刺林包剖面二叠-三叠系间粘土岩研究

二叠-三叠纪之交华南地区普遍发育火山成因的粘土岩,其对于了解二叠纪末大规模火山活动及扬子板块碎屑物质来源具有重要意义。本文对扬子西缘马角坝刺林包剖面飞仙关组底部粘土岩进行了碎屑锆石U-Pb定年及原位Hf同位素测试、X衍射和全岩地球化学分析,以揭示粘土岩的岩石成分及物源信息。镜下特征、X衍射及主量元素特征表明岩石中粘土矿物主要为伊利石,含少量方解石、石英。碎屑锆石年龄显示,特征峰值年龄主要集中在2 615~2 383、1 868~1 328、1 186~778和430~246 Ma,并出现~250 Ma特征高峰,结合原位Hf同位素特征表明锆石与秦岭造山带、华南典型PTB剖面粘土岩及峨眉山大火成岩省具有较高的相似性。同时,通过微量、稀土元素分析发现粘土岩中Zr、Hf、Th、Cr、Co、Ti相对富集,与华南典型PTB粘土岩和峨眉山玄武岩相近,并结合构造背景及锆石特征,综合认为刺林包剖面PTB粘土岩来自于秦岭造山带及龙门山岛链局部剥蚀区、二叠纪末火山活动和峨眉山大火成岩省剥蚀岩体。

A study of the claystone around the Permian-Trassic boundary along the Cilinbao section at Majiaoba, Jiangyou, Sichuan Province

The volcanogenic claystone in the Permian-Triassic boundary plays a significant role in understanding the volcanism and provenance in Yangtze Plate, which is widely distributed in South China. This paper reports zircon U-Pb ages determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) along with Hf isotopes, whole-rock elemental and X-ray diffraction for the samples of claystone in the lower Feixianguan Formation along Cilinbao section. The microscopic characteristics, X-ray diffraction and major elements analysis show that the claystone contains illites and small amounts of calcite and quartz. The detrital zircon age data exhibit a wide range, varying from 2 615 Ma to 246 Ma and concentrated on 2 615~2 383, 1 868~1 328, 1 186~778 and 430~246 Ma, with a~250 Ma peak value. Combined with the in-situ zircon Hf isotope, the authors detected that the zircon characteristics are similar to those in Qinling Mountain, Permian-Triassic boundary sections in South China and Emeishan large igneous province. Indeed, geochemical compositions of the claystone are characterized by relative enrichment of Zr, Hf, Th, Cr, Co and Ti, similar to features around Permian-Triassic boundary sections in South China and Emeishan basalt. Combined with tectonic setting of the end of Permian and detrital zircons, the authors hold that the claystone around the Permian-Trassic boundary along the Cilinbao section at Majiaoba was derived from Qinling Mountain, Longmenshan Islands, volcanism and Emeishan large igneous province.