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Abstract The 2722 ± 2 Ma Archaean megacrystic anorthosite-bearing Haines Gabbroic Complex in the Wawa subprovince of the western Superior Province formed contemporaneously with the spatially associated Greenwater Assemblage of the Shebandowan greenstone belt, Ontario, Canada. Both field and petrographic observations indicate that the Haines Gabbroic Complex and the spatially and temporally associated ultramafic to intermediate volcanic and intrusive rocks of the Greenwater and Burchell assemblages experienced extensive metamorphic recrystallization and alteration as a consequence of greenschist-facies metamorphism. These metamorphic processes resulted in pervasive replacement of igneous minerals by metamorphic counterparts. Metamorphic alteration also mobilised LILE and transition metals and slightly reset the U-Th-Pb and Sm-Nd isotope systems. Consequently, the Haines Gabbroic Complex and Greenwater Assemblage have respective Sm-Nd errorchron ages of 2765 +110/‐280 Ma and 3108 +120/‐380 Ma. Major and trace element compositions suggest that the least altered anorthosites, leucogabbros and gabbros of the Haines Gabbroic Complex are not cogenetic with the Greenwater and Burchell assemblages and have distinct parental magmas and depleted mantle sources. The calc-alkaline andesites of the Burchell Assemblage, the tholeiitic basalts and gabbros of the Greenwater and Burchell assemblages, the transitional gabbros of the Greenwater and Burchell assemblages and the picrites, serpentinites and peridotites of the Greenwater Assemblage are all petrogenetically distinct and have different parental magmas and depleted mantle sources. The pyroxenites are genetically distinct from the other Greenwater Assemblage lithologies, the Burchell Assemblage and the Haines Gabbroic Complex. The Haines Gabbroic Complex crystallised from hydrous, Ca- and Al-rich tholeiitic parental magmas that fractionated from more primitive, hydrous boninitic parental magmas. The initial ε Nd values of the Haines Gabbroic Complex (ε Nd = +3.0 to +4.1) and the Greenwater (ε Nd = +1.8 to +4.1) and Burchell (ε Nd = +2.0 to +3.7) assemblages are indicative of variably depleted mantle sources. The major and trace element characteristics of the Haines Gabbroic Complex and the Greenwater and Burchell assemblages and geological relationships indicate that they formed in a Japan-style mature intra-oceanic continental arc with a large volume of TTGs. Intra-oceanic arc rifting was most likely triggered by slab rollback, followed by mantle plume emplacement. On the basis of rock association and geochemical data, we suggest that the Haines Gabbroic Complex and the Greenwater and Burchell assemblages represent a dismembered subduction-related Archaean ophiolite that marks a Neoarchaean suture zone between the Wawa and Wabigoon subprovinces. Highlights • Haines Gabbroic Complex (HGC) was not cogenetic with the Shebandowan belt • HGC crystallised from hydrous, Ca- and Al-rich tholeiitic parental magmas • These parental magmas fractionated from more primitive, hydrous boninitic magmas • Parent magmas to HGC formed by high-degree partial melting of depleted mantle sources • HGC and adjacent assemblages formed in a Japan-style mature intra-oceanic arc setting [ABSTRACT FROM AUTHOR]

The Neoarchean (ca. 2728 Ma) anorthosite-bearing Doré Lake Complex in the northeastern Abitibi subprovince, Quebec, was emplaced into an association of intra-oceanic tholeiitic basalts and gabbros known as the Obatogamau Formation. The Obatogamau Formation constitutes the lower part of the Roy Group, which is composed of two cycles of tholeiitic-to-calc-alkaline volcanic and volcaniclastic rocks, siliciclastic and chemical sedimentary rocks, and layered mafic-to-ultramafic sills. In this study, we report major and trace element results, and Nd, Sr, Pb and O isotope data for anorthosites, leucogabbros, gabbros and mafic dykes from the Doré Lake Complex and spatially associated basalts and gabbros of the Obatogamau Formation to assess their petrogenetic origin and geodynamic setting. Field and petrographic observations indicate that the Doré Lake Complex and associated volcanic rocks underwent extensive metamorphic alteration under greenschist facies conditions, resulting in widespread epidotization (20-40%) and chloritization (10-40%) of many rock types. Plagioclase recrystallized mainly to anorthite and albite endmembers, erasing intermediate compositions. Metamorphic alteration also led to the mobilization of many elements (e.g., LILE and transition metals) and to significant disturbance of the Rb-Sr and U-Pb isotope systems, resulting in 1935 ± 150 and 3326 ± 270 Ma errorchron ages, respectively. The Sm-Nd isotope system was less disturbed, yielding an errorchron age of 2624 ± 160 Ma. On many binary major and trace element diagrams, the least altered anorthosites and leucogabbros, and the gabbros and mafic dykes of the Doré Lake Complex plot in separate fields, signifying the presence of two distinct magma types in the complex. The gabbros and mafic dykes in the Doré Lake Complex share the geochemical characteristics of tholeiitic basalts and gabbros in the Obatogamau Formation, suggesting a possible genetic link between the two rock associations. Initial εNd (+2.6 to +5.0) and δ18O (+6.1 to +7.9‰) values for the Doré Lake Complex and gabbros of the Obatogamau Formation (εNd = +2.8 to +4.0; δ18O = +7.3 to 8.0‰) are consistent with depleted mantle sources. All rock types in the Doré Lake Complex and the Roy Group share the trace element characteristics of modern arc magmas, suggesting a suprasubduction zone setting for these two lithological associations. On the basis of regional geology and geochemical data, we suggest that the Doré Lake Complex and the Obatogamau Formation represent a dismembered fragment of a suture zone, like many Phanerozoic ophiolites, resulting from closure of a back-arc basin between 2703 and 2690 Ma. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editors discuss various reports within the issue on topics including the landscape evolution during the Quaternary and Cenozoic period in Africa, the Earth's plate tectonic evolution, and the geodynamics in mantle convection and ophiolite obduction.

