Search results for "crust"
showing 10 items of 599 documents
Influence of crustal thickening on arc magmatism:, Nevados de Payachata volcanic region, northern Chile
1993
Neogene through Pleistocene lava flows of the Nevados de Payachata region (lat 18°S) on the Altiplano of northern Chile fall into two discrete chemical groups defined by age and incompatible element concentrations. The Neogene suite (10.5-6.6 Ma) has trace element concentrations comparable to arc magmas erupted on thin crust in central Chile. Pleistocene lava flows (0.29-0 Ma) are enriched in incompatible elements relative to Neogene samples but have similar Sr, Nd, and Pb isotopic ratios. Incompatible element enrichment in mafic rocks reflects deep-crustal or subcrustal processes. Neogene volcanism in northern Chile immediately followed a period of intense crustal thickening. Uplift rates …
Neoproterozoic Ophiolites of the Arabian-Nubian Shield
2004
Publisher Summary Ophiolites of mid-Neoproterozoic age are abundant in the Arabian-Nubian Shield (ANS) of NE Africa and Arabia. The ANS ophiolitic mantle was mostly harzburgitic, containing magnesian olivines and spinels that have compositions consistent with extensive melting. Cumulate ultramafics transition upwards into layered gabbro. Several crystallization sequences are inferred from ANS transition zones and cumulate gabbro sections. In all samples studied, olivine and spinel crystallized first, followed by cpx-plag, cpxopx-plag, and opx-cpx-plag. The ANS in NE Africa and W. Arabia is the largest tract of juvenile continental crust of Neoproterozoic age on Earth. The best preserved oph…
Evidence of magmatic activity related to Middle Jurassic and Lower Cretaceous rifting from northeastern Brazil (Ceará-Mirim): K/Ar age, palaeomagneti…
1992
The Mesozoic magmatic activity in the easternmost part of NE Brazil (Ceara-Mirim) is mainly represented by two-pyroxene tholeiitic dykes; only few dykes have alkaline character. K/Ar ages and palaeomagnetism data indicate that Ceara-Mirim dykes are of Middle Jurassic (175-160 Ma) and Early Cretaceous (140-130 Ma) age. Both Middle Jurassic and Lower Cretaceous dykes have high incompatible-element concentrations and TiO2 with the lower contents generally confined to the Middle Jurassic dykes; rare tholeiitic dykes low in TiO2 and incompatible elements are present. Sr/1bNd isotopic and other chemical data do not support appreciable crustal contamination and in a 87Sr86Sr vs. 143Nd144Nd diagram…
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…
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…
Coupled evolution of back-arc and island arc-like mafic crust in the late-Neoproterozoic Agardagh Tes-Chem ophiolite, Central Asia: evidence from tra…
2002
We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia, south-west of Lake Baikal (50.5°N, 95°E). The majority of samples are high-alumina basalts and basaltic andesites having island-arc affinities. They were derived from an evolved parental magma (Mg#≥0.60, Cr~180 ppm, Ni~95 ppm) by predominantly clinopyroxene fractionation. The parental magma developed from a primary mantle melt by fractionation of about 12% of an olivine+spinel assemblage. The island-arc rocks have high abundances of incompatible trace elements (light rare-earth element abundances up to 100 times chondritic, chondrite-n…
Rapid onset of mafic magmatism facilitated by volcanic edifice collapse: MAFIC MAGMATISM FACILITATED BY VOLCANIC EDIFICE COLLAPSE
2015
Volcanic edifice collapses generate some of Earth's largest landslides. How such unloading affects the magma storage systems is important for both hazard assessment and for determining long-term controls on volcano growth and decay. Here we present a detailed stratigraphic and petrological analyses of volcanic landslide and eruption deposits offshore Montserrat, in a subduction zone setting, sampled during Integrated Ocean Drilling Program Expedition 340. A large (6–10 km3) collapse of the Soufriere Hills Volcano at ~130 ka was followed by explosive basaltic volcanism and the formation of a new basaltic volcanic center, the South Soufriere Hills, estimated to have initiated <100 years after…
Age and tectonic setting of granitoid gneisses in the Eastern Desert of Egypt and south-west Sinai
1994
Strongly deformed and locally migmatized gneisses occur at several places in the southern Eastern Desert of Egypt and in Sinai and have variously been interpreted as a basement to Pan-african (≈900 to 600 Ma) supracrustal and intrusive assemblages. A suite of grabbroic to granitic gneisses was investigated in the Hafafit area, which constitutes an I-type calc-alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so-called ‘Older Granites’ of Egypt. 207Pb/206Pb single zircon evaporation from three samples of the Hafafit gneisses yielded protolith emplacement ages between 677 ± 9 and 700 ± 1…
Geochemical data and zircon ages for rocks in a high-pressure belt of Chu-Yili Mountains, southern Kazakhstan: Implications for the earliest stages o…
2011
Abstract The mechanism and age of Palaeozoic accretion in the Central Asian Orogenic Belt remain poorly constrained. One of the most complex belts extends from the Kokchetav area in northern Kazakhstan to the Kyrgyz northern Tianshan. It represents an assemblage of small blocks with Palaeoproterozoic continental crust, intervening slivers containing early Palaeozoic ophiolites and/or deep-marine sediments, and a number of HP and UHP metamorphic complexes. The HP–LT metamorphic rocks provide important clues for reconstructions of the overall structure and evolution of the accretionary collage. This study is aimed to constrain the metamorphic age and tectonic implications of HP garnet pyroxen…
Compartmentalization of gypsum and halite associated with cyanobacteria in saline soil crusts
2016
The interface between biological and geochemical components in surface crust of a saline soil was investigated using X-Ray Diffraction (XRD), and variable pressure scanning electron microscopy (SEM) in combination with Energy Dispersive X-ray Spectrometry (EDS). Mineral compounds such as halite and gypsum were identified crystallized around filaments of cyanobacteria. A total of 92 genera were identified from the bacterial community based on 16S gene pyrosequencing analysis. The occurrence of the gypsum crystals, their shapes and compartmentalization suggested that they separated NaCl from the immediate microenvironment of the cyanobacteria, and that some cyanobacteria and communities of su…