Archean Subduction Research Articles

Oblique Archean subduction: accretion and exhumation of an oceanic arc during dextral transpression, Southern Volcanic Zone, Abitibi Subprovince Canada

The Archean Abitibi Subprovince, divided into Northern (NVZ) and Southern (SVZ) Volcanic Zones, is characterized by the crustal-scale Destor-Porcupine-Manneville Fault Zone (DPMFZ), which links the SVZ and NVZ, and the Cadillac-Larder-Lake Fault Zone (CLLFZ), which separates the Abitibi and Pontiac Subprovinces. The DPMFZ and CLLFZ represent major deformation zones that record over 60 million years of Archean deformation. The dextral transpressional phase in the NVZ, which represents incipient SVZ deformation, occurred between 2700 and 2692 Ma and is characterized by Southeast (SE)-trending dextral faults (e.g. Macamic Fault). Synorogenic flysch sedimentation is focussed at the interface between the SVZ–NVZ and Pontiac–Abitibi Subprovinces. The SE-trending Parfouru fault, linking the two major trench fault systems, is interpreted as a reactivated leaky transform fault and is associated with synorogenic flysch sedimentation. The early monzodioritic suite of the Preissac-Lacorne batholith was related to DPMFZ thrusting (ca. 2692–2690 Ma). The change from early thrusting to transcurrent motion is recorded in the Duparquet strike-slip basin, which formed between 2690 and 2680 Ma along the DPMFZ, and the Granada pull-apart basin, which evolved between 2680 and 2670 Ma along the CLLFZ. The timing of these basins indicates a diachronous evolution. Renewed thrusting affected the Granada basin but not the Northern Duparquet strike-slip basin, so that a southward migrating deformation front is inferred. Late exhumation resulted in extension along both fault zones and was responsible for the juxtaposition of medium- and low-grade metamorphic rocks. The monzogranitic suite of the Lamotte pluton, which occurred between 2660 and 2642 Ma is linked to exhumation. Final dextral transpression post-dating exhumation produced shearing and folding. The SVZ of the Abitibi Subprovince shows the salient attributes of modern oblique orogenic collisions, with alternating phases of thrusting and strike-slip movement along crustal-scale faults during dextral transpression.

Partial melting in Archean subduction zones: constraints from experimentally determined trace element partition coefficients between eclogitic minerals and tonalitic melts under upper mantle conditions

The partitioning of an extensive suite of trace elements between garnet, clinopyroxene, and hydrous tonalitic melts has been studied experimentally at 1.8 GPa and 1000–1040 °C. The knowledge of trace element partitioning between minerals and coexisting tonalitic melts is essential for geochemical models in order to explain the genesis of Archean tonalite-trondhjemite-granodiorite suites (TTGs). Our experiments were performed on a natural tonalite doped with two different trace element mixtures and with TiO 2 to stabilize titanates. Trace element concentrations of crystals and coexisting melts were measured by laser ablation microprobe. The partition coefficients ( D's) are independent of the trace element concentration over the concentration ranges used. The partition coefficients reported in this paper are a considerable extension of the few available data sets in intermediate to silicic systems. The D-values mostly agree well with previously published data and with predictive models. The garnet partition coefficients show steeper patterns with higher D's for heavy REE, lower D's for highly incompatible elements and lower La/Yb-ratios than previously published data for basaltic systems, whereas clinopyroxene partition coefficients show similar patterns but higher values than for basaltic systems. A set of calculated partition coefficients based on best-fit parabolas was used to model trace element fractionation in melts in equilibrium with rutile-bearing eclogitic residues. The calculated melts reproduce the overall trace element patterns of Archean TTGs, i.e. the melts show strongly fractionated REE patterns, depletions in Nb and heavy REE, and high Zr and Hf concentrations relative to Sm on normalized plots. The calculated residues have depleted trace element patterns with positive Nb anomalies and flat heavy rare earth element patterns similar to rutile-bearing xenolithic eclogites reported from Siberia and West Africa. Restites from eclogite melting are complementary to Archean TTGs and may be an important reservoir of Nb and Ta.

