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

Kamyaran ophiolitic complex located in the northeast Zagros orogeny along the crush zone between Arabian-Iranian plates. This complex outcropped between the Harsin ophiolites in southeast and Sarwabad ophiolite in northwest. Field observation reveal the fact that the Garmab Ophiolite in the northeast of Kamyaran is a tectonic mélange including peridotites and gabbros cut by microgabbroic dikes. Olivine, clinopyroxene and orthopyroxene with chromian spinel made the peridotites minerals with mesh and porphyroclastic fabrics and Gabbros include plagioclase, clinopyroxene and minor amphibole with intergranular, pegmatoidic and interstitial texture. According to the geochemical results, gabbros have tholeiitic to calk-alkaline nature and show the MORB to island arc characteristics. According to the geochemical and geotectonic results, Garmab peridotites plotted to the abyssal peridotites area that represent from the residual mantle spinel lherzolite after extraction of 15–20% partial melting. The break-off of Neo-Tethyan slab and subduction of this slab branch beneath the oceanic lithosphere during cretaceous led to cessation of the Neo-Tethyan subduction beneath the Sanandaj-Sirjan block, and forming arc-back arc basin (second step of subduction) and related rocks in the Kamyaran ophiolite. Presence of tholeiitic to calc-alkaline magmatism is in response to the slab retreat in the Eurasian continental margin.

Basaltic rocks from Marivan-Palangan ophiolite suite (MPO), Zagros suture zone, western Iran, are studied for magmatic evolution, and source rock partial melting processes. The basaltic rocks have remarkable outcrops in the crustal sequence of the MPO and show pillow structure. In terms of microscopic and chemical characteristics, two distinct rock groups are characterized. Based on chemical classification, both groups are basalt. CIPW norm calculations indicate that all basalts are olivine tholeiite (abyssal tholeiite). In terms of the magma series, they also show tholeiitic affinity, somehow close to transition series. Besides, based on the TiO2 content, they belong to high Ti-type ophiolite. Fractionation of varying amounts of olivine, plagioclase, and clinopyroxene has been inferred from Fenner's diagram, especially for group II samples. Although the samples from both groups are similar to E-MORB in multi-elemental diagrams, the show remarkable distinction in HREE and MREE. Melting models show the various degrees of partial melting from a spinel lherzolite source rock (5-25% for the group I and 15-25% for group II). Due to similarity of Sm/Nd, Ce/Sm, Sm/Yb and La/Yb ratios of both groups to enriched mantle type II (EMII) ratios, contribution of EMII-type components to the source of magma is thought to be responsible for the enriched nature of basalts. It is proposed here that variable degrees of partial melting of a spinel-facies peridotite source and contribution of EMII-type components to the source region controlled enriched nature of the ophiolitic basalts of this part of the Neo-Tethyan realm.

The Beshgaz manganese-bearing veins are located in ~45 km northeast of Birdjand, South-Khorasan, and east of Iran. These veins with 0.1-1.5 m thickness and 4-7 m length are engulfed discordantly within Paleocene (Eocene-Oligocene) volcanic-pyroclastic rocks. The host of Mn-bearing veins are dacite to rhyo-dacite tuff, lapilli tuff with andesitic and andsi-basaltic fragments in shear zones. The Mn grade reaches up to 45% in the vein/veinlets. The major Mn ore minerals are pyrolusite, cryptomelane, and psilomelane. Ore minerals showing colloform and open-space filling textures. Amorphous silica is the principal gangue mineral in the Mn ores and the SiO2 content of these veins vary from 2.41 in Mn-bearing veins to 20.98 % in silicic zone. Based on mineralographic and geochemical data, the Mn ores were preliminarily precipitated as amorphous Mn-oxide and hydroxide gels, and gradually psilomelane and then pyrolusite were developed in expense of the primary amorphous minerals. The average ratio of Mn/Fe in Beshgaz manganese-bearing veins is 26.31 and positive correlation of manages with Ba, Sr, U, and Zn in Mn-bearing veins indicate that these veins have formed as epigenetic by hydrothermal fluids.

