Orthogneiss Complex Research Articles

The Neoproterozoic Agudos Grandes granitic batholith, SE Brazil: Inferences on source areas from elemental and Sr-Nd-Pb isotope geochemistry

I-type, K-rich calc-alkaline hornblende-biotite granitoids (HKCA) suites are typical of both pre- and post-collisional tectonic settings, and their petrogenesis is challenging, since they are dominated by rocks with intermediate composition (e.g., 60–68 wt% SiO2; 10–20 vol % mafic mineral) that are neither typical products of crust nor of mantle partial melting. These granitoids are particularly abundant as the main components of Ediacaran batholiths intruding the Neoproterozoic Ribeira Fold Belt in SE Brazil, e.g. constituting ∼70% of the exposition of the Agudos Grandes Batholith, where they were emplaced at 620-610 Ma, as indicated by U–Pb zircon dating from our previous work and new data presented here. Two types of coeval granitoids comprise an expanded suite of mafic to felsic titanite and allanite-rich biotite granitoids (Itapevi-type) and a single occurrence of biotite-muscovite leucogranite (Turvo-type). Elemental and isotope whole-rock data indicate that different sources were involved in the genesis of their parent magmas, with crust with old residence predominating for the HKCA and Itapevi types, as indicated by very negative (−16 to −20) εNd(t) and low initial 206Pb/204Pb (16.4–16.7), and younger metasedimentary rocks as the source of the Turvo leucogranite (εNd(t) (∼-14); initial 87Sr/86Sr = 0.717). This “syn-orogenic” magmatism was immediately followed, as demonstrated by field and geochronological data, by ∼605-600 Ma circumscribed plutons dominated by more fractionated (±muscovite)-biotite granites that derive from melts generated in the middle crust from sources that are slightly younger than those of the HKCA granitoids, as indicated by slightly less negative (εNd(t) (similar to those of the Turvo leucogranite) and higher 206Pb/204Pb (17.1–17.3). After a ∼15 Myr lull, A-type plutons were emplaced in a post-orogenic setting, probably from mantle and crust sources. The syn-to late-orogenic (620-600 Ma) magmatism of the Agudos Grandes was produced by a thermal anomaly, probably associated with a continental arc. A combination of elemental and isotope geochemistry and inherited zircon data suggests that crustal melting was probably induced by ascension of mantle-derived basic magmas, and initiated in hot zones at the lower portions of a previously thickened crust, formed by old Paleoproterozoic to Archean material (the basement of the Apiaí-São Roque Domain of the Ribeira Belt?), and progressed to the middle portions of the crust, where it provoked partial melting of a different crustal domain with younger age and crustal residence (Embu Complex orthogneisses and metasediments).

The REE-Induced Absorption and Luminescence in Yellow Gem-Quality Durango-Type Hydroxylapatite from Muránska Dlhá Lúka, Slovakia

In talc-magnesite veins in serpentinite near Muránska Dlhá Lúka (MDL), Slovakia, yellow euhedral to subhedral crystals apatite of a gem quality occur. It has a composition of hydroxylapatite with F− varying between 0.29 and 0.34 apfu, Cl− in range of 0.02–0.05 apfu and calculated OH− content between 0.62–0.68 apfu. Moreover, [CO3]2− molecules were identified by FTIR and Raman spectroscopy. MDL apatite contains only up to 0.003 apfu As5+ and Si4+ substituting for P5+; Ca is substituted by small amount of Na, Fe2+, Mn2+ (all up to 0.006 apfu), and Rare Earth Elements (REE—in total up to 0.017 apfu). Compared to trace-element composition of similar apatites from Durango, Mexico, the REE content in MDL apatite is around ten times lower with Nd > Ce >> La, its chondrite-normalized REE pattern has almost a horizontal slope and larger negative Eu anomaly. The MDL apatite is richer in Mn, Pb and Li, but poorer in As, V, Th and U. The concentrations of Sr and Y are similar. In the optical absorption spectra, the most prominent bands are at 585–590 nm (Nd3+) and between 600 and 800 nm (Mn2+, Ce3+-SiO3− photochromic center and Nd3+). The photoluminescence spectrum of MDL apatite shows bands between 550 and 620 nm (Dy3+, Sm3+, Pr3+ and also Mn2+) which likely enhance its yellow color. MDL hydroxylapatite likely formed from fluids derived from granitic rocks as evidenced by the chondrite-normalized REE patterns, Li, Mn and Y concentrations. The Sr content reflects the host-rock serpentinite composition. Fluids for its crystallization were likely derived from Muráň complex orthogneisses by the Alpine deformation and recrystallization in greenschist to lower amphibolite facies.

