Committee Bay Belt Research Articles

Deformation history and tectonic significance of the Sanagak Lake shear zone, Boothia Peninsula, Nunavut

The recently recognized Sanagak Lake shear zone (SLsz) is a 165 km long, southwest striking corridor of high-strain rocks that transects the southern portion of Boothia Peninsula, Nunavut. This zone records pervasive deformation (DSL1) at conditions of ∼0.52 GPa and ∼700 °C, and localized deformation (DSL2) at ≥0.5 GPa and 300–500 °C that preserve left lateral and right lateral senses of movement, respectively. Neocrystallized DSL1 titanite in a hornblende-bearing granodiorite yields an age of 1804 ± 6 Ma, interpreted to be the timing of DSL1. The timing of DSL2 is loosely bracketed by 40Ar/39Ar hornblende (1814 ± 3 Ma) and biotite (1743 ± 1 Ma) cooling ages since the deformation temperature falls between the estimated closure temperature of these minerals. Similar rock types and metamorphic conditions on either side of the shear zone rule out the SLsz as a terrane boundary. Rather, strain localization may have been triggered by thermal softening related to the emplacement of a northeast-trending belt of high-temperature granites south of the shear zone between 1840 and 1820 Ma. Deformation and metamorphism at ca. 1.81 Ga south of Boothia Peninsula and in the central Rae (Committee Bay belt) have been attributed to the Superior Province colliding with the southeastern margin of the Rae craton, such that the SLsz may too have formed in response to far-field stresses derived from this collision. The absence of ca. 1.81 Ga tectonic fabrics north of the shear zone indicates that the SLsz marks the northwestern extent of mid-crustal, Trans-Hudson related tectonometamorphism.

Neoarchean continental crust formation and Paleoproterozoic deformation of the central Rae craton, Committee Bay belt, Nunavut

Integrated mapping, structural analysis, and U–Pb geochronology of the Committee Bay area, Nunavut, establish a record of Neoarchean crustal growth followed by penetrative Paleoproterozoic deformation. Supracrustal rocks include a lower ca. 2.73 Ga mafic–ultramafic volcanic-dominated sequence, a middle, economically significant 2.71 Ga intermediate volcanic-bearing sequence with intercalated sulphidized, gold-bearing iron formation, and an upper <2.69 Ga clastic ± komatiite–quartzite sequence. Following an 80 million year hiatus, this succession was intruded by voluminous ca. 2.61–2.57 Ga granodiorite–tonalite–granite ± diorite, which do not appear to have thermally or tectonically affected the supracrustal belt. Instead, three generations of structures record polyphase Paleoproterozoic deformation of the region. D1 structures are consistent with a doubly vergent structural fan developed at ca. 2.35 Ga in response to the Arrowsmith orogeny that affected the western Rae margin. Penetrative D2 structures dominate the map pattern and include northeast-trending, southeast-dipping folds and fabrics within which gold is localized. The general southeast dips of S2 and inclined, northwest-vergent attitude of F2 reflect northwest-directed shortening at 1.84–1.82 Ga. The absence of syn-D2 plutonic rocks in the west and central Committee Bay belt support amphibolite-facies metamorphism as a response to crustal thickening, which, in turn, led to syn-D2 crustal melting in the east. Regionally extensive upright to northwest-vergent D2 structures and associated ca. 1.85–1.82 Ga tectonometamorphism across the Rae craton are attributed to an early stage of the Hudsonian orogeny involving microcontinent collision(s) with its southeastern margin. D3 folds and dextral shearing at ca. 1780 Ma accommodated localized, late-stage compressional strain during final amalgamation of Laurentia.

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

AbstractRegional‐scale 40Ar–39Ar 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. 40Ar–39Ar 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 40Ar–39Ar hornblende ages without significant along‐strike variation. The 40Ar–39Ar 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. M2–D1 and M3–D2). The modelled timing of prograde monazite growth in the migmatite domain suggests that D2 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 40Ar–39Ar 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 40Ar–39Ar 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 40Ar–39Ar 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.

Petrological and in situ SHRIMP geochronological constraints on the tectonometamorphic evolution of the Committee Bay belt, Rae Province, Nunavut

Petrology and in situ SHRIMP monazite geochronology provide new constraints on the polymetamorphic and polydeformational history of the Committee Bay belt, Rae Province, Nunavut, Canada. Rare ca. 2.58 Ga monazite inclusions in garnet porphyroblasts in the eastern part of the belt are interpreted to reflect regional contact metamorphism (M 1) associated with voluminous 2.61–2.58 Ga plutonism. Monazite also occurs as ca. 2.35 Ga inclusions in andalusite in the central supracrustal belt, but is absent from the east. These data complement previous studies to the west, and suggest that the Committee Bay belt represents a ca. 2.35 Ga tilted crustal section that was tectonically thickened ± imbricated during the Arrowsmith orogeny (D 1–M 2). Penetrative reworking of the belt occurred during a diachronous D 2–M 3 tectonometamorphic event which involved northwest-vergent thickening to depths of ∼5 kbar across most of the belt. Although D 2 folds and fabrics across the belt are geometrically correlative, monazite is ∼20 Myr older in the western migmatite domain (ca. 1.86 Ga) than in the western supracrustal belt (ca. 1.84 Ga). This temporal difference is attributed to burial of more radiogenic lithologies in the migmatite domain. Tectonic thickening is attributed to ca. 1.87 Ga collision of Meta Incognita microcontinent with the southeastern flank of the Rae Province. The eastern supracrustal belt exposes similar amphibolite-facies (M 3) northeast-striking, northwest-vergent D 2 structures that are dated at 1.815 Ga. This younger reworking may reflect the influence of a major northward projecting promontory of the Superior Province which continued to affect the softened hinterland of the Rae Province during terminal collision at ca. 1.82 Ga. The central Committee Bay belt records a younger, ca. 1.79 Ga structural overprinting (D 3–M 4) which involved dextral shearing and differential burial to temperatures above monazite growth in a gentle synformal structure formed during the final stages of amalgamation of Laurentia.