Whole Rock Geochemistry of Lavas from Bela Ophiolite, Western Pakistan

The time period and location of the initial collision of India with Asia, and also its tectonic context is a matter of debate. Study of the ophiolites along the westernmost part of the Indian plate will shed light on these controversies. The Bela Ophiolite (BO) is the largest ophiolite in western Pakistan. One hypothesis holds that it was originated from obduction of Neotethyan oceanic plate on the rifted western margin of Indian plate. Another hypothesis suggests suprasubduction-zone (SSZ) environment for formation of these ophiolites that is composed of obducted floor of a back-arc basin and an associated island arc with some oceanic island alkali basalt; while a third hypothesis takes it as originated in a large oceanic fracture zone. The origin of BO, however, is still in argument.

Eighteen lava samples between latitude 26°30xN and 27°30xN are selected from a large number of samples collected from BO for this study. All samples are analyzed with Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) for major and trace elements. These samples present a Calc-alkaline trend in AFM diagram, revealing a possibility of SSZ related origin. The spider diagram pattern (normalized to chondrite) shows that these lava samples can be subdivided into two groups: one group with much enriched Light Rare Earth Element (LREE) and positive Nb and Ta anomaly; the other group with negative Nb and Ta anomaly. The first group could be interpreted as of Ocean Island Basalt (OIB) origin, and the second group SSZ related. These primary results reveal there are two different types of magmas, and indicate a different model for the origin of BO.