ALBERT

Description: The Jurassic Vestfjella dyke swarm at the volcanic rifted margin of western Dronning Maud Land represents magmatism related to the incipient Africa–Antarctica rift zone; that is, rift-assemblage magmatism of the Karoo continental flood basalt (CFB) province. Geochemical and Nd–Sr isotopic data for basaltic and picritic dyke samples indicate diverse low-Ti and high-Ti tholeiitic compositions with Nd (180 Ma) ranging from +8 to –17. Combined with previously reported data on a subcategory of ferropicritic dykes, our new data facilitate grouping of the Vestfjella dyke swarm into seven geochemically distinct types. The majority of the dykes exhibit geochemical affinity to continental lithosphere and can be correlated with two previously identified chemical types (CT) of the wall-rock CFB lavas and are accordingly referred to as the CT1 and CT3 dykes. The less abundant Low-Nb and High-Nb dykes, a relatively enriched subtype of CT3 (CT3-E) dykes, and dykes belonging to the depleted and enriched ferropicrite suites represent magma types found only as intrusions. The chemically mid-ocean ridge basalt (MORB)-like Low-Nb and the depleted ferropicrite suite dykes represent, respectively, relatively high- and low-degree partial melting of the same overall depleted mantle (DM)-affinity source in the sublithospheric mantle. In contrast, we ascribe the chemically ocean island basalt (OIB)-like High-Nb dykes and the enriched ferropicrite suite dykes to melting of enriched components in the sublithospheric mantle. Geochemical modelling suggests that the low-Ti affinity CT1 and CT3, and high-Ti affinity CT3-E magma types of Vestfjella dyke may predominantly result from mixing of DM-sourced Low-Nb type magmas with 〈10 wt % of crust- and lithospheric mantle-derived melts. U/Pb zircon dating confirms synchronous emplacement of CT1 dykes and Karoo main-stage CFBs at 182·2 ± 0·9 and 182·2 ± 0·8 Ma, whereas two 40 Ar/ 39 Ar plagioclase plateau ages of 189·2 ± 2·3 Ma (CT1) and 185·5 ± 1·8 Ma (depleted ferropicrite suite), and a mini-plateau age of 186·9 ± 2·8 Ma (CT3-E) for the Vestfjella dykes raise the question of whether the onset of rift-zone magmatism could predate the province-wide c. 179–183 Ma main stage of Karoo magmatism. Notably variable Ca/K spectra suggest that younger 40 Ar/ 39 Ar plagioclase plateau ages of 173, 170, 164, and 154 Ma are related to crystallization of secondary minerals during the late-stage tectono-magmatic development of the Antarctic rifted margin. The occurrence of rare MORB- and OIB-like magma types in Vestfjella and along the African and Antarctic rifted margins suggests melting of geochemically variable depleted and enriched sublithospheric mantle beneath the Africa–Antarctica rift zone. Our models for the Vestfjella dyke swarm indicate that the voluminous lithosphere-affinity low-Ti and high-Ti rift-assemblage tholeiites could have been derived from MORB-like parental magmas by contamination, which implies sublithospheric depleted mantle as the principal source of the CFB magmas of the Africa–Antarctica rift zone.

Description: To gain insights into the long-standing problem of marginal reversals, we have studied in detail the ~200 m thick marginal zone of the ~3 km thick, mafic Koitelainen Layered Intrusion (KLI). This marginal zone consists of pigeonite gabbros that are chilled against gneisses of the underlying basement but reveals a non-chilled upper contact with orthopyroxenites of the overlying Layered Series. There is a sharp textural and compositional break between the rocks of the marginal zone and the Layered Series of the KLI. The marginal zone is remarkable in showing systematic reverse fractionation trends from the bottom to the very top. These are exemplified by a significant upward increase in whole-rock and pyroxene Mg-number along with normative and actual An content of plagioclase, as well as by an upward decrease in all incompatible components (e.g. TiO 2 , Zr, Y, REE). This is accompanied by a systematic upward decrease in ratios of highly incompatible elements, such as Zr/Y and La/Yb and in initial whole-rock Nd . The uppermost rocks of the marginal zone are very fine-grained and show well-developed plagioclase lamination and ultra-depletion in all incompatible elements (e.g. Zr = 0·5 ppm; Ce = 0·11 ppm). The marginal zone is interpreted as having developed by in situ crystallization of magma in a slowly opening fissure. The origin of reverse compositional trends is attributed to three principal factors, which we refer to as the ‘three- increase model’: (1) an increase in compositional primitivity of the magma that gradually filled the chamber; (2) an increase in the degree of chemical equilibrium among phases associated with increasing distance from the cold country rocks; (3) an increase in the proportion of cumulus minerals in response to more effective removal of evolved liquid from in situ growing crystals with distance from the intrusive contact. The marginal reversal of the KLI is distinct from typical reversals in layered intrusions in that its development was interrupted by the emplacement of a large volume of new magma that was parental to the overlying Layered Series. This new, hot magma caused significant textural and compositional reconstitution of the uppermost gabbroic rocks of the marginal reversal by their partial melting, compaction and recrystallization. This gave rise to a considerable decrease in grain size, appearance of plagioclase lamination, and ultra-depletion in highly incompatible elements. The anomalous nature of these rocks is thus a manifestation of their restitic rather than primary magmatic origin. The proposed ‘three-increase model’ may represent a general explanation for the origin of marginal reversals in many mafic sills and layered intrusions.

