GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 56-6
Presentation Time: 2:50 PM


JOHNSON, Thomas A., Department of Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106, COTTLE, John M., Earth Science, University of California, Santa Barbara, Santa Barbara, CA 93106 and LARSON, Kyle P., Earth, Environmental and Geographic Sciences, University of British Columbia, Okanagan, 3247 University Way, Kelowna, BC V1V 1V7, Canada

Differentiating between individual tectonothermal events in polymetamorphic terranes remains a persistent problem for many studies of orogenic systems. This includes the Himalaya, where recent investigations indicate that although much of the observed metamorphism occurred after 55 Ma, some rocks record evidence of deformation and metamorphism prior to the Cenozoic. Whereas the timing of pre-Himalayan tectonism is constrained to 530-450 Ma, quantitative estimation of metamorphic pressures and temperatures of Paleozoic event(s) is complicated by the overprinting effects of Himalayan metamorphism.

To better understand the conditions and timing of pre-Himalayan metamorphism, we present new results from quartz inclusion thermobarometry and titanite petrochronology of a series of garnet-bearing metapsammites from the Kathmandu Complex in central Nepal. Pre-Cenozoic deformation and metamorphism in the area is partly evidenced by the occurrence of foliated xenoliths of lithologically similar rocks in Cambro-Ordovician granites. Mineral assemblages and tectonic fabrics within the xenoliths indicate they were metamorphosed and deformed at amphibolite facies conditions prior to granite emplacement.

Garnets from the studied samples are zoned with chemically distinct inclusion rich cores and inclusion poor rims, consistent with two periods of crystal growth. Combined titanium-in-quartz thermometry and quartz-in-garnet barometry yields distinct temperatures and pressures for the garnet cores (~600°C, 1.0 GPa) and rims (~550°C, 0.85 GPa). Phase equilibria modeling of the garnet cores and conventional thermobarometry applied to the garnet rims yield similar results. Isotopic analysis of 2 morphologically distinct titanite groups from these samples similarly indicate two periods of growth at 460-440 Ma and ~30 Ma. Petrographic study of the two varieties of titanite indicates that the younger group formed following breakdown of rutile that originally coexisted with the older generation of titanite at pressures and temperatures similar to those determined from the garnet cores. Together these observations highlight the utility of combining inclusion thermobarometry with in-situ petrochronology to distinguish between overprinting tectonothermal events in polymetamorphic terranes.