جستجوی مقالات مرتبط با کلیدواژه "mantle" در نشریات گروه "زمین شناسی"

The Neogene volcanic rocks are exposed in south Arab abad, Lut block. The studied volcanic rocks covered Eocene and Oligocene volcanic lavas and Neogene evaporitic deposits. The base of the Neogene volcanic sequence made of breccia and tuff gradually covered by andesite, trachyandesite and dacitic lavas. These lavas have aphyric-phyric textures and microlitic to hyalomicrolitice in matrix. These lavas show calck alkaline magmatic trend. REE patterns and spider diagrams display enrichement in LREE and LILE relative to HREE and depletion in HFSE which like as subduction zone magmatism. In the tectonomagmatic diagrams they plot of subduction and post-collisions fields. Interpretation of the geochemical data indicates that the studied lavas probably produced from partial melting of lithospheric mantle which had previously enriched by subduction components (fluids and melt). Partial melting of crust, probably involved in generation of acidic lavas. It seems that in the Neogene period, lithosphere thinning induced partial melting of heterogeneous Sub Continental Lithospheric Mantle (SCLM). These processes consequently occurred by lithospheric delamination and asthenospheric upwelling.

The Eocene volcanic rocks which exposed in the NW Sechangi Lut block, including pyroclastic deposits and lavas.  The volcanic rocks in the study area are basalt-basaltic andesite-trachyandesite, trachydacite, dacite and rhyolite-ignimbrite in compositions. Based on the field observations the volcanic rocks erupted in four stages in the aquous to the aerial environments.  The volcanic rocks in this area exposed as alternation of acidic and basic-intermediate rocks like as bimodale volcanism.  Under the petrography investigations the acidic rocks including pyroxenite microenclaves probably the remains of sub continental lithospheric mantle.  In the discriminating magmatic diagrams the volcanic lavas show shoshonitic to high-K calck-alkaline trends.  In the normalized REE patterns and spider diagrams they display enrichments of LREE and LILE in relative to HFSE comparable with subduction related volcanism.  In the tectonic-magmatic diagrams they cluster in continental margins field.  The interpretation of the geochemical diagrams indicate that the basic-intermediate volcanic rocks produced from partial melting of lithospheric mantle which have previously enriched by the subduction components (fluids-melt) which cannot distinguish from the crustal effects.  The acidic rocks rcords some crustal elemnts which indicating the involving of the crust in the generation of the acidic rocks.  It seams that after the collision of the Neo-Tethys branchs in Late Cretaceous and subsequence lithospheric thickening in the Lut block, the Eocene volcanic rocks in the Sechangi area formed in an extentional-collisional environment.

Pyroclastic rocks in the Badam area (E-Mahabad, west Azarbaijan Province), are located at the most extreme northern part of the Sanandaj-Sirjan zone. Petrographic studies revealed that they are basanitic in composition and have porphyric and microlithic porphyric textures. Olivine and clinopyroxne occur as phenocrysts and clinopyroxene and nepheline as microliths. Opaque minerals and glass have occupied the space between different minerals. In classification diagram, they have basanitic composition. Geochemical features such as enrichment in LILE, high LREE concentrations and negative anomalies in Nb, Ta and Y, with positives anomalies in Ba, Pb and Th and enrichment in Th/Yb, all suggest that these rocks have mantle source that are enriched by melt/fluids from the metasomatized subducted oceanic slab. These features are inherited from mantle source and they are not caused by crustal contamination. Regarding MREE and HREE ratios, these basanites originated from 1 % partial melting of a garnet lherzolite source in the garnet stability field (85 Km). Melting trigger in this area is not clear, but old theories such as lithosphere delamination or slab break up are not applicable any more. There is three suggestions about melting in these are: 1) break up of hydrated phases by depth increase, 2) changing and turbulence in convection follows, 3) subduction and dehydration of Arabic passive margin

Gabbrodioritic pluton is located in S-Qorveh in nourthern parts of the Sanandaj-Sirjan Zone. Petrographically, amphibole and plagioclase are main minerals in these rocks. Amphiboles are calcic-type, and their composition varied between ferro-hornblende to magnesio-hornblende. Anortite contents in plagioclases are between 81 to 39 (%) and these crystal are labradore -oligoclase in composition. According to coexist hornblende-plagioclase and Al in amphibole geothermometry methods, temperatures of crystallization are yielded ~723°C. In the later method, data indicates that the investigated rocks were emplaced at average pressure of 3.5 Kbar corresponding to a depth of ~12 Km. Oxygen fugacity was high in magma. Na2O content in amphibole and their calc-alkaline nature show gabbroic diorites are related to subduction setting. Mg#, Al2O3 and TiO2 contents in hornblende indicate mixing of crustal and mantle sources to generate magma of the Darvazeh gabbrodioritic magma. Variations in the composition of plagioclase may be related to H2O content in environment and the role of chemical variation due to input of crustal materials.

SKS phase passes through the mantle as an S wave; it converts to a P wave at the core-mantle boundary (CMB) beneath the source, travels through the fluid outer core, and then converts back to an S wave at the CMB on the receiver side. When a shear wave (SKS) propagates through an anisotropic media, it is partitioned into orthogonally polarized fast and slow shear waves. This property is referred to as shear wave splitting. In this study we used shear wave splitting of SKS phase from teleseismic events to identify and estimate the shear wave anisotropy in the upper mantle of the Alborz region. In this study, to find stable splitting measurement with small error, we selected the automate analysis window by performing a grid search over 210 windows. The selected teleseismic events (and) were recorded by 3 broadband seismic stations, DAMV, CHTH and THKV (belongs to IIEES network) and by 5 short period seismic stations, FIR, DMV, AFJ, MHD and GZV (belongs to IGTU network). They used to determine anisotropy parameters in upper mantle and crust beneath the Alborz region. In this study the average direction of anisotropy in mantle and crust of the Alborz region is estimated about 48±12 degree by study on shear wave splitting of SKS phase. Thus, the fast azimuth of anisotropy oriented approximately in NE-SW direction too. Moreover our results showed that the magnitude of anisotropy is about 1.5±0.4 seconds.