The Malari Leucogranite, Garhwal Himalaya, Northern India: Chemistry, Age, and Tectonic Implications

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In the Garhwal region, India, the Malari leucogranite cuts the South Tibetan detachment system, a large-scale normal fault system at the top of the High Himalaya. The leucogranite crosscuts ductile normal-sense shear fabrics and has experienced relatively little subsolidus brittle deformation or alteration. Its relatively evolved bulk chemical composition, high Rb/Sr ratio, and normative corundum indicate a (meta)sedimentary source, likely the underlying Greater Himalayan sequence. Zircon U-Pb ages, collected by laser-ablation inductively coupled plasma–mass spectrometry (ICP-MS) and corrected for initial U/Th disequilibrium, indicate emplacement at 19.0 ± 0.5 Ma. Thus, ductile normal shear on the South Tibetan detachment system must have ceased by 19 Ma. Studies elsewhere in the Himalaya suggest initiation of South Tibetan detachment system ductile movement not earlier than 24 Ma, and likely ca. 22 Ma. The short duration of extension (≤5 and likely ∼3 m.y.) and early cessation contrast with channel-flow models that predict long-duration ductile normal shear, and large displacements after ca. 20 Ma. Observations are instead better explained by critical taper models, in which internal weakening of the wedge, likely from partial melting, caused a brief interval of flattening and ductile extension in the rear of the wedge.