A metamorphic, structural and geophysical investigation of the Mount Cormack subzone, Central Newfoundland

• Main Author: Deveau, Stuart William
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 1992
• Subjects: Newfoundland
• Online Access: https://research.library.mun.ca/6759/; https://research.library.mun.ca/6759/5/StuartWilliamDeveau.pdf

The Mount Cormack Subzone (MCSZ) is located in central Newfoundland and has been interpreted as a tectonic window which exposes rocks of the Gander Zone through the surrounding Dunnage Zone. Structural evidence suggests that three deformation events caused the present geologic pattern. The first deformation was a result of thrusting of an ophiolite on to the Spruce Brook Formation (SBF) causing recumbent isoclinal F₁ folds in the latter; subsequent tectonic shortening during the second deformation caused the formation of large- and small-scale sub-horizontal F₂ folds. Evidence of low-pressure, high-temperature metamorphism suggests that this phase of crustal shortening was followed by widespread crustal extension. This thinning of the crust was accompanied by crustal melting at depth causing high geothermal gradients in the SBF and the development of mineral reactions which have been recorded as reaction isograds. All earlier structures were refolded during the D₃ deformation which rotated the D₂ structures and the isograds and caused a broad doming. -- The mineralogy preserved in the metasedimentary rocks of the Mount Cormack Subzone indicates that the regional metamorphic grade increases from greenschist to upper amphibolite facies. The reaction isograds crosscut the large-scale second generation folds in the area consistent with early thrusting (emplacement of oceanic crust of Dunnage Zone over Gander Zone) and shortening, followed by structural and thermal doming. The attitude of the S₂ foliation within the terrane is approximately parallel to the edges of the MCSZ and dips away from the centre, consistent with a broad domal structure. -- Five reaction isograds have been mapped within the Mount Cormack Subzone as follows: 1) biotite-muscovite-chlorite isograd; 2) andalusite-biotite-muscovite isograd; 3) sillimanite-biotite-muscovite isograd; 4) sillimanite-K feldspar isograd; and 5) melting isograd defining the beginning of partial melting. Bulk rock compositions lie within the central part of the AFM ...