Search results for "Crust"

Constraining lithospheric flow.

2016

The motion of Earth's tectonic plates—the lithosphere—is driven by the subduction of relatively cold and dense oceanic plates into the mantle. The resulting forces drive the motions of continental plates, but the manner in which this happens depends on the effective viscosities of the lithosphere and mantle. On page 1515 of this issue, Liu and Hasterok ( 1 ) discuss a novel method of constraining viscosities of the lithosphere from geophysical data.

Zircon ages of metamorphic and magmatic rocks within peridotite-bearing mélanges: Crucial time constraints on early Carboniferous extensional tectoni…

2013

Abstract We dated and geochemically characterized peridotite-bearing melanges in the Chinese South Tianshan and within the Main Tianshan Shear Zone. The Yushugou-Tonghuashan melange in the Chinese South Tianshan exposes a tectonic juxtaposition of a diapirically emplaced metaperidotite (predominantly lherzolite) massif with a high-grade metamorphic terrane (ca. 10 km long; protolith age ≥ ca. 445–466 Ma). Metamorphic zircons of a mafic granulite (eNd(t) = 5.0) yielded a weighted mean 206Pb/238U age of 341 ± 8 Ma that we interpret as the time of granulite-facies metamorphism. The youngest zircon rims of an intermediate granulite (eNd(t) = − 4.3) have a mean age of 332 ± 13 Ma that records a …

Kimberlite, carbonatite, and potassic magmatism as part of the geochemical cycle

2013

M20 M21

Subduction-related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India: Constraints from petrol…

2015

Abstract Major and trace elements as well as the first oxygen isotopes are reported on eclogite xenoliths from the Mesoproterozoic KL2 and P3 kimberlite pipes of the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India. Garnets in kyanite-bearing samples are rich in grossular, whereas they are predominantly pyrope-almandines in the bimineralic (kyanite-free) samples. The kyanite eclogite from the P3 pipe is more Mg-rich than those from KL2 pipe. Equilibration temperatures indicate derivation from 4.5 to 5.3 GPa and 1060 to 1220 °C for the KL2 samples and 3.6 GPa, 918 °C for the P3 sample. Garnet rare earth element patterns show two characteristic types, one with relatively lo…

Concept of Biogenic Ferromanganese Crust Formation: Coccoliths as Bio-seeds in Crusts from Central Atlantic Ocean (Senghor Seamount/Cape Verde)

2011

At depths of 2,000 to 3,000 m, seamounts from the Cape Verde archipelago (Central Atlantic Ocean) are largely covered with ferromanganese crusts. Here we studied 60 to 150 mm thick crusts from the Senghor Seamount (depth: 2257.4 m). The crusts have a non lamellated texture and are covered with spherical nodules. The chemical composition shows a dominance of MnO2 (26.1%) and Fe2O3 (38.8%) with considerable amounts of Co (0.74%) and TiO2 (2.1%). Analysis by scanning electron probe microanalyzer (EPMA) revealed a well defined compositional zonation of micro-layers; the distribution pattern of Mn does not match that of Fe. Analysis by high resolution scanning electron microscopy (SEM) revealed…

Further evidence of length- and sex-dependent variation of some morphological characters in Bathyporeia guilliamsoniana (Bate, 1857) (Crustacea: Amph…

2011

Pliocene tourmaline rhyolite dykes from Ikaria Island in the Aegean back-arc region: geodynamic implications

2009

Very rare rhyolite dykes cross-cutting a Miocene I-type biotite-granite were discovered on Ikaria Island in the Aegean back-arc region. Their intrusion postdates exhumation of the granite to brittle crust at about 6.0-3.6 Ma; hence a Pliocene age is inferred. Petrological, geochemical and isotopic arguments indicate an origin through melting of crustal lithologies (tourmaline greywackes/semipelites) with no detectable contribution from asthenospheric sources. Strontium isotope ratios are relatively low unlike values for sediments entering the Hellenic trench but similar to those for certain Miocene Cycladic I-type granites and low-Rb Permo-Carboniferous Cycladic basement acid orthogneisses.…

Mechanism and time of deformation and metamorphism of mylonitic orthogneisses from the Shagou Shear Zone, Qinling Belt, China

1990

Abstract The Shagou Shear Zone south of Xian (Shaanxi Province) is a branch of the Shangdan Fault Zone in the Qinling Belt and documents ductile deformation that affected granitoid rocks. Mylonitization took place under conditions of the lower amphibolite facies at temperatures of ca. 530°C, followed by a phase of stagnation, still under the same metamorphic conditions. The last deformation was a weak event during retrogressive greenschist metamorphism. The emplacement of the granitoid precursors of the mylonite is dated at 211 ± 8 Ma by the U/Pb zircon method, much younger than Nd model ages of 1.2–1.3 Ga, which indicates the involvement of older continental crust, probably in an active co…

Crust–mantle transition and Moho model for Iceland and surroundings from seismic, topography, and gravity data

2005

An increasing data set exists on the nature and thickness of the Iceland crust. This paper relates topography, i.e., elevation and bathymetry (TOP), Bouguer gravity anomalies (BA) and Moho depths to each other to assess the consequences of the “thick crust model” for Iceland in the context of the North Atlantic. Results of regression of TOP and BA vs. Moho depth are converted into “Airy densities” Δρ* (mantle crust density contrasts in the case of ideal Airy isostasy). For Iceland, Δρ* is very low (105±10 kg/m3); for the adjacent continental margins and relicts, it is high and intermediate for the Jan Mayen Ridge. The values are affected by lithosphere cooling and systematic variations of i…

Constraints on mantle source and interactions from He-Sr isotope variation in Italian Plio-Quaternary volcanism

2008

[1] Helium isotope ratios of olivine and pyroxene phenocrysts from Plio-Quaternary volcanic rocks from southern Italy (seven Aeolian Islands, Mt. Vulture, Etna, Ustica, and Pantelleria) range from 2.3 to 7.1 Ra. Importantly, the phenocryst 3He/4He correlate well with whole rock Sr isotopic composition (0.70309–0.70711), reflecting the mixing of two sources. A significant contribution of He from crustal contamination is recorded only occasionally (e.g., pyroxenes from Vulcano). When merged with data from the Roman Comagmatic Province, a remarkably strong near-linear He-Sr isotope correlation is apparent. The general northward decrease in 3He/4He corresponds to an increase in 87Sr/86Sr (and a…