Introduction of meta-harzburgites using thermal originated antigorites in the Gishaki Ophiolitic area, as a part of Iranian Alpine type Ophiolites.

Abstract

In this paper, we investigated two originated metamorphosed harzburgites of the metaperidotite complex from the northern part of the Gishaki area. Harzburgite rocks, as main ultramafic complexes of the studied area, were exposed in the scattered location of the area. Despite intensive alterations, harzburgitic units were discriminated using petrography and Raman spectroscopy studies of relict minerals. There are two meta-harzburgite types of regionally metamorphosed harzburgites (R-Hz) and thermal metamorphosed harzburgites (T-Hz) with different characteristics and antigorite types. Antigorite polymorphs, structurally discriminated using Raman spectroscopy, had certain specific chemical characteristics compared with other serpentine polymorphs in SiO2, MgO, and H2O contents presented as separated fields in scatter diagrams. Reviewed antigorites illustrated general differentiations in themselves, so that polymorphs hosted by T-Hz rocks could be considered as a group with typical thermal antigorite geochemical criteria such as lower H2O ( 0.4 wt. % due to Ni tendency to high temperature phases through the recrystallization process), high SiO2 (because of the  high silica activity in the aureole), lower total octahedral cations (due to pressure reverse dependence), and higher FeO (due to high temperature and Fe2+ activity). The increase in Si and Fe2+ activities cause to lack of Tetrahedral and presence Octahedral Tschermak substitutions as temperature dependent phenomena in thermal antigorites. Diagrams of SiO2 versus calculated H2O, SiO2 versus total octahedral cation units, and FeO versus NiO clearly distinguished thermal antigorites from former usual antigorite polymorphs. Thermal antigorite in the Gishaki serpentinites could be considered as metamorphosed products related to metamorphic aureoles.