Original paper

Metamorphosed mantle peridotites from Central Iran (Jandaq area, Isfahan province)

Torabi, Ghodrat; Arai, Shoji; Koepke, Jürgen

Abstract

Mantle peridotites of the Jandaq ophiolite are composed of metamorphosed lherzolite and harzburgite, with lherzolitic mineralogical and bulk chemical compositions prevailing over harzburgite. They are closely associated and isofacial with amphibolites and Paleozoic metasedimentary rocks (schist and marble). Rock forming minerals of metalherzolites are clinopyroxene, Cr-spinel, olivine, tremolite, chlorite, serpentine, magnetite and calcite. Metaharzburgites are composed of olivine, orthopyroxene, talc, anthophyllite, magnetite, Cr-spinel ± low amounts of chlorite and tremolite. The olivine and orthopyroxene in these rocks are metamorphic products, not relicts of the primary igneous mineralogy. Early relict phases (mantle phases) include clinopyroxene and Cr-spinel. Petrography, mineral chemistry, as well as major and trace element geochemistry of the metamorphic lherzolites indicate that they are relicts of undepleted mantle material and have experienced less than 3% partial melting. Metamorphic assemblages in metaperidotites, mineral chemical studies, together with additional geological constraints, indicate that the Jandaq mantle peridotites are metamorphosed under upper amphibolite facies conditions (8 to 9 kbars and 714 to 737 °C), followed by a retrograde metamorphism under greenschist facies conditions. The maximum temperature of regional metamorphism was not sufficient to replace calcic amphibole (tremolite) by calcic pyroxene. Textures (magnetite inclusions in metamorphic olivines) and associated rock types (meta-listwaenite) indicate the studied rocks were partly altered (M1) before being progressively metamorphosed under upper amphibolite facies conditions (M2). Late-stage retrograde metamorphism (M3) led to formation of chlorite and thin irregular serpentine veinlets, partially to serpentinization of metamorphic olivine and orthopyroxene, and partial replacement of chromian spinel by ferritchromite and magnetite. These three metamorphic episodes can be attributed to the Early, Middle and Late Cimmerian orogenies of Iran, respectively.

Keywords

ophiolitemantle peridotitemetamorphismlherzolitejandaqcentral iran