جستجوی مقالات مرتبط با کلیدواژه « ultramafic » در نشریات گروه « زمین شناسی »

Sahneh, Harsin-Nourabad ophiolits (Kermanshah Provience) located in outer Zagros ophiolitic zone and are parts of the Cretaceous ophiolitic belt, as remnants of southern branch of the Neo-Tethys Ocean obducted over the Arabic–Tauric platform from Oman to northwestern Syria, Cyprus and Antalya. Calculation of Fe/Mg separation coefficient of olivine indicates the mantle origin of this mineral. Textural evidence of ultramafic rocks displays clear evidence of cooling with decompression, partial melting and melt-rock reactions. Tectonomagmatic discrimination diagrams show that peridotite samples are located in olivine- spinel mantle array and SSZ peridotite field. Based on geothermobarometry calibration the temperature and pressure of crystallization of harzburgite and lherzolite rocks were 1100 to 1500 ° C and 12 to 18 Kbar and the wehrlite sample was 1200 to 1500 ° C and 21 to 23 Kbar, which corresponds to the SSZ tectonic environment.

Part of the Ultramafic Sorkh-Band complex along the Kahnuj, Roudan, Minab Ophiolite belts Hormozgan Province at Nazdasht area, contain harzburgaite, dunite, and serpentinit (with dominance harzburgaite). The major mineral in these rocks included olivine, orthopyroxenes and minor mineral such as Spinel. Mesh and granoblastic texture is dominance in these rocks and tectonic pressure effect can be seen as kink band and undulose extinction within the minerals. Results from Electron Microprobe analysis of minerals show olivine with (Fa 90.18-93.6) forsterite composition and rich of magnesium that belong to Alpine type. Also orthopyroxenes show (En 32.8-95.2) enstatite composition and spinel Cr# (53.9-71.02) have chrome. The harzburgites have high Mg# (97.78-84.65) but are poor of Al2O3 (0.71-0.34) and Ca (0.69-0.45), which can be said these rocks are refractory waste that have been remain after the partial melting of more than 25% percent. Low amount of Al and Ti (0.01-0.04) in spinel existence at the Nazdasht harzburgites consider to be characteristic of supra-subduction zone peridotites.

The Hadji-Abad ultramafic complex is located at the north of Hormozgan province and represents a part of Orzuieh-Dowlat Abad colouredmelange in the northern edge of Zagros thrust. The complex contains harzburgites, lherzolites, dunites and chromitites. Harzburgite is the dominant rock type. Evidence such as lobateboundaries, elongation of chromianspinels and pyroxenes, evidence for incongruent melting of orthopyroxenes and exsolution lamellae of clinopyroxenes show that the studied peridotites, experienced high temperature deformation in the upper mantle, and then emplaced in the crust. Different amounts and various shapes of chromite grains occur in all of the studied lithological units. In the harzburgites and lherzolites, disseminated brown chromites are seen as either euhedral isolated crystals or anhedral interstitial ones which crystallized between the other minerals. In the dunites, chromites appear as disseminated black euhedral and subhedral crystal grains which formed within or at the boundaries of olivine grains. In the high grade chromitites, the coarse black euhedralchromites show smooth and triple junction boundaries with cumulative textures, while in the low grade types, disseminated euhedralchromites set in a silicate matrix. Chemical analysis of disseminated chromites in the studied rocks show that maximum amounts of Cr# belong to those exist in the high grade chromitites (80-84) and the minimum are for those in the lherzolites (45-52). Tectonic discrimination diagrams reveal that chemical compositions of chrome spinels from the harzburgites and lherzolites are similar to those exist in the suprasubduction zone mantle peridotites. The host rocks as a part of ophiolites in this environment, suffered 15 to 20% partial melting. Disseminated chrome spinels from the dunite and also the chromitites have been crystallized from boninitic type melts in the same tectonic setting. These evidence show that probably, Hadji-Abad peridotites and their host ophiolitemelange belong to suprasubduction zone upper mantle and the evolutions related to this environment such as melt-peridotite reaction and partial melting have been recorded in their mineral chemistry and textures.

In the central part of the Sanandaj-Sirjan zone, there is an ultramafic rock exposure (hornblendite and pyroxenite) adjacent to Molataleb felsic complex completely located between Azna and Aligoodarz towns. The ultramafic rocks are actually cumulates derived from boninitic magma. During Upper Triassic-Lower Jurassic time the boninitic magma has been originated from mantle wedge as the result of initiation of Neo-Tethys subduction. Later, when subduction was proceeding, the felsic rocks crystallized in Middle Jurassic. Major elements composition of olivine, pyroxene, amphibole and minor plagioclase from the ultramafic rocks reveals crystallization from a sub-alkaline to calc-alkaline magma in a subduction zone setting. Primary minerals have chemical characteristics typical of those derived from a magma with low oxygen fugacity. Different methods for minerals thermobarometry indicate that amphiboles crystallized in relatively low temperature (880°C) but crystallization condition of other minerals corresponds to higher temperatures (1000-1200°c). They were crystallized at pressure condition equal to 5.85 kbar corresponding to the depth of ~17 km. It is not common that a mantle wedge at the depth of ~17 km to be affected by such high thermal gradient during the normal subduction process. Asthenospheric flow around the subducting slab edge during subduction initiation can explain high thermal gradient prevailed the infant mantle wedge. This mechanism corresponds to the boninitic nature of the ultramafic rocks.

