Supracrustal Research Articles

Stratigraphic update, paleotectonic, paleogeographic, and depositional environments of the Crixás Greenstone Belt, Central Brazil

The Crixás Greenstone Belt is a supracrustal rock sequence of the allochthonous Archean-Paleoproterozoic Crixás-Goiás Terrane of the Neoproterozoic Tocantins Province of Central Brazil. The belt occurs between Mesoarchean (2.9-2.8 Ga) primitive orthogneiss complexes intruded by Siderian intracratonic mafic dikes and stocks. The rock succession of the greenstone belt consists of metakomatiites followed by metabasalts, both of greenschist to low amphibolite facies, underlying metasedimentary rocks of low greenschist facies. The greenstone belt is divided into the southern, central, and northwestern sectors, with different rock associations. The northwestern sector tectonically underlies a small exotic block known as the Mina Inglesa Sequence. The sequence consists of a low greenschist facies volcano-sedimentary package similar to a greenstone belt, but of unknown origin. The latest stratigraphic nomenclature for the belt labels its rock package as the Crixás Group. From base to top, the Group comprises the Córrego Alagadinho, Rio Vermelho, and Ribeirão das Antas Formations, made up of metakomatiites, metabasalts, and metasedimentary rock, respectively. The Mina Inglesa Sequence is an independent unit of the Crixás Group, in spite of the similar rock assemblage of both units. Protoliths of the Ribeirão das Antas Formation comprise shallow water black shales with dolomite lenses, laterally wedge-shaped with impure turbiditic sandstones (wakes). Published U/Pb and Sm/Nd whole-rock isotopic data indicate that the metakomatiites and metabasalts erupted during the Mesoarchean (3.0 Ga). Detrital zircon grains from the sedimentary protoliths suggest a mixed Mesoarchean and Rhyacian source-area (3.2 and 2.2 Ga). The U/Pb age from hydrothermal zircon grains of a gold-bearing quartz vein indicates that the sedimentation ended just before 2.1 Ga. Field, drilling, and the isotopic data led the authors to review the stratigraphic content of the Crixás Group by subdividing it into an Upper and Lower Subgroups. The Lower Subgroup comprises the Mesoarchean volcanic section subdivided, from base to top, into the currently adopted Córrego Alagadinho and Rio Vermelho Formations. The Upper Subgroup rests ubnconformable on the Lower Subgroup, and comprisess the Rhyacian metasedimentary section. The Upper Subgroup contains two rock units deposited in different environments. The currently used Ribeirão das Antas Formation is the lower unit, which is redefined as composed of carbonaceous phyllites with dolomite lenses. The upper unit comprises the new proposed Córrego Geral Formation, composed of metagraywackes. The wedge-shaped contact between both sedimentary units indicates that they were coevally deposited. The Paleoproterozoic sedimentary basin consisted of a small depression formed during the sagduction of the volcanic pile, triggered by the diapiric rise of the adjacent orthogneiss protoliths. The sediment deposition started with black shales in response to a slow rise of the seawater and paleoclimatic conditions. An increasing input of siliciclastic turbidites occurred in response to the uplift of a Rhyacian/Mesoarchean source-area that provided a mixed clastic load. Anomalous positive δ13C data from the dolomite lenses indicate that they deposited during the 2.2-2.1 Ga Lomagundi Event, which is compatible with the dominant Rhyacian zircon grains of the graywackes. The Crixás-Goiás Terrane lacks geologic evidence for the existence of an internal Rhyacian crustal block to justify the youngest detrital zircon grains of the graywackes. These observations render uncertainty about the geographic position of the rising crust. Paleoproterozoic crustal shortening of the belt resulted in stacked thin-skinned thrust-faults that host gold orebodies. An eastward age decrease of the crustal shortening from Crixás to an easterly belt leads the authors to propose that the clastic load source could occur to the West of the Terrane. The Mina Inglesa Sequence is an allochthonous block with lithologic and isotopic features that distinguish it from the greenstone belt. The authors also describe that there is no isotopic evidence for Meso- to Neoproterozoic source-areas, neither for the Córrego Geral Formation nor for the Mina Inglesa Sequence. These findings raise a suggestion that the granite-greenstone belt association of the Terrane may represent a craton border that remained stable before its amalgamation to the province during the Neoproterozoic.

