Economic Geology
Kolsoum Fani Chenar; Mansour Ghorbani; Reza Kohansal; Jamshid Lotfian
Abstract
Ali abad chromites mines (north east of Forumad) are as a part of chromites assemblage dependent on ophiolite Sabzevar that are located in NW Sabzevar city. Chromitites deposits in these mines have been located as irregular lenses and layered or banded along with pods of dunite in depleted harzburgite. ...
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Ali abad chromites mines (north east of Forumad) are as a part of chromites assemblage dependent on ophiolite Sabzevar that are located in NW Sabzevar city. Chromitites deposits in these mines have been located as irregular lenses and layered or banded along with pods of dunite in depleted harzburgite. In attention to mineralogy observation, incongruent melting of orthopyroxenes and exsolution lamellae of clinopyroene in the orthopyroxenes, indicate that these rocks were originally formed at mantle pressures and temperatures conditions. Mineralography study on the textures and structures of Ali abad chromites indicate two types main textures that are primary and secondary. The primary textures and structures include massive and disseminated or separated grain. The milonitic, brecciated and tensional textures are from secondary types in chromites. On the basis of geochemical results the structural formula are calculated as the chromite minerals [(Mg,Fe2+)O (Cr,Al,Fe3+)2O3].These studies represent height percent Cr2O3 chromite mineral in disseminated texture than massive textures. Geochemical study on the chromitite area, site them in height Cr types and associated to podiform type that are originated from a parent melt with boninitic character resulting from high degree of partial melting of mantle peridotite in Supra Subduction Zone (SSZ).
R. Kohansal; S. Zolfaghari; M. Ghahraie - Pour
Abstract
The study area is a small part of the Sabzevar structural zone and confined between Semnan and Khorasan provinces. The oldest sedimentary rocks of this region are Middle Jurassic in age with horizons of calc-alkaline rhyolitic- rhyodacitic lavas in between. First appearance of alkaline basaltic lavas, ...
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The study area is a small part of the Sabzevar structural zone and confined between Semnan and Khorasan provinces. The oldest sedimentary rocks of this region are Middle Jurassic in age with horizons of calc-alkaline rhyolitic- rhyodacitic lavas in between. First appearance of alkaline basaltic lavas, intruded by diabasic dikes in this region appeared along with Early Cretaceous limestone. Late Cretaceous rocks include voluminous calc-alkaline rhyodacitic- rhyolitic and trachyandesitic composition as lavas and dikes, and basaltic dikes with abundant pinkish-cream plagic limestone. This has been overlaid by Paleocene rocks. Harker element variation diagrams indicate contamination and differentiation of basic magma, from a depleted mantel, which is also confirmed with spider diagrams. Tectonomagmatic signatures of Middle Jurassic and Upper Cretaceous volcanic rocks are similar to volcanic arc and syn-collisional magmas which have formed by tectonic movements of Middle Cimmerian and Laramid. Furthermore Austrian orogeny has led to development of Lower Cretaceous basic volcanic rocks within continental plate that are demonstrated in binary and ternary trace element diagrams. The Late Cretaceous-Early Tertiary Laramid orogeny leads to closure of the Neo-Tethys basin by folding and uplifting of the study area. Just in the Marri area lack of intrusive rocks pertaining to mature ophiolitic assemblage, presence of volcano-sedimentary rocks in the upper part of this succession from one side, and abundance of pressure and strike-slip faults with similar mechanism of Doruneh fault from other side, suggest an old suture zone and fast closure of this part of Doruneh- Kashmar trough, and development of colored melange in this area.
