The super-giant Oyu Tolgoi porphyry copper-gold deposits in the South Gobi desert, Mongolia, consist of multiple discrete porphyry centers aligned within a north-northeast trending, >6.5 km long, arc-transverse mineralized corridor. The porphyries are linked to a tectono-magmatic event at ~372 Ma within a Devonian to Carboniferous volcanic arc, and U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma. The Oyu Tolgoi district underwent at least three discrete periods of syn- to post-mineral shortening and there is evidence for at least three unconformities within the Paleozoic sequence. Although the deposits were formed in an active orogenic environment characterized by rapid uplift, their preservation is a reflection of climactic effects as well insulation from erosion by rapid burial under mass-wasted and pyroclastic material in the volcaniclastic apron of late-mineral dacitic volcanoes. The porphyry copper-gold deposits are spatially and temporally related to medium- to high-K calc-alkaline quartz monzodiorite (~372 Ma) and granodiorite (~366 Ma) intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC). Adakite-like wholerock compositions as well as zircon grains with high CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and late-mineral porphyry intrusions are different from younger intrusions that are not related to porphyry Cu-Au deposit formation. Moreover, mixed zircon populations within OTIC intrusions indicate that efficient assimilation of material from different host rocks by a convecting magma chamber occurred. Mafic to intermediate volcanic units evolved from tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of marine arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline, regardless of age. Nd-isotopic geochemistry from all suites is consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc and lead isotopic compositions indicate that the sulfides in the porphyry Cu-Au deposits are genetically linked to the Late Devonian magmas. Magma mixing, adakite-like magmatism and rapid uplift and erosion in a juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from other settings in active and fossil volcanic arcs.

Mode of access: World Wide Web. / Thesis (Ph. D.)--University of Hawaii at Manoa, 2005. / Includes bibliographical references (leaves 227-246). / Electronic reproduction. / Also available by subscription via World Wide Web / xiv, 246 leaves, bound ill. (chiefly col.), maps 29 cm

Thesis (M.S.)--University of Hawaii at Manoa, 2006. / Includes bibliographical references (leaves 103-107). / ix, 107 leaves, bound ill. (some col.), col. map 29 cm

Triassic igneous rocks in southeast Queensland show a number of subduction related geochemical characteristics. Extensive calc-alkalic granitoids chains characterise the region and define the ancient arc setting. Despite good evidence that an arc was present, Triassic volcanic rocks are relatively sparse in southeast Queensland. The Mt Marcella Volcanics, of the northern Esk Trough are a previously poorly understood piece of the Middle Triassic convergent margin of southeast Queensland. A three stage model is proposed for the eruptive development of the Middle Triassic (245- 230Ma) volcanic succession that involves; 1) The Middle Triassic basalt, comprising coalesced lava flows covering as much as 500km2 with an estimated eruptive volume in the order of 50km3. 2) The Penwhaupell Volcanic Centre, a concentration of inter-bedded lavas and pyroclastic rocks dominated by dacite that forms a volcanic pile exceeding 2km stratigraphic thickness and representing an eruptive volume of approximately 48km3. 3) The Ettiewyn Caldera, representing the catastrophic culmination of the Mt Marcella Volcanics event, with a sequence of caldera out-flow and in-fill andesite ignimbrites and post-caldera lavas with a total eruptive volume in the order of 130km3. The “Penwhaupell Volcanic Centre” and the “Ettiewyn Caldera” are two new sub-divisions and the proposed names, for the lower and upper sequences of the previously undifferentiated Mt Marcella Volcanics. The Mt Marcella Volcanics magma compositions show cogenetic characteristics that define three evolutionary pathways; 1) a mildly alkali series, from basaltic-andesite to trachy-dacite related through fractionation dominated by plagioclase and clinopyroxene 2) an amphibole series, basaltic-andesite to hornblende dacite through fractionation dominated by plagioclase and hornblende under hydrous conditions, and 3) a pyroxene series, from basaltic-andesite to pyroxene andesite through fractionation dominated by plagioclase and pyroxene. Quantitative petrogenetic models generally support the proposed fractional crystallisation pathway, however weaknesses are acknowledged, with good results for the major elements and REE off-set by generally poor results for the LILE. Despite the inconclusive trace element results for the modelled fractionation, strong geochemical similarities and cogenetic relationships have been established. A typical arc-like geochemical signature including a pronounced Nb depletion characterises the Mt Marcella Volcanics. However, the geochemical character within the Middle Triassic volcanic succession reveals an unusual transition from an OIB character of the Middle Triassic basalts, to the Andean arc character of later Mt Marcella Volcanics. The implications of this could have profound impact on our understanding of how southeast Queensland’s Triassic tectonic setting operated by providing support for hotspot activity rather than subduction-driven activity.

Thesis (M.S.)--Brigham Young University. Dept. of Geological Sciences, 2006. / Includes bibliographical references (p. 25-34).

Thesis (M.S.)--University of North Carolina at Wilmington, 2004. / Includes bibliographical references (leaves : 70-72).

Thesis (Ph. D.)--Rutgers University, 2008. / "Graduate Program in Geological Sciences." Includes bibliographical references (p. 129-133).

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Science, 1982. / Supervised by William J. Jenkins. Includes bibliographical references (leaves 265-290).

Thesis (Ph. D.)--Eidgenössischen Technischen Hochschule Zürich, 1978. / Vita: p. 163.

Thesis--Columbia University. / Bibliography: leaves 121-124.