Oxygen isotope study of migmatites and associated rocks from Nanga Parbat-Harmosh Massif, Pakistan Himalaya.
Collision of the Indian and Eurasian plates (-65Ma) caused the formation of
the Himalayan chain. The Nanga Parbat-Haramosh Massif within the western
Himalaya, Pakistan, is experiencing the fastest uplift anywhere in the world
(-10mmyr-1). The massif contains amphibolite facies pelitic and semipelitic
migmatites with subordinate calc-silicates and amphibolites. Intruded into these
migmatites are very young tourmaline leucogranites (2- l OMa). Only in the central
portion of the massif is the granulite facies sillimanite-K-feldspar-cordierite
assemblage found. The migmatites themselves contain lit-par-lit quartzo-feldspathic
leucosomes (cros) and cross-cutting cordierite-bearing seams (cros). Tourmaline
leucogranites have been derived from muscovite vapour-absent melting of this
basement, while cordierite seams have been derived from biotite-dehydration melting
of these migmatites.
The rapid uplift experienced by the massif reduced the effects of post
migmatisation mineral-mineral oxygen isotope diffusional exchange. Thus, primary
fractionation values are more likely to be preserved. To study the effects of
migmatisation on the fractionation of oxygen isotopes, mineral separates from
mesosomes, leucosomes and seams were analysed by laser-fluorination mass
spectrometry. The use of a Nd: YAG laser required the establishment of a method to
analyse quartz and alumino-silicates. Proxy heating with a standard ferromagnesian
mineral is followed by a mass balance calculation to obtain 818Oquartz/feldspafr om
the mixture. Precision for non-ferromagnesian minerals is ±0.4%o (2sd) for a
standard quartz, NBS-28, using this method.
Results indicate that Mgtrmelt-mesosomaen d Ofeldmelt-mesosomaer e positive. It
is considered unlikely that there was substantial fluid infiltration during
migmatisation because quartz shows large infra- and inter-sample variability,
indicating that 8180 values have not been homogenised by fluids. Any fluids that
were present are likely to have been metamorphic rather than meteoric in origin,
because the latter have particularly low 8180 values (-15%o to -16%o).
Reequilibration between mesosome and leucosome produced diffusion profiles
within quartz. Within quartz, an array of fluid inclusion types (H2O and CO2),
salinities and homogenisation temperatures, suggest that fluids have been present
within the massif over much of its history. Mineral-mineral fractionations suggest
fast cooling in a fluid undersaturated environment.