Ali Polat

Boninite-like volcanic rocks in the 3.7-3.8 Ga Isua greenstone belt, West Greenland : Geochemical evidence for intra-oceanic subduction zone processes in the early Earth

Abstract The 3.7–3.8 Ga Isua greenstone belt of southwest Greenland is characterized by variably metamorphosed, metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks. The voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units. The sedimentary rocks consist mainly of banded iron formation, cherts, conglomerates and siliciclastic turbidites. The least altered metavolcanic amphibolites (the Garbenschiefer unit) from the Central Tectonic Domain of the Isua greenstone belt are characterized by high Mg-number (0.60–0.80), MgO (7–18 wt.%), Al2O3 (14–20 wt.%), Ni (60–645 ppm) and Cr (60–1920 ppm) contents, but low TiO2 (0.20–0.40 wt.%), Zr (12–30 ppm), Y (6–14 ppm) and rare earth element (REE) concentrations. These compositional features collectively represent a coherent mafic to ultramafic suite. Chondrite-normalized REE patterns are concave upward. On the primitive mantle-normalized extended trace element diagrams, they are characterized by relative depletion of Nb, but with an enrichment of Zr, relative to neighboring REEs. Alteration, deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition. The average initial eNd value of these metavolcanic rocks is +2. Collectively, these geochemical characteristics are comparable to those of Phanerozoic boninites. Given the observation that in the Tertiary, boninites are exclusively associated with intra-oceanic subduction environments (e.g., Izu–Bonin–Mariana subduction system), this suggests that intra-oceanic subduction zone-like geodynamic processes were operating as early as 3.7–3.8 Ga.

Geochemistry of Neoarchean (ca. 2.55-2.50 Ga) volcanic and ophiolitic rocks in the Wutaishan greenstone belt, central orogenic belt, North China craton : Implications for geodynamic setting and continental growth

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.

Growth of granite–greenstone terranes at convergent margins, and stabilization of Archean cratons

Abstract Archean granite–greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events. Most Archean granite greenstone terranes appear to have acquired their first-order structural and metamorphic characteristics at convergent plate margins, where large accretionary wedges similar in aspect to the Chugach, Makran, and Altaids grew through offscraping and accretion of oceanic plateaux, oceanic crustal fragments, juvenile island arcs, rifted continental margins, and pelagic and terrigenous sediments. Buoyant slabs of parts of Archean oceanic lithosphere may have been underplated beneath these orogens, forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere. Back-stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges. Dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas (sanukitoid suite) in the mantle wedge, whereas other magmas (tonalite, trondhjemite, granodiorite or TTG suite) appear to have been generated by melting of hot young subducted slabs. Eventual collision of these juvenile orogens with other continental blocks formed anatectic granites, then thickened the crust beyond its ability to support its own mass, which initiated gravitational collapse and decompressional release of syn- to late-tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs, and aided in the cratonization of the granite–greenstone terranes. Deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid- and upper-crust, forming ore deposits and plutons. Most preserved granite–greenstone terranes have been tectonically stable since the Archean, and form the cratonic interiors of many continents.

Assembly of Archean cratonic mantle lithosphere and crust: plume–arc interaction in the Abitibi–Wawa subduction–accretion complex

