Search

Electron-microprobe data were obtained from monazite along five transects within the Longstaff Bluff Formation, a thick package of Paleoproterozoic turbiditic rocks exposed on Baffin Island, northern Canada, and metamorphosed at conditions of the upper greenschist to granulite facies during the Trans-Hudson Orogeny. During prograde metamorphism, the LREE content of monazite progressively decreased, whereas the U, Th, Y and HREE content increased, with sharp changes at the transition from Sil–Kfs rocks to incipiently migmatitic rocks. In situ chemical dating of monazite yielded a range of ages from 1913 to 1741 Ma. The age data form five distinct clusters (1910, 1880, 1845, 1805, and 1765 Ma), of which at least four are interpreted to represent different episodes of monazite growth. The textural relationship and petrographic context of the grains were used to constrain the significance of the data. The two oldest grains (1910 Ma) could be detrital or metamorphic. The 1880 Ma age is interpreted to reflect monazite growth near the onset of regional deformation. The 1845 Ma age cluster is mostly found in the three southern (higher-grade) transects, the 1805 Ma event is widespread throughout the study area, and the 1765 Ma episode of monazite growth has been recognized mostly in the northern (lower-grade) part of the study area. The results suggest a northward progression of metamorphism from granulite- to lower-amphibolite-facies assemblages, with textural data suggesting that the widespread 1805 Ma event was responsible for migmatite formation in the southernmost part of the study area.

New U–Pb geochronological and Sm–Nd isotopic data establish the timing of Neoarchean and Paleoproterozoic magmatic and metamorphic events in northeastern Saskatchewan, and give insight into the late Archean assembly of the central and southern Hearne domains of the western Churchill craton. Tonalite orthogneiss within a migmatite complex is 2714 ± 6 Ma in age. Foliated, tonalite gneiss, typical of tonalitic rock that intruded the migmatite complex and mafic volcanic rocks of the Ennadai Group, is 2698 ± 6 Ma in age. These and a third tonalitic gneiss are correlative with ca. 2.72–2.70 Ga volcanic and plutonic rocks previously documented in the Central Hearne supracrustal belt (CHSB). They have juvenile geochemical signatures (ɛNd > 1.43). A rhyolite dated at 2681 ± 4 Ma is consistent with the age and juvenile nature of a younger volcanic cycle in the CHSB. In contrast, a 2681 ± 3 Ma foliated granite dyke that intrudes the tonalite gneiss contains inherited zircons up to 2.99 Ga and has an ɛNd of −0.48, consistent with a melted older Mesoarchean crustal component similar to that of the southern Hearne domain. Three Paleoproterozoic granites also have Nd isotopic signatures consistent with Mesoarchean precursors. We propose that the abrupt change in geochemical signature of the ca. 2.68 Ga magmatism is due to the accretion of the CHSB onto the southern Hearne domain and a change from oceanic to continental arc magmatism. A 2571 ± 5 Ma in situ monazite age determined for porphyroclastic gneiss in the Striding–Athabasca mylonite zone is linked with latest Archean reactivation along the southeastern margin of the Rae province. The Jones Lake granite, dated at 1.86 ± 0.02 Ga, yielded metamorphic monazite ages from 1821 Ma to 1812 Ma. An amphibolitic gneiss yielded a multi-grain, TIMS, zircon age of 1823 ± 4 Ma. The latter ages agree well with the time of metamorphism determined by in situ monazite SHRIMP ages from three samples of metasedimentary gneiss (1826 ± 8 Ma, 1817 ± 42 Ma and 1809 ± 38 Ma), and coincide with the peak of Hudson granite emplacement in the Western Churchill craton. A diatexitic granodiorite crystallized at 1812 ± 4 Ma. U–Pb titanite analyses from three separate rocks yielded ages of 1778 ± 2 Ma, 1761 ± 2 Ma and 1757 ± 5 Ma, while U–Pb from rutile yielded 1751 ± 12 Ma. The 1830–1800 Ma events appear to be related to terminal collision within the Trans-Hudson Orogen, while the younger titanite and rutile ages are cooling ages coeval with emplacement of the Nueltin igneous suite.

Within the Trans-Hudson Orogen in northern Quebec and southern Baffin Island, parautochthonous Archean basement and Paleoproterozoic cover, as well as allochthonous (accreted) Paleoproterozoic units are exposed in a series of antiformal culminations and complementary synclinoria. The parautochthonous rocks and two assemblages of accreted Paleoproterozoic units (crustal terranes) are separated by major tectonic boundaries and associated deformation zones. Field, petrological, and isotopic data document an increase in the width of the deformation zones and a concomitant increase in P–T conditions of metamorphism from orogenic foreland to hinterland. Tectonostratigraphic and geochronological criteria allow footwall and hangingwall units to be consistently distinguished in spite of the changing structural and metamorphic aspect of the terrane boundaries across the width of the orogen.