Hornblende-bearing gabbroic rocks are quite common in subduction-related magmatic suites and considered to represent magmatic differentiation process in arc magmas (Heliker، 1995; Hickey-Vargas et al.، 1995; Mandal and Ray، 2012). The presence of hornblende as an important mineral phase in gabbroic rocks of subduction zone has been considered either as an early crystallizing mineral from water-bearing mafic magmas (Beard and Borgia 1989; Mandal and Ray، 2012) or as a product of reaction of early crystallized minerals (olivine، pyroxene and plagioclase) and water-rich evolved melt/aqueous fluid (Costa et al.، 2002; Mandal and Ray، 2012). The careful study of petrology and geochemistry of hornblende-bearing gabbroic rocks from Chahak area، of Neoproterozoic age، can provide important information about their petrogenesis. Because of the special characteristics of Chahak hornblende gabbros according to their age and their situation in the main structural units of Iran، their study can present critical keys for the knowledge of geological history of Iran specially central Iran zone. This study carried out in two parts including field and laboratory works. Sampling and structural studies were carried out during field work. Geological map for the study area was also prepared. 65 thin and polished thin sections for petrographical purpose were studied. Major oxides، rare earth elements and trace elements were analyzed for 4 samples (92P-1، 92P-3، B1and B6) from hornblende gabbros on the basis of 4AB1 method using ICP-MS of ACME Laboratory from Canada. In addition، major oxides of three hornblende gabbro samples (89P-62، 89P-59 and 89P-46) were used from Partovifar (Partovifar، 2012). Fariman metamorphic terrains، of Proterozoic age، consist of metamorphosed sedimentary and igneous (plutonic and volcanic) rocks. Hornblende gabbros of the study area include plagioclase، hornblende، biotite pyroxene and olivine as major minerals and apatite، ilmenite and magnetite as minor minerals. In many examples، hornblende and biotite can be seen as corona textures around plagioclase، pyroxene and olivine، while plagioclase، pyroxene and olivine show obviously corrosion features. This can be considered to be formed by the reaction of early formed crystals with aqueous fluid/evolved melt. In some cases، amphiboles show rhythmic overgrowths. The rhythmic amphibole overgrowths represent deep-seated crystallization in a volatile-rich magma under conditions of high but varying gas pressure. In the study area، the most dominant texture of the hornblende gabbros is hypidiomorphic granular، but intergranular and porphyric textures are common too. Based on geochemical data from major and minor elements، studied rocks belong to tholeiite series with meta–aluminous nature. The geochemical behavior of main elements of the studied rocks reveals the normal trend of differentiation in their magma. Chondrite-normalized REE diagram of hornblende gabbros indicates an obvious enrichment of LREE in compare with HREE. MORB-normalized spider diagrams indicate variable enrichment in LILE and depletion in high field strength elements (HFSE). Primitive mantle-normalized spider diagram show negative anomaly for Nb and Zr. Gabbros from southeast of Fariman have an island arc tholeiite nature and based on trace element diagrams، they formed as a result of 3 to 10% partial melting of a garnet lherzolite source. The mineralogy، texture and geochemistry of the studied rocks show striking similarities with gabbroic rocks of subduction zone developed in supra subduction zone of arc-marginal basin setting.

