Anorthosite-mangerite-charnockite-granite Research Articles

Archean to Paleoproterozoic magmatic and metamorphic evolution of the Lofoten – Vesterålen area in northern Norway through zircon U-Pb and Hf isotopic analysis

Neoarchean (2.8–2.5 Ga) and Paleoproterozoic (2.5–1.6 Ga) crust preserves a record of major crustal growth events, the oldest supercontinent assemblies, and the likely transition to modern plate tectonics. The Lofoten–Vesterålen area in Norway underwent a multistage tectono-magmatic history from the Neoarchean to the Paleoproterozoic. In this region three main components are distinguished: 1) a granulite facies zone (GFZ); 2) an amphibolite facies zone (AFZ); and 3) an anorthosite-mangerite-charnockite-granite (AMCG) suite. New zircon U-Pb and Lu-Hf isotope data are used to investigate the evolution of this 2.7–2.6 and 1.9–1.8 Ga crust and evaluate how the developments fit in the global context of Earth’s crustal evolution. In-situ LASS–MC-ICPMS analyses were conducted on zircons to simultaneously determine U-Pb ages and Hf isotope compositions. Solution Hf isotope compositions were additionally obtained on zircons previously dated by ID-TIMS. In general, there is a good agreement between solution and in-situ data. Samples from the GFZ and AFZ both yield Neoarchean (2.7–2.6 Ga) and Paleoproterozoic (1.9–1.8 Ga) populations. Neoarchean populations have εHf(i) of +0.9 to −0.3; Paleoproterozoic populations have εHf(i) around −16 and −7 at ∼1.80 and ∼1.87 Ga, respectively. The AMCG suite was emplaced in two magmatic pulses: the early one at 1.87–1.86 Ga characterized by εHf(∼1.87 Ga) ∼ −7, and the main pulse at 1.80–1.79 Ga has εHf(∼1.80 Ga) between −4.0 and −15.0. Metamorphic overprinting recorded within the GFZ and the AMCG at ∼1.80 Ga is absent in the AFZ, supporting the hypothesis of a tectonic boundary between these domains. The Hf isotope compositions suggest that Paleoproterozoic magmatism was sourced from a combination of juvenile material and incorporated ancient crust, probably in variable proportions, as suggested by the variation in the isotope composition of the AMCG suite. Broadly chondritic Hf isotope compositions in Neoarchean rocks indicate the absence of significantly older crust as a source component. Main crustal growth events recorded in the Lofoten–Vesterålen area coincide with global age peaks and zircon Hf isotope record. The AMCG suite was emplaced during the Columbia supercontinent assembly. Neoarchean and Paleoproterozoic crust in the Lofoten–Vesterålen area exemplifies the elevated preservation potential in collisional tectonic settings.

New palaeomagnetic data for Palaeoproterozoic AMCG complexes of the Ukrainian Shield

A palaeomagnetic study of rocks for two Palaeoproterozoic anorthosite-mangerite-charnockite-granite (AMCG) complexes in the Ukrainian Shield was done to put additional constraints on the interpretation of palaeogeography of Fennoscandia and Volgo-Sarmatia in the Palaeoproterozoic. With this study, 5 sites of Korsun-Novomyrhorod and 3 sites of Korosten AMCG complexes in central and north-western parts of the shield, respectively, were chosen for palaeomagnetic sampling given the geological, modern geochronological and previous palaeomagnetic data. Primary remanent magnetization was isolated on samples of anorthosites, Gabbro, and monzonites within a narrow time interval of U-Pb geochronology dataset of 1.76—1.75 Ga. The palaeomagnetic poles calculated for Korosten and Korsun-Novomyrhorod complexes are almost identical, which indicates that the Volyn and Ingul Domains developed within a single structure of the Ukrainian Shield since at least 1.75 Ga. The new palaeomagnetic pole calculated for all 8 sites (Plat=22.7 °N, Plon=167.4 °E, A95=3.3°) agrees well with previous studies by Elming et al. [2001, 2010]. The selection of the most reliable palaeomagnetic poles for Fennoscandia and Volgo-Sarmatia of this time indicates that the present position of the Ukrainian Shield relative to Fennoscandia is not the same as for about 1.75 Ga, when Fennoscandia occupied a subequatorial position within palaeolatitudes of 5—20 °N, and Volgo-Sarmatia was located close to the equator and rotated relative to Fennoscandia counterclockwise by about 40° compared to its present position.

