Petrological and geochronological constraints on high pressure, high temperature metamorphism in the Snowbird tectonic zone, Canada

Abstract

The upper deck of the East Athabasca mylonite triangle (EAmt), northern Saskatchewan, Canada, contains mafic granulites that have undergone high P–T metamorphism at conditions ranging from 1.3 to 1.9 GPa, 890–960 °C. Coronitic textures in these mafic granulites indicate a near‐isothermal decompression path to 0.9 GPa, 800 °C. The Godfrey granite occurs to the north adjacent to the upper deck high P–T domain. Well‐preserved corona textures in the Godfrey granite constrain igneous crystallization and early metamorphism in the intermediate‐pressure granulite field (Opx + Pl) at 1.0 GPa, 775 °C followed by metamorphism in the high pressure granulite field (Grt + Cpx + Pl) at 1.2 GPa, 860 °C. U–Pb geochronology of zircon in upper deck mafic granulite yields evidence for events at both c. 2.5 Ga and c. 1.9 Ga. The oldest zircon dates are interpreted to constrain a minimum age for crystallization or early metamorphism of the protolith. A population of 1.9 Ga zircon in one mafic granulite is interpreted to constrain the timing of high P–T metamorphism. Titanite from the mafic granulites yields dates ranging from 1900 to 1894 Ma, and is interpreted to have grown along the decompression path, but still above its closure temperature, indicating cooling following the high P–T metamorphism from c. 960–650 °C in 4–10 Myr. Zircon dates from the Godfrey granite indicate a minimum crystallization age of 2.61 Ga, without any evidence for 1.9 Ga overgrowths. The data indicate that an early granulite facies event occurred at c. 2.55–2.52 Ga in the lower crust (c. 1.0 GPa), but at 1.9 Ga the upper deck underwent high P–T metamorphism, then decompressed to 0.9–1.0 GPa. Juxtaposition of the upper deck and Godfrey granite would have occurred after or been related to this decompression. In this model, the high P–T rocks are exhumed quickly following the high pressure metamorphism. This type of metamorphism is typically associated with collisional orogenesis, which has important implications for the Snowbird tectonic zone as a fundamental boundary in the Canadian Shield.

Petrology and fluid inclusions of garnet-clinopyroxene rocks from Paramati in the Palghat-Cauvery Shear Zone System, southern India

“…2b), which has been reported from some eclogites or HP granulites (e.g., Baldwin et al, 2003), is also consistent with the HP origin of the rocks. High CaEs end member mole in the integrated clinopyroxene (~ 8.6%), which is a common feature of HP and even ultrahigh -pressure metamorphic rocks (e.g., Nakano et al, 2007, and references therein), further confirm prograde HP metamorphism, although our clinopyroxene is significantly enriched in CaTs.…”

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

“…Their metamorphic PT parameters of 920-1000 °C at a minimum pressure of 18-20 kbar exceed the corresponding parameters in associated mafic granulites (890-960 °C and 13-19 kbar). The metamorphic processes proceeded in the inland environment; hence, the Snowbird zone is not a palaeosuture [Baldwin et al, 2003[Baldwin et al, , 2004.…”

High-pressure granulites from Cariré, Borborema Province, NE Brazil: Tectonic setting, metamorphic conditions and U–Pb, Lu–Hf and Sm–Nd geochronology

“…Del Lama et al, 2000;Zhao et al, 2001;Baldwin et al, 2003;Zhang et al, 2006;Zhou et al, 2008;Reno et al, 2009;Xiaochun et al, 2009;Sajeev et al, 2010) and are considered to be related to subduction and collisional events. In this context, particularly the metapelitic granulites play an important role because only tectonic processes of subduction and collision can bring those rocks formed on the earth surface into the deep level where they underwent high-pressure granulite facies metamorphism (Tam et al, 2011(Tam et al, , 2012.…”