Anorthosite-mangerite-charnockite-granite Research Articles

The origin of the Palaeoproterozoic AMCG complexes in the Ukrainian shield: New U-Pb ages and Hf isotopes in zircon

The Ukrainian shield hosts two Palaeoproterozoic anorthosite-mangerite-charnockite-granite (AMCG) complexes (the Korosten and Korsun-Novomyrhorod complexes) that intruded Palaeoproterozoic continental crust in north-western and central parts of the shield, respectively. We report results of U-Pb zircon and baddeleyite dating of 16 samples from the Korosten plutonic complex (KPC), and 6 samples from the Korsun-Novomyrhorod plutonic complex (KNPC). Fifteen zircon samples from both complexes were also analysed for Hf isotopes. These new, together with previously published data indicate that the formation of the KPC started at c. 1815Ma and continued until 1743Ma with two main phases of magma emplacement at 1800–1780 and 1770–1758Ma. Each of the main phases of magmatic activity included both basic and silicic members. The emplacement history of the KNPC is different from that of the KPC. The vast majority of the KNPC basic and silicic rocks were emplaced between c. 1757 and 1750Ma; the youngest stages of the complex are represented by monzonites and syenites that were formed between 1748 and 1744Ma. Both Ukrainian AMCG complexes are closely associated in space and time with mantle-derived mafic and ultramafic dykes. The Hf isotope ratios in the zircons indicate a predominantly crustal source for the initial melts with some input of juvenile Hf from mantle-derived tholeiite melts.The preferred model for the formation of the Ukrainian AMCG complexes involves the emplacement of large volumes of hot mantle-derived tholeiitic magma into the lower crust. This resulted in partial melting of mafic lower-crustal material, mixing of lower crustal and tholeiitic melts, and formation of ferromonzodioritic magmas. Further fractional crystallization of the ferromonzodioritic melts produced the spectrum of basic rocks in the AMCG complexes. Emplacement of the ferromonzodioritic and tholeiitic melts into the middle crust and their partial crystallization caused abundant melting of the ambient crust and formation of the large volumes of granitic rocks present in the complexes.

Meso-Neoproterozoic Grenville-Sveconorwegian intracontinental orogen: history, tectonics, geodynamics

The objective of this paper is to represent the main features inherent to Grenville-Sveconorwegian Orogen (GSNO) and to propose a model of tectonic and geodynamic evolution of this orogen based on the results of research concerning similar Precambrian tectonic units in the East European Craton. The studies of the conditions and settings related to origin and evolution of GSNO are of special interest, because it is located geographicaly and in a certain sense ideologically in the center of Rodinia, a supposed Neoproterozoic supercontinent. GSNO originated in the MezoNeoproterozoic in the inner region of the Lauroscandia continent. At present, the synformal tectonic structure of GSNO is divided into two portions: Grenville sector along the southeastern margin of the Canadian Shield, and Sveconorwegian sector in the southwestern Scandinavia. The integrity of Lauroscandia was twice disturbed in the MezoNeoproterozoic when oceanic structures resembling the Atlantic Ocean were formed. Later on, the continuity of the continent was restored with the involvement of oceanic lithosphere subduction and accretion and obduction of the island-arc and oceanic terranes. We distinguish two stages in the GSNO history: (1) ‘preparatory’ stage (from ~1.90 to ~1.16 Ga), and (2) formation of GSNO proper (from ~1.19 to ~0.90 Ga). The manifestations of granulite-facies metamorphism were repeatedly recorded before the Grenville Orogeny at 1.67–1.66, 1.47–1.45, 1.37–1.35, and 1.20–1.18 Ga. The Ottawan stage of the Grenville metamorphism proper is dated between 1.16 and 1.05–1.03 Ga. Metamorphism at the base of Allochthonous Belt corresponds to high-pressure granulite facies and, in a number of places, to hightemperature eclogite facies (800–900 °C at pressure in the range between 14 and 20 kbar). The age of metamorphism of rocks within Paraautochthonous Belt is 1.05–0.95 Ga; metamorphic grade increases from the greenschist facies near the Grenville front to the high-pressure amphibolite facies near Allochthon Boundary Thrust showing an inverted metamorphic zoning. High-pressure granulite-facies metamorphism is characteristic of Sveconorwegian sector; and high-temperature eclogites are observed locally at the base of the allochthonous complexes and within the paraautochthonous complexes. A distinctive feature of GNSO is the abundant occurrence of specific intrusive magmatism. Massifs of anorthosite-mangerite-charnockite-granite (AMCG) and anorthosite-rapakivi granite (ARG) complexes formed 1.8–1.5 Ga ago frame the orogen as a wide arc. In the internal region of GSNO, these complexes were formed successively at 1.16–1.13, 1.09–1.05, 0.99–0.96, and 0.93–0.92 Ga. Later on, after the intrusion, the massifs unevenly underwent granulite-facies metamorphism. The high-temperature magmatism and metamorphism, numerous repeated thermal pulses and enormous crustal body that underwent high-temperature transformation point to a mantle plume as the most adequate source of thermal energy. The model of intracontinental development of GSNO comes into conflict with popular ideas, which assume origination of this orogen as a result of the collision and welding of the formerly distant continents (Laurentia, Baltica and Amazonia), which, as suggested, completed the assembly of Rodinia supercontinent. A conclusion is drawn that the concept of tectonic position and geodynamic evolution of GSNO, which is not a counterpart of the Tibet-Himalayan Orogen, should be revised.

