Geochemistry and petrogenesis of the volcanic rocks from Bagh complex, northern Balochistan, Pakistan

Abstract

The Bagh complex is an assemblage of igneous and sedimentary rocks containing many lithologic units which are bounded by layered parallel thrusts. The volcanic rocks from basalt-chert unit (Bbc) and hyaloclastite-mudstone unit (Bhm) are comprised of pillowed and bedded basalt, hyaloclastite and reworked sediments. Petrography and geochemistry of the basalt from Bbc shows that these rocks are basaltic trachy-andesite, trachy-andesite and foidite with tholeiitic charecter. While those from Bhm are tephrite-basanite, basalt and picro-basalt having alkaline nature. N-MORB-normalized diagram of the tholeiitic rocks have flat pattern of HFS-elements. In contrast enrichment of LIL-elements with the REE having almost flat pattern between 10X and 20X chondritic which are typical of N-MORB. The enrichment of LIL-elements, especially Th and depletion of Nb relative to other incompatible elements are depicting the addition of subduction zone component to a depleted mantle source. The multi element normalised plots of the volcanic rocks from Bhm show a similar pattern to that of OIB. Theese show a marked enrichment in the LIL-elements, depletion in some of the HFS-elements enrichments in LREEs and a marked depletion in the HREEs as compared to NMORB. The geochemical features on tectonic discrimination diagram of tholiiete from Bbc and alkaline rocks from Bhm suggest supra-subduction zone and oceanic-island arc tectonic settings respectively. These rocks were possibly erupted during Cretaceous period in the area which extends from the continental margin of the Indian subcontinent over the Neo-Tethys Ocean floor, and then were obducted with the Muslim Bagh ophiolite over the Indian subcontinent.