Timescales of melt extraction in silicic systems: field relations and petrochronological constraints from a young plutonic-volcanic complex in Central Japan

Tuesday, 15 December 2020: 08:45
Eva Hartung, University of Liverpool, Department of Earth, Ocean and Ecological Sciences, Liverpool, United Kingdom, Federico Farina, University of Milan, Milan, Italy, Daniela Rubatto, University of Lausanne, Lausanne, Switzerland, David Floess, University of Geneva, Geneva, Switzerland, Simon Richard Wallis, University of Tokyo, Bunkyo-ku, Japan, Satoru Harayama, Shinshu University, Matsumoto, Japan and Luca Caricchi, University of Geneve, Earth Sciences, Geneve, Switzerland
Recorded Presentation
Abstract:
Identifying the timescales of melt segregation in crustal reservoirs is fundamental to improve our understanding of large explosive volcanic eruptions. We determine extraction timescales of residual melt in silicic magma mushes using field and petrochronological constraints of the Takidani Pluton (0.8–1.75 Ma) and two associated eruptions in Central Japan (i.e. 1.76 Ma Nyukawa Pyroclastic Flow Deposit and 1.75 Ma Chayano-Ebisutoge Pyroclastic Deposit). Melt extraction is recorded as a gradual transition in texture and composition from equigranular to porphyritic granodiorite in the upper part of the Takidani pluton (Hartung et al. 2017). High precision U-Pb zircon geochronology using chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) give crystallization ages of ca. 1.2 to 1.5 Ma in both, the equigranular and porphyritic, units. In situ core and rim analyses using secondary ion mass spectrometry (SIMS) reveal two distinct zircon crystallization events at ca. 1.45 and 1.20 Ma, respectively, providing maximum timescales of segregation in the pluton of ca. 250 kyr. In the volcanic units, timescales for extraction of rhyolitic melt are drawn from plagioclase relationships. The texture and composition of xenocrystic cores in plagioclase of the Chayano-Ebisutoge rhyolite are identical in texture and composition to phenocrysts of the Nyukawa dacite (and Takidani pluton) indicating that the extraction and eruption of >100 km3 rhyolite occurred in ca. 10 kyr (e.g. time between eruptions). This suggests that in long-lived magmatic systems large rhyolitic eruptions are likely the result of rapid melt extraction at a thermal optimum. The timescales of melt extraction remain largely unresolvable in plutons due to the analytical uncertainties of zircon U-Pb data.

Hartung E, Caricchi L, Floess D, Wallis S, Harayama S, Kouzmanov K, Chiaradia M, 2017. Evidence for residual melt extraction in the Takidani Pluton, Central Japan. J Petrol 58, 763-788.

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