Timing of granulite-facies metamorphism in the eastern Himalayan syntaxis and its tectonic implications

Abstract

We present geochronological evidence in the eastern Himalayan syntaxis (Namche Barwa) for high-pressure (HP) granulite-facies metamorphism and explain its importance for understanding both the deep continental subduction of the Indian plate beneath Asia and its subsequent exhumation. The timing of peak and retrograde metamorphism in part constrains these processes but is debated. We present zircon U–Pb and trace element data on granulite-facies rocks. Zircon cores and rims from a weakly retrograded mafic granulite ( P = 14–18 kbar, T ≈ 800 °C) yield U–Pb ages of 24.0 ± 0.3 Ma and 18.8 ± 0.3 Ma, respectively. Zircon cores and rims from an orthogneiss, the host of the mafic granulite, yield U–Pb ages of 490 ± 3 Ma and 24.2 ± 0.4 Ma, respectively. An amphibolitized mafic granulite gives a U–Pb zircon age of 17.0 ± 0.4 Ma. Combined with petrography, zircon CL images, Th/U ratios and REE patterns, we suggest that the peak metamorphic age for the HP granulite is at ∼ 24 Ma and subsequent moderate- and low-pressure retrograde metamorphism occurred at 19–17 Ma, indicating reasonably rapid exhumation for the HP granulite. The ages of detrital zircons from a metasedimentary rock, another host rock of the mafic granulite, range from ∼ 0.6 to ∼ 2.0 Ga with peak at 0.8–1.2 Ga. The protolith depositional ages for the metasedimentary rock are constrained to be between 490 and ∼ 600 Ma. Our data suggest that the granulite terrane in the eastern Himalayan syntaxis has an affinity with the Greater Himalayan Series. The HP granulite-facies metamorphic events in the eastern Himalayan syntaxis are distinct from ultrahigh-pressure (UHP) metamorphic events in the western Himalayan syntaxis in both age and depth of burial. However, the metamorphic history of the eastern and western Himalayan syntaxes becomes similar after ∼ 24 Ma. The Namche Barwa granulites appear to result from the underthrusting of the Indian plate lithologies beneath the Lhasa block during progressive collisional processes, followed by extrusion and/or exhumation that result from a slab breakoff of the Indian plate during the Miocene.