Petrology and stable isotope geochemistry of the Nanga Parbat-Haramosh Massif, northern Pakistan

The Nanga Parbat-Haramosh Massif in northern Pakistan records the Tertiary metamorphism and dynamics of the Himalayan collision and subsequent overthrusting of the Asian plate onto the Indo-Pakistan plate. The massif consists of an intercalated sequence of para- and orthogneisses, of minor metabasics and calc-silicate rocks, and of post-metamorphic pegmatite dikes. These gneisses are metamorphosed under high pressure upper amphibolite facies conditions. The P-T estimates and the P-T-t paths of the pelitic samples from the massif and the adjacent areas of the Kohistan and Ladakh arcs along two cross-strike transects (Indus and Astore Rivers) through the massif show that the massif followed a compressional (counterclockwise) and the Kohistan-Ladakh arc a decompressional (clockwise) P-T-t path. Geothermobarometry on zoned garnets from core to rim indicates that the massif rocks started their metamorphic history with {dollar}sim{dollar}5.5 kb and {dollar}sim{dollar}650{dollar}spcirc{dollar}C. During collision, the pressure and temperature rose to {dollar}>{dollar}10 kb and about 750{dollar}spcirc{dollar}C. The Ladakh garnets started to grow at {dollar}>{dollar}10 kb, {dollar}sim{dollar}750{dollar}spcirc{dollar}C with subsequent decrease in metamorphic pressure to {dollar}sim{dollar}8.5 kb. After the collision, the massif and the Kohistan-Ladakh rocks equilibrated at similar conditions of {dollar}<{dollar}8 kb and {dollar}sim{dollar}700{dollar}spcirc{dollar}C.; {dollar}sp{lcub}18{rcub}{dollar}O thermometry results are consistent with the phase equilibrium studies of the massif and the Kohistan-Ladakh arc. Temperatures were calculated from {dollar}sp{lcub}18{rcub}{dollar}O fractionations among quartz, garnet, feldspar, biotite, muscovite and amphibole. Among the analyzed samples, the whole rock {dollar}rmdeltasp{lcub}18{rcub}Osb{lcub}SMOW{rcub}{dollar} isotopic values range from 7 to 15.3{dollar}perthous{dollar}, quartz 7.4 to 16.4{dollar}perthous{dollar}, feldspar 7 to 16.1{dollar}perthous{dollar}, garnet 5.3 to 13.7{dollar}perthous{dollar}, biotite 3.9 to 12.6{dollar}perthous{dollar}, muscovite 6.7 to 12.7{dollar}perthous{dollar}, and hornblendic amphibole from 4.4 to 7.2{dollar}perthous{dollar}. Calculation of the {dollar}sp{lcub}18{rcub}{dollar}O composition of fluids that were in equilibrium with different minerals in the temperature range of 500-700{dollar}spcirc{dollar}C reveals that there is one pre-metamorphic and one post-metamorphic fluid activity affecting the isotopic composition of the rocks of the massif. The pre-metamorphic fluids probably originated from an igneous parent, depleting the rocks in {dollar}sim{dollar}2{dollar}perthous{dollar}, especially along the major faults. The post-metamorphic fluids probably originated from prograde metamorphic reactions and were heavy enough to enrich the feldspars up to {dollar}sim{dollar}18{dollar}perthous{dollar}.