Abstract Recent thermodynamic models suggest that direct interaction between mantle plumes and island arcs will enhance long-term arc buoyancy and contribute disproportionately to the crustal record. However, crustal growth models have also proposed a range of differing mechanisms for Archean crust formation that emphasize specific rock types, such as tonalite–trondhjemite–granodiorite (TTG) plutons or high magnesian andesites. Studies in the Abitibi–Wawa subprovince, allow these proposals to be evaluated in the context of the worlds largest greenstone belt. Crustal growth in the southern Superior Province was the product of subduction–accretion tectonics enhanced and modified by mantle plume processes, particularly mantle plume–island arc interaction. High Archean geothermal gradients promoted volumetrically minor slab melts from the earliest phases of the Abitibi–Wawa arc, resulting in the adakite–high magnesium andesite–Niobium–enriched basalt association. However, recent flat subduction models for the formation of adakites also provide important insights into the generation of syn-tectonic Archean TTG batholiths that were probably derived from subducted, rather than accreted, oceanic crust. The distribution of Niobium enriched basalts (NEB) in the southern Superior Province may reflect plume controlled shallow subduction beneath the Abitibi belt that limited melting depths within a restricted asthensopheric mantle tongue. A well-constrained tectonic history, the mantle source requirements of successively-formed components of the Abitibi–Wawa crust, and detailed seismic interpretations of crustal architecture all preclude the presence of an autochthonous mantle lithospheric root beneath the Abitibi–Wawa arc. Instead, the late diapiric ascent of buoyant refractory plume residue and subducted oceanic crust resulted in the coupling of the mantle root and arc crust 10s of million years following batholith emplacement. High-velocity material identified at the base of the crust and centered beneath the Abitibi–Pontiac suture zone is probably Archean aged rather than Proterozoic. It corresponds to minor melts generated during ascent of the plume residue diapir and underplated prior to and during formation of the >200 km thick Abitibi–Wawa continental mantle lithosphere root.

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/Ybn 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/Ybn 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.

Trace element systematics of Mg-, to Fe-tholeiitic basalt suites of the Superior Province: implications for Archean mantle reservoirs and greenstone belt genesis

Magnesian-, to Fe-rich tholeiitic basalts with near flat REE patterns, locally associated with komatiites, are the prevalent rock type in several areally extensive volcanic sequences of the 2.9–2.7 Ga Archean greenstones belts of the Superior Province. In each of five greenstone belt sequences in the Uchi, Wabigoon, Wawa and Abitibi subprovinces, there is a continuous trend in compositions from high to low Mg# (total range, 65 to 21) accompanied by decreasing Cr and Ni contents, but increasing contents of Fe, Th, Nb, Zr, Hf, REE and Y. The total range of La/Smn is 0.60 to 1.40 and HREE patterns are generally flat. Collectively the basalts have a total range of Nb/Lapm from 0.60 to 1.95, and Th/Lapm from 0.61 to 2.36; in three of the sequences there is a negative correlation of Nb/Lapm with La/Smn. Alteration, crustal contamination, thermal assimilation of altered ocean crust, melting processes, and fractional crystallization can all be ruled out as the cause of the spectrum of Nb and Th anomalies. Rather, the anomalies are interpreted in terms of a heterogeneous multi-component mantle plume: positive anomalies represent a residual slab component from oceanic crust processed through a subduction zone and recycled into the mantle, whereas the negative anomalies reflect a recycled slab-derived lithosphere component. These components were likely reactivated in a mantle plume, and/or entrained in a plume. Consequently, greenstone belt formation was preceded by ocean floor spreading, arc formation and recycling into the mantle. These tholeiitic basalt sequences are directly analogous to basalts of Phanerozoic ocean islands or ocean plateaus, such as Iceland and Ontong Java, many of which also possess ranges of Nb/Lapm and Th/Lapm interpreted as mixing of different mantle components. In two of the belts there is clear evidence for intercalated coeval plume and calc-alkaline arc magmatism. Some Mg- to Fe-tholeiitic basalts in these belts have pronounced negative Nb-anomalies where Nb/Lapm<0.6; compositionally these basalts are similar to Phanerozoic primitive arc, or back-arc tholeiites. Jamming of plume-related plateaus into primitive arcs may govern the transition to calc-alkaline sequences.

Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central orogenic belt, North China craton

The 2.55–2.50 Ga Zunhua and Wutaishan belts within the central orogenic belt of the North China craton contain variably metamorphosed and deformed tectonic blocks of peridotites and amphibolites that occur in a sheared metasedimentary matrix. In the Zunhua belt, dunites comprise podiform chromitites with high and uniform Cr-numbers (88). Peridotites and associated picritic amphibolites are characterized by light rare earth element (LREE)–enriched patterns and negative high field strength element (HFSE: Nb, Zr, and Ti) anomalies. They have positive initial ϵ Hf values (+7.9 to +10.4), which are consistent with an extremely depleted mantle composition. Mass-balance calculations indicate that the composition of the 2.55 Ga mantle beneath the Zunhua belt was enriched in SiO 2 and FeO T compared to modern abyssal peridotites. These geochemical signatures are consistent with a suprasubduction zone geodynamic setting. Metasomatism of the subarc mantle by slab-derived hydrous melts and/or fluids at ca. 2.55 Ga is likely to have been the cause of the subduction zone geochemical signatures in peridotites of the Zunhua belt. In the Wutaishan belt, chromitite-hosting harzburgites and dunites display U-shaped rare earth element (REE) patterns and have high Mg-numbers (91.1–94.5). These geochemical characteristics are similar to those of Phanerozoic forearc peridotites. The dunites might have formed by dissolution of orthopyroxene in reactive melt channels, similar to those in modern ophiolites. However, they differ in detail, and they might be residues of Archean komatiites. Following the initiation of an intra-oceanic subduction zone, they were trapped as a forearc mantle wedge between the subducting slab and magmatic arc. Slab-derived hydrous melts infiltrating through the mantle wedge metasomatized the depleted mantle residue, resulting in U-shaped rare earth element (REE) patterns.

Contrasting geochemical patterns in the 3.7-3.8 Ga pillow basalt cores and rims, Isua greenstone belt, Southwest Greenland : implications for postmagmatic alteration processes

Abstract Pillow basalts from the early Archean (3.7 to 3.8 Ga) Isua greenstone belt, West Greenland, are characterized by well-preserved rims and concentric core structures. The pillow rims and cores have different mineral assemblages, and chemical and isotopic compositions. The rims have systematically higher contents of Fe2O3, MgO, MnO, K2O, Rb, Ba, Ga, Y, and transition metals than the cores. In contrast, the cores possess higher concentrations of SiO2, Na2O, P2O5, Sr, Pb, U, Nb, and the light rare earth elements (REEs than the rims). These compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration. Variations of many major and trace element concentrations between the rims and cores of the Isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration. Al2O3, TiO2, Th, Zr, and the heavy REEs display similar values in both rims and cores, suggesting that these elements were relatively immobile during postemplacement alteration. In addition, the rims and cores have distinctive Sm-Nd and Rb-Sr isotopic compositions in that the rims are characterized by higher 143Nd/144Nd and 87Sr/86Sr ratios than the cores. The pillow basalts yield 2569 ± 170 Ma and 1604 ± 170 Ma errorchron ages on 143Nd/144Nd vs. 147Sm/144Nd and 87Sr/86Sr vs. 87Rb/86Sr diagrams, respectively. The Sm-Nd errorchron age may correspond, within errors, to a late Archean tectonothermal metamorphic event recorded in the region. The Sm-Nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of Nd, relative to Sm, during late Archean metamorphism. Although the Rb-Sr errorchron age overlaps with the timing of an early to mid-Proterozoic tectonothermal metamorphic event recorded in the region, because of a considerably large mean square of weighted deviates value and scatter in 86Sr/87Sr and 87Rb/86Sr ratios, this age may not have a precise geological significance. The 1.6 Ga Rb-Sr errorchron is likely to have resulted from the loss of radiogenic 87Sr. Collectively, the Sm-Nd and Rb-Sr data obtained from the 3.7–3.8 Ga Isua pillow basalt rims and cores are consistent with disturbances of the Sm-Nd and Rb-Sr systems by tectonothermal metamorphic events long after their eruption. In contrast to the Sm-Nd and Rb-Sr systems, the Lu-Hf system appears to be largely undisturbed by metamorphism. Five core samples and three rim samples yield a 3935 ± 350 Ma age, within error of the approximate age of eruption (3.7 to 3.8 Ga). Two rim samples that have gained Lu give an age of 1707 ± 140 Ma, within error of the Rb-Sr errorchron age. Initial 176Hf/177Hf ratios of the undisturbed samples at 3.75 Ga lie within ±1 e-unit of the chondritic value, suggesting no long-term depletion in the mantle source of the basalts.

