UHT-HP METAMORPHISM IN AN EXHUMED LOWER CRUSTAL "HOT ZONE"

The >400 km² Upper Deck domain is a 10 km-thick package of UHT-HP felsic and mafic granulites in the Athabasca granulite terrane, western Canadian Shield. Pseudosection modeling in the NCKFMASHT (Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂) system is consistent with the Ky- and Opx-bearing garnet-rich felsic mylonitic granulites representing restitic products of UHT-melting in thickened continental crust. Monazite inclusions in garnet are predominantly depleted in Y, Sm, and Gd and linked to fluid-absent melting of biotite + plagioclase + quartz at P>1.4 GPa. Circa 2.62-2.55 Ga Y-depleted, Th-rich monazite core domains are interpreted as a record of growth in the presence of melt, garnet, ternary feldspar, and Al-rich orthopyroxene at T>950°C via partial melting of Bt-orthogneisses, i.e., Bt + Pl + Qtz + high-Y Mnz → Grt + Opx + Kfs + Melt + low-Y Mnz. Circa 2.58-2.52 Ga low-thorium rim domains locally contain some of the highest positive europium anomalies yet documented in the literature. These rims are inferred to record the loss of plagioclase and recrystallization of orthopyroxene during melt-absent growth of high-pressure grossular-rich garnet concurrent with crustal thickening at 2539 ± 21 Ma (2σ, MSWD = 3.0, n = 4). We conclude that the Upper Deck domain constitutes a viable physical example of the mafic underplate/intraplate or “hot zone” commonly envisaged as the cause of ultrahigh-T metamorphism near the base of continental crust. Felsic granulites and garnetites in this domain are interpreted as the physical record of melt and garnetiferous restite production during emplacement of gabbroic sills. The presence of mafic granulites and crustal eclogites (derived from the intra-plated gabbroic sills), coupled with isobaric-cooling and densification of a garnet-rich package of rocks lends support for the domain also representing the remains of a thickened crustal root. Lower crustal “hot zones” (like the Upper Deck domain) represent one plausible mechanism for the vertical growth and evolution of continental crust through time.