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Abstract: The Fiskenæsset Complex, SW Greenland, contains the world''s best preserved Archean (∼2970Ma) layered anorthosite, leucogabbro, gabbro, and ultramafic association. The complex was emplaced into Archean oceanic crust distal from continental lithosphere and later intruded by tonalites, trondhjemites and granodiorites (TTG) constituting Archean continental crust. The complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions. This study presents new whole-rock major and trace element, petrographic, and SEM (Scanning Electron Microscope)-BSE (backscatter electron) image data for a 45–60m-thick ultramafic sill and cross-cutting hornblendite veins. The ultramafic sill is composed of ∼75% olivine–pyroxene hornblendite, ∼15% pyroxene hornblendite, and ∼10% hornblende pyroxenite. Despite granulite facies metamorphism and multiple phases of deformation, the Fiskenæsset ultramafic sill shares many petrographic characteristics of unmetamorphosed mafic to ultramafic layered intrusions and Alaskan-type ultramafic complexes. Hornblende appears to have originated as an igneous mineral but underwent extensive recrystallization during granulite facies metamorphism. Formation of orthopyroxene–magnetite symplectitic (vermicular) intergrowths, mainly at the expense of olivine, is attributed to chemical reactions between late stage, residual hydrous melts and olivine. Field, geochemical and petrographic observations suggest that the ultramafic sill did not undergo significant fractional crystallization following its intrusion. High MgO (16–31wt.%) contents suggest that the sill was emplaced as a crystal mush, rather than as melt only. Petrographic observations and BSE images indicate that olivine, orthopyroxene, and clinopyroxene were the major crystal phases, whereas hornblende represents the main interstitial liquid phase in the crystal mush. The sill exhibits three different REE patterns, corresponding to three cryptic layers. Although the origin of these REE patterns is not fully understood, they may reflect the intrusion of three batches of magma, containing different hornblende-forming melt/crystal ratios. Geochemical data indicate that the cross-cutting hornblendite veins likely originated from evolved, late-stage melts but were not related to the melts of the ultramafic sill. Large negative Nb anomalies (Nb/Nb*=0.04–0.66) suggest that the Fiskenæsset hornblendites, hornblende pyroxenites, and hornblendite veins were derived from a hydrous sub-arc mantle peridotite. Phanerozoic hornblendites are typically associated with supra-subduction zone ophiolites and magmatic arcs. Accordingly, it is suggested that water was recycled to the source of the Fiskenæsset hornblendites through subduction of altered oceanic crust. Recycling of water to the upper mantle via subduction not only resulted in the generation of hornblendites, but also played an important role in the formation of TTG-dominated Archean continental crust. Therefore, the field, petrographic and geochemical data presented in this study suggest that the origin of the Mesoarchean to Neoarchean terrane in the Fiskenæsset region, SW Greenland, is consistent with subduction zone geodynamic processes, rather than non-uniformitarian, density-driven, vertical, crustal overturn, and diapiric processes. [Copyright &y& Elsevier]

Abstract: The Mesoarchean Fiskenæsset Complex was emplaced as multiple sills and dykes of magma and crystal mush into oceanic crust, forming an association of ca. 550-meter-thick layered anorthosites, leucogabbros, gabbros, hornblendites, pyroxenites, dunites, and peridotites. The complex was intruded by tonalites, trondhjemites and granodiorites (TTG), and underwent multiple phases of deformation. The Fiskenæsset Complex and bordering TTG intrusions were variably affected by granulite facies metamorphism and retrogressed under amphibolite facies conditions. This study presents new field, petrographic, and major and trace element geochemical data for the six major stratigraphic units of the Fiskenæsset Complex at Majorqap qâva, SW Greenland. In addition, new high-precision trace element data are reported for hornblendes from three stratigraphic units. The sampled major lithological units from bottom to top are: Lower Gabbro, Ultramafic, Lower Leucogabbro, Middle Gabbro, Upper Leucogabbro, and Anorthosite. Both whole-rock and hornblende trace element data are consistent with crystallization of the complex from two different parental magmas, originating from different mantle sources. Subsequent differentiation of these parental magmas in the oceanic arc crust resulted in two evolved geochemical rock suites (Suite A and Suite B), which are both characterized by depletions of HFSE (Nb, Ta, Zr and Hf) relative to Th, U, LREE and LILE (Rb, Ba), and enrichments of Sr and Pb relative to MREE; these data are consistent with a magmatic arc geodynamic setting. [Copyright &y& Elsevier]

