Diachronous Palaeoproterozoic deformation and metamorphism in the Committee Bay belt, Rae Province, Nunavut: insights from ~(40)Ar-~(39)Ar cooling ages and thermal modelling

摘要

Regional-scale ~(40)Ar-~(39)Ar data presented in this paper reveal significant across-strike and along-strike age differences in the Committee Bay belt (CBb), Rae Province, Nunavut, Canada, that complement variations in observed monazite ages. ~(40)Ar-~(39)Ar hornblende ages are c. 1795, 1775, and 1750 Ma in the western, eastern and central parts of the Prince Albert Group (PAG) domain respectively. The migmatite domain and Walker Lake intrusive complex are characterized by c. 1750-1730 ~(40)Ar-~(39)Ar hornblende ages without significant along-strike variation. The ~(40)Ar-~(39)Ar data provide important constraints on the cooling history and on thermal modelling that elucidates the controls on diachroneity and metamorphic patterns within the belt. In the western CBb, prograde monazite growth occurred 26 ± 10 Myr earlier in the migmatite domain (1864 ± 9 Ma; peak P-T = 5 kbar-700 °C) than in the PAG domain (1838 ± 5 Ma; peak P-T = 5 kbar-580 °C). Calculations indicate that this earlier monazite growth results from tectonic thickening of higher heat productivity Archean lithologies in the migmatite domain, which undergoes more rapid prograde heating than the less radiogenetic and lower grade rocks of the PAG domain. Granite generation via biotite dehydration melting at 800 °C and 20 km depth is predicted to occur c. 1835 Ma, in agreement with geochronological constraints. The tectonic burial of crustal domains with contrasting radiogenic properties also explains the general congruence of lower to upper amphibolite facies metamorphic zones generated during the two main orogenic cycles (i.e. M_2-D_1 and M_3-D_2). The modelled timing of prograde monazite growth in the migmatite domain suggests that D_2 tectonic thickening began at 1872 ± 9 Ma, some 8 ± 3 Myr before monzazite growth, coeval with the inferred time of collision of the Meta Incognita terrane with the southern Rae Province. Along-strike diachroneity, reflected in 25 Myr younger monazite and ~(40)Ar-~(39)Ar hornblende ages in the eastern relative to the western PAG domain, cannot be accounted for by heat productivity contrasts along the belt. Instead the younger deformation and metamorphism in the eastern CBb was driven by its proximity to the eastern promontory of the Superior Province which collided with the Rae Province at c. 1820 Ma. The ~(40)Ar-~(39)Ar data presented here support the interpretation that the youngest monazite in the CBb crystallized at c. 1790 Ma in the central CBb when this part of the belt was downfolded into a gentle synformal structure while the western part of the belt cooled through ~(40)Ar-~(39)Ar hornblende closure. The results of this study illustrate the important influence of contrasting rock properties on the thermal evolution of orogenic belts and on the temporal record of this evolution.