Palaeoproterozoic Basement Research Articles

Fluid inclusion study of pegmatite and aplite veins of Palaeoproterozoic basement rocks in Bangladesh: Implications for magmatic fluid compositions and crystallization depth

We report the first fluid inclusion data on Palaeoproterozoic basement rocks in Bangladesh for the characterization of magmatic fluid compositions and determination of the crystallization pressure and temperature of the host rock. Fluid inclusions are present as primary and pseudosecondary types in quartz grains within pegmatite and aplite, which occur as veins in dioritic rocks. Both primary CO2-rich and H2O-rich inclusions are present, even in the same inclusion cluster, probably reflecting a single stage of fluid activity during the crystallization of the veins. The melting temperature (Tm) and homogenization temperature (Th) of the dominant carbonic inclusions are in the ranges of −56.6 °C to −58.1 °C and −6.8 °C to +30 °C, respectively; the Th values translate into densities of 0.59-0.97 g/cm3. Rare aqueous fluid inclusions have a final Tm value in the range of 0 °C to −10.8 °C and a Th value in the range of +209.8 °C to +405.5 °C, which corresponds to bulk densities of 0.52-0.97 g/cm3. Isochores of the inclusions and temperatures obtained from the zircon saturation thermometry of pegmatite indicate that the veins crystallized at ∼ 4.8 kbar and 660-670 °C (depth of ∼ 14 km). The results of this study will be useful in understanding the magmatism and metallogeny of felsic igneous rocks of Bangladesh, which are related to the formation of the Columbia supercontinent.

Tectonic evolution of the Western Kunlun orogenic belt in northern Qinghai-Tibet Plateau: Evidence from zircon SHRIMP and LA-ICP-MS U-Pb geochronology

The Western Kunlun Range in northern Qinghai-Tibet Plateau is composed of the North Kunlun Terrane, the South Kunlun Terrane and the Karakorum-Tianshuihai Terrane. Here we report zircon SHRIMP and LA-ICP-MS U-Pb ages of some metamorphic and igneous rocks and field observations in order to provide a better understanding of their Precambrian and Palaeozoic-early Mesozoic tectonic evolution. Based on these data we draw the following conclusions: (1) The paragneisses in the North Kunlun Terrane are likely of late Mesoproterozoic age rather than Palaeoproterozoic age as previously thought, representing tectonothermal episodes at 1.0−0.9 Ga and ∼0.8 Ga. (2) The North Kunlun Terrane was an orogenic belt accreted to the southern margin of Tarim during late Mesoproterozoic to early Neoproterozoic, the two episodes of metamorphisms correspond to the assemblage and breakup of Rodinia respectively. (3) The Bulunkuole Group in western South Kunlun Terrane, which was considered to be the Palaeoproterozoic basement of the South Kunlun Terrane by previous studies, is now subdivided into the late Neoproterzoic to early Palaeozoic paragneisses (khondalite) and the early Mesozoic metamorphic volcano-sedimentary series; the paragneisses were thrust onto the metamorphic volcano-sedimentary series from south to north, with two main teconothermal episodes (i.e., Caledonian, 460–400 Ma, and Hercynian-Indosinian, 340–200 Ma), and have been documented by zircon U-Pb ages. (4) In the eastern part of the South Kunlun Terrane, a gneissic granodiorite pluton, which intruded the khondalite, was crystallized at ca. 505 Ma and metamorphosed at ca. 240 Ma. In combination with geochronology data of the paragneiss, we suggest that the South Kunlun Terrane was a Caledonian accretionary orogenic belt and overprinted by late Paleozoic to early Mesozoic arc magmatism.

Application of sulphur isotopes to discriminate Cu–Zn VHMS mineralization from barren Fe sulphide mineralization in the greenschist to granulite facies Flin Flon–Snow Lake–Hargrave River region, Manitoba, Canada

