SHISHA-PANGMA LEUKOGRANITE, SOUTH TIBETAN HIMALAYA - FIELD RELATIONS,GEOCHEMISTRY, AGE, ORIGIN, AND EMPLACEMENT

The Shisha Pangma pluton forming most of the Xixabangma (8027 m) massi f in south Tibet is one of the 20+ larger leucogranite intrusives that mark the highest structural levels of the Himalayan metamorphic core. The pluton occurs immediately below the Shisha Pangma Detachment, a s trand of the South Tibetan Detachment (STD) system, a low angle (30 de grees) north-dipping normal fault placing Paleozoic black slates atop sillimanite-grade pelites and calc-silicate rocks. K-feldspar augen gn eisses containing fibrolite and sillimanite paragneisses along the foo twall show strong internal S-C fabrics indicative of down-to-the-north extension. The Shisha Pangma leucogranite is a heterogeneous, poly ph ase intrusion with an earlier, foliated biotite-rich phase and a later , tourmaline + muscovite rich phase typically containing the assemblag e: Kfs + Pl + Qtz + Ms + Tur +/- Gt +/- Bt +/- Sil +/- Ap. The highly peraluminous granites have high Sr-87/Sr-86 ratios (0.738-0.750) typic al of pelite-derived anatectites. Nd id-depleted mantle model ages fro m present Nd isotopic data and an assumed crustal Sm-147/Nd-144 of 0.1 0 +/- 0.02) are 1.5-2.2 Ga, indicating a substantial early Proterozoic or older crustal residence age for much of the source material. Xenot imes and monazites from a weakly foliated biotite granite immediately beneath the STD (X8) give consistent U-Pb ages of 20.2 +/- 0.2 Ma. Zir con, uraninite, and monazite from the main Shisha Pangma tourmaline muscovite +/- garnet phase (X20) give an U-Pb age of 17.3 +/- 0.2 Ma. A sill complex above the main leucogranite body is aligned parallel to the metamorphic fabric dipping at 10-30 degrees N, although a few dik es cross-cut the metamorphic fabric beneath the STD. Nowhere do the le ucogranites cut the STD, and the age of normal faulting must largely p ost-date 17.3 +/- 0.2 Ma. Muscovite from the main leucogranite phase h as an Ar-40/Ar-39 plateau age of 16.74 +/- 0.22 Ma. Apatite fission tr ack ages for leucogranite samples from 5800-8000 m range from 12.3 +/- 1.9 to 14.8 +/- 0.8 Ma ( +/- 2 sigma), only slightly younger than the main leucogranite crystallization age. Following crustal melting, ste ep cooling curves (>90-180 degrees C/myr) and rapid exhumation rates ( similar to 4 mm/yr) from 17-14 Ma resulted in removal of at least 12 k m of overburden, both by erosion and normal faulting. If high erosion and exhumation rates correlate with high topography land high precipit ation) these data suggest that the Himalaya reached their maximum topo graphic elevation around 17 Ma.