Archean Terranes Research Articles

Intraplate Seismicity of a Recently Deglaciated Shield Terrane: A Case Study from Northern Ontario, Canada

The economy of northern Ontario, Canada, is heavily dependent on mining, so accurate knowledge of seismicity is important for the safe design and op- eration of mines and other critical facilities, including a proposed underground repos- itory for nuclear waste. In this study, we analyzed 537 cataloged earthquakes that occurred from 1980 to 2006. Seismicity is mainly concentrated in topographically elevated Archean terranes northwest of Lake Superior and in the James Bay and Ka- puskasing regions. We analyzed waveforms to determine the focal depth for 331 re- corded events, using the regional depth-phase modeling (RDPM) method coupled with surface-wave relative-amplitude analysis. The majority of events are shallow ( 350 m), although in the east- ern part this pattern breaks down and some deeper earthquakes (>12 km) are ob- served. Based on a moving-window event-counting technique, we show that distinct spatial clusters of seismicity can be delineated that are statistically significant relative to background seismicity levels. A particularly active cluster is located within James Bay, where focal depths range from a few kilometers to more than 20 km. Another cluster near Kapuskasing contains deep-focus events and may occur along a hot spot track that runs through western Quebec. Near Dryden, a shallow (∼1 km) earthquake swarm concentrated in a 1 × 1 km region commenced in May 2002, faded, and then started up again in February 2003. Shallow mining-induced events are also common around Sudbury, a major world center for nickel mining. The overall pattern of seis- micity appears to correlate with upper-mantle P-wave velocity anomalies, suggesting that lateral variations in mantle rheology may play a significant role in controlling intraplate seismicity of shield areas. It is also likely that crustal stresses caused by glacial isostatic adjustment are an important factor, although the correlation of seis- micity with uplift rate is not as clear. Online Material: Focal depth solutions for earthquakes in northern Ontario, Canada.

Paleo- to Mesoarchean basement recycling and terrane definition in the Northeastern Superior Province, Québec, Canada

The Northeastern Superior Province (NESP) is a 500 km × 700 km segment of plutonic Archean crust generated by 1.2 Ga of tectonomagmatic activity. The NESP exhibits the classical plutonic series of Archean terranes, low-K tonalite–trondhjemite–granodiorite (TTG) and high-K granodiorite–granite–monzogranite (GGM) with unusually large volumes of Px-TTGs (mostly enderbites and subordinate charnockites). Volcanic rocks constitute <10% of the NESP and are dominated by magnesian basalt-komatiite assemblages and bimodal mafic tholeiitic and calc-alkaline felsic tuffs. Mapping, U/Pb geochronological, and Nd isotopic data suggest the presence of two terranes with distinct magmatic histories: the Hudson Bay to the SW, and the Rivière Arnaud to the NE. These terranes are separated by a metasedimentary basin, which may be a paleosuture. The Hudson Bay terrane is a Paleo- to Mesoarchean TTG-greenstone craton (3.8–2.9 Ga), intruded by 2.76–2.74 Ga low-K TTGs and younger 2.72–2.69 Ga high-K GGM and Px-TTGs that have low Nd isotopic values (ɛNd T = +0.45 to −5.03). Mafic to felsic volcanic rocks and mafic to ultramafic plutonic rocks <2.76 Ga also have enriched ɛNd T = +0.40 to −6.90. These radiogenic signatures (as well as inherited zircon cores) record recycling of Mesoarchean (3.8–2.9 Ga) crust, either through direct melting, or by coupled assimilation-fractional crystallization processes. In comparison, the Rivière Arnaud terrane is underlain by younger (<3.0 Ga), more juvenile TTG crust, that is intruded by plutons of low-K TTGs (2.79–2.76 Ga), high-K GGM (2.73–2.72 Ga), and Px-TTGs (2.74–2.72 Ga). Most of these rocks have positive to moderately negative ɛNd T values (+2.34 to −0.67), indicating either a juvenile character, or recycling of fairly juvenile Late Mesoarchean-Early Neoarchean crust. Overall, geochemical and isotopic data of the Late Neoarchean (<2.76 Ga) low-K TTG and Px-TTG series of the NE Superior Province are not consistent with partial melting solely of juvenile basaltic crust in an oceanic setting. The data are most consistent with remelting of a composite crust constituted of older, isotopically enriched mafic–ultramafic rocks and more juvenile metabasaltic rocks. The basaltic component may have been introduced either by crustal underplating, intra-plating or foundering of supracrustal belts. The mafic melts injected into the maturing cratonic nuclei assimilated the older rocks by an AFC-type process and provided heat. The isotopic and geochemical data imply that the high-K GGM series melts were produced by melting at shallower depth of an older composite TTG crust, with less involvement of the metabasaltic component.