The Flin Flon Belt in northern Canada is one of the largest Palaeoproterozoic volcanic-hosted massive sulphide (VHMS) districts in the world, but up to 20 000 km 2 of prospective Palaeoproterozoic basement south of this belt is buried beneath 10 to 100 m of Phanerozoic calcareous cover. The recent acquisition of airborne SPECTREM geophysics data south of the Flin Flon Belt has resulted in the discovery of Cu-Zn sulphide prospects comprising pyrrhotite, pyrite, chalcopyrite, sphalerite and galena, but numerous barren Fe sulphide occurrences comprising only pyrite and pyrrhotite have also been intersected. The problem for explorers is trying to determine whether a barren Fe sulphide intersection that has just been cored is part of a larger Cu-Zn mineralized system, or nothing more than a pyrite–pyrrhotite occurrence. A sulphur isotope study of sulphides from the Flin Flon–Snow Lake–Hargrave River–Talbot area shows that sulphides from the Cu-Zn VHMS deposits have δ 34 S values that range between −1.4 and 6.4‰, with a mean δ 34 S value of 1.6 ± 1.7‰ (2σ error). More than 95% of these samples have δ 34 S values of <3.3‰. In contrast, pyrite and pyrrhotite separates from barren Fe sulphide deposits have δ 34 S values between 1.8 and 10.0‰, with a mean δ 34 S value of 4.3 ± 1.8‰ (2σ error). In this case, >84% of these samples have δ 34 S values of >3.3‰. The results imply that the barren Fe sulphide deposits can be statistically distinguished from Cu-Zn VHMS mineralization based on S isotopic composition, which should make future exploration drilling decisions easier.

Geochronology and nature of the Palaeoproterozoic basement in the Central African Copperbelt (Zambia and the Democratic Republic of Congo), with regional implications

U–Pb SHRIMP zircon age data, together with geochemical analyses, from the basement to the Katanga Supergroup in the Central African Copperbelt reveal the existence of a widespread Palaeoproterozoic magmatic arc terrane. The Lufubu schists represent a long-lived calc-alkaline volcanic arc sequence and, where dated in both Zambia and the Democratic Republic of Congo (DRC), yield ages of 1980 ± 7, 1968 ± 9, 1964 ± 12 and 1874 ± 8 Ma. The oldest dated unit from the region, the Mkushi granitic gneiss from south-east of the Zambian Copperbelt, has an age of 2049 ± 6 Ma. The copper-mineralized Mtuga aplites, which crosscut the foliation in the Mkushi gneisses, have mainly xenocrystic, zoned zircons with cores dated at ca. 2.07–2.00 Ga. Overgrowths on these cores are dated at 1059 ± 26 Ma, which is interpreted as the intrusive age of the aplites. An augen gneiss from the Mulungushi Bridge locality yielded an emplacement age of 1976 ± 5 Ma. The Mufulira Pink Granite has an age of 1994 ± 7 Ma, while the Chambishi granite has been dated at 1983 ± 5 Ma, an age within error of Lufubu schist metavolcanics from elsewhere in the Chambishi basin. The gneisses, granitoids and acid-intermediate calc-alkaline metavolcanics are considered to represent stages in the evolution of one or more magmatic arcs that formed episodically over a 200 million year period between 2050 and 1850 Ma. We suggest naming this assemblage of rocks the “Lufubu Metamorphic Complex”. The rocks of the Lufubu Metamorphic Complex are interpreted to be part of a regionally extensive Palaeoproterozoic magmatic arc terrane stretching from northern Namibia to northern Zambia and the DRC. This terrane is termed the Kamanjab–Bangweulu arc and is inferred to have collided with the Archaean Tanzanian craton during the ca. 2.0–1.9 Ga Ubendian orogeny, to produce a new composite minicontinental entity that we term the “Kambantan” terrane. The Kambantan terrane was accreted onto the southern margin of the Congo craton during the ca. 1.4–1.0 Kibaran orogeny.

Pre-1.91 Ga deformation and metamorphism in the Palaeoproterozoic Vammala Migmatite Belt, southern Finland, and implications for Svecofennian tectonics

