Late Archean Terrane Research Articles

North Atlantic Craton architecture revealed by kimberlite-hosted crustal zircons

Archean cratons are composites of terranes formed at different times, juxtaposed during craton assembly. Cratons are underpinned by a deep lithospheric root, and models for the development of this cratonic lithosphere include both vertical and horizontal accretion. How different Archean terranes at the surface are reflected vertically within the lithosphere, which might inform on modes of formation, is poorly constrained. Kimberlites, which originate from significant depths within the upper mantle, sample cratonic interiors. The North Atlantic Craton, West Greenland, comprises Eoarchean and Mesoarchean gneiss terranes – the latter including the Akia Terrane – assembled during the late Archean. We report U–Pb and Hf isotopic, and trace element, data measured in zircon xenocrysts from a Neoproterozoic (557 Ma) kimberlite which intruded the Mesoarchean Akia Terrane. The zircon trace element profiles suggest they crystallized from evolved magmas, and their Eo- to Neoarchean U–Pb ages match the surrounding gneiss terranes, and highlight that magmatism was episodic. Zircon Hf isotope values lie within two crustal evolution trends: a Mesoarchean trend and an Eoarchean trend. The Eoarchean trend is anchored on 3.8 Ga orthogneiss, and includes 3.6–3.5 Ga, 2.7 and 2.5–2.4 Ga aged zircons. The Mesoarchean Akia Terrane may have been built upon mafic crust, in which case all zircons whose Hf isotopes lie within the Eoarchean trend were derived from the surrounding Eoarchean gneiss terranes, emplaced under the Akia Terrane after ca. 2.97 or 2.7 Ga, perhaps during late Archean terrane assembly. Kimberlite-hosted peridotite rhenium depletion model ages suggest a late Archean stabilization for the lithospheric mantle. The zircon data support a model of lithospheric growth via tectonic stacking for the North Atlantic Craton.

Two cycles of voluminous pyroclastic volcanism and sedimentation related to episodic granite emplacement during the late Archean: Eastern Yilgarn Craton, Western Australia

The thick piles of late-Archean volcaniclastic sedimentary successions that overlie the voluminous greenstone units of the eastern Yilgarn Craton, Western Australia, record the important transition from the cessation in mafic-ultramafic volcanism to cratonisation between about 2690 and 2655 Ma. Unfortunately, an inability to clearly subdivide the superficially similar sedimentary successions and correlate them between the various geological terranes and domains of the eastern Yilgarn Craton has led to uncertainty about the timing and nature of the region's palaeogeographic and palaeotectonic evolution. Here, we present the results of some 2025 U–Pb laser-ablation-ICP-MS analyses and 323 Sensitive High-Resolution Ion Microprobe (SHRIMP) analyses of detrital zircons from 14 late-Archean felsic clastic successions of the eastern Yilgarn Craton, which have enabled correlation of clastic successions. The results of our data, together with those compiled from previous studies, show that the post-greenstone sedimentary successions include two major cycles that both commenced with voluminous pyroclastic volcanism and ended with widespread exhumation and erosion associated with granite emplacement. Cycle One commences with an influx of rapidly reworked feldspar-rich pyroclastic debris. These units, here-named the Early Black Flag Group, are dominated by a single population of detrital zircons with an average age of 2690–2680 Ma. Thick (up to 2 km) dolerite bodies, such as the Golden Mile Dolerite, intrude the upper parts of the Early Black Flag Group at about 2680 Ma. Incipient development of large granite domes during Cycle One created extensional basins predominantly near their southeastern and northwestern margins (e.g., St Ives, Wallaby, Kanowna Belle and Agnew), into which the Early Black Flag Group and overlying coarse mafic conglomerate facies of the Late Black Flag Group were deposited. The clast compositions and detrital-zircon ages of the late Black Flag Group detritus match closely the nearby and/or stratigraphically underlying successions, thus suggesting relatively local provenance. Cycle Two involved a similar progression to that observed in Cycle One, but the age and composition of the detritus were notably different. Deposition of rapidly reworked quartz-rich pyroclastic deposits dominated by a single detrital-zircon age population of 2670–2660 Ma heralded the beginning of Cycle Two. These coarse-grained quartz-rich units, are name here the Early Merougil Group. The mean ages of the detrital zircons from the Early Merougil Group match closely the age of the peak in high-Ca (quartz-rich) granite magmatism in the Yilgarn Craton and thus probably represent the surface expression of the same event. Successions of the Late Merougil Group are dominated by coarse felsic conglomerate with abundant volcanic quartz. Although the detrital zircons in these successions have a broad spread of age, the principal sub-populations have ages of about 2665 Ma and thus match closely those of the Early Merougil Group. These successions occur most commonly at the northwestern and southeastern margins of the granite batholiths and thus are interpreted to represent resedimented units dominted by the stratigraphically underlying packages of the Early Merougil Group. The Kurrawang Group is the youngest sedimentary units identified in this study and is dominated by polymictic conglomerate with clasts of banded iron formation (BIF), granite and quartzite near the base and quartz-rich sandstone units containing detrital zircons aged up to 3500 Ma near the top. These units record provenance from deeper and/or more-distal sources. We suggest here that the principal driver for the major episodes of volcanism, sedimentation and deformation associated with basin development was the progressive emplacement of large granite batholiths. This interpretation has important implication for palaeogeographic and palaeotectonic evolution of all late-Archean terranes around the world.