Early continental crust generated by reworking of basalts variably silicified by seawater

The Archaean continental crust comprises two major groups of silicon-rich granitoids: the tonalite–trondhjemite–granodiorite and granite–monzonite–syenite suites, which differ in their sodium-to-potassium ratios. How these felsic granitoids evolved from their mafic precursors remains elusive and the subject of great debate. Here, we present silicon isotopic constraints on the formation of representative trondhjemitic and granitic plutons from the Kaapvaal craton that range in age from 3.51–2.69 billion years ago. We identified very consistent silicon isotopic signatures, all uniformly 0.1–0.2‰ heavier than rocks of the modern continental crust. This unusual composition is explained by the melting of a mafic source that included significant proportions (15–35 wt%) of silicified basalts, which were common supracrustal rocks before 3 billion years ago. Before the melting event that formed the granitoid magmas at depth, portions of the mafic source rocks were enriched in silica by interaction with silica-saturated seawater. The addition of silica depresses the stability of amphibole at similar water activity, allowing trondhjemitic and granitic melt production at lower temperatures from protoliths with contrasting silica contents: 52–57 and ≥60 wt%, respectively. This explains why granitoids were able to form very early in Earth history but did not emerge in significant amounts on other rocky planets. Granitic continental crust in the Archaean formed from a basaltic source that was enriched in silica due to interaction with the early oceans before melting, according to silicon isotope analyses on rocks from the Kaapvaal craton.

Compositions of Carbonate Rocks and Carbon Isotope Ratios of Graphite in Nulliak Supracrustal Rocks, Labrador: The Oldest Evidence for Life and Seawater Composition

Compositions of Carbonate Rocks and Carbon Isotope Ratios of Graphite in Nulliak Supracrustal Rocks, Labrador: The Oldest Evidence for Life and Seawater Composition

Neoarchaean subduction tectonics in Western Shandong Province, China: Evidence from geochemistry and zircon U–Pb–Hf isotopes of metabasalts

Neoarchaean supracrustal rocks, which were discovered in the Longhuzhai and Menglianggu areas of the Western Shandong Province (WSP) in the Eastern Block of the North China Craton (NCC), are fine‐ to medium‐grained amphibolites that commonly underwent greenschist‐ to amphibolite‐facies metamorphism. LA‐ICP‐MS zircon U–Pb analyses revealed that these metabasalts erupted at 2,527 ± 5 to 2,514 ± 4 Ma. These supracrustal rocks are tholeiitic basalts in chemical composition and exhibit relatively high SiO2 (49.30–53.34 wt%) and MgO (5.24–11.82 wt%) contents but low TiO2 (0.41–0.75 wt%) contents. They display flat chondrite‐normalized rare earth element (REE) patterns ((La/Yb)N = 1.21–2.60) and negative Nb, Ta, and Ti anomalies ((Nb/La)PM = 0.16–0.54), which are similar to those of typical island arc tholeiites (IATs) in subduction‐related settings. These findings, combined with the positive zircon εHf(t) values (+2.51 to +8.90) and εNd(t) values (+0.19 to +3.65), suggest that these metabasalts were derived from the partial melting of a depleted mantle source that was metasomatized by slab‐derived fluids at shallow levels in the upper mantle. Accordingly, the development of arc‐related tholeiitic volcanic rocks within the interior of the WSP imply that the Neoarchaean (2.6–2.5 Ga) subduction‐related slab–wedge interaction mechanism was widespread in the WSP and played a crucial role in the formation of the continental crust and its growth.

Timing of late Neoarchean to late Paleoproterozoic events in the North China Craton: SHRIMP U–Pb dating and LA-ICP-MS Hf isotope analysis of zircons from magmatic and metamorphic rocks in the Santunying area, eastern Hebei