The Spatial Distribution of Garnets and Pyroxenes in Mantle Peridotites: Pressure-Temperature History of Peridotites from the Kaapvaal Craton

cratonic peridotites of the Kaapvaal preserve a temperature–pressure We present a new method for textural analysis of mineral associations path consistent with an origin in an Archean subduction zone. that uses digital back-scattered electron and X-ray images obtained with the electron microprobe to determine the spatial properties of minerals on a two-dimensional surface of the rock at different scale lengths. We determine modal amounts and average grain sizes of KEY WORDS: craton; mantle xenoliths; microprobe; P–T history; spatial each mineral in the thin section without resorting to ellipsoidal analysis; peridotite approximations of grain boundaries, and investigate the spatial relationship of mineral pairs. The method is used to characterize nine mantle xenoliths exhumed within kimberlite pipes in South Africa and to test whether the pyroxenes are spatially correlated INTRODUCTION with the garnets. The spatial association of these minerals is used Mantle xenoliths exhumed within kimberlite pipes in to develop a model for the evolutionary history of the Kaapvaal South Africa may provide clues to the formation and peridotites. The observed distributions can be explained by a two- evolution of the depleted mantle that makes up the stage model. In stage 1, harzburgitic residues are produced by large Kaapvaal craton. One hypothesis that has been proposed extents of partial melting at shallow depths (>60–90 km) and is that the peridotite originally resides deep in the mantle high temperatures (>1300–1400°C). The melting process leading (at depths of perhaps 300 or 400 km) and is transported to this depletion occurs in the garnet stability field where garnet, upwards into the cratonic lithosphere (>180 km depth), clinopyroxene and olivine are consumed and orthopyroxene and where re-equilibration occurs and clinopyroxene (cpx) liquid are produced. The Kaapvaal sample suite shows modal and unmixes from the garnet phase (Haggerty & Sautter, compositional variations consistent with a progressive melt depletion 1990). An alternative idea is that the garnet lherzolites event. In stage 2, the residuum is dragged down to greater depths were originally high-temperature harzburgites that oriby mantle corner flow adjacent to a subducted slab. The most ginated at depths between 100 and 250 km and that both depleted harzburgites descend to 140–160 km depth and are cooled. the cpx and garnet subsequently exsolved from the AlThe least depleted harzburgites end up at shallower depths. The rich orthopyroxene (opx) when the rocks cooled and reresulting stratigraphy is the opposite of what would be expected for equilibrated (Cox et al., 1987). If exsolution of minerals a preserved mantle melt column and is consistent with inversion of has indeed occurred, then a spatial relationship between the melt column as it was dragged around the wedge corner and grains of minerals that were formerly dissolved in each cooled by the subducted slab. The cooling process causes clinopyroxene other should be apparent in the present-day xenoliths. In particular, the first model implies a spatial relationship and garnet to exsolve from the orthopyroxene. Therefore, the depleted

Archean subduction recorded by Re–Os isotopes in eclogitic sulfide inclusions in Kimberley diamonds

Eclogitic sulfide minerals encapsulated in diamonds originating from the deepest part of the continental mantle keel beneath the Kaapvaal craton, southern Africa, and brought to the surface by the Kimberley kimberlites, show low Ni and Os contents and high Re/Os ratios characteristic of a basaltic protolith. The sulfide inclusions with the lowest Os contents give late Archean single grain absolute ages while those with higher Os contents yield a well-constrained 2.9 Ga isochron age and radiogenic initial Os isotope composition ( γ Os=+45). This indicates a significant time gap between basaltic precursor generation and eclogitic diamond crystallization, consistent with extended residence in a near-surface environment prior to subduction associated with accretion of the Kimberley block to the rest of the craton and subsequent diamond formation. These results suggest that subduction-related crustal recycling was already a viable process during continent formation in the middle Archean and may have been implicated in eclogitic diamond formation ever since.

The production of Barberton komatiites in an Archean Subduction Zone

Based upon their geochemical similarity, we propose that the 3.5 Ga Barberton basaltic komatiites (BK) are the Archean equivalents of modern boninites, and were produced by the same melting processes (i.e. hydrous melting in a subduction zone). The Barberton komatiites also share some geochemical characteristics with boninites, including petrologic evidence for high magmatic H2O contents. Experimental data indicates that the Archean sub‐arc mantle need only be 1500–1600°C to produce hydrous komatiitic melts. This is considerably cooler than estimates of mantle temperatures assuming an anhydrous, plume origin for komatiites (up to 1900°C). The depleted mantle residue that generates the Barberton komatiites and BK will be cooled and metasomatised as it resides beneath the fore‐arc, and may represent part of the material that formed the Kaapvaal cratonic keel.

