Amphibole zoning, a record of progressive oxidation during crystallization of mafic microgranular enclaves in the Kurobegawa Granitic Pluton

Abstract

Sulfide, oxide, and amphibole in MME (mafic microgranular enclaves) in the young Kurobegawa Granitic Pluton (2 Ma) indicate that their textural and mineralogical features are characteristic of progressively oxidized magnetite-series granitoids. Hydrous mafic magma intruded into the chamber from the bottom and fragmented to form MME, subsequently vesiculated and floated through the chamber. The MME generally have sharp fine chilled margins and show no in situ mixing between the MME and the host granite. Amphibole, an early phase together with sulfides and plagioclase, began to crystallize just after the mafic magma intruded into the chamber bottom (10 km in depth) and then continued to crystallize as the MME floated. Amphibole shows strongly patchy zoning from brownish green pargasitic core to pale green magnesio-hornblende rim, with a systematic decreasing in Fe²⁺/(Fe²⁺+Mg) with Al^IV decreasing toward the rim. Early sulfide precipitation followed by late -stage magnetite formation and the simultaneous decreasing in Fe²⁺ during amphibole crystallization were resulted from progressive oxidation of iron in minerals. Transformation of sulfur species from SO₂ to H₂S probably oxidized iron in the minerals in this hydrous magma on cooling, as suggested in previous models for large sulfur supplies and magma oxidation in subduction related volcanisms.