Low-pressure crustal anatexis: the significance of spinel and cordierite from metapelitic assemblages at Nanga Parbat, northern Pakistan ALAN WHITTINGTON 1, NIGEL HARRIS 1 & JUDY BAKER 2 1Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK 2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Abstract: Spinel-bearing domains in high-grade metapelitic rocks from Nanga Parbat represent zones of partial melting during biotite breakdown under vapour-undersaturated conditions. Spinel is essentially of the MgA1204-FeA1204 solid solution, and is therefore not stabilized by the presence of trace elements such as zinc, but is restricted to a quartz-absent petrogenesis. A new petrogenetic grid for metapelites has been constructed to allow for both vapour-undersaturated and quartz-undersaturated conditions. This grid contains a quartz- absent invariant point in KFASH that predicts quartz-absent melting will occur in biotite- sillimanite assemblages at low pressures. A P-T pseudosection drawn for bulk compositions with intermediate Fe/Mg ratios shows that at low pressures biotite will break down initially in the presence of quartz, to produce cordierite and K-feldspar and melt. At higher temperatures the quartz-absent reaction will be crossed, and biotite will break down to produce spinel in addition to cordierite, K-feldspar and melt. The sequence of assemblages is biotite-cordierite, biotite-cordierite- spinel, and finally cordierite-spinel. This biotite-absent assemblage is not observed, indicating that the quartz-absent reaction did not go to completion. Limits can be placed on the pressures and temperatures at which the biotite breakdown reaction is crossed, from the coexistence of solid phases and granite melt over a range of water activities. For the Nanga Parbat assemblages, pressures and temperatures lay at about 720°C and 5 kbar at water activity, aiJ2o _> 0.6. These conditions are consistent with a rapidly exhuming terrane, as suggested by isotopic constraints and thermal modelling of the region. Crustal anatexis of metapelitic assemblages in the absence of a free fluid phase usually involves the breakdown of biotite and/or muscovite leading to the coexistence of a melt and anhydrous peritectic phases (Gardien et al. 1995; Le Breton & Thompson 1988; Patifio Douce & Johnston 1991; Vielzeuf & Holloway 1988). In Himalayan lithologies, Miocene leuco- granites have been associated with sillimanite formation during the dehydration melting of muscovite (Harris et al. 1995), but temperatures were insufficient for biotite breakdown to occur. Cordierite-spinel metapelitic assemblages are reported here for the first time from the Hima- layas. The significance of spinel formation and its possible relationship with melt formation are investigated, using standard thermobarometric techniques combined with theoretical calcula- tions in the KzO-FeO-MgO-AlzO3-SiO2-H20 (KFMASH) system. The rocks we have used in this study are from the Nanga Parbat-Haramosh Massif in north- ern Pakistan (Fig. 1). The massif, which marks the western extremity of the High Himalayas, is a polymetamorphic terrain comprising grani- tic orthogneisses and psammitic, pelitic and calcareous paragneisses which have experienced metamorphism both as part of the Indian craton, and more recently during the Himalayan orogeny. The most recent metamorphic episode is associated with rapid exhumation of the mas- sif over the last 10 Ma (Zeitler et al. 1982; Zeitler 1985), which has led to leucogranite genera- tion through fluid-absent muscovite breakdown (Zeitler & Chamberlain 1991; Butler et al. 1997; Whittington et al. 1998) and high-T low-P metamorphic assemblages containing cordierite, sillimanite and K-feldspar. These assemblages fall into two categories: cordierite-K-feldspar leucosomes localized in shear zones, and biotite- sillimanite-K-feldspar 4- spinel domains within migmatitic pelitic 4- gneisses. The leucosomes have been attributed to fluid infiltration and localized fluid-present melting in shear zones (Whittington et al. 1998). This paper investigates the origins of spinel-beating domains in the pelitic gneisses. Petrography Metapelitic gneisses from the core of the Nanga Parbat Massif typically contain biotite, garnet, WHITTINGTON,A., HARRIS, N. & BAKER, J. 1998. Low-pressure crustal anatexis. In: TRELOAR, P. J. & O'BRIEN, P. J. (eds) What Drives Metamorphism and Metamorphic Reactions? Geological Society, London, Special Publications, 138, 183-198.