ACTIVE HYDROTHERMAL SYSTEMS DURING THE RECENT UPLIFT OF NANGA-PARBAT,PAKISTAN HIMALAYA

During the last 10 m.y., the Nanga Parbat Haramosh Massif in the north western Himalaya has been intruded by granitic magmas, has undergone h igh-grade metamorphism and anatexis, and has been rapidly uplifted and denuded. As part of an ongoing project to understand the relationship between tectonism and petrologic processes, we have undertaken an iso topic study of the massif to determine the importance of hydrothermal activity during this recent metamorphism. Our studies show that both m eteoric and magmatic hydrothermal systems have been active over the la st 10 m.y. We suggest that the rapid uplift of the massif created a du al hydrothermal system, consisting of a near-surface flow system domin ated by meteoric water and a flow regime at deeper levels dominated by magmatic/metamorphic volatiles. Meteoric fluids derived from glaciers near the summit of Nanga Parbat were driven deep into the massif alon g the transpressional faults causing delta(18)O and delta D depletions in the gneisses and marked oxygen isotopic disequilibrium between min eral pairs from the fault zones. The discharge of these meteoric fluid s occurs in active hot springs that are found along the steep faults t hat border the massif. At deeper levels within the massif, infiltratio n of low delta(18)O magmatic fluids caused delta(18)O depletions in th e gneisses within the migmatite zone. These low delta(18)O fluids were derived from the young (<4 Ma), relatively low delta(18)O granites (s imilar to 8 parts per thousand) that are found within the core of the massif. Geochronological evidence in the form of fission track and Ar- 40/Ar-39 cooling ages and U/Pb ages on accessory minerals from the gra nites and gneisses provide a constraint on the timing of fluid flow in the surface outcrops we examined. Fluid infiltration in the migmatite zone rocks located along the Tate traverse was coeval with metamorphi sm, granite emplacement, and rapid denudation, in the interval 0.8-3.3 Ma. Finally, we infer from the presence of active hot springs that si gnificant flow systems continue to be active at depth within the centr al portion of the Nanga Parbat-Haramosh Massif.