Geochemical and geochronological constraints on the Mesoproterozoic Red Granite Suite, Kunene AMCG Complex of Angola and Namibia

Anorthosite-mangerite-charnockite-granite (AMCG) suites form large batholiths emplaced during the Proterozoic Eon. Magma source(s), emplacement mechanisms, and the tectonic setting of AMCG suites remain poorly understood. We present new major and trace element geochemistry combined with U-Pb zircon geochronology and Lu-Hf isotopes for the Red Granite Suite, part of the Mesoproterozoic Kunene AMCG Complex (Angola and Namibia), and its Paleoproterozoic host rocks, to elucidate the petrogenesis and tectonic setting of the Kunene Complex granitoids. The studied samples are mostly granite with minor syenite and quartz monzonite. LA-MC-ICPMS U-Pb zircon dates indicate that felsic magmatism was active for at least 90 Myr (1450–1360 Ma) and shows spatial and temporal similarities with emplacement of the main anorthositic body. The geochemistry of the granitoids did not change substantially over time, with predominant alkali-calcic, A-type granites. The εHf(t) of zircon grains in the Red Granite Suite ranges between −11.3 and + 1.6. These results, when considered together with Hf isotopic compositions of zircon from the Paleoproterozoic host rocks (εHf(t) = -11.3 to + 0.4) and xenocrysts in the Red Granite, suggest the suite is derived from the mixing of a Paleoproterozoic crustal component with juvenile mantle-derived material. Syn-magmatic east–west shortening has been documented in both the anorthosites and Red Granites at the southwestern margin of the Kunene Complex in the period 1400–1380 Ma. The results from the Hf isotopes on zircon, combined with the long-lived nature of magmatism, the syn-contractional magmatism, at least locally, and the requirement for extensive crustal melting, suggest that the Red Granite Suite was formed in a convergent margin setting. This challenges the traditional view that A-type granitoids are restricted to anorogenic, extensional settings, and is in agreement with recent views on the petrogenesis of massif-type anorthosites and their associated MCG magmatism.

Chevkinite-group minerals in selected intrusions of the Mazury Complex, North-Eastern Poland: insights into the formation of a titanite-like phase by hydrothermal alteration

Chevkinite-group minerals forming large and common (up to 0.03 vol%) accessory phases in monzodiorites and granodiorites from the Mesoproterozoic anorthosite-mangerite-charnockite-granite (AMCG) suite intrusions, Mazury Complex, north-eastern Poland, range from pristine magmatic types to hydrothermally altered varieties. The unaltered phase is perrierite-(Ce), with the uncommon feature of having Al dominant in the C site. Hydrothermal alteration of the perrierite-(Ce) followed two main trends: one shows depletion in Ca, Fe, Si, Al and Mg, and increasing Ti contents; the other shows increases in Si, Ti and Ca and decreases in light rare-earth elements (LREE), Y and Mg, at about constant Fe content. The second trend resulted in the formation of a phase compositionally similar to titanite. Result of chemical analyses show that the transition from perrierite-(Ce) to the titanite-like phase is sharp; an electron back-scatter diffraction (EBSD) study shows the titanite- like material to be amorphous. The hydrothermal alteration of a chevkinite-group mineral to titanite has commonly been reported in natural sequences but this is the first record of the identification of a titanite-like phase made on the basis of a structural analysis.

