Abstract
Petrologic modeling and in situ SHRIMP monazite geochronology reveal a complex, polycyclic tectonometamorphic history for Southampton Island, Nunavut, that provides further insights into the assembly of the Nuna supercontinent. Although early, cryptic metamorphic events at ca. 2.6–2.5 and 2.3Ga are indicated by monazite ages in an Archean metasedimentary sample, Paleoproterozoic tectonometamorphism is widespread and most profound. Monazite inclusions in garnet from two samples define a lower amphibolite- to upper amphibolite-facies M1–D1 event between 1883±9 and 1879±7Ma. These sampled rocks, as well as pre-1900Ma magmatic rocks, occur on the west side of the island's exposed Precambrian bedrock, outboard of the Hearne craton but on strike with its geophysically defined eastern edge beneath Hudson Bay. This timing and geographic distribution, as well as the orientation of S1 foliations, are consistent with the model of a ca. 1900Ma Rae–Hearne collision, with M1–D1 on Southampton Island reflecting reworking of the Rae upper plate following northeast-directed subduction.Textural variations among monazite dated in two samples tightly bracket the initiation of M2–D2 between 1868±5Ma and 1861±4Ma. M2–D2 monazite crystallized at 1851±5Ma in one sample with a more calcic composition interpreted to have shifted the allanite to monazite reaction to higher temperatures relative to less calcic rocks. Clockwise P–T paths in these recumbently folded high-grade gneisses support a collisional setting, with the timing of M2–D2 consistent with age constraints on Baffin Island for the accretion of Meta Incognita microcontinent to the southeast flank of the Rae craton. The Rae crust of Southampton Island is interpreted as the upper plate during this collision on the basis of the ∼10Myr earlier deformation on Southampton Island compared to the interpreted lower plate (Meta Incognita microcontinent), as well as textural observations in some samples that M2 metamorphism predated D2 deformation. Advective heat transfer would account for the relatively short time inferred between ca. 1870Ma collision and ca. 1868–1861Ma monazite growth during prograde metamorphism.Equant matrix monazite that overgrew the S2 foliation in one sample documents post-D2 metamorphism in the northern part of exposed basement at 1841±4Ma. Four samples from the south record post-D2 monazite growth between 1826±9 and 1815±7Ma. These ages highlight a thermal culmination associated with extensive crustal melting and ca. 1830–1820Ma plutonism attributed to a convergent setting culminating with ca. 1820Ma arrival of the Superior craton. Exhumation and cooling is dated by ca. 1790–1780Ma monazite associated with garnet breakdown.
Published Version
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