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Showing 4 results for Mantle Peridotite

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Volume 12, Issue 1 (4-2004)
Abstract

In north of Anarak area, in the upper Proterozoic ophiolite, exist Massive and Vein type Rodingites. Vein type rodingites have formed in two phases. Massive or Static rodingites have formed during oceanic crust spreading and ophiolite formation, and Vein type or Dynamic rodingites have formed during Anarak ophiolite emplacement and tectonic activities. Abundance order of minerals in massive rodingites is Garnet (Chromian-Andradite), Idocrase, Chlorite, Diopside, Prehnite and Epidote (decreasing). In the first phase of vein type rodingites, the abundance order of minerals is Diopside, Chlorite, Idocrase, and Garnet (Ti-Andradite), and in the second phase is Diopside, Chlorite and Garnet (Andradite). Petrography and geochemical study of minerals show that, during the formation of massive type rodingites, the activity of Calcium, Iron, and Chromium ions in fluids, and during the formation of first phase of vein type rodingites, the activity of Calcium, Silicon, Magnesium, Aluminum, Titanium, and OH- ions in fluids, have been increased. Geochemistry of second phase of vein type rodingites is similar to first phase, but because of absence of Idocrase, and low Titanium in Andradite structure, the activity of Iron, Titanium, and Aluminum ions, is lower than first phase. Generally the diversity of minerals in massive type rodingites is more than vein type rodingites.


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Volume 20, Issue 2 (6-2012)
Abstract

The Surk ophiolitic mélange is located at the south west of central Iran and along the Naein-Dehshir-Baft ophiolitic belt. In petrological sequence, peridotites are nominated the undermost part of rock unit, and then gabbros and volcanic rocks are at the following ranks respectively. The predominant peridotitet of this ophiolitic mélange is harzburgite. Lherzolite that is the primitive mantle rock consists of olivine, orthopyroxene, clinopyroxene as major minerals and chromian spinel and amphibole (pargasite) as accessory mineral. Petrographical characteristics and mineral chemistry of these peridotites show the lithology evaluation from lherzolite to harzburgite and dunite ultimately. The average SiO2 and Na2O content of volcanic rocks in the ophiolitic melange are 70 and 5 weight percent respectively. Petrographical investigations and also classification of these volcanics based on major and trace elements and norm calculations show that these rocks are dacite. REE/chondrite normalized diagrams of these volcanic rocks show horizontal to positively sloping patterns, LREEs are lower than HREEs and their La/Yb ratio is lesser than 1; suggest tholeitic nature of their parent magma. Moreover, Hf/Zr, U and Pb contant of these rocks is higher than the content in the Primitive mantle but the Sr, Ti and Y contant is lower. These differences can be caused by melt migration of subducted oceanic lithosphere on mantle peridotites. By melt/rock reactions clinopyroxene dissolution, incongruent melting of orthopyroxene, formation of replacive olivines and SiO2 increasing in ascending melt will occur. All these characteristics propose that the Surk ophiolitic mélange is a Harzburgite Ophiolitic Type (HOT) which is formed at the supra-subduction (back-arc basin) zone.


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Volume 26, Issue 1 (4-2018)
Abstract

The Kermanshah ophiolite complex is located in outer Zagros ophiolitic belt and consists of both mantle and crustal suites as remnants of southern branch of the Neo-Tethys Ocean in Iran. Chemical composition determinative diagram of minerals indicates that olivine in dunite, harzburgite and lherzolites has forsteritic composition and in wherlitic rocks is chrysolite in composition likewise orthopyroxene and clinopyroxene in these rocks are enstatitic and diopside to augitic composition respectively. Textural evidence such as exsolution lamellae of clinopyroxene in orthopyroxene and  chain-like grains of spinel in minerals of ultramafic rocks of Sahneh, Harsin-Nourabad and Myanrahan region displaying clear evidence of cooling with decompression, partial melting and melt-rock reactions. Very low modal of clinopyroxene, Mg rich olivines and high Cr# content and Cr/Al ratio of chromian spinel from ultramafic cumulates indicate that dunites and harzburgitic rocks have formed by high degrees of partial melting. Calculated TiO2, Al2O3 and FeO/MgO ratio in the parental melt that was in equilibrium with chromian spinels and also all of used discrimination diagrams are compatible with a forearc boninitic magma (such as  picrites or island arc tholeiites) in supra-subduction zone setting.
Ayoub Veisinia, Mohammad Ebrahimi, Mirali Mokhtari, Jamshid Ahmadian, Amirmorteza Azimzadeh,
Volume 26, Issue 3 (10-2018)
Abstract

Northeast Kamyaran ophiolitic complex is mainly composed of harzburgite along with small outcrops of lherzolite. Harzburgites are extensively altered to serpentinite. Small lenses of chromite mineralization are observed in harzburgites. The mineral assemblage present in the study harzburgite and lherzolite are olivine, orthopyroxene and clinopyroxene along with minor amount of spinel. Microprobe studies on the spinel indicate that it is Cr and Al bearing spinel (Cr-spinel) with very high Mg# (55.2-71.9) and Cr# (34.3-45.5) values and low values of TiO2 (with average value of 0.6 wt%). Fe3+ content of the Cr-spinel in the study rocks is high (with average value of 0.8 wt %) indicating that the crystallization occurred at high oxygen fugacity. According to the microprobe data, MgO content of olivine varies between 50.04 and 52.99 wt% and its composition is forsterite (Fo90.68-91.42). Based on the mineral chemistry of spinel, peridotites of Northeast Kamyaran ophiolitic complex belong to residual depleted peridotites which have formed in mid ocean ridge tectonic setting.

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