Paramaka Formation

The Paramaka Formation consists of a series of lavas, tuffs and volcaniclastic sediments with some intercalated chemical sediments, all metamorphosed in the greenschist facies and, close to TTG intrusions, to the amphibolite facies (Bosma et al., Reference Bosma, Kroonenberg, Maas and De Roever1983, Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984; Veenstra, Reference Veenstra1983). Metabasalts with pillow structures were observed in the Saramacca area and in the Rosebel Gold Mine (Fig. 5) and show by their low-K tholeiïtic chemistry ocean floor probably back-arc affinities (Veenstra, Reference Veenstra1983; Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011). The metabasalts from Royal Hill in the Rosebel Gold Mine have flat chondrite-normalised REE patterns enriched in average around 10× to 25× chondrite (Veenstra, Reference Veenstra1983; Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011). La_N/Yb_N ratios vary between 1.70 and 2.10, indicating a very low fractionation of REE (Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011). These form probably the base of the sequence, although their lower contact has not been observed. Higher up, more differentiated meta-andesites, metadacites, metarhyolites and associated tuffs occur with a calc-alkaline signature. Intercalated phyllites, and carbonaceous and ferruginous cherts increase towards the top of the sequence. Meta-andesites are slightly enriched in LREE, the rhyolites much stronger, with chondrite-normalised values for LREE between 97× and 187× chondrite and La_N/Yb_N values that vary between 8.98 and 21.14 (Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011). This shows that volcanism in the Paramaka Formation in and near the Rosebel area recorded a change in tectonic context from a back-arc extensional basin environment to a subduction-dominated environment (Veenstra, Reference Veenstra1983; Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011).

Fig. 5. Pillow structures in Paramaka metabasalts from Poederberg, Stonbroekoe Mountains.

Greenschist-facies metamorphism converted the basalts into massive actinolite–epidote–chlorite–sodic plagioclase greenstones with commonly relict porphyritic, amygdaloidal and fluidal textures, as well as amphibole schists. Andesites and rhyolites are metamorphosed into chlorite and sericite schists. Amphibolite-facies equivalents, commonly found around the TTG plutons, include common amphibolites, clinopyroxene-garnet-bearing metabasalts, banded ironstones (itabirites; Bosma, Reference Bosma1973a), spessartine quartzites (gondites; Holtrop, Reference Holtrop1962; Bosma, Reference Bosma1969), schists with chloritoid, paragonite, staurolite, kyanite and garnet, and some calcsilicate rocks (Bosma et al., Reference Bosma, Kroonenberg, Maas and De Roever1983).

In southern French Guiana two zircon ages have been obtained from metaquartzandesites of the same formation of 2156 ± 6 Ma (U–Pb ion microprobe) and 2137 ± 6 Ma (Pb evaporation) (Delor et al., Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a). No reliable direct age data are available for the oceanic metabasites at the base.

In northwestern Suriname, west of the Bakhuis horst, similar rocks reappear, the first vestige of the western branch of the greenstone belt that continues into Guyana across the Takutu graben. Loemban Tobing (Reference Loemban Tobing1969) refers to them as spilite–quartzkeratophyre suite, and calls them Matapi Formation. Their description resembles that of the Paramaka metabasalts and related metavolcanics, while chemical analyses have shown that the designation ‘spilite’ is incorrect (De Vletter et al., Reference De Vletter, Aleva and Kroonenberg1998).

