Kenoran Orogeny Research Articles

Implication of deformation fabrics of schist-migmatite-gneiss and granite in understanding regional tectonics: Eastern Dharwar Craton (EDC), India

Structural fabrics of schist belt, gneiss, migmatites and younger granitoids are different. Various morphology of migmatites is recognized for the first time in the southern part of EDC along with gneiss. The younger granite plutons are examined from northern through central to southern part of EDC. The paleosome-dominated portions of migmatite were possibly formed from low degree partial melting of older rocks and identified as metatexites. Whereas, the neosome dominated parts were developed by complete melting and marked as diatexites. There is a link between diatexite and granite in the southern part, which is chracterized as catazone segment. The four sub-types of metatexites are observed (viz, patch, dilatant, net and stromatic). The diatexites are dominated by two major structures (viz, schollen or raft and schlieren). Gneisses show lit-per-lit as well as fold and leucocratic material flow features. The litho-structural variation from north to south connotes systematic changes from epizone through mesozone to catazone. The qualitative study of mineral content and textural aspects from rocks at different localities suggest preserved portions of different parts of a pressure–temperature path. However, exact quantification should be possible after thermo-barometry and pseudo section study. The present contribution focuses on documentation of classic representative outcrops and petrographic features of gneissic and migmatitic rocks of EDC. It is observed that the six (6) major ductile deformation stages of Kenoran orogeny implies ductile signature before granite plutonism. Subsequently post granite emplacement deformation phenomenon indicates a transition from ductile to brittle regime and entrance into Hudsonian orogeny.

Structures and Their Tectonic Implications of the Southern Part of Cuddapah Basin, Andhra Pradesh, India

Systematic structural observations in field outcrops can give clues to interpret orogenic events. Deformational structures studied in basement complexes as well as Cuddapah basinal sedimentary rocks indicate at least three events. The formation of N–S trending greenstone belts on gneissic basement complex is related to the oldest Kenoran orogeny. The later developed Hudsonian orogeny is related to opening of basin and numerous younger pluton as well as mafic dike emplacements. The Greenville or Eastern Ghats Orogeny is most intensively imprinted in the Cuddapah basin sedimentary rocks. The crescent shape of the Nallamalai fold belt in the east is due to the E–W compression related to thrusting followed by NNE–SSW compression. The E–W maximum compression (σ1) is related to the Eastern Ghats Orogeny, which is responsible for the development of several sympathetic E–W faults due to N–S extension (σ3). The formation of the Palaeo–Mesoproterozoic supercontinent Columbia is related to the E–W compression and the Napier Complex of east Antarctica connected with India occupying the indentation of the E part of the Cuddapah basin. Palaeostress study from the southern part of Cuddapah basin reveals that the initial E–W compression and associated fracturing produced E–W extensional regime, and subsequent NNE–SSW shear developed conjugate fracture set. However, the initial E–W compression was associated with the N–S extension component to develop broadly E–W parallel normal faults such as Vempalle fault and later strike-slip faults related to changes in the stress field. The proposed model explains the development and evolution of the Cuddapah basin as a new insight into the regional geodynamics.

Emplacement and deformation features of syntectonic leucocratic veins from Rainy Lake zone (Western Superior Province, Canada)

Networks of leucocratic veins are widespread in the Rice Bay–Redgut Bay domain of the Rainy Lake zone at the Wabigoon–Quetico subprovince boundary (Archean Superior Province, Canada). Vein geometries and their cross-cutting relationships with host rocks are indicative of syntectonic emplacement within a regional late Archean transpressional deformation event (Kenoran orogeny). The relative folding and boudinage of the veins compared to their host rocks indicate that they were emplaced syntectonically with the D 2 deformation. Semi-quantitative strain and vorticity analyses confirm the syn-D 2 timing of the veins. The observed structural features are interpreted as being formed during magma emplacement along dilatant fractures and subsequent solid-state deformation under overall ductile deformation conditions. The structures related to emplacement are better preserved in domains of low post-emplacement strain and veins intruded into competent host rocks, whereas the folds and boudins related to post-emplacement deformation are well developed in veins intruded into relatively incompetent host rocks. In domains of high strain, the primary intrusive structures are modified or even locally obliterated due to increasing deformation. It is envisaged that regional kinematics and the rheological state of host rocks are the main controls on the style of vein emplacement and deformation. The variety of observed structures is considered the result of deformation and lithological heterogeneity rather than several distinct magmatic events.