Abstract: This study reviews the geochemical characteristics of Eoarchean to Mesoarchean ultramafic to mafic volcanic rocks (now amphibolites) in SW Greenland and compares them with those of Cenozoic oceanic island arc basalts, to evaluate Archean subduction zone petrogenetic processes. Emphasis is placed on the Th–REE–HFSE (Zr, Ti, and Nb) systematics of the ca. 3800 and ca. 3700Ma arc suites in the Isua greenstone belt, the ca. 3075Ma Ivisaartoq–Ujarassuit greenstone belt, and amphibolites associated with the ca. 2970Ma Fiskenæsset layered anorthosite complex. On N-MORB-normalized diagrams, the Isua, Ivisaartoq–Ujarassuit, and Fiskenæsset volcanic rocks are all characterized by depletion of Nb relative to Th and LREE, consistent with a supra-subduction (forearc–arc–backarc) geodynamic setting. Similarly, on the Th/Yb–Nb/Yb projection, these suites plot within the field of Cenozoic oceanic island arc basalts. On log-transformed immobile trace element ratio (Nb/Th, La/Th, Sm/Th, and Yb/Th) diagrams, they display a trend projecting from MORB (Mid-Ocean Ridge Basalt) to IAB (Island Arc Basalt) on the IAB–CRB (Continental Rift Basalt)–OIB (Ocean Island Basalt)–MORB diagram, as for Cenozoic oceanic island arc basalts. Accordingly, these trace element compositions are interpreted as reflecting the enrichment of Archean depleted upper mantle (MORB-source) by subduction-derived melts and fluids following the initiation of intra-oceanic subduction and arc migration. Concentrations of MgO and Ni in SW Greenland Archean basalts overlap with, but extend to 2 to 4 times higher than, those in Cenozoic oceanic island arc counterparts. In contrast, the majority of Archean basalts have REE and HFSE concentrations 2 to 4 times lower than Cenozoic oceanic island arc basalts, consistent with more depleted sub-arc mantle wedge peridotites in the Archean than in Cenozoic counterparts. Such depletion reflects the extraction of large volumes of mafic to ultramafic melts from hotter Archean mantle. We infer that less refractory compositions of the Present-day depleted upper mantle, the source of MORB and conservative HFSE in arc basalts, have resulted from re-mixing with the less depleted to enriched deeper mantle material transferred to shallower depths by mantle convection and plumes in post-Archean times. Archean basalts in SW Greenland share the negative Nb and Ti anomalies of Cenozoic oceanic island arc basalts. On average, Archean basalts, however, have lower Th/Nb, La/Nb, Sm/TiO2 and Gd/TiO2 ratios than Cenozoic oceanic island arc counterparts. Given that rutile controls the Nb and Ti budgets in arc magmas, the lower Th/Nb, La/Nb, Sm/TiO2 and Gd/TiO2 ratios in SW Greenland basalts are attributed to the mobility of Nb and Ti in slab-derived melts involving rutile fusion in Archean subduction zones. These elements are less mobile in fluids originating from Cenozoic subducting oceanic crust where rutile appears to be generally stable. Accordingly, it is suggested that higher geothermal gradients in the Archean may have provided optimized conditions for slab melting and metasomatism of the sub-arc mantle wedge by slab-derived melts. [Copyright &y& Elsevier]