The Nagssugtoqidian orogen in South-East Greenland: Evidence for Paleoproterozoic collision and plate assembly

The 200 km wide, east-west trending Paleoproterozoic mobile belt of the Ammassalik region of South-East Greenland contains a diverse assemblage of Paleoproterozoic and Archean rocks, variably affected by Paleoproterozoic deformations and high-grade low or high pressure metamorphism. By using previous field and geochemical data combined with new zircon dating and zircon trace element geochemistry, this mobile belt is confirmed as a 1870 to 1840 Ma collisional orogen, which contains one or more 1900 to 1880 Ma magmatic suites and northern and southern Archean basement terranes. The most studied 1900 to 1880 Ma magmatic suite is the Ammassalik Intrusive Complex, which is dominated by diorites (with arc-like geochemical signatures and with Paleoproterozoic Nd depleted mantle model ages), which was intruded into sedimentary rocks, (with predominantly Paleoproterozoic detrital zircons). Both these associated rock types show 1900 to 1880 Ma moderate pressure granulite facies metamorphism (7 kbar, ≥ 850°C). Paleoproterozoic mylonites separate the Ammassalik Intrusive Complex from Archean orthogneisses containing 1870 Ma eclogite (≥ 11 kbar, 650 –700°C) and high-pressure granulite facies assemblages in Palaeoproterozoic diabase dike remnants. Associated with these Archean gneisses are pelitic metasediments, marbles and orthoquartzites (with Archean detrital zircon with complex 1870 to 1740 Ma metamorphic rims) that contain kyanite, thereby also showing high-pressure metamorphism. In the Ammassalik area we propose that one or more Paleoproterozoic magmatic arcs with syn-magmatic moderate pressure, high temperature metamorphism were emplaced over the edge of southern Archean continental crust. This resulted in at least doubling of crustal thickness, causing the transient 1870 Ma eclogite to high-pressure granulite facies metamorphic conditions in the buried southern Archean terrane. Archean orthogneisses in the north of the mobile belt preserve low-pressure Archean granulite facies assemblages, and are interpreted as a different terrane at a higher structural level, which was juxtaposed with the southern eclogite-bearing Archean terrane and the lower pressure metamorphism Paleoproterozoic arc rocks during the collisional event(s). This new information from the Ammassalik region is used in a synthesis of Paleoproterozoic crustal accretion and collisional orogeny in Greenland.

Anisotropy of magnetic susceptibility studies in Tertiary ridge-parallel dykes (Iceland), Tertiary margin-normal Aishihik dykes (Yukon), and Proterozoic Kenora–Kabetogama composite dykes (Minnesota and Ontario)

Mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility (AMS) as a proxy for magmatic flow during intrusion. In Iceland, ridge-parallel basaltic dykes were sampled on each side of the active tectonic boundary. The dykes are < 10 m wide along a 1–2 km strike, and are the result of a single intrusion from 1–2 km deep magma chambers in oceanic crust. Thirteen samples were collected (7 N. American plate; 6 European) and 153 cores were analyzed by AMS and preserve a vertical K max orientation indicating vertical emplacement. The Eocene Aishihik dyke swarm intrudes the Yukon–Tanana terrane in the Yukon province, Canada over an area ~ 200 by 60 km. These dykes were intruded normal to the accretionary margin, are porphyritic andesites, and have an intermediate geochemical signature based on major and trace element analyses. Ten dykes were sampled and 111 cores analyzed using AMS, and the dykes preserve a vertical K max orientation, indicating intrusion was vertical through ~ 30 km of continental crust. The 2.06 Ga Kenora–Kabetogama dykes in northern Minnesota and western Ontario crosscut a variety of Archean terranes (thickness ~ 50 km) in a radiating pattern. The unmetamorphosed basaltic dykes are 1–120 m wide, 10–110 km in length, are vertical in orientation and can be grouped as either being single intrusion or multiple intrusion (composite) dykes. AMS data preserve a vertical K max orientation for the southerly locations (2 dykes, n = 53) and horizontal K max for the remainder to the northwest (15 dykes, n = 194). Maximum magnetic susceptibility axes (4 dykes, n = 92) for composite dykes are scattered and yield inconsistent flow directions with regard to the dyke margin. Almost all of our results are “normal” in that, the magnetic foliation (the plane containing K max and K int, normal to K min) is parallel to the dyke planes, which gives us confidence that the magnetic lineations (i.e., K max orientations) are parallel to magmatic flow.

