Search results for "rock"
showing 10 items of 1160 documents
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…
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…
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…
Geochemical evolution of rift magmas by progressive tapping of a stratified mantle source beneath the Ross Sea Rift, Northern Victoria Land, Antarcti…
1995
Abstract Source compositions of Neogene-Quaternary volcanic rocks from the McMurdo Volcanic Group of the Ross Sea Rift in Northern Victoria Land, Antarctica are constrained by NdSrPb isotopes and trace element ratios in near-primary basalts. The rocks erupted along the western rift margin (Victoria Land Basin) and the western rift shoulder (Transantarctic Mountains). Near-primary basalts show no evidence of crustal contamination, suggesting that their initial NdSrPb isotopes reflect the composition of their mantle sources. The initial isotope ratios of near-primary basalts range from about 87 Sr 86 Sr = 0.70281 to 0.70504 and 143 Nd 144 Nd = 0.51269 to 0.51291 (ϵNd(t) = 1.3–5.5). The 20…
Tectono-metamorphic evolution of the pre-Athabasca basement within the Wollaston-Mudjatik Transition zone, Saskatchewan
2016
The Paleoproterozoic tectono-metamorphic evolution of the pre-Athabasca basement (∼1.7 Ga) within the Wollaston–Mudjatik Transition Zone (WMTZ) (Saskatchewan, Canada) has been characterized using both exposed basement and drill cores from the Wolly–McClean exploration drilling project. The finite ductile strain pattern of the WMTZ results from the superposition of two tectono-metamorphic events M1–D1 and M2–D2. M1–D1 is associated with the development of a gently dipping foliation striking N90°–N100° and a southward decrease in peak pressures from up to 10 kbar (1 kbar = 100 MPa) in the Cochrane River area down to 6 kbar in the Wolly–McClean exploration drilling project. The M2–D2 event is…
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…
Ubendian basement and its late Mesoproterozoic and early Neoproterozoic structural and metamorphic overprint in northeastern Zambia
2004
The Palaeoproterozoic basement in the Muyombe and Luwumbu River areas of northeastern Zambia comprises a WNW‐ESE (to E‐W) trending cordierite‐garnet‐sillimanite granulite unit with numerous enderbite bodies and an amphibolite-facies migmatite unit. Zircons from a biotite metatonalite intruding the granulites were dated at 1960.7±0.4 Ma, and this is time-equivalent with the Nyika granite in adjacent Malawi. Mesoproterozoic intrusions into this basement are represented by a nepheline syenite at Mivula Hill (zircon age: 1360.1±0.8 Ma) and the porphyritic Ntendele biotite metagranite (zircon age: 1329.1±0.6 Ma). The Ntendele granite attains plutonic dimensions north of Muyombe. The Mesoproteroz…
The Lefkada barrier and beachrock system (NW Greece) — Controls on coastal evolution and the significance of extreme wave events
2012
Abstract The Lefkada–Preveza coastal zone, NW Greece, is characterised by an active barrier system and related extensive beachrock sequences. Besides the gradual coastal processes of longshore drift and spit evolution, the presence of active tectonics and the occurrence of tsunamis have been documented in previous studies and are part of the coastal geomorphological system. In this paper, we present the results of detailed multi-proxy sedimentological and geomorphological investigations carried out along the northern part of the barrier system and in back-beach positions. Our findings suggest that extreme wave events contributed to coastal and environmental changes and involved temporary br…
Millstones as indicators of relative sea-level changes in northern Sicily and southern Calabria coastlines, Italy
2011
Abstract New data are presented for late Holocene relative sea-level changes in two coastal sites of Sicily and Calabria, southern Italy. Reconstructions are based on precise measurements of submerged archaeological remains that are valuable indicators of past sea-level position. The archaeological remains are millstone quarries carved on sandstone coastal rocks and nowadays partially submerged which, to the authors’ knowledge, are used for the first time as sea-level markers. Millstones of similar typology are located on the coast of Capo d’Orlando (northern Sicily) and Capo dell’Armi (southern Calabria). When the archeologically-based sea-level position is compared with the shoreline elev…
Fracture stratigraphy and oil first migration in Triassic shales, Favignana Island, western Sicily, Italy
2021
This study aims at evaluating the control exerted by fracture stratigraphy and diagenetic processes on oil first migration through an outcropping, mixed carbonate-siliciclastic succession. The present work included results of sedimentological, paleontological, mineralogical, petrographic, structural, and microstructural analyses carried out on organic-rich shales exposed at the Favignana Island of Sicily, Italy. The analyses focus on Upper Triassic yellowish siltstones and greyish laminated dolomitic limestones, which form a 10's of m-thick succession exposed along the westernmost portion of the Sicilian fold-and-thrust belt. The studied succession deposited in a coastal lagoon associated t…