Abstract

The Bela Ophiolite (BO) is part of the Alpine-Himalayan orogenic belt. Rocks of BO were formed in both MORB and suprasubduction zone settings and include both ultramafic and mafic compositions. The ultramafic rocks were serpentinized after harzburgite. Orthopyroxene is more abundant than clinopyroxene. Serpentine commonly occurs as angular and medium sized fragments exhibiting mesh texture and within veins. Serpentine is associated with variable percentages of chlorite, magnetite, actinolite, talc, brucite and calcite. Shearing and faulting obliterated the primary textures and resultant microfractures permeate the rocks. On an AFM diagram samples from the study area plot in the field of metamorphic peridotite and ultramafic rocks of ophiolitic affinity. On a TAS diagram, the samples reflect a tholeiitic source that was generated at very high temperature, most commonly at a MORB setting. High range of Mg# 97.5 - 82.6 and low values of differentiation index (0.7 - 7.7) suggest primitive mantle compositions. The ultramafic rocks of the study area were serpentinized and subsequently altered into hydrothermal magnesite, hydromagnesite and brucite. The magnesite and associated hydromagnesite indicate formation within a low temperature regime. On a Fe versus Mn plot, and the average Cr/Mg, Fe/Mg and Mn/Mg values revealed that the studied magnesites were genetically affiliated with cryptocrystalline Kraubath type of magnesite.

Key words: Khuzdar, magnesite, Pakistan, petrography, ultramafic rocks.