The Itabuna-Salvador-Curaçá Orogen revisited, São Francisco Craton, Brazil: New zircon U–Pb ages and Hf data support evolution from archean continental arc to paleoproterozoic crustal reworking during block collision

Precambrian orogenic belts are key areas to understand how the continental crust formed and evolved with time, and to constrain when plate tectonics began on planet Earth. The Itabuna-Salvador-Curaçá Orogen, in the São Francisco Craton, is an orogenic belt over 800 km long that is interpreted as the result of collision of four blocks during the Paleoproterozoic. However, its geological history before the final continental collision and the time span of collision are not well constrained. Here, we present new field relationships, in situ zircon U–Pb ages and Hf isotope data for gneisses and granites from the orogen and one of the blocks (Gavião block) that collided during the Paleoproterozoic. Most of the samples were collected along a geological traverse across the northern area of the orogen. The geochronological results indicate ages of 3335 Ma and 3307 Ma for gneisses of the Mairi Complex (Gavião block), 2637 Ma, 2620 Ma and 2581 Ma for gneisses of the Caraíba Complex within the orogen, and ages between 2100 Ma and 2058 Ma for the collisional granitoids from the Itabuna-Salvador-Curaçá Orogen. The slightly negative εHf(t) values (−1.9 and −4.5) for samples of the Mairi Complex suggest contribution from older continental crust in the genesis of the gneisses protoliths. For the Caraíba Complex, positive to negative εHf(t) values (0.9 to −3.7) for the orthogneisses, coupled with similar whole-rock εNd(t) signature from the literature, suggest a continental arc setting for emplacement of the Caraíba Complex orthogneisses. The εHf(t) values (−12.1 and −13.4) for one collisional granitoid indicate it can have originated from the partial melting (reworking) of the Caraíba Complex crust. From the tectonic point of view, the ages of the Caraíba Complex become younger from west to east and this age variation can be interpreted in terms of a continental arc that formed by trench/slab retreat on the edge of the Gavião block. This model is supported also by the occurrence in the Gavião block of coeval (2595 ± 21 Ma) volcanic rocks in the Mundo Novo greenstone belt (interpreted in the literature as formed in a back-arc basin) and some metamorphosed clastic sedimentary rocks of the Saúde Complex with provenance solely from sources older than 2500 Ma. On the other hand, the ages of the collisional granites can be used to constrain the timing and duration of the orogen's evolution. Accordingly, the early collisional granites with ages between 2100 Ma and 2094 Ma set a minimum age for the beginning of collision. The syn-to late-transcurrent granites were emplaced from 2084 Ma to 2060 Ma, and the post-collisional granites from about 2058 Ma until 2027 Ma. Therefore, block collision to form the Itabuna-Salvador-Curaçá Orogen has lasted at least 70 million years.

Triassic magmatism and metamorphism in the Antarctic Peninsula: Identifying the extent and timing of the Peninsula Orogeny

The mid to Late Triassic marks an episode of magmatism, deformation and metamorphism along the proto-Pacific margin of the Antarctic Peninsula and Patagonia. Calc-alkaline magmatism at ~227 Ma developed in a convergent margin setting across the central Antarctic Peninsula and North Patagonian Massif. Two distinct deformation and metamorphic events have been identified at ~221 Ma and ~207 Ma based on new U–Pb zircon ages from a metamorphic orthogneiss complex, investigated for the first time, from the elevated plateau region of central Palmer Land. These two events are interpreted to date the timing of the multi-phase Peninsula Orogeny, which correlates with the Chonide Event of central Patagonia. The onset of deformation in the Antarctic Peninsula is linked to tectonic events in central Patagonia and is associated with non-collisional slab dynamic processes during flat slab subduction in the Late Triassic. No consistent tectonic regime is evident during the mid-to Late Triassic episodes of deformation and metamorphism, but an initial period of extension/transtension (mid-Triassic) and a compressional regime in the Late Triassic is favoured in the Antarctic Peninsula and Patagonia. The newly acquired metamorphic ages provide no clear evidence for a segmented terrane model for the Antarctic Peninsula, as Late Triassic metamorphic ages are identified from across the central and eastern domains. New U–Pb zircon ages are also presented from a suite of strongly foliated granitoids that cross-cut the Triassic orthogneisses. They are dated in the interval 140–136 Ma and represent an episode of Early Cretaceous magmatism, transtensional deformation and widespread metamorphism across northwest and central Palmer Land.

