Migmatitic Orthogneisses Research Articles

Petrofabric and Raman microspectroscopy study of the Mina Afortunada gneiss dome: Mapping its thermal gradient

Black quartzites and migmatitic orthogneisses from the Mina Afortunada dome have been analysed through quartz and graphite petrofabric analysis and Raman microspectrometry on graphitized carbonaceous material. The results permit us to recognize a deformation temperature increase from the dome envelope towards its core marking the transition from basal-<a> to rhomb-<a> quartz intracrystalline slip systems at maximum T of 475 °C, and from rhomb-<a> to prism-<a> at maximum T of 515 °C. The complementary petrofabric and Raman study on graphite discloses a change from basal to prismatic <a> slip systems at maximum T of ca. 470 °C, accompanied by a strengthening of its structural order.The Raman study of large graphite grains revealed a non-random crystallinity organization in them, with domains of weaker lattice structural order (reflecting lower formation T) in core areas and higher crystallinity sectors (higher formation T) at the rims. This finding might reflect a shielding effect during prograde metamorphism, the rim material preventing grain core material from reorganization and increase in its crystallinity. This study shows also that graphite can be a good candidate for pressure-temperature-time path reconstruction in metamorphosed organic-rich rocks.

In Situ Lu–Hf Garnet Dating of Archean Deep Crust Granulites from the Polymetamorphic Grenville Front Tectonic Zone

Abstract Recent advances in geochronological techniques now make it possible to efficiently decipher the timing and duration of geological processes in complex high-grade polymetamorphosed orogenic terranes. This is the case of the Grenville Front Tectonic Zone, which truncates the Superior Craton to the southeast. The zone exposes parautochthonous Archean rocks that underwent mid- to high-pressure granulite facies metamorphism of uncertain age. The metamorphic assemblages have been either interpreted as Archean and associated with the final stages of the Superior Craton assembly or as the result of Mesoproterozoic Grenvillian metamorphism, based on cross-cutting relationships and traditional geochronology methods such as U–Pb zircon and 40Ar–39Ar mica dating. Herein, we revisit the extent of the Grenvillian metamorphic overprint in the parautochthonous domain and provide new age constraints for granulite-facies metamorphic assemblages through in situ garnet dating within migmatitic paragneiss, migmatitic orthogneiss, and mafic granulites, combined with in situ trace element mapping. Six samples, which show bell-shaped and occasionally sharp and oscillatory lutetium growth zoning in garnet, yield garnet Lu–Hf isochrons with identical Archean dates of c. 2.6 Ga. Sparse analyses of the material trend towards Grenvillian ages (c. 1 Ga) in one sample from which garnet shows lutetium zoning consistent with post-growth fluid-assisted disturbance. Overall, our results indicate that the widespread granulite-facies metamorphism within the Grenville Front Tectonic Zone is dominantly late Neoarchean in age, unveiling a rare exposure of Archean lower crust in the southeastern Superior Craton. Our results also point towards a limited Grenvillian metamorphic overprint, though the spatial extent and precise thermal conditions of this metamorphism are still unknown. The results presented herein demonstrate the potential of in situ isotopic geochronology on rock-forming minerals like garnet in polymetamorphic terranes.

Paleoarchean to Neoproterozoic crust formation and migmatization events in the Borborema Province, NE Brazil: Implications for the growth and reworking of the continental crust

The Patos Shear System (PSS) is a 20–40 km wide domain of anastomosing shear zones intercalated with non-mylonitic segments at the core of the Borborema Province, a transpressive Neoproterozoic orogen in NE Brazil, part of the reworked São Francisco Craton. We provide new U-Pb, Lu-Hf and Sm-Nd analyses for non-mylonitic basement migmatitic orthogneisses. Four magmatic events (∼3.40, 3.26, 2.68–2.64 and 2.18 Ga), as well as three metamorphic/migmatization ones (∼3.0, 2.1–2.0 Ga and 570–575 Ma) were evidenced. Metamorphism and leucosome injection at ∼570–575 Ma attest the PSS was subjected to high heat from medium- to high-temperature dextral shearing. A distinct Paleoproterozoic (∼2.1–2.0 Ga) migmatization event is probably related to the amalgamation of Neo-Paleoarchean blocks and an older ∼3.0 Ga metamorphic event may represent crustal thickening induced by the collision between small Paleoarchean landmasses. Paleoarchean magmatic protoliths obtained here were emplaced concomitant with TTGs and high-K rocks regionally, and our Hf data from these rocks, after filtering for isotopic disequilibrium during anatexis, combined with a large compilation, point to an apparent mixing of magma derived from a CHUR-like reservoir with older crust, better reconciled, although not conclusively, with a stagnant-lid scenario dominated by plume activity. Hf data from 2.68 to 2.64 Ga old migmatites plot along a conspicuous Meso-Neoproterozoic “reworking array” and our preferred model is that magmatism between 2.9 and 2.6 Ga took place in an accretionary/collisional continental setting, following renewed juvenile additions between ∼3.2 and 2.9 Ga. At last, the crystallization age of ∼2.18 Ga of an orthogneiss is related to another Wilson Cycle. Our data point to reworking of Archean lithologies during the Paleoproterozoic, as evidenced by inherited Archean zircon grains, but ƐHf and ƐNd close to CHUR suggest involvement of more juvenile material as well, consistent with regional data, and the preferred scenario is that of subduction followed by collision.

