TIMESCALES OF PLUTON EMPLACEMENT AND RHYOLITE MELT EXTRACTION FROM U-PB TIMS DATING OF ZIRCON AND 40AR/39AR THERMOCHRONOLOGY WITHIN THE RISCO BAYO-HUEMUL PLUTONIC COMPLEX

Rhyolite that fuels explosive silicic eruptions is thought to originate in the upper crust via extraction of melt from crystal-rich magma reservoirs. Although these reservoirs may grow incrementally over ca. 10⁴-10⁵ yr timescales, they can be remobilized prior to eruption much more rapidly (ca. 10²-10³ yr). Advances in U-Pb dating of zircon by chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) now permit precise dating of the assembly history and fingerprinting of magmatic processes in young plutons. Connections between the timescales of long-term assembly and transient melt-forming events can thus be investigated. The 150 km³ late Miocene (7.2–6.2 Ma) Risco Bayo-Huemul plutonic complex at 36 ^oS in the Chilean Andes comprises 1-20 km³ domains of calc-alkaline gabbro through high-silica granite emplaced at 3-7 km depth. The mafic-to-intermediate Risco Bayo pluton comprises incrementally-emplaced magma batches that exhibit sharp internal field contacts. In contrast, textural, bulk-rock geochemical, mineralogical, and petrochronologic data suggest domains of the younger, more silicic Huemul pluton preserve near-end members of high-silica melt segregation and complementary residual silicic cumulate formation. U-Pb zircon CA-ID-TIMS geochronology and ⁴⁰Ar/³⁹Ar thermochronology of biotite, amphibole and orthoclase in the granitoids and hornfels wallrock together illuminate the assembly and cooling durations of individual domains. Risco Bayo zircon dates range from 7.193 ± 0.014 to 6.958 ±0.053 Ma, documenting the emplacement of at least three magma batches over 240 kyr. ⁴⁰Ar/³⁹Ar dates indicate that these batches cooled rapidly, yet imposed a modest thermal priming of the crust prior to intrusion of the Huemul pluton. Huemul U-Pb zircon dates span ~190 kyr from 6.384 ± 0.022 to 6.199 ± 0.022 Ma, and suggest that silicic cumulate compositions reach the solidus 7-234 kyr prior to the hypothesized extraction of the high-silica melt domain that caps the complex and is in contact with exposed roof rocks. Numerical simulations are used to test the temporal and thermal viability that Huemul domains might have unmixed from a single parental magma.