Archean emplacement of eclogitic components into the lithospheric mantle during formation of the Kaapvaal Craton

Re‐Os data for selected whole‐rock eclogites and eclogitic sulfide inclusions in diamonds from seven kimberlites on the Kaapvaal‐Zimbabwe cratons yield ages around 3 Ga. The presence of such old eclogitic components in the subcontinental lithospheric mantle is more common than previously thought. Overlap of these ages with those of many cratonic peridotites demonstrates that incorporation of eclogitic materials accompanied or closely followed cratonic lithosphere stabilization. Many of these eclogitic components have fractionated C, N, O, and S isotopic compositions which could have been created at low‐temperatures, implying that some style of Archean subduction involving emplacement of oceanic lithosphere was part of craton keel development.

Magnesian andesites, Nb-enriched basalt-andesites, and adakites from late-Archean 2.7 Ga Wawa greenstone belts, Superior Province, Canada: implications for late Archean subduction zone petrogenetic processes

Magnesian andesites (MA) occur with 'normal' tholeiitic to calc-alkaline basalt-andesite suites in four greenstone belts of the 2.7 Ga Wawa subprovince, Canada. Collectively, the magnesian andesites span ranges of SiO2=56–64 wt%, Mg-number=0.64–0.50, with Cr and Ni contents of 531–106 and 230–21 ppm, respectively. Relative to 'normal' andesites, the magnesian andesites form distinct trends on variation diagrams, with relatively high Th and LREE contents, uniform Yb over a range of MgO, more fractionated HREE, and lower Nb/Thpm and Nb/Lapm ratios. Niobium-enriched basalts and andesites (NEBA; Nb=7–16 ppm), and an Al-enriched rhyolite (adakite) suite are associated in space and time with the magnesian andesites. Nb-enriched basalts and andesites are characterized by high TiO2, P2O5, Th, and Zr contents, variably high Zr/Hf (36–44) ratios, and more fractionated HREE (Gd/Ybcn=1.3–4.1) compared to the 'normal' tholeiitic to calc-alkaline basalt-andesite suites. The adakite suite has the high Al (Al2O3=16–18 wt%), high La/Ybcn (21–43), and low Yb (0.4–1.2 ppm) of Archean tonalite-trondhjemite-granodiorite (TTG) suites and Cenozoic adakites, indicative of liquids derived mainly from slab melting. The basalt-andesite suites are not characterized by normal tholeiitic or calc-alkaline fractionation trends of major or trace elements. Rather, compositional trends can be accounted for by some combination of fractional crystallization and variable degrees of metasomatism of the source of basalt and/or andesites by adakitic liquids. The occurrence of magnesian andesites, Nb-enriched basalts/andesites, and adakites has been described from certain Phanerozoic arcs featuring shallow subduction of young and/or hot oceanic lithosphere. Adakites likely represent slab melts, magnesian andesites the product of hybridization of adakite liquids with mantle peridotite, and Nb-enriched basalts/andesites melts of the residue from hybridization. Geological similarities between the late-Archean Wawa greenstone belts and certain Cenozoic transpressional orogens with the MA-NEBA-adakite association suggest that subduction of young, hot oceanic lithosphere may have played an important role in the production of this arc-related association in the late Archean.

Archean Regional Metamorphism of the Isua Supracrustal Belt, Southern West Greenland: Implications for a Driving Force for Archean Plate Tectonics

