Geochronology of Early to Middle Paleozoic Tectonic Development in the Southwest Newfoundland Gander Zone

Abstract

Southwest Newfoundland's Gander Zone was once considered part of the collided eastern margin of an early Paleozoic Iapetus ocean, distinguished from the rest of the Gander Zone by the extensive exposure of a polydeformed amphibolite to greenschist facies sequence considered Devonian in age, overlying presumed Precambrian basement. It was later found that truly Devonian strata were confined to the Cape Ray Fault system, the local northwestern boundary of the Zone. U-Pb zircon isotopic ages of and on metavolcanic rocks in the upper part of the sequence southeast of the fault system indicate that Ordovician and possibly older rocks underlie the Gander Zone itself. A temporal framework for the three main stages of regional deformation, corresponding to pre-metamorphic recumbent folding and transpression ( and ), is presented using U-Pb and dates of granitoid and metamorphic rocks. ) is bracketed as late Ordovician or Silurian by means of U-Pb zircon isotopic ages on the metavolcanic rocks and a 412 Ma U-Pb minimum age on a post- granitoid intrusion. Spatial domains of contrasting biotite and hornblende cooling ages reflect the and episodes, which led to post-metamorphic cooling following uplift of different segments of the amphibolite facies metamorphic terrane. Early Devonian differential uplift on the Cape Ray Fault in the northeast was associated with and caused rapid post-metamorphic cooling through Ar retention temperatures to yield nearly concordant biotite and hornblende ages between 374 and 388 Ma. Reverse and oblique-slip faulting during produced oblique northwesterly uplift of the still-buried portion of the amphibolite facies terrane against Devonian strata in the southwest part of the Cape Ray Fault zone. Late Devonian to early Carboniferous post-metamorphic cooling in this segment is confirmed by plateau dates of 345-353 Ma for biotite and 361 Ma for hornblende. The Gander Zone as a whole is imprinted with a strong penetrative structural grain similar in pattern to and structures in the southwest Newfoundland Gander Zone. K-Ar mineral dates are likewise earliest Devonian and younger, in contrast to numerous pre-Devonian K-Ar ages in the Humber and the Dunnage Zones. The Gander Zone, already identified as a wide, Acadian ductile shear belt and noted for widely variable metamorphic grade, might represent a mobile zone where coherent relics of the "Iapetus" or prior stages of Appalachian history were exhumed during Devonian orogeny, many after Silurian or earlier tectonic burial.