Petrologic evidence for a terrane boundary in south‐central Circle Quadrangle, Alaska

Abstract

Greenschist to amphibolite facies pelitic, mafic, and aluminous calc‐silicate schists from the south‐central Circle quadrangle, east‐central Alaska, are in contact with carbonaceous quartzite and calcareous phyllite recording diagenetic temperatures. Their mutual contact is best explained as a post‐metamorphic fault, indicating that the Yukon crystalline terrane was, at least in part, assembled after the peak of regional metamorphism. Mineral equilibria within the schists indicate metamorphism at minimum conditions of about 440° to 380°C at 0.3 GPa (3Kbar, biotite grade), 500°C at 0.5 GPa and XCO2 = 0.3 (garnet grade), and 550°C at 0.5 GPa and XCO2 = 0.2 (staurolite‐kyanite grade). Reflectance measurements on vitrinite and inertinite from the carbonaceous unit give temperatures of 170° to 230° ± 50°C.