The high-silica rhyolite lavas of Glass Mountain, California, provide a detailed record of the evolution of the Long Valley magmatic system during the 1.4 m.y. prior to the catastrophic eruption of the voluminous zoned rhyohtic Bishop Tuff at 0.73 Ma. The older lavas of Glass Mountain are extremely evolved, and were erupted from 2.1 to 1.2 Ma with 87Sr/86Sr of 0.707-0.739 and eNd of-3 to-4, whereas the younger lavas are slightly less evolved and were erupted between 1.2 and 0.79 Ma with 87Sr/86Sr of 0.706-0.707 and cNd close to-1, essentially identical to the Sr and Nd isotopic compositions of the Bishop Tuff. Neither the older nor younger lavas display a relationship between the isotopic compositions of Sr and Nd. The Pb isotopic compositions are effectively uniform at 206Pb/204 Pb = 19.13-19.17. A group of older lavas that outcrop in a northwest-trending band near the topographic rim of the caldera collectively define an apparent Rb-Sr isochron age (I) of 2.09 + 0.06 Ma with an intercept of "Sr/"Sr = 0.7060 + 3, whereas analyzed older lavas outcropping outboard of this band define an isochron of 1.90 + 0.02 Ma ((87Sr/86 Sr), = 0.7063 f 4). These isochron ages are identical to the K-Ar ages for oldest erupted rhyolites in the two regions, respectively. The younger lavas away from the caldera define an apparent Rb-Sr isochron age (t) of 1.14 * 0.08 Ma with the same
Abstract:The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U-Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 4 Ma), and on the late Grampian 'Newer Gabbros' (470 9 Ma) of Insch and Morven-Cabrach in Aberdeenshire. These age data, together with the existing 590 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn.(1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic (<600 Ma) age and the Ordovician rocks of the Highland Border Complex.(2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses.(3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected.
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