New U–Pb ages for mafic dykes in the Northwestern region of the Ukrainian shield: coeval tholeiitic and jotunitic magmatism

The palaeoproterozoic Northwestern region of the Ukrainian shield hosts two compositional types of mafic dykes and associated magmatism that intruded at c. 1800–1760 Ma: (1) high-Ni dolerite dykes and layered intrusions of tholeiitic affinity and (2) high-Ti dolerite dykes of jotunitic affinity associated with anorthosite–mangerite–charnockite–granite (AMCG) suites. The jotunitic dykes represent initial melts for basic rocks of the Korosten AMCG plutonic complex, whereas tholeiitic dykes may reflect emplacement of a mantle plume and formation of a large igneous province (LIP). New U–Pb baddeleyite ages indicate that both compositional types can be coeval: the jotunitic Rudnya Bazarska dyke was emplaced at 1793 ± 3 Ma, and the Zamyslovychi tholeiitic dolerite dyke at 1789 ± 9 Ma. In our model, the mantle plume-derived tholeiitic melts (underplate) supplied heat required for melting of the mafic lower crust and the production of jotunitic melts. As formation of the jotunite melts requires pressures in the range 10–13 kbar, either a thickened crust is needed or the lower crust must be subducted, or downthrusted, into the mantle. Alternatively, emplacement and ponding of large volume of tholeiitic melts might cause delamination of the lower crust, its sinking into the mantle, and further fusion to produce jotunitic melts.

Metasedimentary melting in the formation of charnockite: Petrological and zircon U-Pb-Hf-O isotope evidence from the Darongshan S-type granitic complex in southern China

Charnockites are Opx-bearing igneous rocks commonly found in high-grade metamorphic terranes. Despite being volumetrically minor, they show a wide range in both bulk geochemistry and intensive parameters. They form a characteristic component of the AMCG (anorthosite–mangerite–charnockite–granite) suite, but their association with typical S-type granites is less well-known. The Darongshan S-type granitic complex (DSGC) in Guangxi Province, southern China, contains granites varying in mafic silicate mineral assemblages from Bt+Crd (Darongshan suite) to Opx+Grt+Bt+Crd (Jiuzhou suite) and Opx+Crd±Bt (Taima suite), corresponding to a geochemical transition from magnesian calc-alkalic to ferroan calc-alkalic. However, its genesis, even the accurate age of intrusion, remains highly contentious despite intensive research. In order to understand the genesis of charnockite and its genetic relationship with S-type granite; here, we first determined zircon U-Pb ages of each suite using a SIMS on the basis of a detailed petrological study. Zircon U-Pb ages show that all suites of the complex were emplaced contemporaneously at ca. 249Ma. Monazite apparent U-Pb ages are indistinguishable from zircon U-Pb ages within analytical error. Further in situ zircon Hf-O isotope analyses reveal that the granitic complex was dominantly derived from reduced melting metasedimentary rocks (δ18Ozircon=ca. 11‰; εHf(t)zircon=ca. −10; Δlog FMQ≤0; Mn in apatite oxybarometer) with rare material input from the mantle. The variation in δ18O (7.8‰–12.9‰) is more likely a result of hybridization, whereas that in εHf(t) (−31.9 to −1.8) is a result of both hybridization and disequilibrium melting. The variation in mineralogy and geochemistry may be interpreted as a result of entrainment of peritectic garnets from biotite-dehydration melting. Nevertheless, heat input from mantle through basaltic intrusion/underplating is considered to play a major role in high-temperature (>850°C) melting at mid-crustal levels (i.e. the cordierite stable field) for generation of the granitic complex. We interpret that the granites were intruded in a back-arc setting and basaltic magmatism was directly associated with slab roll-back and tearing during the latest Permian and early Triassic times.

The Late Mesoproterozoic Sirdal Magmatic Belt, SW Norway: Relationships between magmatism and metamorphism and implications for Sveconorwegian orogenesis

The Late Mesoproterozoic Sveconorwegian Province is commonly correlated with the continent-collision related Grenville Province in eastern Canada. Recently, however, the evolution of the Sveconorwegian Province in SW Norway has been strongly debated, casting doubt on a direct correlation between these provinces.Metamorphism in SW Norway has traditionally been interpreted as representing a main collisional event between ca. 1030 and 970Ma, followed by a contact metamorphic event at 930Ma. Magmatism has been grouped into a ‘syn-collisional’ suite at 1050–1035Ma, a ‘post-collisional’ suite at 980–930Ma, and an anorthosite–mangerite–charnockite–granite (AMCG) suite at 930Ma. New detailed mapping and geochronology in the area reveal a very different and much more complex evolution, and require re-evaluation of previously presented models. In this paper, we focus on the introduction and description of a newly discovered, ca. 200km×50km magmatic belt, the Sirdal Magmatic Belt (SMB). Previously mapped as granitic gneisses in many areas, the existence of this large, commonly undeformed and unmetamorphosed granitoid batholith was only recognized a few years ago (Slagstad et al., 2013a). Magmatism in this belt between 1060 and 1020Ma precedes and overlaps the main Sveconorwegian metamorphic event(s) that affected the region.Our observations of cross-cutting relationships between previously metamorphosed gneisses and SMB rocks indicate that at least one episode of amphibolite- to granulite-facies metamorphism occurred in the region during or prior to emplacement. A lack of widespread metamorphic overprinting and common preservation of igneous textures in most of the SMB indicate that high-grade Sveconorwegian metamorphism after ca. 1020Ma was local rather than regional in SW Norway.The orogenic evolution of SW Norway is characterized by emplacement of large volumes of granitic magma and more localized UHT metamorphism, which is quite different from the widespread, long-lasting metamorphic evolution observed in the Grenville Province, and may point to different tectonic regimes for the two provinces.

