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In this study, we integrated extensive field, petrographic, whole-rock major and trace element, and Nd–Pb–Sr–O isotope, and zircon U–Pb ages, trace element and Lu–Hf isotope data from the Neoarchean Shawmere Anorthosite Complex and surrounding gneisses to unravel their petrogenetic origin and tectonic history. The ~2765 Ma Shawmere Anorthosite Complex is interpreted to have been emplaced into a sequence of interlayered greywacke and basalt deposited in an intra-continental arc rift system above a north-dipping subduction zone. The complex consists mainly of anorthosite, leucogabbro, gabbro, and hornblendite that were emplaced as several batches of magmas and crystal mushes originating from sub-arc mantle sources. In contrast to the previous studies, our field and petrographic data suggest an igneous origin for the most hornblende in the complex, implying hydrous parental magmas. A hydrous magma origin is also consistent with the high-anorthite content (mostly 70–90%) of the plagioclase in the complex. Percolation of hydrous basaltic melts through gabbroic cumulates in crustal magma chambers led to extensive (>50%) replacement of igneous clinopyroxene by igneous hornblende. Continued subduction resulted in the closure of the intra-arc rift system and the intrusion of the complex by tonalite, granodiorite and diorite between 2765 and 2680 Ma in an Andean-type margin. The complex and surrounding gneisses underwent hornblende granulite-facies metamorphism mainly between 2680 and 2620 Ma, overlapping with mid-crustal east-west extension between 2660 and 2640 Ma. The granulite-facies metamorphism is recorded by the replacement of hornblende, plagioclase and clinopyroxene by garnet and the development of a garnet-orthopyroxene-plagioclase metamorphic assemblage with a granoblastic texture. Tectonic rebounding of mid-crustal rocks to upper crustal levels after 2620 Ma led to the formation of an extensive network of extensional fractures and retrograde metamorphism. Migration of CO2-rich hydrous fluids along the extensional fractures and grain boundaries resulted in the precipitation of many metasomatic minerals mainly at the expense of hornblende and plagioclase, including epidote, clinozoisite, tremolite, actinolite, paragonite, margarite, titanite, quartz, calcite, sillimanite, dolomite, and chlorite. Prevalent replacement of hornblende by garnet during prograde metamorphism and metasomatic replacement of hornblende and plagioclase by retrograde mineral assemblages disturbed the Sm–Nd, U–Th–Pb, and Rb–Sr isotope systems. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

John Tuzo Wilson (1908-1993) was one of the greatest Canadian scientists of the 20th century. His contributions to Earth Sciences, leading the formulation of the theory of plate tectonics, have revolutionized our understanding of how the planet Earth works and evolved over the past 4 billion years. This 50th anniversary special issue of the Canadian Journal of Earth Sciences is dedicated in honour of John Tuzo Wilson, who inspired tens of thousands of students all around the world to study the Earth. This special issue contains 12 papers dealing with various aspects of the 'Wilson Cycle' in the geologic record, plate tectonics, mantle plumes, and how John Tuzo Wilson accepted 'continental drift' and formulated the theory of plate tectonics. The contributions have mostly been made by geoscientists who directly or indirectly associated with John Tuzo Wilson and have contributed significantly to the plate tectonics paradigm. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Earth Sciences is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Abstract: This study reports detailed stable Cr, Sm–Nd and Rb–Sr isotope data for a ca. 1.9Ga old subaerial weathering profile at Schreiber Beach, Ontario, Canada, from which detailed major and trace element signatures and δ 18O values were previously reported. The weathering profile developed on Neoarchean (∼2.7Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (∼1.88Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation. The aim was to investigate the behaviour of chromium during ancient weathering processes and to contribute to the question whether or not stable isotope fractionation accompanied the release of chromium from the weathering rocks at times when atmospheric oxygen was supposedly high enough (in a period following the Great Oxygenation Event (GOE) ∼2.45Ga ago and prior to the deposition of the Gunflint cherts at ∼1.82Ga). These iron-rich cherts belong to one of the last regionally extensive banded iron formations (BIFs) deposited before the ∼1 billion year-long Mesoproterozoic period during which a drastic change in ocean chemistry prevented deposition of BIFs. The gradual textural, mineralogical, and geochemical changes from unweathered basalts to strongly weathered hematite-bearing basalts with increasing stratigraphic height are associated with shifts in the chromium isotope compositions. The δ 53Cr value of unweathered pillow basalt cores (−0.19±0.02‰, 2σ) is within the range of mantle inventory values, whereas weathered brown to green basalts (soils), exhibiting up to 30% lower Cr concentrations compared to unweathered pillow cores, are isotopically lighter (δ 53Cr=−0.35±0.11‰). In contrast, red, hematite-rich basalts and hyaloclastites underlying the brown to green basaltic soils, with the highest δ 18O enrichment in the profile, are isotopically heavier (δ 53Cr=+0.05±0.15‰). Rb–Sr isotope data of weathered basalts define a correlation line with a slope corresponding to an age of 1574±24Ma (MSWD=13) which we interpret to reflect a diagenetic event accompanied by alkaline metasomatism during subsidence/burial of the Gunflint basin. The non-correlation of K enrichment factors with δ 53Cr, δ 18O and other major and trace elements in the weathered rocks indicates that alkaline metasomatism did not affect the geochemical signatures produced during in situ subaerial weathering. Instead, correlations between δ 53Cr and δ 18O, and between δ 53Cr, Ce/Ce*, U and V, indicate coupled mobilization of the redox sensitive elements during oxidative processes and their partial re-deposition at depth from high δ 18O freshwaters. Partial re-precipitation of heavy Cr at depth can be explained on the basis of the weak positive correlation between δ 53Cr and Fe enrichment factors; it is seen as a consequence of the reduction of mobile Cr(VI) and co-precipitation as mixed Fe(III)–Cr(III) oxhydroxides during contemporaneous oxidation of the Fe2+-bearing groundwaters at depth, a process similar to the one governing the deposition of most Precambrian BIFs. Redistribution of certain elements during the weathering process, in this case REEs, is furthermore indicated by a correlation line in a Sm–Nd isochron diagram defined by the Schreiber Beach data with a slope corresponding to an age of 1.93±0.19Ga (MSWD=26), an age which is compatible with the direct geological constraints for the timing of the paleosol formation at this locality. An oxidative atmosphere at ∼1.9Ga, as implied by the results from the Schreiber profile, is furthermore supported by positively fractioned Cr isotopes (δ 53Cr from +0.06 to +0.39‰) recorded in the iron-rich Gunflint Cherts directly above the palaeo-weathered horizons at Schreiber Beach. These values are interpreted to stem from a positively fractionated shallow seawater chromium composition at ∼1.88Ga, potentially reflecting a continental run-off characterized by positive δ 53Cr, and are in accordance with Cr isotope signatures in worldwide BIFs (including the Gunflint Iron Formation). Our results show the potential of Cr isotope studies on ancient paleosols to untangle the presence of oxidative weathering processes, making this isotope system a viable and important tracer for the reconstruction of surface oxygenation in Earth''s history. [Copyright &y& Elsevier]

