Satoru Harayama

Mafic enclaves densely concentrated in the upper part of a vertically zoned felsic magma chamber: The Kurobegawa granitic pluton, Hida Mountain Range, central Japan

The Kurobegawa granitic pluton is a Pliocene batholith exposed over 700–2900 m elevation in the northern Hida Mountain Range of central Japan. The pluton is vertically zoned from the lower granite (a medium-grained equigranular to porphyritic granite, 70–74 wt% SiO 2 ) to the upper granite (a fine-grained porphyritic granite, 72–77 wt% SiO 2 ). Field relationships of the pluton and its textural, mineralogical, and geochemical features indicate that the pluton represents a well-exposed felsic magma chamber that was zoned as a result of fractional crystallization. Densely concentrated mafic microgranular enclaves (MMEs, 0.1–2 m major axis) of basaltic to dacitic composition (54–68 wt% SiO 2 ) occur throughout the pluton except most of the lowermost part. The MMEs generally have sharp, fine-grained, chilled margins and show no in situ mixing with the host granite except for in the northern area where the two magmas locally mixed and mingled to form hybrid rocks. Field relationships reveal that mafic magma intruded into the chamber from the bottom and ascended as a restricted feeder dike through the lower crystal mush. The mafic magma eventually fragmented to form MMEs, and the MMEs rose upward, resulting in their dense distribution throughout the pluton. Mafic silicate phases of the MMEs are amphibole and biotite without anhydrous mafic silicates, and the association magnetite-titanite-quartz occurs extensively, indicating that the mafic magma was originally hydrous and relatively oxidized, characteristic of subduction-related backarc basalt. We infer that the MMEs underwent vapor exsolution by second boiling on cooling during their ascent, reducing their bulk density relative to that of the host felsic magma. Relatively H 2 O-rich MMEs, indicated by the larger modal content of biotite compared to that of amphibole, are abundant in the shallower level of the Kurobegawa pluton. To achieve the MMEs9 dense distribution in the upper granite, we interpret that their flotation was sustained because of bubbles trapped in the segregated, interstitial melt by the MMEs9 chilled crustal margins. Although no evidence for settling, such as load-cast, silicic pipe, and tightly packed structures, is observed, the bubbles may have eventually been lost, with the result that the MMEs became denser and eventually settled onto the then-current top of the crystal mush.

Evidence for Residual Melt Extraction in the Takidani Pluton, Central Japan

The Takidani pluton represents one of a few locations where melt extraction from a crystal mush is preserved in the natural rock record, making it an extremely good case study for investigating the generation of evolved melt reservoirs in the upper crust. Located in the Japan Alps, the Takidani pluton shows a clear vertical zonation consisting of granite and granodiorite in the lower and mid- dle section, a fine-grained porphyritic granitic unit in the upper section and a marginal granodiorite at the roof contact with the host-rock. We present a detailed petrographic and geochemical study using samples collected along a section that traverses the entire vertical section of the pluton. No sharp contacts are found between units. Instead, gradual changes in rock fabric and mineralogy are observed between the lower granodiorite and overlying porphyritic unit. Major and trace elem- ent bulk-rock compositions show sigmoidal variations from the bottom to top of the pluton. Incompatible elements and silica contents increase roofwards within the porphyritic unit. Plagioclase chemistry reveals three main crystal populations (P1, P2 and P3) with Fe contents increasing towards the base of the pluton. Comparison with existing crystallization experiments, thermobarometry and hygrometry indicate that the magmas were emplaced at around 200 MPa, 850–900 C and bulk water contents of 3–4wt %. Whole-rock major and trace element analyses to- gether with mineral chemistry and textural observations suggest that the fine-grained porphyritic unit was extracted from the underlying granodiorite at temperatures between 800 and 740 C and crystallinities of 45–65 wt %. Radiogenic isotopes indicate only minor assimilation (2–6 wt %) and support melt evolution through crystal fractionation. The fine-grained matrix of the porphyritic unit may have been the result of pressure quenching associated with a volcanic eruption.