Geochronology of granites of the western Korosten AMCG complex (Ukrainian Shield): implications for the emplacement history and origin of miarolitic pegmatites

Abstract. The origin of large miarolitic (also known as “chamber”) pegmatites is not fully understood although they may have great economic value. The formation of cavities in magmatic bodies is related to melt degassing and gas or fluid flow through partially solidified magma. In this paper, the origin of the Volyn pegmatite field, located in the Palaeoproterozoic Korosten anorthosite–mangerite–charnockite–granite (AMCG) complex, North-Western region of the Ukrainian Shield, is discussed. Pegmatites of the field host deposits of piezoelectric quartz that is accompanied by gem-quality beryl and topaz. The Volyn pegmatite field is confined to granites located in the south-western part of the Korosten complex and extends for 22 km along the contact with the anorthosite massif within the Korosten plutonic complex. Geological data indicate hybridization of basic melts and partly crystallized granites, as well as direct impact of fluids derived from basic melts on the chamber pegmatites. The new U–Pb zircon ages obtained for granites and pegmatites of the Korosten complex confirm that the rock assemblage in the northern part of the complex crystallized between 1800 and 1780 Ma, whereas rocks in the southern part intruded mainly between 1768 and 1755 Ma. U–Pb zircon ages for granites from the south-western part of the Korosten complex indicate that granites were emplaced at 1770–1765 Ma, a few million years prior to the intrusion of the gabbro–anorthosite massif (1762–1758 Ma), while chamber pegmatites in these granites crystallized at 1760 ± 3 Ma, coevally with the basic rocks. Ultimately, the formation of the chamber pegmatites was related to the reheating of the semi-crystallized granitic intrusion and to fluids migrating from the underlying gabbro–anorthosite massif.

The OPX-Bearing and OPX-Absent granitoids of the Umarizal intrusive suite, a representative example of Ediacaran A-type magmatism in NE Brazil

Ediacaran magmatism represents one of the most prominent features of the Borborema Province (NE Brazil), with the Umarizal intrusive suite a classical example of an anorthosite - mangerite - charnockite - granite (AMCG) association. This study presents new geochemical and petrological data, discusses its magmatic evolution, and compares the outcomes with similar rocks of Western Africa. The Umarizal suite displays mineral associations that crystallizated under high temperature (<900 °C) and pressure (up to 8 kbar) and low oxygen fugacity conditions that favored the formation of fayalite, magnetite, hypersthene, Fe-hornblende and annite. This generated transitional alkaline to calc-alkaline A-type magmas, which have been emplaced at intermediate to upper crust. Based on our results, we concluded that the several lithological componentes of the Umarizal suite resulted from varying degrees of interaction of melts derived from granulite-facies protoliths with crustal materials. It is considered a late-to post-tectonic Brasiliano/Pan-African orogenic context for the emplacement of this AMCG association along extensional shear zones.

Petrogenesis of the peralkaline Flowers River Igneous Suite and its significance to the development of the southern Nain Batholith

AbstractThe Flowers River Igneous Suite of north-central Labrador comprises several discrete peralkaline granite ring intrusions and their coeval volcanic succession. The Flowers River Granite was emplaced into Mesoproterozoic-age anorthosite–mangerite–charnockite–granite (AMCG) -affinity rocks at the southernmost extent of the Nain Plutonic Suite coastal lineament batholith. New U–Pb zircon geochronology is presented to clarify the timing and relationships among the igneous associations exposed in the region. Fayalite-bearing AMCG granitoids in the region record ages of 1290 ± 3 Ma, whereas the Flowers River Granite yields an age of 1281 ± 3 Ma. Volcanism occurred in three discrete events, two of which coincided with emplacement of the AMCG and Flowers River suites, respectively. Shared geochemical affinities suggest that each generation of volcanic rocks was derived from its coeval intrusive suite. The third volcanic event occurred at 1271 ± 3 Ma, and its products bear a broad geochemical resemblance to the second phase of volcanism. The surrounding AMCG-affinity ferrodiorites and fayalite-bearing granitoids display moderately enriched major- and trace-element signatures relative to equivalent lithologies found elsewhere in the Nain Plutonic Suite. Trace-element compositions also support a relationship between the Flowers River Granite and its AMCG-affinity host rocks, most likely via delayed partial melting of residual parental material in the lower crust. Enrichment manifested only in the southernmost part of the Nain Plutonic Suite as a result of its relative proximity to multiple Palaeoproterozoic tectonic boundaries. Repeated exposure to subduction-derived metasomatic fluids created a persistent region of enrichment in the underlying lithospheric mantle that was tapped during later melt generation, producing multiple successive moderately to strongly enriched magmatic episodes.

