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The Mesoarchean Târtoq greenstone belt, southern West Greenland, consists of tectonically imbricated slices of metamorphosed basalt, gabbro, peridotite, and sedimentary rocks and is intruded by felsic rocks (now mylonites) with well-preserved duplex structures, representing a relict accretionary prism. The Târtoq greenstone belt is a remnant of a supra-subduction zone ophiolite that originated as back-arc basin oceanic crust. Following the initiation of intra-oceanic subduction, the back-arc oceanic crust accreted to the overriding plate, forming an accretionary prism. The felsic mylonites are compositionally akin to Archean tonalite-trondhjemite-granodiorite suites. Field observations, along with geochemical and zircon U-Pb age data, indicate that the protoliths of the felsic mylonites were derived from partial melting of back-arc basalts in the accretionary prism and emplaced along thrust faults between 3012 ± 4 and 2993 ± 6 Ma. It is proposed that the partial melting of the basalts likely occurred in response to ridge subduction. The Upper Cretaceous turbiditic greywackes of the Chugach - Prince William accretionary complex in southern Alaska are intruded by Paleogene felsic dykes. These felsic dykes appear to have been derived from partial melting of subducted and (or) accreted oceanic crust during slab window magmatism. Archean granitoid-greenstone terrains share many geological characteristics of Phanerozoic subduction-accretion complexes such as the Alaskan and Altaid subduction-accretion complexes, consistent with the operation of uniformitarian geological processes in the Archean. The Archean Earth might have been dominated by numerous smaller plates and greater ridge length than today that would have resulted in more frequent ridge-accretionary prism interactions and larger volumes of tonalite-trondhjemite-granodiorite generation in subduction-accretion complexes. [ABSTRACT FROM AUTHOR]

The Archean craton of West Greenland consists of many fault-bounded Eoarchean to Neoarchean tectonic terranes (crustal blocks). These tectonic terranes are composed mainly of tonalite–trondhjemite–granodiorite (TTG) gneisses, granitic gneisses, metavolcanic-dominated supracrustal belts, layered anorthositic complexes, and late- to post-tectonic granites. Rock assemblages and geochemical signatures in these terranes suggest that they represent fragments of dismembered oceanic island arcs, consisting mainly of TTG plutons, tholeiitic to calc-alkaline basalts, boninites, picrites, and cumulate layers of ultramafic rocks, gabbros, leucogabbros and anorthosites, with minor sedimentary rocks. The structural characteristics of the terrane boundaries are consistent with the assembly of these island arcs through modern style of horizontal tectonics, suggesting that the Archean craton of West Greenland grew at convergent plate margins. Several supracrustal belts that occur at or near the terrane boundaries are interpreted as relict accretionary prisms. The terranes display fold and thrust structures and contain numerous 10 cm to 20 m wide bifurcating, ductile shear zones that are characterized by a variety of structures including transposed and redistributed isoclinal folds. Geometrically these structures are similar to those occurring on regional scales, suggesting that the Archean craton of West Greenland can be interpreted as a continental scale accretionary complex, such as the Paleozoic Altaids. Melting of metavolcanic rocks during tectonic thickening in the arcs played an important role in the generation of TTGs. Non-uniformitarian models proposed for the origin of Archean terranes have no analogs in the geologic record and are inconsistent with structural, lithological, petrological and geochemical data collected from Archean terranes over the last four decades. The style of deformation and generation of felsic rocks on outcrop scales in the Archean craton of West Greenland and the Mesozoic Sulu orogenic belt of eastern China are similar, consistent with the formation of Archean continental crust by subduction zone processes. [ABSTRACT FROM AUTHOR]

