Geothermometry-geobarometry and ⁴⁰Ar/³⁹Ar incremental release dating in the Sandwich Bay area, Grenville province, eastern Labrador

Van Nostrand, Timothy Stuart (1987) Geothermometry-geobarometry and ⁴⁰Ar/³⁹Ar incremental release dating in the Sandwich Bay area, Grenville province, eastern Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

Three major lithotectonic units have recently been defined in the Grenville Province of eastern Labrador on the basis of contrasting lithologic associations, structural style and metamorphic grade. Two of these units, the Groswater Bay and Lake Melville Terranes are interpreted to represent parts of the parautochthonous and allochthonous units, respectively, which are two regional subdivisions of the Grenvllle Province in Labrador. -- Geothermometry and geobarometry estimates derived from garnet - biotite - plagloclase - Al₂SiO₅ - quartz and garnet - orthopyroxene - clinopyroxene - plagioclase - quartz equilibria of selected paragneiss and mafic lithologies in the Sandwich Bay area suggest contrasting metamorphic P-T conditions of the two terranes. Estimates determined from the Groswater Bay Terrane range from 615°C to 850°C and 10 - 12 kbar, suggesting derivation from depths on the order of 30 to 40 km. In contrast estimates from the Lake Melville Terrane range from 650°C to 900°C and 6 - 10 kbar implying crustal derivation from 20 to 30 km. Microstructural relationships and mineral chemistry of the assemblages in each terrane suggest that the P-T estimates obtained from each terrane are representative of Grenvillian metamorphic conditions. -- Equilibrium P-T paths constructed from the geothermobarometry estimates are interpreted to represent a portion of the uplift trajectory for both the Groswater Bay and Lake Melville Terranes. These paths suggest that both terranes followed a near isothermal uplift path, implying rapid decompression during cooling. -- ⁴⁰Ar/³⁹Ar incremental release radiometric dating of hornblende concentrates from orthogneiss and associated mafic dikes from the study area reveal contrasting ages from both terranes. Age spectra from the Groswater Bay Terrane are generally discordant and are interpreted to result from a combination of the presence of excess argon components and partial gas loss as a result of thermal resetting of the argon system. One sample from the Groswater Bay Terrane yields a reasonably defined plateau age of 1260 +/- 5 Ma. A similar plateau age of 1241 +/- 3 Ma is obtained from a sample immediately south of the Rigolet thrust zone in the Lake Melville Terrane. The significance of these pre-Grenvillian ages is at present not understood since they do not correlate with any major event (thermal or cooling) previously known in the Grenville Province. However, they are tentatively interpreted to date a closure of argon in hornblende at ca. 1250 Ma. Spectra obtained from samples from the southern Lake Melville Terrane record reasonably well defined ⁴⁰Ar/³⁹Ar plateaux. One sample yields a plateau age of 1070 +/- 50 Ma, which is in broad agreement with the 1030 Ma age suggested for the culmination of Grenvillian metamorphism in the Lake Melville Terrane and is thus interpreted to represent the very rapid uplift following Grenvillian thermal overprinting. The most southerly samples yield younger ages of 1020 +/- 8 Ma and 980 Ma +/- 12 Ma and are interpreted to results from slower post-Grenvillian uplift and subsequent cooling through the retention temperature required for argon in the hornblende crystal lattice.

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