Crustal development at the pre-Grenvillian Laurentian margin: a record from contrasting geochemistry of mafic and ultramafic orthogneisses in the Chochocouane River area, Quebec

Mafic and ultramafic orthogneisses from the western Grenville Province of Quebec adjacent to the Archean Superior Province (Canada) display geochemical characteristics that highlight processes that shaped the pre-Grenvillian Laurentian margin through late Archean to Mesoproterozoic time. Group I mafic and ultramafic granulites of the Parautochthonous Belt compositions overlap Archean Abitibi metavolcanic rocks (komatiites to Fe-rich andesites) supporting their autochthonous character. Limited variation of incompatible trace-element ratios with increasing differentiation suggests minimal sialic crust assimilation during petrogenesis and probable formation in an oceanic setting. Group II mafic tholeiites occur within both the Parautochthonous Belt and the Allochthonous Polycyclic Belt. Their compositions differ significantly from those of Archean rocks but overlap those of Proterozoic and modern continental tholeiites. Group III mafic calc-alkaline amphibolites, predominantly in the Allochthonous Polycyclic Belt, display chemical characteristics expected of magma derivation from subducted oceanic crust. In contrast to group I Archean meta-igneous rocks metamorphosed during the Kenoran orogeny, group III rocks do not record an early phase of large-ion lithophile element (LILE) metasomatism and granulite-facies metamorphism. As such, their emplacement is interpreted to postdate the Archean Kenoran metamorphism and to be Proterozoic in age. Group I rocks reflect an Archaean ensimatic greenstone belt environment; group II tholeiites reflect a Proterozoic continental rift environment; and group III rocks reflect a subduction-related source, possibly related to Andean-style, calc-alkaline magmatism.

Autochthonous development of Superior Province greenstone belts?

Models for Superior Province greenstone belt development have changed over the last 30 years from ensimatic, generally autochthonous models to suspect allochthonous terrane models based on the plate tectonic paradigm. Models intermediate between these end-members such as autochthonous terrane models have not been prominent. Within the Superior Province, there is a secular variation from early ca. 3 Ga platformal assemblages with overlying rift-related komatiite–tholeiite units representing breakup of ca. 3 Ga continental fragments to late (ca. 2.7 Ga) pull-apart basins. Autochthonous terranes should demonstrate some sort of logical order such as that described above, whereas allochthonous terranes will be largely oceanic in character and should be essentially random in their occurrence rather than exhibiting secular order. In the face of this situation, this paper reviews evidence from throughout the Superior Province that suggests many, but not all greenstone assemblages developed autochthonously or parautochthonously. The earliest greenstone assemblages in the Superior Province are generally quartzite and carbonate-bearing platform sequences in the North Caribou terrane and the Marmion terrane deposited unconformably upon granitoid or greenstone basement. The evidence for autochthonous/parautochthonous development includes the presence of unconformities between Keewatin type greenstone units such as: (1) basal unconformities between ca. 3 Ga granitoid or volcanic basement and slightly younger platformal assemblages; (2) between ca. 2.9 Ga platformal assemblages and younger platformal and Andean arc assemblages. Stratigraphic patterns such as adjacent greenstone belt assemblages representing sharply differing geodynamic settings with some indication of palinspastic order or progression in the northern, central and southern Superior Province are considered suggestive of orderly, autochthonous progression from platforms through rifting of continental fragments and late assembly during the Kenoran orogeny. Petrogenetic data indicating involvement of older granitoid crust in genesis of greenstone assemblages of a variety of ages throughout the Superior Province suggests the presence of older basement, implying autochthonous or parautochthonous development of many assemblages. Younger greenstone assemblages in the Abitibi–Wawa subprovince and the North Caribou terrane contain xenocrystic zircons similar in age to stratigraphically underlying greenstone assemblages. This pattern has been interpreted to represent autochthonous greenstone development. The current above enumerated evidence for autochthonous development of greenstone assemblages is generally indirect; few actual unconformities between Keewatin type greenstone assemblages have been recognized in the Superior Province. To aid in future recognition of unconformities in Keewatin type greenstone assemblages, examples of unconformities of varying degrees of subtlety in Archean sequences are reviewed. Archean unconformities ranging from sharp angular discordances in the northern Superior Province to subtle stratigraphic patterns, to weathering-related geochemical mobilization along inter-formational contacts are described. These styles of unconformity in Archean sequences suggest previously unrecognized major disconformities and unconformities may well exist between Keewatin type assemblages in the Superior Province generally and the Abitibi–Wawa subprovince specifically. The most important disconformity suggested in the Abitibi–Wawa subprovince is between the ca. 2726 Ma Deloro and the ca. 2708 Ma Tisdale assemblages. The presence of unconformities and disconformities coupled with the indirect lines of evidence cited above suggest an ‘autochthonous’ origin for many Superior Province greenstone belts. This ‘autochthonous’ origin involves either autochthonous or parautochthonous evolution of assemblages at least in part in place. That is, development of ca. 3 Ga platform sequences in selected locales followed by in place evolution of a succession of plume, oceanic and arc assemblages culminating in late pull-apart basin sequences. Implications of the autochthonous model for greenstone belts of the Superior Province, particularly the Abitibi–Wawa subprovince, include the future ability to recognize unconformity bounded sequences, potentially improving regional scale correlation.

