Fluids and Cordierite "Melts" Within the Nanga Parbat - Haramosh Massif, Northern Pakistan

Karen Leslie Geology Department, Royal Holloway University of London, Egham, Surrey, TW20 0EX, UK


Within the Nanga Parbat - Haramosh Massif (NPHM), northwestern Himalayas, pelitic and semipelitic gneisses grade into a suite of young (possibly as young as 4-11Ma, determined by monazites U-Pb ages from schists and gneisses of the NPHM (Smith et al., 1992)) anatectic migmatites towards the core. This group of gneisses and migmatites represents the northernmost outcrop of Indian subcontinental basement (Butler et al., 1989). The mineralogy is quartz, biotite, plagioclase, k-feldspar, ± garnet, ± sillimanite, ± zircon, ± monazite with biotite defining a fabric through the rock mass.

Migmatites from the Upper Riakhot Valley (NPHM) contain cordierite-bearing veins which cross-cut the host fabric. These represent the latest generation of melt formation within this area, indicated because they cross-cut both 2.3 to 7Ma leucogranite dikes (minimum U-Pb zircon ages (Smith et al., 1992)) and quartz-feldspar pods and veins of the lit-par-lit migmatites in the area. Petrographic studies indicate biotite breakdown has occurred to produce peritectic cordierite and a granitic melt. At the pressure and temperature conditions proposed for the formation of these cordierite veins (~600°C, 4.1kbar (Zeitler et al., 1993), ~690°C, 3.5±0.5kbar (Butler et al., 1995)) a fluid phase is essential.

Detailed oxygen isotope work on a cordierite veins and its host rock was carried out and data obtained by laser-fluorination (Mattey and Macpherson, 1993) and performed at Royal Holloway, University of London. The d18O data are consistent with a diffusion profile which can be traced from the vein through the host rock. The profile trends from a maximum d18O value within the vein to a minimum ~5cms into the host. This was initially observed in quartz but has since been noted in biotite and the few feldspar values obtained appear to follow this trend. The inflection point of this profile occurs ~0.3cms into the host, not at the host/vein margin, indicating a component of advective fluid flow was present. This suggests that within parts of the massif there was very local channelised fluid flow whose pathways maybe tracked by the presence of these veins.


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