The rocks of the massif

The Nanga Parbat massif chiefly comprises a suite of metasedimentary paragneisses together a tract of augen orthogneiss, collectively termed the Nanga Parbat Gneisses by early workers (Misch 1949; Wadia 1961; Desio 1976). Much of the central part of the massif consists of rather homogeneous augen orthogneisses. These lie adjacent to high-grade metasediments, including migmatites. There are also widespread occurrences of leucogranite sheets and larger intrusions. For Misch, the transition from metasediments into augen gneiss represented progressive metasomatic granitisation. However, recent work shows the augen gneiss as being a deformed and metamorphosed tonalite/granitoid which had been intruded into the metasediments. Metasediments and augen gneisses are cross-cut by amphibolite sheets which are considered to have been intruded long before Himalayan orogenesis. Current geochronological studies are showing that these sheets are Proterozoic in age. Thus the massif consists of a metamorphic basement, part of the old continental crust of India.

Despite the early history of these gneisses, it is possible to recognise a Tertiary history in these rocks which relates to burial during Himalayan orogenesis and the subsequent exhumation of these metamorphic rocks to the Earth's surface. The rims to monazites from the central part of the massif have yielded ages as young as 10Ma (Smith et al. 1992). Clearly there was some melting of this continental crust to generate the widespread leucogranite bodies within the massif, although those levels currently exposed need not be the sources of the melts. Neverthless, clearly the Nanga Parbat massif experienced high temperatures during the late Cenozoic. Fission track ages on zircon and apatite show the heart of the massif to have cooled very rapidly over the past few Myr (Zeitler et al. 1982, Zeitler 1985). But it is difficult to relate the high-temperature radiometric data from accessory phases to major textural events in the rocks of the massif. There are some local migmatites in the Tato area, near the summit of Nanga Parbat, which post-date deformation of the amphibolite dykes and thus could be of Himalayan in age and associated with the rim ages from monazites. Minor seams (1-2 cm across) containing spinel, cordierite and K-feldspar are texturally the youngest high temperature metamorphic assemblages in the massif (Butler et al. 1997). Whittington et al. (1998) estimate these having formed at about 720 C and 5 kbar assuming a water activity, aH2O >0.6. However, this low-pressure anatexis is only of local significance and has not been described from elsewhere in the massif to date. This is important because, although pre-Himalayan migmatites are found through the region, the Himalayan peak assemblages appear to be restricted to the Tato area. Presumably it is this area that has experienced the greatest and/or most rapid exhumation within the massif and therefore was most prone to decompression melting.

There are however, greater volumes of young granites within the massif that have migrated from their source migmatites. These leucogranites are amongst the youngest such intrusions related to anatexis of continental crust anywhere in the world. A variety of geochronological tools date these as less than 10 Ma (Smith et al. 1992; George et al. 1993; Butler et al. 1992, 1997). There are a few km-sized bodies within the massif but there is a widespread swarm of pegmatitic sheets that cross-cut all other rock units. Geochemically these leucogranites bear all the hallmarks of small-volume batch melts of very old continental crust, through vapour-absent breakdown of muscovite. These young intrusions are useful, not only for deducing the thermal state of the continental crust but in providing time markers for separating young deformation events from much older ones within the massif.

The structure of the Nanga Parbat massif has been interpreted as representing an elongate upright antiform trending north-south (Wadia 1933, Desio 1976). Structural studies along the Indus transect have shown the similar antiform to be a composite structure (Coward 1985; Madin et al. 1989; Treloar et al. 1991; Butler et al. 1992). This antiformal structure is chiefly defined however by the shape of the contact with the overlying Kohistan rocks. The nature of this contact is controversial but it could be part the original ductile shear zone upon which Kohistan was obducted onto the Indian continent (Butler & Prior 1988a). Regionally this structure is termed the Main Mantle Thrust. However, in detail and specifically along the western margin of the massif, the early contact between the massif and Kohistan is strongly modified by much younger structures. These include syn-metamorphic shear zones, high-temperature faults together with much higher-level seismogenic faults marked by broad zones of fracture (Butler & Prior 1988b, Butler et al. 1989). In some places these faults cut poorly consolidated gravels and sands of the modern Indus valley. Consequently the margin is an exceptional natural laboratory for studying the various deformation processes that can act through the middle and upper crust to bring metamorphic rocks to the Earth's surface. It is these areas that form the focus of the western margin field excursion presented here.