Evidence for Silicate–Liquid Immiscibility in Monzonites and Petrogenesis of Associated Fe–Ti–P-rich rocks: Example from the Raftsund Intrusion, Lofoten, Northern Norway

Abstract The 1800 Ma monzonitic to syenitic Raftsund intrusion is the largest intrusive body of the Lofoten–Vesterålen anorthosite–mangerite–charnockite–granite (AMCG) suite. It is composed of three units that can be differentiated based on their textures. This study focuses on the most voluminous, predominantly equigranular, unit consisting of a pigeonite–augite syenite and a fayalite–augite monzonite. The pigeonite–augite syenite is associated with centimeter-scale to hundred-meter scale occurrences of Fe–Ti–P-rich rocks that display sharp to gradational contacts with the surrounding syenite. Iron–Ti–P-rich rocks consist of augite, Fe-rich olivine ± partly inverted pigeonite, apatite, ilmenite, titanomagnetite and sparse pyrrhotite, hornblende and biotite. Partly resorbed ternary feldspar crystals are common toward the contact with the syenite. Microtextures, such as symplectites, encountered at the contact between the syenite and the Fe–Ti–P-rich rocks indicate local disequilibrium between the two rock types. The Fe–Ti–P-rich rocks show large compositional variations but overall are enriched in Ca, Zn, Sc and rare earth elements in addition to Fe, Ti and P compared with the host syenite. Field evidence, whole-rock compositions and textural relationships all suggest that that silicate–liquid immiscibility was involved in the genesis of the Fe–Ti–P-rich rocks. These are interpreted to represent Fe-rich unmixed melts, whereas the syenite is inferred to originate from the crystallization of conjugate Si-rich immiscible melt. The existence of an Fe-rich melt is further supported by the high trace element content of augite from the Fe–Ti–P-rich rocks, showing that they grew from a melt enriched in elements such as Sc and Ti. The fayalite–augite monzonite also displays textural and chemical evidence of silicate liquid immiscibility resulting in unusually variable Zr contents (few hundred ppm to more than 3000 ppm) and the presence of abundant zircon and allanite restricted to millimeter- to centimeter-scale Fe-rich mineral clusters. The most Fe-rich and Si-poor rocks are interpreted to represent the larger proportion of the Fe-rich melt. Liquid immiscibility can be identified at various scales in the pigeonite–augite syenite, from millimeter-size clusters to large-scale bodies, up to hundreds of meters in size, indicating various degrees of separation and coalescence of the Fe-rich melt in the intrusion. The immiscible liquids in the fayalite–augite monzonite consist of an emulsion, with small millimeter- to centimeter-scale droplets of Fe-rich melt, whereas in the pigeonite–augite syenite, Fe-rich melt pockets were able to coalesce and form larger pods. The difference between the two units either results from earlier onset of immiscibility in the pigeonite–augite syenite or reflects a difference in the degree of polymerization of the melt at the time of unmixing. This study emphasizes the importance of silicate–liquid immiscibility in the evolution of intermediate to felsic alkalic ferroan systems and provides a series of arguments that can be used to identify the process in such systems.

Imaging the East European Craton margin in northern Poland using extended correlation processing of regional seismic reflection profiles

