Petrochronologic record of metamorphism and melting in the upper greater Himalayan sequence, Manaslu- Himal Chuli Himalaya, West-Central Nepal

Abstract

New in situ U-Th-Pb monazite ages from metapelites within the upper Greater Himalayan sequence in the Manaslu-Himal Chuli Himalaya provide insight into the tectonometamorphic history of this portion of the mountain range. Monazite grains record a protracted evolution characterized by a three-stage growth history at lower structural levels (ca. 30 Ma, ca. 26-21 Ma, and ca. 18-15 Ma) and a single growth stage between ca. 21 and 16 Ma, following complete resorption of earlier monazite at higher structural levels. All specimens record monazite growth during protracted early to middle Miocene melt crystallization, with monazite at higher structural levels significantly overlapping in age with the young phase of the Manaslu pluton. The metamorphic and anatectic ages reported in this study are consistent with a middle Miocene transition from deep hinterland-style deformation characterized by extending flow in the upper Greater Himalayan sequence to shallower foreland-style deformation characterized by compressing flow in the lower Greater Himalayan sequence. This change outlines a deformational continuum between lateral midcrustal flow beneath Tibet and thrust wedge taper deformation along the mountain front as has been put forth in recent thermo-mechanical and conceptual models. © 2011 Geological Society of America.