TIMING OF METAMORPHISM IN THE MAIN CENTRAL THRUST ZONE IN SOUTH-CENTRAL BHUTAN: INSIGHTS FROM PRELIMINARY GARNET AND MONAZITE GEOCHRONOLOGY

In the Himalayan orogen, major shear zones such as the south-vergent Main Central thrust (MCT) have accommodated a large percentage of India-Asia convergence. Quantifying the temporal evolution of burial and exhumation within shear zones such as the MCT is critical to understanding how the Himalayan orogen evolved. In the eastern Himalaya in Bhutan, the MCT has been mapped where aluminosilicate-bearing, migmatitic Greater Himalayan (GH) gneisses are placed over garnet-bearing schist and quartzite of the Jaishidanda Formation (JF). Shearing on the MCT throughout Bhutan has been bracketed between ~23-15 Ma. In south-central Bhutan, recent work has proposed that the MCT is best defined as a 900 m-thick zone, including 700 m of metamorphosed JF and the lowest 200 m of GH rocks; temperature estimates from these rocks define an inverted metamorphic gradient. To quantify the timing of this metamorphism, we collected in-situ U-Pb monazite dates and Lu-Hf garnet-whole rock isochron ages from three samples within and above the MCT zone. Garnets from a GH paragneiss collected 0.8 km above the GH-JF contact yielded an age of ~33 Ma. Trace element electron microprobe mapping revealed zoning within the monazites that correlates with different 207-corrected ²⁰⁶Pb/²³⁸U dates: Y-poor cores grew between ~20-18 Ma, and Y-enriched rims grew from ~17-16 Ma. Garnets from two JF samples collected 0.4 km below the GH-JF contact yielded ages of ~30 Ma and ~28 Ma. Monazite grains from one of the JF samples showed no zoning and yielded dates between ~24-21 Ma. Garnet ages from within and above the MCT zone are within error, indicating that both the GH and JF rocks underwent burial to garnet-stable depths by ~28 Ma. Within the MCT zone, ~24-21 Ma monazite growth in the JF is interpreted as dating MCT shearing and likely records metamorphism associated with this motion. In GH rocks above the MCT zone, likely near peak, garnet-stable monazite growth occurred from ~20-18 Ma, and garnet-unstable, Y-rich monazite rim growth between ~17-16 Ma is interpreted to date exhumation related to erosion and uplift associated with MCT motion. Additional garnet and monazite ages will allow testing as to how robust the trends of these preliminary results are.