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North and West Pakistan |
National Centre of Excellence in Mineralogy, University of Balochistan, GPO Box 43, Quetta, Pakistan
This is the first detailed report of the well-developed leucocratic rocks, dominantly plagiogranites, from the Bela ophiolite, the most complete and largest ophiolitic outcrop of Pakistan. A wide range of leucocratic rock types occur within the ophiolite including leucogabbro, gabbroic pegmatite, diorite, quartz diorite, trondhjemite, epidote trondhjemite, granite, keratophyre and acidic pegmatite. Two suites of acidic leucocratic rocks are recognized: a very low potash and high soda suite containing magma-fractionated plagiogranite characteristic of most ophiolites, is accompanied by an anatectic suite with higher potash content. Each suite possesses distinct trace element and rare earth contents, together with distinctly different Sr and Nd systematics. These differences indicate differences in tectonic setting for each leucocratic suite. U-Pb isotopic measurements on zircons separated from the acidic rocks yield a crystallization age of 65 ± 1 Ma for both the rock suites. On the basis of geochemical data a suprasubduction zone origin dominated by a marginal basin is proposed for the Bela ophiolite. The plagiogranitic rock suite could have been derived from basic magma by fractional crystallization. The potash-richer granitic rocks may have been generated by crustal anatexis forming a melt that locally invaded the ophiolite.
