Paper No. 139-9
Presentation Time: 3:45 PM
CRUSTAL THICKENING, METAMORPHISM, AND EXHUMATION OF MID-CRUSTAL ROCKS DURING DOMING AND EXTRUSION: INSIGHTS INTO THE LEO PARGIL DOME, NW INDIAN HIMALAYA
Mid-crustal rocks from the NW Indian Himalaya are useful for characterizing the conditions during burial (Eocene-Oligocene), melting and subsequent exhumation (Miocene). Two generations of NW-trending folds are defined by meso-and micro-scale structural data. Kyanite and staurolite overgrowths on F2 crenulation folds record an early phase of Barrovian metamorphism, which ended by 30 Ma. Metamorphic field gradients increase from the garnet zone (499 ± 99°C and 4.5 ± 1.4 kbar to 571 ± 92°C and 7.8 ± 1.4 kbar) to the staurolite-kyanite zone (567 ± 105°C and 6.7 ± 1.6 kbar). In close proximity to the Leo Pargil dome, sillimanite and cordierite overprint staurolite and kyanite porphyroblasts. These overgrows are attributed to the initiation of near-isothermal decompression during top-down-to-the-west displacement on the Leo Pargil shear zone at 23 Ma, a major structure that defines the northwest margin of this northeast trending domal structure. Metasedimentary rocks of the Haimanta Group mantle the dome. Migmatitic gneiss in the central portion of the dome preserves a semi-continuous period of partial melt production between 30 and 18 Ma that fed an extensive injection complex. Data from the Leo Pargil dome is combined with published data, including the Sangla detachment, to develop a two-stage conceptual model for the Eocene-Miocene evolution of the NW Himalaya. The first stage (Eocene-Oligocene) involved crustal thickening and Barrovian metamorphism. The second stage included coeval (23 Ma) initiation of doming in the hinterland and foreland-directed extrusion of the Greater Himalayan Series beneath the Sangla detachment. The conceptual model demonstrates that the distribution of rocks with variable exhumation rates, pressure-temperature paths, and degrees of partial melting during the Miocene were, in part, influenced by shear zones and detachments that formed during the Eocene-Oligocene.