The studied area is located in the north of Rudan city and is a part of Kahnuj - Rudan - Minab ophiolite belt. The studied rock are harzburgite and lherzolite and are composed of olivine, orthopyroxene, clinopyroxene and chrome – spinel (as a minor). According to microprobe data, the amount of MgO in the olivines ranges between %50.17 to %50.55, so these olivines are forsterite. Clinopyroxenes are diopside and spinels contain chrome. Detail microprobe studies on the chrome - spinel of ultramafic lherzolites prove that they are high in Mg#(%71-%77) and Al2O3 (52.98 to 46.31 Wt%) ) and low in Cr# (%0.14 - %0.22). (Cr/Al) ratio in the spinel of lherzolites is 0.19. The amount of Fe3 in the chrome - spinel of ultramafic Peridotites is very low (

Granitoid rocks of the central segment of Sanandaj-Sirjan zone, occurring in Lorestan Province, are parts of  a continental arc setting intruded during Mid Jurassic time. Ultramafic rocks are adjacent to this felsic rocks with olivine, orthopyroxene, clinopyroxene and amphibole as their major rock forming minerals. Microscopic observations revealed rounded shape and occurrence of embayments in the olivines attesting different degree of olivine assimilation. By applying electron microprobe analyzes, the chemical compositions of the melts in equilibrium with minerals were calculated. It was revealed that Mg# of the melt is linearly increased as the minerals crystallized. In addition to magmatic origin of the olivine, this trend clearly shows an uncommon behavior of Mg# in the magma that increased during fractional crystallization. Two different possibilities are examined to explain the Mg# increasing. 1- high oxygen fugacity of the magma that led to early crystallization of Fe-oxides; 2- olivine assimilation during fractional crystallization. The results obtained by geochemical modeling and the increase of Ni during fractional crystallization revealed that olivine assimilation during fractional crystallization is the factor that increased Mg# and Ni content of the magma.

Mafic and ultramafic plutonic igneous bodies، with small and big outcrops، between Shemshak rock units (Jurassic) are observed in east of Imam Zadeh Hashem، in southern Guilan province. Ultramafic cumulates consist of clinopyroxenite، and plagiofer clinopyroxenite، olivine clinopyroxenite، and mafic rocks، based on mineralogy consist of gabbros، olivine gabbros، biotite gabbros and amphibole gabbros. According to geochemical data، studied rocks have tholeiitic nature and in the tectonic setting diagrams، display arc characteristic. The chondrite normalized REE patterns show low enrichment in LREEs relative to HREEs. The negative Nb and Ti anomalies in primitive mantle and MORB-normalized multi-element diagrams of the rocks are characteristic of island arc magmas. Also، enrichment in LILE and depletion of HFSE may indicate a subduction-related tectonic setting. According to geological and geochemical evidence، Imam-Zadeh Hashem ultramafic and sub-alkaline gabbro rocks can be as a part of the Southern Caspian Sea Ophiolite sequence (SCO) that formed in a suprasubduction tectonic system.

Sistan suture zone ophiolitic rocks are remnants of Tethyan oceanic lithosphere between Lut and Afghan continental blocks. This oceanic basin has been existed at least during the Cretaceous. Mineral chemistry from crustal sequence ultramafic-mafic associations and petrographical evidences from parts of these ophiolitic complexes (the ophiolitic rocks located between the south of Birjand and Tchehel Kureh ophiolites) suggest distinct geodynamic setting for the formation of oceanic lithosphere. Plutonic rocks from crustal sequence mainly include ultramafic cumulates (dunite and pyroxenite)، cumulate gabbro، gabbronorite and isotropic gabbro. The crystallization trend in these rocks is variable including earlier crystallization of pyroxene relative to plagioclase and vice versa which implies SSZ (supra-subduction zone) and MORB (Mid-Oceanic Ridge Basalt) - type trends، respectively. The composition of plagioclase in cumulate gabbros and gabbronorite is bytownite and anorthite، respectively. Moreover، the contents of compatible and incompatible elements of clinopyroxene and orthopyroxene in ultramafic cumulates، cumulate gabbros and gabbronorite represent significant differences. The chemical composition of these minerals and crystallization trends indicate that pyroxenites-gabbronorite and troctolite-cumulate gabbros-isotropic gabbros have been formed from two different magma series with SSZ and MORB affinities، respectively. In the study area، it seems that the MORB-type magmatism resulted from partial melting beneath mid ocean ridge produced a depleted mantle. Subsequently، consequence of intra-oceanic subduction the MORB mantle has been suffered higher depletion due to fluid-induced melting in supra subduction zone setting and produced SSZ-type magmatism.

Ghazan mafic-ultramafic intrusive is a small igneous body at the extremity of NW of Sanandaj-Sirjan zone in the NW of Iran، intruded into the early Paleozoic sedimentary rocks. This body includes non-mineralized mafic and mineralized ultramafic part (containing Fe-Ti oxides). The mafic rocks are composed mainly of pegmatite gabbro to micro gabbro، metagabbro and anorthosite، with a simple mineral assemblage (plagioclase، clinopyroxene، orthopyroxene، ilmenite). The ultramafic rocks include pyroxenite and wherlite (with high proportion of ilmenite and titanomagnetite) in accompany by large to small blocks of pure Fe-Ti oxide are widespread in the area. The ultramafic part is located within the intrusion and enclosed by mafic rocks. Based on the results of chemical analyses، the primary magma of mafic-ultramafic rocks، derived from a mantle source. During magma ascent the composition of magma has been changed from alkaline to sub-alkaline. U-Pb dating، of a primary and magmatic zircon، gives an age of 299 Ma and dating on metamorphic and inherited zircons shows maximum age of 541 Ma. The younger age represents the the Upper Paleozoic plutonism in the northern part of the Sanandaj-Sirjan zone. The older ages related to the Precambrian basement rocks or older basement rocks on the way of magma، which are incorporated in the magma as exnocryst. The greenschist facies metamorphism، high-temperature deformation as well as altered rocks are widely observed in the area.