The Isua supracrustal belt (∼3.8 Ga) constitutes the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of more than 1500 rock samples of the Isua belt have enabled us to estimate the extent of regional metamorphism, the petrotectonic environment, and the subduction-zone geothermal gradient in the Archean. The following line of evidence indicates progressive, prograde metamorphism from greenschist (Zone A) through albite-epidote-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeastern part of the Isua supracrustal belt: (1) the systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of the composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) the absence of relict minerals of high-grade amphibolitic metamorphism even in the lowest metamorphic zone. Metabasites of the Isua belt vary extremely in Mg#, causing a complex mineral paragenesis throughout the area. For example, a high FeO content of metabasites expands the stability field of hornblende to both lower and higher grades. The compositional and mineralogical characteristics above also indicate that the Isua supracrustal belt underwent a single regional metamorphic event, involving minor contact metamorphism and mylonitization; however, weak ocean-floor metamorphism and low-grade regional metamorphism during accretion cannot be ruled out. Metamorphic pressures and temperatures are estimated to be 5–7 kbar from garnet-hornblende-plagioclase-quartz geobarometry and 380–550°C from garnet-biotite geothermometry in Zones B to D. These P-T estimates indicate an intermediate P/T ratio metamorphic facies series. Geological investigations and chronological constraints of the Isua metamorphic belt indicate that the regional metamorphism was related to the subduction of Archean lithosphere, and records a geothermal gradient for the Archean subduction zone that is much higher than geotherms for Phanerozoic subduction zones. The high geothermal gradient may have resulted from the young age of subducted lithosphere and high potential temperature of the mantle. The Archean high geothermal gradient led to melting of thick oceanic crust in a thin, descending oceanic plate, creating many huge granitic (tonalite-trondhjemite-granodiorite [TTG]) batholiths. Slab melting changed the oceanic crust (density = 3.07) to denser garnet-bearing assemblages (density = 3.55), implying that TTG melt extraction provided a potential driving force for Archean plate tectonics.

Continental crust formation by crustal delamination in subduction zones and complementary accumulation of the enriched mantle I component in the mantle

A possible mechanism for continental crust formation, which includes slab‐dehydration‐induced mantle melting, basalt magma generation, remelting of the initial basaltic crust, followed by delamination of the residue, is examined by geochemical modeling of dehydration, partial melting, and fluid‐solid reactions. The results suggest that such processes can explain the major and trace element compositions of the andesitic bulk continental crust. Isotopic modeling suggests that a pyroxenitic delaminated component formed at 3–4 Ga possesses Sr‐Nd‐Pb isotopic compositions typical of the EMI component, one of the enriched geochemical endmembers in the mantle. Continental crust formation and complementary accumulation of the EMI reservoir in the deep mantle may have taken place simultaneously in Archean subduction zones.

Niobium-enriched basalts from the Wabigoon subprovince, Canada: evidence for adakitic metasomatism above an Archean subduction zone

Late Archean niobium-enriched basalts from the Central Sturgeon Lake assemblage and Neepawa group of the western Wabigoon subprovince have mantle-normalized Nb/La between 0.8 and 1.3 and Zr/Y between 4 and 7. They are compositionally similar to basalts attributed to adakite metasomatism of mantle wedge regions in Cenozoic subduction zones [Sajona et al., J. Petrol. 37 (1996) 693–726]. In detail, their Sc–REE systematics suggest the Archean basalts were generated above the garnet stability field. An association with adakite-like volcanic rocks, an absence of komatiites and the arc-like attributes of their host sequences suggest a subduction-related origin for the basalts. If current models of adakite and Niobium-enriched basalt genesis are valid, then additional examples of these rocks should be relatively common in other Archean greenstone belts.

Geochemistry of subduction-related mafic to felsic volcanic rocks of the late Archean Wawa greenstone belts, Superior Province, Canada

The late Archean (2.7 Ga) Schreiber-Hemlo and White River-Dayohessarah greenstone belts are characterized by tectonically juxtaposed volcanic and siliciclastic sedimentary rocks, which were collectively intruded by the syn- to post-kinematic, high-Al, high La/Ybn tonalite-trondhjemite-granodiorite (TTG) plutons. The volcanic rocks are composed of two distinct types of sequences: (1) komatiitic to tholeiitic basalts ocean plateau sequences and (2) mafic to felsic volcanic arc sequences. Based on major and trace element abundances, mafic to felsic, arcrelated volcanic rocks of the Schreiber-Hemlo and White River-Dayohessarah greenstone belts are divided into three major suites: (1) mafic to intermediate tholeiitic flows, (2) mafic to intermediate calc-alkaline flows, and (3) felsic calc-alkaline flows. All these suites share positively fractionated REE patterns, and negative anomalies of Nb and Ti. However, some intermediate and felsic volcanic rocks have positive Zr and Hf anomalies. Fractionated REE patterns, and negative Nb and Ti anomalies with respect to the neighbouring REE are all consistent with a juvenile oceanic island arc origins a metasomatized mantle wedge source for the tholeiitic suite. In comparison to tholeiites, the mafic to intermediate calc-alkaline suite is distinct in terms of: (1) enrichment in MgO, Th and LREE, Cr, and Ni; (2) higher Al2O3/TiO2 ratios; (3) more fractionated REE patterns; (4) more pronounced negative Nb and Ti anomalies; and (5) less fractionation between HREE and Sc-V, suggesting deeper and more primitive mantle source compositions for the calc-alkaline suite than the tholeiitic counterpart. Felsic rocks are defined by the enrichment of Zr and Hf, with respect to MREE, and correlate negatively with Sc abundances, suggesting garnet + amphibole ± clinopyroxene residual in the source. The inferred presence of garnet and clinopyroxene in the source is consistent with an independent argument for ecologitic residues in the slab. The existence of overlapping fields between mafic, intermediate, and felsic suites on trace element ratio diagrams suggest that processes controlling the production of these volcanic rocks were more complex than simple slab and/or wedge melting. This complexity may have resulted from a mixture of slab and wedge melts, second stage melting, magma mixing, fractional crystallization, partial equilibration with sub-arc wedge peridotite, crustal contamination, or some combination. Higher MgO, Ni, and Cr contents at a given Mg# in the basalts and andesites are consistent with higher geothermal gradients at Archean subduction zones than Phanerozoic counterparts.