Dom Feliciano Belt orogenic cycle tracked by its pre-collisional magmatism: The Tonian (ca. 800 Ma) Porto Belo Complex and its correlations in southern Brazil and Uruguay

The Porto Belo Complex orthogneisses are dominantly intermediate, high-K calc-alkaline rocks of Tonian age (ca. 800 Ma) that represent the hinterland basement of northern Dom Feliciano Belt. Their significance and correlation to other pre-collisional, Tonian orthometamorphic rocks of the same belt are discussed and integrated in an Occam’s Razor model for the evolution of the Kaoko-Dom Feliciano-Gariep Orogenic System. The model proposes a single, eastward subduction with temporal variations of subduction angle leading to migration of arc-magmatism before the main collision, in the following stages: a) 925–880 Ma early subduction and forearc magmatism; b) 820–770 Ma flat-slab subduction and arc-magmatism migration towards the back-arc region, with generation of arc-related, Tonian magmatic protoliths; c) 765–700 Ma steepening subduction and arc-magmatism migration towards forearc, as the back-arc area is under extension, and rifting is triggered; d) 700–660 Ma accretionary phase, after final consumption of the oceanic plate; e) 660–640 Ma oblique collision leading to basin inversion and double-verging transpression, with hinterland high-T/low-P metamorphism and exhumation; f) 640–580 Ma post-collisional magmatism and shear belt development.

Whole-rock and mineral chemistry characterization of contrasting granitoids, constraints on the source of the Vieirópolis NYF-type pegmatites, Northeastern Brazil

The Vieirópolis pegmatite field is characterized by amazonite pegmatites inserted in the Borborema Province. The Serra Branca pegmatite is hosted by the Serra Branca granites, which intruded partially the Serra Negra granites, and stands out by its amazonite mineralization. To comprehend the crystallization conditions and the source of this pegmatite and associated Serra Branca and Serra Negra granites, petrographic, mineral chemistry, geochronological and whole-rock geochemical data were used. The Serra Branca pegmatite is Cs-, Rb-, Pb-, Nb-, and Ta-rich. The Serra Branca granites (563 ± 2 Ma) are metaluminous to weakly peraluminous, crystallized under high fO2 conditions, pressure of 2.68-2.80 kbar and temperatures ranging from 667-670°C, with many zircon grains showing inherited cores of Paleoproterozoic ages. The Serra Negra granites (594 ± 4 Ma) comprises metaluminous granites crystallized under high fO2 conditions, temperatures within 711-751°C, pressure varying from 4.72 to 5.42 kbar. Both granites resulted from distinct partial melting degrees of a source similar to the Caicó Complex orthogneisses. The Serra Branca pegmatite geochemical data suggests that it was originated by extreme fractionation of the Serra Branca granites magma. The Serra Branca amazonite pegmatite is the first pegmatite mineralogically and geochemically characterized as Nb-Y-F (NYF)-type from the gadolinite subtype in the Borborema Province.

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.

Building Mesoarchaean crust upon Eoarchaean roots: the Akia Terrane, West Greenland

Constraining the source, genesis, and evolution of Archaean felsic crust is key to understanding the growth and stabilization of cratons. The Akia Terrane, part of the North Atlantic Craton, West Greenland, is comprised of Meso-to-Neoarchaean orthogneiss, with associated supracrustal rocks. We report zircon U–Pb and Lu–Hf isotope data, and whole-rock geochemistry, from samples of gneiss and supracrustals from the northern Akia Terrane, including from the Finnefjeld Orthogneiss Complex, which has recently been interpreted as an impact structure. Isotope data record two major episodes of continental crust production at ca. 3.2 and 3.0 Ga. Minor ca. 2.7 and 2.5 Ga magmatic events have more evolved εHf, interpreted as reworking of existing crust perhaps linked to terrane assembly. Felsic rocks from the Finnefjeld Orthogneiss Complex were derived from the same source at the same time as the surrounding tonalites, but from shallower melting, requiring any bolide-driven melting event to have occurred almost simultaneously alongside the production of the surrounding crust. A simpler alternative has the Finnefjeld Complex and surrounding tonalite representing the coeval genesis of evolved crust over a substantial lithospheric depth. Hafnium isotope data from the two major Mesoarchaean crust-forming episodes record a contribution from older mafic Eoarchaean crust. Invoking the involvement of an Eoarchaean root in the growth of younger Mesoarchaean crust puts important constraints on geodynamic models of the formation of the discrete terranes that ultimately assembled to form Earth’s cratons.