Santunying is an important area for revealing nature of the late Neoarchean tectono-magmato-thermal events in the eastern Hebei part of the North China Craton. It is mainly composed of meta-intrusive rocks. Supracrustal rocks sporadically occur in the meta-intrusive rocks. The meta-intrusive rocks are subdivided into the Santunying tonalitic gneiss, Qiuhuayu tonalitic-trondhjemitic gneiss, Xiaoguanzhuang dioritic gneiss and Qingyangshu meta-gabbro. Respectively, SHRIMP U–Pb zircon dating on fourteen samples yielded weighted mean 207Pb/206Pb ages of 2525–2537, 2532–2546, 2530–2544 and ∼2531 Ma for magmatic zircons from them. Dioritic gneiss of the Xiaoguanzhuang gneiss contain abundant 2544–3487 Ma xenocrystic zircons. SHRIMP U–Pb dating on a garnet-biotite gneiss sample yielded a weighted mean 207Pb/206Pb age of 2537 Ma for detrital zircons. All rocks underwent strong metamorphism, deformation and anatexis, resulting in formation of leucosomes and residues, with some leucosomes concentrating to form large veins. They record a strong late Neoarchean event by metamorphic zircon ages of 2489–2519 Ma. Some rocks also record metamorphic zircon ages of 1772–1843 Ma. Magmatic zircons from the magmatic rocks show large variations in εHf(t) values ranging from −1.7 to +8.7. Combined with early studies, conclusions are: 1) Intrusive rocks with the involvement of mantle-derived materials have a narrow range of magmatic zircon ages from 2525 to 2546 Ma, and supracrustal rocks were formed during the same period. 2) Ancient crustal remnants (>2600 Ma) are present, consistent with the late Neoarchean arc magmatism involving older continental crust, similar to Phanerozoic Andean margins. 3) The Archean basement underwent a strong tectonothermal event at the end of the Neoarchean, with the metamorphic zircon ages being 10–30 million years younger than the timing of magmatism, a common feature of the North China Craton. 4) A late Paleoproterozoic tectonothermal event widely occurred in the western part of eastern Hebei, which is linked with regional ductile deformation.

Geochronology of the southern margin of the Bighorn Batholith, Wyoming

The Bighorn Mountains in north-central Wyoming reveal one of the largest exposures of 2800 Ma to 3000 Ma rocks in Laurentia. The northern part of the crystalline core is composed of the composite Bighorn batholith, whereas the central and southern areas of the range expose older gneiss complexes as well as minor supracrustal rocks. We provide new high-resolution LA-ICPMS U-Pb data on zircons sampled from eleven samples of tonalite, granodiorite, mylonite, and migmatite from the southern margin of the Bighorn batholith in the headwaters of the north fork of Paint Rock Creek. These rocks range from strongly foliated to massive and are difficult to subdivide into mappable units in the field because of their lithologic and structural similarities. Several cross-cutting mylonite zones (<10-meter-wide) that trend ~N70°E-N80E and dip steeply are present in the study area. Three distinct age populations are evident: ~2930-2940 Ma, 2905-2915 Ma, and 2880-2890 Ma. Several samples contain xenocrystic zircons >3000 Ma, ranging to 3500 Ma, which indicates assimilation of older crust. Each of the three age populations reported here are older than the previously reported age of ~2850 Ma age for the northern Bighorn Batholith but within the 2890 Ma, 2940 Ma, and 2950 Ma age groupings previously reported for the southern gneiss terrane. Three conclusions can be drawn from these data. First, the Bighorn batholith, at least along the southern margin, contains phases at least 80 million years older than the northern phase of the body, and emplacement was protracted and occurred over a ~100 Ma period. Second, episodes of both intrusion and shearing took place in this area as the various phases of the Bighorn batholith were emplaced. Finally, the existence of inherited zircons within the Bighorn batholith in the age range of ~3.0 Ga to 3.5 Ga indicates that the Bighorn batholith intruded through older crust. This older crust is perhaps a northern extension of Sacawee block in the northern Granite Mountains of central Wyoming, which may underlie the Bighorn Mountains.

A back-arc origin for the Neoarchean megacrystic anorthosite-bearing Bird River Sill and the associated greenstone belt, Bird River subprovince, Western Superior Province, Manitoba, Canada

