Nd–Sr isotopic constraint to the formation of metatexite and diatexite migmatites, Higo metamorphic terrane, central Kyushu, Japan

ABSTRACTMetatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite-facies zones of the Higo low-P/high-T metamorphic terrane. Here we report Nd–Sr isotopic and whole rock composition data from an outcrop in the highest-grade part of the granulite-facies zone, in which diatexite occurs as a 3 m-thick layer between 2 m-thick layers of stromatic-structured metatexite within pelitic gneiss. The metatexite has Nd–Sr isotopes and whole rock compositions similar to those of the gneiss, but the diatexite shows the reverse. The diatexite has a higher εNd(t) and 147Sm/144Nd ratio (εNd(t) = −0.5; 147Sm/144Nd = 0.1636) than the gneiss (εNd(t) = −2.1; 147Sm/144Nd = 0.1287) and metatexite (εNd(t) = −3.1; 147Sm/144Nd = 0.1188). The (87Sr/86Sr)initial and 87Rb/86Sr of the diatexite ((87Sr/86Sr)initial = 0.70568; 87Rb/86Sr = 0.416) are lower than those of the gneiss ((87Sr/86Sr)initial = 0.70857; 87Rb/86Sr = 1.13) and metatexite ((87Sr/86Sr)initial = 0.70792; 87Rb/86Sr = 1.11). The metatexite and gneiss show enrichment of Th and depletion of P and Eu and have a similar chondrite-normalized REE pattern, which shows steep LREE–MREE-enriched and gently declining HREE patterns and negative Eu anomalies, whereas the diatexite shows enrichment of Sr and depletion of Th and Y, and exhibits gently declining LREE and steeply declining HREE pattern and weak Eu depletion. The metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source, whereas the diatexite migmatite included an externally derived melt with a juvenile component. The Cretaceous high-temperature metamorphism of the Higo metamorphic terrane is interpreted to reflect emplacement of mantle-derived basalts under a volcanic arc along the eastern margin of the Eurasian continent, and mass transfer and advection of heat via hybrid silicic melts from the lower crust.【摘要】浅熔混合岩（metatexite）与深熔混合岩（diatexite）广泛分布于日向低P/high-T变质地体（Higo low-P/high-T metamorphic terrane）的上角闪岩相及麻粒岩相带中。本文报道了采自该麻粒岩相带最高变质级露头的钕-锶同位素与全岩成分数据：该露头中，深熔混合岩以3米厚层状产出，夹于两层各2米厚的条带状构造浅熔混合岩（stromatic-structured metatexite）之间，围岩为泥质片麻岩。浅熔混合岩的钕-锶同位素组成与全岩成分与围岩片麻岩近似，但深熔混合岩则呈现相反特征。深熔混合岩的εNd(t)值与147Sm/144Nd比值（εNd(t) = −0.5；147Sm/144Nd = 0.1636）高于片麻岩（εNd(t) = −2.1；147Sm/144Nd = 0.1287）与浅熔混合岩（εNd(t) = −3.1；147Sm/144Nd = 0.1188）。深熔混合岩的初始(87Sr/86Sr)比值与87Rb/86Sr比值（(87Sr/86Sr)initial = 0.70568；87Rb/86Sr = 0.416）低于片麻岩（(87Sr/86Sr)initial = 0.70857；87Rb/86Sr = 1.13）与浅熔混合岩（(87Sr/86Sr)initial = 0.70792；87Rb/86Sr = 1.11）。浅熔混合岩与片麻岩均富集钍（Th）、亏损磷（P）与铕（Eu），且球粒陨石标准化稀土元素配分模式相似：轻稀土元素（LREE）-中稀土元素（MREE）富集且配分曲线陡倾，重稀土元素（HREE）配分曲线平缓，同时具铕负异常；而深熔混合岩则富集锶（Sr）、亏损钍（Th）与钇（Y），轻稀土元素配分曲线平缓、重稀土元素配分曲线陡倾，仅具微弱铕亏损。研究认为，浅熔混合岩形成于源区原位部分熔融，且熔体未发生迁移；而深熔混合岩则包含了外源的新生组分熔体。日向变质地体的白垩纪高温变质作用，被认为对应欧亚大陆东缘火山弧环境下幔源玄武岩的侵位，以及下地壳混合硅质熔体所介导的物质迁移与热平流过程。