Abstract: The Fiskenæsset Complex, SW Greenland, is one of the best preserved layered Archean intrusions in the world, consisting of an association of ca. 550-meter-thick anorthosite, leucogabbro, gabbro, and ultramafic rocks (dunite, peridotite, pyroxenite, and hornblendite). Despite poly-phase deformation and amphibolite to granulite facies metamorphism, primary cumulate textures and igneous layering are well-preserved in the complex. This study reports new major and trace element data for three variably thick (1 to 5m) differentiated (dunite, through peridotite, pyroxenite, gabbro leucogabbro, to anorthosite) sequences (Sequences 1, 2 and 3) in the Sinarssuk area of the Fiskenæsset region. On several variation diagrams, samples from these sequences plot along a well-defined liquid line of descent, consistent with in situ fractional crystallization. The average chemical compositions of these sequences are used to constrain their approximate parental magma compositions. Petrographic observations and geochemical data suggest that Sequences 2 and 3 solidified from evolved magmas that underwent olivine fractionation prior to their intrusion. In contrast, Sequence 1 appears to have been derived from a near-primary parental magma (SiO2 =43wt.%, MgO=20wt.%, Al2O3 =16wt.%, CaO=9.3wt.%, Ni=840ppm, Mg-number=80). The trace element patterns of this parental magma are comparable to those of Phanerozoic boninites, consistent with a supra-subduction zone geodynamic setting. If the relative thickness of ultramafic layers, the sum of dunite, peridotite and pyroxenite layers, in differentiated sequences is taken as an analog for the original complex emplaced into Archean oceanic crust, the Fiskenæsset Complex might have had a minimum thickness of 1000m, with a 500m thick ultramafic unit at the bottom. The thickness of the ultramafic unit in the preserved complex is less than 50m, suggesting that more than 90% of the original ultramafic unit was either delaminated and recycled back into the mantle as a residual cumulate, or was destroyed during thrusting and TTG intrusion. Both the Fiskenæsset Complex and associated tholeiitic basalts display similar Th–Nb–LREE patterns and plot along the same differentiation trend on Zr versus incompatible trace element diagrams, suggesting a possible petrogenetic link between the two suites of rocks. However, basalts do not display the same differentiation trend as the complex on several major and trace element diagrams. In addition, basalts and parental magma to the complex do not plot along the same fractionation line on Al2O3/TiO2 versus incompatible trace element diagrams, implying that the Fiskenæsset layered intrusive rocks were not derived from tholeiitic basalts through fractional crystallization, as previously thought. Accordingly, we infer that the Fiskenæsset Complex and spatially associated basalts were derived from different mantle sources. Parental magmas to the Fiskenæsset Complex originated from a mantle source that was metasomatized by highly aluminous slab-derived melts. Layered anorthosite complexes are mostly restricted to the Archean and typically associated with tonalite–trondhjemite–granodiorite (TTG) gneisses. The petrogenesis of both suites appears to have been controlled mainly by slab melting, endorsing independent evidence for a secular change from slab-dominated melting to wedge-dominated melting in arc magmatism at the end of the Archean, reflecting higher geothermal gradients in Archean arcs than post-Archean counterparts. [Copyright &y& Elsevier]

Abstract: The Archean Fiskenæsset Complex, SW Greenland, consists of an association of ca. 550-meter-thick layered anorthosite, leucogabbro, gabbro, and ultramafic rocks (peridotite, pyroxenite, dunite, hornblendite). The complex was intruded by tonalite, trondhjemite, and granodiorite (TTG) sheets (now orthogneisses) during thrusting that was followed by several phases of isoclinal folding. The trace element systematics of the Fiskenæsset Complex and associated volcanic rocks are consistent with a supra-subduction zone geodynamic setting. The Fiskenæsset anorthosites, leucogabbros, gabbros and ultramafic rocks collectively yield an Sm–Nd errorchron age of 2973±28Ma (MSWD=33), with an average initial εNd =+3.3±0.7, consistent with a long-term depleted mantle source. Regression of Pb isotope data define an age of 2945±36Ma (MSWD=44); and the regression line intersects the average growth curve at 3036Ma. Slightly lower Pb–Pb errorchron age is interpreted as reflecting partial disturbance of the U–Pb system in gabbros, leucogabbros and ultramafic rocks during intrusion of TTGs. Complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca. 3200 and 2650Ma. Zircon ages peak at about 3200, 3100, 3000, 2950, 2820, and 2750Ma. The 3200–3000Ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks. 2950Ma is considered as an approximate intrusion age of sampled TTGs. The 2940–2650Ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the Fiskenæsset region. On the basis of recently published trace element data, and new Nd and Pb isotope and U–Pb zircon age data, a three-stage geodynamic model is proposed to explain the evolution of the Fiskenæsset Complex. Stage 1 represents the formation of depleted shallow mantle source>3000Ma (εNd =+3.3±0.7) for the complex. Stage 2 corresponds to the development of an intra-oceanic island arc between 3000–2950Ma. Stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950–2940Ma. [Copyright &y& Elsevier]