The source of Proterozoic anorthosite and rapakivi granite magmatism: evidence from combined in situ Hf–O isotopes of zircon in the Ahvenisto complex, southeastern Finland

The isotope compositions of massif-type anorthosites in Proterozoic anorthosite–mangerite–charnockite–granite (AMCG) complexes are commonly dominated by crustal values. Olivine-bearing anorthositic rocks in several AMCG suites have, however, been shown to display juvenile character, suggesting that variably depleted mantle reservoirs were involved in their genesis. A coupled in situ zircon Hf–O isotope dataset from the 1.64 Ga Ahvenisto AMCG complex in the 1.54–1.65 Ga Fennoscandian rapakivi granite–massif-type anorthosite province reveals correlated juvenile isotope signals (δ 18 O zrn = 5.4–6.6‰; initial ϵ Hf = −1.1 to +3.4) in the most primitive gabbroic rock type of the suite suggesting a depleted mantle origin for the anorthositic rocks. This signal is not as prominent in the more evolved co-magmatic anorthositic rocks (δ 18 O zrn = 6.3–7.8‰; initial ϵ Hf = −0.8 to +2.0), most probably owing to contamination of the mantle-derived primary magma by crustal material. A rapakivi granite associated with the anorthositic rocks has different isotope composition (δ 18 O zrn = 7.4–8.6‰; initial ϵ Hf = −2.1 to +0.5) that points to a crustal source. Supplementary material: Full analytical results and details and SEM images of the analysed grains with spatial information on both methods are available at: http://www.geolsoc.org.uk/SUP18780 .

Late Paleoproterozoic charnockite suite within post-collisional setting from the North China Craton: Petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes

Abstract Charnockites (pyroxene-bearing granitoids) of magmatic origin in diverse tectonic settings and ranging in age from Mesoarchean to Cretaceous constitute important components of the continental crust. Here we report charnockites displaying both magnesian and ferroan compositions associated with gabbros from an AMCG (anorthosite–mangerite–charnockite–granite) suite in the North China Craton. The orthopyroxene in the magnesian charnockite is characterized by moderate X Mg of 0.63–0.65 (Wo 1–2 En 62–63 Fs 35–36 ), and low Al 2 O 3 content of 0.59–0.71 wt.%. The magnesian charnockites show medium- to high-K contents, and high Mg# (~ 47–69) similar to that of gabbros, whereas the Mg# of the ferroan charnockites is low (~ 6–28). The ferroan charnockites are alkali-calcic to alkalic, and weakly peralkaline to metaluminous, whereas the magnesian charnockites are calcic to calc-alkalic, and metaluminous. Although magnesian charnockites are in general considered to have formed in subduction setting, the medium- to high-K contents, high Mg# values with a wide range, and the highly negative eHf values of the zircons in these rocks (− 8. 4 to − 13.6), suggest inheritance of the arc signature from the melting of ancient arc-related crustal material. The ferroan charnockites show tholeiitic affinity and define a common differentiation trend with the gabbroic anorthosites and likely represent fractionated end-members with or without crustal interaction in a post-collisional rift setting. We present U–Pb age data from zircon grains on seven samples including two ferroan charnockites, three magnesian charnockites, one gabbroic enclave in magnesian charnockite and one gabbroic anorthosite which show emplacement ages of 1748.8 ± 6.4 Ma, 1747.1 ± 9.5 Ma, 1756.4 ± 7.3 Ma, 1756.7 ± 9.2 Ma, 1731 ± 17 Ma, 1731.6 ± 8.2 Ma and 1746.5 ± 7.3 Ma respectively. The negative eHf values (− 1.2 to − 13.6) of zircon grains from these rocks and the older crustal model ages ranging from Mesoarchean to Paleoproterozoic suggest that the magma sources of these rocks involved the melting of ancient crustal components. The age data suggest that the magmatic suite was emplaced within a relatively short time interval between1.73 and 1.76 Ga, during late Paleoproterozoic, placing the rocks suite in a post-collisional scenario, following the amalgamation between the Eastern and Western Blocks of the North China Craton along the Trans-North China Orogen at ca. 1.85–1.80 Ga.

