Search results for "PET"
showing 10 items of 12749 documents
Hafnium-Neodymium isotope, trace element and U-Pb zircon age constraints on the petrogenesis of the 3.44–3.46 Ga Dwalile greenstone remnant, Ancient …
2020
Abstract The geodynamic setting of the 3.44–3.46 Ga Dwalile greenstone remnant (DGR) of the Ancient Gneiss Complex (Swaziland) and its relationship to other Greenstone belts of similar age is investigated in this contribution. We present Hf-Nd isotope, major and trace element data for komatiites, komatiitic basalts, basalts, basaltic andesites as well as felsic volcanic and sedimentary rocks, all metamorphosed at amphibolite-facies grade. Major and trace element compositions of metabasalts and metakomatiites indicate crustal contamination. The best-preserved samples yield 176Lu-176Hf and 147Sm-143Nd isochron ages of ca. 3500 and 3460 Ma, respectively, in line with U-Pb zircon ages of interl…
Generation of early Archaean felsic greenstone volcanic rocks through crustal melting in the Kaapvaal, craton, southern Africa
2013
Abstract High-potassium felsic volcanic rocks interlayered with basalt and komatiite in early Archaean greenstone sequences in the Barberton Greenstone Belt of South Africa and Swaziland, previously considered to be derived from melting of mafic precursors, are shown to be the result of melting of significantly older felsic crust. This is documented by a combination of SHRIMP zircon dating with Hf-in-zircon and whole-rock Lu–Hf and Sm–Nd isotopic data. Zircons from felsic rocks of the oldest Barberton unit, the 3.53 Ga Theespruit Formation, yielded predominantly negative e Hf ( t ) -values, whereas whole-rock e Hf ( t ) - and e Nd ( t ) -data are slightly negative to slightly positive. Simi…
The Nevados de Payachata volcanic region (18�S/69�W, N. Chile) II. Evidence for widespread crustal involvement in Andean magmatism
1990
Volcanism extending over 11 Ma is represented in the rocks of the Nevados de Payachata region, culminating in the formation of two large composite stratocones within the last 500 000 years. Chemically distinct mafic magmas are erupted at a number of parasitic centers. These cannot be related to each other by crystal fractionation and do not appear to be direct parents for the differentiated suites of the composite cones. Two distinct trends are defined by the intermediate and evolved rocks; a high LILE (large ion lithophile element), TiO2 and Ce/Yb lineage among the youngest rocks (including the two major stratocones), and a more typical calc-alkaline trend among the older (>1 Ma) rock type…
Geochemistry and early Palaeogene SHRIMP zircon ages for island arc granitoids of the Sierra Maestra, southeastern Cuba
2004
The Palaeogene volcanic arc successions of the Sierra Maestra, southeastern Cuba, were intruded by calc-alkaline, low- to medium-K tonalites and trondhjemites during the final stages of subduction and subsequent collision of the Caribbean oceanic plate with the North American continental plate. U‐Pb SHRIMP zircon dating of five granitoids yielded 206 Pb/ 238 U emplacement ages between 60.5F2.2 and 48.3F0.5 Ma. The granitoids are the result of subduction-related magmatism and have geochemical characteristics similar to those of magmas from intra-oceanic island-arcs such as the Izu Bonin‐Mariana arc and the New Britain island arc, Lesser Antilles. Major and trace element patterns suggest evol…
Transition from mildly-tholeiitic to calc-alkaline suite: the case of Chichontepec volcanic centre, El Salvador, Central America
1998
Abstract The Chichontepec volcano is a Plio-Pleistocene composite volcano that erupted lavas ranging from high-alumina basalts to dacites. It experienced a caldera-forming paroxysmal eruption during the early Pleistocene. Pre-caldera lavas are mildly tholeiitic and they evolved mainly by low pressure crystal fractionation, notwithstanding the fact that most mafic lavas (low-MgO high-alumina basalts) retain traces of polybaric evolution. Conversely, post-caldera lavas, which are mainly pyroxene andesites, are clearly calc-alkaline, having evolved by open-system crystal fractionation. Sr–Nd isotopic data and trace elements characteristics indicate that the same mantle source was involved in t…