A metamorphic event in the Vammala Migmatite Belt (VMB) at ~1.92 Ga, revealed by SHRIMP U-Pb analyses of both zircon overgrowths and monazite ,i s interpreted as post-depositional and is correlated with the development of the early high-grade schistosity .N either this Early Svecofennian deformation and metamorphism, nor the associated complex folding, is present in the overlying Ta mpere Schist Belt (TSB) sequence, consistent with the VMB being part of a pre-1.91 Ga basement complex. The ~1.92 Ga event provides a maximum deposition age for the TSB ,c onfirm ing earlier age estimates. Earlier stratigraphic correlations between parts of the VMB and TSB ,a nd associated tectonic interpretations, can no longer be sustained.The crustal thickening seen in the VMB ,a nd previously attributed to arc accretion at ~1.89 Ga, is now attributed to accretion of a large Svionian marginal basin during the ‘Early Svecofennian’ orogenic phase at ~1.92 Ga.This is of similar age to the deformation and metamorphism associated with collision in the Lapland-Kola Orogen to the north of the Karelian Province. The well-known post-TSB orogenic phase was also identified in the VMB by a monazite age of 1881±6 Ma. A granitoid intrusion gave an emplacement age of 1888±5 Ma, comparable to the age of granitoid clasts in the upper part of the TSB succession. The detrital zircon data are interpreted to suggest that deposition of the precursor VMB sediments probably took place soon after an earlier pre-depositional metamorphism at ~1.98 Ga, which affected igneous source complexes dated at ~1.99 Ga and ~2.01 Ga. Mafic rocks in the southern part of the VMB ,a nd probably also the Haveri basalts, represent a renewed episode of extensional magmatism, which might correlate with the 1.96–1.95 Ga Jormua and Outokumpu ophiolites. A pre-1.96 Ga older stage basin has an expression in Sweden and complexes of similar age occur in the concealed Palaeoproterozoic basement south of the Gulf of Finland. Similar rocks, deformed and metamorphosed before ~1.96 Ga, might be present beneath the Central Finland granitoid complex and the late Svecofennian granite-migmatite zone ,a nd were possibly more local sources for both the younger stage Svionian basin sediments and the post-1.91 Ga Bothnian Basin sediments. The TSB and other post-accretionary volcanic sequences, and the associated plutonism, are interpreted to reflect a ~40 m.y .e xtensional period, inboard of the contemporaneous active margin, between orogenic phases at ~1.92 Ga and ~1.88 Ga.This interpretation provides a more satisfactory explanation of the major heat input to the crust over a very wide area than does the arc accretion hypothesis.The tectonic evolution of the Svecofennian Province has strong similarities to that of the Palaeozoic Lachlan Fold Belt in eastern Australia.

A multi-element baseline geochemical database from the western extension of the Central Africa Copperbelt in northwestern Zambia

A stream sediment geochemical survey was undertaken in the extreme north-west of Zambia with three primary objectives: (i) to provide a baseline environmental database for an area uncontaminated by heavy industry, mining and intensive farming; (ii) to detect metal anomalies that may indicate potentially economic mineral deposits; and (iii) to provide a tightly controlled geochemical database that can be used as a standard for future mineral exploration programmes throughout central Africa. Three major geological domains underlie the studied area. These are crystalline Neoarchaean to Palaeoproterozoic basement, an overlying sequence of Neoproterozoic Katanga strata within the western part of the Lufilian Arc (a Pan African orogenic belt that hosts major copper–cobalt deposits in the Central African Copperbelt of northern Zambia and the Democratic Republic of Congo [DRC]), and extensive Caenozoic Kalahari Group sediments. Stream sediments collected from each of the three geological domains have distinctive geochemical signatures. Relatively high values for a range of elements including uranium, thorium and yttrium were found in stream sediments within the pre-Katanga basement domain. Enhanced values of a larger number of elements are present in the stream sediments underlain by Katanga strata including significant, previously unknown, copper anomalies that warrant detailed follow-up work. The high metal values in sediments derived from Katanga strata contrasts sharply with sediment samples collected from drainage channels in areas of thick Kalahari sand, which mostly have very low values of all analysed elements. Our results suggest that it will be difficult to detect contamination of streams in the Copperbelt areas of Zambia and the Democratic Republic of Congo due to their naturally high metal concentrations. By contrast, it will be relatively easy to monitor pollution of areas underlain by Kalahari sands as these have naturally very low values of all elements.

U–Pb geochronology of the Leknes Group: an exotic Early Caledonian metasedimentary assemblage stranded on Lofoten basement, northern Norway

The Leknes Group is an amphibolite-facies metasedimentary assemblage thrust onto the Palaeoproterozoic basement of the Lofoten islands west of the Caledonian chain in northern Norway. Its genesis has been enigmatic, and both Sveconorwegian and Silurian ages of metamorphism have been proposed. U–Pb dating now shows that the main metamorphic and structural overprints occurred in the Mid-Ordovician, between the 469 ± 3 Ma age of a metatonalite and the 461 ± 1 Ma age of a titanite crystal co-planar with the main fabric. According to the prevalent structural interpretation this age also dates thrusting of the Leknes Group onto the Lofoten basement. Zircon and rutile in mica schists record this as well as a younger event of zircon growth at 412 ± 2 Ma, the latter coinciding with early stages of extension and enhanced fluid activity; in southern Lofoten this is also associated with the intrusion of 410 ± 3 Ma pegmatites. The Leknes Group shows an affinity with some terranes of peri-Baltican affinity in the Upper Allochthon of the Caledonides. It probably developed in a marginal basin close to Sveconorwegian basement, as shown by detrital zircon ages and published Nd isotopic data, before being thrust onto the Lofoten basement, which appears to be itself a late Caledonian allochthonous terrane.