The Archean (ca. 2.97 Ga) Fiskenæsset layered intrusion, southwestern Greenland, consists of an association of anorthosite, leucogabbro, gabbro, hornblendite, pyroxenite, peridotite and dunite. The intrusion is characterized by well-preserved igneous layering, cumulate texture and primary igneous minerals including olivine, pyroxene, plagioclase, hornblende and chromite. We use new whole-rock ( ) and mineral ( ) oxygen isotopic data for all major lithologic units from the best preserved stratigraphic section of the Fiskenæsset Complex at Majorqap qâva to revisit geodynamic and petrogenetic hypotheses proposed for the origin of Archean terranes. The Fiskenæsset Complex has modern mantle-like whole-rock O-isotope compositions ( ). Average values increase from peridotite ( ), through hornblendite ( ), gabbro ( ), pyroxene hornblendite ( ) and leucogabbro ( ), to anorthosite ( ). These whole-rock isotopic compositions reflect the approximate modal abundances of olivine (average ), hornblende (average ), clinopyroxene (average ) and plagioclase (average ) in each rock type, as a consequence of mineral fractionation in the magma chamber(s). Field relationships and the absence of crustal contamination suggest that the Fiskenæsset Complex formed in an oceanic setting. Subduction zone-like whole-rock trace element signatures and mantle-like and initial values are consistent with formation of these rocks in a juvenile oceanic island arc setting. Field and geochemical data from the Fiskenæsset region and adjacent terranes suggest that the origin of Archean crust in southwestern Greenland is consistent with Phanerozoic-like plate tectonic processes rather than density-driven sinking, delamination and diapiric processes requiring formation of greenstone belts and anorthosite complexes on pre-existing continental crust. Mantle-like values in the ca. 2.97 Ga Fiskenæsset Complex are inconsistent with substantial recycling of continental crust, indicating that the Fiskenæsset rocks originated distal from continental sources. [ABSTRACT FROM AUTHOR]

Abstract: Mesoarchean to Neoarchean orthogneisses (2.95–2.79Ga) in the Fiskenæsset region, southern West Greenland, are composed of an older suite of metamorphosed tonalites, trondhjemites, and granodiorites (TTGs), and a younger suite of high-K granites. The TTGs are characterized by high Al2O3 (14.2–18.6wt.%), Na2O (3.4–5.13wt.%), and Sr (205–777ppm), and low Y (0.7–17.4ppm) contents. On chondrite- and N-MORB-normalized trace element diagrams, the TTGs have the following geochemical characteristics: (1) highly fractionated REE patterns (La/Ybcn =14–664; La/Smcn =4.3–11.0; Gd/Ybcn =1.5–19.7); (2) strong positive anomalies of Sr (Sr/Sr*=1.0–15.9) and Pb (Pb/Pb*=1.4–34.9); and (3) large negative anomalies of Nb (Nb/Nb*=0.01–0.34) and Ti (Ti/Ti*=0.1–0.6). The geochemical characteristics of the TTGs and trace element modeling suggest that they were generated by partial melting of hydrous basalts (amphibolites) at the base of a thickened magmatic arc, leaving a rutile-bearing eclogite residue. Field observations suggest that spatially and temporarily associated tholeiitic basalts (now amphibolites) in the Fiskenæsset region might have been the sources of TTG melts. The high-K granites have steep REE patterns (La/Ybcn =3.8–506; La/Smcn =2.7–18.9; Gd/Ybcn =0.92–12.1) and display variably negative Eu anomalies (Eu/Eu*=0.37–0.96) and moderate Sr (84–539ppm) contents. Four outlier granite samples have variably positive Eu (Eu/Eu*=1.0–12) anomalies. Given that the granodiorites have higher K2O/Na2O than the tonalites and trondhjemites, it is suggested that the granites were derived from partial melting of the granodiorites. It is speculated that the dense eclogitic residues, left after TTG melt extraction, were foundered into the sub-arc mantle, leading to basaltic underplating beneath the lower rust. Melting of the granodiorites in response to the basaltic underplating resulted in the production of high-K granitic melts. Formation of the Fiskenæsset TTGs, the foundering of the eclogitic residues into the mantle, and the emplacement of the high-K granites led to the growth of Archean continental crust in the Fiskenæsset region. [Copyright &y& Elsevier]