Chemical-Abrasion U-Pb zircon geochronology reveals 150 Myr of partial melting events in the Archean crust of the São Francisco Craton

Field observations and CA-LA-ICP-MS U–Pb zircon ages and Hf isotope compositions obtained from migmatitic orthogneisses and granitoids from the Belo Horizonte Complex, southern São Francisco Craton, indicate a major period of partial melting and production of felsic rocks in the Neoarchean. Our observations show that the complex is an important site for studying partial melting processes of Archean crystalline crust. Much of the complex exposes fine-grained stromatic migmatites that are intruded by multiple leucogranitic veins and sheeted dikes. Both migmatites and leucogranite sheets are crosscut by several phases of granitoid batholiths and small granitic bodies; both of which are closely associated with the host banded gneisses. Chemical abrasion followed by detailed cathodoluminescence imaging revealed a wide variety of zircon textures that are consistent with a long-lived period of partial melting and crustal remobilization. Results of U-Pb and Hf isotopes disclose the complex as part of a much wider crustal segment, encompassing the entire southern part of the São Francisco Craton. Compilation of available U-Pb ages suggests that this crustal segment was consolidated sometime between 3000 Ma and 2900 Ma and that it experienced three main episodes of partial melting before stabilization at 2600 Ma. The partial melting episodes took place between 2750 Ma and 2600 Ma as a result of tectonic accretion and peeling off the lithospheric mantle and lower crust. This process is likely responsible for the emplacement of voluminous potassic granitoids across the entire São Francisco Craton. We believe that the partial melting of Meso-Archean crystalline crust and production of potassic granitoids are linked to a fundamental shift in the tectonics of the craton, which was also responsible for the widespread intrusion of large syenitic bodies in the northern part of the craton, and the construction of layered mafic–ultramafic intrusions to the south of the BHC.

Petrographic and geochemical constraints on the evolution of the Matarazzo Sequence, Arroio Grande Ophiolite, Brazil: Evidence from migmatites and marbles

This study comprises the amphibolites, marbles, granitoids, metadiabases, and skarns of the Matarazzo Sequence (Arroio Grande Ophiolite, Dom Feliciano Belt, Brazil), which are here, for the first time, discussed from a migmatite perspective. The contribution presents novel petrographic and elemental geochemical data for the Sequence. Petrographic evidence demonstrates that high degrees of partial melting of the paleosomes (amphibolites) led to the development of millimeter-to meter-sized networks of mesocratic neosome veins (diorite to quartz-monzodiorite). The partial melting of the neosomes, in turn, led to the formation of micrometer-to meter-sized veins and pods of leucosomes (quartz-syenite to monzogranite). Garnet- and epidote-rich skarn layers were generated by the marble/melt interaction. The synkinematic nature of the partial melting event is evidenced by macrostructures such as rotated amphibolite schollen, neosome/leucosome veins concordant to the NE-SW-trending regional shear structures, and accumulation of melt in the hinge of folds. The minimum crystallization temperature of the neosomes and leucosomes are constrained, respectively, at 709 ± 31 and 732 ± 63 °C (Zr saturation thermometry). The geochemical data show that the trace element behavior of the paleosomes, neosomes and leucosomes are similar, strengthening the hypothesis of a genetic relationship and the interpretation that the metadiabases and amphibolites are the protoliths of the diatexites. The geochemical signature of the metadiabases and amphibolites suggest magmatic protoliths developed in an oceanic back-arc setting. The composition of the associated marbles is also consistent with this tectonic setting, reinforcing previous hypotheses that suggest that the studied ophiolite represents metamorphic remnants of an oceanic back-arc basin (South Adamastor paleo-ocean). The minimum metamorphic and obduction age of the Arroio Grande Ophiolite is, so far, marked by the U–Pb crystallization age of the studied leucosomes at 640 Ma. Since the same age is recorded in migmatitic orthogneisses from the southernmost sector of the Punta del Este Terrane in Uruguay, we suggest that the Punta del Este Terrane reached, synchronically, partial melting conditions at 640 Ma.

