زمین شناسی لرستان

Abstract

The ophiolite of Alashtar – Kermanshah axis  is a highly dismembered ophiolite complex that belongs to the Zagros orogenic system. This ophiolitic complex located in western Iran near the northwestern end of the Zagros range.This ophiolite is part of the Crush zone that situated between the the Zagros Folded Belt and Sanandaj-Sirjan Zone .A Crush Zone unit (or High Zagros) has three separate sub-units, which are consist  the Biston limestone (Upper Triassic -Lower Cretaceous ), the  ophiolite, and the Bakhtaran radiolarite.The rocks of this ophiolitic complex consist from bottom to top, peridotite ,Layered gabbros,isotropic gabbros, minor  plagiogranit ,sheeted dike complex, basaltic lavas and sedimentary rocks.

The petrographic studies show that the primary minerals(olivine, plagioclase and pyroxene) of rocks of this ophiolitic complex altered to secondary minerals(serpentine , amphibol , chlorite , sericite and Fe-Oxide).Based on geochemical studies  the  rocks of this ophiolitic complex have tholeiite and calcalalkine magmas signatures. Also the plotted rock samples in different geochemical discrimination diagrams occur in island arc basalt (IAB) field.

These  rocks are marked with depleted pattern in HREE chondrite-normalized and primitive mantle normalized diagrams are characterized by depletion in HFSE  and  enrichment  in  LILE and LREE.These patterns suggest that this ophiolitic complex formed in  Suprasubduction  zone environment.

The results of chemical analysis rocks of this ophiolite show a economic  potential for TiO2 and enriched of MnO and Ni.The Cr and As elements have  anomaly but this rock are without anomaly for Th,U and Co  elements.The Alashtar – Kermanshah axis ophiolite represent the Neo- Tethyan oceanic lithosphere which originally existed between the Arabian-Iranian bloks that in Miocene emplaced.