A Barrovian Overprint Superimposed on Partly Preserved High-Pressure Metamorphic Rocks: Pressure–Temperature–Time Evolution in the Ordovician Corner Brook Collisional Complex (Western Newfoundland, Canada)

Abstract The Corner Brook Collisional Complex in western Newfoundland represents an exhumed part of the partially subducted Humber margin of Laurentia metamorphosed at medium grade and intermediate to high-pressure conditions during collision with outboard arcs. It is interpreted as part of an extruded collisional wedge and consists mainly of metapelitic rocks and minor basement orthogneiss. Its subdivision into different metamorphic zones is supported by calculated PT pseudosections that display a continuous increase in metamorphic grade at intermediate-pressure conditions. The PT paths indicate an evolution at five stages: (I) Burial of peri-Laurentian continental crust after initiation of subduction at the end of the early Ordovician (~470 Ma) during obduction of oceanic crust, which initiated nucleation of garnet at 500°C to 550°C, 6.2 to 9.3 kbar. (II) Maximum, probably Taconic, burial at 520°C to 610°C, 8.2 to 11.7 kbar of one group of samples was followed (III) by extrusion and exhumation with simultaneous thermal relaxation. (IV) Subsequent imbrication within the complex during the Salinic orogeny was accompanied by extensive fluid flow due to prograde dehydration of a second group of rocks at mid-crustal levels during burial. A final stage of thermal relaxation (560–650°C, 6.9–9.2 kbar) created a Barrovian zonation. The ensuing observed peak temperature mineral assemblages are pervasive and only relicts of a former high-pressure stage are preserved locally. Interrelated PT paths of opposing direction can be related to mid-crustal stacking. (V) Finally, post-peak metamorphic folding and faulting of the Barrovian zones created an elongated metamorphic dome transected by brittle thrusts. U–Th–Pbtot dating of monazite inclusions in garnet of one sample yields an age of 438 ± 14 Ma interpreted as the time of garnet growth near peak temperature conditions at intermediate pressures. Also, two white-mica-bearing assemblages were dated by Rb/Sr mineral isochrones at 429 ± 6 Ma and 440 ± 6 Ma, respectively. The U–Th–Pbtot monazite and older Rb/Sr age combined with existing U/Pb ages of rutile suggest the presence of an older age population around 438 ± 4 Ma. Published 40Ar/39Ar ages of white mica and amphibole combined with the younger Rb/Sr age represent a younger age population around 427 ± 4 Ma. Both groups are related to successive separate phases of deformation and recrystallization near and/or after the thermal peak. Extensive fluid influx notably influenced the evolution of various segments of the complex but varied strongly between metamorphic zones.