Characterising the southern part of the Hearne Province: A forgotten part of Canada’s shield revisited

The Hearne craton comprises the southeastern part of the western Churchill craton. The central part of the craton has been well studied in Nunavut and northeastern Saskatchewan in comparison with the southern part of the craton, which is mainly exposed south of the Athabasca Basin in Saskatchewan. Mapping and analytical results indicate that the southern Hearne craton is dominated by a felsic orthogneiss complex derived mainly from tonalites with crystallisation ages in the range 2.81–2.78 Ga and ca. 2.73 Ga. Sm-Nd isotopic analysis suggests that these evolved plutons (mean ƐNd = −3.3) were derived from a Mesoarchean to Paleoarchean source. Other components of the orthogneisses include leucotonalite intrusions dated at ca. 2.664 Ga and regionally extensive K-feldspar phyric granites emplaced between 2.63 and 2.60 Ga. Both of these suites have Sm-Nd isotopic signatures similar to the older tonalites. A suite of granodiorite and quartz diorite, one of which yielded a crystallisation age of 2.654 Ga, has unique Sm-Nd isotopic results with ƐNd values near CHUR. The felsic orthogneiss complex is unconformably overlain by the Black Birch Lake assemblage, which contains dominantly orthquartzites, mafic volcanic rocks and pelites. Detrital zircon signatures in the orthoquartzites are dominated by Archean sources and indicate that the supracrustal rocks were deposited after 2.60 Ga.The central Hearne Province extends into northeastern corner of Saskatchewan. It contains oceanic mafic volcanic rocks of the Ennadai Group that were intruded by juvenile tonalites at 2.71–2.70 Ga. The age and isotopic character of these tonalites contrasts markedly with those in the felsic orthogneiss complex of the southern Hearne Province. A comparison of plutonic suites and supracrustal assemblages of the central and southern Hearne Provinces suggests that there is heterogeneity between the two entities. Analysis of geophysical data, coupled with existing geological maps, suggest that an important tectonic boundary separates them. The current age relationships in the central and southern parts of the Hearne province imply that the two were juxtaposed after 2.60 Ga; however, the time of suturing remains equivocal.

U-Pb and Lu-Hf isotopes of the Archean orthogneiss complex on eastern Hall Peninsula, southern Baffin Island, Nunavut: Identification of exotic Paleo- to Mesoarchean crust beneath eastern Hall Peninsula

Eastern Hall Peninsula on southeastern Baffin Island, lies at the junction of a complex Paleoproterozoic collision between the Rae craton, Meta Incognita microcontinent and the North Atlantic craton from ca. 1.88 to 1.80 Ga. Several different interpretations of crustal correlations and the location of intervening sutures have been proposed based on reconnaissance-scale geologic investigation. Therefore, in this study, complex zircon grains from Archean orthogneiss units on eastern Hall Peninsula were analyzed in-situ to elucidate the detailed magmatic history of the region and assess crustal provenance.Magmatic zircons yielded U-Pb crystallization ages between ca. 2976 and 2720 Ma and metamorphic zircons record tectonothermal disturbances between ca. 2740 and 2700 Ma, a period coinciding with metamorphism documented in adjacent crustal blocks (e.g., west Greenland and northern Labrador). Magmatic rocks older than ca. 2740 Ma generally have positive εHf(t) signatures between 0 and 7 (±2) and depleted mantle model ages of ca. 3.1–3.0 Ga indicating the time that protolith melt was extracted from the mantle. The oldest, granodioritic crust crystallized at ca. 2976 Ma and was then reworked periodically at ca. 2.93, 2.84–2.81 and 2.77–2.69 Ma. Zircons from two orthogneiss samples, with U-Pb crystallization ages younger than ca. 2740 Ma, yielded negative εHf(t) values ranging from −4 to −12 and mean depleted mantle model ages of ca. 3.4 and 3.3 Ga respectively, indicating derivation from an older, potentially exotic, crustal source yet to be identified in outcrop on Hall Peninsula.Synthesizing regional U-Pb data we propose a new regional correlation model that retains the essentials of previous models and incorporates new data from eastern Hall Peninsula and other recent studies. This new tectonic correlation model groups eastern Hall Peninsula, southern Cumberland Peninsula and the Aasiaat domain into a “Core zone” that shared a geologic history prior to 1.90 Ga and potentially prior to 2.75 Ga.