Special Paper: Adakite-Like Rocks: Their Diverse Origins and Questionable Role in Metallogenesis

Based on a compilation of published sources, rocks referred to as adakites show the following geochemical and isotopic characteristics: SiO2 ≥56 wt percent, Al2O3 ≥15 wt percent, MgO normally <3 wt percent, Mg number ≈0.5, Sr ≥400 ppm, Y ≤18 ppm, Yb ≤1.9 ppm, Ni ≥20 ppm, Cr ≥30 ppm, Sr/Y ≥20, La/Yb ≥20, and 87Sr/86Sr ≤0.7045. Rocks with such compositions have been interpreted to be the products of hybridization of felsic partial melts from subducting oceanic crust with the peridotitic mantle wedge during ascent and are not primary magmas. High Mg andesites have been interpreted to be related to adakites by partial melting of asthenospheric peridotite contaminated by slab melts. The case for these petrogenetic models for adakites and high Mg andesites is best made in the Archean, when higher mantle geotherms resulted in subducting slabs potentially reaching partial melting temperatures at shallow depths before dehydration rendered the slab infusible. In the Phanerozoic these conditions were likely only met under certain special tectonic conditions, such as subduction of young (≤25-m.y.-old) oceanic crust. Key adakitic geochemical signatures, such as low Y and Yb concentrations and high Sr/Y and La/Yb ratios, can be generated in normal asthenosphere-derived tholeiitic to calc-alkaline arc magmas by common upper plate crustal interaction and crystal fractionation processes and do not require slab melting. An assessment of several arc volcanic suites from around the world shows that most adakite-like compositions are generated in this way and do not reflect source processes. Similarly, rare adakite-like intrusive rocks associated with some porphyry Cu deposits are the evolved products of extensive crustal-level processing of calc-alkaline basalt-andesite-dacite-rhyolite series magmas. If slab melts contribute to such magmas, their geochemical signatures would have been obliterated or rendered ambiguous by subsequent extensive open-system processes. In Archean terranes, where adakitic and high Al tonalite-trondhjemite-granodiorite (TTG) magma series rocks are more common, porphyry Cu deposits are rare and, where found, are associated with normal calc-alkaline suites rather than adakites. The two different magma series are compositionally distinct in terms of several major and trace element parameters. Common upper plate magmatic processes such as melting-assimilation-storage-homogenization (MASH) and assimilation-fractional-crystallization (AFC) affecting normal arc magmas can be demonstrated to explain the distinctive compositions of most adakite-like arc rocks, including high Mg andesites and especially those rare examples associated with porphyry Cu deposits. In contrast, slab melting can in most cases neither be proved nor disproved and is therefore unsatisfactory as a unique factor in porphyry Cu genesis.

Archean metaterrigenous rocks: Major geochemical constraints

The paper summarizes data on the geochemistry of metaterrigenous rocks from 26 reference Archean territories: the Pilbara and Yilgarn blocks; Isua and Akilia complexes; Wittwatersrand, Swaziland, Pongola, and Yellowknife supergroups; Khapchanskaya and Gimol’skaya groups; Kan, Sharyzhalgai, Chupa, Slyudyanka, and Onot complexes; etc. The general sets of data points and the calculated median values of the concentrations of trace elements and their ratios are compared to those of Archean and post-Archean shales. In Ce/Cr-Co/Hf, Eu/Eu*-GdN/YbN, Ce/Cr-Th/Sc, Th/Sc-Sc, Th-La, La/Sm-Sc/Th, Yb-GdN/YbN, Th/Sc-Cr, Ni-Cr, and some other diagrams, the fields in which the most data points of Archean metaterrigenous rocks group are outlined. The results of this research indicate that there are no values of geochemical parameters that are inherent only in Archean or only in post-Archean fine-grained terrigenous rocks. Within 80–85% confidence levels, most individual compositions of Archean metaterrigenous rocks are characterized by the following geochemical parameters: (1) Th/Sc 0.70–0.75. If the median values are used, these ranges can be further constrained to (i) Th/Sc 25, and (iv) Th < 12 ppm. Compared to PAAS, Archean metaterrigenous rocks are characterized by higher median concentrations of Cr and Ni and the Eu/Eu*, Sc/Th, Cr/Th, and Co/Hf ratios, whereas the Nb, La, Ce, Yb, Hf, Th, and U concentrations and the La/Sm and Ce/Cr ratios of PAAS are, conversely, lower. The median values of the LaN/YbN ratios of reference Archean terranes can be either higher or lower than in PAAS, likely depending on the proportions of various rock types in the sources of the terrigenous material. The medians of the GdN/YbN ratios of ∼60% of the reference Archean metaterrigenous terranes in our databank are slightly higher than the GdN/YbN ratios of PAAS. The median values of the LaN/SmN ratios of Archean terrigenous rocks are mostly slightly lower than the typical PAAS ratios.

New constraints on the seismic structure of West Australia: Evidence for terrane stabilization prior to the assembly of an ancient continent?

