M. R. St-Onge

Constraints on Pb closure temperature in titanite based on rocks from the Ungava orogen, Canada: Implications for U-Pb geochronology and P-T-t path determinations

On the basis of multiequilibrium metamorphic pressure-temperature ( P-T ) estimates, we have determined that 660–700 °;C is a reasonable minimum estimate of the closure T of Pb diffusion in titanite, a temperature significantly higher than previously suggested. Activity-corrected reactions that produce titanite define narrow T stability fields (≈40 °;C) that correspond closely to metamorphic P-T determinations derived for coronitic garnet-clinopyroxene-hornblende samples. Growth of titanite, occurring between 1789 and 1814 Ma in the underthrust Archean basement of the Paleoproterozoic Ungava orogen (northern Quebec), is associated with retrograde metamorphism during underthrusting of the Archean basement (Approx.)1 b.y. after formation of the original granulitic assemblages.

Correlation of Archaean and Palaeoproterozoic units between northeastern Canada and western Greenland: constraining the pre-collisional upper plate accretionary history of the Trans-Hudson orogen

Abstract Based on available tectonostratigraphic, geochronological, and structural data for northeastern Canada and western Greenland, we propose that the early, upper plate history of the Trans-Hudson orogen was characterized by a number of accretionary–tectonic events, which led to the nucleation and growth of a northern composite continent (the Churchill domain), prior to terminal collision with and indentation by the lower plate Superior craton. Between 1.96 and 1.91 Ga Palaeoproterozoic deformation and magmatism along the northern margin of the Rae craton is documented both in northeastern Canada (Ellesmere–Devon terrane) and in northern West Greenland (Etah Group–metaigneous complex). The southern margin of the craton was dominated by the accumulation of a thick continental margin sequence between c. 2.16 and 1.89 Ga, whose correlative components are recognized on Baffin Island (Piling and Hoare Bay groups) and in West Greenland (Karrat and Anap nunâ groups). Initiation of north–south convergence led to accretion of the Meta Incognita microcontinent to the southern margin of the Rae craton at c. 1.88–1.865 Ga on Baffin Island. Accretion of the Aasiaat domain (microcontinental fragment?) in West Greenland to the Rae craton resulted in formation of the Rinkian fold belt at c. 1.88 Ga. Subsequent accretion–collision of the North Atlantic craton with the southern margin of the composite Rae craton and Aasiaat domain is bracketed between c. 1.86 and 1.84 Ga (Nagssugtoqidian orogen), whereas collision of the North Atlantic craton with the eastern margin of Meta Incognita microcontinent in Labrador is constrained at c. 1.87–1.85 Ga (Torngat orogen). Accretion of the intra-oceanic Narsajuaq arc terrane of northern Quebec (no correlative in Greenland) to the southern margin of the composite Churchill domain at 1.845 Ga was followed by terminal collision between the lower plate Superior craton (no correlative in Greenland) and the composite, upper plate Churchill domain in northern and eastern Quebec at c. 1.82–1.795 Ga. Taken as a set, the accretionary–tectonic events documented in Canada and Greenland prior to collision of the lower plate Superior craton constrain the key processes of crustal accretion during the growth of northeastern Laurentia and specifically those in the upper plate Churchill domain of the Trans-Hudson orogen during the Palaeoproterozoic Era. This period of crustal amalgamation can be compared directly with that of the upper plate Asian continent prior to its collision with the lower plate Indian subcontinent in the early Eocene. In both cases, terminal continental collision was preceded by several important episodes of upper plate crustal accretion and collision, which may therefore be considered as a harbinger of collisional orogenesis and a signature of the formation of supercontinents, such as Nuna (Palaeoproterozoic Era) and Amasia (Cenozoic Era).

Syn-tectonic magmatism and the development of compositional layering, Ungava Orogen (northern Quebec, Canada)

Abstract Layer- and foliation-parallel emplacement of granitic veins was an important process in the regional development of compositional layering in now-exhumed middle crustal sections of both Archean and Paleoproterozoic age in the northern Ungava peninsula, Quebec (Canada). In the Archean Superior Province, diorite and tonalite plutons were penetratively deformed and metamorphosed at granulite-facies conditions coeval with voluminous granitic magmatism. The Paleoproterozoic Narsajuaq arc contains evidence for contemporaneous magmatism, transpressional deformation and granulite-facies metamorphism prior to its collision with the Superior Province basement. In both plutonic domains, regional compositional layering is defined by (1) metre to kilometre-scale alternation of generally well foliated tonalite and quartz diorite bodies; and (2) centimetre to kilometre-scale, variably deformed granitic veins and sheets that lie parallel to layering/tectonic foliation in the host rocks. Syn-tectonic intrusion of a substantial portion of the veins along extension fractures sub-parallel to layering/foliation (i.e. at high angle to the regional shortening direction) is interpreted to have occurred due to the combination of a strong anisotropy and high magma pressures. Compositional layering generated and/or enhanced by this process may contribute to the overall seismic reflectivity of the middle and lower crust.