Melting of a subducting slab and production of high‐mg andesite magmas: Unusual magmatism in SW Japan at 13∼15 Ma

Characteristic high‐Mg andesite magmas were produced in the SW Japan arc at 13∼15 Ma that was synchronous with the commencement of subduction of a very young (<11 m.y.) lithosphere of the Shikoku Basin. Numerical simulation suggests that temperature at the surface of such a young subducting plate is high enough for partial melting both of the subducting sediments and oceanic crust at the beginning of the subduction. High‐Mg andesite magmas were likely to be produced by interaction between silicic slab melts and the overlying mantle wedge. HMA magmas may be commonly produced in the Archean subduction zones under relatively high mantle temperature conditions, contributing to making continental crusts.

Fractionation of trace elements by subduction-zone metamorphism — effect of convergent-margin thermal evolution

Differential chemical/isotopic alteration during forearc devolatilization can strongly influence the cycling of volatile components, including some trace elements, in subduction zones. The nature and magnitude of this devolatilization effect are likely to be strongly dependent on the thermal structure of individual convergent margins. A recent model for metamorphism of the Catalina Schist, involving progressive underplating (at ≤45 km depths) of rock packets metamorphosed along successively lower- T prograde P-T paths in a rapidly cooling, newly initiated subduction zone, affords a unique evaluation of the effects of varying prograde P-T paths on the magnitudes of devolatilization and chemical/isotopic alteration of subducting rocks. In the Catalina Schist, the most extensive devolatilization occurred in metasedimentary rocks which experienced prograde P-T paths encountering the epidote-blueschist facies (>350°C at 9 to 12 kbar) or higher- T conditions; such rocks are depleted in ‘fluid-mobile’ elements such as N, B, Cs, As, and Sb relative to protoliths. Removal of these elements resulted in changes in B/(Be, Li, La, Zr), Cs/Th, Rb/Cs, As/Ce, Sb/Ce, and C reduced/N, and increases in δ 15N and δ 13C. The relative susceptibilities of the “fluid-mobile” elements to loss along increasingly higher- T P-T paths can be categorized. Boron and Cs show the greatest susceptibility to low- T removal by fluids, showing >50% depletion in even lawsonite-blueschist-facies metasedimentary rocks which experienced relatively low- T prograde metamorphic paths. In rocks which experienced higher- T paths, As and Sb (likely in sulfides) show the greatest depletions (>90%); N, Cs, and B (largely in micas) occur at ∼25% of protolith contents in even partially melted amphibolite-facies rocks. Variations in B/Be, Cs/Th, As/Ce, and Sb/Ce among arcs from differing convergent-margin thermal regimes, and conceivably some cross-arc declines in these ratios, are compatible with evidence from the Catalina Schist for varying degrees of element removal as a function of prograde thermal history. In relatively cool subduction zones (e.g., Kuriles, Marianas, Aleutians, southern Alaska) with thermal regimes similar to that which formed the low-grade units of the Catalina Schist (and blueschist-facies rocks in the Franciscan Complex), forearc devolatilization is less profound, B, Cs, As, Sb, and N are more likely to be deeply subducted, and enriched in arc lavas, and significant devolatilization occurs at the blueschist-to-eclogite transition. High-grade units could reflect thermal evolution analogous to that of relatively warm subduction zones (e.g., Cascadia) and back-arcs in which arc lavas are depleted in B, Cs, As, and Sb due to prior removal by forearc devolatilization. The results of this study also imply less efficient recycling of these elements during the warmer Archean subduction which resulted in greater slab melting and production of abundant trondhjemite-tonalite magmatic suites.