Description: : In situ Lu–Hf (laser ablation microprobe–inductively coupled plasma mass spectrometry (LAM-ICPMS)) and U–Pb (LAM-ICPMS, secondary ionization mass spectrometry (SIMS)) analyses of zircon, and whole-rock Sm–Nd isotope analyses were performed on rocks formed during magmatic events in three Archaean complexes in the Karelian Province of Fennoscandia (Pudasjärvi, Koillismaa and Iisalmi). These complexes have U–Pb ages ranging from 3.5 to 2.6 Ga. In Pudasjärvi, sparse xenocrystic cores give ages of 3.6–3.7 Ga and initial 176 Hf/ 177 Hf suggesting influence of a crustal component T ≥ 4.0 Ga (assuming a CHUR-like mantle source). Ages and Nd and Hf isotope patterns indicate magmatic events at 3.6–3.7 Ga (Siurua, Pudasjärvi with ≥4.0 Ga precursor), 3.2 Ga (Iisalmi, Koillismaa), 2.8 Ga (Pudasjärvi) and 2.7 Ga (Pudasjärvi, Iisalmi). In the Meso- and Palaeoarchaean events, there is no evidence of sources equivalent to present-day depleted mantle; such sources were, however, involved in the 2.8–2.7 Ga events. Hf and Nd are strongly correlated. Contrasts between the Archaean complexes indicate that they evolved separately until c . 2.7 Ga. The age and Hf pattern of ≤2.8 Ga rocks in the Karelian Province is compatible with a scenario in which the Karelia, Superior, Yilgarn and Slave cratons were part of a late Archaean supercontinent, but does not constitute proof of the existence of such a supercontinent. Supplementary material: U–Pb and Lu–Hf data are available at http://www.geolsoc.org.uk/SUP18430 .

Description: The Kunene Complex of Namibia-Angola is one of the largest anorthosite massifs on Earth (up to 18,000 km 2 ), consisting of several distinct anorthosite and leucotroctolite intrusions. The Namibian portion of the Kunene Complex measures ~80 x 50 km, ~4,000 km 2 , and is dominated by the Zebra Mountain lobe, a ~16-km-thick dome-like mass of interlayered, relatively unaltered dark leucotroctolite with relatively altered, "white," anorthosite. Past studies and the present work have found evidence for intrusion of two distinct phases of dark leucotroctolite into the white anorthosite, namely a relatively early, deformed, phase dated at 1363 ± 17 Ma (U-Pb in baddelyite), and a relatively later and undeformed phase whose absolute age remains unknown. The Kunene leucotroctolites are among the least evolved troctolites known from anorthosite complexes, with olivine containing 59 to 77 mol % forsterite and up to 1,700 ppm Ni, and plagioclase containing 56 to 69 mol % anorthosite. Our isotope data from the troctolites indicate a relatively small crustal component ( 18 O, ~5.3–7.3; 34 S, 0.5–1; and Nd T , 0.9–1.8), whereas Nd and oxygen isotope data from the white anorthosites, published by other workers, showed a slightly larger crustal component (e.g., Nd T as low as –3; 18 O up to 7.5). In the periphery of the Kunene Complex are several, relatively small (〈10 km 2 ), mafic-ultramafic intrusions comprising peridotite, pyroxenite, gabbro, troctolite, and anorthosite. Some of these bodies are Ni-Cu-PGE mineralized, including the Ohamaremba troctolite, the Oncocua pyroxenite, and the Ombuku peridotite-gabbronorite. The latter additionally contains a massive chromitite layer. A new U-Pb baddelyite age of 1220 ± 15 Ma for Ohamaremba indicates that the latter postdates the main Kunene Complex by ~140 Ma. The relative enrichment in MgO, Cr, and Ni, and the O, Nd, and S isotope characteristics of Kunene magmatism suggest that the primary magmas were predominantly mantle-derived picrites or basalts. The massif-type anorthosites formed through ascent of feldspathic slurries followed by downward draining of residual liquid. Subsequent magma pulses formed troctolitic sills within the anorthosite plutons and mafic-ultramafic satellite intrusions in the periphery of the anorthosites. The recurring nature of Kunene mafic-ultramafic magmatism results from several successive mantle upwellings. Partial mantle melts ascended through reactivated translithospheric lineaments along the southern margin of the Congo craton.

