COMPREHENSIVE PETROGRAPHIC AND GEOCHEMICAL CHARACTERIZATION OF DIORITES, HORNBLENDITES AND AMPHIBOLITES FROM THE KOHISTAN ISLANAD ARC SEQUENCE NEAR TIMERGARA, DIR (LOWER), PAKISTAN

Abstract

Well exposed outcrops of the Kohistan Island Arc (KIA) sequence are located along the hanging wall of Main Mantle Thrust (MMT) in Dir (Lower). Among the KIA sequence the most abundant rock unit is Southern Kamila Amphibolites, diorites and hornblendites. Detail field investigation was carried it in the proposed study area and observed that the area has undergone through severe tectonic deformations. The examined rock unit possess multiple shear zone and are cross cutted by variable veins. Shearing reflects the activeness of MMT, and rooted veins demonstrates crustal thickening and crustal anatexis. The inspected amphibolites are banded as well massive in nature represents variable protoliths for their genesis. Massive amphibolites are intruded and cross cutted by leucogranite pegmatites veins. The unmetamorphosed nature of these pegmatites exhibits its post collisional origin as a result of crustal thickening. Petrographic study reveals that examined rock has experienced medium grade metamorphism, evident from the development of hornblende and Ca-rich plagioclase. Analytical data interpretation suggests that protolith for the investigated rock unit was tholeiitic basalt, that has been metamorphosed under regional dynamo-thermal amphibolite grade metamorphosed during collisional phase of Himalayan orogeny. Petrographically the amphibolites are mainly composed of hornblende and plagioclase with subordinate amount of quartz and alkali feldspar. Biotite, muscovite, chlorite, epidote and opaque occur as accessory mineral constituents. Diorites are dominantly composed of plagioclase with variable proportion of quartz and alkali feldspar. The other minor constituents observed in the representative samples of diorites include biotite, hornblende, muscovite, and apatite, zircon, garnet and opaque. Hornblendites of the investigated complex are essentially composed of hornblende (with variable amounts of plagioclase and pyroxene), and minor amount of opaque minerals, mostly magnetite and, locally, apatite, rutile, quartz, sphene, chlorite, epidote and carbonate.  Paragenetic interpretation of some of the observed minerals exhibits as a product of alteration such as chlorite, epidote and carbonate.