Tectonics of the Higher- and Tethyan Himalaya, Upper Kaghan Valley, NW Himalaya, Pakistan: Implications of an early collisional, high-pressure (eclogite facies) metamorphism to the Himalayan belt

The Upper Kaghan Nappe is situated in the western syntaxis area. It belongs to the Higher Himalaya, delimited by the Main Central Thrust and the Indus Suture. The basement is overlain by two Tethyan covers, metamorphosed during the Himalayan orogen. Structurally, at least two major phases of deformation occur. Eclogites in Upper Kaghan show X{dollar}sb{lcub}rm jd{rcub}{dollar} contents of omphacite of up to 0.431. Garnets are almandine-rich and co-exist with amphiboles, phengite and rutile. Geothermobarometry calculates mean temperatures of 650{dollar}spcirc{dollar}C {dollar}pm{dollar} 50{dollar}spcirc{dollar}C at pressures up to 17.5 kbar. Cathodoluminescence observations show unseen sedimentary structures. XRD calculations of the mineralogical content of the carbonates distinguish between stratigraphic units. Geochemically, the eclogites and amphibolites discriminate to tholeiitic (to sub-alkalic), within plate, continental flood basalts. The Higher Himalayan basement is a peraluminous, "S"-type granite and the Himalayan leucogranites show many similarities with the basement and appear to be derived from these granites. {dollar}partialsp{lcub}18{rcub}{dollar}O and {dollar}partialsp{lcub}13{rcub}{dollar}C isotopes on carbonates show distinctive chemo-stratigraphical trends. New isotope ages of the Upper Kaghan nappe are: Sm/Nd on eclogite is 49 {dollar}pm{dollar} 6 Ma; Rb/Sr on eclogite is 43 {dollar}pm{dollar} 1 Ma; U/Pb on rutile is 39-40 {dollar}pm{dollar} 1 Ma; K/Ar analysis range from 35.2 {dollar}pm{dollar} 1.4 Ma to 42.7 {dollar}pm{dollar} 1.4 Ma for hornblende and 26.3 {dollar}pm{dollar} 0.7 Ma for biotite. Ar/Ar analysis of phengite and amphibole in eclogite suggests incorporation of excess argon. Sm/Nd whole rock analysis, which pertain to give eclogite protolith age, are 337 Ma although the M.S.W.D. is high. {dollar}varepsilonsb{lcub}rm Nd{rcub}{dollar}t = 270 varies between {dollar}-{dollar}9.9 to +4.9 with the more positive values occurring at the higher metamorphic grades. Whole rock initial {dollar}sp{lcub}87{rcub}{dollar}Sr/{dollar}sp{lcub}86{rcub}{dollar}Sr ratios suggest continental crust evolved signatures. The cooling history of the Upper Kaghan nappe is determined at 13{dollar}spcirc{dollar}-14{dollar}spcirc{dollar}C/Ma. Fluid inclusion studies show three systems of secondary inclusions, the product of metamorphic retrogression. The nappe shows variation in exhumation rates, related to a change in tectonic transport direction. The early, high pressure phase suggests initial Indian plate impingement at 65 Ma.