Rhonite in undersaturated alkaline gabbroic rocks, Central Alborz, North Iran: petrography and mineral chemistry

The Kamarbon alkaline gabbroic intrusion crops out in Central Alborz, north Iran, along the northern margin of the Alpine-Himalayan belt. The gabbroic intrusion includes theralites at marginswhich replace with teschenites toward the center. In teschenitic rocks, the main minerals are diopside, clinopyroxene, and rhnite. In this research, the occurrence of rhnite is reported in Kamarbon teschenitic gabbros, and also its mineralogical properties, paragenesis, and source magma are investigated. Based on whole rock and microprobe analysis data, we try to clarify the formation of Kamarbon gabbro and the crystallization condition of rhnite. In teschenitic gabbros, rhnite reveals the composition (Na, Ca)(1.97)(Ti, Al-VI, Fe+3, Fe+2, Mn, Mg)(5.99)(Si, Al-IV)(6.02)O-20. On the basis of petrographical observations and mineral chemistry, we suggest that the teschenites were formed in distinctive lower pressures and temperatures than theralites, below 1.9 kbar and 1075 degrees C. Rhnite was crystallized (at the mentioned P-T condition) as a primary phase, in the late stage of crystallization at shallow depth corresponding with 6-10 km, in teschenites. Important factors of the rhnite crystallization in undersaturated magmas can be regardarded as Al and Ti enrichment and Si depletion; the same enrichment and depletion are also observed in the associated clinopyroxenes.