Island arc-related, back-arc basinal, and oceanic-island components of theBela Ophiolite-Melange complex, Pakistan

The Bela ophiolite-melange complex (BOMC) of Pakistan is a late Cretaceous (similar to 66 Ma) suprasubduction-zone composite ophiolite. Its abundant m afic volcanic and hypabyssal rocks are divisible into three compositionally distinct suites-an ensimatic volcanic island-are (suite 1) dominated by lo w-Ti (Ti < 7200 ppm) tholeiites, a back-are basinal assemblage (suite 2) co ntaining high-Ti (>10,000 ppm Ti) rocks, and high-Ti oceanic-island alkalin e basaltic rocks (suite 3). Compared to suites 1 and 2, suite 3 rocks are h igh in light rare-earth elements (LREE), Zr, Nb, and Zr/Y, and low in Cr, Z r/Nb, Y/Nb, and Ti/Nb. In Zr/Y ratios, suite 1 < suite 2 < suite 3. The geo graphic distribution of the three BOMC suites defines a northern (north of latitude 26 degrees 50' N) island-are terrane characterized by island-are t holeiites (IAT) and a southern basinal terrane containing back-are basinal basalts (BABB). Alkaline basalts apparently represent oceanic seamounts, an d occur at isolated sites in both terranes. Some oceanic-island basalts (OI B) are emplaced in younger sediments overlying the ophiolite. As exceptions to the general distribution, a few tectonic blocks of high-Ti basalt crop out in the northern terrane, and a few low-Ti basalts occur in the southern terrane. Representative massifs with a completely intact ophiolite stratig raphic sequence include the Lak Baran massif in the are terrane and the Bor a Jhal massif in the basinal terrane. The are terrane contains more extensi ve chromite deposits than does the basinal terrane. Are chromites possess h igher Cr/(Cr+Al) than do chromites of the basinal terrane, indicative of mo re refractory mantle source rocks and higher degrees of partial melting. Nd -Sr isotopic data on 10 BOMC lithologies support their arc-related nature a nd derivation from an enriched source mantle.