青藏高原北羌塘盆地东部二叠纪高Ti玄武岩的地球化学特征

Geochemical characteristics of Permian high-Ti basalt in the eastern part of the northern Qiangtang basin, Qinghai-Tibet Plateau.

在北羌塘盆地东部莫云地区早二叠世栖霞期尕笛考组中首次发现高Ti玄武岩。在当郎寨剖面上,火山岩主要产于尕笛考组第2和第4岩性段,第3岩性段中火山岩呈夹层产出,火山岩形成于浅海环境。岩石类型包括杏仁状玄武岩、粗玄岩、块状玄武岩、橄榄玄武岩和安山玄武岩,属碱性玄武岩系列。该火山岩岩石地球化学特征表现为高FeO,极高TiO2,较高FeO/MgO值,而Al2O3、MgO和Mg″值则低,LILE(Rb、Sr、Ba)和HFSE(Nb、Ta、Zr、Hf、Th、P2O5)富集,REE总量高,显示出LREE富集的配分型式,Eu表现为弱的负异常或无异常,具板内碱性玄武岩的地球化学特征。以上特征表明,火山岩形成于陆块边缘拉张带(初始裂谷)环境。

High-titanium basalt was for the first time found in the Early Permian Chihsian Gadikao Formation in eastern part of the northern Qiangtang basin, Qinghai-Tibet Plateau. At the Danglangzhai section, volcanic rocks mainly occur in the Second and Fourth members of the Gadikao Formation, while in the Third Member volcanic rocks occur as intercalations. Volcanic rocks formed in a neritic environment. The rock types include amygdaloidal basalt, dolerite, massive basalt, olivine basalt and andesite-basalt, which belong to the alkali basalt series. Geochemically, these volcanic rocks are characterized by high FeO^＋, very high TiO2, higher FeO^＋/ MgO values and low Al2O3, MgO and Mg＂, as well as enrichment in LILE （P,b, Sr and Ba） and HFSE （ Nb, Ta, Zr Hf, Th and P2O5） and total REE, exhibiting LREE-enriched distribution patterns and a week negative Eu anomaly or no Eu anomaly. They show the geochemical characteristics of intraplate alkali basalt. The above-mentioned characteristics suggest that the volcanic rocks at the Danglangzhai section formed in a continental-margin extensional zone （incipient rift）.