Thrusting, magmatic intraplating, and metamorphic core complex development in the Archaean Belleterre-Angliers Greenstone Belt, Superior Province, Quebec, Canada

doi:10.1016/0301-9268(94)90029-9

Precambrian Research

Volume 68, Issues 3–4, August 1994, Pages 183-200

https://doi.org/10.1016/0301-9268(94)90029-9 Get rights and content

Abstract

The Belleterre-Angliers Greenstone Belt in the Pontiac Subprovince is the youngest and southernmost of the greenstone belts in the Archaean Superior Province of the Canadian shield. The greenstone belt has been disrupted into three fragments: from north to south the Baby, Belleterre and Lac des Bois groups, respectively.

The Belleterre-Angliers Greenstone Belt has sheared contacts with younger Pontiac metasediments and older tonalites. The Pontiac metasediments dip beneath the Baby Group and are tectonically interlayered with the tonalites. Thus the Belleterre-Angliers belt is interpreted to be a thrust sheet (6 km thick in the north, thinner in the south) above a mid-crustal duplex consisting of metasedimentary and tonalite imbricates. The thrust sheet was transported from the north, but age and geochemical constraints rule out derivation from the Abitibi Subprovince. It probably roots in a band of ultramafic rocks that mark a major south-vergent thrust in the northern Pontiac Subprovince.

After emplacement the Belleterre-Angliers sheet and the adjacent Pontiac metasediments were deformed by a system of linked, extensional shears and transfer faults that border dome or antiformal areas in a style similar to Phanerozoic metamorphic core complexes. The orientation of shear zones and foliations indicates that the regional far-field stresses were compressional both before and after extensional shearing. Since the system of extensional shears is intruded by a suite of monzodiorite, granodiorite, syenodiorite and syenite plutons (with high Mg number) it is concluded that crustal inflation caused by the injection (intraplating) of large volumes of magma perturbed the regional compressive stress field and facilitated extensional faulting.

