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The collaborative federal–provincial Southern Indian Lake project in north-central Manitoba covered an area of more than 3500 km2 of the Trans-Hudson orogen. Regional-scale geological mapping, sampling, and lithogeochemical, isotopic, and geochronological studies resulted in the identification of distinct assemblages of supracrustal rocks and varied episodes of plutonism. A granodiorite gneiss dated at ca. 2520 Ma is interpreted to represent the basement of the Southern Indian domain and is considered a separate crustal domain, named the Partridge Breast block. The Churchill River assemblage is composed of juvenile pillow basalt with intervening clastic sedimentary rocks, possibly a reflection of plume magmatism related to initial rifting of the Hearne craton margin. The Pukatawakan Bay assemblage consists mainly of massive to pillowed, juvenile metabasaltic rocks and associated basinal metasedimentary rocks. The Partridge Breast Lake assemblage is dominated by continental-arc volcanic and volcaniclastic rocks associated with basinal metasedimentary rocks. The Strawberry Island assemblage, consisting of arenite and polymictic conglomerate, is interpreted to have been deposited in a foreland-basin basin or intra-orogen pull-apart basin environment. The Whyme Bay assemblage is characterized by fluvial-alluvial orogenic sediments and is temporally linked to the Sickle Group rocks in the Lynn Lake greenstone belt. Granitoid rocks, dominantly monzogranite and granodiorite, range in age from ca. 1890 to 1830 Ma and occur throughout the Southern Indian domain, and intermediate and mafic intrusions of similar ages are also present. In this paper, we integrate these new data into a tectonic framework for the Southern Indian domain of the Trans-Hudson orogen in Manitoba. [ABSTRACT FROM AUTHOR]

The Flin Flon – Athapapuskow Lake area, situated in the Flin Flon Greenstone Belt, Manitoba, consists of ocean-floor and island-arc assemblages, deformed and metamorphosed during the Trans-Hudson Orogeny (∼1.86–1.69 Ga). A new map of metamorphic mineral assemblages and isograds has been compiled that reveals a largely coherent regional metamorphic sequence increasing in metamorphic grade from prehnite–pumpellyite to amphibolite facies. Regional metamorphism postdates most of the deformation within the area, with the exception of the reactivation of major block-bounding faults. The regional prograde sequence has been subdivided into 10 metamorphic zones, separated by 9 isograds, that describe the transition from prehnite–pumpellyite to greenschist to amphibolite facies. The formation of contact metamorphic aureoles, pre-dating regional metamorphism, record conditions up to amphibolite facies. Equilibrium phase diagrams for the island-arc (low-Mg) and ocean-floor (high-Mg) assemblages were calculated and allow for the evaluation of the modelling techniques and determination of pressure–temperature conditions. Discrepancies between the modelling predictions and natural observations occur due to (1) limitations in the thermodynamic models for some of the complex minerals (e.g., amphibole); and (2) metastable persistence of some minerals to higher grade due to sluggish reaction kinetics. Notwithstanding these discrepancies, the modelling suggests that metamorphosed mafic rocks in the Flin Flon – Athapapuskow Lake area reached about 430–480 °C and 3.0–4.5 kbar. Peak metamorphic conditions within contact aureoles that preceded regional metamorphism did not exceed 500 °C (at a pressure between 2.7 and 4.4 kbar). The metamorphic field gradient records a transition from 250–300 °C/1.5–2.3 kbar to 430–480 °C/3–4.5 kbar (100–150 °C/kbar), defining a geothermal gradient of approximately 25–31 °C/km. [ABSTRACT FROM AUTHOR]

In western Superior province, the North Caribou terrane (NCT) constitutes a Mesoarchean proto-continent heavily overprinted by Neoarchean magmatism and deformation resulting from the western Superior Province accretion. Locally, along the southern margin of the NCT, Mesoarchean (~3.0 Ga) rift sequences are preserved. These sequences are of key importance to our understanding of the early tectonic evolution of continental crust. The Wallace Lake greenstone belt is located at the southern margin of the NCT and includes the Wallace Lake assemblage, the Big Island assemblage, the Siderock Lake assemblage, and the French Man Bay assemblage. The Wallace Lake assemblage exposes one of the best-preserved Mesoarchean rift sequences along the southern margin of the NCT. The volcano-sedimentary assemblage (3.0–2.92 Ga) exposes arkoses derived from the uplift of a tonalite basement in a subaqueous environment, capped by carbonate and iron formation. Mafic to ultramafic volcanic rocks exhibiting crustal contamination and derived from plume magmatism cap this rift sequence. The Wallace Lake assemblage exhibits D1 Mesoarchean deformation. The Big Island assemblage comprises mafic volcanic rocks of oceanic affinity that were docked to the Wallace Lake assemblage along northwest-trending D2 shear zones. The timing of volcanism and docking of the Big Island assemblage remain uncertain. The Siderock Lake and French Man Bay assemblages were deposited in strike-slip basins related to D3 and D4 stages of movement of the transcurrent Wanipigow fault (<2.709 Ga). Regionally, the Wallace Lake assemblage correlates with the Lewis–Story Rift assemblage observed in Lake Winnipeg, whereas the Big Island assemblage appears to correlate with the Black Island assemblage observed in the Lake Winnipeg area. Thus, the North Caribou terrane appears to preserve vestiges of a Mesoarchean rifted succession together with overlying Neoarchean allochthonous, juvenile, volcanic successions over a considerable distance along its present-day southern margin. [ABSTRACT FROM AUTHOR]

