BACK-ARC BASIN SIGNATURES REPRESENTED BY THE SHEETED DYKES FROM THE MUSLIM BAGH OPHIOLITE COMPLEX, BALOCHISTAN, PAKISTAN

R. H. SIDDIQUI, J. M. MENGAL, K. HOSHINO, Y. SAWADA, I. A. BROHI

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 Jurassic- Cretaceous Ceno-Tethyan convergence zone.


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