Tectonometamorphic evolution along the Iapetus suture zone in Newfoundland: Evidence for polyphase Salinic, Acadian and Neoacadian very low- to medium-grade metamorphism and deformation

Abstract

The Red Indian Line (RIL) in central Newfoundland is the suture, where the main tract of the Iapetus Ocean was closed at ~452 Ma during accretion of the peri-Gondwanan Victoria arc with the composite active Laurentian margin. The protracted deformation history of this soft collision started at ~471 Ma with accretion of oceanic terranes to the active composite Laurentian margin. After Iapetus closure both colliding active margins were progressively deformed and metamorphosed during Silurian and Devonian (Salinic, Acadian and Neoacadian orogenic cycles).Peak conditions of the very low- to medium-grade, heterogeneously distributed metamorphism were determined by pseudosection techniques within the range of 2–7 kbar, 230–450 °C during increase of the metamorphic field gradient from ~12 °C/km to ~32 °C/km over time. Multiple metamorphic crystallisation stages were dated by white mica 40Ar/39Ar spot and plateau ages, additional Rb-Sr mineral isochrons involving white mica and one U/Pb age of titanite. All resulting ages between 439 ± 4 Ma and 356 ± 16 Ma postdate the closure of Iapetus.Results differ along two transects: The oldest ages of 443–421 Ma (Salinic orogenic cycle) were observed along the northern transect through the RIL zone with minimal younger overprint. Hence low temperature, intermediate to high pressure conditions (4.0–7.0 kbar, 230–340 °C) achieved during Taconic-Salinic underthrusting are well preserved. During Acadian dextral transpression the Taconic-Salinic structural wedge was tilted subvertically.In contrast, rocks along the southern transect through the RIL zone mainly show Acadian ages of 408–390 Ma with local preservation of older ages. Acadian deformation occurred under low temperature/low pressure conditions (~250–450 °C, 2.5–4.6 kbar). Also Silurian terrestrial cover rocks were buried under these conditions. Acadian-Neoacadian deformation (393–340 Ma) becomes younger towards the northwest and progressively localized in transcurrent fault zones. This final foreland deformation at shallow crustal level established the Acadian/Neoacadian orogenic front in central Newfoundland slightly northwest of the RIL.