青藏北部新生代钾质火山岩微量元素和Sr、Nd同位素地球化学研究

STUDY ON TRACE ELEMENT AND Sr, Nd ISOTOPIC GEOCHEMISTRY OF CENOZOIC POTASSIC VOLCANIC ROCKS IN NORTH TIBET

通过微量元素和Sr、Nd同位素组成研究揭示了青藏高原北部第三纪火山岩在成因上与初始裂谷作用有关;第四纪火山岩的原始岩浆来自地球化学异常的富集地幔源区,在融体上升和喷发过程中又受到地壳的污染。

The Cenozoic potassic volcanic rock zone in north Tibet can be divided into the southern and northern subzones. Geochemically they belong to typical post-collision types. Tertiary volcanic rocks occurring in the southern subzone are characterized by quite low ratios of ^(87)Sr/ ^(86)Sr(0.704634～0.706296) and higher ratios of ^(143)Nd/^(144)Nd(0.51259～0.51267), and consistent with alkaline volcanic rocks in the East African rift zone and related to an initial rift zone within the Qiangtang terrane in genesis. Southward A-type subduction of the Tarim terrane is an important geological baekground for Quaternary volcanic eruption in the northern subzone. ^(87)Sr/^(86)Sr and ^(143)Nd/^(144)Nd ratios of these potassic volcanic rocks are 0.707573～0. 710540 and 0.512221～0.512365,respectively, which suggests that the primary magmas might have been derived from an heterogeneous enriched sources developing in a subcontinental lithosphere; but they were contaminated by crust during rising and eruption.