Island Arc–Related, Back-Arc Basinal, and Oceanic-Island Components of the Bela Ophiolite-Mélange Complex, Pakistan

Abstract

The Bela ophiolite-mélange complex (BOMC) of Pakistan is a late Cretaceous (∼66 Ma) suprasubductionzone composite ophiolite. Its abundant mafic volcanic and hypabyssal rocks are divisible into three compositionally distinct suites—an ensimatic volcanic island-arc (suite 1) dominated by low-Ti (Ti < 7200 ppm) tholeiites, a back-arc basinal assemblage (suite 2) containing high-Ti (>10,000 ppm Ti) rocks, and high-Ti oceanic-island alkaline basaltic rocks (suite 3). Compared to suites 1 and 2, suite 3 rocks are high in light rare-earth elements (LREE), Zr, Nb, and Zr/Y, and low in Cr, Zr/Nb, Y/Nb, and Ti/Nb. In Zr/Y ratios, suite 1 < suite 2 < suite 3. The geographic distribution of the three BOMC suites defines a northern (north of latitude 26°50' N) island-arc terrane characterized by island-arc tholeiites (IAT) and a southern basinal terrane containing back-arc basinal basalts (BABB). Alkaline basalts apparently represent oceanic seamounts, and occur at isolated sites in both terranes. Some oceanic-island basalts (OIB) 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 stratigraphic sequence include the Lak Baran massif in the arc terrane and the Bora Jhal massif in the basinal terrane. The arc terrane contains more extensive chromite deposits than does the basinal terrane. Arc chromites possess higher Cr/(Cr+Al) than do chromites of the basinal terrane, indicative of more refractory mantle source rocks and higher degrees of partial melting. Nd-Sr isotopic data on 10 BOMC lithologies support their arc-related nature and derivation from an enriched source mantle.