S.J.A. Brown

SHRIMP U-Pb dating of the preeruption growth history of zircons from the 340 ka Whakamaru Ignimbrite, New Zealand: Evidence for >250 k.y. magma residence times

The Whakamaru group ignimbrites are products of one of the largest caldera-forming eruptions from the central Taupo Volcanic Zone, New Zealand, ca. 340 ka. Imaging of zircons separated from low-silica rhyolite pumice from Whakamaru group ignimbrite reveals a complex magmatic crystallization history; some grains contain resorbed cores. Cores and rims of individual zircons were dated using the sensitive high-resolution ion microprobe. Due to the high U contents of the zircons, 206 Pb count rates were sufficiently high to obtain 206 Pb/ 238 U ages with precisions of 5%–10% (1 σS) using longer than usual 206 Pb + count times, and this has allowed zircon core and rim ages to be distinguished. Zircon rim and outer growth zone ages range from within 1 σS error of the eruption age to 449 ± 20 ka, whereas zircon cores produced ages to 608 ± 20 ka, indicating magmatic residence times exceeding 250 k.y. for some zircons. The data are consistent with a prolonged evolution of the magma system over hundreds of thousands of years, involving magma replenishment and mixing with residual phases of older silicic magma chambers, in addition to crystal fractionation processes, prior to eruption. The study has shown that 206 Pb/ 238 U dating using high-resolution ion microprobe can be used to determine zircon preeruptive growth histories in Quaternary lavas and pyroclastic rocks.

Crystallisation ages in coeval silicic magma bodies: 238U-230Th disequilibrium evidence from the Rotoiti and earthquake flat eruption deposits, Taupo volcanic zone, New Zealand

Abstract 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 km3 magma) at Okataina volcano was followed, within months at most, by the Earthquake Flat eruption (∼10 km3 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 238U–230Th 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 ( 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.

SHRIMP U–Pb in zircon geochronology of the Chor granitoid: evidence for Neoproterozoic magmatism in the Lesser Himalayan granite belt of NW India

Abstract Porphyritic and peraluminous granitoids of the Lesser Himalayan granite belt fringe the High Himalaya from Nepal to Pakistan. Magmatic zircons in the Chor granitoid in Himchal Pradesh define a SHRIMP U–Pb age of 823±5 Ma. This Neoproterozoic age contrasts with Cambro-Ordovician ages for other dated Lesser Himalayan granites, but is coeval with granite magmatism in the South China craton. Neoproterozoic magmatism within the northern margin of Gondwanaland may provide a more local source for detrital zircons in High Himalayan metasediments during the Neoproterozoic and early Paleozoic than the recently postulated East African orogen.

The Late Archaean Melita Complex, Eastern Goldfields, Western Australia: shallow submarine bimodal volcanism in a rifted arc environment

Abstract The Melita Volcanic Complex is a Late Archaean bimodal rhyolite/basalt volcanic succession within the Gindalbie Terrane in the Eastern Goldfields Province of the Yilgarn Craton. The Melita Complex has been dated by ion probe at 2683±3 Ma (95%) and forms part of a distinctive 2681–2692-Ma volcanic association that records bimodal (basalt/rhyolite) and calc–alkaline intermediate-silicic volcanism at several discrete volcanic centres, and which locally hosts volcanic massive sulphide mineralisation (Teutonic Bore). Approximately 3 km of stratigraphic thickness is exposed in the Melita area. The upper 1–1.5 km of the exposed succession is dominated by subaqueously resedimented volcaniclastic sandstones and breccias, rhyolite flows and sills. Primary subaerial pyroclastic deposits including ignimbrites have not been identified in this study, although subaerial explosive activity is indicated by the occurrence of accretionary lapilli and the abundance of vitric material (shards) and pumice fragments in resedimented deposits. The lower part of the succession is dominated by pillowed to massive basalt lavas, and in situ and resedimented mafic hyaloclastites. Mafic extrusive and intrusive rocks are tholeiitic with trace element concentrations similar to modern arc tholeiites. Felsic volcanic rocks at Melita are dacite to high-silica rhyolite. They are highly enriched in incompatible elements (particularly high field strength element-enriched), compared to other felsic associations in the Eastern Goldfields Province, representing evolved partial melts of heterogeneous intermediate arc-type crust. The volcanic facies and geochemistry of volcanic rocks at Melita are consistent with those observed in modern intra-arc or arc-rift settings, and the succession is interpreted to represent the initial stages of back-arc rifting within a complex convergent margin.