Petrogenesis of the Orthogneisses of the Mantiqueira Complex, Central Ribeira Belt, SE Brazil: An Archaean to Palaeoproterozoic Basement Unit Reworked During the Pan-African Orogeny

The Occidental terrane of the central segment of the Brasiliano-Pan-African Ribeira belt comprises two crustal scale thrust sheets (Andrelândia and Juiz de Fora domains) taken as reworked Neoproterozoic products of the São Francisco cratonic margins. Pre-1.8 Ga orthogneisses and associated rocks of the Mantiqueira Complex comprise the basement for rocks of the Andrelândia Depositional Cycle within the Andrelândia tectonic domain. Geochemical data indicate that the Mantiqueira Complex comprises rocks that can be grouped as follows: intermediate to acid calc-alkaline rocks and a transitional basaltic series. On the basis of quantitative analysis of the lithogeochemical data, these lithotypes cannot be related. Statistical and/or petrological criteria made it possible to define suites and/or groups within each one of those units and to constrain petrogenetic models based mostly on their REE data. Simple least-square regression analysis indicates that the basic rocks are unlikely to constitute a single suite themselves. The results of the geochemical modelling presented in this work suggest that crustal partial melting rather than fractional crystallisation is the most likely petrogenetic process associated with the rocks of the Mantiqueira Complex. The partial melting processes might have taken place under oxidising conditions, typical of tectonic settings associated with the generation of calc-alkaline rocks.

U–Pb (LA–ICP-MS) dating of detrital zircons from Cambrian clastic rocks in Avalonia: erosion of a Neoproterozoic arc along the northern Gondwanan margin

Most Neoproterozoic and Early Palaeozoic tectonic syntheses place Avalonia and related peri-Gondwanan terranes facing an open ocean along the northern margin of Gondwana, thereby providing important constraints for palaeocontinental reconstructions during that time interval. However, the precise location of Avalonia along the margin and its position relative to other peri-Gondwanan terranes is controversial. We present laser ablation–inductively coupled plasma mass spectrometry U–Pb data for detrital zircons from Cambrian clastic rocks in two localities in Avalonia: the Antigonish Highlands of Nova Scotia (62 analyses) and the British Midlands (55 analyses). The data from both samples are very similar, and taken together indicate an overwhelming dominance of Neoproterozoic ( c . 580–680 Ma) or Early Cambrian source rocks with minor older Neoproterozoic clusters at c . 710 Ma or of Mesoproterozoic age, three Palaeoproterozoic zircons and one Archaean zircon. The zircons can all be derived from local Avalonian sources. The Neoproterozoic zircons are attributed to erosion of the underlying Avalonian arc. Mesoproterozoic and Palaeoproterozoic zircons of similar ages are also found in Avalonian Neoproterozoic clastic rocks and their presence in the Cambrian clastic rocks could represent recycling of Neoproterozoic strata and do not necessarily imply the presence of Mesoproterozoic or Palaeoproterozoic basement rocks within their respective drainage basins. Comparison with the data from the Neoproterozoic arc-related clastic sequences suggests significant differences between their respective drainage systems. Whereas the Neoproterozoic data require extensive drainage systems, the Cambrian data can be attributed to localized drainage systems. The change in drainage patterns could reflect rifting and isolation of Avalonia from Amazonia between c . 585 and 540 Ma. Alternatively, it might reflect the creation of topographical barriers along the northern Gondwanan margin, in a manner analogous to the Cenozoic rise of the Andes or the creation of the Basin-and-Range topography in the Western USA.

