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

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 quartzabsent invariant point in KFASH that predicts quartz-absent melting will occur in biotitesillimanite 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-cordieritespinel, 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.

Petrologic and thermal constraints on the origin of leucogranites in collisional orogens

“…Therefore, if the muscovite-tourmaline granites were generated by MDM, then the two-mica granites must have been generated by BDM at approximately the same conditions, also below 10 kbar. In the Nanga Parbat massif, Pakistan, spinel-and cordierite-bearing migmatites formed by anatexis of metapelites at ~5 kbar (Whittington et al 1998), showing that biotite-breakdown conditions can also be reached at rather low pressures. Harris & Massey (1994) estimated these to have been the conditions of melting in the Himalayan migmatites.…”

The exhumation of gneiss domes in bivergent wedges: Geometrical concepts and examples from the Himalayan syntaxes

“…Structural evidence from the interior of the Nanga Parbat-Haramosh Massif indicates a vertical minimum principle stress orientation (Butler et al, 1997), consistent either with radial spreading of the Himalayan arc or transpression across the southern Karakorum fault system (McCaffrey and Nabelek, 1998;Seeber and Pêcher, 1998). Exhumation through the bivergent wedge geometry has produced Plio-Pleistocene, granulite-grade metamorphism, partial melting, and rapid cooling (e.g., Zeitler, 1985;Zeitler et al, 1993;Smith et al, 1994;Winslow et al, 1995;Whittington et al, 1998;Whittington et al, 1999;Schneider et al, 1999a;Schneider et al, 2001;Zeitler et al, 2001a). These recent events have not entirely erased the earlier high-pressure history of rocks in the massif core, some of which record earlier metamorphism at pressures at or exceeding 10 kbar (e.g., Winslow et al, 1995;Whittington et al, 1999;Poage et al, 2000), implying that the wedge is probably asymmetric (see previous section).…”

“…Observations of spinel-cordierite assemblages in the core of the Nanga Parbat-Haramosh Massif have been interpreted as recording biotite dehydration melting on a microscopic scale (Whittington et al, 1998). Although there is no fi eld evidence supporting widespread or volumetrically signifi cant biotite dehydration melting within the Nanga Parbat-Haramosh Massif at the present time, this could be the next stage in the evolution of the western syntaxis.…”

Crustal anatexis and its relation to the exhumation of collisional orogenic belts, with particular reference to the Himalaya

“…The spinel-cordierite intergrowths (1), which appear in small (mm-sized) bands in pelitic gneisses in the core of the NPHM, with fabricforming biotite breaking down, have been interpreted as recording quartz-absent biotite dehydration melting (Whittington et al, 1998). Calculated P-T conditions for this reaction were 7108C at 5 kbar, with locally reduced water and silica activities required to allow the formation of spinel under these conditions, which indicate the earlier removal of a melt phase rich in both these components.…”