Paleoproterozoic tectonic assembly of Northeast Laurentia through multiple indentations

Recent detailed geological mapping and UPb geochronological studies in the Torngat, Ungava and Baffin orogens of Northeast Laurentia have provided new information regarding the tectonic assembly of this region between ∼1.92 and 1.74 Ga. In contrast with an earlier hypothesis whereby the geometry of Archean continental fragments was thought to have resulted entirely from northerly indentation of the Superior Province and consequent extrusion tectonics, interpretation of recently acquired data shows that Superior Province indentation was the last of multiple collisional events during the formation of Northeast Laurentia. In the present model, we propose that Southeast Rae and North Rae provinces were separate prior to ∼1.92 Ga, when regional tectonism changed from extension to contraction. Westerly subduction and westward translation of the Archean Disko terrane and Nain Province between the North Rae and Southeast Rae provinces resulted in the development of a continental magmatic arc in the Cumberland batholithic complex and in the Torngat orogen, and deposition of the upper sequences of the Penrhyn-Piling-Karrat sequences in the Foxe and Rinkian belts of the Baffin orogen (1880–1895 Ma). Continental collision between the Disko terrane and North Rae Province produced shortening across the Rinkian and Foxe belts at, or before, 1876 Ma. Meanwhile, early arc magmatism in the Ungava orogen (1861–1898 Ma) accompanied foredeep or transtensional basin formation in the New Quebec orogen (∼1870–1883 Ma) and the onset of deformation in the southeastern Nagssugtoqidian orogen (∼1883 Ma). Nain-Southeast Rae provinces collision at ∼1860 Ma across the Torngat orogen caused renewed granitic magmatism and metamorphism in the Cumberland batholithic complex between 1857 and 1853 Ma. Initiation of felsic volcanism in the Makkovik-Ketilidian orogen at this time indicates a shift in the locus of plate subduction from the Southeast Rae Province to the southern margin of the Nain Province. In the Ungava orogen, northerly subduction changed to southerly subduction resulting in the generation of a second arc suite at between 1844 and 1826 Ma. Formation of the de Pas batholith in the hinterland of the New Quebec orogen at ∼1840 Ma accompanied arc-continent and Superior-Southeast Rae collision from 1845 to 1829 Ma. At this time, accretion of arc material in the Makkovik-Ketilidian orogen was contemporaneous with high-grade sinistral shearing along the reactivated Torngat orogen and the northwestern Nagssugtoqidian orogen. Southeast-directed thrusting along the southeastern margin of the Cumberland batholithic complex, and reactivation of the Rinkian belt is predicted to have also occurred at this time. At ⩽1826 Ma, the onset of arc-continent collision across the Ungava orogen initiated closure of the North Rae-Southeast Rae provinces juncture and generation of the Ford Lake plutonic suite. Subsequent Superior-Southeast Rae continental collision (∼1810 Ma) resulted in dextral “escape tectonics” in the Foxe and Rinkian belts, and thick-skinned folding of basement and cover sequences in the Ungava orogen. High-grade metamorphism and granitoid magmatism in the Makkovik-Ketilidian orogen also occurred. Reactivation of the Nain-Southeast Rae (east-directed thrusting) and Superior-Southeast Rae (west-directed thrusting) margins at ∼1790 Ma resulted in “pop-up” of the Southeast Rae Province between its more rigid neighbors. Final cooling and the emplacement of post-tectonic granitoid rocks took place at ∼ 1740–1790 Ma.

Early partitioning of Quebec: Microcontinent formation in the Paleoproterozoic

The distribution of allochthonous versus parautochthonous carbonate platforms, combined with the timing of initial continental-rift magmatism versus the timing of subsequent rifting of the continental margin, provides tectonostratigraphic evidence for the formation and isolation of the oldest documented microcontinent, perhaps as a consequence of the impingement of a mantle plume onto a cratonic margin during the Paleoproterozoic. Initial rifting of the Archean nucleus of North America in eastern Canada is constrained to have been diachronous between 2.17 and 2.03 Ga. Renewed rifting of a segment of the continental margin, 292 to 120 m.y. later, was accompanied by the emplacement of ultramafic layered sills, the accumulation of komatiitic and alkalic basalts, the deposition of banded iron formations and the isolation of a microcontinent and its 1.93 Ga continental shelf succession (subsequently accreted to the telescoped continental margin during collisional orogenesis).