This study presents extensive new field, petrographic, and whole-rock major and trace element and Sm–Nd and Pb isotope data for the Neoarchean megacrystic anorthosite-bearing Bird River Sill, spatially associated Neoarchean supracrustal rocks of the Bird River greenstone belt, and the Mesoarchean Maskwa Lake TTG Batholith I, Manitoba, Canada. Field observations indicate that the 2743 Ma Bird River Sill was emplaced into the Northern Lamprey Falls Formation in an oceanic setting and subsequently intruded by the ca. 2725 Ma Maskwa Lake Batholith II. The Northern Lamprey Falls Formation is in fault contact with the overlying Peterson Creek Formation, which in turn is unconformably overlain by the Bird River Formation. The peridotites of the Bird River Sill were subjected to intense alteration and ductile shearing, resulting in the redistribution of many major and trace elements and resetting of their Sm–Nd and U–Th–Pb isotopic systems. The gabbro, leucogabbro, and anorthosite units of the sill largely retain their primary igneous textures and minerals, REE and HFSE compositions, and Sm–Nd and Pb isotope signatures. These geochemical and petrographic data indicate that these units of the Bird River Sill crystallised from a hydrous, Ca- and Al-rich tholeiitic magma that fractionated from a hydrous primitive arc tholeiitic parental magma. The major and trace element geochemistry and initial eNd values of the Bird River Sill (+ 0.53 to + 4.72) and the Northern Lamprey Falls Formation (+ 1.40 to + 2.66) of the Bird River greenstone belt crystallised from parental magma batches that were derived by partial melting of variably-depleted mantle sources. High-magnesian andesites from the Bird River Formation formed by partial melting of subducting oceanic crust and/or slab sediments and subsequent interaction with sub-arc mantle peridotite, whereas the Peterson Creek Formation dacites and Maskwa Lake TTG Batholith I tonalites formed by partial melting of juvenile lower arc crust. The major and trace element geochemistry of the Bird River Sill and greenstone belt and the Maskwa Lake TTG Batholith I, and geological relationships and lithological associations point to a transition from continental arc to continental back-arc magmatism along the southern active margin of the North Caribou terrane at ca. 2743 Ma, resulting in the formation of the Maskwa Lake microcontinent that occurred prior to subsequent continental arc magmatism along the southern margin of this microcontinent. Combined field observations and geochemical data indicate that the Bird River Sill and Bird River greenstone belt are a dismembered Archean subduction-related ophiolite that marks a suture zone between the Winnipeg River subprovince and the Maskwa Lake microcontinent.

Bulk rock and zircon geochemistry of granitoids from the Chotanagpur Granite Gneissic Complex (CGGC): implications for the late Paleoproterozoic continental arc magmatism in the East Indian Shield

The north-eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) exposes a vast expanse of felsic orthogneisses (~ 1450 Ma magmatic crystallization age) containing a variety of metamorphosed supracrustal rocks and augen gneiss. However, origin and timing of emplacement of the protolith of the augen gneiss are not well constrained. Owing to lack of detailed study and paucity of exposures, the antiquity of the basements of the CGGC also remains poorly understood. To address this issue, we present bulk rock geochemical data and the U–Pb and Lu–Hf isotope composition of zircon from augen gneisses from two adjoining areas within the CGGC. Our obtained data suggest that the augen gneiss have been derived from hybrid source involving older crustal rocks and metasomatised mantle. The bulk rock and zircon geochemistry also suggest that the protolith of the augen gneiss was emplaced in an early Paleoproterozoic basement in a continental arc setting. U–Pb ages of the magmatic zircon grains constrain the age of intrusion at ca. 1710 Ma and ca. 1635 Ma. The timing of this late Paleoproterozoic felsic magmatism overlaps with the timing of the regional high-to-ultrahigh-temperature granulite facies metamorphism in the CGGC.

Geochemistry and tectonic setting of the supracrustal rocks from the central part of the Bundelkhand craton, India

Supracrustal rocks (mafics and ultramafics) occurs along with banded iron formation, and felsic volcanics around Babina, Dhaura, and Mauranipur linear east-west trends in central part of the Bundelkhand craton represent Archean crust. The mafic and ultramafic rocks geochemically classified into Komatiite and Basaltic Komatiite and have high-Fe Tholeiitic in composition which may relate with the primitive mantle. The major and trace element geochemistry of mafic and ultramafic rocks correspond to hydrated mantle with wedge tectonic sources and ocean ridge geological characteristics.

Insights into orogenic processes from drab schists and minor intrusions: Southern São Francisco Craton, Brazil