Variation in technical properties of granitic rocks with metamorphic conditions

Granitic rocks constitute a global raw material key asset for aggregate production. Their technical performance is, however, highly variable and knowledge on the aggregate functionality of granites in different geological settings is incomplete. This study investigates systematic variations in resistance to fragmentation (Los Angeles value) and wear (Micro-Deval value) for granitic rocks along a 150 km long east–west trending metamorphic gradient across the Eastern Segment of the Sveconorwegian Province in Scandinavia. In essence, the metamorphic gradient represents a transition from pristine granite to stromatic migmatitic orthogneiss and granulite. Along this profile, the aggregate functionality is governed by the behaviour of quartz and feldspar during metamorphic recrystallisation. Two critical metamorphic variables were identified: temperature and hydrous fluids. Recrystallisation at comparably low metamorphic temperatures, ≤ 600 °C, result in irregular grain shapes and fine-grained quartz aggregates that in turn result in low Los Angeles and Micro-Deval values, i.e., high-quality materials for road construction. At higher metamorphic temperatures, > 600 °C, grain coarsening and smoothening of grain boundaries result in significantly higher Los Angeles and Micro-Deval values. At high temperatures, high availability of hydrous fluids promoted extensive partial melting and post-deformational crystallisation which resulted in remarkably poor aggregate performance, with Los Angeles and Micro-Deval values consistently above 40% and 12%, respectively. In contrast, deformation at high temperature but low availability of hydrous fluids resulted in inequigranular textures with high proportions of very fine-sized crystals, interlocking textures, and aggregates with Los Angeles and Micro-Deval values down to 19% and 5%. The consistent variation in metamorphic microtextures correlate with macro-fabrics at the outcrop scale, allowing assessment of aggregate functionality already in the field. For migmatitic gneisses, the proportion and strain state of leucosome are the most critical parameters.

The Maravilha Orthogneiss: High-K calc-alkaline Cryogenian evidence of the pre- to early- collisional magmatism in the southern PEAL domain, SE Borborema Province

The study area is in the southern East Pernambuco–Alagoas Domain, Borborema Province, Southern sub-province, northeastern Brazil. The Maravilha Orthogneiss comprises an elongated intrusion of ~200 km2, with an ESE-WNW major axis, ranging in composition from tonalite to sienogranite, emplaced into metasedimentary rocks of the Inhapi Sequence, in the southern part of the Pernambuco-Alagoas Domain. Diorite sheets, amphibolite, leucocratic dikes and quatz-feldspar pegmatite veins are common. The Maravilha Orthogneiss show SiO2 contents ranging from 55.9 to 65.55 wt%, and Mg# (MgO/FeO + MgO) of 6.28–9.12. It has magnesian, metaluminous, and medium to high-K calc-alkaline composition. Chondrite-normalized spidergrams are characterized by troughs at Ba, Rb, Sr, Th, Nb and Ta. Chondrite-normalized REE patterns are moderately fractionated, with (Ce/Yb)N ratios ranging from 2.28 to 17.29 and display discrete negative Eu anomalies. This rock shows volcanic-arc geochemical signature. U–Pb zircon SHRIMP data yielded an age of 646 ± 5 Ma, interpreted as the crystallization age of the protolith. The Geochemical and isotopic data suggest that the source of the Maravilha orthogneiss protolith involved mixing of crustal and mantle melts. The resulted magma evolved by amphibole, plagioclase and apatite fractionation. A sample of migmatitic orthogneisses of the Inhapi Sequence basement show U–Pb zircon crystallization age of 983 ± 7.2 Ma. The lack of zircon grains with Tonian age in the studied orthogneiss indicates no contribution of Tonian crust in their source. The Maravilha Orthogneiss is a record of a pre-to early-collisional calc-alkaline magmatism with arc-related geochemical signature emplaced during the beginning of the Brasiliano Orogeny compressive stage.

PETROGRAFIA E LITOQUÍMICA DAS ROCHAS MIGMATÍTICAS DA PORÇÃO NORTE DO CINTURÃO SALVADOR-ESPLANADA-BOQUIM, CRÁTON DO SÃO FRANCISCO, BRASIL

&#x0D; &#x0D; 4Doutor em Geologia, Programa de Pós-Graduação em Geologia, Núcleo de Geologia Básica (NGB), Instituto de Geociências, Universidade Federal da Bahia (UFBA), Salvador-BA, Brasil.&#x0D; ORCID: https://orcid.org/0000-0002-2143-5594&#x0D; Email: mmm@ufba.br&#x0D; &#x0D; Resumo&#x0D; A área de estudo localiza-se na porção extremo norte do Cinturão Salvador-Esplanada-Boquim, nas porções sul do estado de Sergipe e nordeste do estado da Bahia, Brasil. Este Cinturão apresenta três faixas litológicas de rochas metamórficas denominadas Complexo Migmatítico Rio Real-Riachão do Dantas (CMRR), Complexo Granulítico Esplanada-Boquim (CGEB) e Complexo Gnáissico/Migmatítico-Granulítico Costa Atlântica (CGGCA). Esse artigo se restringe aos estudos petrológicos da primeira faixa de migmatitos que foi subdividida em Ortognaisse Migmatítico Granítico Leste (OMGL) e Ortognaisse Migmatítico Granítico Oeste (OMGO). Ambas, macroscopicamente e petrograficamente são similares, cujas paragêneses minerais indicam a ocorrência de processos metamórficos de médio grau, da fácies anfibolito com faixas retrometamórficas. Litoquimicamente, notou-se características particulares das porções claras e escuras de cada um desses tipos de migmatitos e que, embora tenham sofrido migmatização, mantiveram as características dos protólitos, permitindo identificar as porções félsicas como provenientes de magma variando entre cálcio-alcalino normal a cálcio-alcalino de alto K.