Geology and Chemistry of the Early Proterozoic Purtuniq Ophiolite, Cape Smith Belt, Northern Quebec, Canada

The two-billion year-old Purtuniq ophiolite comprises pillowed mafic flows, sheeted mafic dykes, gabbros, and minor plagiogranites, and an extensive suite of layered mafic and ultramafic cumulate rocks; depleted mantle rocks have not been observed. The tectonically dismembered ophiolite is similar in most physical and chemical respects to Phanerozoic ophiolites, and represents direct evidence for modern-style plate tectonic processes in the early Proterozoic. The preservedcrustal thickness is of the order of 7. 5–8 km; but may originally have been as much as 9–10 km thick. The pillowed volcanic rocks and sheeted dykes are tholeiitic, with rare earth and other trace element abundances most similar to modern MORBs. The cumulate rocks a,lso follow a tholeiitic trend in major element composition. The mafic nature of the volcanic pile, the absence of extensive pyroclastic rocks, and the presence of a well developed sheeted dyke complex stratigraphically underlying the volcanic rocks supports an oceanic spreading center origin for the ophiolite. Modally graded layers and adcumulate textures observed in thin section suggest that the layered mafic and ultramafic rocks are dominantly the result of cumulus crystallization. Cryptic compositional variations in relict igneous minerals, such as forsterite content in olivine, and chromium and titanium content of clinopyroxene do not vary systematically with stratigraphic height in the cumulate pile. This is thought to record the periodic input of fresh batches of primitive magma into the magma chamber. Nd-isotopic data suggest that two time-integrated depleted mantle sources were responsible for the generation of the ophiolite, one highly depleted (∈Nd + 4. 6 to + 5. 3), the other less so (∈Nd +2. 5 to +3. 6). Each source produced a suite of cumulate rocks and sheeted mafic dykes. The mutually intrusive nature of sheeted dykes from the two suites suggests that the two sources operated simultaneously, and in close proximity to one another. The physical extent of the ocean basin in which the ophiolite was generated is not well constrained.

Nd isotopic and geochemical signature of the Paleoproterozoic Trans-Hudson Orogen, southern Baffin Island, Canada: implications for the evolution of eastern Laurentia

Abstract The southern Baffin Island segment of the Trans-Hudson Orogen comprises a three-part crustal architecture. The lowest structural level (level 1) includes ca. 2.88 Ga tonalite-diorite-granite orthogneisses of the parautochthonous Superior Province. Level 2 comprises monzogranite-tonalite-diorite orthogneisses of the 1.86–1.82 Ga Narsajuaq arc. At the highest structural level (level 3), ca. 1.95 Ga tonalite-monzogranite orthogneiss, ca. 1.93 Ga shelf metasediments of the Lake Harbour Group, and dominantly psammitic metasediments of the Blandford Bay assemblage are tectonically imbricated and cross-cut by 1.86–1.85 Ga monzogranitic to tonalitic rocks of the Cumberland batholith. Level 2 Narsajuaq arc felsic orthogneisses display strongly fractionated rare earth elements, with La n /Yb n >100, e Nd (1.85 Ga) values ranging from −12 to −19, and T DM ages from 2.7 to 3.6 Ga, consistent with derivation by underplating and melting of strongly fractionated crust bearing components as old as Paleoarchean, such as parts of the Nain Province or the northernmost Superior Province. The Narsajuaq arc rocks on southern Baffin Island show more evolved Nd isotopic compositions and strongly fractionated REE profiles relative to correlated rocks on the Ungava Peninsula in northern Quebec. The north-south change in Nd isotopic and REE geochemical signatures is consistent with northward subduction beneath an Archean cratonic nucleus. At structural level 3, the peraluminous Cumberland batholith shows La n /Yb n mostly e Nd (1.85 Ga) values from −2.7 to −7.4, and T DM ages from 2.35 to 3.08 Ga. The data are consistent with pervasive assimilation of Lake Harbour Group metasediments ( e Nd (1.85 Ga)=−4.5 to −7.3) and lesser amounts of Ramsay River orthogneiss ( e Nd (1.85 Ga)=−5.2 to −16.5). Lake Harbour Group Nd isotopic and REE compositions are similar to the Tasiuyak metasedimentary gneiss of Labrador, and support correlation of the two units. The Blandford Bay metasediments show Nd isotopic evidence for contribution of Mesoarchean detritus ( e Nd (1.85 Ga)=−10.9 to −13.4), and may be derived from Archean Nain Province. The distinctive lithologies, Nd isotopic signatures, REE chemistry and crystallization ages of structural levels 2 and 3 metaplutonic rocks suggest that they represent two separate Paleoproterozoic terranes showing distinctive crustal evolution histories involving separate and distinctive Archean crustal nuclei. Tectonic models for the eastern Trans-Hudson Orogen require two spatially-distinct 1.86–1.82 Ga subduction systems reworking distinct Archean cratonic margins in order to account for the differences between Paleoproterozoic orthogneisses on southern Baffin Island. One subduction system would have produced the Narsajuaq arc of level 2, by pervasive assimilation of crustal rocks with e Nd (1.85 Ga) lower than −11.6 and La n /Yb n >100. The differences between the less evolved Nd isotopic compositions of the Narsajuaq arc of Ungava Peninsula and the highly evolved Nd isotopic compositions of the Narsajuaq arc rocks on Baffin Island may explained by: (i) subduction beneath a northward-thickening Archean cratonic block; or (ii) by a relict north–south age boundary in the Archean cratonic margin. A second subduction zone would have produced the Cumberland batholith by melting and assimilation of a cratonic margin less fractionated REE and more elevated e Nd values. Data from structural levels 2 and 3 are consistent with correlation of these rocks across the eastern segments of the Trans-Hudson Orogen.

