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-<i>P</i>/high-<i>T</i> 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 ε_Nd(t) and ¹⁴⁷Sm/¹⁴⁴Nd ratio (ε_Nd(t) = −0.5; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1636) than the gneiss (ε_Nd(t) = −2.1; ¹⁴⁷Sm/¹⁴⁴Nd = 0.1287) and metatexite (ε_Nd(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 <i>in situ</i> 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 <i>via</i> hybrid silicic melts from the lower crust.

**摘要**：条痕状混合岩（metatexite）与深熔混合岩（diatexite）广泛分布于日高低压力-高温度（low-P/high-T）变质地体的上角闪岩相带与麻粒岩相带内。本次研究报道了采自麻粒岩相带最高变质级部位一处露头的钕-锶（Nd-Sr）同位素与全岩组成数据；该露头中，深熔混合岩以3米厚层状产出，夹于泥质片麻岩中两套2米厚的条带状构造条痕状混合岩层之间。条痕状混合岩的Nd-Sr同位素组成与全岩地球化学特征与该套片麻岩近似，而深熔混合岩则与之截然相反。深熔混合岩的ε_Nd(t)值与¹⁴⁷Sm/¹⁴⁴Nd比值（ε_Nd(t) = -0.5；¹⁴⁷Sm/¹⁴⁴Nd = 0.1636）高于片麻岩（ε_Nd(t) = -2.1；¹⁴⁷Sm/¹⁴⁴Nd = 0.1287）与条痕状混合岩（ε_Nd(t) = -3.1；¹⁴⁷Sm/¹⁴⁴Nd = 0.1188）。深熔混合岩的初始⁸⁷Sr/⁸⁶Sr比值与⁸⁷Rb/⁸⁶Sr比值（初始⁸⁷Sr/⁸⁶Sr = 0.70568；⁸⁷Rb/⁸⁶Sr = 0.416）低于片麻岩（初始⁸⁷Sr/⁸⁶Sr = 0.70857；⁸⁷Rb/⁸⁶Sr = 1.13）与条痕状混合岩（初始⁸⁷Sr/⁸⁶Sr = 0.70792；⁸⁷Rb/⁸⁶Sr = 1.11）。条痕状混合岩与片麻岩均表现为钍（Th）富集、磷（P）与铕（Eu）亏损，且二者具有相似的球粒陨石标准化稀土元素配分模式：轻重稀土分馏显著，轻-中稀土（LREE-MREE）富集且重稀土（HREE）平缓递减，同时具负铕异常；而深熔混合岩则表现为锶（Sr）富集、钍（Th）与钇（Y）亏损，其稀土配分模式为轻稀土平缓递减、重稀土陡直递减，仅具微弱铕亏损。研究认为，条痕状混合岩形成于原地部分熔融过程，且熔融体未从源区发生迁移；而深熔混合岩则包含了源自外部的、带有新生组分的熔融体。日高变质地体的白垩纪高温变质作用，被认为是欧亚大陆东缘火山弧环境下地幔源玄武岩侵位，以及下地壳混合硅质熔体引发的热平流与质量转移过程的响应。