The Shishkhid ophiolite, northern Mongolia: A key to the reconstruction of a Neoproterozoic island-arc system in central Asia
2005
Abstract The Shishkhid ophiolite is a well-preserved 13 km-thick mafic-ultramafic assemblage which comprises (from bottom to top): mantle tectonites (∼6 km), layered and isotropic gabbro (∼4.5 km), sheeted dykes (up to 0.5 km), a bimodal assemblage of basalt and rhyolite (up to 0.7 km), as well as andesitic pyroclastic rocks (∼2 km). The volcanic rocks are overlain by a 3 km-thick sedimentary sequence showing progressive subsidence of the volcanic edifice after cessation of volcanism. The sedimentary unit is unconformably overlain by Ediacaran-Cambrian platform sediments. SHRIMP U-Pb dating of magmatic zircons from a rhyolite of the lower volcanic unit has yielded a concordant 206 Pb/ 238 U…
Devonian to Permian plate tectonic cycle of the Paleo-Tethys Orogen in southwest China (II): Insights from zircon ages of ophiolites, arc/back-arc as…
2009
Abstract New SHRIMP zircon ages for ophiolites, arc/back-arc assemblages and within-plate igneous rocks constrain the evolution of the Paleo-Tethys Orogen of southwest China. Ophiolites represent remnants of the Paleo-Tethys ocean, which was divided by the Simao terrane into two tracts, the main ocean in the west (current position) and a branch in the east. The main ocean has been inferred to originate from Gondwana break-up in the Early Devonian, and the heritage is preserved as the Changning-Menglian ophiolite (SSZ-type). The oceanic branch is represented by the Ailaoshan (NMORB-type)–Jinshajinag (EMORB-type) ophiolites. Zircons from a diabase and a plagiogranite of the Ailaoshan ophiolit…
Petrological and structural significance of the Santa Elena-Nicoya ophiolitic complex in Costa Rica and geodynamic implications
1999
Detailed structural and petrological investigations were carried out on the Santa Elena-Nicoya ophiolitic complex in Costa Rica, which is represented by a classic association of mantle tectonites (Santa Elena Peninsula), a cumulitic gabbroic-plagiogranite sequence, and basaltic volcanics and sills (Matapalo and Esperanza Units of the Nicoya Peninsula), with discontinuous radiolarian covers. Despite biostratigraphically different ages (Late Jurassic for Matapalo and Mid-Late Cretaceous for Esperanza) in both units, the major and trace element data, Sr-Nd isotope systematics, and mineral phase crystallization order show identical tholeiitic fractionation trends, where the marked Fe-Ti enrichm…
Plume chemistry provides insights into mechanisms of sulfur and halogen degassing in basaltic volcanoes
2004
Abstract This paper deals with sulfur, chlorine and fluorine abundances in the eruptive volcanic plume of the huge October 2002–January 2003 eruption of Mount Etna, aiming at relating the relevant compositional variations observed throughout with changes in eruption dynamics and degassing mechanisms. The recurrent sampling of plume acidic volatiles by filter-pack methodology revealed that, during the study period, S/Cl and Cl/F ratios ranged from 0.1–6.8 and 0.9–5.6, respectively. Plume S/Cl ratios increased by a factor of ∼10 as volcanic activity drifted from paroxysmal lava fountaining (mid- and late November) to passive degassing and minor effusion (early January), and then decreased to …
Low-pressure fractionation of the Nyiragongo volcanic rocks, Virunga Province, D.R. Congo
2004
Abstract A new petrogenetic model for the Nyiragongo volcanics based on petrological and geochemical studies of a wide range of highly alkaline and silica-undersaturated fractionated rocks (Mg# 23–44) invokes low-pressure fractionation occurring mainly within the Nyiragongo volcanic edifice. The most primitive Nyiragongo rocks seem correspond to a pyroxene-nephelinitic melt which evolved to melilite nephelinite and melilitite by fractionation of leucite, together with varying amounts of nepheline. Leucite nephelinite, leucitite and leucite tephrite represent rock types originating partly by phenocryst accumulation. Mineral and bulk rock geochemistry of pyroxene nephelinites indicate an alre…