Bemau Ultramafitite

Closely associated with the Paramaka Formation ultramafic–mafic intrusive bodies occur, such as the Bemau ultramafic complex in the Saramacca area, described in great detail by Veenstra (Reference Veenstra1983). Similar bodies are found in the De Goeje Mountains in southeast Suriname, and the name De Goeje Gabbro was formerly used to indicate all punctual gabbroic and ultramafic bodies in Suriname (Bosma et al., Reference Bosma, Kroonenberg, Maas and De Roever1983, Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984; De Vletter, Reference De Vletter and De Vletter1984). However, new data have shown that there are at least two generations of gabbroic bodies: (1) those associated with the greenstone belt, such as the Bemau and De Goeje complexes, which have been dated in French Guiana at around 2147 Ma (Tampok gabbro, Pb–zircon evaporation; Delor et al., Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a), and (2) a much younger suite in western and central Suriname, the Lucie Gabbro, dated at 1985 Ma (see below). De Roever & Bosma (Reference De Roever and Bosma1975) suggested that the Bemau-type bodies should not be included in the De Goeje Gabbro, although this has not been heeded in their later papers. To avoid confusion we prefer to abolish the name De Goeje Gabbro altogether (De Roever, Reference De Roever2014) and introduce Bemau Ultramafitite for those in the greenstone belt and Lucie Gabbro for the younger suite.

The Bemau ultramafic complex in Saramacca consists of websterites, dunites, clinopyroxenites and their metamorphosed equivalents, ultramafic schists with serpentine, chlorite, tremolite, talc and carbonate. They are associated with minor metagabbros and quartzdiorites. Both the ultramafics and the metagabbro and quartzdiorite show enrichment of LREE similar to modern andesites and high-alumina basalts (Veenstra, Reference Veenstra1983). The De Goeje Gabbro in southeast Suriname shows similar REE patterns (Bosma et al., Reference Bosma, Maas and De Roever1980). The geochemical data suggest that the Bemau ultramafics result from fractional crystallisation of gabbroic–andesitic parent magma by early separation of olivine, magnetite, clinopyroxene and minor orthopyroxene, and later intercumulus crystallisation of amphibole, phlogopite and plagioclase. The metagabbro and quartzdiorite represent the residual liquids after separation of the cumulates. It is conceivable that the Bemau Ultramafitite bodies represent the feeder pipes of the subduction-related andesites in the Paramaka Formation (Veenstra, Reference Veenstra1983; De Roever, Reference De Roever2014).

Kabel Tonalite

Large parts of the central greenstone belt are occupied by ellipsoidal batholiths of tonalites, trondhjemites and granodiorites, designated as Kabel Tonalite by Bleys (Reference Bleys1951), after Kabel village, now submerged in the Afobaka storage lake. The contacts with the surrounding Paramaka rocks are conformable in such a way that the metavolcanics, usually in the amphibolite facies, wrap around the outlines of the intrusions, as is clearly visible from the LANDSAT imagery (Kroonenberg & Melitz, Reference Kroonenberg and Melitz1983). This configuration suggests diapiric ascent of the TTG magma and contemporaneous deformation of the surrounding metavolcanics (Veenstra, Reference Veenstra1983), a classic feature in many other Archean and Paleoproterozoic greenstone belts in the world. The westernmost body, the Saramacca batholith, has been studied in great detail by Veenstra (Reference Veenstra1983).

Tonalites are concentrated along the contact zones, while trondhjemites and granodiorites occupy the central part of the batholith. A similar concentric pattern is reported from the Central Guiana Batholith in French Guiana (Delor et al., Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a), consistent with the diapiric nature of the batholith. The TTG rocks often display a foliated aspect, especially near the contact zones. In the field the tonalite shows sharp intrusive contacts with the ultramafic Bemau rocks (Veenstra, Reference Veenstra1983). Elsewhere there are transition zones with banded migmatitic gneisses and amphibolites.

Tonalites contain hornblende and biotite as mafic minerals, trondhjemites only biotite. Oligoclase is the dominant feldspar in most rocks. Textures are clearly magmatic. Their K₂O/Na₂O ratios are 0.24 for tonalites and 0.38 for trondhjemites, and they plot as VAG (volcanic arc granite) in the Rb/Yb+Ta discriminant plot of Pearce et al. (Reference Pearce, Harris and Tindle1984) (De Vletter & Kroonenberg, Reference De Vletter and Kroonenberg1987). They show a calc-alkaline differentiation trend (Holtrop, Reference Holtrop1969), and are chemically indistinguishable from diapiric tonalite bodies in Archean greenstone belts (Veenstra, Reference Veenstra1983). The whole range of tonalite zircon–Pb evaporation ages in neighbouring French Guiana is around 2.18–2.16 Ga in the north and south of the country, with younger ages around 2.15–2.13 Ga in central French Guiana. From the continuation of the southernmost Suriname tonalite batholith into French Guiana a zircon–Pb evaporation age of 2141 ± 8 Ma has been obtained (Delor et al., Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a). In Amapá, granitoids associated with subduction furnished ages between 2.19 and 2.13 Ga (Avelar et al., Reference Avelar, Lafon and Delor2002; Rosa-Costa et al., Reference Rosa-Costa, Lafon and Delor2006, Reference Rosa-Costa, Monié, Lafon and Arnaud2009).