The Orosirian Cuiú-Cuiú magmatic arc in Peixoto de Azevedo domain, Southern of Amazonian craton

The Peixoto de Azevedo domain (PAD) is the southernmost portion of the Tapajós-Parima province in the Amazonian craton. It is constituted by Paleoproterozoic rocks (2.05–1.85 Ga) of the Cuiú-Cuiú magmatic arc (CCMA), and by post-orogenic to anorogenic granitoids. The rocks with magmatic arc signature in the PAD are correlated with those of the Mundurucus orogeny (2.04–1.96 Ga) in the Tapajós domain, where several authors propose the evolution of at least three magmatic arcs. However, in this work, the rocks of PAD are interpreted as part of a single magmatic arc of Orosirian age, the CCMA. Geochronological, geochemical and petrographical data suggest that CCMA has evolved within a continuous long-lived subduction process, in the approximate range of 2.05–1.95 Ga, at the border of an Archean terrain. This older basement is represented by the Gavião gneiss, an inlier whose U-Pb LA-ICP-MS zircon age is 2825 ± 9 Ma. Additionally Nd-TDM data (2.76–2.5 Ga) and some Archean inherited and detrital zircon crystals indicate that CCMA developed on an Archean basement. The CCMA is divided into: Cuiú-Cuiú complex (CCC; 2050–1990 Ma), which marks the main phase of subduction in the evolution of CCMA and is represented by migmatitic orthogneisses, quartz diorites, tonalites and granodiorites; metaplutonic Pé Quente suite and volcanic rocks of the Jarinã formation (2000–1970 Ma), which both form an expanded calc-alkaline serie, typical of continental margin arcs, of tonalitic, granodioritic, monzo- and syenogranitic composition; and Nhandu intrusive suite (1970–1950 Ma), composed of high-K calc-alkaline to shoshonitic syeno- and monzogranites, whose isotopic data (ϵNd from −0.95 to −7.6) indicate strong participation of crustal sources in its generation. Nhandu intrusive suite shows chemical characteristics of magma generated in late to post-tectonic setting and marks the final phase of the CCMA evolution. The metamorphic peak in the area reached high amphibolite facies, probably between 2015 Ma and 1990 Ma (main age range of the migmatitic gneisses). After the orogenic compressive phase, the region experienced tectonic quiescence, which was followed by post-orogenic to anorogenic extension, approximately between 1905 and 1850 Ma. This later event is represented by expressive acid plutonism of Guarantã do Norte and Matupá granitoid suites, and subordinately by basic rocks of Flor da Serra intrusive suite.

Description and hierarchy of ductile deformation events in the Camalaú region, state of Paraíba, central portion of the Alto Moxotó Terrane, Borborema Province, Brazil

The Borborema Province (NE Brazil) has a long history of deformation, magmatism, and metamorphism during the Precambrian Eon. Thus, it represents a natural laboratory to investigate widely developed ductile-deformation markers, such as major fold belts and large-scale shear zones. This paper describes the structural evolution of a complex folded area of the Alto Moxoto Terrane, Central Borborema Province. In this region, para-derived rocks of the Sertânia Complex are intruded by orthogneisses and migmatitic orthogneisses. The Geophysical magnetometric expressions of the Borberma Province are characterized by strong aligned and folded magnetic lineaments and integrated structural analysis allowed us to identify three ductile deformation episodes. Brittle tectonics is also present, but not described. D1 stage represents the basement structural framework restricted to local structural windows of ancient orthogneisses and migmatites that are not mappable at the working scale. D2 deformation is widespread throughout the Central Borborema Province, producing nappes that mark top-to-NW tectonic vergence. Foliation and lineation attitudes are compatible with progressive deformation from tangential to strike-slip tectonics (i.e. D2 to D3). The latter is associated with the NE-SW Xinxo and Congo-Cruzeiro do Nordeste sinistral strike-slip shear zones, producing refold patterns that resemble Type-3 interference geometry. Based on airborne magnetic geophysical imaging interpretations and mesoscopic observations, we suggest that progressive deformation strongly affected the region and overprinted at large-scale early formed rocks.

940 Ma Anatexis in 1726 Ma Orthogneiss in the Northern Margin of the Bhilwara Belt and Significance for the Precambrian Evolution in Northwest India

The northwestern domains of India record Proterozoic orogenies that reflect global cycles of convergence and extension. A garnet-biotite migmatitic orthogneiss hosted within the Agucha-Kekri Shear Zone sandwiched between the Bhilwara Belt and the North Delhi Fold Belt (NDFB) has two zircon populations yielding U-Pb ages of 1726 and 938 Ma. The older age is correlated with the intrusion of the migmatite protolith, consistent with the partial melting event recorded in gneisses in the southern margin of the Bhilwara Belt. The younger age is interpreted as the age of partial melting and migmatization. Petrographical observations and pressure-temperature (PT) pseudosection analyses indicate incongruent melting of biotite and plagioclase in the gneiss-produced garnet, potash feldspar, and melt under water-fluxed conditions. The peak conditions of ~9 kbar and ≥700°C estimated for the partial melting are similar to those of coeval migmatization recorded at the northwestern margin of the Bhilwara Belt, but lower than those in the adjacent NDFB. This is interpreted to indicate formation of a migmatitic front along the northwestern margin of the Bhilwara Belt while this was being underthrust under the NDFB. Migmatization under similar PT conditions and, at the same time, estimated for the Central Indian Tectonic Zone implies the presence of several loci of crustal amalgamation leading to the final architecture of peninsular India during Rodinia formation.

