华北克拉通恒山地区晚太古代变质火山岩的地球化学特征及构造意义

对恒山地区出露的变质火山岩开展岩相学、地球化学、锆石U-Pb年代学和Hf同位素研究,旨在确定其形成时代,探讨岩石成因及地球动力学意义。锆石U-Pb定年结果表明,这些变质火山岩喷发于新太古代晚期(2508±20Ma)。地球化学分析结果显示,恒山变质火山岩由变质玄武岩和变质玄武安山岩组成,SiO₂(45.51%～62.67%)、FeO_T (4.43%～15.72%)和MgO (3.75%～8.14%)含量变化幅度大,是幔源岩浆经单斜辉石、角闪石和磁铁矿分离结晶的产物。这些变质火山岩富集轻稀土(LREE)和大离子亲石元素(LILE),亏损重稀土(HREE)和高场强元素(HFSE),具有相对高的Th含量和Th/Yb比值,呈现钙碱性岛弧火山岩的特征。结合不相容元素比值Nb/Yb,Zr/Yb和(Nb/La)_N的特点,推测其来源于俯冲带具流体交代特征的富集地幔源区。结合区域构造背景,推断恒山变质火山岩形成于新太古晚期大陆弧环境。

Geochemical Features and Tectonic Significance of Late ArcheanMetavolcanic Rocks in Hengshan Area,North China Craton

Petrographic, geochemical, zircon U-Pb geochronogical and Hf isotopic data are presented for the exposed metavolcanic rocks in Hengshan region to determine their eruption age, petrogenesis and geodynamic significance in the Trans-North China Orogen, North China Craton. Zircon U-Pb dating of the rocks yielded a weighted mean 207Pb/206Pb age of 2508 ± 20 Ma, indicating that their protoliths were erupted in the late Neoarchean. Geochemically, the Hengshan metavolcanic rocks are composed of basalt and basaltic andesite and marked by variable SiO₂ (45.51%–62.67%), FeOt (4.43%–15.72%) and MgO (3.75%–8.14%) contents, indicative of product of derivation from a mantle source with subsequent fractional crystallization of clinopyroxene, hornblende and magnetite. These metavolcanic rocks are characterized by enrichments in light rare earth elements (LREE) and large-ion lithophile elements (LILE), as well as depletions in heavy rare earth elements (HREE) and high-field-strength elements (HFSE), with relatively high Th content and Th/Yb values, similar to those of the typical calc-alkaline arc-like volcanic rocks. Such arc-like geochemical signatures, together with some incompatible elemental ratios (e.g., Nb/Yb, Zr/Yb and (Nb/La)N), reveal their origination from a sub-arc enriched mantle wedge variably metasomatized by the slab-derived dehydration fluids. Combined with the region geological background, these Hengshan metavolcanic rocks were developed on a continental arc setting in the late Neoarchean.