Abstract: This study reports petrographic, major and trace element, and oxygen isotopic data for a subaerial weathering profile at Schreiber Beach, Ontario, Canada. The weathering profile developed on Neoarchean (ca. 2.7Ga) pillow basalts and is unconformably overlain by the Paleoproterozoic (ca. 1.88Ga) Gunflint Chert and basal conglomerates. This stratigraphy suggests that the basalts were uplifted and subaerially weathered prior to deposition of the Gunflint Formation. Rocks at Schreiber Beach have been classified as pillow cores, pillow rims, hyaloclastites, weathered red basalts, and weathered brown to green basalts. There are gradual textural, mineralogical, and geochemical transitions from unweathered basalts to intensely weathered hematite-bearing basalts with stratigraphic height. Rocks at Schreiber Beach underwent seafloor hydrothermal alteration and greenschist facies regional metamorphism in the Neoarchean. Following the subaerial weathering, they were buried and re-metamorphosed to greenschist facies in the Paleoproterozoic. Given the preservation of primary igneous textures and trace element patterns in pillow cores, they are used as a protolith to estimate element mobility during seafloor alteration, metamorphism, and subaerial weathering. Chloritized pillow rims and hyaloclastites are characterized by moderate losses or gains of major (Si, Ca, Na, Fe, Ti, K) and trace (LILE, REE, HFSE, and transition metals) elements during Neoarchean seafloor alteration. Alteration of these rocks at relatively low temperatures is indicated by enrichment in 18O (δ18O=+10.4 to +12.7‰) relative to fresh basalt (~+5.5‰). Subaerial weathering resulted in significant textural, mineralogical (e.g., hematite precipitation), and geochemical variations in metamorphosed basalts, causing additional redistribution of most major and trace elements and further enrichment in 18O. The highest δ18O values (+16.1 to +17.1‰) occur at the silica-rich base of the weathering profile. Many elements, including LILE, REE and HFSE, are strongly enriched (>100%) or depleted (up to 96%) relative to their original abundances. Alkaline metasomatism, which is a characteristic of Precambrian paleosols, is also evident at Schreiber Beach, where extreme enrichment in K (280 to 1000%), Rb (390 to 660%), and Cs (1160 to 2560%) occurs in the weathering profile. [Copyright &y& Elsevier]

