四川盆地西北缘早-中三叠世间白云质泥岩碎屑锆石U-Pb年代学、地球化学及地质意义

早-中三叠世时期四川盆地西北缘雷口坡组底部发育的一套白云质泥岩与该时期华南及川黔地区所分布的“绿豆岩”截然不同。本文对该套白云质泥岩进行了碎屑锆石U-Pb定年、X衍射及主微量地球化学分析，以揭示白云质泥岩的岩石成分及物源信息。通过碎屑锆石年龄显示，特征峰值主要集中在473～236 Ma，972～750 Ma，1 436～1 114 Ma，1 983～1 628 Ma和2 635～2 253 Ma，与川西中-上三叠统地层有较高的相似性，而与川东存在较大区别，结合岩相古地理特征指示物源为秦岭造山带、扬子板块西缘龙门山岛链局部剥蚀区。镜下特征、X衍射及主量元素特征则表明岩石中含少量白云石，粘土矿物主要为伊利石。通过微量、稀土元素分析则显示白云质泥岩碎屑物质主要来自于长英质岩石，且川西中-上三叠统地层有向稳定构造型碎屑物源过渡的趋势。白云质泥岩与川西中-上三叠统地层REE配分曲线相似，而与“绿豆岩”存在明显差异，指示川西三叠纪碎屑来源较为相似，但受到华南火山活动的影响较小。

Detrital zircon U-Pb geochronology,geochemistry and implication for provenance of the dolomitic mudstone in the northwestern margin of Sichuan Basin in the Early-Middle Traissic,SW China

The dolomitic mudstone from the lower of the Leikoupo Formation in the northwestern margin of Sichuan Basin which is grayish-yellow and lacking siliceous pisolite, had been different from the sedimentary volcanic tuff（locally called “green-bean rock”）in South China region during the Early-Middle Traissic. The dolomitic mudstone has been studied with methods of LA-ICP-MS U-Pb isotope dating, geochemical analysis and X-ray diffraction. The detrital zircon age data yield a wide range, varying from 2 635 Ma to 236 Ma with the population groups of 473～236 Ma, 750～972 Ma, 1 436～1 114 Ma, 1 983～1 628 Ma and 2 635～ 2 253 Ma. The results of U-Pb isotope dating are similar to the stratum of Middle-Upper Triassic in Sichuan in addition to the eastern area, indicating the dolomitic mudstone from the lower of the Leikoupo Formation were mainly derived from Qinling Mountains, the westhern of Yangtze Craton, probably little influenced by the volcanic activity in South China. The feature of X-ray diffraction and major elements shows that the dolomitic mudstone contains dolomite and the clay minerals are illite. Diagrams of Co/Th-La/Sc and La/Th-Hf show that the provenance of the dolomitic mudstone is felsic rocks. The dolomitic mudstone from the Leikoupo Formation and the stratum of the western Sichuan Middle-Upper Triassic have similar chondrite-normalized REE patterns, which showing they have the same provenance, but had a greater distinction in “green-bean rock”.