Record of modern-style plate tectonics in the Palaeoproterozoic Trans-Hudson orogen

The Trans-Hudson orogen of North America is a circa 1,800 million year old, middle Palaeoproterozoic continental collisional belt. The orogen may represent an ancient analogue to the Himalayan orogen, which began forming 50 million years ago and remains active today. Both mountain belts exhibit similar length scales of deformation and timescales of magmatism and metamorphism. A notable divergence in this correlation has been the absence of high-pressure, low-temperature metamorphic rocks in the Trans-Hudson compared with the Himalaya. It has been debated whether this absence reflects a secular tectonic change, with the requisite cool thermal gradients precluded by warmer ambient mantle temperatures during the Palaeoproterozoic, or a lack of preservation. Here we identify eclogite rocks within the Trans-Hudson orogen. These rocks, which typically form at high pressures and cool temperatures during subduction, fill the gap in the comparative geologic record between the Trans-Hudson and Himalayan orogens. Through the application of phase equilibria modelling and in situ U–Pb monazite dating we show that the pressure–temperature conditions and relative timing of eclogite-facies metamorphism are comparable in both orogenies. The results imply that modern-day plate tectonic processes featuring deep continental subduction occurred at least 1,830 million years ago. This study highlights that the global metamorphic rock record (particularly in older terrains) is skewed by overprinting and erosion. The timing of onset of modern-style plate tectonics on Earth is unclear. Identification of eclogite rocks—typically formed during subduction—in the Trans-Hudson orogen implies modern-style tectonics may have been active 1,830 million years ago.

Terrane boundaries within Trans-Hudson Orogen (Quebec–Baffin segment), Canada: changing structural and metamorphic character from foreland to hinterland

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.

Long-lived continent-ocean interaction in the Early Proterozoic Ungava orogen, northern Quebec, Canada

The Early Proterozoic Ungava orogen contains evidence for the development of a longlived (>80 m.y.) and presumably wide ocean that was subsequently destroyed during plate convergence leading to arc-continent collision. A magmatic arc was built on 2.00 Ga oceanic crust above a north-dipping subduction zone between about 1.90 and 1.86 Ga. Development of a south-verging continental thrust belt containing units associated with prior rifting of the Archean Superior province basement probably records impingement of the upper plate against the underthrust continental margin at ca. 1.87 Ga. Subduction of the Superior province continental crust is inferred to have led to a reversal in subduction polarity. A younger, south-dipping subduction zone is suggested by a suite of 1.85-1.83 Ga quartz diorite to granite plutons that intrude the upper- plate oceanic crust and the older arc. The arc-continent collision resulted in a minimum of 65 km of tectonic overlap of the composite arc9s plutonic core on the autochthonous footwall basement.