Svalbard: a fragment of the Laurentian margin

Abstract The last decade of structural and isotope-age dating studies in Svalbard and East Greenland has provided strong support for the close correlation of these segments of the Caledonide Orogen, as had previously been inferred from stratigraphic evidence. Prior to Tertiary opening of the Norweigan-Greenland Sea, Svalbard’s Caledonian terranes were an essential part of the Laurentian margin, as witnessed not only by the Early Palaeozoic depositional environments and fauna, but also by the character of the Palaeoproterozoic basement, the Meso- to Neoproterozoic cover, the evidence of late Grenvillian tectono-thermal activity, Caledonian structural style and timing of movements, Caledonian granitic magmatism and Old Red Sandstone (ORS) deposition. Recently published maps of East Greenland show the hinterland allochthons of central East Greenland to strike out obliquely into the continental shelf. The hypothesis promoted here requires that they continue offshore northwards, extending to the northern edge of the NE Greenland shelf and that most of the Svalbard terranes were northerly continuations of the East Greenland Caledonides. Only along the west coast of central Spitsbergen are ‘foreign’ terranes exposed that have affinity with Pearya, having been located north of the North Greenland foldbelt, apparently unrelated to Laurentia, prior to Ellesmerian Orogeny. The unambiguous affinity of the Svalbardian and Greenlandian (Laurentian) Caledonides contrasts markedly with the Timanide evolution of northeastern Baltica. It confirms previous interpretations that an important Caledonian suture-zone transgresses northeast-wards across the Barents Sea, separating Laurentian domains in the NW from the Timanides of Baltica in the SE. The Timanides of northeastern Europe are truncated by, and terminate in the Barentsian Caledonides of the Barents Shelf.

Ubendian basement and its late Mesoproterozoic and early Neoproterozoic structural and metamorphic overprint in northeastern Zambia

The Palaeoproterozoic basement in the Muyombe and Luwumbu River areas of northeastern Zambia comprises a WNW–ESE (to E–W) trending cordierite–garnet–sillimanite granulite unit with numerous enderbite bodies and an amphibolite-facies migmatite unit. Zircons from a biotite metatonalite intruding the granulites were dated at 1960.7 ± 0.4 Ma, and this is time-equivalent with the Nyika granite in adjacent Malawi. Mesoproterozoic intrusions into this basement are represented by a nepheline syenite at Mivula Hill (zircon age: 1360.1 ± 0.8 Ma) and the porphyritic Ntendele biotite metagranite (zircon age: 1329.1 ± 0.6 Ma). The Ntendele granite attains plutonic dimensions north of Muyombe. The Mesoproterozoic Mafinga Group occurs in two major belts imbricated in the basement, i.e., the Makutu Range belt and Nyika escarpment belt, both trending NE–SW, nearly at right angles to the Ubendian structures in the basement. Numerous smaller tectonic slices of the Mafinga Group, in the Ubendian basement gneisses farther south of the two major belts, document a significant areal extent of this unit. Structures and metamorphic effects (greenschist-facies, biotite ± garnet zone) imprinted in the Mafinga Group rocks correlate with those in the Irumide belt farther west near Isoka and to the north, in the Mafinga Hills. A mylonitic foliation and newly-formed mineral assemblages (biotite zone) in the Ntendele granite correspond to deformation and metamorphism of the Mafinga Group. Mylonitization affecting the basement complex near the Mafinga Group slices resulted in strongly but heterogeneously sheared domains with a corresponding Irumide-age structural and metamorphic overprint. 40Ar/ 39Ar dating of muscovite from mylonitic gneisses derived from basement rocks and from a Mafinga Group phyllite point to closure of the K–Ar system in the interval 860–890 Ma. These early Neoproterozoic ages correlate with published muscovite and biotite K–Ar ages of biotite gneisses and muscovite schists from other parts of the Irumide belt in NE Zambia. They probably indicate slow cooling rates after main Irumide tectonism and associated magmatism around 1015 Ma. The 40Ar/ 39Ar data show that the Muyombe area was not affected by Pan-African thermal overprinting on a regional scale. In view of contrasting relations between the area studied and published results from northern Malawi, where both structural and thermal Pan-African overprints are documented, it is suggested that the NW–SE trending Mugesse shear zone in northern Malawi and along the border with NE Zambia is an important structural boundary separating contrasting crustal domains, i.e. the Ubendian belt, Irumide belt, and East African Orogen.

The LATEA metacraton (Central Hoggar, Tuareg shield, Algeria): behaviour of an old passive margin during the Pan-African orogeny