We present a new, near-comprehensive survey of the variations in seismic structure across the West Australian craton at the scale of the main terrane groups. Analyzing data from distant earthquakes recorded at temporary and permanent stations located across the region, we found the best-fi tting structure by modeling the conversions from P- to S-wave motion (the receiver function) that take place as the seismic energy travels upward through the lithosphere. Such methods can be used to delineate the extent of cratonic and orogenic terranes in regions where geological exposure of the surface is limited, and they provide an effective alternative to active-source seismic techniques for deep crustal targets. The seismic structure is consistent within several of the individual Archean terranes, most notably the Pilbara, Murchison, and Southern Cross. These terranes are underlain by lower crust of low seismic velocity and show a sharp seismic Moho. The structure shows signifi cant contrasts between neighboring terranes; thus, major tectonic units have a velocity profi le that is a signature of that terrane or terrane group. We infer that the seismic structure of the Archean crust and upper mantle was fixed before craton assembly and preserved through the subsequent collision and accretion of the tectonic units that formed the West Australian craton.

Zircon geochronology and Sm–Nd isotopic study: Further constraints for the Archean and Paleoproterozoic geodynamical evolution of the southeastern Guiana Shield, north of Amazonian Craton, Brazil

The eastern part of the Guiana Shield, northern Amazonian Craton, in South America, represents a large orogenic belt developed during the Transamazonian orogenic cycle (2.26–1.95 Ga), which consists of extensive areas of Paleoproterozoic crust and two major Archean terranes: the Imataca Block, in Venezuela, and the here defined Amapá Block, in the north of Brazil. Pb-evaporation on zircon and Sm–Nd on whole rock dating were provided on magmatic and metamorphic units from southwestern Amapá Block, in the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80–2.79 Ga) and during the Neoarchean (2.66–2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26–2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian orogenic cycle, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated at 2.22 Ga, 2.18 Ga and 2.05–2.03 Ga. Most of the ε Nd values and T DM model ages (2.52–2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components. The Archean Amapá Block is limited in at southwest by the Carecuru Domain, a granitoid-greenstone terrane that had a geodynamic evolution mainly during the Paleoproterozoic, related to the Transamazonian orogenic cycle. In this latter domain, a widespread calc-alkaline magmatism occurred at 2.19–2.18 Ga and at 2.15–2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the eastern Guiana Shield. Nevertheless, T DM model ages (2.50–2.38 Ga), preferentially interpreted as mixed ages, and ε Nd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. In addition, the Carecuru Domain contains an oval-shaped Archean granulitic nucleus, named Paru Domain. In this domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism are recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga. The lithological association and the available isotopic data registered in the Carecuru Domain suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian orogenic cycle, which was accreted to the southwestern border of the Archean Amapá Block.

Archean Geodynamics and Environments.: K. BENN, J-C. MARASCHEL, AND K.C. CONDIE, Editors. Pp. 320. AGU Geophysical Monograph 164, American Geophysical Union, Washington D.C. 2005. ISBN 0-87590-429-7. Price US$89.

Archean terranes are important from an economic viewpoint because they host world-class Archean orogenic gold, paleoplacer gold, VHMS, komatiite-associated Ni-Cu, and enriched BIF iron ores; world-class post-Archean intrusion-related PGE-Ni-Cu, IOCG, and diamond deposits; and a variety of regolith-related deposits, such as bauxite, nickeliferous laterite, and beach sands. Many other ore provinces, such as Carlin and those containing most giant nickel districts, are sited adjacent to Archean cratonic margins. Archean terranes are also natural laboratories in which to answer some of the fundamental questions concerning processes that operated during the early evolution of our planet, when the first continental crust formed in what was, undoubtedly, a hotter and more primitive Earth. These questions include the nature of processes operating within the mantle, the nature of early lithosphere, and the processes of formation, stabilization, and preservation of continents as related to the geodynamic processes …

U–Pb ages constraining structural development of an Archean terrane boundary in the Lake of the Woods area, western Superior Province, Canada

Layered gneisses in the Winnipeg River subprovince contain magmatic zircon with U–Pb ages of 3317 ± 9 and 3055 ± 4 Ma at Tannis Lake, and ~3170 and 3255 ± 5 Ma at Cedar Lake, indicating widespread occurrence of Mesoarchean crust. This is in contrast to the well-documented Neoarchean age of the western Wabigoon subprovince. Further geochronology using both SHRIMP (sensitive high resolution ion microprobe) and ID-TIMS (isotope dilution thermal ionization mass spectrometry), combined with structural observations, in the Kenora area and Lake of the Woods greenstone belt show the effects of juxtaposition of these two terranes. Isoclinally folded gneiss north of the subprovince boundary zone near Kenora gives a magmatic age of 2882 ± 2 Ma with 3051 ± 6 Ma inheritance. Ages of syntectonic dykes show that asymmetric refolding of these gneisses occurred between 2717 ± 2 and about 2713 ± 1 Ma. Subsequent regional vertical flattening and horizontal extension are dated at 2708 ± 2 Ma by syntectonic tonalite sheets. These events are broadly coeval with deposition of orogenic sediments in the Warclub Group and a first stage of regional folding (age brackets of 2716–2709 Ma) in the Lake of the Woods greenstone belt to the south. A second stage of folding and regional faulting in the greenstone belt occurred about 2695 ± 4 Ma and is approximately coeval with open upright folding in the Winnipeg River subprovince. These observations are consistent with overthrusting and collapse of a Mesoarchean continental terrane by a juvenile Neoarchean arc terrane over the time span 2717–2695 Ma.