U-Pb SHRIMP dating of basement rocks of the Iriri-Xingu domain, Central Amazonian province, Amazonian craton, Brazil

Abstract The Iriri-Xingu domain, located in the central part of the Amazonian craton, consists of extensive occurrences of Paleoproterozoic volcanic rocks and granites with published ages of ca. 1990 to 1840 Ma, which show a strong crustal contribution for their magmas. Exposures of basement rocks are small and rare. Samples from two areas were dated in this work by U-Pb SHRIMP in zircon. In the northern Maribel area, a high-grade pelitic paragneiss presented an age of 2160 ± 8 Ma and the leucosome of a migmatitic orthogneiss, probably a diatexite, crystallized at 2149 ± 20 Ma. These Rhyacian ages suggest that it is part of the Bacaja domain, related to the Trans-Amazonian cycle. The Morro Grande area occurs in the central part of Iriri-Xingu domain, where a high-grade muscovite gneiss with a protolith of 2120-2180 Ma is dated at 1982 ± 7 Ma, and the leucosome of a migmatitic orthogneiss is crystallized at 1979 ± 8 Ma. These are the first records of high-grade metamorphism at ca. 1980 Ma in the Central Brazil shield and may be related to the generation of this Orosirian felsic magmatism, which is widespread throughout the nearby Ventuari-Tapajos province.

Grenvillian-aged reworking of late Paleoproterozoic crust of the southern North Australian Craton, central Australia: Implications for the assembly of Mesoproterozoic Australia

LA–ICP–MS U–Pb monazite and zircon geochronology from metapelites and migmatitic orthogneiss along the central southern margin of the North Australia Craton (NAC), central Australia, reveal the presence of a regional-scale Grenvillian-aged (ca. 1130Ma) deformation system. Grenvillian-aged deformation extends over a strike length of over 110km from the southern Aileron Province in the vicinity of Alice Springs to the Teapot Granite Complex within the Warumpi Province. There is also evidence for Grenvillian-aged migmatisation and resetting of monazite further west in the Mount Liebig area, which may extend the Grenvillian-aged footprint to a strike-length of at least 250km. Deformation associated with Grenvillian-aged reworking produced map-scale east–west trending folds and strongly foliated, steeply dipping shear zones that define the structural architecture of the interface region between the Aileron and Warumpi Provinces. Shallow, westerly-plunging folds associated with partial melting have evolved into shear zones and mylonites that record south-side up movement. Phase equilibria modelling of age-constrained garnet–biotite±sillimanite±cordierite-bearing metapelites from the southern Aileron and Warumpi Provinces suggest Grenvillian-aged metamorphism reached temperatures in the range of 775–820°C and pressures of 5–5.5kbar, corresponding to thermal gradients of ∼130–165°Ckbar−1. The extensive Grenvillian-aged system reworks and overprints late Paleoproterozoic (ca. 1650–1630Ma) high-grade metamorphic rocks. The Grenvillian-aged system occurs above a south dipping lithospheric-scale interface that has been geophysically imaged to depths of at least 200km. This feature was interpreted to represent a fossil subduction zone of late Paleoproterozoic age. However, the presence of regional-scale Grenvillian-aged deformation in the crust above this feature suggests that it may instead be Grenvillian-aged. If that is the case, deformation along the southern margin of the NAC may record suturing of the NAC with the Grenvillian-aged Musgrave Province to the south. This would effectively place the long-lived, Late Mesoproterozoic ultrahot orogen that is the Musgrave Province in southern central Australia in an upper plate tectonic setting, linked to the convergence of the NAC with the South Australian Craton.

