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    埃达克岩与皖中沙溪斑岩铜矿的成矿作用

    The adakite and mineralization of the Shaxi porphyry copper-gold deposit, Central Anhui

    • 摘要: 提要:埃达克岩是一组从安山质到长英质系列岩石,具有高硅(≥56%)、富钠(3.5.%≥ Na2O ≥7.5 %)及低的K2O/Na2O (~0.42), 高Mg# (~0.51)和高Ni (20×10-6~40×10-6)和Cr(30×10-6~50×10-6) 的岩浆岩;其主要特点还有高锶(>400×10-6),最高可以达到3000×10-6,轻重稀土强烈分异 ((La/Yb)N>10) 以及非常低的重稀土元素含量 (Yb < 1.8 ×10-6, Y≤18 ×10-6),具有明显Nb、Ta负异常的火山岩和侵入岩。埃达克岩构造环境仍存在很多争议。主要的观点:(1)年轻的俯冲洋壳 (< 25 Ma) 在俯冲过程中脱水熔融;2)加厚下地壳环境下的镁铁质下地壳(如榴辉岩和石榴辉石岩)底垫岩浆作用或拆沉作用下熔融。与埃达克岩石有关的矿产资源主要为铁、铜、金、钼矿床。在对安徽省沙溪斑岩型铜金矿床岩石和同位素地球化学资料分析和处理基础上,笔者认为沙溪侵入岩属于埃达克岩,其成因是燕山晚期西太平洋板块俯冲至扬子地块深部所导致的洋壳板片熔融作用有关,洋壳俯冲过程的大量流体参与,所携带的幔源大离子亲石元素及铜金物质在地壳浅部形成adakite岩石,同时形成该区斑岩型铜金矿床。

       

      Abstract: Abstract:Adakites are intermediate to felsic igneous rocks, andesitic to rhyolitic in composition (basaltic members are lacking). They have trondhjemitic affinities (high-Na2O contents and K2O/Na2O 0.5) and their Mg no. (0.51), high contents of Ni (20×10-6-40×10-6) and Cr (30×10-6-50×10-6) contents are higher than in typical calc-alkaline magmas. Sr contents are high (>300 ×10-6, until 2000 ×10-6) and REE show strongly fractionated patterns with very low heavy REE (HREE) contents (Yb≤1.8×10-6, Y≤18 ×10-6). These rocks are depleted in Nb and Ta compared with other igneous rock. The tectonic formation of adakites are still disputed at present, the points of views are: 1) the melting of young oceanic crust during the subduction;2) delamination or foundering of dense mafic lower crust rocks (e.g., eclogite and garnet pyroxenite) in mafic lower crust to the mantle during continental orogenesis. Consequently, the Fe-Cu-Mo mineralization is closed related to the formation of adakites. On detailed review of geochemical data and geological environment in Shaxi porphyry Cu-Au deposit (central Anhui), we fund the close link between adakite and the regional Cu-Au mineralization. The mineralized model for Shaxi porphyry Cu deposit can be explained by the subduction of the western Pacific plate in the early Cretaceous, whose process released large ion lithophile element (LILE)-rich fluids rise up into the mantle wedge, inducing both its metasomatism and partial melting, trigger a large mount of Cu-Au and fluids materials to the crust environment and melting the subduction plate formed the adakitic assemblage and Shaxi porphyry Cu-Au deposit.

       

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