Historically, the Tuareg shield is divided into three parts bordered by mega-shear zones with the centre, the Central Polycyclic Hoggar, characterized by Archaean and Palaeoproterozoic lithologies. Nearly 10 years ago, the Tuareg shield was shown to be composed of 23 displaced terranes [Geology 22 (1994) 641] whose relationships were deciphered in Aı̈r to the SE [Precambr. Res. 67 (1994) 59]. The Polycyclic Central Hoggar terranes were characterized by the presence of well preserved Archaean/Palaeoproterozoic and Neoproterozoic lithologies. We show here that the terranes from Central Hoggar (Laouni, Azrou-n-Fad, Tefedest, Egéré-Aleksod) belonged to a single old passive margin, to which we gave the acronym name LATEA, which behaved as a craton during the Mesoproterozoic and the Early-Middle Neoproterozoic but was partly destabilized and dissected during the Late Neoproterozoic as a consequence of its involvement as a passive margin in the Pan-African orogen. An early Pan-African phase consisted of thrust sheets including garnet-bearing lithologies (eclogite, amphibolite, gneiss) that can be mapped and correlated in three LATEA terranes. In the Tin Begane area, P– T– t paths have been established from >15 kbar––790 °C (eclogite) to 4 kbar––500 °C (greenschist retrogression) through 12 kbar––830 °C (garnet amphibolite) and 8 kbar––700 °C (garnet gneiss), corresponding to the retrograde path of a Franciscan-type loop. Sm–Nd geochronology on minerals and laser ablation ICP-MS on garnet show the mobility of REE, particularly LREE, during the retrograde greenschist facies that affects, although slightly, some of these rocks. The amphibolite-facies metamorphism has been dated at 685 ± 19 Ma and the greenschist facies at 522 ± 27 Ma. During the thrust phase, the Archaean–Palaeoproterozoic basement was only locally affected by the Pan-African tectonics. LATEA behaved as a craton. Other juvenile terranes were also thrust early onto LATEA: the Iskel island arc at ≈850 Ma to the west of LATEA, the Serouenout terrane in the 700–620 Ma age range to the east. No subduction-related magmas have intruded LATEA during this epoch, which behaved as a passive margin. During the main Pan-African phase (625–580 Ma), LATEA was dissected by mega-shear zones that induced several hundreds km of relative displacement and allowed the emplacement of high-K calc-alkaline batholiths. Smaller movements continued till 525 Ma, accompanied by the emplacement of subcircular plutons with alkaline affinity. Here is dated the Ounane granodiorite (624 ± 15 Ma; 87Sr/ 86Sr i =0.70839 ± 0.00016; 6WR, MSWD=0.87) and the Tisselliline granite (552 ± 15 Ma; 87Sr/ 86Sr i =0.7074 ± 0.0001; 5WR, MSWD=1.4). Nd isotopes indicate a preponderant Palaeoproterozoic crustal source for these two plutons: ε Nd=−14 to −21 at 624 Ma and T DM=1650–2320 Ma for Ounane and ε Nd=−13 to −15 at 555 Ma and T DM=1550–1720 Ma for Tisselliline. Our model links these intrusions to a linear lithospheric delamination along mega-shear zones, allowing the hot asthenosphere to rise, melt by adiabatic pressure release and inducing the melting of the Palaeoproterozoic and Archaean lower crust. The LATEA cratonic microcontinent remained however sufficiently rigid to preserve Archaean and Palaeoproterozoic lithologies as well as Middle Neoproterozoic oceanic thrust sheets. This corresponds to the notion of metacraton [J. African Earth Sci. 34 (2002) 119], i.e. a craton that has been remobilized during an orogenic event but is still recognizable dominantly through its rheological, geochronological, isotopic and sometimes petrological characteristics.

The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton

Since 1980, several diverging models for the Pan-African fold-belt of the Anti-Atlas mountains have been proposed. Differences mainly result from diverging interpretations of geological, geochemical and geochronological data. However, the fundamental point is the lack of a detailed lithostratigraphic framework of the Neoproterozoic successions, supported by geochronological data and regional correlations. Furthermore, these models did not evolve from multidisciplinary studies which are essential in the case of this belt, whose structure is complex and diversified. We discuss here some points taken as a base by Ennih and Liegeois to conclude that the entire Anti-Atlas is underlain by Eburnian crust and that the northern limit of the West African craton (WAC) and its passive margin is located at the South Atlas Fault (SAF). This led the authors to propose a new model of the Anti-Atlas belt, which also explains the origin of the Khzama-Bouazzer ophiolite. 1. Evidence for a Palaeoproterozoic basement in Saghro?