Tectonic reactivation of transverse basement structures in the Grenville orogen of SW Quebec, Canada: Insights from gravity and aeromagnetic data

NW-SE trending, transverse lineaments, including the lithospheric-scale Mont-Laurier lineament, are interpreted from regional Bouguer gravity of the Grenville orogen of SW Quebec and adjacent Superior Craton in southeastern Canada. These lineaments, transverse to the ENE trending Grenville orogen, are inferred to correspond to Palaeoproterozoic structures in Archaean basement that have played an important role: (i) in the development of volcano-sedimentary back-arc basins along this segment of the Laurentian margin; (ii) on the geometry of thrust sheets and folds formed during thrusting in the ca. 1.23–1.2Ga Elzevirian orogeny and incorporation of the basins within the orogen; (iii) on reorientation of early-formed structures in the Central Metasedimentary Belt of Quebec (CMB-Q) during ca. 1.19–1.17Ga post-Elzevirian orogenic collapse; and (iv) for development of syn-plutonic deformation corridors and shear zones at the onset of the emplacement of the Morin anorthosite-mangerite-charnockite-granite (AMCG) suite. In the CMB-Q, a 100km wide megakink zone developed during ca. 1.19–1.17Ga differential, post-Elzevirian orogenic collapse in the upper-most nappe above transverse sinistral shear corridors 10–20km wide located in an underlying thrust sheet or “lower-deck”. Emplacement of 1.17Ga Chevreuil intrusive suite preferentially occurred within the megakink zone, starting late in the post-Elzevirian collapse and culminating during a switch to local shortening early in (and in part as a consequence of) the emplacement of the voluminous Morin anorthosite and associated AMCG-suite plutons. The Labelle deformation zone separating the CMB-Q and Morin terrane is interpreted as a post-Shawinigan, reverse shear zone that truncates folded lithological layering in the eastern CMB-Q and western Morin terrane that is either subsequently folded above the Mont-Laurier lineament during its further reactivation, or developed as a curved shear zone stepping across the Mont-Laurier lineament. The Grenville Province of SW Quebec therefore provides an example of strain partitioning and distinct deformation responses at different crustal levels during reactivation of basement structures.

Late Palaeoproterozoic mafic dyking in the Ukrainian Shield of Volgo-Sarmatia caused by rotation during the assembly of supercontinent Columbia (Nuna)

The Ukrainian Shield comprises the exposed crust of the large Palaeoproterozoic protocraton Volgo-Sarmatia, which together with the Fennoscandian crustal segment constitutes the East European Craton (“Baltica”). Geological and geophysical data indicate that 1.80 to 1.75Ga mafic dykes related to anorthosite–mangerite–charnockite–granite (AMCG) plutons are widespread within the Ukrainian Shield. We examined their ages, distribution patterns, orientations and compositions in three different crustal blocks (Volyn, Ingul and Azov), and found close spatial relationships with major strike-slip fault systems developed during two phases of extension. The early, 1.80–1.77Ga, generation of mafic dykes mostly follows NW (330±20°) and more rarely N–S- or E–W-trending faults corresponding to major NE–SW extension (the Submoshorino phase). These dykes contain olivine dolerites, picrites, camptonites, lamprophyres, kimberlites and other rocks belonging to tholeiitic and subalkaline jotunitic series. The compositions of these dykes differ between the host blocks, but all feature upper mantle geochemical signatures such as high contents of Ni and Cr, and positive values of εNd(1800) up to +2.8. High degrees of REE fractionation indicate deep levels of mantle melting, which is particularly characteristic of the Ingul block as marked by the most extensive and dense mafic dyke swarms. The later, 1.76–1.75Ga, dyke swarms occur close to the most voluminous AMCG suites of similar age and were emplaced during the second (Korsun) phase of faulting when all the older strike-slip fault zones were reactivated and partly transformed to tensional faults by E–W extension. These dyke swarms mainly trend 030±20°. They are jotunitic and their isotopic signatures indicate a greater participation of crustal sources in the parent melts. The overall transtensional tectonic setting of the mafic dyking associated with the AMCG magmatism in Volgo-Sarmatia was created by convergent tectonics and postcollisional collapse of the thickened lithosphere, as well as by mantle delamination coupled with the rotation of Volgo-Sarmatia between 1.80 and 1.75Ga. This agrees with palaeomagnetic reconstruction suggesting rotation(s) of Volgo-Sarmatia during its protracted oblique docking with Fennoscandian terranes and Laurentia as supercontinent Columbia (Nuna) was assembled.

Reconnaissance geochronology and geochemistry of the Mont-Tremblant gneiss of the Morin terrane, Grenville Province, Québec

The Mont-Tremblant gneiss is a granulite-facies quartzofeldspathic orthogneiss unit in Morin terrane of the southwestern Grenville Province. These gneisses are the oldest rocks in the Morin terrane and country rocks to intrusions of the 1.15 Ga anorthosite-mangerite-charnockite-granite (AMCG) suite. The Mont-Tremblant gneiss may also be basement to the Grenville Supergroup, but until now it has not been directly dated. Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) geochronology of zircon from two penetratively deformed samples of Mont-Tremblant gneiss yields igneous ages of 1324 ± 38 Ma and 1333 ± 32 Ma (2σ), and one sample shows metamorphic zircon growth during the Shawinigan orogeny at 1159.4 ± 15.6 Ma. Geochemically, Mont-Tremblant gneiss samples are calc-alkaline, granitic, and show hallmarks of an evolved arc environment. These characteristics are consistent with the Mont-Tremblant gneiss being correlative with the Geon 14 Lacoste and Bondy suites in the Central Metasedimentary Belt of Quebec to the west, but contrast with the more juvenile tonalitic Geon 14 calc-alkaline suites in the Adirondack Highlands and other Grenville outliers in the Appalachian Mountains. This transition may reflect differences in the lithosphere along strike of the Laurentian margin during Geon 14 subduction.

