Gabbronorite from Jijal Complex, Kamila Amphibolite Belt and Chilas Complex, Northern Pakistan: Implications for Arc Genesis

Rocks of gabbronoritic composition occur in three principal tectono-stratigraphic units forming the lower and middle parts of the Kohistan Island arc (KIA). These include the Jijal complex (JC), the Kamila Amphibolite belt (KAB) and the Chilas complex (CHC). The Jijal complex constitutes the lowermost part and hence is regarded as the root zone of KIA. Its north-eastern part adjacentto KAB contains gabbronorite as a minor component in the form of small irregular patches and layers within garnet granulite. The JC gabbronorite is sub-equigranular, medium to coarse grained, largely massive and consists of variable amounts of plagioclase(53-71 %), orthopyroxene (14-27 %) and clinopyroxene (11-19 %) as essential constituents and accessory to minor amounts of amphibole (1-9 %), opaque ore (1-6 %) and orthoclase (1-4 %). The occurrence and distribution of biotite, epidote, chlorite, clay, sericite, muscovite, quartz and actinolite in the studied samples suggest their formation through alteration and/ or reaction between pre-existing minerals. In many cases, these minerals are disposed such that a variety of simple and complex corona structures are produced. The principal petrographic features (modal composition, optical properties of the major mineral phases, exsolution in pyroxenes, products of alteration and reactions and the resulting corona textures) of the JC gabbronorite are broadly similar to gabbronorites from both the KAB and CHC. Although the observed similarities could reflect identical physico-chemical conditions during subsolidus or metamorphic re-equilibration, the possibility of a genetic relationship among gabbronorites from all the three tectono-magmatic units of the KIA (i.e. the JC, KAB and CHC) cannot be ruled out.