Conditions of metamorphism in high-grade pelitic gneisses from the Frontenac Axis, Ontario, Canada

Abstract

The Frontenac Axis is a southeasterly extension of the Grenville province of the Canadian Shield, which connects to the Adirondack Mountains in New York. A mineralogic and petrologic study of the high-grade pelitic gneisses in three sampled areas in the Westport and Gananoque map areas was undertaken to examine the dynamic nature of metamorphic and postmetamorphic history in the area. The mineral assemblages indicate pressures of 4–6 kbar, (4–6 × 105 kPa), temperatures of 675–800°C, and [Formula: see text] during peak metamorphism based on experimentally determined phase equilibria. Chemographic relations reveal that the highest grade rocks in southeastern Ontario (cordierite–orthopyroxene zone) are in the northeast corner of the Gananoque map area and in the area northwest of Gananoque. On the scale of a thin section, chemical equilibration has not been achieved as supported by chemical zoning, chemical heterogeneity, and absence of exchange equilibrium. On the scale of a rock, graphical analysis indicates that the continuous variation in the Mg/(Mg + Fe) ratio of ferromagnesian phases (assemblage: biotite + sillimanite + cordierite + garnet + quartz + alkali feldspar) is in response to temperature, which conceals any evidence of local gradients in [Formula: see text]. Different temperatures are recorded across a rock because of reequilibration during cooling. The direction of displacement of reactions that produce (1) the corona textures; (2) the chemical zoning associated with corona textures; and (3) the chemical zoning related to exchange reactions between adjacent phases, is consistent with a decrease in temperature, pressure, or both. Intermingling of incompatible assemblages and the widespread occurrence of six-phase AFM assemblages are due to disequilibrium produced by incomplete reaction during uplift and cooling.

A grandidierite-sapphirine association from India

“…For example, the present author found small grains (0.01 x 0.05 to 0.04 x 0.18 mm) of grandidierite (Table I) associated with sillimanite, Lonker, 1980, andpers. comm., 1982), which is located in the Grenville Province.…”

“…In addition to the assemblages described here, grandidierite-kornerupine associations are reported from Natal (de Villiers, 1940) and Zambia (Vrfina, 1979), where kornerupine formed after grandidierite. In the pelitic gneiss from the Westport quadrangle, kornerupine is found in a layer free of sillimanite except for rare sillimanite inclusions in garnet (see also Lonker, 1980, Table I), while grandidierite is found sparingly with sillimanite in a contiguous layer. Tourmaline occurs in both layers.…”

Contrasting assemblages and petrogenetic evolution of corona and noncorona gabbros in the Grenville Province of western Labrador

“…Davidson also noted that there is a gradation of corona types from orthopyroxenepargasite (+ spinel) assemblages across the boundary zone between the Central Gneiss Belt and the Central Metasedimentary Belt. Furthermore, corona gabbros have not been recorded from the low-pressure granulite-facies region of the Central Metasedimentary Belt near the St. Lawrence River, where P has independently been estimated at 4-6 kbar (Lonker 1980).…”

Retrograde cation exchange in garnets during slow cooling of mid crustal granulites and the P-T-t trajectories from the Mont Laurier region, Grenville province Quebec

“…Upper amphibolite to granulite facies marbles with associated orthogneisses, amphibolites, quartzites and metapelites comprise the Frontenac Terrane/New York Lowlands (Figure 2 -VU;Carmichael et al 1978;Lonker 1980;Davidson 1986;Anovitz and Essene 1990). The metamorphic grade increases to the southeast, reaching low pressure, upper granulite facies in the Frontenac terrane.…”

“…(6) 3Fe2AUSi5Oi8 = 2Fe3Al 2 Si3Oi2 + 4Al 2 Si05 + 5SiO 2 Fe-cordierite aimandine aluminosilicate quartz Previous geobarometers based on the assemblage cordierite-garnet-sillimaiiite-quartz (Hutcheson et al 1974;Thompson 1976a;1976b;Holdaway and Lee 1977;Newton and Wood 1979;Martignole and Sisi 1981;Lonker 1981;Bhattacharya and Sen 1985) resulted in calculated pressures that disagreed (Bhattacharya 1986). Bhattacharya (1986) re-examined these geobarometers, and removed the inaccuracies that caused these disagreements.…”