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The metasedimentary subprovinces of the Superior Province provide critical insights into the tectonic style and thermal state prevailing during its Neoarchean cratonic assembly. The Pontiac subprovince consists of metamorphosed turbiditic sequences with minor interlayered mafic volcanic rocks that are intruded by abundant mafic to felsic plutonic rocks of different geochemical affinities, and for which the source, timing, deformation, and relationship to metamorphism and gold endowment are poorly constrained. Petrological, geochemical, isotopic (Lu-Hf zircon), and geochronological (U-Pb zircon and monazite) data reveal three plutonic events consisting of 1) c. 2764 Ma medium-pressure TTG gneiss, interpreted as the local basement to the Pontiac supracrustal sequences; 2) syn- to late- sedimentary c. 2690–2670 Ma juvenile (mean εiHf = 4.02 ± 1.12) monzonite sanukitoid and highly potassic calc-alkaline sills and plutons; and 3) voluminous c. 2670–2620 Ma S-type granite sourced by protracted partial melting of sedimentary rocks at depth. This succession in the intrusive record combined with syn-depositional volcanism and subsequent Barrovian metamorphism of the metasedimentary host rock is consistent with a transition from a continental rift setting to crustal thickening via terrane assembly and challenges early models of arc accretion.
Synsedimentary rifting and basaltic-komatiitic volcanism in the Pontiac subprovince, Superior craton (Canada): Implications for Neoarchean geodynamics
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These cm- to m-scale intrusions occur throughout the Pontiac and Abitibi subprovinces (e.g., Wyman and Kerrich, 1993; Mathieu et al., 2017; Perrouty et al., 2017; Koïta, 2019). Volcanic rocks in the southern part of the Pontiac subprovince include ultramafic to felsic volcanic rocks of the Belleterre-Angliers belt (Fig. 2; Sawyer and Barnes, 1994). They have been dated from 2690 to 2682 Ma (U-Pb zircon ages by Mortensen and Card, 1993) and have geochemical signatures reflecting magmatism in an extensional environment coupled with varying degrees of crustal assimilation (Koïta, 2019).
New field observations from the Pontiac subprovince, Superior Province, supported by whole-rock geochemistry provide strong evidence for extensional magmatism and volcanism during deposition of the ca. 2685 Ma Pontiac sediments. Contacts between basaltic and komatiitic volcanic rocks and sedimentary rocks are concordant and weakly deformed, inconsistent with an allochthonous emplacement model proposed for volcanic successions in the Pontiac subprovince. Structural observations and aeromagnetic imagery show that the volcanic rocks are present as thin (<500 m), laterally continuous (>30 km) conformable successions within the turbiditic succession and were emplaced prior to the earliest deformation of the Pontiac subprovince. Sediment-matrix breccias with fluidal and blocky igneous clasts identical to the compositions of the mafic–ultramafic rocks occur along intrusive contacts with Pontiac Group wacke. These textures are interpreted as peperite, representing the interaction of magma or lava with wet, unconsolidated sediment. Peperite was observed along the margins of tholeiitic-komatiitic rocks and calc-alkaline lamprophyre intrusions, indicating that these magmas were broadly synchronous with the deposition of the Pontiac sediments. The volcanic rocks are characterized as tholeiitic basalt and Al-depleted to Al-undepleted komatiite, have high Nb/Th, flat to positive light rare earth element (REE) slopes, and flat heavy REE slopes, consistent with partial melting of a mantle plume. The calc-alkaline lamprophyre intrusions have low Nb/Th and prominent negative light and heavy REE slopes, consistent with partial melting of a previously metasomatised mantle. The presence of autochthonous komatiitic and tholeiitic rocks is consistent with the Pontiac subprovince developing as an extensional basin within the Abitibi subprovince in response to an upwelling mantle plume ca. 2.7 Ga. This model is incompatible with uniformitarian evolutionary models for the Superior craton interpreting the sedimentary subprovinces as accretionary prisms that formed during subduction processes. Non-uniformitarian models invoking stagnant-lid tectonics predict the formation of extensional basins and ascent of depleted mantle-derived magmas associated with upwelling mantle plumes, but the ability to produce source-metasomatism magmas without subduction is highly contested. The evolution of the Pontiac basin may thus reflect a transitional period in the Neoarchean where stagnant-lid and proto-subduction styles of tectonism were operating intermittently.
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Citation Excerpt :
Crustal thickening and subsequent extensional collapse have been proposed as additional ways to produce crustal melt. In many cases, GGM granites are broadly co-eval with, or directly post-date compressional deformation (Kusky, 1993; de Ronde and de Wit, 1994; Sawyer and Barnes, 1994; Nelson, 1997; Zegers et al., 1998b). Dirks and Jelsma (1998) suggested that crustal melting in the Zimbabwe craton was a direct consequence of crustal stacking.
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Mechanisms for folding of high-grade rocks in extensional tectonic settings
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The maximum horizontal extension direction during orogenic collapse may be sub-parallel (e.g. Betic Cordillera, Dewey, 1988; Andes, Dalmayrac and Molnar, 1981; Himalaya, Burchfiel et al., 1992; south Scandinavian Caledonides, Fossen, 1992, 2000), perpendicular (e.g. Tibetan plateau, Dewey, 1988) or oblique (Central Andes, Dewey, 1988) to the direction of initial convergence across the orogen. Selverstone (1988) and Sawyer and Barnes (1994) describe cases where normal displacements are interpreted synchronously on structures striking at a high angle to each other, suggesting local variations in the maximum extension direction. Because of such variations in strain patterns, axes of folds formed during orogenic collapse may therefore be sub-parallel or oblique to those formed during previous crustal shortening.
This review of structures developed in extensional high-grade terrains, combined with results of centrifuge analogue modelling, illustrates the range of fold styles and mechanisms for folding of amphibolite to granulite facies rocks during rifting or the collapse of a thrust-thickened orogen. Several extensional fold mechanisms (such as folding within detachment shear zones) are similar to those in contractional settings. The metamorphic P–T–t path, and not fold style or mode of formation, is therefore required to determine the tectonic setting in which some folds developed. Other mechanisms such as rollover above and folding between listric normal shear zones, and folding due to isostatic adjustments during crustal thinning, are unique to extensional tectonic settings.
Several mechanisms for folding during crustal extension produce structures that could easily be misinterpreted as implying regional contraction and hence lead to errors in their tectonic interpretation. It is shown that isoclinal recumbent folds refolded by open, upright folds may develop during regional extension in the deep crust. Folds with a thrust sense of asymmetry can develop due to high shear strains within an extensional detachment, or from enhanced back-rotation of layers between normal shear zones. During back-rotation folding, layers rotated into the shortening field undergo further buckle folding, and all may rotate towards orthogonality to the maximum shortening direction. This mechanism explains the presence of many transposed folds, folds with axial planar pegmatites and folds with opposite vergence in extensional terrains.
Examples of folds in high-grade rocks interpreted as forming during regional extension included in this paper are from the Grenville Province of Canada, Norwegian Caledonides, Albany Mobile Belt and Leeuwin Complex of Western Australia, Ruby Mountains in the Basin and Range Province of Nevada, the Atâ Sund area of Greenland, the Napier Complex of Enderby Land in East Antarctica and the Kigluaik Mountains in western Alaska.
U-Pb geochronology of Archean metasedimentary rocks in the Pontiac and Abitibi subprovinces, Quebec, constraints on timing, provenance and regional tectonics
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U–Pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the Pontiac subprovince and from the Cadillac, Kewagama and Lac Caste Groups within the southern Abitibi subprovince. Youngest detrital zircon ages from these sequences are 2685, 2686, 2687, and 2693 Ma, respectively. The Lac Fournière pluton, which intrudes Pontiac Group metasedimentary rocks, is dated at 2682±1 Ma. A similar age was previously measured from a pluton that cuts the Kewagama Group. These data bracket deposition of greywacke in the Pontiac and Abitibi subprovinces to 2685±3 Ma. In contrast, the youngest zircon found in a volcaniclastic unit interbedded with Timiskaming Group metawacke is 2673±2 Ma, while the age of a pluton that intrudes the Timiskaming is 2672±1 Ma. Thus, the Timiskaming Group is distinctly younger than the other metasedimentary rocks. The normalized probability distributions of detrital zircon ages from the Pontiac and the Abitibi metasedimentary rocks, as well as of dated igneous rocks from the southern Abitibi subprovince, all show that their major zircon populations formed in the age range 2685–2710 Ma. The distributions for the Abitibi metaturbidites and dated igneous rocks are very similar, suggesting that metasedimentary rocks in the Abitibi belt were mostly derived from the surrounding granite–greenstone terrain. The largest peak of the Pontiac distribution is a few million years younger than that from the Abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older, pre-Abitibi components (2750–2850 and 3020–3030 Ma). Deposition of the ca. 2685 Ma sediments was virtually syn-deformational. Based on previous field, geochemical and isotopic studies, as well as the detrital zircon age data, the Pontiac Group may represent a foreland basin that may have developed from an accretionary prism. The source of much of the sediment may have been an uplifted extinct continental arc to the north. The arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin, now partly preserved in the southern volcanic zone of the Abitibi subprovince. Metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones. These were brought up by high angle faults, which also localized deposition of non-marine (Timiskaming-type) sediments. It is tentatively suggested that Pontiac-type metasedimentary rocks extend most of the way beneath the Abitibi subprovince, the oldest part of the basin emerging as the Quetico Group in the western Superior province.