Metamorposed Archean epithermal Au-As-Sb-Zn-(Hg) vein mineralization at the Campbell Mine, northeastern Ontario

The Campbell mine and the adjacent Red Lake mine are developed in an Archean gold deposit hosted within volcanic rocks of the Red Lake greenstone belt, situated in the western part of the Uchi subprovince of the Superior province. Geochronological evidence suggests that the Campbell-Red Lake deposit formed after volcanism but prior to late felsic plutonism and regional metamorphism, between 2722 and 2710 Ma. The orebodies at Campbell mine are largely hosted in epithermal-style veins and vein stockworks associated with strike-slip faults.Complex, superimposed phases of wall-rock alteration occurred prior to the emplacement of main stage veins and were controlled by both primary and secondary (contact, fault, and fracture induced) permeability in mafic and ultramafic rocks as well as secondary permeability in rhyolite and diorite. Widespread pervasive carbonatization and potassic (biotite) alteration with distal chloritic alteration is well developed in basalt and ultramafic rocks. In basalt, this alteration was in turn partly overprinted by silicification and aluminous alteration consisting of zoned mineral assemblages of quartz-andalusite-sericite-chloritoid-cordierite-margarite occurring within bleached zones, and quartz-chlorite-chloritoid-garnet-cummingtonite assemblages occurring in flanking chloritic zones. Similarly, zoned aluminous mineral assemblages also occur in diorite and rhyolite. In ultramafic rocks, the aluminous alteration is represented by sericite-fuchsite-cordierite assemblages, which grade out toward chlorite-anthophyllite-cordierite assemblages. This zoning appears to reflect the bulk composition of zoned premetamorphic aluminous (argillic) alteration.Main-stage carbonate (dolomite to ankerite)-quartz veins exhibit well-developed open-space-filling textures characterized by colloform and crustiform banding and cockade breccia infill textures typical of veins which formed in a near-surface environment. Fissure veins occur within fault zones themselves, whereas large snowbank (banded carbonate-quartz) veins formed in dilatant sites associated with a releasing bend in the faults. Fissure veins and snowbank veins are best developed in basalt, whereas veinlet stockwork and sheeted veinlet zones occur in ultramafic rocks adjacent to main-stage vein sites in basalt. The main-stage veins partially sealed remaining permeable zones, and proximal early alteration assemblages were overprinted by distal chloritic alteration as the hydrothermal system collapsed inward. The main-stage veins were then overprinted by silicification + or - tourmaline within breccias and veinlet stockworks.Mineralization is associated with microstockwork sulfide veinlets, strongly disseminated sulfide, and sulfide cemented breccias which cut silicified zones in veins and wall rocks. Native gold occurs on its own or with sulfides in fractures cutting silicification. Arsenopyrite is commonly associated with gold and occurs in ore zones with pyrite, pyrrhotite, and magnetite. Stibnite and sphalerite commonly occur in high-grade zones. Mineralized zones hosted in fissure veins and wall-rock replacement zones tend to be narrow (<1 m wide) and in narrow fault zones. Mineralization associated with brecciated snowbank veins are typified by multiple brecciation and fracturing events and are formed in dilatant sites at a high angle to fault zones. Mineralized zones are cut by late hydrothermal breccia dike and pipelike bodies as well as late quartz-carbonate veins and faults.The zoned aluminous alteration, open-space-filling textures of the main-stage veins, sheeted veinlet zones and stockwork fracturing, multiple phases of hydrothermal breccias, and anomalous Au-Ag-As-Sb-Hg-Zn-K at the Campbell mine are characteristics similar to those of Phanerozoic low sulfidation epithermal deposits. The mineralized environment has been deformed (flattened and stretched) and metamorphosed to middle to upper greenschist facies during the Kenoran orogeny (ca. 2650 Ma).

