Geology and tectonic setting of the volcaniclastic succession of the Upper Cretaceous, western Sulaiman fold belt, Pakistan

Abstract

The volcaniclastic rocks of the Upper Cretaceous Bibai Formation are exposed through out the Ziarat district near Muslimbagh ophiolite, within the western part of the Sulaiman Thrust-Fold Belt east of the Quetta Syntaxes. These volcanic rocks generally comprises basic volcanic rocks, volcanic conglomerate and breccias, sandstone, mudstone and ash beds, deposited by various processes of sediment gravity flows on the western margin of the Indian Plate and indicate deposition by turbidity currents in over bank (-levee) complex between channels. The mudstone, possessing occasional thin sandstone and siltstone beds in lower part and profusion of shallow marine fauna in upper part, indicate deposition in lower fan / basin plane conditions and also an overall swallowing-up trend of the succession. Limestone, interbedded with volcaniclastic facies in lower part of the formation, is very finely crystalline (bio-micritic) possessing foraminifera of the Globotruncana family suggest deposition during calm periods when gravity flows had been suspended intermittently. Paleo-current pattern indicate a southsouthwest paleo-flow direction and a source area to the northnortheast of Bibai Peak. Based on characters of various rocks associations, their vertical and lateral organization, paleocurrent pattern and composition of detritus. The Bibai Formation developed on the slope of a series of seamounts (hotspot volcanoes). Seamounts developed on sea floor of the northwestern margin of the Indo-Pakistan Plate. Within the Bibai Formation, it is dominantly composed of volcaniclastic sediments and rarely lava flows, as the in-situ volcanic rocks. Detailed petrography and geochemical analyses of clasts of the volcanic conglomerate and sandstone were carried out to determine the origin and provenance of volcaniclastic sediments of the Bibai Formation. Volcanic conglomerate contains clasts of alkali basalt, picrite, trachy basalt, tepherite/ phonolite, trachy andesite, dolerite, diorite and granodiorite, which are varieties of the alkaline magma suite. Sandstones are also dominantly composed of the basaltic rock fragments and pyroxene. XRF data of both major and trace elements were plotted in various discrimination diagrams of the volcanic and associated intrusive rocks indicate that the analyzed samples fall in the field of within-plate alkali basalt or close to it suggests that the volcaniclastic sediments of the Bibai Formation were derived from a volcanic terrain composed of alkali basalts originated by hotspot volcanism. Trace elements presented in various spider diagrams suggest that the parent magma was enriched in mantle source and confirm that the fragments of the volcanic conglomerate of the Bibai Formation were derived from a hotspot related (within-plate setting) volcanic terrene. The paleocurrent data confirms that these sediments were derived from the volcanic to the northnortheast of the study area.

Journal of Nepal Geological Society, 2007, Vol. 36 (Sp. Issue) p.6