⁴⁰Ar/³⁹Ar age constraints on deformation and metamorphism in the main central thrust zone and Tibetan slab, eastern Nepal Himalaya

Abstract

In eastern Nepal, structural and petrologic observations suggest that movement on the Main Central Thrust (MCT), the development of an inverted metamorphic sequence, and leucogranite plutonism were genetically related. Samples from across the MCT zone and its hanging wall, the Tibetan Slab, were collected from four locations in the Everest region for 40 Ar/ 39 Ar studies: (1) the MCT zone, (2) the lower Tibetan Slab, (3) the upper Tibetan Slab away from intrusive rocks, and (4) the upper Tibetan Slab adjacent to and including leucogranitic material. Within the MCT zone, muscovite and hornblende yield cooling ages of 12.0±0.2 Ma and 20.9±0.2 Ma, respectively. K‐feldspar gives a minimum age of 8.0±0.2 Ma (closure temperature (T C ) =220±15°C). In the lower Tibetan Slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7.7±0.4 Ma and 7.5±0.6 Ma and a K‐feldspar minimum age of 6.4±0.8 Ma (T C = 225±20°C). An adjacent gneiss yields a 9.1±0.2 Ma biotite isochron and a K‐feldspar minimum age of 3.6±0.2 Ma (T C = 210±50°C). In the upper Tibetan Slab, samples collected >200 m from visible intrusives yield a biotite isochron age of 20.2±0.2 Ma and a complex hornblende age spectrum suggestive of excess argon. At the fourth location an augen gneiss, a pegmatite, and a tourmaline‐bearing leucogranite, all in mutual contact, yield indistinguishable mineral ages. The average biotite and muscovite isochron ages for these samples are 17.0±1.4 and 16.6±0.2 Ma, respectively. The minimum age for K‐feldspar from the leucogranite is 15.5±1.8 Ma. As thermobarometric data from the MCT zone in this area suggest synmetamorphic deformation at temperatures of ∼500°–550°C, the MCT hornblende (T C = ∼500°C) dates this event at ∼21 Ma. Diffusion experiments on hornblendes from the MCT zone provide data which support a maximum duration of peak metamorphic temperatures of ≤2 Ma. Biotite ages as old as ∼20 Ma from the upper Tibetan Slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the MCT zone. Very young ages in a ductile shear zone in the lower Tibetan Slab suggest that there has been deformation within the Tibetan Slab that postdates major movement on the MCT.