Illite crystallinity and bo spacing analyses of rocks from the Lesser Himalaya show that the upper Paleocene-lower Eocene Subathu Formation, deposited immediately prior to or early in the India-Asia collision, shows distinctly lower metamorphic grades than the (anchiepizone) upper Precambrian-lower Paleozoic rocks. The latter have been affected by a precollision metamorphic event, the probable age of which is Paleozoic, although a Mesozoic age is not ruled out. Thermal effects of the Main Central thrust are restricted to a section ∼1300 m below the thrust; therefore, the Le Fort model for metamorphic inversion and granite formation needs modifying because the exposed Main Central thrust footwall rocks could not have generated the fluids responsible for anatectic melting.

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