Strike-slip juxtaposition of ca. 2.72 Ga juvenile arc and &gt;2.98 Ga continent margin sequences and its implications for Archean terrane accretion, western Superior Province, Canada

The North Caribou terrane of the western Superior Province attained continental thickness (~35 km) by 2997 Ma. It records a subsequent 300 million years history of continental fragmentation, arc magmatism, and terrane accretion. At Lake Winnipeg the ~2978 Ma Lewis–Storey quartzite–komatiite–iron formation assemblage marks Mesoarchean breakup. Unlike the relatively continuous 2980–2735 Ma stratigraphic record of the Red Lake and Birch–Uchi greenstone belts to the east, little of this interval is recorded at Lake Winnipeg. Rather, two belts of younger, juvenile rocks are tectonically juxtaposed: the Black Island assemblage of isotopically depleted, 2723 Ma basalt, and calc-alkaline andesite; and Rice Lake greenstone belt of basalt, calc-alkaline andesite, and dacite (2731–2729 Ma). Collectively these terranes represent a short-lived island-arc–back-arc system that docked with the southwestern North Caribou margin along a northwest-trending, dextral, transpressive, D1 suture. This zone is marked by the highly deformed coarse clastic Guano Island sequence (&lt;2728 Ma) that contains detritus of North Caribou affinity and is interpreted as a strike-slip basin deposit. Younger clastic sequences, including the Hole River (&lt;2708 Ma), San Antonio (&lt;2705 Ma), and English River (&lt;2704 Ma) assemblages, occur in east–west belts that may have been deposited during the terminal collision (D2, D3) between the North Caribou terrane and continental crust of the Winnipeg River terrane to the south. Several terrane docking events within a framework of north-dipping subduction and continental arc magmatism appear necessary to explain structural and stratigraphic relationships in the 2735–2700 Ma interval.

Archean greenstone-tonalite duality: Thermochemical mantle convection models or plate tectonics in the early Earth global dynamics?

Mantle convection and plate tectonics are one system, because oceanic plates are cold upper thermal boundary layers of the convection cells. As a corollary, Phanerozoic-style of plate tectonics or more likely a different version of it (i.e. a larger number of slowly moving plates, or similar number of faster plates) is expected to have operated in the hotter, vigorously convecting early Earth. Despite the recent advances in understanding the origin of Archean greenstone–granitoid terranes, the question regarding the operation of plate tectonics in the early Earth remains still controversial. Numerical model outputs for the Archean Earth range from predominantly shallow to flat subduction between 4.0 and 2.5 Ga and well-established steep subduction since 2.5 Ga [Abbott, D., Drury, R., Smith, W.H.F., 1994. Flat to steep transition in subduction style. Geology 22, 937–940], to no plate tectonics but rather foundering of 1000 km sectors of basaltic crust, then “resurfaced” by upper asthenospheric mantle basaltic melts that generate the observed duality of basalts and tonalities [van Thienen, P., van den Berg, A.P., Vlaar, N.J., 2004a. Production and recycling of oceanic crust in the early earth. Tectonophysics 386, 41–65; van Thienen, P., Van den Berg, A.P., Vlaar, N.J., 2004b. On the formation of continental silicic melts in thermochemical mantle convection models: implications for early Earth. Tectonophysics 394, 111–124]. These model outputs can be tested against the geological record. Greenstone belt volcanics are composites of komatiite–basalt plateau sequences erupted from deep mantle plumes and bimodal basalt–dacite sequences having the geochemical signatures of convergent margins; i.e. horizontally imbricated plateau and island arc crust. Greenstone belts from 3.8 to 2.5 Ga include volcanic types reported from Cenozoic convergent margins including: boninites; arc picrites; and the association of adakites–Mg andesites- and Nb-enriched basalts. Archean cratons were intruded by voluminous norites from the Neoarchean through Proterozoic; norites are accounted for by melting of subduction metasomatized Archean continental lithospheric mantle (CLM). Deep CLM defines Archean cratons; it extends to ∼ 350 km, includes the diamond facies, and xenoliths signify a composition of the buoyant, refractory, residue of plume melting, a natural consequence of imbricated plateau-arc crust. Voluminous tonalites of Archean greenstone–granitoid terranes show a secular trend of increasing Mg#, Cr, Ni consistent with slab melts hybridizing with thicker mantle wedge as subduction angle steepens. Strike-slip faults of 1000 km scale; diachronous accretion of distinct tectonostratigraphic terranes; and broad Cordilleran-type orogens featuring multiple sutures, and oceanward migration of arcs, in the Archean Superior and Yilgarn cratons, are in common with the Altaid and Phanerozoic Cordilleran orogens. There is increasing geological evidence of the supercontinent cycle operating back to ∼ 2.7 Ga: Kenorland or Ur ∼ 2.7–2.4 Ga; Columbia ∼ 1.6–1.4 Ga; Rodinia ∼ 1100–750 Ma; and Pangea ∼ 230 Ma. High-resolution seismic reflection profiling of Archean terranes reveals a prevalence of low angle structures, and evidence for paleo-subduction zones. Collectively, the geological–geochemical–seismic records endorse the operation of plate tectonics since the early Archean.