Armina Formation

The Armina formation as used by GMD (1977) and Bosma et al. (Reference Bosma, Kroonenberg, Maas and De Roever1983) has had a long and confusing history of past name changes, due to uncertainty about its depositional environment and its stratigraphic position with regard to the Paramaka and Rosebel Formations and the Kabel TTG batholiths (cf. Doeve, Reference Doeve1957; D'Audretsch, Reference D'Audretsch1957; O'Herne, Reference O'Herne1969a). Recent studies in the Rosebel Gold Mines (Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011) and along the Marowijne River, including the type locality Armina Falls (Naipal & Kroonenberg, Reference Naipal and Kroonenberg2016), have clarified its origin and stratigraphic position to a great extent.

The Armina Formation consists of regularly alternating sequences of low-grade metagreywacke and phyllite, called flysch facies by Bosma & Groeneweg (Reference Bosma and Groeneweg1973) and since then indeed recognised as metaturbidites (Bosma et al., Reference Bosma, Kroonenberg, Maas and De Roever1983, Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984; Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011; Naipal & Kroonenberg, Reference Naipal and Kroonenberg2016; Watson, Reference Watson2008, Fig. 6). The sequences along the Marowijne river show individual flow units of 10 cm to 5 m thickness, starting with a coarse-grained graded metagreywacke bed, and topped by a few centimetres of fine-grained metasiltstone or phyllites with parallel or convoluted lamination, cross-bedding and locally climbing ripples. Three different metaturbidite facies have been distinguished on the basis of field, petrographic and diagenetic features, showing northwards slightly increasing maturity. Metamorphic grade also increases northwards, ranging from chlorite–sericite-rich assemblages to biotite- and garnet-bearing ones, though all within the greenschist facies. Conspicuous calcsilicate nodules in one of the three metaturbidite facies consist of garnet, actinolite, clinozoisite and plagioclase. Chemically the variability within the individual flow units is greater than between the different facies. Monomineralic clasts are mainly quartz and (igneous) plagioclase grains. Lithic clasts are Paramaka metavolcanics, chert and phyllites, but also tonalite or trondhjemite fragments, suggesting that the TTG batholiths were already exhumed when the turbidites were deposited (Naipal & Kroonenberg, Reference Naipal and Kroonenberg2016). This is corroborated by the juvenile isotopic character of metagreywackes in French Guiana (positive ε(Nd)_t values), which might testify to the erosion of the TTG granitoids, although metapelites give negative values (Delor et al., Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a).

Fig. 6. Armina Formation metaturbidite sequence at the type locality, Marowijne River.

The Armina Formation metaturbidites in the Rosebel Gold Mine have conspicuous conglomeratic intercalations, consisting mainly of well-rounded, tectonically flattened metavolcanic and metagabbroic clasts, with little evidence for tonalitic sources (Watson, Reference Watson2008; Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011; Naipal & Kroonenberg, Reference Naipal and Kroonenberg2016). Chondrite-normalised REE profiles from the mine are LREE-enriched, showing values varying between 44× and 213× chondrite and La_N/Yb_N values between 8.4 and 33.1, with an average value of 17.9, comparable to the value of 16.1 for calc-alkaline metavolcanic rocks (Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011). According to the latter authors detrital zircons in the Armina Formation would indicate a maximum age of 2127 ± 7 Ma for the deposition of the metaturbidite sequence, citing Milési et al. (Reference Milési, Egal, Ledru, Vernhet, Thiéblemont, Cocherie, Tegyey, Martel-Jantin and Lagny1995). However, Milési et al. (Reference Milési, Egal, Ledru, Vernhet, Thiéblemont, Cocherie, Tegyey, Martel-Jantin and Lagny1995) refers to this age as belonging to the Rosebel Formation.