Trace element and Sm–Nd systematics of volcanic and intrusive rocks from the 3 Ga Lumby Lake Greenstone belt, Superior Province: evidence for Archean plume–arc interaction

The metavolcanic Lumby Lake belt comprises mafic tholeiites intercalated with thin felsic pyroclastic units. Al-undepleted komatiites are present towards the top of the stratigraphy. Identification of Al-depleted pyroclastic komatiites associated with chemical and siliciclastic sedimentary rocks indicates the upper portion of the 3 Ga Lumby Lake stratigraphy is directly comparable to a stratigraphic sequence developed on a paleoregolith in the nearby Steep Rock greenstone belt. The lower portion of the Lumby Lake sequence therefore represents a rarely preserved association of komatiite–tholeiite and calc alkaline volcanism developed prior to rifting episodes identified in ∼3 Ga terranes of the northern Superior Province. Al-undepleted komatiites are characterised by elevated MgO (18–24 wt.%) and Ni (600–1500 ppm) contents, in conjunction with variable LREE depletion (La/Sm n =0.5–0.8). Intercalated spinifex textured komatiitic basalts possess lower MgO (10–11 wt.%) and Ni (150–180 ppm) and flat to weakly enriched LREE (La/Sm n =0.9–1.1). Pyroclastic Al-depleted komatiites (Al 2O 3/TiO 2=4–5) are strongly LREE enriched (La/Yb n =6.7–10.5) with variable HFSE anomalies. Compositionally uniform tholeiites with variably depleted to enriched LREE (La/Sm n =0.8–1.2) and minor HFSE anomalies dominate the stratigraphy of the belt. A distinct subset of tholeiites, occurring towards the centre of the belt, is characterised by low Al 2O 3/TiO 2 ratios, LREE enrichment and the absence of HFSE anomalies. Minor intermediate (SiO 2=53–64 wt.%) volcanic rocks with pronounced REE fractionation (La/Yb n =0.8–1.1), high Al 2O 3/TiO 2 and Zr/Y ratios also occur throughout the belt. Two distinct subtypes of felsic pyroclastic rocks are recognised intercalated sporadically throughout the stratigraphy. Both types display pronounced LREE enrichment (La/Sm n =3.9–6.1) but Type 1 has strongly fractionated HREE patterns (Gd/Yb n =1.5–4.6) whereas, Type 2 HREE patterns are generally flat (Gd/Yb n =1.7–1.9). Intrusive counterparts to both subtypes have been identified within the Marmion Lake batholith to the south of the greenstone belt. The felsic subtypes may be derived from similar parental magmas by variable degrees of hornblende involvement during fractionation processes. Sm/Nd isotope systematics from a range of rock compositions have yielded ε Nd values of +2 to +5, typical of the range for Archean volcanic and intrusive rocks. Komatiite–tholeiite associations within Archean terranes are interpreted as the result of plume related magmatism likely in a geodynamic setting comparable to Phanerozoic oceanic plateaux. In contrast intermediate and felsic rocks are typical of calc alkaline suites generally attributed to Archean subduction related environments. The coeval eruption of the two magma suites in the Lumby Lake belt is best interpreted as the result of the sporadic subduction of plume-modified oceanic spreading centres over 10s of m.y. and the eventual impingement of a mantle plume upon an active subduction zone. This process can also account for the young age of basement material relative to overlying rift sequences in 3 Ga terranes of the Superior Province.

