Petrogenesis of metatexite and diatexite migmatites determined using zircon U–Pb age, trace element and Hf isotope data, Higo metamorphic terrane, central Kyushu, Japan
Petrogenesis of metatexite and diatexite migmatites determined using zircon U–Pb age, trace element and Hf isotope data, Higo metamorphic terrane, central Kyushu, Japan
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DOI：
10.1111/jmg.12073
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发表时间：
2014-04
影响因子：
3.4
通讯作者：
K. Maki;T. Yui;K. Miyazaki;M. Fukuyama;K.‐L. Wang;U. Martens;M. Grove;J. Liou
中科院分区：
文献类型：
--
作者：
K. Maki;T. Yui;K. Miyazaki;M. Fukuyama;K.‐L. Wang;U. Martens;M. Grove;J. Liou
变质岩和辉长岩混合岩广泛分布于肥后低磷/高温变质地体的上部角闪岩和麻粒岩相带内。在这里，我们报告了来自麻粒岩相带最高品位部分露头的数据，其中，泥质片麻岩内的泥质片麻岩内 2 m 厚的间质结构变质岩层之间有 3 m 厚的辉长岩层。混合岩和片麻岩含有相同的峰值矿物组合：黑云母+斜长石+石英+石榴石+钾长石，含退行绿泥石±白云母和一些副矿物钛铁矿±金红石±钛矿+磷灰石+锆石+独居石±黄铁矿±硫化锌±方解石。计算出的变质 P-T 条件为 800-900 °C 和 9-12 kbar。硅藻土中的锆石形成拉长的自形晶体，具有振荡分区，但没有核-缘结构。来自片麻岩和变质岩的锆石形成自形-亚面体晶粒，包括由薄边缘覆盖的继承核心。变长岩中的过度生长边缘的 Th/U 比值低于辉长岩中的锆石，并产生 116.0 ± 1.6 Ma 的 206Pb/238U 年龄，该年龄比源自辉长岩中锆石的 110.1 ± 0.6 Ma 206Pb/238U 年龄更老。辉长岩中的锆石具有不同的稀土元素含量，具有向上凸起的模式和平坦的归一化重稀土元素，而变质岩和片麻岩中的过度生长边缘具有陡峭的富含重稀土元素的模式；然而，这两种类型都有相似的正 Ce 和负 Eu 异常。变质岩的过度生长边缘中的 176Hf/177Hf 比率变化更大，并且通常低于辉长岩中锆石的值。根据 U-Pb 年龄、微量元素和 Hf 同位素数据，变质岩中的锆石边缘被解释为是在深熔过程中继承的原岩锆石部分溶解后，从局部衍生的熔体中结晶出来的，而辉长岩中的锆石被解释为是从包含外部衍生成分的熔体中结晶出来的。通过综合混合岩和泥质片麻岩的锆石和岩相数据，变杂岩混合岩被解释为是通过原位部分熔融形成的，其中熔体没有从源头迁移，而辉长岩混合岩包括外部衍生的幼年组分。肥后变质地体的白垩纪高温变质作用被解释为反映了欧亚大陆东缘火山弧下源自地幔的玄武岩的侵位以及来自下地壳的混合硅质熔体带来的热量的平流。中地壳深度高温区深熔熔体的峰后结晶发生在 c 区间。 116–110 Ma，由变辉岩和辉辉岩混合岩的锆石年龄差异表明。
Metatexite 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 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 migmatites and gneiss contain the same peak mineral assemblage of biotite + plagioclase + quartz + garnet + K‐feldspar with retrograde chlorite ± muscovite and some accessory minerals of ilmenite ± rutile ± titanite + apatite + zircon + monazite ± pyrite ± zinc sulphide ± calcite. Calculated metamorphic P–T conditions are 800–900 °C and 9–12 kbar. Zircon in the diatexite forms elongate euhedral crystals with oscillatory zoning, but no core–rim structure. Zircon from the gneiss and metatexite forms euhedral–subhedral grains comprising inherited cores overgrown by thin rims. The overgrowth rims in the metatexite have lower Th/U ratios than zircon in the diatexite and yield a 206Pb/238U age of 116.0 ± 1.6 Ma, which is older than the 110.1 ± 0.6 Ma 206Pb/238U age derived from zircon in the diatexite. Zircon from the diatexite has variable REE contents with convex upward patterns and flat normalized HREE, whereas the overgrowth rims in the metatexite and gneiss have steep HREE‐enriched patterns; however, both types have similar positive Ce and negative Eu anomalies. 176Hf/177Hf ratios in the overgrowth rims from the metatexite are more variable and generally lower than values from zircon in the diatexite. Based on U–Pb ages, trace element and Hf isotope data, the zircon rims in the metatexite are interpreted to have crystallized from a locally derived melt, following partial dissolution of inherited protolith zircon during anatexis, whereas the zircon in the diatexite is interpreted to have crystallized from a melt that included an externally derived component. By integrating zircon and petrographic data for the migmatites and pelitic gneiss, 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 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 advection of heat via hybrid silicic melts from the lower crust. Post‐peak crystallization of anatectic melts in a high‐T region at mid‐crustal depths occurred in the interval c. 116–110 Ma, as indicated by the difference in zircon ages from the metatexite and diatexite migmatites.