Taffra Schist

The contact zone between the Armina Formation and the Patamacca Two-Mica Granite (see below) in northeast Suriname is about 5 km wide, and is characterised by well-foliated coarse-grained staurolite–garnet–biotite schists, dubbed Taffra Schists in the past (Schols & Cohen, Reference Schols and Cohen1951, Reference Schols and Cohen1953). Locally they contain kyanite, andalusite or fibrolite. While these rocks are generally considered to be contactmetamorphic equivalents of the Armina Formation, we prefer to consider them as a separate unit, as the characteristic turbiditic sedimentary structures of the Armina Formation are no longer discernible in them, and in some areas they border Paramaka-type rocks. Moreover, the pronounced schistosity and the mineral assemblages suggest regional rather than contact metamorphism of the pelitic protoliths. No modern data are available.

Patamacca Two-Mica Granite

At least five individual plutons of two-mica granite, called Patamacca Granite by Yang (Reference Yang2014), intrude into the Armina Formation in northeast Suriname, causing aureoles of staurolite–garnet–biotite schists. The granites are slightly gneissose, and near the margins of the granites many lens-shaped metasedimentary enclaves are found, with a strike similar to that of the adjoining metasediments (Bosma et al., Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984). These features, together with the schistosity and medium-pressure mineral assemblages in the adjoining Taffra Schist, suggest that the ascent of these plutons may also have been diapiric, just like the Kabel Tonalite. The contact zone between the Taffra Schist and the granite contains numerous pegmatite veins, some of which carry amblygonite, cassiterite, tantalite and beryl (Montagne, Reference Montagne1964). Based on major element composition the Patamacca Granite can be classified as a peraluminous S-type granite, pointing to derivation from a high-grade metapelitic parent lithology. Combined major and trace element signatures suggest an origin in a syn-collisional tectonic setting (Yang, Reference Yang2014). The small leucocratic garnet-bearing two-mica granite in the Rosebel Gold Mine area (Brinck, Reference Brinck1955) is peraluminous, has a highly enriched REE distribution and has the most calc-alkaline affinity of all the rocks at the mine (Watson, Reference Watson2008). In French Guiana a U–Pb (SIMS) age of 2060 ± 4 Ma was obtained on zircon from the Petit Saut two-mica granite from the same belt, and monazite from the same granite gave a U–Th–Pb (EPMA) age of 2059 ± 23 Ma (Delor et al., Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a).

Rosebel Formation

The uppermost metasedimentary formation in the Marowijne Greenstone Belt is the Rosebel Formation, named by Schols & Cohen (Reference Schols and Cohen1951) for the Rosebel Savanna area, the site of the present-day open-pit Rosebel Gold Mine. The Rosebel Formation overlies the Armina and Paramaka Formations unconformably with a basal metaconglomerate, as has been observed in the mine (Watson, Reference Watson2008; Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011). Higher in the sequence conglomeratic intervals also occur. The most characteristic component is a cross-bedded greyish quartz-rich meta-arenite, with magnetite grains concentrated at the base of the foresets (Fig. 7). The metaconglomerates are polymictic, quartz-rich, but with also abundant phyllite clasts (Fig. 8) supposedly derived from the underlying Armina Formation (Bosma et al., Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984).

Fig. 7. Cross-bedded Rosebel meta-arenite, with magnetite concentrated at the base of the foresets. Royal Hill pit, Rosebel Gold Mine.

Fig. 8. Rosebel Formation meta-arenite with phyllite and quartz pebbles (left: parallel to bedding, right: perpendicular to bedding). Royal Hill pit, Rosebel Gold Mine.

