Burwell domain of the Palaeoproterozoic Torngat Orogen, northeastern Canada: Tilted cross-section of a magmatic arc caught between a rock and a hard place

The northern segment of the Palaeoproterozoic Torngat Orogen is unique relative to the southern segment in that it is underlain by a 1910-1885 Ma suite of diorite-tonalite-granite rocks (DTG suite) emplaced into the margin of the Archaean Nain Province. Field and geochemical data indicate that it represents a subduction-generated Andean-type magmatic arc. Slivers of MORB-type, tholeiitic amphibolites and metasedimentary rocks within the DTG suite are interpreted as remnants of a backarc basin. Arc rocks are preserved in a tilted section from homogeneous, foliated tonalites at mid-amphibolite facies in the east, through gneissic tonalites, to massive orthopyroxene-bearing granitoid rocks of the Killinek charnockitic suite in the west. This assemblage is separated from the Archaean Rae Province, with which Nain Province collided, by a wide belt of turbiditic sedimentary rocks, now at granulite facies, known as the Tasiuyak gneiss. A younger suite (c. 1865 Ma) of mafic tonalites and megacrystic metagranites within the Nain arc suite represents either syn-collisional plutons or late additions to the continental magmatic arc. The southern segment of the orogen differs from the northern segment because it preserves no evidence of the DTG suite on Nain Province margin, but instead contains a widespread suite of calc-alkaline plutons across the Tasiuyak gneiss/Rae Province margin, dated as 1880 Ma. We develop two testable models to account for these along-strike variations and the presence of arc magmas on both sides of the suture. In Model A, a flip in subduction polarity from east-dipping at 1910-1885 Ma to west-dipping at c. 1885 Ma is invoked, in which the c. 1865 Ma suite of plutons in the northern segment are interpreted to represent syn-collisional magmas generated during accretion of the Burwell arc onto Nain Province. In Model B, the younger plutons in the northern segment may be interpreted as a late phase of the arc built on Nain Province, which we suggest may have been accomplished during a phase of double subduction immediately prior to collisional orogeny. The Nain-Rae collision at c. 1870-1860 Ma formed a doubly vergent thrust wedge, burying part of the arc in Burwell domain to depths of 35 km. During subsequent sinistral shear deformation at 1845-1822 Ma, Nain Province was buried obliquely beneath the still hot arc to depths of 40 km. At 1798-1780 Ma, crustal-scale disharmonie folding of the northern segment resulted in the simultaneous exhumation of the deeply-buried granulites of the arc and of the northwestern margin of the Nain Province across the Komaktorvik shear zone (KSZ). The KSZ was the site of significant translation between the Burwell domain and unreworked portions of the Nain Province, but it does not represent a fundamental plate boundary.