Abstract. In NE Poland, Eastern European Craton (EEC) crust of Fennoscandian affinity is concealed under a Phanerozoic platform cover and penetrated by sparse, deep research wells. Most of the inferences regarding its structure rely on geophysical data. Until recently, this area was covered only by the wide-angle reflection and refraction (WARR) profiles, which show a relatively simple crustal structure with a typical three-layer cratonic crust. ION Geophysical PolandSPAN™ regional seismic programme data, acquired over the marginal part of the EEC in Poland, offered a unique opportunity to derive a detailed image of the deeper crust. Here, we apply extended correlation processing to a subset (∼950 km) of the PolandSPAN™ dataset located in NE Poland, which enabled us to extend the nominal record length of the acquired data from 12 to 22 s (∼60 km of depth). Our new processing revealed reflectivity patterns, which we primarily associate with the Paleoproterozoic crust formed during the Svekofennian (Svekobaltic) orogeny, that are similar to those observed along the BABEL and FIRE profiles in the Baltic Sea and Finland, respectively. We propose a mid- to lower-crustal, orogeny-normal lateral flow model to explain the occurrence of two sets of structures that can be collectively interpreted as kilometre-scale S–C′ shear zones. The structures define a penetrative deformation fabric invoking ductile extension of hot orogenic crust in a convergent setting. Localized reactivation of these structures provided conduits for subsequent emplacement of gabbroic magma that produced a Mesoproterozoic anorthosite–mangerite–charnockite–granite (AMCG) suite in NE Poland. Delamination of thickened orogenic lithosphere may have accounted for magmatic underplating and fractionation into the AMCG plutons. We also found sub-Moho dipping mantle reflectivity, which we tentatively explain as a signature of the crustal accretion during the Svekofennian orogeny. Later tectonic phases (e.g. Ediacaran rifting, Caledonian orogeny) did not leave a clear signature in the deeper crust; however, some of the subhorizontal reflectors below the basement, observed in the vicinity of the AMCG Mazury complex, can be alternatively linked with lower Carboniferous magmatism.

NEW RESULTS OF POLYMETALLIC, PGE AND REE MINERALIZATION RESEARCH IN THE SUWAŁKI ANORTHOSITE MASSIF (NE POLAND)

Suwałki Anortosite Massif (SAM) occurs in the crystalline basement of NE Poland within 200 km of the magmatic, Mesoproterozoic AMCG (anorthosite–mangerite–charnockite–granite) rock suite terrane called the Mazury Complex. SAM was discovered as a result of the drilling research of the prominent negative magnetic and gravimetric anomalies. There is an extensive negative anomaly of both potential fields related to the anorthosite massif. Gravimetric anomaly is surrounded by the bands of positive anomalies caused by rocks with elevated densities, such as granitoids, monzondiorites and granodiorites. A negative magnetic anomaly is surrounded by the bands of positive anomalies with significant amplitudes, particularly strongly marked from the south, west and north. Positive magnetic anomalies are associated with the presence of rocks with proven strong magnetic susceptibility due to the content of ferrolites (ilmenite-magnetite rocks) with accompanying Fe-Cu-Ni-Co sulphide mineralization. Fe-Ti-(V) ore deposits in the SAM were discovered in the early 1960s, in the region of Krzemianka and Udryn, but also Jeleniewo and Jezioro Okrągłe, under a thick overburden of Phanerozoic sedimentary rocks within small positive magnetic anomalies. These deposits were documented in about 100 deep boreholes to a depth of 2300 m, and the resources in C1 + C2 category were estimated for about 1.5 billion tons of titanium-magnetite ores with vanadium, mainly in the Krzemianka and Udryn ore fields. The model age obtained by the Re-Os NTIMS method for Fe-Ti-V ores and sulphides from the Krzemianka and Jezioro Okrągłe ore deposits was 1559 ±37 Ma with an initial ratio of 187Os/188Os = 1.16 ±0.06. This age was recognized as the age of the entire Suwałki Massif. Despite many years of research, the deep structure and the form of the massif has not been fully recognized. At present, geophysical and geological 3D modelling of borehole data is carried out using the OasisMontaj (Geosoft) software package. The 3D model is generated in the GeoModeller 3D application (Intrepid Geophysics) in order to recognize the geological correctness and interpretation of magnetic-gravity anomalies of the whole massif and its cover.