Minor altered intrusions and drab retrogressed schists can easily be overlooked in geological studies but this contribution explores these rocks within the Archaean and Palaeoproterozoic southern São Francisco Craton (SSFC), Brazil using geological relationships and accessory mineral in situ analyses in the context of cratonic assembly. Three magmatic pulses are documented: i) Archaean and ii) Palaeoproterozoic felsic intrusions, both hosted by Archaean protoliths, and iii) Palaeoproterozoic felsic intrusions in Palaeoproterozoic supracrustal sequences. Archaean felsic intrusions confirm the Palaeoarchaean age of the mafic/ultramafic sequence of the Rio das Velhas Greenstone Belt and Rhyacian intrusions mark the collisional stage of the Mineiro Belt with the SFC at c. 2130 Ma. Greenstone belt schists show a wide distribution of rounded ‘soccer ball’ Archaean detrital and metamorphic zircon grains ranging in age from 3200 to 2750 Ma with an interpreted overprinting high-grade metamorphic event at c. 2700 to 2680 Ma. Most high-grade metamorphic rims have Th/U > 0.1, negative εHf(t) values and REE pattern consistent with eclogite/granulite metamorphic facies, reinforcing the hypothesis of a dehydrated-refractory crust formed during the stabilization of the SSFC, even though no such protoliths are preserved. This event links crustal thickening and partial melting of Archaean lower crust. Archaean rutile crystals from the greenstone belt schist were reset during the Palaeoproterozoic event but still preserve the early Archaean high-grade metamorphic signature. The presence of unstable ilmenite replaced by rutile in the schist, associated to felsic intrusions with the same age at c. 2130 Ma suggest high pressure, low temperature prograde metamorphism during the collisional stage of the Palaeoproterozoic orogen. Elongate and prismatic zircon grains from the Rhyacian intrusions have low εHf(t) signature and crystallised from partial melting of sedimentary protoliths. Accretionary events produced thicker and more differentiated crust by the end of Rhyacian time. Easily overlooked rocks in this study, when studied, have revealed a rich multi-event history of cratonic evolution.

Mesoarchaean to Neoproterozoic (3.2–0.8 Ga) crustal growth and reworking in the Aravalli Craton, northwestern India: Insights from the Pur-Banera supracrustal belt

The Banded Gneissic Complex (BGC) of the Aravalli Craton in Rajasthan, northwestern India, forms the Archaean basement to the major, extensively studied, Proterozoic-age metasedimentary sequences of the Aravalli and Delhi Supergroups. However, the poorly documented deformation history of the basement and other supracrustal sequences, and insufficient geochronological work, limit a holistic understanding of the evolution of the Aravalli Craton. Here we present structural analysis and zircon and monazite geochronology of the relatively poorly studied Pur-Banera supracrustal belt in the Bhilwara region of south-central Rajasthan. The belt comprises paragneisses, carbonates, calc-gneisses, banded iron formations, garnetiferous schists, and amphibolites, deposited over Aravalli Supergroup rocks and locally also granite. A single detrital zircon core from the Pur-Banera paragneisses yields a 207Pb/206Pb age of 3159 ± 8 Ma (1σ), indicating a Mesoarchaean protolith. Magmatic zircon grains from the granite, here termed the Malola Granite, provide a 207Pb/206Pb crystallisation age of 2538 ± 11 Ma (95% confidence). The Pur-Banera supracrustals have experienced three phases of deformation, D1–D3, each of which produced a tectonic fabric. Zircon and monazite geochronology provides complementary ages. The detrital zircons, derived from both magmatic and metamorphic protoliths, provide ages of 3.2–1.7 Ga. Monazite ages of 1.3–0.8 Ga record multiple post-depositional tectonothermal events. Textural and compositional evidence and U–Th–total Pb monazite ages indicate that D1 deformation occurred at ca. 1.30 Ga as a result of NW–SE-directed shortening, forming upright folds on the primary foliation that resulted in a penetrative fabric in the gneisses and schists. Subsequent NNE–SSW-directed deformation (D2) between 1.13 and 1.05 Ga produced WNW–ESE-trending vertical folds in the gneisses and crenulations in the schists. Asymmetrical folds developed due to N–S dextral shearing at 0.85–0.80 Ga constitute the last phase of deformation (D3). The Pur-Banera supracrustal rocks probably represent a rift-fill sequence, of Mesoproterozoic age (thus temporally equivalent to the Delhi Supergroup), deposited on an Archaean and Palaeoproterozoic basement, and repeatedly reworked during 1.30–0.80 Ga.

A holistic model for the origin of orogenic gold deposits and its implications for exploration

