Geology topics
T. R. Ireland
Publications and source records attributed to T. R. Ireland.
Late Pleistocene granodiorite beneath Crater Lake caldera, Oregon, dated by ion microprobe
Variably melted granodiorite blocks ejected during the Holocene caldera-forming eruption of Mount Mazama were plucked from the walls of the climactic magma chamber at ∼5 km depth. Ion-microprobe U-Pb dating of zircons from two unmelted granodiorite blocks with SHRIMP RG (sensitive high-resolution ion microprobe–reverse geometry) gives a nominal 238 U/ 206 Pb age of 100 − 80 + 78 "> 100 + 78 − 80 100−80+78 ka, or 174 − 115 + 89 "> 174 + 89 − 115 174−115+89 ka when adjusted for an initial 230 Th deficit. SHRIMP RG U-Th measurements on a subset of the zircons yield a 230 Th/ 238 U isochron age of 112 ± 24 ka, considered to be the best estimate of the time of solidification of the pluton. These results suggest that the granodiorite is related to andesite and dacite of Mount Mazama and not to magmas of the climactic eruption. The unexposed granodiorite has an area of at least 28 km 2 . This young, shallow pluton was emplaced in virtually the same location where a similarly large magma body accumulated and powered violent explosive eruptions ∼7700 yr ago, resulting in collapse of Crater Lake caldera.
530 Ma zircon age for ophiolite from the New England orogen: Oldest rocks known from eastern Australia
New ion microprobe data provide constraints on the timing of formation of ophiolitic rocks in the New England tectonic collage in eastern Australia. Results for analyses of magmatic zircons from plagiogranite of the Weraerai terrane ophiolite at Upper Bingara give a 206pb/238|j ag e Qf 53Q ± 6 Ma (2a). This plagiogranite is the oldest rock from eastern Australia yet identified. Existing tectonic models suggest that progressively younger crust was accreted to the eastern margin of Gondwanan Australia throughout the Paleozoic. This cannot be reconciled easily with the Cambrian age for these ophiolitic rocks, which are juxtaposed between Devonian terranes and are at least 1000 km east of the nearest lithologically similar rocks of comparable age. We speculate that younger, thin-skinned terranes may have been thrust westward over the continental freeboard of eastern Australia during the late Paleozoic.