What the ca. 1.83 Ga gedrite-cordierite schists in the crystalline basement of Lithuania tell us about the late Palaeoproterozoic accretion of the East European Craton

ABSTRACTIn the western East European Craton (EEC), southern Lithuania, a suite of fine-grained, thinly bedded rocks of unusual composition has been shown to have originally comprised intermediate and felsic volcanic rocks. They extruded at ca. 1.83 Ga and were hydrothermally altered prior to metamorphism, which converted them into garnet-, gedrite-, anthophyllite-, staurolite- and cordierite-bearing schists. After the rocks have experienced a 630°C and 7 kbar metamorphism, they were uplifted to 15 km (5 kbar) probably at ca. 1.73 Ga. They were reheated to 640°C at ca. 1.50 Ga (monazite age). The monazite age of ca. 1.50 Ga is coeval with the emplacement of the neighboring 1.50 Ga Anorthosite-Mangerite-Charnockite-Granite (AMCG) Mazury complex. The ca. 1.83 Ga volcanic suites in Lithuania and northern Poland, together with the Oskarshamn-Jönköping belt (OJB) in south-central Sweden, may belong to the same chain of volcanic island arcs, and thus provide information on the evolution of the entire western EEC. The ca. 1.50 Ga metamorphic reworking and the replacement of the Mazury AMCG suite may have been triggered by the Danopolonian orogeny further west and, at a larger scale, accretion of the continental margin of Columbia.

Spatial and temporal links between shawinigan accretionary orogenesis and massif anorthosite intrusion, southern grenville province, New York, U.S.A.

Synthesis of new and existing field, structural, geochemical, isotopic, and geochronologic data in the Adirondack Highlands of the southern Grenville Province indicate a suite of arc plutonic rocks (Piseco Lake granite suite) was intruded into basement rocks from ca. 1205–1180 Ma due to northward subduction beneath the Laurentian margin. Although rocks of similar age and origin are found in the Adirondack Lowlands and elsewhere in the Highlands, they occur in the greatest volume and are most intensely deformed in the Piseco Lake Shear Zone (PLSZ). This structure separates the Adirondack Highlands from ca. 1350-1300 Ma tonalitic gneisses in the southern Adirondacks. Tectonites in the PLSZ record the deformation of a continental arc batholith during collision with an outboard terrane which was accreted to Laurentia during the Shawinigan Orogeny. Internal to the PLSZ, a 10–15 km wide, subvertical zone of subhorizontally lineated L-S and L-tectonites (Piseco Lake gneisses) was derived primarily from megacrystic arc plutonic protoliths and documents sinistral relative motion. Intrusion of the arc granitoids, deformation and anatexis (ca. 1180–1160 Ma) throughout the entire Adirondack region immediately preceded and overlapped the intrusion of the anorthosite-mangerite-charnockite-granite (AMCG) suite at ca. 1165-1145 Ma in the Adirondack Highlands. We hypothesize that delamination or slab detachment, in combination with an irregular Laurentian margin, triggered disruption of the underlying lithosphere, allowed ascent of enriched asthenosphere, and facilitated the intrusion of voluminous AMCG rocks in the Adirondacks north of the PLSZ. Other large anorthosite massif complexes in the southern Grenville Province, including the Morin and Lac St. Jean bodies near the Laurentian margin, have also been linked to Shawinigan convergent structures and were an important part of accretionary processes documented in the Grenville Orogen.

Magma-driven, high-grade metamorphism in the Sveconorwegian Province, southwest Norway, during the terminal stages of Fennoscandian Shield evolution

