NEW U-PB AGE AND TRACE ELEMENT COMPOSITION OF YOUNG METAMORPHIC ZIRCON RIMS FROM THE UHP TSO MORARI COMPLEX, NW HIMALAYA, DISTINGUISHES PEAK FROM RETROGRADE METAMORPHISM

The ultrahigh-pressure (UHP) Tso Morari Complex (TMC) sits in the footwall of the Indus-Yarlung suture zone in the NW Himalaya. The timing of metamorphism during subduction and exhumation in the complex is critical to constraining the age of the India-Asia collision. Leech et al. (2005) reported mean U-Pb ages for thin outer rims of sectioned zircon between 53.3 ± 0.7 Ma and 47.5 ± 0.5 Ma for the age of peak UHP through retrograde metamorphism, and used these data to calculate the minimum age for the start of continental collision at 57 ± 1 Ma. Recently published results for the TMC have reignited debate on the age of metamorphism and thus, the timing of India-Asia collision. We used the same TM38 sample analyzed for the results described in Leech et al. (2005) and performed new SIMS U-Pb depth-profiling analyses to target only the outermost ~1.5 micron rims of zircon. Our results yield a mean age of 44.9 ± 0.7 Ma; adjacent spots for REE analyses yielded positive, enriched HREE profiles with negative Eu anomalies and corresponding Ti-in-zircon temperatures of ~550° to 680° C. Sharp boundaries between zircon domains are clearly resolved with CL and BSE imaging of TM38 zircons, and there is a large age difference between rims and protolith core ages; any mixing during depth-profiling through rims is clear. The positive HREE profiles imply the period of zircon growth in the TMC at c. 45 Ma to be retrograde. We suggest that the 47-43 Ma peak ages and flat heavy REE profiles with no Eu anomaly recently reported by Donaldson et al. (2013) on sectioned zircons, and interpreted as the age of UHP metamorphism of the TMC, may actually represent mixing between zircon rims and cores. The Leech et al. (2005) continental collision age of 57 ± 1 Ma assumed the TMC represents the leading edge of India. However, numerical modeling of Warren et al. (2008) suggests all exhumed material is derived from the central part of the pro-continental margin at least 200 km inboard of the ocean-continent transition. Recalculating using the geometric model of Leech et al. (2005) and the 53.3 ± 0.7 Ma age of peak UHP metamorphism yields a revised collision age of 61.1 ± 1.0 Ma.