The late Archean Schreiber–Hemlo and White River–Dayohessarah greenstone belts, Superior Province: collages of oceanic plateaus, oceanic arcs, and subduction–accretion complexes

The late Archean (ca. 2.80–2.68 Ga) Schreiber–Hemlo and White River–Dayohessarah greenstone belts of the Superior Province, Canada, are supracrustal lithotectonic assemblages of ultramafic to tholeiitic basalt ocean plateau sequences, and tholeiitic to calc-alkaline volcanic arc sequences, and siliciclastic turbidites, collectively intruded by arc granitoids. The belts have undergone three major phases of deformation; two probably prior to, and one during the assembly of the southern Superior Province. Imbricated lithotectonic assemblages are often disrupted by syn-accretion strike-slip faults, suggesting that strike-slip faulting was an important aspect of greenstone belt evolution. Field relations, structural characteristics, and high-precision ICP–MS trace-element data obtained for representative lithologies of the Schreiber–Hemlo and White River–Dayohessarah greenstone belts suggest that they represent collages of oceanic plateaus, juvenile oceanic island arcs, in subduction–accretion complexes. Stratigraphic relationships, structural, and geochemical data from these Archean greenstone belts are consistent with a geodynamic evolution commencing with the initiation of a subduction zone at the margins of an oceanic plateau, similar to the modern Caribbean oceanic plateau and surrounding subduction–accretion complexes. All supracrustal assemblages include both ocean plateau and island-arc geochemical characteristics. The structural and geochemical characteristics of vertically and laterally dismembered supracrustal units of the Schreiber–Hemlo and White River–Dayohessarah greenstone belts cannot be explained either by a simple tectonic juxtaposition of lithotectonic assemblages with stratified volcanic and sedimentary units, or cyclic mafic to felsic bimodal volcanism models. A combination of out-of-sequence thrusting, and orogen-parallel strike-slip faulting of accreted ocean plateaus, oceanic arcs, and trench turbidites can account for the geological and geochemical characteristics of these greenstone belts. Following accretion, all supracrustal assemblages were multiply intruded by syn- to post-tectonic high-Al, and high-La/Yb n slab-derived trondhjemite–tonalite–granodiorite (TTG) plutons. The amalgamation processes of these lithotectonic assemblages are comparable to those of Phanerozoic subduction–accretion complexes, such as the Circum-Pacific, the western North American Cordilleran, and the Altaid orogenic belts, suggesting that subduction–accretion processes significantly contributed to the growth of the continental crust in the late Archean. The absence of blueschist and eclogite facies metamorphic rocks in Archean subduction–accretion complexes may be attributed to elevated thermal gradients and shallow-angle subduction. The melting of a hotter Archean mantle at ridges and in plumes would generate relatively small, hot, and hence shallowly subducting oceanic plates, promoting high-temperature metamorphism, migmatization, and slab melting. Larger, colder, Phanerozoic plates typically subduct at a steeper angle, generating high-pressure low-temperature conditions for blueschists and eclogites in the subduction zones, and low-La/Yb n granitoids from slab dehydration, and wedge melting. Metasedimentary subprovinces in the Superior Province, such as the Quetico and English River Subprovinces, have formerly been interpreted as accretionary complexes, outboard of the greenstone belt magmatic arcs. Here the greenstone–granitoid subprovinces are interpreted as collages of subduction–accretion complexes, island arcs and oceanic plateaus amalgamated at convergent plate margins, and the neighbouring metasedimentary subprovinces as foreland basins.

A link between deformation history and the orientation of reflective structures in the 2.68–2.83 Ga Opatica belt of the Canadian Superior Province

True three‐dimensional reflector orientations can be derived from prestack seismic reflection data where a seismic profile is particularly crooked. This is accomplished by estimating a measure of coherency along travel time trajectories defined by the azimuth, dip, and depth of a reflector and the medium velocity. Results from Lithoprobe line 48, located in the Opatica belt of the Archean Superior Province, differentiate reflectors with two distinct orientations, which coincide with the attitudes of two deformational fabrics mapped at surface. Assuming a connection between reflectivity and strain induced by tectonic processes, the reflectors with NNE strikes and shallow dips toward the east are correlated with surface evidence for early west vergent thrusting in the Opatica belt. Other reflectors, which strike ENE‐WSW and dip shallowly to the north and to the south, indicate that most of the reflectors in the southern Opatica and beneath the Abitibi greenstone belt at middle and lower crustal levels formed during a later, approximately N‐S shortening event. Mantle reflections previously interpreted as a relict suture of an Archean subduction zone dip to the north at around 30°–45° and are also associated with this N‐S event. The distribution of reflector orientations estimated at the crooked parts of line 48 indicates that much of the Opatica crust was reworked during the N‐S shortening event, although a region of the middle and lower crust, characterized by the earlier D1 reflectors, is preserved in the central part of the belt.