In Landsat imagery the Rosebel and Armina Formations show strikingly different morphologies (Cohen & Van der Eijk, Reference Cohen and Van der Eijk1953; O'Herne, Reference O'Herne1969b; Kroonenberg & Melitz, Reference Kroonenberg and Melitz1983). Soils on Rosebel Formation are whitish and sandy, while those in the Armina Formation are reddish and clayey. Nevertheless distinction between the two formations is sometimes difficult in the field, especially when less mature sediments are intercalated (cf. Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011). Naipal & Kroonenberg (Reference Naipal and Kroonenberg2016), using factor analysis on major element analytical data, show that real Rosebel rocks are consistently lower in Fe and Na than Armina sediments, and that many samples from the mine may have been misclassified by Daoust et al. (Reference Daoust, Voicu, Brisson and Gauthier2011) as Rosebel.

Sedimentary structures in the Rosebel metasandstones suggest deposition in a fluvial environment. The higher maturity of the sediments suggests provenance from a more weathered hinterland; the magnetite may be all that remains from deep chemical weathering of the Paramaka metavolcanics. This means that an interval of uplift, erosion and deep weathering must have occurred between the deposition of the Armina turbidites and the Rosebel fluvial sands. The equivalent in French Guiana, the Upper Detrital Unit, is supposed to have been deposited in pull-apart basins formed in a late stage of transpressional deformation of the greenstone belt (Milési et al., Reference Milési, Egal, Ledru, Vernhet, Thiéblemont, Cocherie, Tegyey, Martel-Jantin and Lagny1995; Vanderhaeghe et al., Reference Vanderhaeghe, Ledru, Thiéblemont, Egal, Cocherie, Tegyey and Milési1998; Delor et al., Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a). The U–Pb geochronology of detrital zircons on these rocks in French Guiana suggests a maximum age of deposition of 2115 ± 4 Ma (Milési et al., Reference Milési, Egal, Ledru, Vernhet, Thiéblemont, Cocherie, Tegyey, Martel-Jantin and Lagny1995; Daoust et al., Reference Daoust, Voicu, Brisson and Gauthier2011), and possible derivation from the Kabel Tonalite.

In western Suriname, west of the Bakhuis horst, the sericite quartzites and oligomictic quartz-rich metaconglomerates of the Ston Formation crop out next to the Paramaka-like Matapi ‘spilites’ (Loemban Tobing, Reference Loemban Tobing1969; see above). They are more mature than the Rosebel Formation because of the total absence of feldspar, and show a higher degree of quartz recrystallisation, obliterating the clastic fabric of the rock. Metaconglomerates also include, next to quartz pebbles, metachert-like fragments probably derived from Paramaka-like chemical metasediments. In contrast to the eastern limb of the greenstone belt they show open folding, and have been observed by Loemban Tobing (Reference Loemban Tobing1969) to be intruded by sills of the overlying Dalbana metavolcanics (see below), although later unpublished reports suggest the Ston to be conformably overlain by Dalbana metavolcanics (see below). Locally the quartzites show contact metamorphism by the intrusion of surrounding Wonotobo Granites (see below). Gibbs & Barron (Reference Gibbs and Barron1993) correlate the Ston Formation with the Muruwa Formation in Guyana. No modern geochronological data are available.

Sara's Lust Gneiss

Along both its northern and its southwestern flank the greenstone belt is bordered by a zone of high-grade metamorphic, often migmatitic gneisses and amphibolites (Bosma et al., Reference Bosma, Kroonenberg, Maas and De Roever1983, Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984). Outside the greenstone belt many mappable enclaves of similar rocks occur, in the vast expanses occupied by the Older Granites in central Suriname (see below). We call these rocks Sara's Lust Gneiss, after the crushed rock quarry at the old plantation with that name along the Suriname River (Fig. 9). On the 1977 map these rocks received the same signature as the high-grade rocks in the Coeroeni area, but recent data show that they are probably over 100 Ma older, and therefore preferably go with their own name.

Fig. 9. Intensely folded migmatitic Sara's Lust Gneiss at type locality. Black arrow indicates calcsilicate nodule with carbonate core.

