In the Himalayan orogen, Greater Himalayan (GH) rocks were buried to mid-to lower-crustal levels and are now exposed across the strike of the orogen. Within the eastern Himalaya, in the Kingdom of Bhutan, the GH is divided into structurally lower (lower-GH) and upper (upper-GH) levels by the Kakhtang thrust (KT). Pressure-temperature estimates from lower-and upper-GH rocks collected on two transects across the KT yield similar P-T-structural distance trends across each transect. In the eastern transect, temperatures are similar (from 730 to 650°C) over a structural thickness of~11 km, but peak pressures decrease from~10 to 6 kbar with increasing structural level. In comparison, peak temperatures in the central Bhutan transect are similar (from 730 to 600°C), but pressures decrease from 10 to 6.5 kbar with increasing structural level over a structural thickness of~6 km. The structurally highest sample reveals slightly higher pressures of 8.0 kbar in comparison to pressures of 6.5 kbar for samples collected from within the KT zone,~4 km below. Within each transect, there are increases in pressure AE temperature within the overall upright P-T gradient that may demarcate intra-GH shear zone(s). These P-T results combined with evidence that the timing of initial melt crystallization becomes older with increasing structural level suggest that the intra-GH shear zones emplaced deeper GH rocks via progressive ductile underplating. These shear zones, including the KT, likely aided in the initial emplacement and construction of the GH as a composite tectonic unit during the Late Oligocene to Early Miocene, from c. 27 to 16 Ma.
Studies of pre-Grenvillian (1.4-1.3 Ga) plutons offer insight into the dynamics of arc amalgamation and backarc rifting prior to continental collision during the Ottawan orogeny. The Central Metasedimentary Belt boundary thrust zone (CMBbtz) is a northeast-southwest-trending thrust zone consisting of metaplutonic thrust sheets enveloped in gneissic tectonites and calcitic-dolomitic marble. Tonalitic CMBbtz thrust sheets (Dysart and Redstone), located in the southern Ontario Grenville Province (Canada) are made up of upper amphibolite facies, foliation-concordant metatonalite (+ amphibole ± biotite ± accessory zircon and titanite) and amphibolite (± biotite ± clinopyroxene). These thrust sheets are thought to have formed and amalgamated onto the Laurentian margin prior to Ottawan orogeny. Major and trace element analyses show that the metatonalite rocks have calc-alkaline affi nity and amphibolite rocks have both calc-alkaline and tholeiitic affi nities, suggesting an arc environment. Laser ablation-inductively coupled plasmamass spectrometry (LA-ICP-MS) geochronology of zircon from the two thrust sheets yield igneous ages of ca. 1350-1300 Ma for diorite and granodiorite, ca. 1150-1100 Ma ages from Dysart tonalite interpreted to record metamorphic zircon growth, and a ca. 1086 Ma Ottawan metamorphic age from a Dysart amphibolite. The ca. 1150-1100 Ma metamorphic event has not been previously documented within these thrust sheets of the CMBbtz, but correlates well with thermal events in the allochthonous Parry Sound domain to the west, as do ca. 1350 Ma igneous ages of tonalite in both areas. These data support the hypothesis that the CMBbtz and Parry Sound domain may have been initially linked. Widespread ca. 1350 Ma crust along with distinct 1460-1400 Ma depleted mantle model ages (T DM ) are also consistent with a shared genesis with the Dysart-Mount Holly suite in New York and Vermont, and support the correlation between the CMBbtz thrust sheets and the Adirondack Highlands-Mount Holly belt as a rifted arc. Alternatively, the CMBbtz thrust sheets and the Adirondack Highlands-Mount Holly belt may represent contemporaneous arc development at different parts of the convergent margin; however, we support the correlation between the CMBbtz thrust sheets and the Adirondack-Highlands-Mount Holly belt.
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