西藏仲巴县特提斯喜马拉雅早白垩世日朗组玄武岩地球化学特征及其构造意义

摘要

Lower Cretaceous volcaniclastic litharentites are widely distributed in the Tethyan Himalaya,while less research was reported about the provenance of these volcaniclastic litharentites.In this paper,we report sedimentray and geochemical characteristics of basalts in Rilang Formation of central-western Tethayan Himala-ya.Major and trace element compositions show that these basalts belong to alkaline basalt.They are enriched in LREE relative to HREE,similar geochemically to Ocean Island Basalt (OIB)and intra-plate basalt.The con-tents of Nb fall between the upper crust and lower crust.These basalts have a relatively low content of Th than that of lower crust and relatively high Th /Nb ratio,low Ce /Pb ratio,which indicates that these basalts suffered variable crustal contamination.Sedimentary and geochemical characteristics of these basalts are apparently dif-ferent from seamounts basalt in the Yarlung-Zangbo ophiolitic mélange belt.The tectonic setting discrimination diagrams based on trace elements suggest that basalts of the Rilang Formation formed in intracontinental rift en-vironment.Combining the petrology and depositional environments of sedimentary rocks,we conclude that these basalts maybe the one of the provenance of volcaniclastic litharentites of the Rilang Formation.%特提斯喜马拉雅地层中广泛分布早白垩世火山碎屑岩，但对这套火山碎屑岩的源区缺乏有力的约束。在特提斯喜马拉雅中西段仲巴地区白垩系日朗组地层中发现一套玄武岩夹层，该玄武岩为碱性玄武岩，表现为 LREE 富集的分布型式，与典型的 OIB 和区域上的板内玄武岩类似。玄武岩 Nb 含量介于下地壳与上地壳之间，Th 含量略低于下地壳，具有较高的 Th／Nb 比值和较低的 Ce ／Pb，指示岩浆在演化过程中遭受了一定程度的地壳混染，与雅鲁藏布蛇绿岩混杂岩带中的海山明显不同。构造环境判别图解显示玄武岩形成于大陆板内裂谷环境，结合日朗组地层沉积环境的分析，该玄武岩可能为日朗组火山碎屑岩提供物源。