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Archean lower crustal granulites are exposed as fault-bounded blocks in the Varpaisjarvi area, central Finland. The prevailing rock types in these blocks are igneous enderbites, tonalitic-trondhjemitic-granodioritic migmatites and mafic granulites. Although metamorphosed at moderate pressures (8-10 kbar), the Varpaisjarvi granulites are not, in a geochemical sense, good representatives of depleted...
Archean greenstone belts from the northern Superior Province, ranging in age from 3.0 to 2.9 Ga, comprise an association of komatiite-tholeiite sequences, some of which were erupted onto continental crust, intercalated with felsic volcanic rocks. Komatiites from the Uchi subprovince are characterised by variable LREE enrichment and negative Nb anomalies consistent with contamination of Munro-type...
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...
The metavolcanic Lumby Lake belt comprises mafic tholeiites intercalated with thin felsic pyroclastic units. Al-undepleted komatiites are present towards the top of the stratigraphy. Identification of Al-depleted pyroclastic komatiites associated with chemical and siliciclastic sedimentary rocks indicates the upper portion of the 3 Ga Lumby Lake stratigraphy is directly comparable to a stratigraphic...
To test existing lateral accretion models for the Archean Kaapvaal Craton in South Africa, metapelites and leucocratic granitoids from two different provinces, the granulite facies Southern Marginal Zone of the Limpopo Belt and the northern part of the adjacent greenschist to amphibolite facies Kaapvaal Craton, have been studied. Both provinces resemble each other in terms of isotope and trace element...
Stepwise HF-dissolution experiments on five Archean zircon populations reveal very systematic patterns of Pb discrimination, releasing at one point Pb with artificially too-old 207 Pb/ 206 Pb ages. The experiments involved a first HF dissolution step for 1 h at room temperature and evaporation on a hot-plate that produces Pb with young 207 Pb/...
Geochemical data from the ~2.9-Ga Northern Pickle assemblage of the Pickle Lake greenstone belt in the northern Superior Province distinguish three distinct volcanic suites. Suite I, from the north of the assemblage, comprises tholeiitic basalts with unfractionated rare earth elements (REE). Suite II basalts are light REE (LREE)-enriched with negative mantle-normalized Nb anomalies relative to Th...
A suite of 14 diamond-bearing and 3 diamond-free eclogite xenoliths from the Newlands kimberlite, South Africa, have been studied using the Re-Os isotopic system to provide constraints on the age and possible protoliths of eclogites and diamonds. Re concentrations in diamond-bearing eclogites are variable (0.03-1.34 ppb), while Os concentrations show a much more limited range (0.26-0.59 ppb). The...
Lower crustal xenoliths recovered from Eocene to Cambrian kimberlites in the central and southern Slave craton are dominated by mafic granulites (garnet, clinopyroxene, plagioclase+/-orthopyroxene), with subordinate metatonalite and peraluminous felsic granulites. Geothermobarometry indicates metamorphic conditions of 650-800 o C at pressures of 0.9-1.1 GPa. The metamorphic conditions are...
The Earth's continents are cored by Archean cratons underlain by seismically fast mantle roots descending to depths of 200 + km that appear to be both more refractory and colder than the surrounding asthenospheric mantle. Low-temperature mantle xenoliths from kimberlite pipes indicate that the shallow parts of these cratonic mantle roots are dominated by refractory harzburgites that are very...
The global Archean record preserves ca. 35 large cratonic fragments and a less well defined number of smaller slivers. Most Archean cratons display rifted margins of Proterozoic age and therefore are mere fragments of supercratons, which are defined herein as large ancestral landmasses of Archean age with a stabilized core that on break-up spawned several independently drifting cratons. The tectonic...
The late tectonic evolution of the Slave craton involves extensive magmatism, deformation, and high temperature-low pressure (HT-LP) metamorphism. We argue that the nature of these tectonic events is difficult to reconcile with early, pre-2.7 Ga development and preservation of a thick tectosphere, and suggest that crust-mantle coupling and stabilization occurred only late in the orogenic development...
The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al 2 O 3 /TiO 2 and CaO/Al 2 O 3 ratios of 15-18 and 1.1-1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb) N ratios. In contrast, the Al-depleted...
Three Paleoproterozoic A-type rapakivi granite suites (Jamon, Serra dos Carajás, and Velho Guilherme) are found in the Carajás metallogenic province, eastern Amazonian craton. Liquidus temperatures in the 900–870 °C range characterize the Jamon suite, those for Serra dos Carajás and Velho Guilherme are somewhat lower. Pressures of emplacement decrease from Jamon (3.2±0.7 kbar) through Serra dos Carajás...
Early Paleoproterozoic extension in the Archean craton of the Fennoscandian shield led to the emplacement of several 2.44 Ga layered gabbroic intrusive complexes in northern Finland and adjacent Russia. Closely associated with them are felsic rocks of similar age: (1) the Sirniö Group volcanic rocks on top of the Koillismaa layered complex; (2) a quartz alkali feldspar syenite at Kynsijärvi near the...
The world's oldest diamond deposits occur in ∼2.67 Ga dikes and heterolithic breccias emplaced into greenstone belts of the Wawa and Abitibi Subprovinces, southern Superior Province, Canada. Thousands of white to yellow microdiamonds and macrodiamonds to 5 mm in width have been recovered by non-contaminating fusion techniques. The host rocks exhibit petrographic and compositional features that are...
The Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt contains well-preserved primary magmatic structures, such as pillow lavas, volcanic breccias, and clinopyroxene cumulate layers (picrites), despite the isoclinal folding and amphibolite facies metamorphism. The belt also includes variably deformed gabbroic to dioritic dykes and sills, actinolite schists, and serpentinites. The Ivisaartoq rocks...
The tectonic style and viability of modern plate tectonics in the early Earth is still debated. Field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the Archean continue to accumulate. Here, we present the first numerical modeling results that address for a hotter Earth the viability of subduction, one of the main requirements...
The longevity of the continental lithosphere mantle is explained by its unusual composition. This part of the mantle is made up mainly of forsterite-rich olivine (Fo92–94), with or without orthopyroxene, and it is essentially anhydrous. The former characteristic makes it buoyant, the latter makes it viscous, and the combination of these features that allow it to remain isolated from the convecting...
Depth distributions of seismic velocities and their directional dependence (anisotropy) in the crust and mantle beneath cratons yield essential constraints on processes of their formation and evolution. Despite recent progress in mapping the lateral extent of cratonic roots around the globe, profiles of seismic velocities within them remain uncertain. In this study we employ a novel combination of...
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