TIMING AND SOURCE OF MELTING AT THE EASTERN EDGE OF THE GURLA MANDHATA CORE COMPLEX, NW NEPAL HIMALAYA
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Located at the eastern edge of the Gurla Mandhata core complex in the NW Nepal Himalaya, the Chuwa Khola exposes stromatic metatexite, calc-silicate gneiss, augen orthogneiss, and schlieren structured diatexite, all crosscut by a leucogranitic dyke and sill network that culminates in the peraluminous Chuwa granite at the structurally highest level. Distributed shear sense indicators are consistent with top-to-the-southwest ductile extrusion of the Himalayan metamorphic core as observed elsewhere in the Himalaya. In situ U-Th/Pb monazite petrochronology conducted on the Chuwa granite yields an interpreted mean crystallization age of ca. 18 Ma, with no ages younger than 16.5 ± 0.6 Ma. The dyke and sill network yields an interpreted mean crystallization age of ca. 19.5 Ma, with a minimum age of 15.8 ± 0.6 Ma. The southernmost migmatite samples yield similar mean dates interpreted to be the migmatization age (ca. 18.5 Ma), but also contain monazite ages as young as 12.8 ± 0.5 Ma. Yttrium content of the monazite in the migmatite increases with decreasing age, suggesting garnet breakdown and retrograde metamorphism from ca. 18 Ma to 12 Ma. The dyke and sill network are interpreted as a feeder system to the overlying Chuwa granite, based on their similar (ca. 18 Ma) age and geochemical signature. Migmatization occurred as young as ca. 13 Ma as a result of orogen-parallel extension. It is suggested that the dyke and sill network and the coeval Chuwa granite were emplaced during southward-directed crustal extrusion prior to orogen-parallel extension, whereas the Chuwa Khola migmatite rocks record the transition from orogen perpendicular extrusion of the Himalayan metamorphic core to orogen parallel extensional exhumation at ca. 15 Ma to 13 Ma. The Gurla Mandhata core complex and the Chuwa Khola rocks have been affected by a coeval thermal pulse typical of southward extrusion of the Himalayan metamorphic core from ca. 22 Ma to 16 Ma. Although, the Gurla Mandhata core complex records top-to-the-west/northwest sense of shear at ca. 15 Ma to 13 Ma, the Chuwa Khola exhibits dominant top-to the southwest suggesting the existence of a strain partitioning boundary between the two areas.
URI for this recordhttp://hdl.handle.net/1974/28716
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