Transformation of Archean structural inheritance at the Grenvillian Foreland Parautochthon Transition Zone, Chibougamau, Quebec

The easternmost portion of the Archean Abitibi greenstone belt adjacent to the Grenville Front in the Chibougamau area displays distinctive structural features related to crustal-scale contraction during the Grenvillian orogeny. The "Foreland parautochthon transition zone," (FPTZ) a 3–15 km wide zone of Archean rocks, shows a Grenvillian deformation and metamorphic history that overprint fabrics inherited from the Archean Kenoran orogeny. The deformation history of this zone can be divided into two stages. During stage 1, a major crustal-scale deformation zone is responsible for a first uplift episode of deep Archean crust. During this stage, the Grenvillian deformation reactivated and emphasized the Archean fabrics. Amphibolite-grade mylonitic fabrics are interpreted to be an enhancement of the Archean regional schistosity, which remains axial planar to the regional folds. Archean rocks with well-developed former fabrics are strongly affected by Grenvillian non-coaxial flow associated with southeast-dipping stretching lineations. The northwestward transport direction, deduced from shear-sense indicators, is compatible with the signature of the Grenvillian orogeny. Stage 2 is responsible for the formation of the N015°-trending, late Grenvillian faults that have dissected the stage 1 deformation zone. Steep east-dipping narrow zones of retrograde low-temperature mylonites indicate a reverse, east-side-up movement that has contributed to the uplift of deeper Archean structural levels. In a later stage of brittle deformation, pseudotachylite injections and cataclasis, resulting from sinistral lateral offsets in a dominantly brittle regime, are superimposed on the mylonitic structures in response to a modification of the transport direction at the end of the Grenvillian orogeny. Even if high-grade metamorphism can be partly explained by the uplift of deep Archean crust, the structural signature associated with amphibolite mylonites within the FPTZ is interpreted as a product of the Grenvillian orogeny.

Differentiation and source of the Nipissing Diabase intrusions, Ontario, Canada

Major and trace element data are presented for 2.2 Ga Proterozoic diabase sills from across the Nipissing magmatic province of Ontario. In situ differentiation of the magma coupled with assimilation of Huronian Supergroup roof sediments is responsible for the variation in composition between quartz diabase and granophyric diabase seen within many of the differentiated intrusions. Uniform trace element and isotope ratio signatures, such as La/Sm (2.8 – 3.7) and εNdCHUR (−2.7 to −5.9) characterize chilled margins and undifferentiated quartz diabases. These chemical signatures suggest the existence of a single magma source that was parental to intrusions throughout the magmatic province; this magma has higher La/Sm and lower Ti/Y than primitive mantle and is displaced towards the composition of shales. Most chilled diabases and quartz diabases have a similar Mg# (0.64 and 0.60) and Ni content (98 and 127 ppm), and it is argued that the magma differentiated at depth and was emplaced as a uniform low-Mg magma. The Wanapitei intrusion and Kukagami Lake sill are an exception in that although the quartz diabase has La/Sm similar to the Nipissing magma type, which suggests that they came from the same source, the Mg# (0.68–0.71) and Ni content (130–141 ppm) are higher, which may suggest that they are either slightly more primitive examples of the normal Nipissing magma or that cumulus hypersthene has been resorbed. The light rare earth element enriched signature of the Nipissing magmas was perhaps introduced from the continental crust as the magma migrated from the mantle to the surface, but a remarkably constant and large amount (&gt;20%) of crustal contamination would be required. An addition of 1 –3% shale to the source of a transitional mid-ocean ridge basalt type magma can broadly reproduce the compositional features of the Nipissing magma type. The source characteristics were perhaps imparted during subduction accompanying the terminal Kenoran orogeny.