Re-Os isotopic compositions of Midcontinent rift system picrites: implications for plume – lithosphere interaction and enriched mantle sources

Picrites and tholeiites from the Mamainse Point Formation, a 5.3 km thick section of Keweenawan (1100 Ma) volcanic and sedimentary fill on the eastern flank of the central portion of the Midcontinent rift system, contain a nearly continuous record of rift magmatic activity. Picrites occur primarily in the lowermost two units of the formation. In this study, they are compared to rarely exposed, slightly older Keweenawan basalts from the North Shore Volcanic Group and the Powder Mill Group to constrain mantle source compositions during early phases of rift magmatic activity. The most primitive picrites analyzed have low Re content (0.069–0.18 ppb), high Os content (0.8–2.1 ppb), and low 187Re/188Os (0.28–1.18). A Re–Os isochron with an age of 1128 ± 54 Ma and an initial 187Os/188Os of 0.1267 ± 0.0013 (γOs = +5.7) was obtained from a 24-point isochron on all but two analyzed samples. The Re–Os data, regressed separately for the older basalts, and the groups 1 and 2 samples from the Mamainse Point Formation, have barely resolvable initial 187Os/188Os that decrease up-stratigraphy from initial γOs(1100) of +12.2 to +6.2 and +4.2, respectively, and couple with changes in initial Nd isotopic composition. These data can be explained by mixing of melts of an enriched mantle plume and unradiogenic continental lithospheric mantle. A radiogenic initial Os isotopic composition (γOs of +8 or higher) for the Keweenawan plume marks the first known appearance of demonstrably radiogenic plume-derived magmas on Earth. Plume-derived magmas with radiogenic Os signatures are more common later. The radiogenic Os signatures of Keweenawan plume magmas may mark the appearance of melts derived from mantle containing recycled slab components from late Archean subduction.

Archean oceanic flake tectonics

Assuming that the Archean mantle was hotter and the oceanic crust thicker than at present, we estimate that the Archean oceanic lithosphere had a "sandwich" rheology, with the lower crust behaving as a soft, ductile layer. Accordingly, the Archean oceanic lithosphere had a rheological structure similar to present‐day continental lithosphere but unlike present‐day oceanic lithosphere. The thickness of the ductile layer is a decreasing function of age, while its strength is an increasing function of age. The buoyancy problem in Archean subduction could be overcome by delamination of oceanic upper crust, but mass balance calculation indicates that most Archean oceanic crust must have been recycled. Where the right dynamic conditions existed for delamination, oceanic flake tectonics may have been an important process leading to the preservation of oceanic upper crust in "greenstone belts" that are characteristic of Archean terrains.

The role of shoshonitic and calc-alkaline suites in the tectonic evolution of the Carajás District, Brazil

In the Carajás District, the Bahia Prospect and Grão Pará volcanosedimentary sequences, both of Archean age, have been the object of extensive petrochemical studies for major, trace, and rare earth elements. In spite of the widespread spilitization that affected the volcanic mafic rocks, the petrochemical nature of the mafic volcanism may still be assessed on the basis of the less mobile elements like P, Cr, Zr, Nb, Ti, and REE. The Bahia Prospect basalts have a petrochemical affinity with Phanerozoic island-arc calc-alkaline basalts. The Grão Pará basalts and basaltic andesites, on the other hand, have a petrochemical affinity with Phanerozoic basalts of a mature island arc environment. The petrochemical data presented here lead to a new geotectonic interpretation of the area, where the volcanic sequences are genetically linked to Archean subduction processes.

Evidence for Archean subduction and crustal recycling, Wyoming province

The Beartooth Mountains of the northern Wyoming province contain a suite of incompatible-element-enriched amphibolites of andesitic composition that contains both calcalkalic and tholeitic varieties. These amphibolites formed about 2800 Ma and show trace-element and lead isotopic signatures that suggest an origin similar to some modern andesites produced in convergent margin environments. In particular, these rocks show depletions of the high-field-strength elements (e.g., Nb, Ti, Zr) and enriched Pb isotopic compositions indicative of recycling of Pb from a crustal reservoir into the mantle. These data and other geologic considerations suggest that the northern Wyoming province may have been the site of a Late Archean convergent continental margin.