Petrology and Evolution of an Archaean Metamorphic Aureole in the Slave Craton, Canada

• Published in: Journal of Petrology
• Main Authors: RAMSAY, C. R., KAMINENI, D. C.
• Format: Text
• Language: English
• Published: Oxford University Press 1977
• Subjects: Articles; Canada; Yellowknife
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/18/3/460; https://doi.org/10.1093/petrology/18.3.460

In the Slave Craton of northern Canada, extensive areas were metamorphosed in broad aureoles (typically ca. 10–15 km wide) around granitie batholiths emplaced about 2575 m.y. ago. Meta-greywackes and meta-pelites from two areas traversing one of these aureoles near Yellowknife have been studied. New petrographic data are given and integrated with previously published mineralogical data to elucidate the metamorphic history of the area. Metasediments in the aureole contain the concentrically zoned succession of index minerals chlorite, biotite, cordierite, gedrite, andalusite, sillimanite. In addition, garnet, staurolite, and paragenetically late andalusite occur more irregularly, and cummingtonite characterizes subordinate calcic rock-types. The chemistry of all these minerals is given and their origins discussed. The aureole evolved by the development and decay of a thermal dome. This was a continuous process, but three recognizable metamorphic phases can be correlated as follows with established deformational phases. The cycle began with a deformation phase (D 1 ) unaccompanied by metamorphism. This evolved into D 2 which was accompanied by broad regional metamorphism M 2 (characterized by the index succession chlorite, biotite, garnet, staurolite) as thermal doming began. With continued updoming of the isotherms, the third phase (D 3 ) produced only minor folding but caused major metamorphic recrystallization (M 3 ), culminating in the emplacement of granite at the core of the thermal dome. A concentric zonation of the metamorphic index minerals biotite, cordierite, gedrite, andalusite+sillimanite was superimposed on earlier assemblages. This M 3 phase occurred at lower pressure (2.5–3.5 kb) than M 2 because of erosional unloading, but the temperatures were more extreme, ranging up to about 700 °C. With deformation then complete, the thermal dome decayed, and minor mineralogical changes occurred in this (M 4 ) decay phase. The region has since been effectively stable.