A combined U–Pb and field mapping study of the Island Lake greenstone belt has led to the recognition of three distinct supracrustal assemblages. These assemblages record magmatic episodes at 2897, 2852, and 2744 Ma. Voluminous plutonic rocks within the belt range in age from 2894 to 2730 Ma, with a concentration at 2744 Ma. U–Pb data also show that a regional fault that transects the belt, the Savage Island shear zone, is not a terrane-bounding structure. The youngest sedimentary group in the belt, the Island Lake Group, has an unconformable relationship with older plutons. Sedimentation in this group is bracketed between 2712 and 2699 Ma. This group, and others similar to it in the northwestern Superior Province, is akin to Timiskaming-type sedimentary groups found throughout the Superior Province and in other Archean cratons. These data confirm that this belt experienced a complex geological history that spanned at least 200 million years, which is typical of greenstone belts in this area. Age correlations between the Island Lake belt and other belts in the northwest Superior Province suggest the existence of a volcanic "megasequence". This evidence, in combination with Nd isotopic data, indicates that the Oxford–Stull domain, and the Munro Lake, Island Lake, and North Caribou terranes may have been part of a much larger reworked Mesoarchean crustal block, the North Caribou superterrane. It appears that the Superior Province was assembled by accretion of such large independent crustal blocks, whose individual histories involved extended periods of autochthonous development. [ABSTRACT FROM AUTHOR]

Western Superior Lithoprobe seismic-reflection line 1 exhibits a broad region of northward-dipping reflectors in the Uchi subprovince, which gives way to southward-dipping reflectors farther north in the Berens River sub province. Mafic metavolcanic rocks across the region of northward-dipping reflectors exhibit a decline in metamorphic pressure, from pressures of 6 kbar (1 kbar = 100 MPa) in the south to only 2 kbar 80 km to the north. This indicates that the southern edge of the Uchi subprovince has undergone significantly more unroofing than regions farther north. The differential unroofing is not consistent with a doubly vergent thrusting origin for the northward- and southward-dipping reflector pattern. It could result from a crustal-scale synform, of which the region of northward-dipping reflectors would make up the southern limb. Metamorphic pressures from samples off the seismic line, however, provide only limited support for a regional synform, and suggest that much of the pressure variation may result from deformation associated with motion on late faults that are widespread in the western Superior Province. These faults occur in a WNW-striking set with dextral offsets and an ENE-striking set with sinistral offsets. They could result from north–south compression and east–west extension, provided the faults have rotated towards the east–west direction during deformation. Regional tilting and (or) jostling of crustal blocks is attributed to deformation associated with the fault rotation. Motion on the faults and the associated deformation of intervening fault blocks may be important contributors to the present crustal architecture of the western Superior Province, including the surface distribution and form of the greenstone belts. [ABSTRACT FROM AUTHOR]

Evidence for both vertical and horizontal movement is well preserved in the Cross Lake greenstone belt in the northwestern Superior Province. The vertical movement components are concentrated in high-strain zones along pluton–greenstone contacts and are characterized by pluton-side-up or greenstone-side-down movement, and the geometry, kinematics, and strain distribution are consistent with a vertical tectonic model involving diapirism and sagduction. The horizontal components are concentrated in major east-southeast-trending dextral high-strain zones and in subordinate northeast-trending sinistral, antithetic high-strain zones and can be readily explained by a horizontal tectonic model involving dextral transpression. Results of a detailed structural analysis indicate that the vertical and horizontal tectonism were more or less synchronous, and there was a transition from dominantly vertical tectonism at the early stages to dominantly horizontal tectonism at the late stages. The Cross Lake Group, consisting of Timiskaming-type sedimentary rocks, was deposited in a synclinal keel between granitoid domes associated with vertical tectonism. It is suggested that synchronous vertical and horizontal tectonism was a common process in the Neoarchean and might represent a transition from dominant vertical tectonism in the Mesoarchean (and Paleoarchean?) to dominant horizontal tectonism in the Proterozoic and Phanerozoic. [ABSTRACT FROM AUTHOR]