THE TECTONOMETAMORPHIC EVOLUTION OF THE GREATER HIMALAYAN SEQUENCE ALONG THE ZANSKAR SHEAR ZONE, NW INDIA

The Greater Himalayan Sequence (GHS) has been exhumed along the Zanskar Shear Zone (ZSZ) in the Zanskar region of the western Himalaya.Granites and metapelites collected along the the ZSZ in the Suru River valley provide how and when the GHS rocks exhumed and were deformed. There are two suites of Paleozoic granites deformed within the ZSZ: Pan-African Cambrian–Ordovician granites at the cores of gneiss domes and Mississippian–Permian granites related to Panjal Traps magmatism. Anatectic leucogranite crystallization from c. 28 to 16 Ma occurred within the GHS. The metamorphic grade in the GHS increases from biotite to sillimanite zone from E to W along the Suru River toward the Suru dome and comprises Qz + Kfs + Pl + Bt + Ms ± Grt ± Sil ± Ky ± St ± Chl ± Tur ± Rt ± Zrn ± Opq. Rotated garnets, recrystallized quartz grains, irregular grain boundaries, kink bands, microfolds, and deformation bands suggest different deformation mechanisms and temperatures corresponding to changing strain with distance from the ZSZ. Isochemical phase diagrams using Perple_X, mineral chemistries, and electron backscatter diffraction, combined with geo/thermochronology data from U-Pb and ⁴⁰Ar/³⁹Ar, methods reveal the exhumation history of the GHS rocks within the ZSZ, and tests the mid-crustal channel flow model. I have modeled metapelite mineralogy, using an internally consistent thermodynamic database, several well-calibrated mineral solution models, and the program Perple_X suite of programs to compute pseudosections. Solution models include biotite, chlorite, staurolite, feldspar, white mica, garnet, and ideal cordierite; and were modeled over the pressure-temperature range of 0.4-1.5 GPa and 573-1273°C. According to the calculations using Perple_X, the maximum pressure for garnet growth in sample ZH-35 is approximately 1.3 GPa. ⁴⁰Ar/³⁹Ar dating of muscovite and biotite constrains cooling and exhumation ages of the GHS at ~20-19 Ma and ~15 Ma, respectively. A metamorphic pressure-temperature-time-deformation history of the GHS rocks, generated in light of these combined data, fills gaps in the understanding of the tectonic and metamorphic evolution of the GHS in the Suru River Valley which is less well-studied than the comparable evolutionary history of the GHS in the eastern Himalaya.