PETROLOGY OF THE CRUSTAL PLUTONIC ROCKS OF NAWEOBA BLOCK, ZHOB OPHIOLITE, BALOCHISTAN, PAKISTAN

Abstract

Zhob Ophiolite complex is composed of three detached blocks named Omzha, Ali Khanzai and Naweoba blocks. The crustal plutonic section of the Naweoba block is mapped and divided into gabbro and granite. Based on petrographical studies, the gabbros fell in the domain of gabbro, gabbronorite, and hornblende gabbro while granitic rocks fell in the vicinity of quartz-rich granitic rocks, granodiorite, plagiogranite and tonalite. Gabbroic rocks cover the maximum area of the crustal plutonic section and are usually medium-grained while at many places the grain size is quite large to be seen with naked eyes. Minerlogically gabbroic rocks consist of orthopyroxene, clinopyroxene, amphibole and plagioclase. These rocks maybe the fragments of main crustal plutonic section of the Zhob ophiolite. The granitic rocks having mafic minerals dominted in the eastern portion, while the felsic minerals dominted ones are in the west. The eastern side of the granitic body is compact and massive compared to western portion which is quite altered and shattered. Granitic rocks are composed of plagioclase, alkali feldspar and quartz where rutile and Cr-spinel exist in trace amounts. The gabbros of Naweoba block may have formed in a magma chamber as a result of fractional crystallization. While the granites maybe a late magmatic differentiate from the same magma chamber. The close correlation of gabbroic and granitic rocks of Naweoba block with Muslim Bagh, Khanozai and Bela ophiolites suggests their formation in supra subduction zone setting.