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

Deveau, Stuart William (1992) A metamorphic, structural and geophysical investigation of the Mount Cormack subzone, Central Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

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 diagram resulting in cordierite and staurolite being relatively rare. -- Mineral assemblages from both sides of the bathogradic reaction: -- biotite + garnet + andalusite + vapour ⇄ -- staurolite + muscovite + quartz + sillimanite occur in rocks of appropriate composition and temperature, indicating a pressure of approximately 3.2 kilobars at the bathograd. This bathograd is approximately coincident with the sillimanite-biotite-muscovite reaction isograd. Pressure increases slightly with increasing metamorphic grade (i.e. higher pressures occur on the high grade side of the isograd), consistent with interpretations that either the high-grade areas underwent greater post-metamorphic uplift or that the syn-metamorphic Through Hill Granite dragged up its metamorphic aureole during emplacement. -- Results of geothermobarometry from samples from the Mount Cormack terrane indicate that pressure estimates range from 2.5 - 3.5 kilobars and temperature estimates vary from 525 - 600 ゚C for andalusite-biotite-garnet assemblages, and up to 650 ゚C for sillimanite-biotite-garnet assemblages. These estimates indicate high geothermal gradients of up to 50 ゚C/km in the high-grade assemblages. Such high geothermal gradients are consistent with the suggestion that crustal thinning played an important role in the metamorphic development of the Mount Cormack Terrrane. P-T vectors from zoned minerals from high-grade rocks are shallow, indicating that the amount of syn-metamorphic uplift and erosion was small. Exhumation must have been delayed until after substantial post-metamorphic cooling, and is not recorded in the mineral assemblages. -- Interpretation of gravity and magnetic anomalies in the Mount Cormack Subzone indicates a model for the subsurface geology that is consistent with that observed at the surface. Large magnetic and gravity anomalies are associated with the ophiolitic rocks, and there appears to be little effect from the other units in the study area. The 1985 gravity profile is asymmetric; the anomaly associated with the Coy Pond Complex is much smaller than that associated with the Pipestone Pond Complex, suggesting that the Partridgeberry Hills Granite (or similar low-density body) extends beneath this ophiolite. -- The magnetic low directly northwest of the Coy Pond Complex results from the fact that the dip of the ophiolite is similar to the inclination of the overall magnetic field. Small-scale magnetic anomalies within the Mount Cormack Subzone are interpreted as traces of bedding defined by cordierite-magnetite sub-assemblages. The attitude of these features is consistent with structural measurements made at the surface. The magnetic anomaly associated with the high-grade pelites may be due to the breakdown of hematite to form magnetite in the presence of reducing fluids.

Item Type: Thesis (Masters)
URI: http://research.library.mun.ca/id/eprint/6759
Item ID: 6759
Additional Information: Bibliography: leaves 198-206.
Department(s): Science, Faculty of > Earth Sciences
Date: 1992
Date Type: Submission
Geographic Location: Canada--Newfoundland and Labrador, Central
Library of Congress Subject Heading: Geology, Structural; Geology--Newfoundland and Labrador, Central; Metamorphism (Geology)--Newfoundland and Labrador, Central

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