A trace element and chemical Th–U total Pb dating study in the lower Belt-Purcell Supergroup, Western North America: Provenance and diagenetic implications

Comprehensive geochemical and monazite chemical age data on the Mesoproterozoic Belt-Purcell Supergroup reveals information on provenance age(s), and the sources of interbedded argillites and sandstones. Compliance of rare earth element (REE) and multi-element patterns of argillites to post-Archean upper continental crust (PA-UCC), and Cr–Ni, and Th/Sc–Sc systematics are consistent with a dominantly post-Archean source area. Sandstones have the same geochemical fingerprint as argillite for the Appekunny and Grinnell formations in the lower Belt-Purcell Supergroup sequence, albeit variably depleted by detrital quartz. Sandstone developed in a separate high-energy environment, and argillite and sandstone became interbedded during storms, accounting for the sharp interbedding of the two facies. Detrital monazite chemical Th–U total Pb ages from the Appekunny and Grinnell sandstones, and argillites of the former, cluster at ∼ 1700 Ma. Paleocurrents and chemical ages support a Laurentian Paleoproterozoic provenance, likely the Yavapai, Mazatzal and Central Plains provinces. Other potential sources could have been terranes at ∼ 1875–1750 Ma that cooled through the blocking temperature of monazite ∼ 1700 Ma such as the Wyoming Province. Rare > 2500 Ma monazite ages in argillite and sandstone stem for minor contributions from Archean terrane(s). Diagenetic overprint in the Belt-Purcell rocks is widespread and displayed geochemically as heavy REE enrichment relative to light REE, normalized to PA-UCC. There are two populations of monazites: the first has detrital textures, with chemical ages > ∼ 1400 Ma, higher ThO 2 and Y, and lower LREE/HREE content than < ∼ 1400 Ma monazites with euhedral texture, ages up to ∼ 900 Ma post-deposition, with lower ThO 2 and Y, and higher LREE/HREE content. The second population is interpreted as the product of protracted migration of basinal brines that mobilized HREE, as documented in several Proterozoic sedimentary basins.

Global 1° × 1° thermal model TC1 for the continental lithosphere: Implications for lithosphere secular evolution

This paper reports a new 1° × 1° global thermal model for the continental lithosphere (TC1). Geotherms for continental terranes of different ages (> 3.6 Ga to present) constrained by reliable data on borehole heat flow measurements (Artemieva, I.M., Mooney, W.D. 2001. Thermal structure and evolution of Precambrian lithosphere: a global study. J. Geophys. Res 106, 16387–16414.), are statistically analyzed as a function of age and are used to estimate lithospheric temperatures in continental regions with no or low-quality heat flow data (ca. 60% of the continents). These data are supplemented by cratonic geotherms based on electromagnetic and xenolith data; the latter indicate the existence of Archean cratons with two characteristic thicknesses, ca. 200 and > 250 km. A map of tectono-thermal ages of lithospheric terranes complied for the continents on a 1° × 1° grid and combined with the statistical age relationship of continental geotherms ( z = 0.04 ⁎ t + 93.6, where z is lithospheric thermal thickness in km and t is age in Ma) formed the basis for a new global thermal model of the continental lithosphere (TC1). The TC1 model is presented by a set of maps, which show significant thermal heterogeneity within continental upper mantle, with the strongest lateral temperature variations (as large as 800 °C) in the shallow mantle. A map of the depth to a 550 °C isotherm (Curie isotherm for magnetite) in continental upper mantle is presented as a proxy to the thickness of the magnetic crust; the same map provides a rough estimate of elastic thickness of old (> 200 Ma) continental lithosphere, in which flexural rigidity is dominated by olivine rheology of the mantle. Statistical analysis of continental geotherms reveals that thick (> 250 km) lithosphere is restricted solely to young Archean terranes (3.0–2.6 Ga), while in old Archean cratons (3.6–3.0 Ga) lithospheric roots do not extend deeper than 200–220 km. It is proposed that the former were formed by tectonic stacking and underplating during paleocollision of continental nuclei; it is likely that such exceptionally thick lithospheric roots have a limited lateral extent and are restricted to paleoterrane boundaries. This conclusion is supported by an analysis of the growth rate of the lithosphere since the Archean, which does not reveal a peak in lithospheric volume at 2.7–2.6 Ga as expected from growth curves for juvenile crust. A pronounced peak in the rate of lithospheric growth (10–18 km 3/year) at 2.1–1.7 Ga (as compared to 5–8 km 3/year in the Archean) well correlates with a peak in the growth of juvenile crust and with a consequent global extraction of massif-type anorthosites. It is proposed that large-scale variations in lithospheric thickness at cratonic margins and at paleoterrane boundaries controlled anorogenic magmatism. In particular, mid-Proterozoic anorogenic magmatism at the cratonic margins was caused by edge-driven convection triggered by a fast growth of the lithospheric mantle at 2.1–1.7 Ga. Belts of anorogenic magmatism within cratonic interiors can be caused by a deflection of mantle heat by a locally thickened lithosphere at paleosutures and, thus, can be surface manifestations of exceptionally thick lithospheric roots. The present volume of continental lithosphere as estimated from the new global map of lithospheric thermal thickness is 27.8 (± 7.0) × 10 9 km 3 (excluding submerged terranes with continental crust); preserved continental crust comprises ca. 7.7 × 10 9 km 3. About 50% of the present continental lithosphere existed by 1.8 Ga.