Insights on Archean continental geodynamics from gravity modelling of granite–greenstone terranes

Abstract This work investigates continental geodynamics, in the Middle and Late Archean, by way of gravity models of cratonic regions that range in age from ca. 3.4 to ca. 2.7 Ga. Selected regions of three major cratons are compared: the Pilbara craton, Australia (3.4–3.2 Ga), the Yilgarn craton, Australia (2.7 Ga) and the Abitibi Subprovince (2.7 Ga), which is part of the Superior Province, Canada. The modelled regions include large tonalite–granodiorite batholiths hosted within greenstones. Each region is modelled using several 2-D gravity profiles to obtain a 3-D picture of the first order structure of the upper to middle crust. In the 3.4 Ga Pilbara craton, some modelled batholiths have well defined roots that extend to depths greater than 10 km whereas others have roots extending to less than 6 km depth. The greenstones form keels, some extending to 10 km depth. The models for the 2.7 Ga Yilgarn craton and the Abitibi Subprovince show no major differences between the two younger terranes. In them, batholiths share a commonly thin shape with shallow roots that are not as well defined as those of the Pilbara batholiths. The rarity of deep roots and a thickness of 5–6 km are characteristics of the ca. 2.7 Ga batholiths. The surrounding greenstones form keels of up to 10 km depth, as in the modelled regions of the older Pilbara craton. Comparison of the results from the older and younger cratons suggests the following. (1) The presence (Middle Archean) or quasi-absence (Late Archean) of deep batholith roots may indicate changes in continental geodynamics and in the processes of cratonic formation and batholith assembly from 3.4 to 2.7 Ga. The differences in batholith shapes may be interpreted as the result of an evolution in the processes involved in crustal differentiation from a crustal-scale diapirism-type model prior to 2.8 Ga to a more plate tectonic-like model after 2.8 Ga. At least, the model results suggest that if crustal diapirism were an important process in the Archean, it may have been limited to the period preceding 2.8 Ga. (2) Greenstones in the Middle and Late Archean terranes form deep keels, which are consistent with the crustal diapirism model in the case of the older greenstones, and may be explained by crustal folding in the younger terranes.

Gold Deposits in Metamorphic Belts: Overview of Current Understanding,Outstanding Problems, Future Research, and Exploration Significance

Gold Deposits in Metamorphic Belts: Overview of Current Understanding,Outstanding Problems, Future Research, and Exploration Significance

Testing the model of late Archean terrane accretion in southern West Greenland: a comparison of the timing of geological events across the Qarliit nunaat fault, Buksefjorden region