Tectonic implications of the discovery of a Shawinigan ophiolite (Pyrites Complex) in the Adirondack Lowlands

The Pyrites Complex in the Adirondack Lowlands domain of the Grenville Province forms the core of a large NE-trending, elongate, winged structure ∼15 km long dominated by highly deformed metagabbro, amphibolite, and hornblende schist. A previously unrecognized km-scale boudin of metamorphosed ultramafic rocks associated with the belt is described. It is largely replaced by secondary hydrous minerals, but retains relict igneous textures and some primary minerals such as augite and chromite, and is cut by several 1–2-m-wide lamprophyre dikes. The ultramafic rocks are interpreted as part of an obducted ophiolite complex on the basis of its structural contact with a belt of rocks including marine metasedimentary rocks, pyritic gneisses, metagabbros, and amphibolites with mid-ocean ridge basalt chemistry and were emplaced within a collapsing backarc basin of Shawinigan age. Small (50–200 μ) zircon crystals separated from peridotite and pyroxenite yield minimum ages (1140 ± 7 and 1197 ± 5 Ma) and constrain the timing of metamorphic and thermal events associated with the Shawinigan orogenesis and anorthosite-mangerite-charnockite-granite (AMCG) plutonism. Neodymium T DM ages from the Pyrites Complex range from 1440 to 2650 Ma, are not compatible with derivation from a typical depleted mantle reservoir, and suggest, along with incompatible element concentrations, that these rocks record mantle enrichment, presumably during subduction beneath the leading edge of Laurentia. Rifting and development of oceanic crust between the southern Adirondack Highlands and Lowlands, coincident with a similar backarc rifting in the Central Metasedimentary Belt at ca. 1300 Ma, are proposed. Three mafic suites in the Adirondack Lowlands are distinguished by their field relations, age, geochemistry, and Nd isotopic systematics and reflect the various stages of evolution of the Trans-Adirondack backarc basin. Within the Lowlands interleaved evaporites, metasedimentary and possible metavolcanic rocks, calc-alkaline and transitional plutonic rocks, and ophiolitic rocks of the Pyrites Complex provide constraints on the tectonic processes and sedimentary response to development of a backarc basin, magmatic arc, foredeep sedimentation, and ophiolite obduction during Shawinigan convergence from ca. 1200–1160 Ma, which culminated in slab breakoff and plate delamination resulting in intrusion of the AMCG suite throughout the Frontenac-Adirondack terrane and beyond.

Differentiating Shawinigan and Ottawan orogenesis in the Central Adirondacks

The rocks at Chimney Mountain provide a rare glimpse into primary intrusive relations and exceptionally well-preserved pre-Shawinigan metasedimentary rocks in the Adirondack Highlands despite a strong Ottawan thermal overprint. A near vertical contact between granite (ca. 1172 Ma) and a shallowly dipping and structurally intact sequence of quartzose to calc-silicate metasedimentary rocks is exposed on the southern flank of Chimney Mountain in the Central Adirondacks. The contact is marked by foliation truncation and a metasomatic aureole with randomly orientated porphyroblasts of enstatite rimmed by anthophyllite (max. 5 cm) and phlogopite (max. 2 cm), and a zone of granular, quartz-rich rock. The granite is non- to weakly foliated and has a shallow, north-plunging, mineral lineation as do the metasedimentary rocks. Zircons separated from a diopside-bearing quartzite (82% SiO2; 0.75 m thick) are of variable size (up to 400 μm), equant, and contain, on average, >1000 ppm uranium. Scanning electron microscope investigation indicates that there is little variation in a uniformly dark cathodoluminescence response, no discernible cores or rims, few inclusions, and partially faceted to round morphologies. Zircon U-Pb sensitive high-resolution ion microprobe (SHRIMP II) ages of 1042 ± 4 Ma and 1073 ± 15 Ma are coincident with Ottawan metamorphic ages from the Adirondack Highlands. Zircons from the intrusive granite yield large cores with typical anorthosite-mangerite-charnockite-granite (AMCG) ages (ca. 1171.6 ± 6.3 Ma) and sparse, thin younger rims (ca. 1060–1090 Ma) readily distinguishable by cathodoluminescence. Despite the younger zircon ages, the metasedimentary rocks and their fabric must predate the crosscutting granite. The thermal effect of the Ottawan event was likely enhanced by volatile fluxing and resulted in recrystallization and resetting of zircons in the metasedimentary rocks. However, it had limited effects on zircons in the granite and produced only thin metamorphic rims emphasizing the importance of local geochemical conditions to the response of zircon to metamorphism. Elzevirian or Shawinigan fabrics are preserved as the dominant foliation; the lineation and folding is likely late (post-1170 Ma) Shawinigan or Ottawan (ca. 1050 Ma). Titanites from the same metasedimentary sequence yield a range of 238U/206Pb ages from 969 to 1077 Ma, with a maximum probability age of 1035 Ma, similar to other titanites in the Adirondack Highlands. Ottawan paleotemperatures, estimated by zirconium in titanite thermometry, range from 787 to 818 °C.

