Back-Arc Basin Signatures represented by the Sheeted Dykes from the Muslim Bagh Ophiolite Complex, Balochistan, Pakistan

Abstract

The sheeted dyke complex is about 1000 meter thick and overlies the mafic cumulate sequence of the Muslim Bagh Ophiolite Complex. This sheeted dyke complex is represented by four phases of metadolerites, the earlier phase shows compositional banding of hornblende and plagioclase. The second phase of metadolerite which cut the earlier phase neither shows foliation nor compositional banding. The third phase which cuts the above two phases has foliation and is relatively more mafic in nature. The final phase is represented by dykes which not only cut the above three phases but also to the plagiogranites. The middle part of sheeted dyke complex is intruded by numerous small bodies of plagiogranites (An 8-30). The pillow basalts are not found on top of the sheeted dyke complex. The petrochemical study of metadolerites show that they belong to low-K quartz tholeiite series. In trace elements chemistry they are enriched in LILE and depleted in HFSE relative to N-MORB, and their average Nb/Y, Zr/Nb, K/Rb, Ba/Zr, Ce/Ba, La/Sm, La/Ce and La/Nd ratios are more consistent with back-arc basin basalt relative to N-MORB or island arc basalts (IAB). Their LILE enriched N-MORB normalized patterns and moderately LREE enriched Chondrite normalized REE patterns are also consistent with back-arc basin basalts. It is further suggested that all the Tethyan ophiolite complexes and oceanic island arcs including Muslim Bagh, Waziristan, Semail, Zagros, Chagai-Raskoh and Kohistan-Ladakh were developed in a single but segmented Late JurassicCretaceous Ceno-Tethyan convergence zone.