First evidence of a Barrovian-type metamorphic regime in the Ross orogen of the Byrd Glacier area, central Transantarctic Mountains

The Selbome Group comprises two metamorphic rock units, the muscovite dolomite bearing Madison Marble and the biotite-muscovite quartz-calcite Contortion Schist, which contains thick tenses of variably deformed metabasalts and metaconglomerates. Petrological and structural data indicate a polyphase metamorphic evolution including: i) an early stage of upper greenschist regional metamorphism (P = similar to 0.15-0.3 GPa; T = similar to 380-450 degrees C), ii) prograde metamorphism during D, up to amphibolite facies peak conditions (P = 0.58-0.8 GPa, T = similar to 560-645 degrees C), iii) syn-D-2 unloading-cooling retrograde metamorphism, iv) a post-D-2 contact metamorphic overprint at variable T between 450 and 550 degrees C and similar to 0.2 GPa connected to the emplacement of granitic plutons and felsic dyke swarms. Geochronological data constrain the polyphase syn-D-1/D-2 evolution between similar to 510 and 492 Ma. A similar metamorphic path, including a medium P stage but at lower T conditions, is documented in greenschist facies metabasalts within the Byrd Group in the Mount Dick area. The metamorphic pattern and close lithostratigraphic matching between Selbome Group and Byrd Group sharply contrast with the high-grade Homey Formation that is exposed north of the Byrd Glacier and corroborate the hypothesis that the Byrd Glacier discontinuity marks a first-order crustal tectonic boundary crossing the Ross orogen.