峨眉山大火成岩省米易晚二叠世正长岩的成因及其对地幔柱−岩石圈相互作用的启示

Petrogenesis of Miyi Late Permian syenites and implications for plume-lithosphere interaction in the Emeishan large igneous province

摘要

扬子板块西缘广泛发育晚二叠世岩浆活动，包括大规模的溢流玄武岩以及与其时空紧密联系的碱性岩。峨眉山大火成岩省（ELIP）碱性岩具有从钠质到高钾质的复杂成分，其成因仍存在争议。本文采集四川省米易地区的钠质到高钾质正长岩，通过岩石学和地球化学研究，探讨地壳物质在米易正长岩复杂成分变化中的作用，并进一步约束 ELIP 地幔柱−岩石圈相互作用过程。米易正长岩由碱性长石、斜长石、霓辉石构成，其中霓辉石具有富集高场强元素和亏损大离子亲石元素的特征，以及明显的 Ta、Sr 和 Eu 的负异常。锆石 U-Pb 年代学揭示米易正长岩的年龄为（258.8±1.7） Ma （MSWD = 0.78），属于峨眉山地幔柱活动晚期。米易正长岩具有中等的 SiO₂ 质量分数（58.91%～66.19%），高的全碱含量［w（Na₂O +K₂O） = 9.48%～11.50%］，铝饱和指数为 1.20～1.34，莱特碱度率为 2.17～5.14，为典型的碱性岩。岩石相对富集轻稀土元素和亏损重稀土元素，（La/Yb）_N = 9.38～16.50。岩石具有富集的 Sr-Nd-Pb 同位素组成［（ 87Sr/⁸⁶Sr）_t = 0.707 26，ε_Nd（t）= −1.87，（ 206Pb/²⁰⁴Pb）_t = 18.835］。热力学模拟结果表明：米易正长岩可以由高钛玄武质岩浆分离结晶产生，分离结晶的矿物组合为～35% 斜长石、～ 17% 单斜辉石、～11% 橄榄石和～4% 氧化物。微量元素和同位素模拟结果表明，米易正长岩是由受到少量地壳物质混染（～10%）的高钛玄武质岩浆经过 30%～70% 分离结晶产生。因此本文认为在 ELIP 晚期拉张背景下，由于地幔柱来源的幔源岩浆的结晶分异及其与地壳物质的相互作用，形成了 ELIP 内带广泛发育的钠质到高钾质正长岩。

Abstract

The western margin of the Yangtze Block is widely characterized by extensive magmatic activities that occurred during the Late Permian, including large-scale flood basalt and alkaline rocks that were closely associated in both time and space. These alkaline rocks of the Emeishan large igneous province (ELIP) exhibits variable compositions ranging from sodic type to ultrapotassic type, and their petrogeneses are controversial. Therefore, through petrology and geochemistry analyses, this study focused on syenite, ranging from sodic type to ultrapotassic type, in the Miyi region, Sichuan Province, to explore the role of crustal materials in the process of composition transformation of Miyi syenites, and further constrain the plume-lithosphere interaction of the ELIP. The Miyi syenites are composed of alkali feldspar, plagioclase, and aegirine-augite. The aegirine-augite exhibits characteristics of enrichment in high-field-strength elements, depletion in large-ionic-lithophile elements, and negative anomalies in Ta, Sr, and Eu. Zircon U-Pb dating reveals that the Miyi syenites have an age of (258.8 ± 1.7) Ma (MSWD = 0.78), corresponding to the late stage of Emeishan mantle plume activity. The Miyi syenites display moderate SiO₂ (58.91% to 66.19%), high AK (Na₂O +K₂O = 9.48% to 11.50%), A/CNK values of 1.20 to 1.34, and A.R values of 2.17 to 5.14. The syenite is relatively enriched in light rare earth elements and depleted in heavy rare earth elements, with (La/Yb)_N = 9.38 to 16.50. The rocks exhibit evolved Sr-Nd-Pb isotope compositions, with ( 87Sr/⁸⁶Sr)_t = 0.707 26, ε_Nd(t) = −1.87, and ( 206Pb/ 204Pb)t = 18.835. The thermodynamic modeling results indicate that the Miyi syenites could have been produced by fractional crystallization of high-Ti basaltic magma. The fractionated mineral assemblage consists of ～35% plagioclase, ～17% clinopyroxene, ～11% olivine, and ～4% oxide. The results of the trace element and isotopic modeling indicate that the Miyi syenites were derived from high-Ti basaltic magma that underwent 30% to 70% fractional crystallization with minor crustal assimilation (～ 10%). Therefore, this paper argues that in the extensional settings of the late stage of the ELIP, fractional crystallization of mantle-plume-derived magma, and its interaction with crustal material led to the widespread development of sodic-type to ultrapotassic-type syenites within the inner zone of the ELIP.

关键词

峨眉山大火成岩省 / 碱性岩 / 同化混染和分离结晶作用 / 地幔柱−岩石圈相互作用

Key words

Emeishan large igneous province / alkaline rocks / assimilation and fractional crystallization / plume-lithosphere interaction

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基金资助

国家自然科学基金青年科学基金(42203053)

四川省自然科学基金(23NSFSC1557)