Recently it has been argued that the Sveconorwegian orogeny in southwest Fennoscandia comprised a series of accretionary events between 1140 and 920 Ma, behind a long-lived, active continental margin characterized by voluminous magmatism and high-grade metamorphism. Voluminous magnesian granitic magmatism is recorded between 1070 and 1010 Ma (Sirdal Magmatic Belt, SMB), with an apparent drop in activity ca. 1010-1000 Ma. Granitic magmatism resumed ca. 1000-990 Ma, but with more ferroan (A type) compositions (hornblende-biotite granites). This ferroan granitic magmatism was continuous until 920 Ma, and included emplacement of an AMCG (anorthosite-mangerite-charnockite-granite) complex (Rogaland Igneous Complex). Mafic rocks with ages corresponding to the spatially associated granites suggest that heat from underplated mafic magma was the main driving force for lower crustal melting and long-lived granitic magmatism. The change from magnesian to ferroan compositions may reflect an increasingly depleted and dehydrated lower crustal source. High-grade metamorphic rocks more than similar to 20 km away from the Rogaland Igneous Complex yield meta-morphic ages of 1070-1015 Ma, corresponding to SMB magmatism, whereas similar rocks closer to the Rogaland Igneous Complex yield ages between 1100 and 920 Ma, with an apparent age peak ca. 1000 Ma. Ti-in zircon temperatures from these rocks increase from similar to 760 to 820 degrees C ca. 970 Ma, well before the inferred emplacement age of the Rogaland Igneous Complex (930 Ma), suggesting that long-lived, high-grade metamorphism was not directly linked to the emplacement of the latter, but rather to the same mafic underplating that was driving lower crustal melting. Structural data suggest that the present-day regional distribution of high-and low-grade rocks reflects late-stage orogenic doming.

Prolonged high-temperature, low-pressure metamorphism associated with ∼1.86 Ga Sancheong–Hadong anorthosite in the Yeongnam Massif, Korea: Paleoproterozoic hot orogenesis in the North China Craton

Elevated heat flow in association with mafic magmatism in an orogenic belt commonly leads to high-temperature, low-pressure (HTLP) metamorphism and the production of granulite-facies assemblages. We studied such a HTLP complex in the vicinity of the Sancheong–Hadong anorthosite-mangerite-charnockite-granite (AMCG) suite, Yeongnam Massif, Korea, in order to constrain the P–T conditions, timing, and duration of metamorphism. This complex primarily consists of massif-type anorthositic-gabbroic bodies emplaced at ∼1.87–1.86 Ga and a series of country rocks comprising orthopyroxene-bearing gneisses as well as anatectic granites and migmatites. Migmatitic gneisses were studied in detail because melt-related features are abundant and well preserved; for example, inclusion-rich peritectic phases such as cordierite or K-feldspar are characteristic for the prograde melt-forming stage, whereas biotite-quartz symplectites mantling garnet or orthopyroxene represent the cooling stage consuming melt. Pseudosection P–T analyses of migmatitic gneisses suggest peak metamorphic conditions of 810–840 °C and 5.9–6.2 kbar, followed by near-isobaric cooling and melt crystallization at ∼780 °C and ∼5.5 kbar. SHRIMP (sensitive high-resolution ion microprobe) U–Th–Pb ages of zircon and monazite from six migmatitic gneisses are in the range of 1870–1854 Ma. The oldest age, recorded only in high Y monazite, suggests that prograde metamorphism commenced at ∼1870 Ma. In contrast, melt crystallization had culminated by 1860–1855 Ma producing widespread leucosomes and anatectic granites. Our results suggest that high thermal gradient (∼40 °C km−1) attending the granulite-facies metamorphism is attributable to coeval, pulse-like emplacement of anorthositic-gabbroic magmas. Moreover, the HTLP metamorphism lasted over a period of ∼15 Ma, indicating a long-lived process corresponding to the late stage of Paleoproterozoic (∼1.95–1.85 Ga) hot orogenesis in the North China Craton.

Magmatic evolution and source of a Proterozoic rapakivi granite complex in the North China Craton: New evidence from zircon U−Pb ages, mineral compositions, and geochemistry