Abstract: New major and high-precision ICP-MS trace element data on the Neoarchean (ca. 2970Ma) layered Fiskenæsset Complex and associated volcanic rocks, southern West Greenland, provide new constraints on the petrogenesis and geodynamic setting of the complex. The complex appears to have been emplaced as multiple sills of magma and crystal mush into oceanic crust (tholeiitic basalts and gabbros), forming an association of ca. 550m thick anorthosite, leucogabbro, gabbro, and peridotite layers. The Fiskenæsset Complex and the associated volcanic rocks were intruded by Neoarchean tonalite, trondhjemite, and granodiorite (TTG) sheets during thrusting that was followed by several phases of isoclinal folding. Despite the intense deformation and amphibolite to granulite facies metamorphism, primary cumulate textures and igneous layering are locally well preserved throughout the complex. The presence of calcic-plagioclase (An75-95) and igneous amphibole in anorthosites, gabbros and leucogabbros, and hornblendite veins in peridotites suggests a hydrous magma source(s). The major and trace element compositions of tholeiitic basalts (amphibolites) suggest that they are petrogenetically related to the Fiskenæsset Complex by fractional crystallization. The trace element systematics of the least-altered anorthosites, gabbros, leucogabbros, peridotites, tholeiitic basalts, and calc-alkaline high-magnesian andesites (HMA) is collectively consistent with a supra-subduction zone geodynamic setting. On the log-transformed tectonic discrimination diagram, including La/Th, Sm/Th, Yb/Th, and Nb/Th ratios, tholeiitic basalts display a trend projecting from mid-ocean ridge basalt (MORB) field to island arc basalt (IAB) field. This trend is interpreted as reflecting a transition from the Neoarchean depleted upper mantle to a subarc mantle wedge following the initiation of intra-oceanic subduction and arc migration. Collectively, on the basis of field relationships, petrographic features, and geochemical characteristics, the Fiskenæsset Complex is interpreted as a fragment of a Neoarchean oceanic island arc. On the basis of REE patterns, anorthosites are divisible into four major groups: (1) Group 1 displays moderately depleted to slightly enriched LREE and HREE patterns; (2) Group 2 possesses strongly enriched LREE and moderately depleted HREE patterns; (3) Group 3 has strongly enriched LREE and depleted HREE patterns; and (4) Group 4 (garnet anorthosite) exhibits concave-upward REE patterns; that appear to have resulted from contamination of anorthositic magma by basaltic rocks or magma. The different REE patterns in Group 1, 2, and 3 anorthosites are interpreted to reflect various depths of partial melting and variably enriched (in incompatible elements) source compositions. The majority of anorthosites and leucogabbros were derived from depths above the garnet stability field. [Copyright &y& Elsevier]

Abstract: Neoarchean (ca. 2700Ma) Wawa greenstone belts, Superior Province, contain two lithologically and geochemically distinct volcanic associations: (1) a plume-derived oceanic plateau association; and (2) a subduction-derived oceanic island arc association. The oceanic plateau association is composed of three groups: (1) voluminous tholeiitic basalts with near-flat REE patterns; (2) volumetrically minor moderately LREE-depleted tholeiitic basalts and Al-undepleted komatiites; and (3) volumetrically minor moderately to strongly LREE-enriched transitional to alkaline basalts and Al-depleted komatiites. This geochemical diversity is comparable to those of Phanerozoic oceanic plateaus and ocean islands. The oceanic island arc association is characterized mainly by variably LREE-enriched and Nb-depleted (relative to Th and La) tholeiitic to calc-alkaline basalts, andesites, dacites and rhyolites, with minor adakites, magnesian andesites, and Nb-enriched basalts. All these lavas are intruded by subduction-derived late- to post-tectonic gabbros, tonalites, trondhjemites, granodiorites, and monzonites. Within the Wawa subprovince the Schreiber–Hemlo greenstone belt is dominated by plume-derived oceanic plateau volcanic rocks (60–80%), whereas the Winston Lake and Manitouwadge greenstone belts are composed dominantly of subduction-derived (70–90%) volcanic rocks. New major and trace element, and Sm–Nd isotope data are presented for volcanic rocks from the Schreiber–Hemlo, Winston Lake, and Manitouwadge greenstone belts, to address their petrogenetic origin. In addition, new major and trace element data are reported for subduction-derived gabbros from the Schreiber–Hemlo greenstone belt. The geochemical diversity of the oceanic plateau association is consistent with a compositionally heterogeneous mantle plume source. Depleted initial Nd isotopic compositions (ɛ Nd =+1.7 to +2.2) indicate that transitional to alkaline basalts were derived from a long-term LREE-depleted mantle source. It is shown that the origin of transitional to alkaline lavas can be explained by mixing of LREE-depleted and LREE-enriched melts in an upwelling mantle plume. Depleted to enriched LREE patterns in the plume-derived association are consistent with variable degrees of melting that took place over a range of temperatures and pressures as the plume ascended towards the surface. On the basis of field relationships and geochemical data, it is suggested that the geodynamic evolution of the Schreiber–Hemlo greenstone belt began with the initiation of a subduction zone at the margin of an oceanic plateau at about 2725Ma, forming an intra-oceanic island arc. In response to plate reorganization and subducted slab rollback, the island arc underwent extension leading to the opening of a backarc basin. Hydrothermal alteration of backarc oceanic crust resulted in the formation of volcanogenic massive sulphide (VMS) deposits in the Winston Lake and Manitouwadge greenstone belts. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Geological investigation of the Neoarchean (2.55–2.50 Ga) Wutaishan greenstone belt in the central orogenic belt of the North China craton has provided new information on the geodynamic origin of this belt and its mineral deposits. Structural, geochronological, and geochemical characteristics of the Wutaishan greenstone belt suggest that it formed in a forearc tectonic environment at ca. 2.55 Ga and accreted to the Eastern continental block at ca. 2.50 Ga. A ridge subduction model is proposed to explain several unique geological features of the Wutaishan greenstone belt, such as the generation of dunites and chromitite-hosting harzburgites with U-shaped rare earth element (REE) patterns, formation of volcanogenic massive sulphides (VMS) and banded iron formations (BIF), extrusion of mafic to felsic volcanic rocks, and intrusion of tonalite-trondhjemite-granodiorite plutons (TTG). Anomalously high geothermal gradients in the subarc mantle-wedge beneath the Wutaishan forearc may have increased its buoyancy, resulting in its accretion to the continental crust. We propose that ridge subduction also played an important role in the growth of Archean continental crust. In this model, the origin of Archean TTG is genetically linked to eclogites through partial melting of accreted and/or underplated oceanic plateaus and normal oceanic crust under amphibolite to eclogite metamorphic conditions by upwelling of an anomalously hot asthenospheric mantle window resulting from ridge subduction. TTG suites intruding Archean accretionary complexes formed the nuclei of intra-oceanic island arcs; subsequent juxtaposition of these arcs resulted in the lateral growth of Archean continental crust. [ABSTRACT FROM AUTHOR]

