Thermal metamorphism of the mantle peridotites from Paleozoic accretionary prism by the Eocene pluton (NE of Anarak, Kal-e-Kafi area, Central Iran)

Thermally metamorphosed mantle peridotites from Central Iran

Abstract

The Eocene pluton cross cut the Paleozoic accretionary wedge in the Kal-e-Kafi area (NE of Anarak, Isfahan province, Central Iran). Metaperidotites of this Paleotethys- related accretionary prism are consist of metalherzolite, metaharzburgite and metadunite and are associated with the listwaenites and metasediments (schist and marble).

Rock-forming minerals of the Kal-e-Kafi metaperidotites are olivine (forsterite and chrysolite, Mg#= 0.86-0.92 with CaO<0.02 wt%), orthopyroxene (enstatite, Mg#= 0.84-0.86), clinopyroxene (Mg#= 0.90-0.94, Al₂O₃ 0.03-2.08 wt%), tremolite, tremolitic-hornblende and magnesio-hornblende, anthophyllite, serpentine (antigorite with Mg#= 0.82-0.95), talc, chlorite (clinochlore, Mg#= 0.94-0.95), chromian spinel (Cr#= 0.58-0.72), magnetite and plagioclase (Anorthite An_0.91-0.95). All these minerals are metamorphic products and are not relicts of the primary igneous mineralogy, except the inner parts of Cr-spinels. The main textures of these rocks are porphyroblastic, granoblastic, nematoblastic, poikiloblastic, jack-straw and mesh texture.

Chemical composition of core of Cr-spinels reveal that the protolith of the studied rocks were mantle peridotites which belong to the depleted and moderately depleted peridotite series. Mineral chemistry of clinopyroxenes (Cr₂O₃= 0-0.06 wt%, Al₂O₃= 0.03-2.08 wt%, average TiO₂ value of 0.036 wt%, and trace element patterns in the normalization diagrams) indicate their metamorphic nature

The results of the field study (association of metaperidotites with Paleozoic schist and marbles), petrography evidences (the presence of metamorphic mineral assemblages of tremolite + olivine and olivine + orthopyroxene + anthophyllite), chemical characteristics of minerals (the average forsterite content of olivine neoblasts is ~90; extensive variation in MnO content of olivines; the high value of Al₂O₃ in chlorite; low Cr₂O₃, Al₂O₃ and TiO₂ contents of clinopyroxenes, and presence of anorthite plagioclase) and thermobarometry calculations, show that the peridotites of the Kal-e-Kafi area have suffered a regional metamorphism in P-T condition of greenschist facies in the Paleozoic, then intrusion of the Kal-e-Kafi pluton, caused to a progressive contact metamorphism in 630 to 750 °C and at a pressure less than 1 kbar (pyroxene hornfels facies) in the Eocene. In the next stage, a retrograde metamorphism led to the formation of chlorite from tremolite and partial serpentinization of metamorphic olivines and orthopyroxenes.