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: The Mesoarchean Ivisaartoq greenstone belt consists of tholeiitic pillow basalts, picrites, boninites, gabbroic to dioritic dykes and sills, actinolite schists and serpentinites. In addition, the belt includes volumetrically minor siliceous volcaniclastic sedimentary rocks, cherts, biotite schists, and quartzitic gneisses. Siliceous volcaniclastic rocks yield an average U–Pb zircon age of 3075±15Ma, and pillow basalts and gabbros yield a Sm–Nd errorchron age of 3080±160Ma, constraining the maximum age of the belt. The Ivisaartoq rocks underwent high-temperature seafloor hydrothermal alteration, resulting in extensive epidote-diopside calc-silicate metasomatism. Th–REE–Nb systematics of the least metasomatized volcanic rocks are consistent with a supra-subduction zone geodynamic setting, displaying 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 depleted upper mantle to a subarc mantle wedge following the initiation of intra-oceanic subduction and arc migration. Many pillow basalts in the Ivisaartoq greenstone belt contain numerous millimetre- to centimetre-long white polycrystalline ellipsoidal inclusions, called ‘ocelli’, set in a dark green mafic matrix (basaltic host). On the basis of new major and trace element, Nd isotope, and petrographic data, we re-interpret the origin of these inclusions and suggest that they were derived from the melting of hydrated mafic to ultramafic rocks in the lower oceanic crust during magma–crust interaction. Some pillow basalts contain rare small inherited magmatic zircons revealing Eoarchean (3706±8 to 3717±4Ma) 207Pb206Pb ages. In the least metasomatized pillow basalts, and gabbros and diorites there is a strong negative correlation between initial Nd isotopic composition (ε Nd =+0.76 to +3.10) and depleted mantle model ages (T DM =3100–3800Ma). Post-magmatic metasomatic alteration can be ruled out as the cause of this negative correlation, suggesting the contamination of the Ivisaartoq rocks, or their mantle source, by Eoarchean (~3700Ma) continental crust or sediments derived from such crust. Four possible processes of crustal contamination might have taken place during or prior to the emplacement of the Ivisaartoq belt: (1) interaction with continental crust during magma ascent; (2) mixing between the depleted upper mantle and recycled sediments derived from Eoarchean continental crust; (3) mixing between the depleted upper mantle and delaminated lower continental crust; and (4) assimilation of continentally-derived sediments during eruption. It is unlikely that recycled Eoarchean zircons and their host rocks could have survived through multiple events of partial melting in the Mesoarchean upper mantle. A tectonic slice of metasedimentary rocks exposed between the Ivisaartoq belt and the Eoarchean (3600–3870Ma) Isukasia terrane contains magmatic zircon grains up to 3800Ma old. Accordingly, we argue that Eoarchean zircons are likely to have been derived from the protoliths of these metasedimentary rocks. The negative correlation between initial ε Nd and T DM in pillow basalts, gabbros and diorites, however, can be better explained by contamination of the mantle source through hydrous melts and/or fluids, derived from Mesoarchean subducted (recycled) sediments, than by contamination at the surface. [Copyright &y& Elsevier]