The article focuses on the studies by T. J. Nagel and J. Adam and colleagues on the methods that are being used to address the geodynamic setting of Eoarchean tonalite-trondhjemite-granodiorites (TTGs) in oceanic island arcs. It says that the studies show the consistency of the geochemical description of Eoarchean TTGs with the melting of subduction-derived mafic lavas. It adds that both studies strengthen the genetic link between the formation of Archean TTGs and subduction zone processes.

Constraining the timing of tectonothermal events in the Proterozoic Miaowan Ophiolite Complex (MOC) and associated rocks in the southern Huangling dome, Yangtze craton, is critical for understanding late Mesoproterozoic and Neoproterozoic tectonic evolution (ca. 1.2–0.9 Ga) of South China and its relationship with the formation of the Rodinian supercontinent. The MOC consists of metamorphosed dunite and harzburgite, isotropic and layered gabbros, diabasic sheeted dike, basalt, plagiogranite, and calc-silicate-bearing siliceous and carbonaceous mylonitic rocks. In this study, we present new field, petrographic, geochemical, geochronological and isotopic data for these metasedimentary rocks and mylonitic metasandstone between the ophiolite and underlying flysch sequence, to unravel their origin and tectonic significance for the Yangtze craton. The MOC and associated sedimentary rocks underwent amphibolite facies metamorphism. Geochemical data and field relationships indicate that the mylonitic metasandstone was derived from both the autochthonous rocks of the underlying Yangtze craton and the allochthonous MOC during the accretion of the ophiolite to the craton. The calc-silicate-bearing siliceous and carbonaceous rocks are interpreted as tectonic slices of metasomatized and mylonitized chert and limestone, respectively, deposited on the basaltic crust in a Neoproterozoic ocean. Cores of igneous detrital zircons in the calc-silicate-bearing siliceous and carbonaceous mylonitic rocks have yielded two distinct age groups including a 967–1105 Ma (Mean = 1009 Ma) group and a 1011–1095 Ma (Mean = 1054 Ma) group. The majority of initial ε Hf ( t ) values (+9.2 to +14.1) in the zircon cores are similar to those of zircons from the gabbro, diabase and plagiogranite in the MOC, indicating that zircons in the mylonitic rocks were mainly derived from the MOC. Metamorphic overgrowth ages in zircon rims suggest that the latest tectonothermal event in the Precambrian basement rocks of the Yangtze craton took place between 942 and 935 Ma. These metamorphic ages are comparable to those of the Grenvillian-aged collisional events recorded in orogenic belts worldwide. Accordingly, we suggest that this youngest tectonothermal event represents the latest amalgamation time of the heterogeneous tectonic blocks of the Yangtze craton, which were associated with the assembly of the Rodinian supercontinent. [ABSTRACT FROM AUTHOR]

