松潘-甘孜褶皱带南部上三叠统物源及构造抬升: 碎屑锆石年代学和Hf同位素证据

碎屑锆石年代学不但能够限定地层沉积开始的最大时限，还能为示踪沉积物源区提供关键信息。中国西南部的松潘-甘孜褶皱带广泛出露一套巨厚的三叠纪复理石沉积，其物源区和可能存在的同期抬升与剥蚀历史并未得到很好约束。本文获得的松潘-甘孜褶皱带南部雅江地区上三叠统四套地层（由老至新分别为侏倭组、新都桥组、两河口组和雅江组）5件砂岩样品的碎屑锆石U-Pb年龄和锆石Hf同位素数据表明，最年轻锆石年龄指示侏倭组从~229Ma后开始沉积，新都桥组则从~223Ma后开始沉积。碎屑锆石年龄频谱图显示四套地层都具有中奥陶世-早泥盆世（465~398Ma）和中二叠世-晚三叠世（271~225Ma）的年龄峰。除两河口组外的其他三套地层还具有较强的古元古代（1.90~1.86Ga）和新元古代（872~712Ma）的年龄峰。锆石Hf同位素显示松潘-甘孜褶皱带南部上三叠统小于300Ma的锆石颗粒主要来自峨眉山大火成岩省和义敦岩浆弧。本文物源区示踪结果表明，华南板块和义敦地体可能为松潘-甘孜褶皱带南部地层的主要物源区。晚三叠世由于周缘地体的强烈汇聚，松潘-甘孜褶皱带在小于~18Myr的时间内经历了快速的隆升和剥蚀作用，剥蚀产生的碎屑物质被搬运至四川盆地的西缘再沉积。

Provenance and tectonic uplift of the Upper Triassic strata in the southern Songpan-Ganzi fold belt, SW China: Evidence from detrital zircon geochronology and Hf isotope

Detrital zircon geochronology can not only help to determine the maximum depositional ages of sedimentary strata but also provide key constraints on the sediment provenance. The provenance of the Late Triassic flysch in the Songpan-Ganzi fold belt, SW China and the uplift history possibly occurred at slightly later remain debated. This paper reports new detrital zircon U-Pb ages and Hf isotopic data of five sandstone samples from the Upper Triassic Zhuwo, Xinduqiao, Lianghekou, and Yajiang formations (upward in section) in Yajiang, southern Songpan-Ganzi fold belt. The youngest detrital zircon ages indicate that the deposition of Zhuwo and Xinduqiao formations initiated around 229Ma and 223Ma, respectively. The probability density plots show that these four formations collectively yield age peaks of Middle Ordovician to Early Devonian (ca. 465~398Ma) and Middle Permian to Late Triassic (ca. 271~225Ma). Except for one sample from the Lianghekou Formation, the other samples have age peaks of Paleoproterozoic (ca. 1.90~1.86Ga) and Neoproterozoic (ca. 872~712Ma). The Hf isotopic data indicates that younger than 300Ma grains are mainly sourced from the Emeishan large igneous province to the east and the Yidun magmatic arc to the west. Our provenance study reveals that the Late Triassic sediments in the southern part of the Songpan-Ganzi fold belt is probably derived from the South China Block and the Yidun Terrane. This paper proposes that, during the Late Triassic, the Songpan-Ganzi fold belt may have experienced rapid uplift and exhumation within ~18Myr due to the strong convergence of surrounding terranes, following by the sediment delivery to the western margin of the Sichuan Basin.