The problem of deep carbon—An Archean paradox

Black carbon-rich chert dikes are common features of early Archean terranes and are especially prominent on the Pilbara Craton of Western Australia. The dikes cut across the local stratigraphy before truncating abruptly at specific stratigraphic horizons where they can be seen to interfinger with the stratigraphic succession. These uniquely Archean dike complexes are enigmatic and raise significant questions about the source of the carbon they contain—is it biogenic, and therefore indicative of the early biosphere, or abiotic, and perhaps indicative of prebiotic conditions that led to the appearance of the first life on Earth?The Strelley Pool Chert, which is exposed on the Pilbara Craton of Western Australia, forms part of the Warrawoona Group and rests unconformably on older ultramafic volcanics. The formation, which is generally less than 30m thick, consists of a relatively consistent lithologic succession found over a wide area. Locally, a basal transgressive sandstone is overlain by a sequence of carbonate, barite?, black and white laminated chert and grey–green chert which is locally stained blue green by weathered fuchsite. The formation is intersected by normal faults about which the depositional successions thickens and thins. The mineralogy and geochemistry (including stable isotope data) of the underlying dike complex closely parallels that of the lithologies preserved in the Strelley Pool Chert. Similarly, the depositional sequence of minerals in voids within the dike complex parallels that of the depositional sequence above.This new data from the Strelley Pool Chert and its associated dike complex show that the black chert dikes are an integral part of a hydrothermal mineralogical complex. The complex accumulated around vents fed by growth-fault-controlled dikes that extended to a depth of more than 2km below the Archean paleo-seafloor. The mineralogy of the sedimentary succession reflects deposition during the declining temperature regime of an evolving hydrothermal system driven by an intruding granitoid complex below. The fault system and associated hydrothermal complex evolved in the tensional environment developed during doming above the intermittently intruding granitoid complex that forms the framework of the Pilbara Protocontinent.Carbonaceous compounds contained within the black chert dikes could have been generated abiotically by a process similar to Fischer–Tropsch synthesis at elevated temperatures (200–300°C) and pressures (500bar) at a paleo-depth of 2km within the hydrothermal system. The “primordial medium” in which life first evolved may thus have been a silica-rich slurry with a low pH and a high CO2 content which contained basic abiotic organic building blocks fundamental to the origins of life. The abiotic organic output from these hydrothermal systems may overwhelm any early biospheric geochemical signal and may therefore be the most difficult environment in which to recognise the early biospheric record.

Calcite strains, kinematic indicators, and magnetic flow fabric of a Proterozoic pseudotachylyte swarm, Minnesota River valley, USA

