Serpentinization of Koçali Ophiolite in the Southeast Anatolian Orogenic Belt (Turkey)

Abstract

The Koçali ophiolite is a part of the Southeast Anatolian Orogenic Belt ophiolites. The Koçali ophiolite is represented by serpentinized mantle peridotites, gabbros, sheeted diabase dykes and basalts.  They are later intruded by arc-granitoids. The serpentinized mantle peridotites consist of relicts of olivine and orthopyroxene + serpentine minerals (lizardite + chrysotile + brucite) + Cr-spinel ± carbonate minerals. Two types of serpentine mineralogies and textural relationships are observed. They are: 1- lizardite mesh-textured vein networks with relict olivine cores, 2) bastite texture with serpentinization of orthopyroxene. The high LOI values (8.1 to 16.01%) indicate high degrees of serpentinization. Relatively low Al2O3/SiO2 (<0.03), generally high MgO/SiO2 (between0,86 to1,26), high  MgO (>35 wt%), low Al2O3 (<1.2 wt%) and high Mg# values ( 88 to 90) indicate that the protoliths underwent significant partial melting. The serpentinites are depleted in REE concentrations compared to chondrite values. HREE less depleted than LREE. They revealed generally a small decrease from LREE to MREE (La/Sm= 2,636263 in average,) and a progressive enrichment from MREE to HREE (Sm/Lu=0,466771 in average). The overall depleted bulk concentrations in REE compositions are consistent with high melt extraction is expected for fore-arc/mantle wedge serpentinites.