DETECTION OF INHERITED MONAZITE IN THE MANASLU LEUKOGRANITE BY PB-208TH-232 ION MICROPROBE DATING - CRYSTALLIZATION AGE AND TECTONIC IMPLICATIONS/

Although leucogranites are among the least petrologically variable of all igneous rocks, ironically they are among the most difficult to rel iably date. The High Himalayan leucogranites have been the subject of numerous geochronological investigations because of their interrelatio nship with the most significant tectonic features of that mountain bel t. For a variety of reasons linked to the minimum melt composition of these leucogranites, these dating studies have not been entirely succe ssful. We report results of a new ion microprobe dating method based o n the decay of Th-232 to Pb-208 in monazite that has directly revealed the presence of inherited Pb in monazite from the Manalsu granite, ca sting doubt on its previously accepted age. Monazite ages obtained fro m this leucogranite yield two distinct populations, a large number of ages with a normal distribution and mean age of 22.4 +/- 0.5 Ma (+/-2 S.E.) that we interpret to be the crystallization age, and a smaller i nherited fraction with an age of ca. 600 Ma. Because formation of the granite is thought to be related to slip on the Main Central Thrust, b oth this date and a second less precise result from a structurally sim ilar pluton near Mt. Everest indicate that the Main Central Thrust was active in the interval 24-22 Ma, but do not constrain its initiation. Together with crosscutting relationships, these data require that mov ement on the North Himalayan Fault occurred prior to 22 Ma at both loc ations.