High-pressure (HP), granulite-facies thrusting in a thick-skinned thrust system in the eastern Grenville Province, central Labrador

Krauss, Jason B. (2002) High-pressure (HP), granulite-facies thrusting in a thick-skinned thrust system in the eastern Grenville Province, central Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

In the eastern Grenville Province of central Labrador, the Grenvillian Grand Lake thrust system (GLTS) comprises part of a thick-skinned thrust system separating the allochthonous and parautochthonous Grenvillian terranes. North-northwest directed thrusting on the GLTS during the Grenvillian orogeny carried the Labradorian (1710-1600 Ma) rocks of the Cape Caribou River Allochthon (CCRA) and the overlying Mealy Mountains terrane (MMT), onto a footwall composed of similar aged granitoid gneisses of the Groswater Bay and Lake Melville Terranes (GBT and LMT). The CCRA is principally composed of an AMCG suite, with the GLTS comprising a basal shear zone about 1km thick characterized by mylonite and straight gneisses developed under granulite- and upper amphibolite-facies conditions. Lithologies in the GLTS include highly strained dioritic to granitic orthogneiss (straight gneiss) comprising alternating mafic and felsic layers consisting of a mineralogy composed of garnet, clinopyroxene, amphibole, plagioclase, quartz and minor orthopyroxene which locally defines the stretching lineation. The footwall rocks of the GBT and LMT were metamorphosed under upper amphibolite facies conditions during Labradorian orogenesis. -- Field studies and published U-Pb data indicate that the extent of mylonitization and recrystallization induced by thrusting along the GLTS diminish rapidly upward into the allochthon and downward into the footwall rocks. U-Pb metamorphic ages within the CCRA and footwall rocks are Labradorian (1710-1600 Ma), whereas U-Pb metamorphic ages from the recrystallized rocks of the GLTS define two Grenvillian populations at 1040 and ~1010 Ma, which are correlated with the Ottawan and Rigolet orogenies respectively. -- Detailed analysis of appropriate, low variance mineral assemblages within the GLTS and adjacent footwall and hangingwall rocks was carried out to determine the P-T conditions of thrusting during the Grenvillian orogenesis using the TWEEQU software. Resultant P-T estimates associated with Grenvillian thrusting in the GLTS yield a consistent grouping of ~875 ± 50°C and 14 kbar using garnet-clinopyroxene and garnet-amphibole thermobarometry on the porphyroblastic assemblage, and 700 ± 50°C and 6-9 kbar using garnet-amphibole thermobarometry on the matrix assemblage, consistent with textural and mineralogical evidence of decompression. On the basis of core-rim P-T vectors in single grains it is concluded that non-synchronous closure of the geobarometric and geothermometric reactions occurred in some samples, and thus the highest P-T estimates are 'apparent peak' estimates that underestimate the true peak conditions. Although unquantified, the degree of underestimation of peak conditions is considered to be small on the basis of independent petrological considerations. -- Detailed petrography has revealed a wide range of fabrics elements that are related to thrusting, but a precise link between the P-T estimates and the available U-Pb database is not currently possible. A tentative tectonic model for this part of the Grenville orogen involves formation of a crustal-scale orogenic wedge by thrust imbrication during the Ottawan orogeny (~1040 Ma), with development of high-pressure (HP) granulites (~875 °C and ~14 kbar) at the base of the wedge. The HP granulites were uplifted up a crustal-scale ramp and exhumed during both Ottawan (~1040 Ma) and Rigolet times (~1010-1000 Ma). Recrystallization in the GLTS during the Rigolet orogeny took place below the closure temperature for the Fe-Mg exchange geothermometers employed and must have been principally driven by ingress of H₂O and dynamic recrystallization. It is inferred that rapid uplift, exhumation and cooling of the orogenic wedge were facilitated by normal faulting at the top of the wedge, which brought the Mealy Mountains terrane, virtually unmetamorphosed during the Grenvillian orogenesis, into tectonic contact with the CCRA with the HP granulite-facies GLTS shear zone at its base. -- This study points out the importance of integration of metamorphic petrology with geochronology in tectonic studies and emphasizes the significance of deformation as a mechanism to induce recrystallization and resetting of geothermobarometers and U-Pb systematics in dry metamorphic rocks.

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