Abstract

Penetratively cleaved slate–phyllite within supracrustal sequences (Seal Lake and Letitia Lake groups) in northernmost portions of the Grenville parauthochthon in central Labrador record 1018 ± 15 and 985 ± 14 Ma whole-rock 40Ar/39Ar plateau ages, which are interpreted as closely dating deformation and a concomitant low-grade metamorphism. The 40Ar/39Ar signature of Grenvillian tectonothermal effects is variably recorded in more southerly portions of the parautochthon. Locally within the extensive North Pole Brook igneous suite, there is no significant metamorphic retrogression. In these areas, igneous hornblendes record ca. 1650 Ma plateau ages similar to those suggested for emplacement of most of the suite by previously reported Rb–Sr whole-rock isochron and U–Pb zircon analyses. Low-temperature retrogression of igneous assemblages is observed throughout most of the North Pole Brook igneous terrane. Hornblende concentrates from five relatively undeformed samples are characterized by internally discordant 40Ar/39Ar spectra reflecting variable extraneous argon contamination and minor development of retrogressive blue–green amphibole. Biotite spectra are generally more concordant but display a wide range in total-gas and plateau ages (1534–2419 Ma) as a result of variable extraneous argon contamination. The North Pole Brook suite is progressively more deformed southwestward in the Grenville parautochthon, where it is largely represented by blastomylonitic orthogneisses that record hornblende plateau ages of 991–1013 Ma. Where not contaminated with extraneous argon, biotite records ages identical to those of coexisting hornblende, suggesting rapid post-Grenvillian metamorphic cooling. Paragneiss exposed in northeastern portions of the parautochthon displays a heterogeneous Grenvillian tectonothermal record. Muscovite within mylonitic rocks, developed along internal imbrication zones, yields plateau ages of 1046 and 1088 Ma. Hornblendes within amphibolite horizons in the paragneisses show little rejuvenation and record pre-Grenvillian thermal histories (1873 and 1315 Ma plateau dates). Hornblende within statically metamorphosed amphibolite near the structural base of the Wilson Lake allochthon records an early Grenville (1112 Ma) plateau age. Biotite within the allochthon has been previously shown to be regionally contaminated with extraneous argon components. The 40Ar/39Ar results support previous hypotheses that suggest that northwestward emplacement (at ca. 1100 Ma?) of the Wilson Lake and other allochthons was partially responsible for the burial and associated polydeformation and metamorphism of the Grenville parautochthon. The 985–1018 Ma low-grade metamorphism and penetrative deformation recorded in supracrustal sequences in the northernmost parautochthon are markedly younger and must reflect late Grenvillian orogenic activity with different tectonic implications.

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