Shawinigan arc magmatism in the Adirondack Lowlands as a consequence of closure of the Trans-Adirondack backarc basin

The Antwerp-Rossie metaigneous suite (ARS) represents arc magmatism related to closure of the Trans-Adirondack backarc basin during Shawinigan collisional orogenesis (ca. 1200–1160 Ma). The ARS is of calc-alkaline character, bimodal, and lacks intermediate compositions. Primarily intruding marble and pelitic gneiss, the ARS is spatially restricted to the Adirondack Lowlands southeast of the Black Lake fault. On discrimination diagrams, the ARS samples plot primarily within the volcanic arc granite fi elds. Incompatible elements show an arc-like signature with negative Nb, Ta, P, and Zr and positive Cs, Pb, La, and Nd anomalies relative to primitive mantle. Neodymium model ages (TDM, depleted mantle model) range from 1288 to 1634 Ma; the oldest ages (1613–1634) and smallest epsilon Nd (eNd) values are found in proximity to the Black Lake fault, delineating the extent of Laurentia prior to the Shawinigan orogeny. The epsilon Nd values at crystallization (1200 Ma) plot well below the depleted mantle curve. Geochemical and isotopic similarities to the Hermon granitic gneiss (HGG) (ca. 1182 Ma) and differences from the Hyde School Gneiss–Rockport Granite suites (1155–1180 Ma) suggest that arc plutonism rapidly transitioned into A-type AMCG (anorthosite-mangeritecharnockite-granite) plutonism. Given the short duration of Shawinigan subduction, apparently restricted extent of the ARS (Adirondack Lowlands), location outboard of the pre-Shawinigan Laurentian margin, intrusion into the Lowlands supracrustal sequence, bimodal composition, and recent discovery of enriched mantle rocks in the Lowlands, it is proposed the ARS formed as a consequence of subduction related to closure of a backarc basin that once extended between the Frontenac terrane and the Southern Adirondacks.

LATE- TO POST-TECTONIC SETTING OF SOME MAJOR PROTEROZOIC ANORTHOSITE - MANGERITE - CHARNOCKITE - GRANITE (AMCG) SUITES

Recent zircon-based geochronological investigations in the Adirondack Mountains, in the state of New York, demonstrate that 1155 ± 6 Ma massif anorthosite was emplaced during the post-contractional phase (1160–1140 Ma) of the ca. 1200–1140 Ma Shawinigan orogeny. Emplacement of many other anorthosite – mangerite – charnockite – granite (AMCG) suites also correlates with the waning stage of orogeny and includes the ca. 1155 Ma Morin and Lac-Saint-Jean complexes, the ca. 1650 Ma Mealy Mountain complex, and the ca. 1450 Ma complexes of eastern Labrador. The correlation also applies to the ca. 930 Ma Rogaland anorthosite complex of Norway and the ca. 1060–1020 Ma late- to post-Ottawan anorthosites of central Quebec and the Appalachians of Virginia and southeastern Pennsylvania. These correlations suggest models involving delamination of overthickened orogenic lithosphere by foundering or convective removal, followed by asthenospheric ascent, and ponding of gabbroic melt at the crust–mantle interface. Orogen rebound following delamination results in stable, dynamically balanced settings in which gabbroic magma evolves slowly at high pressure to produce high-aluminum pyroxene and coarse, intermediate plagioclase characteristic of massif anorthosite. Related melting of the lower crust produces mangeritic and charnockitic magma. Ultimately, both anorthositic and granitic magmas ascend, and lower-pressure fractionation yields the classic AMCG suites. Transtensional reactivation of lithospheric-scale shear zones and old sutures also correlates with important AMCG magmatism and provides conduits for gabbroic magma that ponds at the crust–mantle interface or in the deep crust to produce AMCG suites. Flat-slab subduction, back-arc extension, slab breakoff, and hotspots represent alternative settings that can account for gabbroic underplating and fractionation resulting in AMCG suites, if consistent with geochronological constraints.

U–Pb and Lu–Hf isotope systematics of lower crust from central-southern Mexico – Geodynamic significance of Oaxaquia in a Rodinia Realm

Combined U–Pb ages and Lu–Hf isotope systematics of individual zircon grains from lower crustal rocks of late Mesoproterozoic (Grenville) age, reveal a long geologic evolution of the Oaxaquia terrane (also Oaxaquia) of E and S Mexico. Zircon grains from migmatites, which are host rocks of the AMCG (anorthosite-mangerite-charnockite-granite) suite, have complex internal structures. They contain zoned cores, surrounded by high-U zones, and low-U metamorphic overgrowths. Most of these grains crystallized at ∼1.2 Ga, but some cores are as old as ∼1.35 Ga. Igneous crystallization of the protoliths was followed by migmatization and by granulite facies metamorphism at ∼1.0 Ga, and late stage retrogression. Lutetium–hafnium isotope systematics reveal that both AMCG and host rocks have a limited range of 176Hf/ 177Hf (i) (0.28215–0.28224) with ɛ Hf(1.2 Ga) ≈ +5 to +7 indicating crust formation from a depleted source throughout the Oaxaquia terrane in the early Mesoproterozoic. Zircon from one particular exposure that contains inherited components as old as 1.8-1.5 Ga, has lower 176Hf/ 177Hf (i) (0.28207–0.28209) with ɛ Hf(1.2 Ga) ≈ +2. The Hf isotope ratios of these zircons are comparable with zircon from the Garzón Massif (Colombian Andes). The data indicate that Oaxaquia comprises (1) a crustal precursor that was formed probably as an oceanic arc at 1.4-1.2 Ga, here referred to as “typical Oaxaquia”, and (2) distinct crustal slices with influence from older continental crust that were probably thrusted over “typical Oaxaquia” during the 1.0 Ga orogeny. Migmatization is interpreted to have occurred during the accretion of “typical Oaxaquia” to Amazonia. This event was followed by extension and AMCG intrusion at ∼1.01 Ga, and then the collision of Amazonia with Baltica (assemblage within Rodinia), which caused granulite facies metamorphism elsewhere in the Oaxaquia terrane at ∼0.99 Ga.

