川西前陆盆地晚三叠世早期物源与龙门山的有限隆升——碎屑锆石U-Pb年代学研究

摘要

四川盆地西部(川西)前陆盆地晚三叠世物源区过去常认为主要为其西部的龙门山和松潘-甘孜褶皱带,仅少数学者认为主要物源区位于东部和北部.本文利用碎屑锆石LA-ICP-MS U-Pb法对川西上三叠统小塘子组(T3t)不同地区的4个样品进行了研究,发现川西上三叠统小塘子组碎屑锆石年龄主要集中在2550～2400Ma,1900～1750Ma,～1000Ma,850～700Ma,500～400Ma,300～250Ma,250～213Ma 等7个范围,其中代表扬子西缘来源的850～700Ma年龄锆石在T3t沉积时在川西地区广泛分布,而松潘-甘孜地区却分布较少,表明龙门山在T3t沉积时已经隆升.此外,850～700Ma的锆石数量虽众多却并不占绝对优势,相比之下1900～1750Ma年龄的锆石数量更多,经分析认为1900～1750Ma年龄的锆石主要来自秦岭造山带,表明川西上三叠统小塘子组沉积物质主要来自其北东方向和北部,龙门山晚三叠世早期隆升规模有限.%The provenance of the early upper Triassic sediments in the Western Sichuan Foreland Basin was often considered to be the Longmen Mountains based on the analysis of gravelimbrications. However, gravel imbrications are not reliable without other supporting evidence such as the foreset beddings, crescent type cross-beddings and the distribution of detrital zircon ages. This paper discussed the early Late Triassic sediment provenance of the Western Sichuan Foreland Basin based on the U-Pb dating results of detrital zircons. Four samples ( total 301 zircon grains)of T3t (Xiaotangzi Formation) were analyzed using LA-ICP-MS method. The results yield a wide range of ages, varying from 213Ma to 3706Ma with the main population groups of 2550 ～ 2400Ma, 1900 ～ 1750Ma, 850 ～700Ma,500 ～400Ma, 300 ～ 250Ma, Meanwhile, the lack of 250 ～ 213Ma and ～ 1000Ma. The predominant detrital zircon ages group of 1900 ～ 1750Ma and much less significant 2550 ～ 2400Ma group indicate that the early Late Triassic sediments of the Western Sichuan Foreland Basin were mainly derived from the Qinling mountains rather than North China block. Plentiful zircon ages from 850 ～700Ma are found in T3t (Xiaotangzi Formation), which are lack in the Songpan-Ganze area, indicating the Longmen Mountain had already been uplifted in late Carnian and Longmen Mountains had already became a barrier between the Western Sichuan Foreland Basin and the Songpan-Ganze area.Furthermore, the Longmen Mountains were not the main provenance of the Western Sichuan Foreland Basin in early Late Triassic and its tectonic movements were not extensive enough to make the Longmen Mountains to be one of the main provenances of the Songpan-Ganze area.