The rocks in the northern segment of the belt are predominantly migmatitic hornblende–biotite gneisses, biotite–plagioclase gneisses, garnet–biotite gneisses and quartzofeldspathic gneisses, with minor amphibolites, locally with garnet or clinopyroxene, and furthermore pelitic sillimanite–biotite–muscovite gneisses and calcsilicate rocks, clearly of supracrustal origin (Fig. 9). In the southwestern segment of the belt near the De Goeje Mountains andalusite–cordierite sillimanite schists, garnet-biotite gneiss and pyroxene gneisses are also found (Van Eijk, Reference Van Eijk1961). Metamorphism is mainly in the higher amphibolite facies, but some gneisses and amphibolites in the southern belt and in the enclaves contain orthopyroxene, testifying that granulite facies conditions were reached (Barink, Reference Barink1975; Ho Len Fat, Reference Ho Len Fat1975). The gneisses in the northern belt are not simply the higher-grade equivalents of the adjacent Taffra Schists because the latter are mainly pelitic, whereas the Sara's Lust Gneiss is mostly tonalitic in composition. They might, however, be the high-grade metamorphic equivalents of the Paramaka meta-andesites and related rocks.

In the continuation of the southern gneiss belt into southern French Guiana tonalitic gneisses of the Tamouri Complex, associated with gneisses with varying amounts of biotite, garnet and sillimanite, are dated between 2155 and 2165 Ma (Delor et al, Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a). No age data are available from the northern belt. In the Île de Cayenne, French Guiana, ages of over 2200 Ma have been found for trondhjemite gneiss, amphibolite and gabbro (Vanderhaeghe et al., Reference Vanderhaeghe, Ledru, Thiéblemont, Egal, Cocherie, Tegyey and Milési1998; Delor et al., Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a). These are the oldest rocks found in the greenstone belt so far, but it is uncertain whether Sara's Lust Gneiss can be correlated with them as Delor et al. (Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a) suggest.

Gran Rio Granite

The most common granites among the Older granites are medium- to coarse-grained biotite granite and hornblende–biotite granite, in part with alkali feldspar megacrysts. They may show vague banding and migmatitic ghost structures. This rock unit was named Gran Rio Granite by IJzerman (Reference IJzerman1931). It crops out extensively along the Gran Rio River and the Tapanahony River and its tributaries (Haug, Reference Haug1966; Groeneweg, Reference Groeneweg1969; Barink, Reference Barink1975; Ho Len Fat, Reference Ho Len Fat1975). Chemical data are not available. Recently four samples from this unit in southeastern Suriname have been dated by Lafon (Reference Lafon2013) using the ²⁰⁷Pb–²⁰⁶Pb evaporation method for zircons (see Table 2). The age data are slightly younger than the youngest rocks of the the greenstone belt, and therefore we consider these granites as representing the deeper anatectic products of the greenstone belt.

Table 2. Geochronology of Older granites from central Suriname (for sample location see Fig. 4).

Pikien Rio Pyroxene Granite

Within the area of the Older granites, as along the Pikien Rio (Ho Len Fat, Reference Ho Len Fat1975) irregular-shaped outcrops of pyroxene granites occur, often in association with enclaves of migmatitic, in part granulite-facies gneisses. Clinopyroxene is the most common mafic mineral, but locally orthopyroxene is also present. The latter rocks may be called charnockites. Charnockites and pyroxene granites are also widespread in the Bakhuis Mountains in West Suriname (Kabalebo Charnockite), and on the 1977 map they were indicated with the same legend unit as those described here. However, a recent ²⁰⁷Pb–²⁰⁶Pb age obtained by Lafon (Reference Lafon2013) shows that the Pikien Rio pyroxene granites are just as old as the Gran Rio granites and more than 100 Ma older than the Kabalebo Charnockite (cf. Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015, Reference Klaver, de Roever, Thijssen, Bleeker, Söderlund, Chamberlain, Ernst, Berndt and Zeh2016; see also below). There is a striking concentration of younger gabbroic intrusions in the pyroxene granites.