• The Pipestone Lake anorthosite complex consists of anorthosite, gabbro and leucogabbro. • The Pipestone Lake anorthosite complex formed in a continental back-arc setting. • The anorthosite complex and associated greenstone belt represent an Archean suture zone. • The anorthosite complex and greenstone belt were intruded by Neoarchean arc granitoids. • The Cross Lake-Pipestone Lake terrain originated at a Neoarchean convergent plate margin. The Neoarchean Cross Lake-Pipestone Lake terrain, northwestern Superior Province, consists of the Cross Lake greenstone belt, the Pipestone Lake anorthosite complex, and multiple generations of granitoids. The Cross Lake greenstone belt includes an assemblage of basalt, picrite, gabbro, and sedimentary rocks (Pipestone Lake Group) that formed between 2790 Ma and 2760 Ma, and an assemblage of polymictic conglomerate, sandstone, and siltstone (Cross Lake Group) that were deposited between 2704 Ma and 2691 Ma. The Pipestone Lake anorthosite complex is composed of gabbro, megacrystic anorthosite and leucogabbro, and mafic dykes. The Pipestone Lake Group and the Pipestone Lake anorthosite complex are intruded by ca 2760 Ma and younger granitoids. The trace element systematics of the granitoids suggest that they originated in a volcanic-arc to syn-collisional tectonic setting, whereas the trace element signatures of the basalt, picrite, gabbro, anorthosite and leucogabbro association are consistent with a back-arc setting. Whole-rock initial εNd values, zircon initial εHf values and 207Pb/206Pb zircon ages for the granitoids and the anorthosite complex are consistent with long-term depleted mantle sources and variable degrees of crustal contamination during their interaction with Mesoarchean to Paleoarchean continental crust. Zircon trace element data indicate that the granodiorite and the anorthosite complex formed in a continental crust to oceanic crust transition zone at an arc-related setting. On the basis of the field relationships, and trace element and isotopic data, we suggest that the Pipestone Lake anorthosite complex and the spatially and temporally associated Pipestone Lake Group formed in a continental back-arc setting. During and after the closure of the back-arc basin, both the Pipestone Lake Group and the anorthosite complex were intruded by arc-derived granitoids. Accordingly, we interpret the association of the Pipestone Lake Group and Pipestone Lake anorthosite complex as a Neoarchean suture zone. Following the collision and closure of the back-arc basin, the region was dissected by strike-slip faults resulting in the formation of pull-apart basins and deposition of the Cross Lake Group as Timiskaming-type sedimentary rocks. [ABSTRACT FROM AUTHOR]

Assesses whether the assemblage of the Archean Schreiber-Hemlo greenstone belt in Superior Province, Canada, can be interpreted as an Archean tectonic melange using a combination of detailed structural mapping and geochemistry of sedimentary, volvanic and intrusive rocks. Information on Superior Province; Deformation phases and its relationship to the formation of a tectonic melange in the belt's Schreiber assemblage.

The Neoarchean (2742.8 ± 0.8 Ma) megacrystic anorthosite-bearing Mayville layered intrusion of the Bird River greenstone belt in the Superior Province of Canada is well preserved and well exposed. Given its good preservation, we studied the Mayville Intrusion through a combination of extensive field and petrographic observations, mineral chemistry, and whole-rock major and trace element and Sm-Nd and U-Th-Pb isotopic analyses to unravel its petrogenesis and geodynamic setting, and determine whether its amphibole is of igneous origin and indicative of hydrous magmatism. The Mayville Intrusion intrudes pillow basalts and gabbros of the Mayville Assemblage, which was erupted onto and emplaced into the Mesoarchean phase of the Maskwa Lake tonalite-trondhjemite-granodiorite (TTG) Batholith. Some of the anorthosites and leucogabbros of the Mayville Intrusion have undergone extensive shearing and hydrothermal alteration and had their Sm-Nd and U-Th-Pb isotope systems reset. Despite metamorphism and deformation affecting the Mayville Intrusion, it has largely retained its primary igneous minerals and textures, and trace element and Sm-Nd and Pb isotope compositions. Abundant interstitial magmatic amphibole oikocrysts in the anorthosites and leucogabbros exhibit whole-grain optical continuity and well-preserved igneous grain boundaries with magmatic plagioclase, indicating that the Mayville Intrusion crystallized from a hydrous magma. The Mayville Intrusion has variably positive ε Nd (2743 Ma) values (+0.7 to +3.9) that are indicative of its derivation from a depleted mantle source. The anorthosites and leucogabbros of the Mayville Intrusion crystallized from a hydrous Ca- and Al-rich tholeiitic parental magma that fractionated from a hydrous primitive arc tholeiitic parental magma. These lithologies were derived by partial melting of a depleted, shallow hydrous sub-arc mantle wedge source. The parental magmas to the gabbros, leucogabbros and anorthosites of the Anorthosite to Leucogabbro Zone subsequently experienced minor contamination by Mesoarchean TTGs (Maskwa Lake TTG Batholith). The field relationships and trace element geochemistry of the Mayville Intrusion and its proximity to the continental arc Maskwa Lake TTG Batholith indicate that it formed in a Chilean-style continental back-arc geodynamic setting. The findings of this study demonstrate that hydrous arc magmatism and modern-style plate tectonic processes have been operating since at least the Late Archean. [ABSTRACT FROM AUTHOR]

The article announces that Tiffani A. Fraser and Matt P. Hutchison won the 2017 Best Paper Award from the Canadian Science Publishing and the Geological Association of Canada.

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]