The prevailing tectonic model for the Archean gneiss complex of southern West Greenland holds that terranes with different metamorphic histories and protolith ages were juxtaposed between ∼2.80 and 2.72 Ga. This model was tested by using detailed mapping and U–Pb geochronology to compare the timing of geological events across the Qarliit nunaat fault, the boundary between terranes in the Buksefjorden region. The fault was interpreted as a terrane boundary mostly because it is a narrow zone of mylonitic rocks that separates granulite (or retrograde amphibolite) facies gneisses of the Tasiusarsuaq terrane (TT) from prograde amphibolite facies gneisses of the Akulleq terrane (AT), and it separates tonalitic gneisses of different age (3.87–3.60 and 2.80 Ga in the AT versus 2.92–2.86 Ga in the TT). Dating of similar homogeneous gneisses in each terrane yielded ages of 2843±3 Ma in the TT (tonalite) and 2824±2 Ma in the AT (granodiorite, Ikkattoq gneiss). Granulite facies metamorphism in the TT is thought to have coincided with intrusion of a 2805±2 Ma orthopyroxene pegmatite, and zircon and titanite from nearby amphibolitic gneiss yielded ages of 2779±2 and 2660±10 Ma, respectively. Metamorphic minerals in the AT are younger: zircon and titanite from amphibolitic gneiss are 2717±3 and 2510±10 Ma, respectively, and monazite from pelitic schist is 2732±5 and 2708±4 Ma. A concordant pegmatite layer that underwent a substantial amount of shearing along the Qarliit nunaat fault was dated at 2725±2. Other pegmatites that intruded relatively late in the structural history crystallized at 2710±2 in the TT and 2541±2 Ma in the AT. These results support the prevailing tectonic model by showing that (i) similar homogeneous gneisses in each terrane are different age; (ii) ∼2805 Ma granulite facies metamorphism in the TT predated 2740–2700 Ma amphibolite facies metamorphism in the AT; and (iii) shearing along the terrane boundary occurred after ∼2725 Ma.

Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation — Archean to present: a review: R. Kerrich & K. F. Cassidy, Ore Geology Reviews, 9(4), 1994, pp 263–310

Abstract Late Archean (ca. 2.7 Ga) lode gold mineralization in the Superior Province of Canada and Yilgarn Craton of Australia was late kinematic, and syn to late peak metamorphic, and intimately related in space and time to accretionary tectonic processes. Gold mineralization in the Superior Province was coeval with late-kinematic diachronous accretion of allochthonous subprovinces from north to south over ca. 2710 to 2670 Ma. Timing of gold genesis is constrained by field relationships, and rigorous lower ages of cross-cutting dikes. Gold vein formation is an integral part of the thermal and rheological history of deposit host rocks, such that, in general, deposits in greenschist facies rocks have greenschist facies assemblages and occur in brittle-ductile structures, whereas counterparts in amphibolite facies rocks feature amphibolite facies alteration assemblages and generally occur in ductile structures. Gold mineralization in the southeastern Yilgarn Craton is also temporally related to craton-scale compressional events. There is abundant evidence for thermal, structural and hydrothermal disturbance post orogeny in the Yilgarn Craton, with most ArAr, RbSr and reset UPb zircon ages corresponding to known periods of tectonism in adjacent terranes. In addition, gold-related sulphides typically have reset model Pb ages recorded by secondary isochrons. Accordingly, anomalously young UPb ages on rutile and titanite and ArAr ages on hydrothermal silicates in deposits that post-date peak metamorphism by 100 to 240 m.y. are interpreted to reflect multiple secondary hydrothermal events that also locally remobilized primary ore metals. These variably young, reset ages are also recorded in lithological units of undisputed primary age, including VMS deposits, syn-tectonic granitoids and komatiites. These variably young ages provide an exact parallel for reset secondary ages in gold deposits. Mineralization interpreted to have formed 100 to 240 m.y. after terrane accretion based on scattered ages from non-robust isotopic systems or minerals is not compatible with the above relationships, but rather reflects intermittent secondary reactivation of the primary ore hosting structures. The southern Superior Province has experienced thermal, structural and hydrothermal disturbances since termination of S-type magmatism at ca. 2645–2611 Ma, reflected in KAr, RbSr and SmNd ages 100 to 240 m.y. younger than UPb zircon ages on the same lithological units, and reversed apparent blocking sequences. Eight different models, with distinct time connotations, exist for lode gold deposits in the late Archean terranes. The TTG model linking mineralization in the southern Abitibi Subprovince to synvolcanic tonalite-trondhjemite granodiorite batholiths (2695–2685 Ma) can be ruled out on geochronological grounds, as the deposits are most closely associated in space and time to late-kinematic, calc-alkaline and shoshonitic rocks (2685–2670 Ma). Two “late gold” models based on the non-concordant young ages, involving either delayed thermal rebound or young events in Kapuskasing-like mid-crust can be reinterpreted as late kinematic primary mineralization with multistage isotopic resetting. Both young gold models are based on geochronology alone, and ignore tectonic, structural and metamorphic relationships that provide evidence of deposits forming in thermal and rheological equilibrium with their host terranes. Five models (“early gold” models) for lode gold deposits in late-Archean terranes, namely granulization, cratonization, two gold-shoshonite schemes and the crustal continuum model, explicitly relate mineralization to processes resulting from terrane accretion. All five are consistent with rigorously constrained ages of some gold deposits, including primary mineralization at Val d'Or as given by hydrothermal zircons. Lode gold deposits in the Proterozoic Trans-Hudson orogenic belt (ca. 2.0 to 1.7 Ga) also closely follow terrane collision and metamorphism. Similarly, lode gold deposits in late Paleozoic orogenic belts of Appalachia and the Lachlan fold belt of southeastern Australia show temporal relationships of mineralization to collisional tectonics, metamorphism, magmatism and deformation. The temporal relationships of mineralization to collisional tectonics is also repeated in the Mesozoic-Cenozoic Cordilleran belt of the North American continent and the Cenozoic Alpine-Himalayan chain. Giant lode gold metallogenic provinces formed at three times in Earth history, in the late-Archean (2.7-2.6 Ga), late Paleozoic (450–340 Ma) and Mesozoic-Cenozoic. These times correspond to accretionary tectonic assembly of a supercontinent, in external orogens. Minor gold deposits also formed in space and time with collisional tectonics in internal orogenic belts of the supercontinent cycle. Consequently, the local temporal relationships of lode gold mineralization to collisional tectonics and post-peak metamorphism is also part of the secular development of a larger global geodynamic cycle.

Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation — Archean to present: A review

Late Archean (ca. 2.7 Ga) lode gold mineralization in the Superior Province of Canada and Yilgarn Craton of Australia was late kinematic, and syn to late peak metamorphic, and intimately related in space and time to accretionary tectonic processes. Gold mineralization in the Superior Province was coeval with late-kinematic diachronous accretion of allochthonous subprovinces from north to south over ca. 2710 to 2670 Ma. Timing of gold genesis is constrained by field relationships, and rigorous lower ages of cross-cutting dikes. Gold vein formation is an integral part of the thermal and rheological history of deposit host rocks, such that, in general, deposits in greenschist facies rocks have greenschist facies assemblages and occur in brittle-ductile structures, whereas counterparts in amphibolite facies rocks feature amphibolite facies alteration assemblages and generally occur in ductile structures. Gold mineralization in the southeastern Yilgarn Craton is also temporally related to craton-scale compressional events. There is abundant evidence for thermal, structural and hydrothermal disturbance post orogeny in the Yilgarn Craton, with most ArAr, RbSr and reset UPb zircon ages corresponding to known periods of tectonism in adjacent terranes. In addition, gold-related sulphides typically have reset model Pb ages recorded by secondary isochrons. Accordingly, anomalously young UPb ages on rutile and titanite and ArAr ages on hydrothermal silicates in deposits that post-date peak metamorphism by 100 to 240 m.y. are interpreted to reflect multiple secondary hydrothermal events that also locally remobilized primary ore metals. These variably young, reset ages are also recorded in lithological units of undisputed primary age, including VMS deposits, syn-tectonic granitoids and komatiites. These variably young ages provide an exact parallel for reset secondary ages in gold deposits. Mineralization interpreted to have formed 100 to 240 m.y. after terrane accretion based on scattered ages from non-robust isotopic systems or minerals is not compatible with the above relationships, but rather reflects intermittent secondary reactivation of the primary ore hosting structures. The southern Superior Province has experienced thermal, structural and hydrothermal disturbances since termination of S-type magmatism at ca. 2645–2611 Ma, reflected in KAr, RbSr and SmNd ages 100 to 240 m.y. younger than UPb zircon ages on the same lithological units, and reversed apparent blocking sequences. Eight different models, with distinct time connotations, exist for lode gold deposits in the late Archean terranes. The TTG model linking mineralization in the southern Abitibi Subprovince to synvolcanic tonalite-trondhjemite granodiorite batholiths (2695–2685 Ma) can be ruled out on geochronological grounds, as the deposits are most closely associated in space and time to late-kinematic, calc-alkaline and shoshonitic rocks (2685–2670 Ma). Two “late gold” models based on the non-concordant young ages, involving either delayed thermal rebound or young events in Kapuskasing-like mid-crust can be reinterpreted as late kinematic primary mineralization with multistage isotopic resetting. Both young gold models are based on geochronology alone, and ignore tectonic, structural and metamorphic relationships that provide evidence of deposits forming in thermal and rheological equilibrium with their host terranes. Five models (“early gold” models) for lode gold deposits in late-Archean terranes, namely granulization, cratonization, two gold-shoshonite schemes and the crustal continuum model, explicitly relate mineralization to processes resulting from terrane accretion. All five are consistent with rigorously constrained ages of some gold deposits, including primary mineralization at Val d'Or as given by hydrothermal zircons. Lode gold deposits in the Proterozoic Trans-Hudson orogenic belt (ca. 2.0 to 1.7 Ga) also closely follow terrane collision and metamorphism. Similarly, lode gold deposits in late Paleozoic orogenic belts of Appalachia and the Lachlan fold belt of southeastern Australia show temporal relationships of mineralization to collisional tectonics, metamorphism, magmatism and deformation. The temporal relationships of mineralization to collisional tectonics is also repeated in the Mesozoic-Cenozoic Cordilleran belt of the North American continent and the Cenozoic Alpine-Himalayan chain. Giant lode gold metallogenic provinces formed at three times in Earth history, in the late-Archean (2.7-2.6 Ga), late Paleozoic (450–340 Ma) and Mesozoic-Cenozoic. These times correspond to accretionary tectonic assembly of a supercontinent, in external orogens. Minor gold deposits also formed in space and time with collisional tectonics in internal orogenic belts of the supercontinent cycle. Consequently, the local temporal relationships of lode gold mineralization to collisional tectonics and post-peak metamorphism is also part of the secular development of a larger global geodynamic cycle.