The term orogenic gold deposits has been widely accepted, but there has been continuing debate on their genesis. Early syn-sedimentary or syn-volcanic models and hydrothermal meteoric-fluid models are now invalid. Magmatic-hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships. The most plausible models involve metamorphic fluids, but the source of these fluids is equivocal. Intra-basin sources within deeper segments of the hosting supracrustal successions, the underlying continental crust, subducted oceanic lithosphere with its overlying sediment wedge, and metasomatized lithosphere are all potential sources. Several features of Precambrian orogenic gold deposits are inconsistent with derivation from a continental metamorphic-fluid source. These include the presence of hypozonal deposits in amphibolite-facies domains, their anomalous multiple sulfur isotopic compositions, and problems of derivation of gold-related elements from devolatilization of dominant basalts in the sequences. The Phanerozoic deposits are largely described as hosted in greenschist-facies domains, consistent with supracrustal devolatilization models. A notable exception is the Jiaodong gold deposits of China, where ca. 120-Ma gold deposits are hosted in Precambrian crust that was metamorphosed over 2000 million years prior to gold mineralization. Other deposits in China are comparable to those in the Massif Central and elsewhere in France, in that they are hosted in amphibolite-facies domains or clearly post-date regional metamorphic events imposed on hosting supracrustal sequences. If all orogenic gold deposits have a common genesis, the only realistic source of fluid and gold is from devolatilization of a subducted oceanic slab with its overlying gold-bearing sulfide-rich sedimentary package, or the associated metasomatized mantle wedge, with CO2 released during decarbonation and S- and ore-related elements released from transformation of pyrite to pyrrhotite at about 500 °C. Although this model satisfies all geological, geochronological, isotopic, and geochemical constraints, and is consistent with limited computer-based modeling of fluid release from subduction zones, the precise mechanisms of fluid flux are model-driven and remain uncertain. From an exploration viewpoint, the model re-emphasizes the ubiquitous occurrence of orogenic gold deposits in subduction-related orogenic belts and importance of continental-scale lithosphere-tapping fault and shear zones to focus large volumes of auriferous fluid. It confirms the importance of the consistent spacing between world-class deposits, broadly equivalent to the depth of the Moho, as derived from empirical observations.

Petrologia, geoquímica e geocronologia do Granodiorito Presidente Kennedy: contextualização na evolução do Cinturão Araguaia

Diversos corpos graníticos têm sido identificados no extremo leste do Cinturão Araguaia (CA) em seu domínio de mais alto grau metamórfico (Grupo Estrondo). Tais granitos têm sido interpretados como de posicionamento sin a tardi-cinemático em relação à tectônica principal e metamorfismo regional da evolução do CA, no final do Neoproterozóico. Neste contexto o Granodiorito Presidente Kennedy (GPK) localizado a 6 km a noroeste da cidade de Presidente Kennedy (TO) compõe a forma de um stock elíptico, que abrange uma área aflorante cujo diâmetro maior é de aproximadamente 10 km, e o menor de 5 km, encaixado em micaxistos do Grupo Estrondo e parcialmente coberto por rochas sedimentares da Formação Pimenteiras. O estudo petrográfico identificou rochas hololeucocráticas (M<6) granodioríticas e monzograníticas equigranulares, de cor cinza levemente rosada, e com textura granular hipidiomórfica e alotriomórfica. Dados litoquímicos indicam que o GPK tem caráter levemente peraluminoso, com elevados valores SiO2, Al2O3 e álcalis e baixos valores de MgO, Fe2O3t e TiO2. O estudo geocronológico de evaporação de Pb em zircão forneceu idade 539 ± 5 Ma, interpretada como idade mínima de cristalização do GPK. Os estudos petrográficos, geoquímicos e geocronológicos do GPK, indicam semelhanças com os demais granitos ocorrentes no CA. Os estudos isotópicos pelo método Sm-Nd definiram idades modelo (TDM) de 2,13 Ga e 2,17 Ga para o GPK e valores εNd negativos indicando que essa rocha tem proveniência de fonte crustal, gerada da fusão de crosta antiga Paleoproterozóica. Os estudos comparativos indicam semelhanças entre este e os demais granitos do CA, possivelmente cogenéticos relacionado à fase principal do metamorfismo do CA, no limete Neoproterozóico-Paleozóico.

Lithostratigraphy of the Mesoproterozoic Windpoort Granite (Spektakel Suite), western Namaqualand, South Africa

AbstractThe Windpoort Granite is a porphyritic, leucocratic granite belonging to the Spektakel Suite, a group of late- to post-tectonic granites intruded into the orthogneisses and supracrustal metamorphic rocks in western Namaqualand. Like other granites of this type, it is devoid of penetrative tectonic foliation, at most displaying a magmatic foliation parallel to the boundaries of the intrusion. Its main characteristic setting it apart from other Spektakel Suite granites in western Namaqualand is its tightly packed arrangement of small stubby alkali feldspar phenocrysts. Its U-Pb LA-ICPMS age of 1087 ± 11 Ma agrees with the age of other Spektakel Suite granites showing similar field relationships. It classifies geochemically as a highly potassic monzogranite with more evolved compositions than all other plutons of the Spektakel Suite.