Archaean metasedimentary rocks from the northwestern Pontiac Subprovince of the Canadian shield: chemical characterization, weathering and modelling of the source areas

The northwestern Pontiac Subprovince is dominated by a monotonous greywacke sequence, metamorphosed and deformed during the Late Archaean Kenoran orogeny. Limited chemical weathering of the source rocks is suggested by: (1) low values of the chemical index of alteration, (2) no preferential leaching of Ca, Sr and Ba, (3) well defined correlations among MgO, K 2O and Al 2O 3, and (4) small variations of Cr/Ni. The source was dominantly felsic as indicated by high Th/Sc (average=0.39), La/Co (1.06), Zr/Cr (0.7), Zr/Y (11.2) and SiO 2/MgO (19.4). The composition of these metasedimentary rocks can be modelled by mixing of three end-member components: (1) tonalites/trondhjemites (∼ 65%) in order to explain the SiO 2, Al 2O 3, Th, U, Zr, Sr, and Ba abundances, Na K ratios, strong depletion in HREE and HFSE relative to LREE and LILE, low Th, Y and K 2O, and fractionated La/Yb (19.4), (2) komatiites (∼ 5%) in order to explain the high Cr (220 ppm) and Ni (66 ppm) contents, and (3) tholeiitic basalts (∼ 30%) accounting for the observed TiO 2 (0.62 wt%), V (144 ppm) and Sc (17 ppm) concentrations. The source region may be considered as a deeply eroded greenstone belt intruded by abundant tonalite-trondhjemite plutons; possibly now located to the south (and east) in the Grenville Province or possibly to the north, underlying the dominantly volcanic Abitibi Subprovince.

U–Pb geochronology of the eastern Abitibi Subprovince. Part 2: Noranda – Kirkland Lake area

U–Pb zircon ages for 15 volcanic and plutonic units in the Noranda and Kirkland Lake areas help constrain the history of volcanism, plutonism, sedimentation, and deformation in the south-central part of the Abitibi belt. Volcanism occurred over an interval of at least 50 Ma, beginning with the deposition of the volcanic and volcaniclastic units within the Pacaud Structural Complex at 2747 Ma. Following a period of apparent quiescence, magmatism resumed at 2730–2725 Ma with the eruption of volcanic rocks in the Normétal and Lac Abitibi area. From 2715 until about 2698 Ma, volcanism occurred sporadically throughout much of the area, culminating in the eruption of the Blake River Group from 2703 to 2698 Ma. Several large intrusive bodies yield ages that indicate that they are plutonic equivalents of the Blake River Group. Plutons that are considered to have been emplaced during the Kenoran orogeny give ages that are only slightly younger than the youngest volcanic units of the Blake River Group, emphasizing the very rapid onset of Kenoran deformation following the cessation of volcanic activity.The Cléricy syenite, dated at 2682 ± 3 Ma, postdates the main period of Kenoran deformation in this area and intrudes sedimentary rocks of the Kewagama Group which contain detrital zircons as young as 2687 Ma. These data suggest that the Kewagama Group is the same age as late sedimentary sequences such as the Timiskaming Group and may have been deposited in a similar tectonic setting.

U–Pb geochronology of the eastern Abitibi Subprovince. Part 1: Chibougamau–Matagami–Joutel region

U–Pb zircon, titanite, and baddeleyite ages are reported for 20 volcanic and plutonic rock units in the Chibougamau, Matagami, and Joutel areas of the northern Abitibi belt. Pretectonic magmatism occurred in three main pulses. Volcanic rocks of the Obatagamau Formation in the Chibougamau area were erupted at 2759 Ma. Following a possible hiatus in igneous activity, volcanism, accompanied by the emplacement of layered mafic intrusions, occurred in the Chibougamau, Matagami and Joutel areas in the period of 2730–2722 Ma. In the Chibougamau area, a slightly younger period of plutonism between 2720 and 2712 Ma is thought to be comagmatic with felsic volcanic rocks in the uppermost part of the Roy Group. Syntectonic intrusive rocks in the northern Abitibi belt range in age from 2701 to 2693 Ma. These ages are broadly comparable to ages obtained for syntectonic plutons elsewhere in the Abitibi belt, suggesting that tectonism associated with the Kenoran orogeny occurred at approximately the same time throughout the belt.

Relationship between northeast-trending regional faults and Archean mesothermal gold-copper mineralization; Cooke Mine, Abitibi greenstone belt, Quebec, Canada

