西南天山托云地区白垩纪-早第三纪玄武岩地球化学及其成因机制

Geochemistry and genesis of Cretaceous-Paleogene basalts from the Tuoyun Basin,Southwest Tianshan Mountains

托云玄武岩主要分布于托云盆地东侧,按形成时代可分为白垩纪玄武岩和早第三纪玄武岩。白垩纪玄武岩包括早白垩世玄武岩和晚白垩世玄武岩,以碱玄岩和碱性橄榄玄武岩为主,碱性程度高;早第三纪玄武岩包括早第三纪玄武岩及脉岩,以碱性橄榄玄武岩、碱性橄榄辉绿岩为主,碱性程度低。所有岩石稀土元素(REE)、微量元素分布模式相似,REE均为向右陡倾型,富集不相容元素。白垩纪玄武岩的∑REE、Rb、Ba、Th、K、Sr、Nb和Ta等元素富集程度均高于早第三纪玄武岩,相容元素富集程度大体较低。微量元素和同位素特征显示,玄武岩起源于与洋岛玄武岩源区相似的富集地幔源。玄武岩中赋存有交代地幔捕虏体,这表明玄武岩浆可能是交代地幔经不同程度部分熔融的产物。微量元素的特征同时显示,早白垩世玄武岩部分熔融程度较低,早第三纪玄武岩部分熔融程度较高,且在不断的部分熔融过程中,形成的岩浆又有结晶分异作用发生。托云玄武岩形成于大陆板内拉伸环境,在形成过程中经历了较弱的壳幔相互作用。

The basalts from the Tuoyun Basin are distributed in the eastern part of the basin, which include the Cretaceous basalts and the Paleogene basalts based on the forming ages. The Cretaceous basalts including the early and late Cretaceous basalts are predominantly composed of tephrites and alkaline basalts with high alkaline degree; the Paleogene basalts that consist of Paleogene basalts and intrusions are mainly a set of alkaline basalts with low alkaline degree. Chemically, the basalts show strong enrichment of LREE and incompatible elements with ∑REE, LILE (e.g. Rb, K, Sr and Th) and HFSE (e.g. Nb and Ta) of the Cretaceous basalts being higher than that of the Paleogene basalts. According to the characteristics of trace elements and isotope, the source of the basalt is similar to the OIB source. It is deduced that the basaltic magma are the product of metasomatized mantle with different partial melting degrees based on the metasomatized mantle xenoliths contained in the basalts. The characteristics of trace elements also indicate that the Early Cretaceous basalts are the product of low degree partial melting and the Paleogene basalts of high degree partial melting with fractional crystallization occurred on the magma evolutionary processes. The basalts in the Tuoyun Basin formed in intracontinental plate extending environment and underwent weak crust mantle interaction during the period of magma ascending to the surface through the thinner lithosphere.