MID ORDOVICIAN METAMORPHISM IN THE GANDER ZONE (CENTRAL NEWFOUNDLAND, CANADA): BACK-ARC RELATED CRUSTAL ANATEXIS?

The Mount Cormack Subzone in Newfoundland is a tectonic window of Gander zone rocks surrounded by Tremadocian Penobscot ophiolites (ca. 492 Ma) and stitched by Arenig granites. Migmatitic quartzo-feldspathic gneiss, amphibolite and orthogneiss occupy the core of this subzone. A previously reported U-Pb zircon age of 462 ± 3 Ma from a migmatitic gneiss, supposedly dated metamorphism in this basement window. This rock has striking lithological similarities to Siluro-Devonian upper amphibolite facies Gander Zone rocks of southwest Newfoundland. To test the age of metamorphism we analyzed monazite and titanite from an anatectic melt pocket and an amphibolite respectively. The monazite provided a concordant age of 462 ± 1 Ma, which is within error with the titanite age of 460 ± 4 Ma of the amphibolite. These new ages confirm the previous interpretation of the zircon age and corroborate that crustal anatexis in the Mount Cormack Subzone was coeval with the widespread 470-460 Ma felsic volcanism in the surrounding Exploits Subzone of the Dunnage Zone. This reinforces the hypothesis that crustal melting was a result of a high geothermal gradient linked to the Arenig / Llanvirn back-arc extensional events, and suggests that the Gander basement is the source region of the felsic melts extruded in the overlying Exploits Subzone.