Update to the South African Stratigraphic Code and Guidelines for Standardised Lithostratigraphic/Lithodemic Descriptions

Summary The South African Committee for Stratigraphy (SACS) is the official body in South Africa which regulates the nomenclature and definition of lithostratigraphic and lithodemic rock units, unconsolidated deposits and soils. The robust framework set in the existing stratigraphic code and guide for publication by the Council for Geoscience (CGS) have for many years provided leadership in this field, but there are a number of aspects in need of reconsideration. One example is the non-compliance of our nomenclature for high-grade supracrustal rocks with international (and South African) stratigraphic codes and another is manuscript format changes necessitated by publishing SACS descriptions in the SAJG. Any stratigraphic code is a living document that should be able to accommodate changing views and circumstances. This review provides a consensus opinion from experts on how to deal with changing needs.

Paleoproterozoic metamorphism in the northern Fennoscandian Shield: age constraints revealed by monazite

ABSTRACTWe have identified two contrasting styles of Paleoproterozoic metamorphism in the northern part of the Fennoscandian Shield. The Karelia and Lapland-Kola Provinces, comprising Archean and overlying Paleoproterozoic supracrustal rocks, show a typical medium pressure Barrovian-style metamorphism with commonly found kyanite-bearing mineral assemblages and ITD (isothermal decompression) PT paths. In the juxtaposed Svecofennia Province metamorphism represents low pressure-high temperature Buchan style with garnet-cordierite migmatites and intercalated andalusite-cordierite and andalusite-staurolite schists and sillimanite-muscovite gneisses. The retrograde PT paths show only a moderate uplift during cooling.U-Pb age determinations on monazite were made using the LA-ICP-MS from more than 80 samples from metasedimentary rocks. The sampling covered most parts of the Paleoproterozoic bedrock in Finland. The analyses reveal three peaks at c. 1.91 Ga, 1.86–1.88 Ga and at 1.79–1.81 Ga. The oldest, c. 1.91 Ga monazites are mostly found in the Lapland-Kola Province which is located in the northernmost Finland. In the Karelia Province where the Paleoproterozoic is underlain by Archean bedrock monazite yielded ages of 1.76‒1.81 Ga with only a few older exceptions in samples showing a spread of 207Pb/206Pb ages from c. 1.92–1.81 Ga. The Karelia Province underwent tectonic thickening, where monazite ages of around 1.80 Ga mostly represent exhumation near the temperature maximum.In the Svecofennia Province monazite ages vary from c. 1.89 to 1.78 Ga. In the Western Finland Subprovince the monazite ages in high-grade migmatites are mostly 1.86‒1.88 Ga but within the older migmatite areas there are lower grade zones where monazite yields ages of c. 1.80 Ga. Some samples also show a spread of 207Pb/206Pb ages from 1.89‒1.86 Ga to c. 1.78 Ga. In the Southern Finland Subprovince most ages are either 1.80‒1.78 Ga, especially in the andalusite grade schists, or the sample shows a spread of 207Pb/206Pb ages from c. 1.88 to 1.78 Ga. Only in the eastern part of the Southern Finland Subprovince there are rocks which yield merely 1.86‒1.89 Ga ages. Low pressure-high temperature metamorphism and lack of high or medium P/T rocks in the Svecofennia Province refers rather to accretionary than collisional processes.

Metamorphic P–T paths for the Archean Caozhuang supracrustal sequence, eastern Hebei Province, North China Craton: Implications for a sagduction regime

Archean supracrustal rocks of amphibolite-facies occur as enclaves within granitoids gneiss domes and belts between domes, representing collision or sagduction regimes. In order to distinguish between tectonic regimes using metamorphic patterns, systematic data on metamorphic evolution and zircon age dating are presented for the Caozhuang supracrustal sequence of eastern Hebei Province, China, which occurs as enclaves in gneisses. A garnet biotite gneiss records a P–T path involving pre-peak isobaric heating to peak conditions at 780–800 °C and 10–11 kbar (medium-P/T type), followed by decompression to 5–6 kbar. Two Mg/Al-rich schists from the same locality show low P/T conditions of 6–7 kbar/690–750 °C and 4–7 kbar/750–780 °C, similar to that of the decompression stage for the biotite gneiss. Zircon age dating suggests that the amphibolite-facies metamorphism occurred at ∼2.49 Ga, coeval with TTG magmatic activities. Combined with a summary of the metamorphism from Kaapvaal and Pilbara Cratons, we concluded that Archean amphibolite-facies supracrustal rocks occurring as enclaves and belts show different metamorphic patterns. The former exhibits the metamorphic patterns the same as those recovered for the Caozhuang rocks, characteristic of pre-peak isobaric heating to 600–800 °C at 10–12 kbar followed by isothermal decompression with the overprinting of medium- and/or low-P/T assemblages in different enclaves. The latter shows medium- or high-P/T peak conditions mostly lower than 600–650 °C followed by decompression, lacking the pre-peak heating. These metamorphic patterns could be good indicators of a sagduction regime.

