The tectonic significance of (U,Th)/Pb ages of monazite inclusions in garnet from the Himalaya of central Nepal

Abstract

Textural observations, chemical zoning, and 196 in situ LA-ICP-MS 232Th/ 208Pb and 238U/ 206Pb dates of monazite inclusions in Greater Himalayan garnets demonstrate that inclusions are frequently composite grains composed of multiple generations of monazite. Cenozoic prograde and retrograde monazite are commonly intergrown, and both of these components are frequently intergrown with lower Paleozoic monazite. Because zones in most inclusions are too small to date, and because some inclusions show little zoning of Y and Th, chemical characterization alone is often insufficient to guide interpretation of the crystallization significance of Th/Pb dates. Comparison of 238U/ 206Pb and 232Th/ 208Pb dates thus is critical for interpreting the tectonic significance of dates derived from these composite inclusions. Most Cenozoic monazite crystallized between 42–29 and 22–12 Ma, and these age ranges may record prograde and retrograde metamorphism of Greater Himalayan rocks, respectively. Dates younger than ∼ 12 Ma are discordant, without exception. The < 12 Ma dates come from the eastern side of the Annapurna Range and record limited monazite growth during retrograde metamorphism and metasomatism of these Greater Himalayan rocks. Microcracks allow communication between the interior and exterior of garnet, affecting monazite inclusions by facilitating Pb loss, dissolution, recrystallization, and intergrowth of younger monazite.