Nd-isotope systematics of ~2.7 Ga adakites, magnesian andesites, and arc basalts, Superior Province: evidence for shallow crustal recycling at Archean subduction zones

Abstract An association of adakite, magnesian andesite (MA), and Nb-enriched basalt (NEB) volcanic flows, which erupted within ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts, has recently been documented in ∼2.7 Ga Wawa greenstone belts. Large, positive initial ϵNd values (+1.95 to +2.45) of the adakites signify that their basaltic precursors, with a short crustal residence, were derived from a long-term depleted mantle source. It is likely that the adakites represent the melts of subducted late Archean oceanic crust. Initial ϵNd values in the MA (+0.14 to +1.68), Nb-enriched basalts and andesites (NEBA) (+1.11 to +2.05), and ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts and andesites (+1.44 to +2.44) overlap with, but extend to lower values than, the adakites. Large, tightly clustered ϵNd values of the adakites, together with Th/Ce and Ce/Yb systematics of the arc basalts that rule out sediment melting, place the enriched source in the sub-arc mantle. Accordingly, isotopic data for the MA, NEBA, and ‘normal’ arc basalts can be explained by melting of an isotopically heterogeneous sub-arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late Archean subduction zones up to 200 Ma prior to the 2.7 Ga arc. If the late Archean Wawa adakites, MA, and basalts were generated by similar geodynamic processes as their counterparts in Cenozoic arcs, involving subduction of young and/or hot ocean lithosphere, then it is likely that late Archean oceanic crust, and arc crust, were also created and destroyed by modern plate tectonic-like geodynamic processes. This study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the Archean upper mantle. In addition, the results of this study indicate that the Nd-isotope compositions of Archean arc- and plume-derived volcanic rocks are not very distinct, whereas Phanerozoic plumes and intra-oceanic arcs tend to have different Nd-isotopic compositions.

Variability of Nb/U and Th/La in 3.0 to 2.7 Ga Superior Province ocean plateau basalts: implications for the timing of continental growth and lithosphere recycling

Extensive volcanic sequences of tholeiitic basalts, having near-flat REE patterns and spatially associated with komatiites, occur in many Archean Superior Province greenstone belts; they are considered to be fragments of intraoceanic volcanic plateaus derived from a plume. Basalts from the 2.9–3.0 Ga Lumby Lake greenstone belt, carefully screened for minimum alteration, have variable Nb/U ratios of 36 to 58. Least-altered basalts from the Abitibi and Wawa greenstone belts also have variable Nb/U ratios of 25 to 50 and 28 to 42, respectively, compared to an average value of 47 for modern ocean basalts. In the Abitibi suite Nb/U correlates positively with Nb/Lapm but negatively with Th/Lapm. Alteration can be ruled out as the cause of Nb/U variation, as there are no correlations of Nb/U with LOI or Eu/Eu*, and Nb/U correlates with Nb/Th in all three suites of basalts. Numerous lines of evidence indicate that crustal contamination can be eliminated as the cause of Nb/U variability, especially for samples with Nb/U > 36. High Nb/U ratios can be explained by recycling ocean crust processed through a subduction zone (high Nb/U and Nb/Lapm, low Th/Lapm) into the mantle source of the basalts, whereas low Nb/U ratios can be accounted for by recycling complementary subarc mantle lithosphere, or continental crust (low Nb/U and Nb/Lapm, high Th/Lapm) into the mantle source. For the entire population of basalts, Th/Lapm ratios generally <1 may result from recycling the residue of slab-derived Archean-type tonalites having high Th/La ratios. Nb/U ratios as high as the 47 ± 10 range of modern ocean basalts have been found in a 2.7 Ga volcanic belt of the Yilgarn craton. Taken together, the results signify very early growth of the continental crust, rather than episodic growth over several Ga.