Tectonic setting of the Bela Ophiolites, southern Pakistan

Abstract

The Bela Ophiolites consist of a 3–5-km-thick sequence of basaltic pillow lava, inter-flow sedimentary rocks (chert, argillite, limestone) and diabase-gabbro sills. Debris of serpentinite (up to kilometer-sized blocks), serpentinite-carbonate breccia, and less commonly of basalt, gabbro, peridotite, pelagic limestone and their metamorphic equivalents occur throughout the ophiolite sequence and may form extensive horizons. The age of the ophiolites, as indicated by pelagic microfauna, is Aptian-early Maestrichtian. The lava flows belong to a spilite-keratophyre-basaltic andesite suite of tholeiites enriched in Fe, TiO 2 and other trace elements. The lava flows, although similar to the E-type MORB, differ in details such as La/Ta ratio ranges (9.62–18.04 for the Bela lavas versus 10–15 for the MORB). The diabase-gabbro sills are similar to the associated lava flows. However, due to their relative enrichment in certain elements (e.g., Fe, Ti, Hf, Th and Ta, etc.), they may represent more fractionated late melts. A comparison of the stratigraphic and structural features of the Bela Ophiolites with modern oceanic environments reveals that these ophiolites probably originated in a large oceanic fracture zone (leaky transform). This is based on lithotectonic features such as the common occurrence of debris of foliated serpentinite and serpentinite-carbonate-breccia-bearing rocks in the ophiolite sequence and on geodynamic considerations. The fracture zone ancestral to the Bela Ophiolites probably acted as a boundary between the Indian and the Neo-Tethys plates during the Cretaceous, analogous to the present-day Owen fracture zone which separates the Indian and the Arabian plates. This ancestral fracture zone was destroyed when transform movement gave way to oblique convergence, culminating in the ophiolite obduction in Paleocene-Early Eocene time. The ophiolite obduction on the West Indian continental margin was preceded by alkaline (WPB) type volcanism which is represented by the Porali basalts, the Bibai lavas and the Deccan basalts.