吉林省西保安二叠纪E-MORB型基性岩岩石成因及其对古亚洲洋演化的启示

Petrogenesis of Permian E-MORB type mafic rocks in Xibao’an area, Jilin Province and its implications for evolution of Paleo-Asian Ocean

摘要

斜长角闪片岩是西保安岩组的主要岩石类型之一, 为探讨其形成时代与构造背景, 利用岩相学、岩石地球化学和锆石 U-Pb 年代学对其进行研究。结果表明, 斜长角闪片岩主要矿物成分为角闪石和斜长石, 具有低硅、钾, 高镁、钙特征, 其原岩为基性岩浆岩, 具拉斑系列 - 钙碱性系列过渡特征, 呈稀土总量偏低、轻微的轻稀土元素富集和平坦的重稀土配分模式, 其 (La/Yb)_N值介于 2.15~2.40, 微量元素无明显的 Nb-Ta 异常, 与 E-MORB 相似。斜长角闪片岩样品中岩浆成因锆石的 εHf(t) 值为 +7.6~+14.1。岩石地球化学特征显示, 明显富集 Rb、Ba 等大离子亲石元素, 并具有以下关键元素比值特征: Nb/Ta=13.6~16.0, Zr/Hf=37.5~38.8, La/Ta=10.4~11.7, La/Nb=0.74~0.81, Th/Sm=1.80~2.35, Th/Yb=2.41~2.80。上述特征共同表明, 其岩浆来源于曾遭受俯冲沉积物熔体交代的亏损软流圈地幔源区。锆石 U-Pb 定年显示其结晶年龄为 274±4 Ma, 表明形成时代为早二叠世末期。综合区域地质资料, 认为该 E-MORB 型基性岩形成于古亚洲洋板块低角度前 进式俯冲或弧 - 陆碰撞、华北板块北缘东段地壳由减薄向增厚转变阶段, 为局部伸展背景下的岩浆活动产物。

Abstract

Amphibolite schist is one of the main rock types in the Xibao’an rock formation. To investigate its formation age and tectonic setting, we conducted petrographic, whole-rock geochemical, and zircon U-Pb geochronological analyses. The results show that the amphibole-plagioclase schist is mainly composed of hornblende and plagioclase, with low Si and K contents and high Mg and Ca contents. Its protolith is basic magmatic rock, exhibiting transitional characteristics between tholeiitic and calc-alkaline series. The total rare earth element content is relatively low, with slight enrichment of light rare earth elements and a flat heavy rare earth element distribution pattern. The (La/Yb)_N ratio ranges from 2.15 to 2.40, and the trace elements show no significant Nb-Ta anomalies, resembling E-MORB. The magmatic zircons in the amphibolite schist samples have εHf(t) values ranging from +7.6 to +14.1. The whole-rock geochemical characteristics reveal significant enrichment in large-ion lithophile elements (e.g., Rb and Ba) and distinctive elemental ratios: Nb/Ta = 13.6~16.0, Zr/Hf = 37.5~38.8, La/Ta = 10.4~11.7, La/Nb = 0.74~0.81, Th/Sm = 1.80~2.35, and Th/Yb = 2.41 ~ 2.80. Collectively, these features indicate that the magma was derived from a depleted asthenospheric mantle source that had been metasomatized by melts from subducted sediments. Zircon U-Pb dating yields a crystallization age of 274 ± 4 Ma, suggesting that the rock formed during the late Early Permian. Integrated with regional geological data, it is proposed that this E-MORB-type basic rock was generated in a local extensional setting, associated with either low-angle progressive subduction of the Paleo-Asian oceanic plate or an arc-continent collision event during the transitional stage from crustal thinning to thickening in the eastern segment of the northern margin of the North China Craton.

关键词

二叠纪 / 斜长角闪片岩 / 西保安岩组 / 华北板块北缘东段 / 古亚洲洋

Key words

Permian / amphibolite schist / Xibao’an rock formation / Eastern segment of the northern margin of the North China Block / Paleo-Asian Ocean

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基金资助

国家自然科学基金项目(42302244)

山东省自然科学基金项目(ZR2025MS549)