The Paleoproterozoic (ca. 1900 Ma) Lynn Lake greenstone belt of northern Manitoba, Canada, has been previously characterized as comprising a series of tectonically juxtaposed intra-oceanic-derived metavolcanic rocks. The results of more recent local and regional studies, however, support a significant contribution of continental crust during formation of the metasedimentary, metavolcanic, and intrusive igneous rocks that comprise the majority of the Lynn Lake greenstone belt. The tectonic model previously proposed for the Lynn Lake greenstone belt, however, did not consider the geodynamics of the Lynn Lake greenstone belt in the context of all available data. In this study, we report the results of outcrop mapping and petrographic analysis, as well as major, minor, and trace element geochemical analyses for 54 samples from the Northern terrane, and integrate and compare the results with data from previously published studies. These data are used to recharacterize the metavolcanic rocks and to develop a new geodynamic model for the formation of the Lynn Lake greenstone belt. Ultramafic to intermediate rocks in the vicinity of the MacLellan Au–Ag deposit are characterized primarily by E-MORB-like trace element characteristics and Th–Nb–La systematics, which are interpreted to be the result of a primary, plume-derived melt interacting with continental lithosphere at a thinned (i.e., rifted) continental margin. Similarly, the majority of the mafic to intermediate rocks that comprise the Lynn Lake greenstone belt are characterized by flat to E-MORB-like trace element patterns and Th–Nb–La systematics, which are consistent with mantle plume-derived, contaminated, oceanic continental rift or rifted margin setting rocks. This study suggests that the metavolcanic rocks of the Lynn Lake greenstone belt were derived via rifting between the Superior and Hearne Cratons, which resulted in the formation and growth of the Manikewan Ocean. Alternatively, the metavolcanic rocks could have been derived from an upwelling plume generated from a back-arc-style subduction zone that formed between the Rae and Hearne Cratons during the closure of the Snowbird Ocean, which resulted in rifting of the passive Hearne margin. [ABSTRACT FROM AUTHOR]

Mafic granulitic and amphibolitic boudins dispersed in Archean felsic gneisses are widely distributed in the Central Orogenic Belt (COB) and the Eastern Block of the North China Craton (NCC) and are considered to constitute deformed mafic dike swarms. Previous studies have demonstrated that the mafic dikes in the Zanhuang Complex of the NCC intruded the fabrics of an Archean mélange belt and were boudinaged during younger deformation. Igneous zircons from an undeformed mafic dike yield a 207Pb/206Pb age of 2535 ± 30 Ma, which is interpreted as the crystallization age. In addition, pegmatites cutting across the mafic dikes in the field also yield an igneous zircon 207Pb/206Pb age of 2504 ± 16 Ma, providing strong evidence that the mafic dikes in the NCC intruded during the Neoarchean. Metamorphic zircons from one deformed mafic dike sample yield a metamorphic 207Pb/206Pb age of 2090 ± 83 Ma, and another four samples from deformed mafic dikes have a consistent metamorphic zircon age of ca. 1850 Ma, indicating that the mafic dikes underwent at least two generations of Paleoproterozoic metamorphism of ca. 2.1 Ga and ca. 1.85 Ga. Previously reported trace element systematics of the mafic dikes are consistent with an arc-related lithospheric mantle source region, rather than an ocean island basalt (OIB)-like source region. The new whole rock Nd isotopic composition (ɛNd(t) = + 0.71 to + 3.70) is relatively more evolved compared to that of the depleted mantle at 2.5 Ga, indicating an enriched lithospheric mantle source. Accordingly, the mafic dikes are proposed to have been formed in a subduction-related environment and their enriched mantle source was metasomatized by the melts and fluids derived from the subducted slab. Based on previous studies of the NCC and new geochronological and isotopic data in this contribution, a new comprehensive tectonic model is proposed for the evolution of the NCC between 2.7 Ga and 1.85 Ga: (1) from 2.7 to 2.5 Ga, an oceanic arc terrane belt developed in what is now the COB of the NCC while a passive margin sequence was deposited on the western edge of the Eastern Block on the margin of the intervening ocean; (2) the oceanic arc terrane collided with the passive margin sequence at 2.5 Ga above a west-dipping subduction zone; (3) the subduction polarity was reversed at ca. 2.5 Ga and a new east-dipping subduction zone gave rise to the intrusion of the 2.5 Ga mafic dikes of the NCC; (4) the final collision between the Western Block and the Eastern Block occurred at ca. 2.1 Ga; (5) the whole NCC then collided with the Columbia Supercontinent along the north margin of the craton at 1.9-1.8 Ga. [ABSTRACT FROM AUTHOR]

Highlights: [•] The Archean Fiskenæsset Complex, SW Greenland, contains well-preserved igneous minerals. [•] The complex underwent amphibolite to granulite facies metamorphism. [•] Trace elements were immobile on mineral scale during metamorphism. [•] Two distinct petrogenetic suites of rocks are recognized based on hornblende composition. [•] The trace element systematics of hornblendes suggest complex formed at a convergent-plate margin. [ABSTRACT FROM AUTHOR]