U–Pb and 40Ar/ 39Ar geochronology of the Saint-Urbain and Lac Allard (Havre-Saint-Pierre) anorthosites and their associated Fe–Ti oxide ores, Québec: Evidence for emplacement and slow cooling during the collisional Ottawan orogeny in the Grenville Province

Intrusive suites containing anorthosite–mangerite–charnockite–granite (AMCG) are a characteristic feature of Mesoproterozoic magmatic activity in the Grenville Province of eastern North America. In this study, the U–Pb and 40Ar/ 39Ar geochronology of two AMCG suites (Saint-Urbain and Havre-Saint-Pierre), and associated rutile-bearing hemo-ilmenite deposits, within the allochthonous polycyclic belt of the Grenville Province in Québec are used to constrain their crystallization ages and cooling histories, and to assess their tectonic setting. Both AMCG suites, separated by 700 km along the St. Lawrence River, have similar crystallization ages (ca. 1050–1060 Ma) and durations of magmatism (10–15 million years). The 450 km 2 Saint-Urbain anorthosite is a composite intrusion with a distinctly younger central leuconorite (1046.2 ± 3.1 Ma) compared with anorthosite to the north and south (1055.0 ± 2.4 Ma and 1053.6 ± 2.6 Ma, respectively). The age of a deformed oxide–apatite gabbronorite (1057.4 ± 1.5 Ma) along the northwest margin of the intrusion is within analytical uncertainty of the anorthosites, whereas the oldest intrusion in the Saint-Urbain area is the regionally extensive Saint-Anne du Nord orthopyroxene granodiorite (1060.8 ± 2.8 Ma). The large 11,000 km 2 Havre-Saint-Pierre anorthosite suite comprises a number of intrusive phases, including the 2700 km 2 Lac Allard anorthosite, which contains the giant Lac Tio ilmenite deposit and Big Island ilmenitite dyke. U–Pb zircon ages of three anorthosites from the Lac Allard intrusion, including the host to the Lac Tio deposit, overlap within error (1061.6 ± 3.0 Ma, 1060.5 ± 1.9 Ma, 1057.4 ± 8.4 Ma) and the crosscutting Big Island dyke yields an age of 1052.9 ± 6.5 Ma. These results indicate that the Havre-Saint-Pierre anorthosite should be divided into an older geon 11 part (ca. 1129 Ma) and a younger geon 10 part (ca. 1060 Ma). Average cooling rates of 3–4 °C/million years were estimated for the two AMCG suites by combining crystallization ages (U–Pb zircon) and cooling ages (U–Pb rutile, 40Ar/ 39Ar biotite/plagioclase) and reflect emplacement of the intrusions into a hot, long-duration orogen. Emplacement of these two AMCG suites at ca. 1050–1060 Ma is related to regional extensional tectonics near the end of peak metamorphism associated with the collisional Ottawan orogenic phase of the Grenville orogeny. The results of this study provide strong support for proposals that link AMCG magmatism associated with the Grenville orogeny to convective thinning or delamination of the lithosphere and subsequent melting of upwelling asthenospheric mantle.

Timing of anatexis in the eastern Adirondack Highlands: Implications for tectonic evolution during ca. 1050 Ma Ottawan orogenesis

U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of zircons from migmatitic metapelites in the eastern Adirondack Highlands demonstrate that substantial anatexis took place ca. 1050 Ma during the late, but still high-grade, ca. 1090–1050 Ma Ottawan orogeny. These results contrast with data from metapelites of the southwestern Adirondack Highlands and Adirondack Lowlands, which indicate that anatexis occurred ca. 1200–1160 Ma, during the Shawinigan orogeny and subsequent (ca. 1155 Ma) anorthosite-mangerite-charnockite-granite (AMCG) magmatism. Ca. 1180–1150 Ma zircons from this western regime do not contain ca. 1050 Ma (Ottawan) metamorphic overgrowths. The absence of ca. 1050 Ma Ottawan anatexis and overgrowths in the Adirondack Lowlands is explained by the observation that, consistent with titanite cooling ages, post–1150 Ma temperatures did not exceed ~700 °C. In the southwestern Adirondack Highlands, the absence of ca. 1050 Ma anatexis can be accounted for by earlier dehydration of metapelites during ca. 1180–1150 Ma Shawinigan-AMCG metamorphism. The occurrence of ca. 1050 Ma anatexis and formation of metamorphic zircons in the eastern Adirondacks cannot be explained by higher temperatures, because geothermometry indicates that all of the Adirondack Highlands reached a nearly uniform 750–800 °C during this period. Accordingly, the occurrence of ca. 1050 Ma Ottawan anatexis in the eastern regime is interpreted to be the result of: (1) infl uxes of hydrous fl uids at ca. 1050 Ma, or (2) decompression melting during late extensional exhumation. A recently recognized low-angle late Ottawan (ca. 1050 Ma) fault system may have provided both fl uid conduits and footwall decompression. The sense of displacement along the shear zone has not yet been unequivocally determined, but preliminary investigations of kinematic indicators demonstrate normal displacement. Thus, this structure may be an analogue of the ca. 1050 Ma northwest-dipping Carthage–Colton zone normal fault system that juxtaposed the Adirondack Lowlands against the Adirondack Highlands. In this context, these fault zones form a quasi-symmetrical core complex or gneiss dome, within which the high-grade core of the Adirondack Highlands was tectonically exhumed. A similar east-dipping, along-strike normal fault in Quebec (Tawachiche shear zone) exhumed the eastern Morin and Lac Taureau terranes at ca. 1050 Ma and may merge with the eastern Adirondack shear zone described here.