Geology and correlation of the central Irumide belt

The Irumide Fold belt of Zambia consists of a Palaeo to Mesoproterozoic complex of gneisses and granite gneisses and a supracrustal sequence of quartzites and pelites. Although no direct correlation is possible, the metasedimentary sequence is tentatively equated with the Manshya River Group described in the NE. The basement to the Irumide belt consists of the Bangweulu Granites to the North, and the Mkushi gneiss basement (MGB) to the Southwest. Age constraints on the Bangweulu Block are limited, but the Mkushi Gneiss has been dated at ∼2050 Ma. A detrital provenance study on a quartzite of the Manshya River Group near Mkushi indicates derivation from terranes of up to 3180 Ma, with a maximum age for the Manshya River Group set by the youngest detrital grain at 1941 Ma. Detrital cores from paragneisses and migmatites in the Serenje area indicate a more uniform detrital source for the sedimentary protolith of 2050–2000 Ma. A more direct age constraint on the Manshya River Group has been provided through an age of 1880 Ma on a concordant rhyolite in the metasedimentary sequence near Chinsali. The Manshya River Group consists of a succession of four pelite and four quartzite Formations. The granitoids, which make up a large portion of the Irumide belt, can be subdivided into an older suite of leucocratic gneisses and biotite-granites, and a younger suite of alkaline, often porphyritic K-feldspar granites. Oversimplified lithostratigraphic division of the “crystalline basement” in the southwest of the Irumide belt has lead to an overestimation of the Palaeoproterozoic basement (MGB). The extent of the Palaeoproterozoic basement awaits re-evaluation through fieldwork. The main structural trend of the Irumide belt is northeast and is related to extensive crustal shortening during the main stage of the Irumide orogeny. Tectonic transport is directed towards the northwest, with southwest directed backthrusting locally developed to define a double verging overall structure. The compressional stage is characterised by amphibolite grade metamorphism and accompanied by widespread granite magmatism and anatexis.

Relationships between Archaean and Palaeoproterozoic crystalline basement complexes in the southern part of the East Greenland Caledonides: an ion microprobe study

The Caledonian orogenic belt in East Greenland encompasses rock complexes with a long-lived evolution of structural and metamorphic events. Due to the strong Caledonian overprint in the region it has hitherto proved difficult to unravel the earlier history. New U–Pb zircon analyses have revealed that the crystalline basement complexes can be divided into Archaean and Palaeoproterozoic terrains. The former terrain yields U–Pb ages between ≈2700 and 2800 Ma, while the latter yields ages between ≈1900 and 2000 Ma. New Nd isotopic analyses suggest that the late Archaean terrain contains juvenile crust together with gneisses incorporating various amounts of slightly older continental crust (3000 Ma). The Archaean basement terrain is exposed in the southern part of the study area, south of 72°50′N; it is cut by Palaeoproterozoic intrusions ranging in size from centimetre-scale dykes to kilometre-size plutons. A low-strain lens in the Archaean basement preserves an ≈2700-Ma-old leucosome granite. North of 72°50′N the basement terrain is Palaeoproterozoic (≈2000 Ma) in age, part of a major Palaeoproterozoic province that extends all the way up to 81°N. In the study area (72–73°N) it is not possible to differentiate between the two basement terrains on field criteria. Geochemical data suggest that the transition from Archaean to Palaeoproterozoic basement is a result of Palaeoproterozoic subduction of oceanic crust beneath the Archaean basement. No Caledonian zircons or zircon rims were found in the crystalline basement rocks suggesting that conditions for new zircon growth were not optimal during Caledonian reworking, or that the Caledonian overprint was weak in the western part of the fold belt.

Precise U–Pb zircon ages from Alderney, Channel Islands: growing evidence for discrete Neoproterozoic magmatic episodes in northern Cadomia

The northernmost exposures of rocks formed during the Late Neoproterozoic Cadomian orogeny in the Channel Islands–northern France region occur on Alderney. The island mainly comprises foliated quartz diorite, once considered to be 2 Ga, pre-Cadomian basement, and an undeformed basic to intermediate plutonic complex. A precise age of 610±2 Ma, based on U–Pb analyses of single and small groups of zircons, for the foliated Fort Tourgis quartz diorite demonstrates that the oldest rocks were emplaced and deformed during a Cadomian magmatic event. The age is virtually identical to ages from similar, foliated syntectonic quartz diorite bodies on the islands of Guernsey and Sark and at La Hague (north Normandy), indicating that this magmatic and deformational event was regional in extent. Discordant zircon xenocrysts define an upper intercept age of c. 2 Ga indicating the presence of Palaeoproterozoic basement at depth. Single zircons from the undeformed Bibette Head granodiorite give a precise U–Pb age of 572±1 Ma. This age is closely similar to that for the emplacement of the Northern Igneous Complex of Guernsey. The emerging data indicate that Cadomian magmatism in the northern Channel Islands region was not a protracted continuum, but occurred during two distinct, short-lived events separated by c. 30–40 my.

