Nd isotopic and geochemical signature of the Paleoproterozoic Trans-Hudson Orogen, southern Baffin Island, Canada: implications for the evolution of eastern Laurentia

Abstract

The southern Baffin Island segment of the Trans-Hudson Orogen comprises a three-part crustal architecture. The lowest structural level (level 1) includes ca. 2.88 Ga tonalite-diorite-granite orthogneisses of the parautochthonous Superior Province. Level 2 comprises monzogranite-tonalite-diorite orthogneisses of the 1.86–1.82 Ga Narsajuaq arc. At the highest structural level (level 3), ca. 1.95 Ga tonalite-monzogranite orthogneiss, ca. 1.93 Ga shelf metasediments of the Lake Harbour Group, and dominantly psammitic metasediments of the Blandford Bay assemblage are tectonically imbricated and cross-cut by 1.86–1.85 Ga monzogranitic to tonalitic rocks of the Cumberland batholith. Level 2 Narsajuaq arc felsic orthogneisses display strongly fractionated rare earth elements, with La n /Yb n >100, ε Nd (1.85 Ga) values ranging from −12 to −19, and T DM ages from 2.7 to 3.6 Ga, consistent with derivation by underplating and melting of strongly fractionated crust bearing components as old as Paleoarchean, such as parts of the Nain Province or the northernmost Superior Province. The Narsajuaq arc rocks on southern Baffin Island show more evolved Nd isotopic compositions and strongly fractionated REE profiles relative to correlated rocks on the Ungava Peninsula in northern Quebec. The north-south change in Nd isotopic and REE geochemical signatures is consistent with northward subduction beneath an Archean cratonic nucleus. At structural level 3, the peraluminous Cumberland batholith shows La n /Yb n mostly<50, negative Eu anomalies, weakly fractionated HREE, ε Nd (1.85 Ga) values from −2.7 to −7.4, and T DM ages from 2.35 to 3.08 Ga. The data are consistent with pervasive assimilation of Lake Harbour Group metasediments ( ε Nd (1.85 Ga)=−4.5 to −7.3) and lesser amounts of Ramsay River orthogneiss ( ε Nd (1.85 Ga)=−5.2 to −16.5). Lake Harbour Group Nd isotopic and REE compositions are similar to the Tasiuyak metasedimentary gneiss of Labrador, and support correlation of the two units. The Blandford Bay metasediments show Nd isotopic evidence for contribution of Mesoarchean detritus ( ε Nd (1.85 Ga)=−10.9 to −13.4), and may be derived from Archean Nain Province. The distinctive lithologies, Nd isotopic signatures, REE chemistry and crystallization ages of structural levels 2 and 3 metaplutonic rocks suggest that they represent two separate Paleoproterozoic terranes showing distinctive crustal evolution histories involving separate and distinctive Archean crustal nuclei. Tectonic models for the eastern Trans-Hudson Orogen require two spatially-distinct 1.86–1.82 Ga subduction systems reworking distinct Archean cratonic margins in order to account for the differences between Paleoproterozoic orthogneisses on southern Baffin Island. One subduction system would have produced the Narsajuaq arc of level 2, by pervasive assimilation of crustal rocks with ε Nd (1.85 Ga) lower than −11.6 and La n /Yb n >100. The differences between the less evolved Nd isotopic compositions of the Narsajuaq arc of Ungava Peninsula and the highly evolved Nd isotopic compositions of the Narsajuaq arc rocks on Baffin Island may explained by: (i) subduction beneath a northward-thickening Archean cratonic block; or (ii) by a relict north–south age boundary in the Archean cratonic margin. A second subduction zone would have produced the Cumberland batholith by melting and assimilation of a cratonic margin less fractionated REE and more elevated ε Nd values. Data from structural levels 2 and 3 are consistent with correlation of these rocks across the eastern segments of the Trans-Hudson Orogen.