The tectonometamorphic histories of basement and cover units in the eastern Trans-Hudson Orogen attest to the importance of structural, geohydrological, and magmatic controls on the attainment of metamorphic conditions in a convergent-plate-margin setting. In northern Quebec, two metamorphic suites are recognized in the parautochthonous (lower-plate) Superior Province basement. An older metamorphic suite comprises arc-related, granulite-facies assemblages (less than 5 kbar and 860 to 920°C) dated at ca. 2.70 Ga. A younger, overprinting, collision-related metamorphic suite comprises amphibolite-facies assemblages (7.7 kbar at 640°C to 9.8 kbar at 715°C) dated at ca. 1.80 Ga. Within the overlying Paleoproterozoic Cape Smith Belt (lower-plate, south-verging thrust belt), thermal-peak mineral growth is syn- to post-thrusting; a relatively high-pressure, greenschist- to amphibolite-facies metamorphism (6.3 kbar at 400°C to 9.1 kbar at 575°C) is interpreted to be a result of ca. 1.80 Ga collision-related thickening. At higher structural levels, upper-plate mid-crust plutonic and metasedimentary units of the 1.86–1.82 Ga Narsajuaq arc contain granulite-facies assemblages (7–10 kbar and 800–900°C) retrograded to amphibolite-facies conditions (7–9 kbar and 700–775°C) during collision-related thrusting. At the structurally highest level, granulite-facies metamorphism of the (upper-plate) Lake Harbour Group and Blandford Bay assemblage is related to emplacement of the 1.86–1.85 Ga Cumberland batholith. Between ca. 1820 and 1795 Ma, accretion and collision-related thickening resulted in retrogression of the granulite-facies assemblages in the Narsajuaq arc, Lake Harbour Group and Blandford Bay assemblage, thermal peak metamorphism of lower-plate cover rocks, and re-equilibration of the parautochthonous Superior Province basement, all at greenschist- to amphibolite-facies conditions. The integration of field, thermobarometric and U–Pb data establishes a causal relationship between arc plutonism and development of granulite-facies assemblages in a convergent-plate-margin setting. In contrast, the attainment of greenschist- to amphibolite-facies conditions at mid-crust levels during plate collision is primarily a function of structural and geohydrological controls.

The Grenville orogenic cycle, between ∼ 1190 and 980 Ma, involved accretion of magmatic arcs and/or continental terranes to the Laurentian craton. A transect across the western Central Gneiss Belt, Georgian Bay, Ontario, which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20–30 km, offers some insights on the thermal and mechanical behavior of the lower crust during the development of the Grenville orogen. Prior to Grenvillian metamorphism, this part of Laurentia consisted largely of Meso-proterozoic (∼ 1450 Ma) granitoid orthogneisses, granulites, and subordinate mafic and supracrustal rocks. Grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed (∼ 1160 Ma) Parry Sound domain over the craton sometime between 1120 Ma and 1080 Ma. This stage of transport was followed by out-of-sequence thrusting and further convergence along successively deeper, foreland-propagating ductile thrust zones. A major episode of extension at ∼ 1020 Ma resulted in southeast directed transport of allochthonous rocks along the midcrustal Shawanaga shear zone. The final stage of convergence involved deformation and metamorphism in the Grenville Front Tectonic Zone at ∼ 1000–980 Ma. Peak metamorphism along most of the transect at 1065–1045 Ma followed initial transport of allochthonous rocks over the craton by 15–35 m.y. Regional cooling, which postdated peak metamorphism by >70 m.y., was probably delayed by the combined effects of late-stage extension and convergence. Transport of allochthons at least 100 km over the craton was accomplished along a weak, migmatitic decollement; further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement. Extensional deformation was associated with distributed ductile flow, the formation of regional transverse folds with axes parallel to the stretching direction, and reactivation of the allochthon-parautochthon thrust boundary as an extensional decollement. The extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the Central Gneiss Belt.

The Cartier Batholith, exposed about 50 km northwest of Sudbury, Ontario, Canada, is a late Archean K-rich granitic batholith. Along with the Levack Gneiss Complex, it forms the footwall of the North Range of the Sudbury Igneous Complex. The Cartier Batholith, dominantly monzogranitic to granodioritic, is relatively homogeneous, unfoliated, and contains K-feldspar megacrysts. It shows several distinctive chemical features, including high concentrations of Zr, Th, U and LREE, and low concentrations of Nb, Ti, and HREE. Several discriminants suggest a post-orogenic tectonic environment for the batholith. This inference is consistent with a UPb age of 2642 ± 1 Ma obtained on zircon, only slightly younger than the inferred timing of high-grade metamorphism of the Levack Gneiss Complex. The Cartier Batholith could have formed as a result of approximately 33% partial melting of the Levack Gneiss Complex.

