Low-pressure crustal anatexis: the significance of spinel and cordierite from metapelitic assemblages at Nanga Parbat, northern Parkistan

Abstract

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 MgAl 2 O 4 -FeAl 2 O 4 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, a H 2 O ≥ 0.6. These conditions are consistent with a rapidly exhuming terrane, as suggested by isotopic constraints and thermal modelling of the region.