Significance of ~ 1.5 Ga zircon and monazite ages from charnockites in southern Lithuania and NE Poland

The presence of 1.52–1.50 Ga charnockites from the anorthosite–mangerite–charnockite–granite (AMCG) Mazury complex in southern Lithuania and NE Poland, in the western East European Craton (EEC) is revealed by secondary ion mass-spectrometry (SIMS) and EPMA geochronology. Early 1.85–1.82 Ga charnockites are related to major orogeny in the region whereas the newly studied charnockites intrude the already consolidated crust. The 1.52–1.50 Ga charnockite magmatism (SIMS data on zircon) was followed by high-grade metamorphism (EPMA data on monazite), which strongly affected the surrounding rocks. The 1.85–1.81 Ga zircon cores in Lazdijai and 1.81 Ga monazite domains in the Lanowicze charnockites represent the protolith age of a volcanic island arc. The 1.52–1.50 Ga charnockite magmatism and metamorphism are likely related to the distal, Danopolonian, orogeny further to the west, at the margin of Baltica. The c.1.52–1.50 Ga AMCG magmatism and metamorphism in the western EEC as well as the paired accretionary-rapakivi suites in Amazonia, may be the inboard manifestations of the same early Mesoproterozoic orogeny associated with the juxtaposition of Amazonia and Baltica during the amalgamation of the supercontinent Columbia.

Evidence for a pulse of 1.45 Ga anorthosite–mangerite–charnockite–granite (AMCG) plutonism in Lithuania: implications for the Mesoproterozoic evolution of the East European Craton

AbstractSeveral subcropping anorthosite–mangerite–charnockite–granite (AMCG) plutonic suites are aligned along E–W trending lineaments in the Lithuanian part of the East European Craton. The Rukai quartz monzodiorite from the Nemunas suite yields a zircon U–Pb intrusion age of 1447 ± 5 Ma, and the Geluva granite an age of 1445 ± 8 Ma, both obtained using secondary ion mass spectrometry. These rocks are 50 Myr younger than the 1.53–1.50 Ga Mazury AMCG complex in southern Lithuania and northern Poland.The Nemunas and Geluva AMCG rocks correlate in age with Bornholm granitoids in the Baltic Sea and Blekinge granites in southern Sweden, and are similarly aligned along E–W trending lineaments. This regional 1.45 Ga magmatic event across the Baltic Sea may be regarded as an inboard manifestation of the accretionary 1.50 Ga Danopolonian orogeny (cf. Pol. Mineral. Soc. Spec. Publ., 2005, 26:18) farther west.

Grenville-age A-type and related magmatism in southern Laurentia, Texas and New Mexico, U.S.A.

Large Grenville-age plutons are known to be present in the 1340–1370 Ma basement in Texas and eastern New Mexico, but their distribution and compositional ranges are poorly known. New U–Pb zircon dates (SHRIMP-RG, Stanford University) on subsurface samples have identified widespread presence of compositionally diverse Grenville-age (1070–1110 Ma) plutonic rocks. In the Texas Panhandle, core from a > 175 m thick, sill-like gabbroic intrusion yielded a date of 1081 ± 8.3 Ma. In situ differentiation of this tholeiitic magma led to a ∼ 7-fold increase in incompatible element concentrations as REE patterns remained essentially flat. Trace element data suggest an E-MORB-like source. To the south, alkali–feldspar granite with A-type affinities was recovered from drill core from the Abilene gravity minimum near Albany, Texas. It yielded a date of 1078 ± 23 Ma; similar to undeformed (post-orogenic) granites in the Llano uplift. Further west, an anorthosite xenolith from the Eocene Three Sisters intrusion in El Paso, Texas yielded a date of 1068 ± 30 Ma which is within error of the 1110 ± 19 Ma age determined for the main stage of the nearby A-type Red Bluff granitic suite. Xenoliths from Potrillo maar volcano in the center of the southern Rio Grande rift include monzonitic xenoliths with granulitic texture which yielded dates of ∼ 1072 Ma. In contrast, Potrillo maar xenoliths with igneous textures yielded U–Pb dates of ∼ 27 Ma. Development of the granulitic texture is interpreted to reflect granulite facies metamorphism during Tertiary time (26–27 Ma) on the basis of age zoning and U concentrations in the zircon crystals. Our results show that Grenville-age magmatism in the Texas and New Mexico subsurface was widespread and was coeval with syn- and post-deformation granites in the Llano uplift of central Texas. The compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were “A-type”. These magmatic affinities are thought to be associated with crustal extension rather than with subduction. Moreover, the presence of anorthosite xenoliths in the El Paso-Kilbourne Hole region, the broadly tholeiitic character of mafic rocks, and the A-type nature of the granitic rocks are broadly similar to AMCG (Anorthosite–Mangerite–Charnockite–Granite) suites present in the Grenville province of eastern Laurentia.