18O Fractionation in Feldspars from the Nanga Parbat-Haramosh Massif, Northern Pakistan

Abstract

The Nanga Parbat-Haramosh Massif in northern Pakistan represents the northernmost exposure of the Indian plate that has been juxtaposed against the Kohistan Ladakh island arc along Raikot-Sassi and Astak faults. The massif is a complex mixture of paragneisses, orthogneisses, minor metabasics, calc-silicate rocks, and post metamorphic pegmatite dikes. These gneisses are metamorphosed under high pressure upper amphibolite facies conditions (5.5-10 kb, 650-750^oC; Khattak, 1990).

The isotopic compositions of the rocks and their constituent minerals in the massif and adjacent areas of the Kohistan-Ladakh arc along the Indus and Astore Rivers varies as following: whole rock δ¹⁸O_SMOW = 7-15.3%o, quartz = 7.4-16.4%o; feldspar =7-16.1%o; garnet = 5.3-13.7%o; biotite = 3.9-12.6%o; muscovite = 6.7-12.7%o; and hornblendic amphibole = 4.4-7.2%o (Khattak, 1994). Feldspars isotopic compositions show maximum scatter indicating refractory nature of the minerals. ¹⁸O thermometry results based on coexisting quartz-feldspar fractionation curves show disequilibrium in some of the samples. Calculation of the ¹⁸O composition of fluids that were in equilibrium with different minerals in the temperature range of 500-700^oC 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 ~2%o, 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 ~1.8%o. Retrograde paths indicate that the massif was probably quick in its upward flight from depths of around 35 kms as shown by pressure estimates of Khattak and Stakes (1993).