Crystallisation ages in coeval silicic magma bodies: ~(238)U-~(230)Th disequilibrium evidence from the Rotoiti and Earthquake Flat eruption deposits, Taupo Volcanic Zone, New Zealand

摘要

The timescales over which moderate to large bodies of silicic magma are generated and stored are addressed here by studies of two geographically adjacent, successive eruption deposits in the Taupo Volcanic Zone, New Zealand. The earlier, caldera-forming Rotoiti eruption (>100 km~3 magma) at Okataina volcano was followed, within months at most, by the Earthquake Flat eruption (～10 km~3 magma) from nearby Kapenga volcano; both generated non-welded ignimbrite and coeval widespread fall deposits. The Rotoiti and Earthquake Flat deposits are both crystal-rich high-silica rhyolites, with sparse glass-bearing granitoid fragments also occurring in Rotoiti lag breccias generated during caldera collapse. Here we report ~(238)U-~(230)Th disequilibrium data on whole rocks and mineral separates from representative Rotoiti and Earthquake Flat pumices and the co-eruptive Rotoiti granitoid fragments using TIMS and in situ zircon analyses by SIMS. Multiple-grain zircon-controlled crystallisation ages measured by TIMS from the Rotoiti pumice range from 69 ± 3 ka (<63 μm size fraction) to 76 ± 6 (125-250 μm fraction), with a weighted mean of all size fractions of 71 ± 2 ka. SIMS model ages from single zircons in pumice range from 50 ± 24 ka to >350 ka, with a pronounced peak at 70-90 ka. The weighted mean of isochrons is 83 ± 14 ka, in accord with the TIMS data. One glass-bearing Rotoiti granitoid clast yielded an age of 57 ± 8 ka by TIMS (controlled by Th-rich phases that, however, are not apparently present in the juvenile pumices). Another glass-bearing Rotoiti granitoid yielded SIMS zircon model ages peaking at 60-90 ka, having a similar age distribution to the pumice. Age data from pumices are consistent with a published 64 ± 4 ka eruptive age (now modified to 62 ± 2 ka), but chemical and/or mineralogical data imply that the granitoid lithics are not largely crystalline Rotoiti rhyolite, but instead represent contemporaneous partly molten intrusions reflecting different sources in their chemistries and mineralogies. Similarly, although the Earthquake Flat eruption immediately followed (and probably was triggered by) the Rotoiti event, age data from juvenile material are significantly different. A multiple-grain zircon-controlled crystallisation age measured by TIMS from a representative pumice is 173 ± 5 ka, while SIMS model ages range from 70_(26)~(+34) ka to >350 ka, with a peak at 105 ka. These age data coupled with previously published geochemical and isotopic data show that the Rotoiti and Earthquake Flat deposits were erupted from independent, unconnected magma bodies.