Crystalline rocks of the Spinatizha area, Pakistan

During the Cretaceous an andesitic arc developed across south Asia facing the Tethys Ocean. Remnants of this arc are preserved in Iran, Afghanistan, and the Chagai Hills and Kohistan, Pakistan. West of the Chaman fault near Spinatizha, Pakistan (33° 33′N, 66° 23′E) a terrain of crystalline rocks is exposed that links the Chagai Hills portion of this arc with the Kandahar portion of it in Afghanistan. Four units are present. (1) The Spinatizha Metamorphic Complex includes orthogneiss, greenschist, amphibolite, metavolcanics, marble and foliated muscovite granite. Extreme variation in rock type and degree of metamorphism characterises the entire complex. It is the oldest unit west of the Chaman fault in Pakistan. (2) The Bazai Ghar Volcanics consist of weakly deformed tuffs, flow breccias, and other coarse-grained pyroclastics of andesitic-arc type. Andesite flows and at least one silicic welded tuff are also present. The Bazai Ghar Volcanics are everywhere separated from the Spinatizha Metamorphic unit by granitic intrusions and a major fault. (3) Both the above units are intruded by a series of calc-alkaline granitic plutons ranging from diorite to granite. The silicic plutons generally intrude the more mafic ones. The Bazai Ghar Volcanics and related intrusions are probably equivalent to the Cretaceous (?) Sinjrani volcanics and the Cretaceous and younger intrusions of the Chagai Hills. (4) Along the fault zone between the volcanic and metamorphic rocks is a small area of previously unknown clastic sedimentary rocks: conglomerates and slates. The unit is of Palaeogene age but cannot yet be correlated with known units. The Spinatizha crystalline terrain extends south along the Chaman fault into Afghanistan and is covered by the Helmund desert to the west. It is the eastern continuation of the calc-alkaline arc terrain of the Chagai Hills dragged by oroclinal flexing into the Chaman transform zone. To the north it connects with the Kandahar volcanic arc. The metamorphic complex may represent the basement on which the arc terrain rests, only exposed due to strong vertical uplift near the Chaman fault.