Traces of Archean Subduction in the Tarim Craton, Northwest China: Evidence from the Tuoge Complex and Implications for Basement Formation

AbstractThe Tuoge Complex, the oldest basement block exposed in the northeastern Tarim Craton, northwest China, consists mainly of supracrustal rocks, tonalite-trondhjemite-granodiorite (TTG) gneis...

Revisit and comparative analysis of the typical graphite deposits in the Paleoproterozoic khondalite series, western North China Craton: Implications for genesis, depositional environment and prospecting potential

The phaneritic graphite deposits in the western North China Craton can be delineated into an E-W trending belt, which is consistent with the Paleoproterozoic khondalite series. To interpret why graphite is not ubiquitous among the Paleoproterozoic supracrustal rocks, we revisit and comparatively analyze the typical graphite deposits in the Paleoproterozoic khondalite series employing sequence stratigraphy, chronology, tectonic deformation and metamorphism for the khondalite series and carbon isotope analysis for graphite. Our renewed profile of the western graphite belt (WGB) reveals the following: 1) A greater abundance of organic matter was trapped within the lower part of the khondalite series than in the middle and upper parts; consequently, all the known deposits are restricted to the lower part of the khondalite series. 2) As the protolithic basin evolved from a passive continental margin into an active continental margin, subduction-related magmatism provided the influx of heat needed to transform the organic matter into amorphous graphite. 3) Continental collision-induced high-grade metamorphism and associated magmatism finally reworked the original amorphous graphite into the present-day phaneritic graphite deposits.

Extreme δ18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga

Research Article| April 25, 2019 Extreme δ18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga Adrien Vezinet; Adrien Vezinet 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Emilie Thomassot; Emilie Thomassot 2Centre de Recherches Pétrographiques et Géochimiques, UMR 7358, Université de Lorraine, CNRS, 54500 Vandoeuvre-lès-Nancy, France3Institute for Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8038 Zurich, Switzerland Search for other works by this author on: GSW Google Scholar D. Graham Pearson; D. Graham Pearson 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Richard A. Stern; Richard A. Stern 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Yan Luo; Yan Luo 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Chiranjeeb Sarkar Chiranjeeb Sarkar 1Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Search for other works by this author on: GSW Google Scholar Geology (2019) 47 (7): 605–608. https://doi.org/10.1130/G46086.1 Article history received: 04 Feb 2019 rev-recd: 25 Mar 2019 accepted: 27 Mar 2019 first online: 25 Apr 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Adrien Vezinet, Emilie Thomassot, D. Graham Pearson, Richard A. Stern, Yan Luo, Chiranjeeb Sarkar; Extreme δ18O signatures in zircon from the Saglek Block (North Atlantic Craton) document reworking of mature supracrustal rocks as early as 3.5 Ga. Geology 2019;; 47 (7): 605–608. doi: https://doi.org/10.1130/G46086.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The most ancient rocks in the geological record provide insights into the processes that shaped the evolution and composition of the first continental masses. To better constrain these processes, we made a detailed study of a ca. 3.86 Ga felsic meta-igneous rock from the Eoarchean Saglek Block (North Atlantic Craton) that experienced high-grade metamorphism at ca. 3.5 Ga. Our robust zircon-isotope plus trace-element analyses reveal metamorphic zircon domains with δ18O values up to +9‰ at ca. 3.5 Ga, which are the highest values so far measured in any pre–3.0 Ga zircons, metamorphic or igneous, extracted from unambiguous (meta)igneous host rocks. Such elevated zircon δ18O signatures clearly document the involvement of mature supracrustal precursors (mafic volcanics ± clastic/chemical sediments) during the reworking of 3.86 Ga crust at ca. 3.5 Ga. This study provides unequivocal evidence for hydrosphere–crust interactions and reworking processes resulting in metamorphic zircon growth at ca. 3.5 Ga, namely 1 Ga before the Archean-Proterozoic transition. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.