巴楚辉绿岩墙的岩石成因及其对塔里木大火成岩省岩浆演化的启示

Petrogenesis of the mafic dykes from Bachu and implications for the magma evolution of the Tarim large igneous province,NW China

早二叠世塔里木大火成岩省主要有两期岩浆活动:以柯坪玄武岩为代表的喷出岩形成较早(~290Ma),以巴楚超基性-基性-长英质侵入体和岩墙为代表的侵入岩形成较晚(~280Ma)。本研究选取代表晚期岩浆活动的巴楚辉绿岩墙为研究对象,旨在为认识塔里木地幔柱活动及与岩石圈的相互作用提供更多信息。巴楚辉绿岩XHZ-10样品中的锆石均为继承性锆石,不能用来限定其侵位时代。然而,野外关系表明巴楚辉绿岩墙属于塔里木大火成岩省的第二期岩浆活动。巴楚辉绿岩墙经历了橄榄石、辉石和磁铁矿的分离结晶。Nd同位素比值与SiO2和La/Nb比值的相关性以及继承性锆石年龄(2480~717Ma,主要集中在~800Ma)表明,新元古代基底的同化混染作用可以解释其εNd具有较大的变化范围(-2.6^+5.1)。受混染程度最小的样品具有与洋岛玄武岩(OIB)极其相似的微量元素特征,并且具有亏损的Nd同位素组成(εNd=^+5),暗示其来源于富集的对流地幔源区,与早期(~290Ma)具有岩石圈地幔源区特征的玄武岩形成鲜明对比。本文提出地幔柱侧向流动模型以解释两期岩浆的时空分布和地球化学差异:当地幔柱上升至塔里木岩石圈底部时,巨厚的岩石圈地幔(>140km)会阻碍其减压熔融;但不断积累在岩石圈底部的地幔柱物质提供足够的热,促使早期富集交代的岩石圈地幔发生部分熔融,形成~290Ma的玄武岩;地幔柱物质将向岩石圈厚度较薄的边缘地区侧向流动,发生减压熔融,形成的熔体侵位到地壳中形成超基性-基性-长英质侵入岩和辉绿岩墙,并诱发地壳的熔融形成长英质火山岩。

The Early Permian Tarim large igneous province （TLIP） in Northwest China consists of two main magmatic phases： the earlier one （ ～ 290Ma） represented by basahs in Keping, and the later one （ ～ 280Ma） by uhramafic-mafic-felsic intrusions and mafic dykes in Bachu. In order to identify mantle and crustal components involved in the genesis of the TLIP and to understand the plumelithosphere interaction, we performed zircon U-Pb dating, geochemical analyses on the Bachu dykes. Zircons from sample XHZ-10 are all inherited in origin, thus yielding no constraints on eruption age. Field relationship, however, suggests that the Bachu dykes were emplaced during the second episode of TLIP magmatism. The Bachu dykes experienced fractionation of olivine, clinopyroxene and magnetite. Correlations among εNd and SiO2 and La/Nb, as well as the age spectrum of inherited zircons （2480 ～717Ma, with a peak at ～800Ma） suggest that some dykes with low εNd may have been assimilated by Neoproterozoic basements en route to the surface. Nevertheless, the least-contaminated samples show OIB-like trace element compositions and high εNO （ ±5 ）, consistent with a derivation from a convective mantle source. This is in contrast with the earlier phase of magmatism （～ 290Ma）, which was mainly derived from an enriched sub-continental lithospheric mantle （SCLM）. We propose here a model involving plume lateral deflection to account for these two discrete magmatic phases in the TLIP. The earlier phase formed as a response of impact of a mantle plume at the base of the SCLM. Partial melting of the enriched lithospheric mantle was triggered by conductive heating of the impregnating plume. The later phase was generated by decompression melting of the mantle plume, as a result of deflection of the plume towards the margins of the Tarim craton where the lithosphere is relatively thin.