Polyphasal foreland-vergent deformation in a deep section of the 1 Ga Sveconorwegian orogen

Metamorphic belts in Precambrian shields expose deep interiors of orogens and are often challenging to interpret in tectonic terms. The Eastern Segment of the 1.1–0.9Ga Sveconorwegian orogen is a metamorphic belt, which was metamorphosed at high-pressure granulite and upper amphibolite facies at 35–40km depth and shows highly complex fold patterns. We use detailed structural analysis in combination with U–Pb SIMS dating of complex zircon to identify the structural and tectonic evolution in a composite migmatitic orthogneiss complex of the Eastern Segment. We link four fold phases to late-orogenic foreland-vergent flow, and date D2 and D3 at 0.97–0.95Ga. Leucosome and mesosome of felsic metasupracrustal migmatitic gneiss contain igneous zircon that dates the crystallization of the source rock or protolith at 1695±8Ma, and 1690±8Ma, respectively, demonstrating a temporal link to unmetamorphosed or little metamorphosed igneous rocks east of the Sveconorwegian orogen (the Transscandinavian Igneous Belt). Early migmatization attributed to Hallandian orogenesis is dated by formation of secondary zircon in two leucosome samples at 1402±12 and 1386±7Ma. The pre-Sveconorwegian structure (Sc), which is strongly overprinted by Sveconorwegian deformation and migmatization, is a composite coarse gneissic layering made up of a primary compositional layering and (variably present) Hallandian leucosome veins. The dominant foliation, a pervasive gneissic banding (S1), is axial planar to intrafolial F1 folds and developed as a result of tectonic overprint of Sc; S1 is associated with a strong ESE-trending aggregate stretching lineation (L1). S1 and L1 were folded by asymmetric SE-vergent F2 folds during foreland-vergent flow. Crystallization of Sveconorwegian zircon in syn-F2 leucosome dates this phase at 970±5Ma. The sequence was subsequently deformed by symmetric and asymmetric F3 folds that are S- to SE-vergent. Syn-F3 leucosome, mineral parageneses and microtextures associated with D3 show that this deformation occurred under still high temperatures. The last ductile phase (D4) also involved the generation of leucosome synkinematic with N-S trending folds that deformed all previous structures under amphibolite facies conditions. K-feldspar-rich, originally coarse-grained and strongly deformed metapegmatite contain two generations of zircon: texturally old 1414±5Ma cores and fragments, and voluminous Sveconorwegian envelopes, and new grains that demonstrate the presence of melt as late as 958±7Ma. Ductile structures are similar in metasupracrustal and metaplutonic orthogneiss complexes. Likely, these units were tectonically juxtaposed during D1, while D2–D4 structures reflect a common tectonic evolution after their emplacement. We interpret D1–4 structures as recording WNW-ESE convergence (D1) and ESE-vergent flow (D2 and D3), followed by E-W gentle upright folding (D4). Sveconorwegian 0.98–0.96Ga foreland-vergent deformation, accompanied by migmatization at all four stages, was responsible for formation of the polyphasal deformation pattern in this part of the orogen.

Migmatization related to mafic underplating and intra- or back-arc spreading above a subduction boundary in a 2.0–1.8 Ga accretionary orogen, Sweden

Absolute ages of migmatization and protolith formation, and constraints on the timing of ductile deformation in two major lithotectonic units in the south-western part of a 2.0–1.8Ga orogenic belt in the Fennoscandian Shield, south-eastern Sweden, have been determined using U–Pb ion probe analysis of different generations of zircon in veined gneisses and leucocratic granite. Detrital and xenocrystic zircon in paragneiss and garnet-bearing leucogranite, respectively, in the Bergslagen lithotectonic unit show ages of 2.1–2.0Ga and 1.9Ga. Deposition of the sedimentary material occurred during or after a subduction-related magmatic event at 1.91–1.87Ga. Two orthogneiss protoliths formed during this magmatic event around 1.88Ga while most zircon in the leucosome in a third migmatitic orthogneiss was inherited from a 1.85Ga igneous protolith. A polyphase tectonothermal evolution with anatexis under low-P metamorphic conditions around 1.86Ga (M1) and 1.84–1.81Ga (M2) is inferred for the migmatitic gneisses in the Bergslagen unit; garnet-bearing leucogranite crystallised around 1.84–1.83Ga, close in time to major folding of the M1 gneissic fabric. A previously unrecognised 1.86–1.85Ga ductile deformational event under medium-grade metamorphic conditions has been identified in the adjacent lithotectonic unit to the south (Småland lithotectonic unit), close in time to the M1 event in the Bergslagen unit to the north.By constraining the timing of anatexis and comparing with information bearing on crustal thickness, excess mass at depth and the character and age of magmatic activity, it is inferred that anatexis in the Bergslagen lithotectonic unit is related to pulses of mafic underplating, during the early stages of two separate, subduction-related magmatic episodes after the 1.91–1.87Ga magmatic event. It is suggested that each pulse was related to intra- or back-arc spreading above a subduction boundary, which had entered a retreating mode with separate, long periods (20–50Ma) of extension or transtension. This study challenges the need to invoke crustal thickening related to plate collision at 1.9–1.8Ga as a mechanism to explain high-grade metamorphism in the southern part of the 2.0–1.8Ga orogen. Instead, a solely accretionary tectonic model involving an overriding plate along an active continental margin with significant extensional or transtensional crustal deformation is preferred.