The Cauaburi magmatic arc: Litho-stratigraphic review and evolution of the Imeri Domain, Rio Negro Province, Amazonian Craton

A lithostratigraphic review of the Cauaburi Complex was carried out by means of field, tectono-metamorphic and geochemical data, which were the basis for the sub-division of the Cauaburi Complex orthogneisses into the Santa Izabel do Rio Negro, Cumati and São Jorge facies. These rocks crop out between São Gabriel da Cachoeira and Santa Izabel do Rio Negro, Amazonas, Brazil. The gneisses of the Santa Izabel do Rio Negro and Cumati facies are metaluminous and of calc-alkaline affinity; in turn, the rocks of the São Jorge facies are peraluminous and of alkaline affinity. They vary from (amphibole)-biotite granodiorites/monzogranites (Cumati and Santa Izabel do Rio Negro facies) to spessartite-bearing biotite monzogranites (São Jorge facies). The Cauaburi Complex geochemical signature is compatible with that of granites generated in collisional settings (magmatic arc?) and its evolution is related to three distinct tectono-metamorphic events: D1, causing foliation S1, which developed during the Cauaburi Complex syn-tectonic emplacement in the Cauaburi Orogeny; D2/M2, causing foliation S2, which was generated under amphibolite facies conditions (717.9 °C and 5.84 kbars), and the emplacement of I- and S-type granite during the Içana Orogen, and low-temperature D3, associated with the K'Mudku Event, which caused foliation S3 and reworking via transcurrent shear zones under greenschist facies conditions.

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.

The Connemara Antiform and its possible Silurian history

The Connemara Antiform has iconic status in the geology of western Ireland. With its eastto-west trend and eastward plunge and the disposition of Dalradian stratigraphy about its axis, it forms the dominant structure in the present-day landscape and is a focal feature of geological maps of the area. Its genesis has long been ascribed to D4, the last main major deformation phase of the Ordovician Grampian Orogeny that affected the Dalradian rocks of Connemara. But does the structure have such a straightforward and simple origin? This paper investigates the possible role of middle late Silurian deformation in the origin of the Connemara Antiform. Overview of the geology of Connemara Connemara was part of the south-eastern Laurentian margin Dalradian sequence that experienced a major short-lived early Ordovician orogenic episode, the Grampian Orogeny, caused by collision with, and obduction of, an arc that developed above oceanward-directed subduction (Chew 2009). The Connemara Terrane became detached from the Laurentian margin and now lies to the south of the obducted arc and the South Mayo Trough (Fig. 1). The translation of the Connemara Terrane from the Laurentian margin to its current position is indicated by an influx of coarse clastic rocks of Connemara provenance into the c. 464Ma Bunnacunneen Member of the Mweelrea Formation of the South Mayo Trough (McConnell et al. 2009). Dewey (2005) provides a succinct account of the local plate-tectonic, metamorphic and structural history of the Connemara Terrane and the South Mayo Trough during the Grampian Orogeny, and Graham et al. (1989), Leake and Tanner (1994), McConnell et al. (2002) and Long et al. (2005) provide a comprehensive overview of the detailed geology of the area. The Grampian Orogeny began with the subduction of rifted Laurentian margin sediments beneath a continent-facing oceanic arc c. 478Ma (Dewey 2005). The Connemara Dalradian sediments were buried in a thick nappe pile of D2 and D3 folds and experienced high-pressure, lowtemperature Barrovian metamorphism (Leake and Tanner 1994). Palaeomagnetic studies indicate that the D3 folds originally dipped c. 208 south (Robertson 1988). Between 474Ma and 470Ma (Friedrich et al. 1999) a suite of metagabbros and orthogneisses (the Metagabbro Orthogneiss Complex) were injected into the deforming nappe pile during D2 D3 in an east west belt that extends for more than 80km along the southern margin of the terrane (Fig. 1). The high-temperature metagabbros and orthogneisses drove sillimanite-grade, hightemperature, low-pressure Buchan metamorphism. Migmatites resulting from the metagabbro heat pulse formed at c. 6kbar but crystallised at much lower pressures of c. 3.5kbar, indicating rapid exhumation (Barber and Yardley 1985). Rapid exhumation may have been facilitated by extensional collapse. The Renvyle Bofin Slide, a lowangle post-D3 extensional detachment, has, on the Irish Journal of Earth Sciences 32 (2014), 71 78 doi: 10.3318/IJES.2014.32.71 # 2014 Royal Irish Academy

Uvá complex, the oldest orthogneisses of the Archean-Paleoproterozoic terrane of central Brazil