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Thrusting, magmatic intraplating, and metamorphic core complex development in the Archaean Belleterre-Angliers Greenstone Belt, Superior Province, Quebec, Canada

Abstract

Magmatism and extension: the thermal and mechanical effects of the Yellowstone hotspot

J. Geophys. Res.

Cyclic volcanicity and sedimentation in the evolutionary development of the Archean greenstone belts of shield areas

Geol. Soc. Aust. Spec. Publ.

Cordilleran metamorphic core complexes-from Arizona to southern Canada

Annu. Rev. Earth Planet. Sci.

Cartes géologiques-Feuille Ville-Marie 31M

Nickel-copper sulfide occurrences from the Belleterre-Angliers belt of the Pontiac Subprovince and the use of Cu/Pd ratios in interpretation platinum-group element distributions

Econ. Geol.

Instrumental neutron activation analysis by collecting only one spectrum: results for international geochemical reference samples

Geostand. Newsl.

Structural framework of the western Pontiac Subprovince: an integrated geological and geophysical approach

Orogen parallel and transverse shearing in the Opatica Belt, Quebec: implications for the structure of the Abitibi Subprovince

Can. J. Earth Sci.

Crustal structure and kinematic framework of the Pontiac Subprovince: an integrated structural and geophysical study

Can. J. Earth Sci.

Evolution of Archean volcanic-sedimentary sequences of the western Wabigoon Subprovince and its margins: a review

Late Archaean thrusting in the northwestern Pontiac Subprovince, Canadian shield

Precambrian Res.

A review of the Superior Province of the Canadian Shield, a product of Archean accretion

Precambrian Res.

U-Pb and Pb-Pb geochronolgy of the Baby-Belleterre greenstone belt and associated Pb-Cu-Ni deposits, Pontiac Subprovince

Geol. Assoc. Can. Mineral. Assoc. Can. Abstr.

Discrete Proterozoic structural terranes associated with low-P, high-T metamorphism, Anmatjira Range, Arunta Inlier, central Australia: tectonic implications

J. Struct. Geol.

Cordilleran metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression

Geology

Leucogranites of the Himalaya/Karakoram: implications for magmatic evolution within collisional belts and the study of collision-related leucogranite petrogenesis

J. Volcanol. Geotherm. Res.

Age constraints on deposition and provenance of Archean sediments in the southern Abitibi and Pontiac subprovinces from U-Pb analyses of detrital zircons

Lithoprobe Rep.

U-Pb geochronology of the Opatica metaplutonic-gneiss belt, Quebec, and its relationship to the Northern Abitibi

Geol. Assoc. Can. Mineral. Assoc. Can. Abstr.

Evolution of the south-central part of the Archean Abitibi Belt, Quebec, Part I. Stratigraphy and paleogeographic model

Can. J. Earth Sci.

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

Can. J. Earth Sci.