Two lithotectonic assemblages in southern Britt domain have different histories of plutonism, metamorphism, structural development, and mafic dyke emplacement. These differences are accounted for by postulating that a cryptic Grenvillian thrust separates the assemblages. Amphibolite-facies extensional shear along the Central Britt shear zone (CBSZ) overprinted the thrust, obscuring kinematic evidence for its existence. The structurally lower Bayfield–Nadeau Island assemblage contains orthogneiss suites of disparate age, lesser amounts of supracrustal rocks deposited before intrusion of the youngest orthogneiss, pre-Grenvillian and Grenvillian metamorphic assemblages, and at least three mafic dyke suites. The overlying Ojibway – Sand Bay assemblage contains only younger orthogneiss with Grenvillian metamorphic assemblages, volumetrically important supracrustal rocks that are younger than the youngest orthogneiss, and lacks cross-cutting mafic dykes.Comparable tectono-stratigraphic changes are present across the thrust boundary separating the Ojibway – Sand Bay assemblage and the basal Parry Sound assemblage. Extensional shearing did not strongly overprint this boundary and it therefore serves as a relatively unmodified analogue of the overprinted boundary.Extension on the CBSZ overlapped formation of transverse ductile folds (hinges parallel to the extension–transport direction). These folds and the CBSZ dominate the crustal architecture at this level and are interpreted to be late orogenic structures formed during a gravity-assisted shape adjustment of the orogenic wedge. Thermal softening of the lower crust caused by thrust thickening may have allowed this to occur.

Systematic mapping of a transect along the well-exposed shores of Georgian Bay, Ontario, combined with the preliminary results of structural analysis, geochronology and metamorphic petrology, places some constraints on the geological setting of high-grade metamorphism in this part of the Central Gneiss Belt. Correlations within and between map units (gneiss associations) have allowed us to recognize five tectonic units that differ in various aspects of their lithology, metamorphic and plutonic history, and structural style. The lowest unit, which forms the footwall to a regional decollement, locally preserves relic pre-Grenvillian granulite facies assemblages reworked under amphibolite facies conditions during the Grenvillian orogeny. Tectonic units above the decollement apparently lack the early granulite facies metamorphism; out-of-sequence thrusting in the south produced a duplex-like structure. Two distinct stages of Grenvillian metamorphism are apparent. The earlier stage (c. 1160–1120 Ma) produced granulite facies assemblages in the Parry Sound domain and upper amphibolite facies assemblages in the Parry Island thrust sheet. The later stage (c. 1040–1020 Ma) involved widespread, dominantly upper amphibolite facies metamorphism within and beneath the duplex. Deformation and metamorphism recently reported from south and east of the Parry Sound domain at c. 1100–1040 Ma have not yet been documented along the Georgian Bay transect. The data suggest that early convergence was followed by a period of crustal thickening in the orogenic core south-east of the transect area, with further advance to the north-west during and after the waning stages of this deformation.

[1] The Trans-Hudson Orogen (THO) of North America and the Himalaya-Karakoram-Tibetan Orogen (HKTO) of Asia preserve a Paleoproterozoic and Cenozoic record, respectively, of continent-continent collision that is notably similar in scale, duration and character. In THO, the tectonothermal evolution of the lower plate involves (1) early thin-skinned thrusting and Barrovian metamorphism, (2) out-of-sequence thrusting and high-T metamorphism, and (3) fluid-localized reequilibration, anatexis, and leucogranite formation. The crustal evolution of the Indian lower plate in HKTO involves (1) early subduction of continental crust to ultrahigh pressure (UHP) eclogite depths, (2) regional Barrovian metamorphism, and (3) widespread high-T metamorphism, anatexis, and leucogranite formation. The shallow depths of the high-T metamorphism in HKTO are consistent with early to mid-Miocene ductile flow of an Indian lower plate midcrustal channel, from beneath the southern Tibetan Plateau to the Greater Himalaya. Melt weakening of the lower plate in THO is not observed at a similar scale probably due to the paucity of pelitic lithologies. Tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks, emplacement of Andean-type plutonic suites, and high-T metamorphism. Syncollisional to postcollisional events include emplacement of garnet-biotite-muscovite leucogranites, anatectic granites, and sporadic metamorphism (up to 90 Myr following the onset of collision in THO). Comparing the type and duration of tectonothermal events for THO and HKTO supports the notion of tectonic uniformitarianism for at least the later half of dated Earth history and highlights the complementary nature of the rock record in an older “exhumed” orogen compared to one undergoing present-day orogenesis.