Geology and geochemistry of Indian plate rocks south of the Indus Suture, Besham area, northern Pakistan Public Deposited

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  • The Himalaya are the geologic manifestation of continental collision, and in northern Pakistan the Main Mantle thrust (MMT) is a major suture along which the collision occurred. A newly recognized basement uplift near Besham village in southern Kohistan, adjacent to the MMT, is bounded on the east and west by north-trending high angle fault zones. The uplift exposes basement rocks that are significantly different from any of the other plutonic and metamorphic rocks of southern Kohistan and that are not seen elsewhere in the Pakistan Himalaya west of the Nanga Parbat-Haramosh massif (NPHM). Rocks of the Besham area are subdivided from oldest to youngest into five groups. The oldest rocks are (1) the Besham group, metasediments and heterogeneous gneisses. In conjunction with field evidence, major, trace, and rare earth element analyses of Besham gneisses suggests that the gneisses formed in situ from a sedimentary protolith. The presence of both quartzofeldspathic gneiss and sodic quartzofeldspathic gneiss in the Besham group may be attributable to variable protolith composition. The Besham group was intruded by (2) mafic dikes that were subsequently metamorphosed to amphibolites. Geochemical data suggests that these tholeiitic dikes have island arc geochemical affinities. (3) The third group of rocks compose the Lahor granitic complex, which includes cogenetic, small granitic intrusions and associated pegmatites; the Shang granite, the Dubair granite and the Shorgara pegmatite. Unconformably lying upon these three units is (4) the Karora group, which comprises conglomeratic, calcareous, and carbonaceous sedimentary rocks. The Karora group provides evidence for more than one metamorphic event in the Besham area basement uplift, i.e., the Karora group is metamorphosed to middle greenschist facies, in contrast with the underlying units, which are metamorphosed to epidote- amphibotite facies. The youngest unit observed in the Besham area basement uplift is (5) a relatively undeformed leucogranite that intrudes both the Karora group and the Besham group. The leucogranite is a previously unrecognized unit. The metamorphic and granitic rocks of the Besham area uplift may be correlative with basement exposed in the Nanga Parbat-Haramosh massif. Specifically, the quartzofeldspathic gneisses of the Besham group may correlate with the Nanga Parbat gneisses, and the amphibolites found in the Besham area may correlate with mafic dikes of the massif. Further study of both the Besham area uplift and the NPHM can provide a better understanding of Precambrian basement rocks of northern Pakistan.
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