It is now well-established that the Tertiary metamorphism in the High Himalaya was abruptly terminated by a phase of particularly rapid cooling at around 18-20Ma, as evidenced by Ar-Ar cooling ages on metamorphic micas (Vance and Kelley, 1994; Harrison et al., 1992). The metamorphic history of the High Himalaya prior to this time, however, remains largely unknown, particularly the duration and intensity of the Tertiary metamorphism. While published monazite ages from leucogranites imply crustal anatexis that produced small volumes of melt at around 20-22Ma (Noble and Searle, 1995), it is not clear whether this anatexis was the result of a short thermal pulse or whether the large-scale isograd pattern was established at this time. Despite this dearth of knowledge, a substantial amount of effort thas been expended on attempting to relate the regional metamorphism in the Himalaya to heat sources for granite production. Here we present U-Pb and Rb-Sr analyses of metamorphic garnets from the Zanskar Himalaya in NW India in order to establish the pre-20Ma history.
The Zanskar Himalaya preserve a sequence of metamorphic rocks varying in metamorphic grade from ga-zone to sill-zone, exposed between the Main Central Thrust and a large normal fault, the South Tibetan Detachment System. In common with the rest of the Himalaya, Ar-Ar ages on micas evidence a period of rapid cooling around 18-22 Ma (Vance and Kelley, 1994) and monazite ages on leucogranites (Noble and Searle, 1995) imply crustal anatexis just before this time. The garnets analysed are from metapelites in a transect through the metamorphic sequence from ga-zone to upper ky-zone. The most complete isotopic dataset comes from a large (17mm) grain from the ky-sill zone boundary which has been separated into core and rim fractions for analysis. Regression of the Pb-Pb data for core, rim and whole rock yields an age of 655 ± 50Ma (MSWD = 5.8). Two separate core analyses and a whole rock yield a 238U-206Pb isochron corresponding to an age of 157 ± 3Ma (MSWD = 0.34) while the two rim analyses and the whole rock yield an isochron age of 54 ± 2 (MSWD = 0.5). The Rb-Sr data yield intermediate ages of 396Ma for the core and 236Ma for the rim. A second large garnet (10mm) from the kyanite zone exhibits similarly complex Rb-Sr systematics with the core age being 240Ma and the rim being 177Ma. Six other much finer-grained samples from varying grades yield 238U-206Pb and Rb-Sr ages of 110-25Ma, while their Pb-Pb ages are clearly Pre-Cambrian.
The first order interpretation of the data is that the age of garnet growth in Zanskar is not Tertiary. While the garnet isotopic data are complex and cannot relate in a simple manner to growth, they are most readily interpreted in terms of partial disturbance in the Tertiary after initial garnet growth in a much earlier metamorphic event. The important question that then arises is how much of the Himalayan metamorphism is really Tertiary in age. The large garnets that preserve the much older ages display major element chemistry that corresponds to classical growth profiles and the major elements have not re-equilibrated signifcantly after that growth. Given the above "ages" these profiles must represent a much earlier equilibrium than the melting event that produced leucogranites in the Tertiary. In that case, the pressures and temperatures that metamorphic minerals record, at least in this part of the Himalaya, cannot be related in a simple way to the Tertiary granite-producing event. The second order observation is that such pronounced isotopic disequilibrium as preserved in these garnets implies a short-lived thermal pulse.
Harrison, T.M., Copeland, P., Kidd, W.S.F. & Yin, A., Science 255, 1663-1670 (1992).
Noble, S. & Searle, M.P., Terra Abstracts 7, 352 (1995).
Vance, D. & Kelley, S.P., Min. Mag. 58A, 930-931 (1994).