Original paper

Metapicrites of the Bayazeh Ophiolite (Central Iran), a trace of Paleo-Tethys subduction-related mantle metasomatism

Nosouhian, Nargess; Torabi, Ghodrat; Arai, Shoji


The western part of the Central-East Iranian microcontinent (CEIM) hosts the Anarak, Jandaq, Bayazeh and Posht-e-Badam ophiolites of Paleozoic age. The Bayazeh ophiolite is situated in the Eastern margin of the Yazd Block and is exposed along the Bayazeh Fault. This ophiolite consists of serpentinized peridotites, metagabbro, metamorphosed ultrabasic dykes, metapicrite, serpentinite and metalistwaenite which are covered by Late Paleozoic schist and marble. Mineral association of the metapicrites in the Bayazeh Ophiolite is olivine (completely altered to serpentine), clinopyroxene (diopside), phlogopite, apatite, prehnite, amphibole (tremolite, actinolite and tremolitic hornblende), chlorite (clinochlore, penninite and diabantite), ilmenite and magnetite. Matrix glass is significantly devitrified and chlorite is present throughout the matrix. Clinopyroxene and phlogopite occur as relicts of the primary igneous mineralogy. Petrography and trace element composition of clinopyroxenes indicate near-simultaneous crystallization of clinopyroxene and plagioclase from the magma. The ultramafic character of the Bayazeh metapicrites is shown by their high MgO (25.8 to 28.0 wt%) and low SiO2 (37.5 to 39.4 wt%) contents. They are characterized by high Mg# (80.61 to 81.60), Ni (975 to 1020 ppm) and Cr (1300 to 1431 ppm) contents, suggesting that this melt closely approached the composition of a primitive mantle-derived melt. High-field strength element (HFSE) enrichment, high Mg# and Ni values, enrichment in light rare earth elements (LREE) (e.g. [La/Yb]CN=11.65-12.31), associated with a large variation of large ion lithophile element (LILE) concentrations indicate metasomatic enrichment of an asthenospheric mantle source with a subduction-related components prior to melting. Geochemical characteristics show that the metapicrites were generated by partial melting of a metasomatized asthenospheric amphibole-bearing spinel lherzolite. The presence of phlogopite as a primary hydrous mineral together with high LILE/HFSE ratios (e.g. Sr/Sm = 63.01-104.86) and a negative Ti anomaly reveal the role of previously subducted oceanic crust. Subduction of the Paleo-Tethys from the Early to the Late Paleozoic is the cause of volatile enrichment and mantle metasomatism. Involvement of hydrous fluids related to Paleo-Tethys subduction, enabled the peridotite source to melt despite the absence of abnormal thermal condition.


bayazehcentral iranmetapicriteophiolitepaleo-tethyspaleozoic