Near Granite Falls, Minnesota sub-parallel pseudotachylyte, mafic dikes, and calcite veins crosscut Archean granulite facies rocks in the Minnesota River valley adjacent to the north-dipping Yellow Medicine Shear Zone (YMSZ; N80°E) that separates the Montevideo and Morton tectonic terranes. The docking of these two Archean terranes occurred prior to intrusion of the 2.067 Ga Kenora-Kabetogama dike swarm as demonstrated by aeromagnetic anomalies (correlated with field exposures) that cross the YMSZ without offset. Tectonic adjustments along the YMSZ associated with the Penokean Orogeny (∼ 1.8 Ga) are likely responsible for pseudotachylyte formation. Pseudotachylyte is exposed in 22 sub-parallel veins (∼ N80°E, 90°) each less than 2 cm wide across an outcrop width of 45 m. The pseudotachylyte matrix is commonly banded, and contains crystal fragments (quartz, plagioclase, amphibole, rutile, apatite, ilmenite, ulvöspinel), magnetite microlites, flow banding swirls, amygdules (filled with calcite, ankerite and siderite), collapsed vesicles, and abundant lithic clasts. Pseudotachylyte formed in a number of phases. Kinematic reconstruction is complex, utilizing winged porphyroclasts, S-C structures in the country rock, and fault drag indicators along the pseudotachylyte zones. Dextral motion along the YMSZ is the most common observation. Mechanically twinned calcite within amygdules in the pseudotachylyte preserves horizontal shortening normal to the pseudotachylyte strike. Calcite veins are apparently contemporaneous with the pseudotachylyte; one set preserves twinning strains identical to the calcite amygdule strains, and the second set contains a horizontal, vein-parallel (N70°E) shortening strain. The pseudotachylyte contains a flow fabric, as determined by AMS techniques, that is a proxy for vertical flow ( K max is vertical). The Kenora-Kabetogama dikes, identified geochemically, are locally parallel to the pseudotachylyte and the adjacent YMSZ tectonic suture and preserve a vertical-to-horizontal, dike-parallel AMS fabric from east (Franklin) to west (Granite Falls). Hornblende andesite dikes (055°, 1.8 Ga) are not found south of the suture, are not associated with pseudotachylyte and have a different paleopole and AMS fabric.

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.

Rare-earth elements in Oligo-Miocenic pelitic sediments from Lagonegro Basin, southern Apennines, Italy: implications for provenance and source area weathering

The Lagonegro Units are a part of the southern Apennines orogenic wedge. The age of the Lagonegro successions ranges from lower–middle Triassic to Oligo-Miocene. During late Cretaceous and Oligocene the deposition of calcareous-clastic sediments occurred interbedded with shales (Flysch Rosso Fm). During Oligocene and early Miocene, in the Mediterranean area, an important variation of the tectonic regime occurred, and siliciclastic sediments of the Numidian Basin unconformably lay on the Meso-Cenozoic units of the Lagonegro Basin. In the Lucanian Apennine, the Aquitanian–Langhian Numidian Flysch Fm overlies the Flysch rosso Fm. The shales of the Flysch rosso Fm have a peculiar geochemical fingerprint relative to typical shales of post-Archean age. The abundance of Ni and Cr is significantly higher and the HREE chondrite-normalized patterns are steep with a (Gd/Yb)ch>2. A supply of material from the African Archean terranes could be the cause. The palaeo-weathering indices record changes at the source, reflecting variations in the tectonic regime. The oldest samples are derived from an environment in which steady-state weathering conditions prevailed, whereas the youngest samples are related to non-steady-state weathering conditions. This difference could record deformational events that affected the Mediterranean area during the Oligocene and early Miocene. The sample at the top of the studied log has very high silica content and an abundant coarse grain-sized fraction. This suggests that this sample belongs to the Numidian Flysch Fm. The geochemical proxies of this sample are different from those associated with samples from the Flysch rosso Fm, indicating that the source-area of the Numidian Flysch Fm did not include the Archean terranes.

What controls gold distribution in Archean terranes?

Despite intensive efforts, determination of a predictable pattern of gold distribution to enhance process understanding and simplify exploration has remained elusive. The pattern of gold distribution along the Archean Boulder-Lefroy shear zone in the Yilgarn craton of Western Australia provides insights into gold distribution in one of the world's richest provinces. Four world-class gold deposits, including the giant Golden Mile at Kalgoorlie, are regularly spaced along the shear zone, providing a window into understanding large-scale Archean hydrothermal systems. Relationships among gold endowment, shear-zone trend, and geologic complexity, as measured by fractal dimensions, indicate that (1) ore fluids were focused into those parts of the shear zone with greatest misorientation from the regional trend, which define dilational sites during shearing and became the sites of world-class gold deposits; and (2) the degree of complexity contrast along the shear zone modulated gold endowment.

Quest for palladium: Application of the Groves hypothesis to Precambrian terranes of northeast North America

A compilation of all types of magmatic deposits in Precambrian terranes of northeast North America revealed a continental-scale metallic zonation. Nickel is homogeneously distributed. Palladium and chromite are centered on the Archean craton, as predicted by the Groves hypothesis, whereas low-Ti Cu-U–rare earth element iron oxide deposits occur just south and east of the Archean and Paleoproterozoic margin, within the Grenville province. Ti is concentrated along this margin in association with anorthosite. Nickel is still abundant there, but palladium and chromite are remarkably absent. This Grenvillian Ni-Pd-Cr paradox suggests that the fractionation process invoked in the classical plume model is inadequate to explain the geochemical characteristics of the anorthosite parental magmas. The crustal-tongue melting model seems to better explain the metallic zonation observed at the margin of the Archean and Proterozoic terranes.