Metamorphic rocks in SW Arak composed from two groups including regional and contact metamorphism. Regional metamorphic rocks are composed of slate, phyllite and micaschists and locted in green-schist facies and contact metamorphic rocks are composed of spotted schist and hornfels respectively. Mineralogically, these rocks are composed of quartz, chlorite, muscovite, biotite and cordierite. Petrographic studies of these rocks show that cordierite mineral formed during contact metamorphism. On the basis of mineral chemistry, the Chlorite has ripidolite compositions and muscovite is rich in the muscovite end-member. Based on calculations, chlorite mineral in regional metamorphic rocks in the study area have formed at temperature of 387-416 ºC and low-to medium pressure. According to the microscopic studies, field observations and obtained analyses of samples from different protoliths, the rocks have been sedimentary and graywacke or shale type. Major and trace elements concentration (e.g. K2O, TiO2, Rb, Ni) indicates that the primary igneous rocks was acidic (andesite to rhyodacite). In comparison with average of upper continental crust, the rocks of studied area of P, Sr, Ba, Ti negative anomalies and of Rb, K positive anomalies that cause replacements of these elements during metamorphism and alteration. Discriminations diagrams of tectonic setting indicate a subduction Zone. Using major oxides elements, the continental active margins considered for metapelites source.

The plutonic crustal sequence of Garagoli-Bagjar, located in NE Sabzevar, is a part of ophiolitic association in the outer part of Central Iran microcontinent. This sequence comprises cumulative olivine gabbro, pyroxene gabbro, isotropic gabbro, foliated gabbro and diabasic sheeted dike complex, cross cut by the wherlitic intrusion, pegmatite gabbro and plagiogranite as small bodies, dikes and veins. This sequence is comparable to upper gabbroic part of the well known ophiolite complex. These rocks construct a large tectonic slics thrusted over the serpentinized peridotites and tectonically located under serpentinites. The cumulative gabbros change gradually to isotropic gabbro upwards and at the same time isotropic gabbro change generally to diabasic sheeted dike complex. The structural measurements of magmatic layering and strike of sheeted dikes, wherlitic intrusions, pegmatite gabbro and plagiogranite revealing E-W direction in the north of Qaragoli and N-S direction in SW Solymanieh might be comparable to the formation of plutoinc oceanic crust in fractures zone. The microscopic studies exhibit that mesocumulate to heteradcumulate textures are abundant which indicate the open magmatic chamber of the supra-subduction zones. The crystallization trends show diversity from olivine-plagioclase-clinopyroxene-orthopyroxene-amphibole trend (MORB like) to olivine-clinopyroxene-orthopyroxeneplagioclase- amphibole trend (as arc type). The study of the chemical analyses indicates diverse compositions and primitive mantle normalized spider diagrams show depletion of Nb with different enrichement of LREE. The low-Ti character, locations in arc related cumulate and non cumulate fields, island arc to MORB tectonomagmatic setting remark their supra subduction origin which formed in Late Cretaceous.

Mantle sequences exposed in west of Fannuj- Maskutan area comprise of lherzolite and porphyroclastic cpx-bearing harzburgite in the lower part and recrystallized fine-grained lherzolite with chromitite lenses in the upper parts of the sequence. Petrography studies and microprobe data show evidence of melt/peridotite interactions, post-melting processes and subsolidus interactions associated with the appearance of two generations of deformed primary pyroxene-olivine and fine-grained pyroxene-olivine amphibole neoblasts. Second generation of minerals formed as inclusion, interstitial and fine-grained. These two groups of minerals have different geochemical characteristics, i.e., the first group are comparable with abyssal peridotites and the second group are comparable to suprasubduction peridotites. Therefore, the chemical compositions of different generations of minerals show different petrogenesis for ultramafic rocks of the Fannuj- Maskutan ophiolitie complex. Interpretation of whole rock chemical data indicate that these rocks have a depleted MORB mantle source which underwent 5-15% partial melting. Rare earth element patterns normalized with chondrite standard values and compared with patterns of depleted MORB mantle (DMM), indicate enrichment in LREE/MREE ration and show U-shape patterns. Thus, the peridotites of the Fannuj-Maskutan ophiolitie have experienced multistage evolution and show characteristics of abyssal environment to suprasubduction zone. It might be said that transition from abyssal environment to suprasubduction has been affected by fluids derived from the subducted slab.