Abstract: Mafic volcanic rocks in the 3.8–3.7 Ga Isua greenstone belt, southern West Greenland, contain randomly distributed 1 to 10-centimeters long white spheroidal structures. In this study, these structures are called ‘ocelli’. In the western part of the belt, ocelli-bearing lavas are enclosed in basaltic to picritic flows (MgO=9–21 wt.%) with a subduction zone geochemical signature. The ocelli are composed predominantly of polycrystalline Na-plagioclase and quartz, with minor hornblende and biotite, whereas the surrounding amphibolite matrix (basaltic host) is composed mainly of hornblende, Ca-plagioclase, and quartz. The ocelli are devoid of radial or concentric internal structure, and display all stages of coalescence. Contacts between the ocelli and surrounding amphibolite matrix are sharp to gradational. Compositionally, the ocelli are calc-alkaline dacites (SiO2 =62.9–72.0 wt.%; MgO=0.60–3.50 wt.%; Ni=58–143 ppm; Cr=250–510 ppm), whereas the surrounding matrix is tholeiitic basalt (SiO2 =46.6–50.6 wt.%; MgO=8.70–12.30 wt.%; Ni=119–175 ppm; Cr=330–600 ppm). In terms of major element composition, the Isua ocelli closely resemble plagiogranites in Phanerozoic supra-subduction zone ophiolites. Field and petrographic observations, and geochemical data (SiO2 =54.2–60.7 wt.%; MgO=3.95–7.72 wt.%; Ni=127–158 ppm; Cr=500–570 ppm) on the transitional areas between the ocelli and the matrix suggest magma mixing between dacitic and basaltic melts. On a chondrite-normalized diagram, the basaltic host is characterized by variably depleted LREE patterns (La/Smcn =0.30–0.94; Gd/Ybcn =1.03–1.45), whereas the dacitic ocelli display LREE-enriched patterns (La/Smcn =1.30–2.60; Gd/Ybcn =1.32–2.58). The strongly depleted REE patterns in the basaltic host are attributed to LREE loss during carbonate alteration. Partial melting of a forearc mantle wedge is favoured for the origin of the protolith of the basaltic host. The geochemical characteristic of the ocelli cannot be explained by post-magmatic alteration, slab melting, fractional crystallization of tholeiitic melts, or liquid immiscibility. We suggest that the dacitic ocelli might have been derived from hydrous melting of the fragments of oceanic crust (high-Mg volcanic rocks) that fell into the magma chamber, suggesting magma–crust interaction in the early Earth. Formation of dacitic volcanic rocks by partial melting of altered oceanic crust may have played an important role in the generation of felsic crust in the early Archean. [Copyright &y& Elsevier]

Abstract: We report trace element, samarium (Sm)–neodymium (Nd) and lead (Pb) isotopic data for individual micro-and mesobands of the Earth''s oldest Banded Iron Formation (BIF) from the ∼3.7–3.8 Ga Isua Greenstone Belt (IGB, West Greenland) in an attempt to contribute to the characterization of the depositional environment and to the understanding of depositional mechanisms of these earliest chemical sediments. Rare earth element (REE)-yttrium (Y) patterns of the individual mesobands show features of modern seawater with diagnostic cerium (Ce/Ce⁎), presodymium (Pr/Pr⁎) and Y/holmium (Ho) anomalies. Very low high field strength elements (HFSE) concentrations indicate essentially detritus-free precipitation. Uranogenic Pb isotope data define a correlation line with a slope of 3691±41 Ma, indicating that the uranium (U)–lead (Pb) system remained closed after the formation of this BIF. High 207Pb/204Pb relative to 206Pb/204Pb ratios compared to average mantle growth evolution models are a feature shared by BIF, penecontemporaneous basalts and clastic volcanogenic metasediments and are indicative of the ultimate high-μ (238U/204Pb) character of the source region, an essentially mafic Hadean protocrust. Sm–Nd isotopic relations on a layer-by-layer basis point to two REE sources controlling the back-arc basin depositional environment of the BIF, one being seafloor-vented hydrothermal fluids (εNd (3.7 Ga)∼+3.1), the other being ambient surface seawater which reached its composition by erosion of parts of the protocrustal landmass (εNd(3.7 Ga)∼+1.6). The validity of two different and periodically interacting water masses (an essentially two-component mixing system) in the deposition of alternating iron- and silica-rich layers is also reflected by systematic trends in germanium (Ge)/silicon (Si) ratios. These suggest that significant amounts of silica were derived from unexposed and/or destroyed mafic Hadean landmass, unlike iron which probably originated from oceanic crust following hydrothermal alteration by deep percolating seawater. Ge/Si distributional patterns in the early Archean Isua BIF are similar to those reported from the Paleoproterozoic Hamersley (Western Australia) BIF, but overall Ge concentrations are about one order of magnitude higher in the Archean BIF. This seems consistent with other lines of evidence that the ambient Archean seawater was enriched with iron relative to Proterozoic and recent seawater. [Copyright &y& Elsevier]

