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桑秀组玄武岩仅仅分布在特提斯喜马拉雅带中段东部,古地理属大印度北缘。分别采用XRF、ICP-MS和全岩同位素稀释法对这些玄武岩的主元素、微量元素和Sr-Nd同位素进行了系统分析,并用来推论其成因。结果显示,桑秀组玄武岩除了MgO含量和相容元素含量较低表明其为演化岩浆以外,其高TiO2、TFeO和P2O5含量(分别平均为3.46%、10.90%和0.51%)、轻重稀土明显分馏(La/Yb)N=8.4～10.2、Ti/V、Ti/Y和Zr/Y比值高以及富集Ba和Th等不相容元素和高场强元素等特征与洋岛玄武岩(OIB)相似;桑秀组玄武岩高Sr(87Sr/86Sr)t=0.707370～0.709904和较低εNd(t)(=-1.71～2.00)同位素组成以及微量元素地球化学指标等又显示了岩石圈地幔物质的印记;微量元素特征显示桑秀组玄武岩为大陆边缘裂谷背景下的碱性玄武岩,在源区物质低度部分熔融过程中不断有橄榄石等铁镁矿物的分离结晶,没有遭受地壳混染;桑秀组玄武岩的地球化学特征和同位素组成可与印度东缘的Rajmahal玄武岩、印度洋90°E海岭玄武岩和KerguelenOIB对比,提出桑秀组玄武岩可能是岩石圈地幔与地幔热柱或热点物质相互作用的产物,这种地幔热柱或热点可能与Kerguelen热点的早期活动有联系。
The basalts of the Sangxiu Formation are only distributed in the middle part of the Tethyan Himalayas belt. The ancient geography belongs to the northern margin of Great India. The main elements, trace elements and Sr-Nd isotopes of these basalts were systematically analyzed by XRF, ICP-MS and all-rock isotope dilution method, respectively, and used to infer the genesis. The results show that the basalts in the Sangxiu Formation have high contents of TiO2, TFeO and P2O5 (averagely 3.46%, 10.90% and 0.51%, respectively) except for the low contents of MgO and compatible elements, which are obviously fractionated by light and heavy rare earths (La / Yb) N = 8.4-10.2, Ti / V, Ti / Y and Zr / Y ratios are high and enrichment of incompatible elements such as Ba and Th and high-field-strength elements are similar to oceanic basalts (OIB) (87Sr / 86Sr) t = 0.707370 ~ 0.709904 and lower εNd (t) (= -1.71 ~ 2.00) basalts in the Sangxiu Formation also show lithospheric mantle material imprints; The trace element characteristics show that the basalts in the Sangxiu Formation are basic basalts in the continental rift background. The separation and crystallization of olivine and other iron-magnesium minerals continue to occur during the low-level partial melting of the source region and are not subject to crustal contamination. The geochemical characteristics and isotopic compositions of the basalts can be compared with the Rajmahal basalts in the eastern margin of India and the 90 ° E Ridge basalts and Kerguelen OIBs in the Indian Ocean. It is suggested that the basalts in the Sangxiu Formation might be the product of interaction between lithospheric mantle and hot mantle plumes or hot objects. This mantle hot column Or hot spots may be linked to early events at Kerguelen Hotspots.