In the Chibougamau-Chapais area, a spatial relationship exists between Cu-Au mineralization contained in east-west-trending shear zones and northeast-striking faults. This has led many authors to propose a genetic link between the Cu-Au Opemisca deposits at Chapais, such as the Cooke mine the main Au producer, and the northeast-striking Gwillim Lake fault. Structural analysis at the Cooke mine shows that the gold mineralization is located in sulfide-rich quartz veins hosted by northwest-southeast and east-west brittle-ductile reverse-oblique shear zones at a high angle to the northeast faults. The mineralized shears were produced by a subhorizontal, north-south-directed shortening compatible with the Kenoran orogeny. The shears are offset by oblique northeast sinistral faults. These faults, as well as the regional Gwillim Lake and Chibougamau-Copper faults, are products of the same subhorizontal, north-south-directed shortening and regional strain regime.This study demonstrates that although the Gwillim Lake fault system crosscuts the mineralization found at the Cooke mine, it could still have played a key role in the development of dilatancy favoring the circulation of mineralizing fluids. In such a scenario, the Gwillim Lake fault is thought to be a product of a progressive Kenoran deformation event to which both the east-west and north-east faults or shears are related. However, crosscutting relationships and strain compatibility indicate that the Gwillim Lake fault formed slightly later than the gold-hosting shear zones in a progressive deformation event. Nonetheless, the presence of this northeast regional structure in the vicinity of gold-copper or copper-gold deposits is significant.

Archaean orogenic ultrapotassic magmatism: an example from the southern Abitibi greenstone belt

The Cléricy Pluton in the southern Abitibi greenstone belt of Québec is a syenitic intrusion cross-cut by nordmarkitic dykes. It is probably one of the oldest known ultrapotassic intrusions (about 2.7 Ga). It crystallized in a crustal environment under low P H 20 < 2.5 kbar) and low f O 2 conditions. Most trace elements behaved as compatible elements suggesting that sphene and apatite were involved together with clinopyroxene during low-pressure crystal fractionation. The large range of initial isotopic ratios (calculated for 2.68 Ga, ϵ Nd −1.9 to + 2.3), as well as the trace element distribution, suggest that the syenitic pluton is the product of differentiation from a parental magma with a composition similar to that of Timiskaming-type Archaean shoshonites contaminated by a crustal component. On the other hand, the nordmarkitic dykes are probably the result of crustal anatexis of metasedimentary rocks and some addition of the syenitic magma. The Cléricy peralkaline syenite differs chemically from typical intraplate syenites in being saturated with respect to quartz, and displaying lower Ti and HFSE contents. It is related to an ultrapotassic volcano-plutonic event, which characterized the last 20 Ma of igneous activity in the Abitibi greenstone belt. It may represent an increased rate of convergence of small heterogeneous plates during a subduction-driven accretion tectonic event which possibly marked the culmination of the Kenoran Orogeny in the Superior Province of the Canadian Shield.

Paleomagnetic evidence for proterozoic tectonism in the Kapuskasing Structural Zone, Ontario

The Kapuskasing Structural Zone (KSZ) in the Superior Province of north‐central Ontario is thought to be an oblique cross‐section through Middle Archean crust. Samples from 30 sites in granulite gneisses and anorthosites of the KSZ have been analyzed using alternating field and thermal demagnetization methods. Three remanent magnetization components were found with mean directions of: D = 23°, I = 50° (k =52, α95 = 11°, N = 5 sites) for the A component; D = 73°, I = −68° (k = 50, α95 = 5°, N = 18) for the B component; and D = 332°, I = −59° (k = 53, α95 = 17°, N = 3) for the C component. V. Constanzo‐Alvarez and D. J. Dunlop found similar A and B components in less metamorphosed rocks from the same units to the immediate west. They attributed the A component to uplift at 2.55 Ga with subsequent 15° to 30° WNW tilt and the B component to thermochemical overprinting at 1.1 Ga as the KSZ was reactivated during Keweenawan rifting and volcanism. We also attribute the A component to uplift during the 2.55 Ga Kenoran Orogeny with concomitant 10°±5° WNW downward tilting before intrusion of the 2.45 Ga Matachewan dikes, to give its present paleopole of 33°E, 67° (dp = 6°, dm = 9°, N = 13) using data from both studies. We attribute acquisition of the B component to uplift at about 1.88 Ga, as suggested by recent thermal modeling and tectonic syntheses, to give a paleopole of 124°W, 30°N (dp = 7°, dm = 8°, N = 20). The C component is found in only three sites and may be either an hybrid of a 1.1 Ga Keweenawan CRM with the B component or a fault‐block tilted B component near the Ivanhoe Lake cataclastic zone.