The Archean-Paleoproterozoic terrane of central Brazil is an exotic and allochthonous part of the Tocantins Province, a large Brasiliano/Pan-African orogen of the South American Platform formed during the Brasiliano orogeny. The terrane amalgamated to the province during the late stages of the orogeny as a crustal segment consisting of six Archean orthogneiss complexes and five low-grade metamorphic, in part Paleoproterozoic (Rhyacian) greenstone belts. The Uvá complex is the southernmost orthogneiss association of the Archean-Paleoproterozoic terrane of central Brazil. New U–Pb LA-ICP-MS data from zircon crystals show that the complex formed at least during two magmatic stages. The older consists of polydeformed tonalite and granodiorite batholitic and diorite stock protoliths with igneous age of 3040 Ma to 2930 Ma. The youngest comprises tonalite, monzogranite and granodiorite tabular bodies formed between 2876 and 2846 Ma. As compared to the orthogneisses of the northern portion of the terrane, both the oldest and youngest granitogenesis stages of the Uvá complex are, in average, about 150 Ma older. This suggests that the northern and southern orthogneisses formed during different times as independent crustal segments, but when and why they amalgamated is still under investigation.

Geoquímica e petrogênese do Plúton Serra da Macambira, Neoproterozoico da Faixa Seridó, Província Borborema (NE do Brasil)

O plúton Serra da Macambira intrude ortognaisses do Complexo Caicó e metassupracrustais do Grupo Seridó, no Domínio Rio Grande do Norte da Província Borborema, compreendendo enclaves intermediários (quartzo monzonitos e biotita tonalitos), monzogranitos porfiríticos, sienogranitos e monzogranitos equigranulares, diques tardios de granitos e pegmatitos. Os quartzo monzonitos contêm microclina, plagioclásio, biotita, hornblenda e pouco quartzo, enquanto os biotita tonalitos não apresentam hornblenda. Os granitos porfiríticos e equigranulares portam biotita e raramente hornblenda, mostram texturas mirmequítica e pertítica, além de plagioclásio zonado, que caracteriza a relevância da cristalização fracionada na sua evolução. Os diques graníticos são hololeucocráticos, com textura granofírica indicando condições de colocação hipabissais. Os granitos equigranular e porfirítico mostram características geoquímicas similares, sendo interpretados como produtos de diferenciação do mesmo magma. Esses granitos apresentam padrão de Elementos Terras Raras (ETR) com anomalia negativa de Eu, enriquecimento em ETR leves, razão (La/Yb)N entre 8,9 e 24,6 e YbN entre 8,2 e 45,4, variando de ligeiramente metaluminosos a ligeiramente peraluminosos e seguindo a trajetória cálcio-alcalina de alto-K. Modelos geoquímicos utilizando elementos maiores e traços sugerem que o magma foi formado a partir de 20-35% de fusão parcial da crosta continental inferior, deixando um resíduo composto por K-feldspato, plagioclásio, quartzo, hornblenda, ortopiroxênio, apatita, magnetita, ilmenita e zircão. A evolução magmática envolveu 20-25% de cristalização fracionada, gerando um cumulato de plagioclásio, K-feldspato, biotita, hornblenda, titanita, magnetita, apatita, zircão e alanita, formando as rochas que constituem o plúton. Diagramas discriminantes aliados a feições texturais e estruturais permitem posicioná-lo em contexto de geração e colocação tardi- a pós-tectônico, durante o colapso da orogênese Brasiliana.

Caracterização estrutural e petrológica de metatonalitos e metadioritos do Complexo Arroio dos Ratos na sua seção-tipo, região de Quitéria, RS

O Complexo Arroio dos Ratos faz parte do embasamento do Batólito Pelotas, no Escudo Sulrio-grandense. O mapeamento geológico-estrutural de sua seção-tipo, na região de Quitéria, RS, levou à identificação de um conjunto de rochas tonalíticas a dioríticas foliadas, denominado Associação 1 (A1), intrudido discordantemente por rochas de composição similar. O embasamento destes tipos litológicos compreende gnaisses calcissilicáticos, gnaisses tonalíticos e rochas metavulcano-sedimentares. A A1 é composta por granada-biotita metatonalitos de granulação média a grossa e por metadioritos a metatonalitos de granulação fina, com granada restrita aos termos mais diferenciados. As relações de campo indicam que os termos finos constituem diques sinplutônicos intrudidos nos demais. Em ambos os grupos, a textura granoblástica é atribuída à recristalização sob temperaturas compatíveis com as da fácies anfibolito superior a granulito. A foliação S tem direção NW-SE a EW e a sua dispersão é atribuída 1 principalmente aos efeitos das dobras F2 , cujo eixo tem caimento baixo a médio para WSW, coincidente com a posição da lineação de estiramento, LX . Retirando-se os efeitos das dobras F2 , a geometria original da foliação na A1 teria sido sub-horizontal, com mergulho suave para WSW. O sentido de movimento dado por estruturas de macro e microescala indica topo para E. Os metatonalitos e metadioritos da A1 são meta a peraluminosos e pertencem à série cálcio-alcalina de médio a alto-K. O comportamento dos elementos maiores e traços e o padrão de fracionamento moderado dos ETR, com enriquecimento dos leves em relação aos pesados, indicam ambiente de arco continental maduro para o magmatismo desta associação e a mesma é comparável a associações TTG. Dados preliminares situam a A1 no Paleoproterozóico e o conjunto das informações permite compará-las aos ortognaisses do Complexo Encantadas, embora estes registrem com mais intensidade o metamorfismo regional orogênico. Assim, presume-se que as rochas da A1 representem o registro da fase mais madura do mesmo arco continental onde teria se gerado o Complexo Encantadas.

