Abstract The Takidani pluton (1·1–1·6 Ma) represents a shallow magmatic reservoir at the base of an exhumed caldera floor. The deposits of two large caldera-forming eruptions, the Nyukawa Pyroclastic Flow Deposit (1·76 Ma; crystal-rich dacite) and the Chayano Tuff and Ebisutoge Pyroclastic Deposits (1·75 Ma; a sequence of crystal-poor rhyolite), are distributed concentrically around the pluton. We use major and trace element chemistry of whole-rock, glass and minerals to show that (1) the crystal-rich dacite (>400 km3 DRE; dense rock equivalent) is the erupted portion of a shallow mush zone constituting the Takidani pluton and (2) the crystal-poor rhyolite (>100 km3 DRE) was extracted from a deeper part of this vertically extended magmatic plumbing system. Whole-rock geochemistry indicates that the Nyukawa and Takidani compositions were produced dominantly through crystal fractionation of amphibole, pyroxene and plagioclase in the mid to lower crust and subsequently emplaced in the upper crust prior to eruption and solidification, respectively. The crystal-poor Chayano–Ebisutoge rhyolite (>100 km3 DRE) is compositionally distinct from the Nyukawa and Takidani magmas and its generation is associated with a substantial contribution of crustal melts. However, plagioclase and orthopyroxene textures and chemistry provide strong evidence that the ascending rhyolite percolated through the upper Takidani–Nyukawa mush zone prior to eruption. Overgrowth of ‘rhyolitic plagioclase’ on ‘xenocrystic dacitic plagioclase’ typical of the Takidani–Nyukawa magmas indicates that the extraction and accumulation of the rhyolitic melts could have occurred in less than 10 kyr (i.e. the time between eruptions) prior to eruption, providing maximum timescales for pre-eruption storage. Overall, our findings show a progressive growth and thermal maturation of a vertically extended magmatic plumbing system over hundreds of thousands of years and imply that large volcanic eruptions can occur in relatively short succession without dramatic changes in the plumbing system, thus complicating the identification of signs of an impending large eruption.
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 middle 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 element 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 3–4 wt %. Whole-rock major and trace element analyses together 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.
Abstract The thermal evolution of a contact aureole is strongly dependent on the emplacement mode of the intrusion that is causing the thermal anomaly. Inferences on the emplacement mode can be made if solid temperature estimates are available. The contact aureole of the Western Adamello Tonalite provides a unique combination of magma ascent close to the host rock and suitable rock chemistry. We use three mineral reactions at different distances in the aureole in combination with phase petrology to estimate temperatures. These temperatures are not reproducible by thermal models considering the emplacement of the intrusion as a single batch. The external zone of the intrusion represents a feeder conduit in which magma was transported. We investigated the thermal effect of the conduit on the host rocks using thermal models. Different thermal profiles for the aureole are obtained by varying flow/no-flow times, conduit thickness and magma temperature. We show that only a few combinations match the temperature constraints in the aureole, and that the amount of magma transported through the conduit can be calculated. A comparison with time-averaged fluxes of recent volcanoes and the presence of reworked volcanic sediments in the surroundings of the Adamello batholith indicate that the calculated rates and volumes are plausible. Supplementary material: Tables comparing sample compositions with literature experimental data are available at http://www.geolsoc.org.uk/SUP18847
Abstract Pre‐ A lpine garnets of V ariscan age from metapelitic basement units in N orthern I taly were strongly retrogressed at near‐surface conditions prior to A lpine contact metamorphism. The replacement by sheet silicates caused a significant volume increase during retrogression, producing pervasive fracturing. Up to several hundreds of angular fragments formed from each crystal. Electron backscatter diffraction analysis documents a maximum misorientation of ~22° of some fragments as a result of local rotation during fracturing. New garnet growth is observed on the garnet fragments during contact metamorphic overprinting, resulting in garnet clusters. Fragments can be identified due to calcium‐rich domains. Fragment orientations were inherited, and only minor new nucleation occurred. These garnets develop features typically associated with multiple nucleation models, but here they reflect multiple metamorphic events. We propose that clusters can be indicative of multiple metamorphic events, which were separated by a period of intense retrograde alteration.
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