Here we present Sm-Nd, Re-Os, and Pb isotopic data of carefully screened, least altered samples of boninite-like metabasalts from the Isua Supracrustal Belt (ISB, W Greenland)that characterize their mantle source at the time of their formation. The principal observations of this study are that by 3.7–3.8 Ga melt source regions existed in the upper mantle with complicated enrichment/depletion histories. Sm-Nd isotopic data define a correlation line with a slope corresponding to an age of 3.69 ± 0.18 Gy and an initial ϵNd value of +2.0 ± 4.7. This Sm-Nd age is consistent with indirect (but more precise) U-Pb geochronological estimates for their formation between 3.69–3.71 Ga. Relying on the maximum formation age of 3.71 Gy defined by the external age constraints, we calculate an average ϵNd [T = 3.71 Ga] value of +2.2 ± 0.9 (n = 18, 1σ) for these samples, which is indicative of a strongly depleted mantle source. This is consistent with the high Os concentrations, falling in the range between 1.9–3.4 ppb, which is similar to the estimated Os concentration for the primitive upper mantle. Re-Os isotopic data (excluding three outliers) yield an isochron defining an age of 3.76 ± 0.09 Gy (with an initial γOs value of 3.9 ± 1.2), within error consistent with the Sm-Nd age and the indirect U-Pb age estimates. An average initial γOs [T = 3.71 Ga] value of + 4.4 ± 1.2 (n = 8; 2σ) is indicative of enrichment of their source region during, or prior to, its melting. Thus, this study provides the first observation of an early Archean upper mantle domain with a distinctly radiogenic Os isotopic signature. This requires a mixing component characterized by time-integrated suprachondritic Re/Os evolution and a Os concentration high enough to strongly affect the Os budget of the mantle source; modern sediments, recycled basaltic crust, or the outer core do not constitute suitable candidates. At this point, the nature of the mantle or crustal component responsible for the radiogenic Os isotopic signature is not known.Compared with the Sm-Nd and Re-Os isotope systems, the Pb isotope systematics show evidence for substantial perturbation by postformational hydrothermal-metasomatic alteration processes accompanying an early Archean metamorphic event at 3510 ± 65 Ma and indicate that the U-Th-Pb system was partially opened to Pb-loss on a whole rock scale. Single stage mantle evolution models fail to provide a solution to the Pb isotopic data, which requires that a high-μ component was mixed with the depleted mantle component before or during the extrusion of the basalts. Relatively high 207Pb/204Pb ratios (compared to contemporaneous mantle), support the hypothesis that erosion products of the ancient terrestrial protocrust existed for several hundred My before recycling into the mantle before ∼3.7 Ga.Our results are broadly consistent with models favoring a time-integrated Hadean history of mantle depletion and with the existence of an early Hadean protocrust, the complement to the Hadean depleted mantle, which after establishment of subduction-like processes was, at least locally, recycled into the upper mantle before 3.7 Ga. Thus, already in the Hadean, the upper mantle seems to be characterized by geochemical heterogeneity on a range of length scales; one property that is shared with the modern upper mantle. However, a simple two component mixing scenario between depleted mantle and an enriched-, crustal component with a modern analogue can not account for the complicated and contradictory geochemical properties of this particular Hadean upper mantle source. [Copyright &y& Elsevier]

The article discusses research being done on ocean island basalt (OIB)-like trace element patterns for Eoarchean basaltic amphibolites from Innersuartuut Island, southwestern Greenland. It references the study "Eoarchean Within-Plate Basalts From Southwest Greenland," by F. E. Jenner, V. C. Bennett, G. Yaxley, C. R. L. Friend and O. Nebel in the 2013 issue of "Geology." The study examined geological record in the Archean craton of the region which was affected by subduction.

