川西江浪穹窿二叠系变玄武岩的地球化学特征与岩石成因

Geochemistry and petrogenesis of the Permian metabasalts in the Jianglang dome, western Sichuan

关键词:  变玄武岩  地球化学  岩石成因  二叠系  江浪穹窿  川西

Key words:  metabasalt  geochemistry  petrogenesis  Permian  Jianglang dome  western Sichuan

摘要:

318江浪穹窿二叠系地层中发育一套顺层产出的变玄武岩,主要由角闪石(~80%)、斜长石(~15%)与少量的石英(< 3%)、磁铁矿(~2%)等组成,具有明显的枕状构造。为深入探讨变玄武岩的岩石成因,本文对其进行了主微量元素分析。结果显示,变玄武岩具有低的SiO₂(平均44.14%)与TiO₂(平均1.79%)含量、高的^TFe₂O₃(平均13.95%)和MgO(平均11.64%)含量,Mg^#值介于65.6~58.3;稀土配分型式显示为轻微的右倾型,Ce异常(Ce/Ce^*平均0.89)与Eu异常(Eu/Eu^*平均1.03)不明显;富集大离子亲石元素Rb、Ba和U,亏损高场强元素Nb、P、Zr和Hf;低的La/Sm值(2.71~2.26)、(Th/Ta)_PM值(1.36~1.14)与(La/Nb)_PM值(1.78~1.33)。综合分析认为,该套变玄武岩属于洋底玄武岩,可能是古特提斯洋的洋壳残余。岩浆源区为亏损地幔混有少量的富集地幔组分,岩浆上升侵位过程中没有遭受地壳物质的混染。与峨眉山低钛玄武岩(LT1)的对比显示,该套变玄武岩并非晚二叠世峨眉山玄武岩。

Abstract:

A succession of bedded metabasalts occurs in the Permian strata in the Jianglang dome, western Sichuan. These rocks have well-defined pillow structures, and are composed mainly of hornblende (~80%), plagioclase (~15%) and minor quartz (< 3%) and magnetite (~2%). The results of research in this study have disclosed that the metabasalts are characterized by low SiO₂ (an average of 44.14%) and TiO₂ (an average of 1.79%), high ^TFe₂O₃ (an average of 13.95%) and MgO (an average of 11.64%); slightly right-leaning REE distribution patterns and unmarkedly Ce and Eu anomalies; enrichment of large ion lithophile elements Rb, Ba and U, and depletion of high field strength elements Nb, P, Zr and Hf; low La/Sm ratios (2.71-2.26),(Th/Ta)_PM ratios (1.36-1.14) and (La/Nb)_PM ratios (1.78-1.33). The above-mentioned results show that the Perian metabasalts in the Jianglang dome may be assigned to ocean-floor basalts, and remnants of palaeo-Tethyan oceanic crust. The magmas may be derived from the depleted mantle mixed with minor enriched mantle components, and were not contaminated with crustal matter during the ascending processes. Instead, they may be subjected to the crystallization differentiation of ilmenite. Compared with the Emeishan basalts, the metabasalts in the study area are not the products of the Late Permian Emeishan basaltic magma activities.