Age and composition of Archean crystalline rocks from the southern Madison Range, Montana: Implications for crustal evolution in the Wyoming craton

The southern Madison Range of southwestern Montana contains two distinct Precambrian lithologic assemblages: (1) a complex of tonalitic to granitic gneisses that has been thrust over (2) a medium-grade metasupracrustal sequence dominated by pelitic schist. Crystallization ages for the protolith of a granodioritic gneiss that intruded the metasupracrustal sequence ([approximately]2.6 Ga)-along with an intercalated meta-andesite ([approximately]2.7 Ga) confirm the sequence as Archean. Chemical (major and trace element), isotopic (Rb-Sr, Sm-Nd, Pb-Pb), and geochronologic (U-Pb zircon) data for selected components of the gneiss complex indicate two groups of gneisses: an older, tonalitic to trondhjemitic group ([approximately]3.3 Ga) and a younger, mostly granitic group ([approximately]2.7 Ga). Both groups of gneisses exhibit the radiogenic Pb and nonradiogenic Nd isotopic signature characteristic of Middle and Late Archean rocks from throughout the Wyoming province. The older gneisses, in particular, appear to be compositionally, isotopically, and chronologically comparable to other Middle Archean gneisses from the northern part of the province (for example, Beartooth Mountains). The Late Archean gneisses, however, exhibit some distinct differences relative to their temporal counterparts, including (1) trace-element patterns that are more suggestive of crustal melts than subduction activity and (2) higher initial Sr isotopic ratios that suggest more involvement of older crustmore » in their petrogenesis. These comparisons suggest that the juxtaposition of Late Archean terranes in the northern Wyoming province was the result, at least in part, of intracratonic processes. 41 refs., 6 figs., 2 tabs.« less

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

The late Archean Opatica granitoid-gneiss belt is situated within the northern Abitibi Subprovince, along the northern margin of the Abitibi greenstone belt. Approximately 200 km of structural section was mapped along three traverses within the previously unstudied Opatica belt. The earliest preserved structures are penetrative foliations and stretching and mineral lineations recording regional ductile shearing (D1). Late-D1 deformation was concentrated into kilometre-scale ductile fault zones, typically with L > S tectonite fabrics. Two families of lineations are associated with D1, indicating shearing both parallel and transverse to the east-northeast trend of the belt. Lineations trending east-northeast or northwest–southeast tend to be dominant within domains separated by major fault zones. In light of the abundant evidence for early north–south compression documented throughout southern Superior Province, including the Abitibi greenstone belt, D1 is interpreted in terms of mid-crustal thrusting, probably resulting in considerable crustal thickening. Movement-sense indicators suggest that thrusting was dominantly southward vergent. D2 deformation resulted in the development of vertical, regional-scale dextral and sinistral transcurrent fault zones and open to tight upright horizontal folds of D1 fabrics. In the context of late Archean orogenesis in southern Superior Province, the tectonic histories of the Abitibi and Opatica belts should not be considered separately. The Opatica belt may correlate with the present-day mid-crustal levels of the Abitibi greenstone belt, and to crystalline complexes within the Abitibi belt. It is suggested that the Abitibi Subprovince should be viewed, at the regional scale, as a dominantly southward-vergent orogenic belt. This work demonstrates that structural study of granitoid-gneiss belts adjacent to greenstone belts can shed considerable light on the regional structure and structural evolution of late Archean terranes.