The Scott Lake deposit: a contact-metamorphosed volcanogenic massive sulfide deposit, Chibougamau area, Quebec

The Scott Lake volcanogenic massive sulfide deposit lies near the margin of a large, early kinematic granitoid intrusion in the vicinity of Chibougamau, Quebec. The deposit was contact metamorphosed by the intrusion, and subsequently it was metamorphosed to the greenschist facies during the Kenoran Orogeny. Pyrite, magnetite, and sphalerite are the most abundant metallic minerals, and minor amounts of chalcopyrite, pyrrhotite, and loellingite are also present. Both pyrite and magnetite locally occur as porphyroblasts up to several centimetres in diameter. Metamorphic textures developed in the massive sulfide ore appear to have formed during contact metamorphism, and they remained intact through the subsequent regional event. However, silicate minerals (biotite and possibly amphibole) that grew during contact metamorphism were largely retrograded during regional metamorphism. The presence of biotite indicates that contact metamorphism took place at 400°–500 °C. Application of the sphalerite geobarometer gives a pressure of approximately 4.5 kbar (450 MPa), which probably reflects the later regional event.

Early Proterozoic activity on Archean faults in the western Superior province— Evidence from pseudotachylite

Major transcurrent faults in the Superior province developed in the Late Archean at the close of the Kenoran orogeny. Reactivation of some of these faults late in the Early Proterozoic is indicated by Rb-Sr analyses of pseudotachylite from the Rainy Lake-Seine River and Quetico faults in the Rainy Lake region of Minnesota and Ontario. Fault veins of pseudotachylite and immediately adjacent country rock at two localities yielded subparallel isochrons that are pooled for an age of 1947 ±23 Ma. K-Ar and Rb-Sr biotite ages register earlier regional cooling of the terrane at about 2500 Ma with no evidence of younger thermal overprinting at temperatures exceeding 300 °C. Accordingly, the 1947 ±23 Ma age is interpreted as dating the formation of the pseudotachylite. Reactivation of existing faults at this time was caused by stresses transmitted from margins of the Superior province where compressional tectonic events were occurring.

Geology and geochemistry of Huronian rhyolites and low-Ti continental tholeiites from the Thessalon region, central Ontario

Bimodal volcanism associated with early phases of Huronian rifting in central Ontario, dated about 2450 Ma, produced low-Ti tholeiitic basalts and two varieties of crustally derived calc-alkaline rhyolite. Early tholeiites are characteristically highly evolved, have Mg* values from 30 to 50, and display pronounced enrichment in large-ion lithophile elements (LILE) and light rare-earth element (LREE) in comparison with modern oceanic basalts, fractionated heavy rare-earth element (HREE) patterns, and low Ti, Zr, P, Nb, Ba, and K abundances. Ti/Zr ratios rise progressively in early basalts and associated basaltic andesite fractionates from about 35 in early flows to 55 in central units. Late basalts also carry enriched LILE and LREE, but, in contrast to early types, have average Mg* values greater than 50 and lower rare-earth element (REE) abundances with flat HREE patterns. They also display negative Ba, Nb, and P anomalies on chondrite-normalized distribution diagrams, but lack low K, Zr, and Ti contents. Their Ti/Zr ratios of about 80 approach chondritic levels. Melting models suggest the differences are explained by lower degrees of fusion (as low as 10%) in a hydrated, LILE- and LREE-enriched peridotite during generation of the early basalts, leaving a residue containing appreciable garnet, amphibole, Ti oxides, zircon, and apatite.Erupted simultaneously with the basalts were two distinctive rhyolite types: (1) a low-LILE, high-LREE group (25% of analysed specimens), derived by −20% melting of granulitic siliceous tonalitic gneiss, presumably at deep crustal levels, and (2) a high-LILE, low-LREE group (75%), derived, probably at shallower levels, by ≤ 30% melting in granitic rocks with pegmatitic or leucogranitic compositions. Mutual magma mixing of basalts and rhyolites during early stages of volcanism produced abundant hybrid andesites, but the frequency of contamination is much lower in later units.Hypothetical subcontinental source compositions, calculated from the Raleigh equation, suggest that the Huronian mantle had already undergone a complex history. Low Ba, Nb, P, Ti, and depleted HREE abundances compared with abundances for modern oceanic basalts suggest that a basaltic melt had already been withdrawn from this source during Archean time. Subsequently, an episode of hydrous metasomatism enriched the source in LILE and LREE. The latter event resulted from (1) subcontinental mantle metasomatism by previous Archean subduction, (2) mantle metasomatism during the terminal Archean Kenoran Orogeny, or (3) a wave of hydrous metasomatism accompanying Huronian mantle convection immediately preceding volcanism.

