Late Permian blueschist from Anarak ophiolite (Central Iran, Isfahan province), a mark of multi-suture closure of the Paleo-Tethys ocean

Abstract

Anarak Paleozoic ophiolite is located in western part of the Central-East Iranian Microcontinent. This metaophiolite is covered by Paleozoic schist and marble. Blueschists of the Anarak ophiolite are exposed along the northern Anarak east-west main faults and are considered as remnants of the Paleo-Tethys suture zone in Central Iran. Anarak blueschists are formed by metamorphism of primitive basic lavas. In some cases, they preserve the primary pillow structure. Petrography and microprobe analyses show that they are composed of riebeckite, actinolite, plagioclase (albite), sphene, magnetite, white mica and apatite. Secondary minerals are chlorite (pycnochlorite), epidote, pyrite and calcite. Mineralogical assemblages are consistent with blueschist facies metamorphism, which is followed by a retrograde metamorphism in greenschist facies. Estimation of the metamorphic conditions suggests 300-450 oC and 4-9 kbar.

Whole rock geochemical analyses show that these rocks can be classified as alkaline basalts. Chondrite-normalized rare-earth element (REE) patterns of the studied rocks display 10-150 times enrichment, high light REE and relatively low heavy REE contents. These geochemical characteristics are representative of mantle-derived magmas. Primitive mantle normalized spidergram of the Anarak samples exhibit negative anomalies of Ba, U, K and Sr, and positive anomalies of Cs, Rb, Th, Nb, Ta and Zr. Similar geochemical features of all analyzed rocks indicate that they were all derived by more than 12% partial melting of an enriched/carbonated garnet lherzolite and underwent similar degree of partial melting. Geochemically, the studied blueschists resemble intraplate alkali-basalts. The presence of Paleozoic ophiolitic rocks along the main faults of central and northern Iran are indicative of a multi-suture closure of the Paleo-Tethys ocean.