塔里木盆地西北缘柯坪地区二叠纪玄武岩喷发时限:来自碎屑锆石的证据

The Eruptive Timing of Permian Basalts in Keping Area,the Northwest Margin of Tarim Basin,China:Evidence from Detrital Zircons

塔里木盆地西北缘柯坪地区玄武岩由夹于二叠纪沉积岩系中的两套玄武岩组成,被普遍认为是塔里木大火成岩省的组成部分。对其喷发时限,目前有270~290 Ma和290 Ma前后两种认识。这些认识不仅在时限上存在明显差别,也与该区现在被广泛接受的地层划分方案不协调。针对这一问题,采用碎屑锆石定年方法,对采自3个剖面不同层位7个砂岩样品的碎屑锆石进行了LAICP-MS U-Pb定年,结合已有其他资料,试图通过限定夹有玄武岩的碎屑岩沉积时代,间接探讨该区玄武岩喷发时限。采自下部玄武岩之下的3个碎屑岩样品都含有数量可观的年龄介于275~290Ma的碎屑锆石,指示其沉积时代不早于早二叠世晚期;采自下部玄武岩中陆源碎屑岩夹层的1个砂岩样品,除了含有20粒年龄介于280~290Ma的碎屑锆石外,还含有11粒年龄介于270~279Ma的碎屑锆石;采自两套玄武岩之间陆源碎屑岩的3个砂岩样品都含有大量年龄介于260~280 Ma的碎屑锆石。这些碎屑锆石的年龄表明,柯坪地区夹有玄武岩的二叠纪地层沉积时代不会早于早二叠世晚期,很可能是中二叠世至晚二叠世早期。因此,该区的这些沉积岩所夹玄武岩不可能是在早二叠世期间喷发的。考虑到这些碎屑锆石从结晶经过隆升剥蚀搬运再到沉积需要一定的时间,以及两套玄武岩之间和上部玄武岩之上的沉积岩中所含的古生物化石,推测柯坪地区玄武岩喷发时限可能为中—晚二叠世。这些资料揭示出,塔里木盆地西北缘柯坪地区玄武岩的喷发时限有可能与峨眉山玄武岩是大致相同的。

Permian basalts in Keping area,the northwestern margin of Tarim Basin,as part of Tarim large igneous province,consist of two interlayers in Permian sedimentary sequences.Up to now,there are two different viewpoints on the eruptive timing of those basalts in publications,one is 270-290 Ma,another one is about 290 Ma,which are controversial each other and discordant with biostratigraphy of the area.The eruptive timing of the basalts based on the sedimentary ages of Permian strata holding these basalts,which are constrained by detrital zircon dating data of 7samples from 3sections obtained by LA-ICP-MS zircon U-Pb dating method,was discussed.Three sandstone samples from sedimentary sequences under the lower basalts contain a moderate numbers of zircons with the ages of 275-290 Ma,implying that the oldest deposition age of the sandstone is not earlier than the late of the Early Permian.One sample（Kp10-1）from the sedimentary interlayer of the lower basalts contains 11 younger zircons with the ages of 270-279 Ma,and 20 grains with the ages of 280-289 Ma among 94 zircons.Three samples from sedimentary sequences between lower and upper basalts yield a big number of zircons with the ages of 260-280 Ma.These younger detrital zircon ages show that the depositing timing of Permian strata holding the basalts in Keping area is not early than the late of Early Permian,probably from Middle Permian to the early of Late Permian.Thus,it is impossible that the eruption of basalts in the area took place during Early Permian.Considering the fossils in the sedimentary sequences between two layers of basalts and above the upper basalts,and the long time from crystallisation through uplifting,denudation,and transportation to deposition of the detrital zircons,it is speculated that the eruption of basalts in Keping area took place in Middle-Late Permian,which means that the eruption of Keping basalts is possibly simultaneous with Emeishan basalts in South China.