Petrogenesis of metatexite and diatexite migmatites determined using zircon U-Pb age, trace element and Hf isotope data, Higo metamorphic terrane, central Kyushu, Japan

摘要

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 ℃ 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 ~(206)Pb/~(238)U age of 116.0 ± 1.6 Ma, which is older than the 110.1 ± 0.6 Ma ~(206)Pb/~(238)U 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 HREEenriched patterns; however, both types have similar positive Ce and negative Eu anomalies. ~(176)Hf/~(177)Hf 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.