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]

Abstract: Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu–Hf and Sm–Nd during mantle melting. Eoarchean rocks, for instance those from southern West Greenland, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf–Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800Ma old tholeiitic metabasalts and gabbros from the ∼3700Ma and ∼3800Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial εHf–εNd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (−0.7 to +6.3 and −0.8 to +4.4, respectively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm–Nd and Lu–Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium–Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm–Nd space, indicating early disturbance of the Sm–Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element compositions of the metabasalts indicate an arc setting and a strong source overprint by melt-like subduction components. It is likely, that the source overprint may have caused partial decoupling of the εHf–εNd values, due to selective addition of Nd as observed in modern subduction settings. In this case, the most radiogenic initial εNd and εHf isotope values characterize the most depleted mantle sources, and less radiogenic values would reflect increased contributions of isotopically more enriched subduction components. However, the most depleted samples still exhibit decoupled Hf–Nd compositions, making a case for the presence of even older mantle heterogeneities. A proposed superchondritic composition of the silicate Earth (SCHEM), however, cannot account for the most depleted sample compositions. Conversely, a depleted upper mantle formed by crystallization of perovskite-rich cumulates in the early Hadean may well explain these observed compositions. A literature survey reveals an overlap in initial Hf–Nd compositions between southern West Greenland TTGs and the metabasalts analyzed here. This overlap suggests a genetic relationship between these lithologies, where the TTGs may have inherited their unusual Hf–Nd compositions from mafic precursors isotopically similar in composition to the Isua tholeiites. [Copyright &y& Elsevier]

Abstract: Growing evidence from the accessible geological record reveals that crust–mantle differentiation on Earth started as early as 4.4Ga. In order to assess the extent of early Archean mantle depletion, we obtained 176Lu–176Hf, 147Sm–143Nd, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial εHf(3720) values from −0.7 to +1.5 and initial εNd(3720) values from +1.6 to +2.1. Initial εHf(3720) values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial εNd(3720) values from −0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO2, Al2O3/TiO2 and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial γOs(3720) values for some of the samples. In addition to the positive εHf(3720) values, the least disturbed samples exhibit positive εNd(3720) values and a co-variation of εHf(3720) and εΝd(3720) values. Based on these observations, it is argued, that the most depleted samples with initial εHf(3720) values of up to +12.9 and high 176Lu/177Hf of ∼0.05 to ∼0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb–Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotope data for Hadean zircons and combined 142Nd–143Nd isotope data for early Archean rocks. [Copyright &y& Elsevier]

[Display omitted] • Deciphering the Eoarchean geological record must come from observation, not modelling. • Tectonic interleaving (including mantle over crustal rocks) shows crustal shortening. • Greenland's Eoarchean mafic rocks formed by fluid-fluxing of depleted upper mantle. • Formation of Greenland's Eoarchean crust matches convergent plate boundary processes. • Greenland's Eoarchean geology is inconsistent with stagnant-lid/heat-pipe models. It is 50 years since the landmark paper where Black et al. (1971) presented whole-rock Pb-Pb and Rb-Sr isotopic evidence for some rocks in Greenland surviving from Earth's first billion years; the ≥ 3700 Ma Amîtsoq gneisses. This overturned ideas prevalent at that time that the young Earth was far too violent for such ancient rocks to survive. In the following few years it emerged how 'normal' this early Earth appeared to be, with a retained hydrosphere (oceans) by 3700 Ma and 'continental' crust dominated by rather normal granitic sensu lat o rocks. By several decades ago this led to a vision of Eoarchean lithosphere development being controlled by a geodynamic regime with definite similarities to that in the Phanerozoic, particularly with evidence for lateral lithosphere movement (mobile lid) such as exemplified by arc-like magmatism and Barrovian metamorphism (the latter requiring tectonic crustal thickening). However, the literature in recent years is increasingly emphasising model-driven non-uniformitarian stagnant lid geodynamic scenarios to explain the Eoarchean geological record. This paper reviews the broad range of field and laboratory evidence extracted from the Eoarchean geological record in Greenland. We argue that the non-uniformitarian sagduction within a stagnant lid regime interpretation does not fit Eoarchean facts. [ABSTRACT FROM AUTHOR]