Mineral Chemistry Perspective of Nain Ophiolite Mélange, Central Iran

The present study documents detailed mineral chemi-stry perspective of Nain ophiolite melange (NOM) of Central Iran with an aim of deciphering the mineral systematics and understanding geothermobarometric equilibration. The NOM covers ~600 km2 and is located at the northwest margin of Central Iranian Microcontinental block. NOM is represented by a sheared, tectonized and serpentinized peridotite in-truded by coarse-grained pegmatitic gabbroic dykes, layered gabbro, sheeted dolerite dykes (with typical rodingite alteration) and pillow basalts. Plagioclase in pillow basalt is albitic and indicates its spilitic affinity, while pyroxene is typically quad pyroxene (augite to diopside). Amphiboles belong to calcic group and range from actinolite to magnesio hornblende. Ilme-nite is the characteristic opaque phase. Clinopyroxene thermometry records a temperature span of 1100–1300C, while amphibole thermometry records 979–1145C. Two-feldspar thermometry also records a similar thermometric range. Amphibole barometry shows higher pressure of equilibration for mantle pegmatite in general and a very low equilibration pressure for sheeted dyke. Pyroxene compositions typically indicate a calc-alkaline basaltic (orogenic) parentage. NOM signifies lherzolite ophiolite type in a chromite-free environment and it is analogous to an idealized ophiolite succession, but has been emplaced in the form of discrete tectonic melange.