A Laurentian provenance for the Dalradian rocks of north Mayo, Ireland, and evidence for an original basement–cover contact with the underlying Annagh Gneiss Complex

Abstract: Metasediments of the early Dalradian Grampian Group (Erris Group) and probable equivalents (Inishkea Division) structurally overlie Palaeoproterozoic to Neoproterozoic Annagh Gneiss Complex orthogneisses in NW Mayo, Ireland. Sm–Nd isotopic data suggest a Palaeoproterozoic source for the metasediments. K-feldspar and granitoid clasts from the Doonamo Formation, Erris Group, record U–Pb zircon ages of c . 1740 Ma and c . 980 Ma, respectively. These ages are within error of the c . 1730–1750 Ma Mullet gneisses and c . 990 Ma Grenvillian migmatitic leucosomes in the underlying Annagh Gneiss Complex. U–Pb detrital zircon data reveal that the Erris Group was deposited after c . 955 Ma, with predominant input from c . 1640, c . 1500 and c . 990 Ma interpreted Laurentian sources (Labradorian, Pinwarian and Grenvillian terranes, respectively). Limited detrital zircon data from the Inishkea Division yield similarly aged detritus and a tentative maximum depositional age of c . 1005 Ma. Correlation of both sequences with the Scottish Dalradian Grampian Group is considered valid based on detrital zircon U–Pb and whole-rock Sm–Nd data. The clast ages support the hypothesis that the Dalradian unconformably overlies the Annagh Gneiss Complex in north Mayo, whereas the detrital zircon data imply more distal Laurentian sources. Dalradian deposition is thereby tied to the margins of Laurentia. Supplementary material: U–Pb zircon (secondary ion mass spectrometry) data and descriptions of dated zircon grains are available at http://www.geolsoc.org.uk/SUP18414 .

The complex age of orthogneiss protoliths exemplified by the Eoarchaean Itsaq Gneiss Complex (Greenland): SHRIMP and old rocks

