Paper No. 95-9
Presentation Time: 9:00 AM-1:00 PM
ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPIC COMPOSITIONS OF AMA DRIME MIGMATITIC GNEISSES: IMPLICATIONS FOR THE ASSEMBLY OF THE PALEOPROTEROZOIC SUPERCONTINENT COLUMBIA
Paleoproterozoic migmatitic gneisses are ubiquitous along the entire Himalayan stretch, extending from Sinagar in the east to the south Karakoram in the west. Timing and origin of these temporally and spatially-related high-grade metamorphic rocks is an important element to better develop the global paleogeographic reconstruction models during 2.1-1.8 Ga which is the time for the assembly of the supercontinent Columbia. We provide U-Pb ages and Hf isotopic compositions of the ortho and paragneisses from Ama Drime Nepal, exposed within the High Crystalline Series. These results are then integrated with regional data to better understand the Paleoproterozoic tectonic evolution of the Archean crust. Three orthogneisse samples yielded zircon U-Pb weighted mean ages of 1820 ± 13 Ma, 1858 ± 13 Ma and 1857 ± 1 Ma. Two zircon spots yielded Archean inherited ages of 2485 ± 19, and 2536 ± 11 Ma. One paragneiss sample yielded a U-Pb weighted mean age of 454 ± 11 Ma. Detrital zircons from another paragneiss sample yielded dates varying from 490 to 276 Ma. Three samples from the ~1.82 Ga orthogneisses yielded near-chondritic initial Hf isotopic compositions with weighted mean εHf (t) of -1 ± 0.54, 1 ± 1.7, and 2 ± 0.53. The Ordovician-Silurian paragneiss samples yielded negative εHf (t) values range from -6.2 to -1.9. The orthogneiss samples yielded Hf depleted mantle ages (TDM) ranging from 2.3 to 2.1 Ga while the younger paragneisses yielded the TDM ages of 1.2 Ga and 1.5 Ga. These data suggest that the protolith of the Paleoproterozoic orthogneisses formed by the partial melting of juvenile Archean crust. While zircons in the Ordovician-Silurian paragneisses indicate the subsequent partial melting of preexisting metamorphic rocks (~1.1-0.9 Ga) in the region. The regional correlation of geochronological and Hf isotopic data of the Paleoproterozoic zircons along the entire stretch of the Himalaya indicate the presence of the ca. 1.8 Ga collision orogeny formed during the assembly of the supercontinent Columbia. We propose that the regional spread of 1.8 Ga orthogneiess are the result of the collision of the south China block with the northern margin of India. This collision orogen has been previously overlooked in the Paleogeographic reconstruction models of supercontinent Columbia.