Probable Archean nepheline syenite plutons in the Superior Province adjacent to the Labrador Trough

Three nepheline-bearing plutons have recently been discovered in the Superior Province northwest of Schefferville. They are apparently related miaskitic nepheline syenites, varying to leucocratic nepheline syenite, juvite, and rarely ijolite. All three plutons are crudely zoned with an increasing nepheline content towards the centres. The foliation and lineation in these rocks are the same as those in the surrounding country rocks. Together with folding, microscopic comminution, and metamorphic recrystallization, this structure suggests that these plutons were intruded prior to or during the Kenoran Orogeny. This conclusion is consistent with a K/Ar biotite minimum age of 2398 ± 72 Ma (3%) obtained for one sample.

Petrogenesis of Archean granitoid plutons from the Kenora area, English River Subprovince, Northwest Ontario, Canada

In the Kenora area, the emplacement of granitoid plutons and associated minor intrusions into older tonalitic gneiss and remnant mafic supracrustal rocks, represents the second half of a protracted history during the Kenoran Orogeny. The oldest granitoid pluton is the Marginal granodiorite (2735 ± 65 Ma) which separates gneiss of the English River Subprovince from supracrustal rocks of the Wabigoon Subprovince. This unit has a narrow, elongate form and a gradational boundary with the gneiss. These features, and the unit's age, initial Sr ratio and REE patterns that are analytically indistinguishable from those of the tonalitic gneiss, are reasons for interpreting the Marginal granodiorite as being derived by ‘frictional reconstitution’ of tonalitic gneiss, resulting from heating during movement along the Kenora fault. The Melick tonalite (2630 ± 50 Ma), and the Lulu and Dalles granodiorites (2630 ± 10 Ma) are interpreted as the next additions to the area. A correlation between decreasing age, increasing initial Sr ratio, increasing size of intrusive body, and progressive depletion of heavy REE may indicate that the Kenora tonalitic gneiss, Melick tonalite, Lulu granodiorite and Dalles granodiorite represent a continuum of genetically related plutonism. The changes in heavy REE suggest that progressively younger source rocks (all comparable to Archean tholeiite) underwent transformation from high-pressure granulite to eclogite. The Dalles granite is modelled by fractional crystallization of the Dalles granodiorite. The youngest plutons are the Muriel diorite-granodiorite (2660 ± 25 Ma) and the Austin granite (2631 ± 63 Ma), both interpreted as being derived from upper crustal rocks. A sequence of dioritic rocks occupies an intermediate structural and lithological position between the amphibolite-tonalite gneiss and the Muriel granodiorite. The model advanced is that the diorites represent the ‘roots’ of major diorite plutons, and indicate derivation by partial melting of amphibolite (and perhaps also low pressure granulite) and tonalitic gneiss. The melt, and some residual material, was mobilized upward along structurally controlled paths. The Austin granite is a potassic pluton with a higher initial Sr ratio than other units in the area. It is interpreted as being derived by extensive partial melting of rocks comparable to the Lulu or Dalles granodiorite.

Evolution of the south-central segment of the Archean Abitibi Belt, Quebec. Part II: Tectonic evolution and geomechanical model

In this paper, we describe the relations between the paleogeographic and tectonic evolution of the southwestern part of the Archean Abitibi and Bellecombe belts. Volcanism in the Abitibi Belt created a very thick, anisotropic plate composed of competent volcanic rocks and broken by the Duparquet–Destor break. The depocenters of the upper division of diverse volcanic rocks subsided about 10 km relative to their surroundings, and some central volcanic complexes within this division were consolidated by synvolcanic plutons and their thermal metamorphic aureole. The Cadillac break, a normal fault, separated the Abitibi and Bellecombe belts. The latter consisted of comparatively incompetent sedimentary rocks on top of a basement composed of ultramafic–mafic flows.North–south compression of the volcanic terrain during the Kenoran Orogeny produced a set of flexure folds, F1, that curve around the consolidated cores of central volcanic complexes generally in an easterly direction. Synclinoria nucleated at the deeply subsident depocenters of the upper diverse division. Further north–south flattening and subvertical stretching produced the east-trending F2 folds, their axial-plane schistosity S2, and local superposed schistosities S3 and S4. Southward verging recumbent folds suggest that the Bellecombe Belt simultaneously was pulled northward below the Abitibi Belt. During the orogeny, the Duparquet–Destor and Cadillac breaks were transformed to thrust faults; the Duparquet–Destor break also shows minor (&lt; 3 km) right-lateral strike slip. Diapiric rise of late- to post-kinematic plutons locally distorted earlier schistosities.