Description: The Monts de Cristal Complex of Gabon consists of several igneous bodies interpreted to be remnants of a tectonically dismembered, 〉100 km long and 1–3 km wide, ultramafic–mafic intrusion emplaced at 2765–2775 Ma. It is the most significant mafic–ultramafic layered complex yet identified on the Congo Craton. The complex consists largely of orthopyroxenite cumulates, with less abundant olivine-orthopyroxenite and norite, and rare harzburgite and dunite. Mineral compositions (Fo ol 84, Mg# Opx 85, An plag 60–68, Cr/Fe chromite 1–1·45) and whole-rock data suggest that the parent magma was a low-Ti basalt containing approximately 10% MgO and 0·5% TiO 2 . Trace element and Rb–Sr and Sm–Nd isotope data indicate the presence of an enriched component, possibly derived from crustal contamination of a magma generated in the sub-lithospheric mantle. Most rocks show a highly unusual pattern of strong Pt enrichment (10–150 ppb) at low concentrations of Pd (1–15 ppb), Au (1–2 ppb), Cu (1–20 ppm), and S (〈500 ppm), suggesting that unlike in most other PGE-rich intrusions globally, platinum in the Monts de Cristal Complex is not hosted in magmatic sulfides. Synchrotron X-ray fluorescence mapping has revealed the location of buried small Pt particles, most of which are associated with As. We propose that this constitutes some of the strongest evidence yet in support of magmatic crystallization of a Pt–As phase from S-undersaturated magma.

Description: The Raahe–Ladoga Shear Complex is a major crustal structure representing the Archaean–Palaeoproterozoic boundary in the Fennoscandian Shield. The complex developed during the Svecofennian Orogeny ( c. 1.9 – 1.8 Ga) beginning with regional thrust tectonic phases D 1 and D 2 , followed by large-scale shearing events D 3 and D 4 . The Pielavesi Shear Zone is a vertical north–south-trending shear zone within the Raahe–Ladoga Shear Complex formed during regional D 3 shearing and later reactivated during the regional D 4 phase. Three north–south-trending elongate granitoid intrusions were selected as representative of silicic melts that intruded the transtensional Pielavesi Shear Zone during the regional D 3 phase. The oriented magmatic fabric of the granitoids indicates that they intruded coeval to the deformation event. The zircon U–(Th)–Pb secondary ionization mass spectrometry (SIMS) ages of these intrusions (1888 ± 4, 1884 ± 6 and 1883 ± 5 Ma) overlap within error and provide a direct age for the regional D 3 deformation. Hf(T) (–1.1 to +3.4) and Nd(T) (–1.2 to +0.4) values from these granitoids are both consistent with a predominantly juvenile source affected by a minor Archaean component. U–(Th)–Pb SIMS analyses of metamorphic monazite formed within a crosscutting blastomylonite provide an age for the regional D 4 phase and associated fluid activity of 1793 ± 3 Ma. Supplementary material: Analytical methods and tabulated analytical data are available at https://doi.org/10.6084/m9.figshare.c.3498501

Description: We have studied a group of granitoids from the Western Karelia subprovince of the Fennoscandian Shield. This group is referred to as quartz syenites, but shows compositional variation from syenites to quartz monzonites, with a small number of monzonites and granites. Compositionally studied rocks are alkali and alkali-calcic, and magnesian, mostly metaluminous. Characteristically, they have a high content of alkaline (Na, K), large ion lithophile elements (LILE) (Ba, Sr), high-field strength elements (HFSE) (TiO 2 , Zr, Ce), as well as a low content of Mg, Ni and Cr, by which they can be distinguished from sanukitoid and quartz diorite suites of the Karelia Province. These quartz syenites were emplaced between 2.74 and 2.66 Ga, representing late-phase intrusions overlapping in age with the sanukitoids, the quartz diorites and the leucogranitoids. Initial whole-rock Nd values of quartz syenites vary from 1.8 to –1.8, and do not indicate a significant contribution of considerably older crust. Oxygen-isotope data for zircon indicate a varying mantle source ( 18 O 5.35–7.15), with a contribution from source(s) with elevated 18 O values. Our data provide constraints on compositionally diverse Neoarchaean magmatism in the Archaean Karelia Province. The late Archaean evolution of the Western Karelia subprovince resembles that of the Neoarchean domains worldwide with respect to granitoid composition and temporal distribution. Supplementary material: Tables detailing geochemical analyses, analytical data for the five age samples and oxygen-isotope analyses from this study are available at https://doi.org/10.6084/m9.figshare.c.3459771