Leucosome distribution in migmatitic paragneisses and orthogneisses: A record of self-organized melt migration and entrapment in a heterogeneous partially-molten crust

The thickness and spatial distribution of foliation-parallel leucosomes in metric to decametric scale interlayered units of migmatitic paragneiss and orthogneiss from the Fosdick migmatite–granite complex in West Antarctica are quantified along one-dimensional transects. This study demonstrates that leucosomes in stromatic metatexite migmatites have thickness and spacing distributions consistent with being sampled from a power-law (scale-invariant) distribution. However, leucosome distribution in the paragneisses and orthogneisses yields different scaling exponents and the largest leucosomes in orthogneiss are thicker than those in the paragneiss by approximately half an order of magnitude. The difference in the spatial distribution and maximum thickness of leucosomes between the two rock types is attributed to the decimetric scale of inherited compositional layering in migmatitic paragneiss that restricted the development of larger leucosomes compared with an absence of such heterogeneity in migmatitic orthogneiss that allowed thicker leucosomes to form. Phase equilibria modeling of the protoliths of the paragneisses and orthogneisses shows that at Cretaceous peak metamorphic conditions the spectrum of metasedimentary protolith compositions could have produced 8–48 vol.% melt and the range of igneous protolith compositions could have produced 3–17 vol.% melt at the crustal depth exposed, which is generally less than the volume of leucosome at outcrop (43–72 and 39–67 vol.%, respectively, in the paragneiss and orthogneiss). This discrepancy indicates that the Fosdick complex acted as both a source of melt production and a zone of melt entrapment, whereby some of the melt derived from deeper in the crust has partially crystallized during migration to shallower levels in the crust. The observed power-law behavior of leucosomes is consistent with the hypothesis that intracrustal differentiation by anatexis and granite magmatism is scale-invariant and represents a self-organized critical system. The interaction of this critical system with the compositional layering in the paragneisses and the interlayering between paragneiss and orthogneiss accounts for the three-dimensional distribution of leucosome in stromatic metatexite migmatites.

Partial melting of the Archaean Thrym Complex of southeastern Greenland

High-grade Archaean rocks of the remote Thrym Complex of southeastern Greenland expose a lower crustal section that formed at a depth of >30km at temperatures reaching 1000°C and pressures approaching 1.5GPa. The complex is part of the North Atlantic Craton, which is bound to the north by the Trans Hudson–Nagssugtoqidian–Ammassalik Orogen and to the south by the Makkovik–Ketilidian Orogen. The rocks in the complex include orthogneiss, migmatitic orthogneiss with banded or nebulitic fabrics, migmatite with a breccia-like structure (agmatite), and narrow bands of lensoidal mafic and ultramafic rocks (granulites) with associated widespread garnet-bearing mafic granulite, and a minor proportion of garnet–cordierite–sillimanite–biotite gneiss. The latter high-grade rocks form part of the substrate for the emplacement of the protoliths for orthogneiss that was synchronous with the ca. 2790–2700Ma Skjoldungen Orogeny.At least two suites have been recognised in the Thrym Complex based on geochemistry. The oldest is a tholeiitic suite of mafic to ultramafic granulites, and the other is a calc-alkaline suite that includes felsic to intermediate orthogneiss. Most of the orthogneiss has a granodioritic (oldest) to quartz monzodioritic (younger) composition with rare monzogranitic, tonalitic, monzonitic and quartz dioritic protoliths. Historically, the orthogneiss units have been called TTGs, although monzogranite and tonalite are rare, and no trondhjemite is observed. In addition, they could also be called adakitic on geochemical grounds. Instead of using this terminology, which has genetic connotations, the orthogneiss is referred to as predominantly granodioritic to quartz-monzodioritic.Assuming that modern tectonics operated during the Neoarchaean, at least some of the mafic and ultramafic granulites and orthogneiss exposed in the Thrym Complex represent the root zone of a magmatic-arc that might have been relaminated around 2760Ma during the collisional Skjoldungen Orogeny.

Análise estrutural e metamórfica da região de Sucuru (Paraíba): implicações sobre a evolução do Terreno Alto Moxotó, Província Borborema

Inserted on the Alto Moxoto Terrane of the Borborema Province, the Sucuru region (Paraiba state, Brazil) includes two dominantly pre-Ediacaran tectono-stratigraphic domains, bounded by an expressive thrust shear zone (Carmo SZ). The first domain has a metaplutonic nature, being formed by granodioritic to migmatitic orthogneisses (Floresta Complex) cut by several intrusive suites. These suites comprise a unit of mafic-ultramafic nature (Malhada Vermelha), a second one mainly of granitic-granodioritic composition (Pedra d'Agua) and a last suite whose composition ranges from syenitic to syenogranitic (Serra da Barra). On the second domain predominate migmatitic paragneisses from of the Sertânia Complex. All these units were cut by Cambrian-Ediacaran felsic dykes and A-type granites. Three tectonic events were recognized. The first Dn episode represents a thrusting with tectonic transport to NW-NNW, being important the Sucuru and Carmo shear zones. The Dn+1 episode is a transcurrent event of Ediacaran age, being important in regional context the Coxixola and Congo shear zones. The final progressive late Ediacaran-Cambrian Dn+2 episode, culminated with the emplacement of the Sucuru dyke swarm and the Prata and Serra da Engabelada A-type granites. Petrographic evidences show that the Dn episode reached its metamoprhic peak on the granulitic or eclogitic facies, whose paragenesis was after re-equilibrated to the amphibolite during the Dn+1 event. The Dn+1 episode developed mylonitic corridors on the amphibolite facies with associated migmatization, while the Dn+2, event reached a greenschist facies restrict to influence of the late and post-tectonic intrusives. The occurrence of garnet amphibolites with symplectite texture along the metamorphic path Mn-Mn+1 suggests that the thrust event represented a high pressure metamorphic episode, indicating a probable Paleoproterozoic suture.

