Relationship Between Deformation and Monazite Ages: Two Counter- Examples in the Himalaya

H A Smith, C P Chamberlain, T Allen (Earth Sciences, Dartmouth College, Hanover NH 03755), P K Zeitler (Earth and Environmental Sciences, Lehigh Univ., Bethlehem PA 18015)

Monazite has found use as a geochronometer for metamorphic processes, processes intimately coupled with deformation in developing orogens. While the role deformation might play in initially forming monazite is unknown, the prograde metamorphism of pelites has been shown to result in the genesis of metamorphic monazite. The U-Pb system in such monazite then gives a direct age for onset of metamorphism, as well as offering constraints on the timing of deformation.

In Pakistan, we have used such ideas in two separate regions: the Nanga Parbat-Haramosh massif (NPHM) of the Indian plate and in the Karakorum range of the Asian plate. Although it has been shown that the NPHM is a region of rapid, accelerating uplift and denudation over the past 10 Ma, expectations were that the timing of Himalayan metamorphism would be broadly similar to that shown for the central Himalayas: mineral cooling ages there give younger bounds on the age of metamorphism of ~30-40 Ma. However, metamorphic rocks of the NPHM gives much younger monazite U- Pb ages of 4-11 Ma. We have been able to confirm none of the known scenarios for resetting the U-Pb system in monazite. Instead, we find independent corroborating evidence of Neogene metamorphism: young zircon U-Pb ages from leucogranite dikes and from the metasediments, as well as consistently younger fission-track ages.

Such is not the case for the Karakorum. Undeformed, post-metamorphic igneous units constrain deformation and metamorphism to being older than ~35 Ma, while 2 upper amphibolite facies metamorphic samples give concordant U-Pb monazite ages of 6-7 Ma. Although other geochronologic data are scarce, such young ages are enigmatic. We will use these two counter-examples to speculate on the viability of the U-Pb system in monazite as influenced by thermal and deformational perturbations.

1993 EOS, Transactions of the American Geophysical Unions 74(16): 123.