PETROLOGY OF THE MAFIC VOLCANIC ROCKS OF THE MAKRAN ACCRETIONARY WEDGE, SOUTHEAST PAKISTAN

A vast area of the Makran accretionary wedge in southwest Pakistan, mapped as the Oligocene-Early Miocene Panjgoor Formation, consists of mostly abyssal plain turbidites with isolated thin slivers of mafic volcanic rocks, agglomerate, purple colored shales, cherts and polymict conglomerate exposed along major thrust zones. The mafic volcanic rocks, are medium-fine grained and porphyritic. These rocks are basaltic-basaltic andesite and contain primary plagioclase, alkali-feldspar, orthopyroxene, clinopyroxene and opaque minerals. Ferromagnesian minerals are mostly altered, however, primary structures and textures suggest pyroxene, olivine and possibly hornblende. They exhibit varying degrees of pervasive low-grade alteration / metamorphism up to greenschist facies with secondary minerals such as calcite, chlorite, minor amounts of prehnite, pumpellyite, iron oxides, apophyllite and clay minerals. Secondary minerals are indicative of a seafloor alteration sequence with temperatures reaching up to 350^oC and low pressures, which is characteristic of pre-emplacement oceanic basalts. The agglomerates are composed mainly of pebbles of mafic volcanic (basaltic) rocks with minor amounts of granite and rhyolite. They are interbedded with volcaniclastic sandstone, which is also composed predominantly of basaltic fragments and minor amounts of granitic and rhyolitic fragments. The occurrence of mafic volcanic rocks of basaltic composition, along with agglomerate, purple colored shale and chert, as thin wedge-shaped isolated outcrops or crushed material within the major thrust zones, may represent ophiolitic mélanges. We propose this sequence formed in response to the accretion of the oceanic floor of the Gulf of Oman during its northward subduction beneath the southern margin of the Eurasian Plate.