Field studies integrated with cathodoluminescence petrography and SHRIMP U–Pb dating of zircons from >150 orthogneisses and metatonalites from the Eoarchaean Itsaq Gneiss Complex (southern West Greenland) shows that only a minority contain ≥3840 Ma zircons, whereas the majority carry only younger ones. Rocks containing ≥3840 Ma zircons vary from very rare single-phase metatonalites to more common complexly banded tonalitic migmatites. The former metatonalites have simple oscillatory-zoned ≥3840 Ma zircon with limited recrystallisation and overgrowth, whereas the more common migmatites have much more complicated zircon populations with both ≥3840 Ma and 3650–3600 Ma oscillatory-zoned zircon, more extensive recrystallisation and widespread complex core-rim multiple growth relationships. With only 100–160 ppm Zr in the tonalites and likely melt generation temperatures of >1000 °C, the experimentally determined zircon solubility–melt composition relationships established by other workers shows that the precursor melts to the Itsaq Complex tonalites were strongly undersaturated in zircon, thus any entrained xenocrystic zircon would have been rapidly dissolved. Therefore, the ≥3840 Ma oscillatory-zoned zircons crystallised out of tonalitic melt and gives magmatic age of the rock in which they occur. With an established igneous age of ≥3840 Ma established from such relationships, we interpret the correlated variation between the field nature of these rocks and their zircon petrography/age structure as due to superimposition onto ≥3840 Ma tonalite protoliths of variable amounts of heterogeneous strain, heterogeneous distribution of melt patches formed during in situ anatexis at up to ∼800 °C, plus granitic veining. This explains why geologically simple metatonalites have simple zircon populations, whereas complex orthogneisses have complex zircons. The large amount of integrated field, geochemical and zircon data rule out an alternative interpretation, that the ≥3840 Ma zircons represent an igneous xenocrystic component, present in younger rocks to varying degrees. If this were true, then the structurally simple (less reworked) rocks should still display complex zircon populations. Gneisses with ≥3840 Ma zircon are commonest on Akilia and neighbouring islands, in Itilleq fjord (∼65 km east Akilia) and on the north of Ivisaartoq (∼150 km northeast of Akilia). These include from Itilleq a 3891 ± 6 Ma gneissic tonalite (with minor neosome)—which is currently the oldest rock recognised in the Itsaq Gneiss Complex. Overall, the ≥3840 Ma tonalites are a widespread and unevenly distributed in the Itsaq Gneiss Complex, and they are a volumetrically minor component compared with ∼3800, 3750 and 3700 Ma tonalite generations. Using the subset of our data covering Itilleq and the neighbouring fjords, migmatite samples with ≥3800 Ma igneous zircon are mutually exclusive from migmatite samples with ∼3700 Ma igneous zircon. This suggests that prior to an amalgamation event followed by 3660–3600 Ma high-grade metamorphism, ≥3840 Ma tonalites might have resided in a terrane discrete from ∼3700 Ma tonalites. This is in accord with interpretation of the non-migmatised part of the Complex in the Isua area, where a terrane of ∼3800 Ma tonalites with a minor associated ≥3840 Ma component and a terrane with ∼3700 Ma tonalites were tectonically juxtaposed at ∼3660 Ma.

Mesoarchean assembly and stabilization of the eastern Kaapvaal craton: A structural‐thermochronological perspective

Understanding the construction and stabilization of Archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early Earth, as well as for the subsequent evolution of continents. We provide new constraints for the amalgamation and stabilization of the eastern Kaapvaal craton in the vicinity of the Barberton greenstone belt circa 3.3–3.1 Ga. Isotope dilution thermal ionization mass spectrometry U‐Pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high‐grade orthogneiss terranes from low‐grade supracrustal rocks and to relate this movement to the timing of differential exhumation. New and existing data are consistent with accretion at ∼3.23 Ga within an oblique subduction plate boundary manifested in the BGB as an asymmetric flower‐like structural geometry. For >100 Ma following terrane assembly, transform boundary tectonics, manifested partially as transtension on reactivated faults, led to large‐scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths. Stabilization of this portion of Archean lithosphere involved at least a three‐stage process: (1) creation of a thick and rigid mantle lithosphere during crustal growth, subduction, and terrane assembly from circa 3.30 to 3.23 Ga, (2) generation of a rigid crust during strike‐slip and transtensional tectonics coupled with migration of granites and heat‐producing elements into the upper crust between circa 3.2 and 3.1 Ga, and (3) erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2.9 Ga.

Calc-Alkaline Magmatism at the Archean–Proterozoic Transition: the Caicó Complex Basement (NE Brazil)

The Paleoproterozoic metaplutonic rocks of the Caico* Complex Basement (Serido* region, NE Brazil) provide important and crucial insights into the petrogenetic processes governing crustal growth and may potentially be a proxy for understanding the Archean^ Proterozoic transition. These rocks consist of high-K calc-alkaline diorite to granite, with Rb^Sr, U^Pb, Pb^Pb and Sm^Nd ages of c. 225^215 Ga. They are metaluminous, with high YbN, K2O/Na2Oand Rb/Sr, low ISr ratios, and are large ion lithophile elements (LILE) enriched. Petrographic and geochemical data demonstrate that they belong to differentiated series that evolved by low-pressure fractionation, thus resulting in granodioritic liquids.We propose a model in which the petrogenesis of the Caico* Complex orthogneisses begins with partial melting of a metasomatically enriched spinel- to garnet-bearing lherzolite (with high-silica adakite melt as the metasomatic agent), generating a basic magma that subsequently evolved at depth through fractional crystallization of olivine, followed by low-pressure intracrustal fractionation. A subduction zone setting is proposed for this magmatism, to account for both negative anomalies in high field strength elements (HFSE) and LILE enrichment. Mantle-derived juvenile magmatism with the same age is also known in the SaÐ o Francisco andWest Africa cratons, as well as in French Guyana, and thus the Archean^Proterozoic transition marks a very important continental accretion event. It also represents a transition from slab-dominated (in the Archean) to wedge-dominated post-Archean magmatism.