An Archean mélange belt has been recognized in the Zanhuang Complex, Central Orogenic Belt (COB), North China Craton (NCC). All units of the mélange belt are intruded by a ca. 2.5Ga granite pluton and cross-cut by undeformed 2.5Ga pegmatites, which constrains the Archean formation age of the mélange. Some exotic mafic–ultramafic blocks are tectonically dispersed in the metasedimentary matrix to the mélange and mainly consist of metabasaltic rocks, deformed pillow lavas with epidosite cores, gabbros, and ultramafic rocks. Both metabasalts and epidosites are characterized by enrichment in light rare earth elements (LREE) and pronounced negative Nb and Zr anomalies on chondrite- and primitive mantle- normalized diagrams. These geochemical characteristics are consistent with supra-subduction zone geochemical signatures. Based on the pillow structures and seafloor hydrothermal alteration, combined with the geochemical characteristics, an intra-oceanic arc–forearc setting is proposed for the formation of these exotic blocks. Furthermore, a west-dipping intra-oceanic subduction zone is proposed to have formed in an ocean between the Eastern Block and an arc terrane prior to 2.5Ga. The collision between the arc and the Eastern Block of the NCC happened at ca. 2.5Ga, resulting in the formation of the Archean mélange belt in the Zanhuang Complex. All of the units of the mélange belt were intruded by late mafic dikes, which are present as boudinaged or lenticular shapes in the mélange and surrounding gneisses and are intruded by younger ca. 2.5Ga pegmatites. Geochemical characteristics of the late mafic dikes are consistent with an arc-related mantle source region, rather than an OIB-like source. Accordingly, pending determination of their precise age, an arc-polarity reversal event is proposed to have occurred following the arc–continent collision, resulting in east-dipping subduction beneath the newly accreted arc and Eastern Block, forming the swarm of arc-affinity mafic dikes. The new subduction zone further resulted in ponded magma below the accreted arc/Block, which induced the partial melting of thickened crust and the intrusion of ca. 2.5Ga granitic and pegmatitic melts into the mélange and the mafic dikes. The mafic dikes were then deformed and metamorphosed when the remaining open part of the ocean between the arc-collision modified margin of the Eastern Block collided with the Western Block of the NCC. [ABSTRACT FROM AUTHOR]

Abstract: The Cretaceous Xigaze ophiolite is best exposed at the central Yarlung Zangbo Suture Zone (YZSZ, Tibet) which also includes the Gangdese arc and the Xigaze forearc basin. This study reports new geochronological and geochemical data for this ophiolite to revisit its geodynamic and petrogenetic evolution. The Xigaze peridotites have low CaO and Al2O3 contents and U-shaped Rare Earth Element (REE) patterns, suggesting that they are residues after moderate to high degrees of partial melting and were modified by infiltration of Light Rare Earth Element (LREE)-enriched boninitic melts. The Xigaze crustal rocks belong to two groups: Mid-Ocean Ridge Basalt (MORB)-like rocks and boninitic rocks showing a uniform LREE depletion and flat to LREE enrichment on chondrite-normalized patterns, respectively. Geochemically, both groups show the influence of subducting oceanic slab-derived fluids. LA-ICPMS zircon U–Pb and Lu–Hf analyses from dolerite and quartz diorite dikes, which intruded into the mantle peridotite, and dolerite sheeted sills show that they were generated between 127 and 124Ma. The zircons possess positive εHf(t) values ranging from +7.5 to +17.3. Taking into account the geological and geochronological characteristics of the central-western YZSZ, we propose that ophiolites in this region formed in a forearc spreading setting through rapid slab rollback during subduction initiation between 130 and 120Ma. Following this stage of spreading, the forearc was stabilized and the zone of melting migrated beneath the Gangdese arc producing the voluminous Late Cretaceous granitoids displaying depleted mantle-type Hf isotopic compositions. Our model provides a new explanation for the generation and evolution of forearc-type ophiolites. [Copyright &y& Elsevier]

Abstract: The North Anatolian Fault Zone (NAFZ) is a 1200km long dextral strike-slip fault zone forming the boundary between the Eurasian and Anatolian plates. It extends from the Gulf of Saros (North Aegean) in the west to the town of Karlıova in eastern Turkey. Although there have been numerous geodetic studies concerning the crustal deformation, velocity field and the slip rate of the NAFZ along its western and central segments, geodetic observations along the eastern section of the NAFZ are sparse. In order to investigate the GPS velocities and the slip rate along the eastern part of the NAFZ, a dense GPS network consisting of 36 benchmarks was installed between Tokat and Erzincan on both sides of the fault zone and measured from 2006 to 2008. Measurement results indicate that the slip rate of the NAFZ increases westwards within about 400km from 16.3±2.3mm/year to 24.0±2.9mm/year, in consistence with the observation that the Anatolian block is being pulled by the Hellenic trench rather than being pushed by the Arabian plate as a result of continental collision between the Arabian and Eurasian plates in eastern Turkey since late Miocene. Modelling the GPS velocities shows that fault locking depth increases also in the same direction from 8.1±3.3km to 12.8±3.9km. Slip rate decreases as moving off the Hellenic trench. An average slip rate of 20.1±2.4mm/year and a locking depth of 12.5±3.5km are also estimated for the entire study area by using all of the GPS measurements obtained in this study. The GPS velocities are in good agreement with the kinematic models created by paleomagnetic studies in the region and complete the overall picture. [Copyright &y& Elsevier]