Discovery of coesite–eclogite from the Nordøyane UHP domain, Western Gneiss Region, Norway: field relations, metamorphic history, and tectonic significance

AbstractUltrahigh‐pressure (UHP) rocks from the Western Gneiss Region (WGR) of Norway record subduction of Baltican continental crust during the Silurian to Devonian Scandian continental collision. Here, we report a new coesite locality from the island of Harøya in the Nordøyane UHP domain, the most northerly yet documented in the WGR, and reconstruct the P–T history of the host eclogite. The coesite–eclogite lies within migmatitic orthogneiss, interpreted as Baltica basement, that underwent multiple stages of deformation and partial melting during exhumation. Two stages of metamorphism have been deduced from petrography and mineral chemistry. The early (M1) assemblage comprises garnet (Pyr38–41Alm35–37Grs23–26Spss1) and omphacite (Na0.35–0.40Ca0.57–0.60Fe2+0.08–0.10Mg0.53Fe3+0.01AlVI0.40–0.42)2(AlIV0.03–0.06Si1.94–1.97)2O6, with subordinate phengite, kyanite, rutile, coesite and apatite, all present as inclusions in garnet. The later (M2) assemblage comprises retrograde rims on garnet (Pyr38–40Alm40–44Grs16–21Spss1), diopside rims on omphacite (Na0.04–0.06Ca0.88–0.91Fe2+0.09–0.13Mg0.81–83Fe3+0.08AlVI0.03)2(AlIV0.07–0.08Si1.92–1.93)2O6, plagioclase, biotite, pargasite, orthopyroxene and ilmenite. Metamorphic P–T conditions estimated using thermocalc are ∼3 GPa and 760 °C for M1, consistent with the presence of coesite, and ∼1 GPa and 813 °C for M2, consistent with possible phengite dehydration melting during decompression. Comparison with other WGR eclogites containing the same assemblage shows a broad similarity in peak (M1) P–T conditions, confirming suggestions that large portions of the WGR were buried to depths of ∼100 km during Scandian subduction. Field relations suggest that exhumation, accompanied by widespread partial melting, involved an early phase of top‐northwest shearing, followed by subhorizontal sinistral shearing along northwest‐dipping foliations, related to regional transtension. The present results add to the growing body of data on the distribution, maximum P–T conditions, and exhumation paths of WGR coesite–eclogites and their host rocks that is required to constrain quantitative models for the formation and exhumation of UHP metamorphic rocks during the Scandian collision.

Field relationship of high-grade Neo- to Mesoarchaean rocks of South-East Greenland: Tectonometamorphic and magmatic evolution

South-East Greenland forms part of the North Atlantic Craton and is characterized by migmatitic orthogneisses, narrow bands of mafic granulite, ultramafic and possible meta-sedimentary rocks, and alkaline-carbonatitic intrusive rocks. Mafic granulite, meta-sedimentary and ultramafic rocks form the basement for the emplacement of granitic intrusions at ca. 2865Ma that lasted episodically until ca. 2790Ma and continuously during 2750–2700Ma. The area is structurally complex with evidence of at least seven deformation events including reclined and mushroom-like fold interference patterns. An older (>2790Ma) foliation formed in granitic rocks and the basement during the Timmiarmiut Orogeny (DT). Deformation associated with the ca. 2790–2700Ma Skjoldungen Orogeny folded this early foliation, and is associated with a penetrative foliation that is refolded progressively in a northeast–southwest oriented stress field. The orientation of the stress field progressively rotated into a northnorthwest–southsoutheast orientation during the last stages of the orogeny. The orogeny is also characterized by syn-deformational anatexis at granulite-facies (at approximately 800°C and 5–8kbar, ca. 2790–2740Ma), which decreased to the amphibolite-facies at ca. 2730Ma.The late- to post-tectonic granite and alkaline rocks assigned to the Skjoldungen Alkaline Province intruded the central-northern part around 2710Ma. This was followed by north–south extensional deformation during the Singertat Stage forming discrete shear-zones at greenschist-facies grades, which is coeval with the emplacement of pegmatite, ijolite, and carbonatite emplacement during ca. 2680–2650Ma.Similar lithology and tectonic processes in the Tasiusarsuaq Terrane of southern West Greenland and the Lewisian Complex in Scotland suggest a possibly large Archaean terrane at that time, which, taking the present size, at least covered around 500–600km in an east–west direction and approximately 200km in a north–south direction.