Typical Proterozoic rapakivi granite of the Northern Hemisphere occur in Precambrian shield areas, and are commonly associated with anorthosites, mangerites, and charnockites, thus belonging to the AMCG (anorthosite–mangerite–charnockite–granite) suite. A similar suite is represented in the North China Craton (NCC) by the Shachang rapakivi granite and Damiao anorthosite complexes. The Shachang complex consists of amphibole–biotite rapakivi granite, porphyritic biotite granite, coarse-grained biotite granite, medium- to fine-grained biotite granite, and two-mica granite (listed from the earliest to the latest phases). New zircon U–Pb dating shows that the complex was formed over the period 1700–1682 Ma. The mafic minerals of the complex are calcic amphibole (ferroedenite–ferrohastingsite) and biotite (annite–siderophyllite), and the compositions of the amphibole suggest that the matrix of the amphibole–biotite rapakivi granite crystallized at a pressure of ∼4 kbar. The temperatures of crystallization, obtained from the compositions of amphibole, biotite, and zircon, decreased from ∼780 °C for the amphibole–biotite rapakivi granite to ∼660 °C for the two-mica granite. Our study of the Shachang complex indicates an evolution from early potassium-rich magmas to late silicon-rich magmas. The early amphibole–biotite rapakivi granite crystallized under relatively high temperatures (760–780 °C) and low fO2, while the later rocks formed under lower temperatures (760–660 °C) and high fO2. The Shachang rapakivi granite and Damiao anorthosite complexes contain different mafic minerals and show diverse trace element ratios and disparate geochemical evolutionary trends, suggesting different sources. Zircon Lu–Hf and whole-rock Sm–Nd isotopic data for the Shachang complex are relatively homogeneous, with εHf(t) and εNd(t) values varying from −9.8 to −3.3 and −6.4 to −3.4, respectively. Corresponding two-stage Hf model ages and Nd depleted mantle model ages vary from 2650 to 2850 Ma and from 2480 to 2840 Ma, respectively. These model ages are similar to the ages of Neoarchean lower crustal metamorphic rocks in the NCC (2500–2800 Ma). In addition, the compositions of the amphiboles and biotites in the granite in the Shachang complex indicate derivation from the crust. All the evidence points to the Shachang rapakivi granite complex having been formed from magmas derived from the lower crust of the NCC, rather than from the fractionation of mantle-derived mafic magmas with minor crustal contamination. This paper provides a detailed case study of the Shachang complex that will further our understanding of the sources of Proterozoic rapakivi granite in the Northern Hemisphere that belong to the AMCG suite of rocks.

In situ U–Pb and Lu–Hf isotopic studies of zircons from the Sancheong–Hadong AMCG suite, Yeongnam Massif, Korea: Implications for the petrogenesis of ∼1.86 Ga massif-type anorthosite

Isotopic and geochemical characteristics of Proterozoic anorthosite-mangerite-charnockite-granite (AMCG) suite have long been used for tracing the mantle–crustal source and magmatic evolution. We analyzed Lu–Hf isotopic compositions of zircon from the Sancheong–Hadong AMCG complex, Yeongnam Massif, Korea, in order to understand tectonomagmatic evolution of the Paleoproterozoic AMCG suite occurring at the southeastern margin of the North China Craton (NCC). The anorthositic rocks in this complex, associated with charnockitic and granitic gneisses, were recrystallized to eradicate magmatic features. In situ SHRIMP (sensitive high-resolution ion microprobe) U–Pb analyses of zircon from a leuconorite and an oxide-bearing gabbroic dyke yielded weighted mean 207Pb/206Pb ages of 1870±2Ma and 1861±6Ma, respectively. Charnockitic, granitic, and porphyroblastic gneisses yielded weighted mean 207Pb/206Pb zircon ages of 1861±6Ma, 1872±6Ma, and 1873±4Ma, respectively. These crystallization ages, together with our previous geochronological data for anorthosites (1862±2Ma), are indicative of episodic AMCG magmatism over an ∼10Ma interval. Initial εHf(t) values of zircon analyzed from five anorthositic rocks and four felsic gneisses range from +2.1 to −6.1 and −0.3 to −5.4, respectively. Zircon Hf isotopic data in combination with available whole rock Sr–Nd isotopic data suggest that anorthositic parental magma was most likely derived from a mantle source and variably affected by crustal contamination. This crustal component is also reflected in charnockitic-granitic magmas produced primarily by the melting of lower crust. Taken together, the AMCG magmatism at 1.87–1.86Ga in the Yeongnam Massif is most likely a late orogenic product of Paleoproterozoic NCC amalgamation tectonically linked to assembly of the Columbia supercontinent.

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.