Affinités géochimiques et implications géodynamiques des dykes basiques de la boutonnière d'Iguerda-Taïfast (Anti-Atlas central, Maroc)

Geochimical affinities and geodynamic implications of mafic dykes of Iguerda-Taffat inlier (central Anti-Atlas, Morocco). In the inlier of Iguerda-Taïfast (Anti-Atlas, Morocco) the Neoproterozoic mafic rocks consist dominantly of dolerite dykes intruded the Palaeoproterozoic basement and overlain by the late Neoproterozoic series. Major and immobile trace elements (HFS and REE elements) allow to identify two groups of mafic rocks : (i) the first is tholeiitic, (ii) the second shows a transitional affinity with more fractionated chondrite normalized REE patterns . Their geoche-mical signatures display similarities with those of intraplate series found in young continental rift structures (they are enriched in incompatible elements, excepted Y and Yb, relative to N-MORB, but they are depleted in Nb). This preorogenic magmatism can be related to the begining of rifting in the northern border of the West-African craton, which allows the emplacement of bassins of the Anti-Atlas Pan-African belt. This rifting can be related, at the same period, to the opening of an oceanic domain in Siroua and Bou-Azzer areas.

Surface and subsurface facies architecture of a small hydroexplosive, rhyolitic centre in the Mesoproterozoic Gawler Range Volcanics, South Australia

At Menninnie Dam, South Australia, a drilling program has revealed a complete section through the subsurface feeder system and erupted products of a small, hydroexplosive, rhyolitic centre within the Mesoproterozoic Gawler Range Volcanics. Porphyritic rhyolite intruded near-vertical faults in the Palaeoproterozoic basement and at less than a few hundred metres depth, interacted with fault-hosted (hot?) groundwater. Hydrofracturing of the wall rock occurred in advance of and at the margins of the rhyolitic intrusions. The rhyolitic intrusions have peperitic margins and grade into discordant lithic-rich PB facies. The advancing fragmentation front intersected the palaeosurface, triggering phreatic eruptions that deposited a poorly sorted, lithic-rich explosion breccia. Rhyolite then rose to the surface through the intrusive breccias and shallow-seated magma–water interaction occurred in the conduit within <50m of the surface. As the magma discharge rate increased, “dry” explosive activity prevailed. A fall deposit, the top of which is welded, was deposited close to the vent, and in more distal locations (>800m from the inferred source), the products include muddy sandstone and pumice breccia. At the end of the eruption, rhyolitic lava was extruded in the form of a small dome. The presence of contemporaneous Pb–Zn–Ag mineralisation in the wall rocks suggests that an active hydrothermal system may have been involved in the formation of the Menninnie Dam hydroexplosive volcanic centre.

Exhumation of eclogite-bearing rocks by thrusting and upper crustal extension in the Western Gneiss Region of the Scandinavian Caledonides

In the Western Gneiss Region (WGR) of the Scandinavian Caledonides, eclogites crystallized in Palaeoproterozoic basement gneisses of the margin of continent Baltica during Scandian collisional orogeny with Laurentia. Underthrusting of the Baltoscandian ma

Rapakivi-related basement structures in the Baltic Sea area; a regional approach

The largest massifs of rapakivi granites and related rocks in the western part of the East European Craton are located within the junction area of the Baltic Sea and the Gulfs of Bothnia, Finland and Riga, and along the Gulf of Finland. The rapakivi massifs are of Palaeoproterozoic to Mesoproterozoic (Subjotnian) age, ranging from about 1.67 to 1.5 Ga. In the framework of the Palaeoproterozoic basement tectonics, all major and minor occurrences of rapakivi magmatism are areally limited to within the boundaries of the Svecofennian juvenile crustal domain and its frontiers. The most voluminous of the intrusive bodies are located within the originally thickest (55–65 km) primary part of Svecofennia, where they are coupled with crustal thinning to 40–45 km and mafic underplating anomalies. Only minor rapakivi bodies have intruded the originally thinner (40–45 km) peripheral Palaeoproterozoic crust. The largest igneous subprovinces, namely the Wiborg and Riga-Åland rapakivi massifs, are furthermore connected to specific regional Bouguer gravity anomalies. These anomalies exhibit complex patterns due to the rapakivi-related reworking of the upper mantle and the crust. The bimodal rapakivi and related upper crustal magmatism was systemised in such a way that satellite bodies with minor stock and dike suites became located concentrically around the main igneous bodies. Considering the different ages and locations of the igneous suites, an internal subprovincial distribution occurs in the province. The time of formation for each subprovincial cluster is no longer than 50 m. y. The timing of birth and extinction of the different subprovinces rarely coincide with each other.