Grenvillian Metamorphism of Monocyclic Rocks, Georgian Bay, Ontario, Canada: Implications for Convergence History

Abstract

The Parry Sound and Shawanaga domains of the Central Gneiss Belt along Georgian Bay, Ontario, contain monocyclic rocks that originated at or near the southeastern margin of Laurentia between ca. 1450 and 1120 Ma. Their deformation and metamorphism are entirely attributable to Grenvillian orogenesis. Metamorphic assemblages, fabrics, and P-T-t paths from these rocks therefore provide important constraints on Grenvillian thermal and tectonic history. Rocks in the interior and basal Parry Sound assemblages of the northern Parry Sound domain were metamorphosed to granulite-facies conditions during an early phase of Grenvillian tectonism, i.e., at ca. 1161 and 1163 Ma, respectively. The most likely setting for high-P -high-T granulite-facies metamorphism in the interior Parry Sound assemblage was at or near the base of crust that was underplated by voluminous mafic magma. In contrast, heat advected from anorthosite could account for intermediate-P granulite-facies metamorphism in the basal Parry Sound assemblage. In the upper part of the basal Parry Sound assemblage, retrogression from the granulite to upperamphibolite facies likely occurred in response to thrust emplacement of the interior Parry Sound assemblage onto this part of the basal Parry Sound assemblage at ca. 1159 Ma. In the lower part of the basal Parry Sound assemblage, thrust deformation and reequilibration at lower-amphibolite-facies conditions took place at ca. 1120 Ma. In the southern Parry Sound domain, a highly attenuated sequence of quartzites and mafic rocks, deposited some time after 1140-1120 Ma, was affected by upper-amphibolitefacies metamorphism before or at 1080 Ma. Northwest of and structurally below the Parry Sound domain, rocks in the Shawanaga domain were affected by eclogite-facies metamorphism at ca. 1090-1085 Ma, suggesting deep burial or partial subduction of the Laurentian margin beneath the Central Metasedimentary Belt at this time. Widespread upper-amphibolite-facies metamorphism after ca. 1080 Ma was associated with a major phase of northwest-directed thrusting and crustal thickening. Sillimanite-grade conditions in the Shawanaga domain were maintained until at least ca. 1020 Ma, the time of major extensional deformation. Data on metamorphic grade and age are consistent with progressive northwest-directed juxtaposition of lithotectonic assemblages metamorphosed at progressively later times, and may thus record progressive or multistage convergence at the southeastern margin of Laurentia. Evidence for multiple phases of Grenvillian high-grade metamorphism in the Parry Sound and Shawanaga domains suggests that construction and interpretation of P-T-t paths from these rocks require careful assessment of timing and overprinting relationships. Derived P-T-t paths suggest that exhumation of interior Parry Sound granulites resulted from thrusting soon after peak metamorphism, whereas those for rocks of the Shawanaga domain suggest that exhumation was likely associated with both thrusting and extension.

Age and isotopic geochemical characteristics of Archean carbonatites and alkaline rocks of the Baltic shield

“…The next stage (onset of tectonic exhumation) corresponds to the temperature range from 700 to 800 °C and pressures of 10-17 kbar [Cox et al, 2002;Indares, Dunning, 2004]. In the southwestern Grenville Province, the highest pressure and temperature are confined to the bases of tectonic slices, and lower values are related to their inner regions [Wodicka et al, 2000]. The metamorphism of rocks in the Para-autochthonous belt proceeded, as a rule, at a moderate pressure, whereas locally occurring high-pressure granulite and eclogite mineral assemblages were formed at pressures exceeding 12 kbar.…”

Post-convergent structures in lower parts of the 1090–1050 Ma (early-Ottawan) thrust-sheet stack, Grenville Province of Ontario, southern Canadian Shield

“…The thicknesses of such boudinaged layers might correspond, for example, to those of individual lithostructural domains delineated in the CGB ( Fig. 3; Culshaw et al 1997;White et al 2000;Wodicka et al 2000). We address this information gap, and the timing of boudinage, in this study.…”

“…In the western Grenville Province, the stack includes parts of the Central Gneiss Belt (CGB; Fig. 3), which was assembled at ϳ1090 Ma, as well as the previously imbricated Parry Sound domain, which is composed of 1160 Ma granulite-facies thrust sheets (Davidson et al 1982;Wodicka et al 1996Wodicka et al , 2000Culshaw et al 1997) and may originally have comprised part of the Composite Arc Belt (CAB) to the south . Recently, Culshaw et al (2010) concluded that the Parry Sound domain acquired an amphibolite-facies ductile sheath of well-layered gneiss during early-Ottawan northwest-directed thrusting that overprinted ϳ1160 Ma shear-zone fabrics at the margins of the domain.…”

Meso-Neoproterozoic Grenville-Sveconorwegian intracontinental orogen: history, tectonics, geodynamics

“…Для объяснения наблюдаемых соотношений пород различного уровня метамор-физма Н. Водичка с коллегами предполагают слож-ную последовательность взаимных надвиганий и погружений тектонических пластин в процессе ак-креционных и коллизионных событий [Wodicka et al, 2000]. Между тем приведенные в статье этих ав-торов геологические карты четко фиксируют при-уроченность наиболее высоких значений давления и температуры к основанию тектонических чешуй, а более низких значений -к их внутренней области.…”