Bakhuis Granulite

The granulites are mainly intermediate or mafic in composition and show marked banding on the centimetre to decimetre scale. The dominant compositional banding is inherited from the supracrustal protolith, whereas finer parallel banding is caused by incipient migmatisation. The protolith was in part sedimentary, as indicated by the presence of partly mappable intercalations of pelitic gneisses, sillimanite quartzites, spessartine quartzites and calc-silicate granulites. Part of the mafic granulites probably are of volcanic origin, as they plot in the island arc tholeiite field in trace element discrimination diagrams (Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015). Intermediate granulites predominate. They are dark-grey, banded rocks, with granoblastic plagioclase and quartz besides orthopyroxene, with clinopyroxene and hornblende in darker bands, and with only orthopyroxene in coarse-grained leucosome bands. The plagioclase typically is antiperthitic. Perthitic alkali-feldspar and mesoperthite occur mainly in rather rare felsic granulites. Mafic granulites comprise both hornblende-rich pyroxene amphibolites and nearly hornblende-free (pyroxene-rich) granulites, commonly with some biotite and more rarely with garnet.

Stondansi Gneiss

The metapelitic gneisses, commonly rich in sillimanite and migmatitic, are called after the large occurrence near the Stondansi Falls in the Nickerie River. In a rather large metapelite area in the northeast of the BGB mineral assemblages orthopyroxene + sillimanite + quartz and sapphirine + quartz have been found, characteristic for UHT metamorphism (De Roever et al., Reference De Roever, Lafon, Delor, Rossi, Cocherie, Guerrot and Potrel2003a; Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015). Elsewhere the metapelitic gneisses mainly show an assemblage cordierite + sillimanite, also formed at UHT conditions. Orthopyroxene crystals show a marked zonation, with up to 10% Al₂O₃ in the core, and considerably lower Al₂O₃ along the rim. This indicates an anticlockwise cooling path after peak UHT metamorphism (De Roever et al., Reference De Roever, Lafon, Delor, Rossi, Cocherie, Guerrot and Potrel2003a). Cordierite in the gneisses is commonly partially or fully replaced by fine-grained sillimanite, Al-poor orthopyroxene and biotite, with additional kyanite or, more rarely, andalusite. This points to an anticlockwise cooling path. In these cordierite pseudomorphs a new beryllium-bearing Mg–Al silicate, surinamite, was discovered by De Roever et al. (Reference De Roever, Kieft, Murray, Klein and Drucker1976).

Werekitto Gneiss

Quartzofeldspathic gneisses cover a large area in the Coeroeni Gneiss belt, and are here designated as Werekitto Gneiss. Robert Schomburgk (Reference Schomburgk1845) mentions that the Pianoghotto indians living at the headwaters of the Koetari River called the decomposed gneiss they used for grinding ‘Were Kitto’, and in the Werekitto Falls in the Corantijn River these rocks are exposed. This unit encompasses a great variety of hornblende–biotite gneisses and biotite gneisses of tonalitic, trondhjemitic to granitic composition, which usually show some compositional banding, suggesting a largely supracrustal origin. They show commonly signs of incipient migmatisation and there are also some cross-cutting veins with alkalifeldspar megacrysts. Some biotite–plagioclase gneisses may represent semi-pelitic protoliths. Metamorphism is in the amphibolite facies (Kroonenberg, Reference Kroonenberg1976).

Amotopo Gneiss

The second most common rock types in the Coeroeni Gneiss Belt, as seen in the Corantijn river near the newly established Trio village of Amotopo, are low-pressure amphibolite-facies pelitic gneisses, characterised by sillimanite, biotite and muscovite, often with either cordierite or garnet. Cordierite is often concentrated in centimetre-sized cordierite–quartz clots. These gneisses are strongly migmatitic as a rule. A striking feature is that cordierite is often replaced by higher-pressure minerals, such as green biotite, muscovite, staurolite, andalusite, kyanite, garnet and ferrogedrite, testifying to a second, static, higher pressure phase of metamorphism (Kroonenberg, Reference Kroonenberg1976). The pelitic gneisses are commonly associated with decimetre-sized bands or boudins of amphibolites, with hornblende and plagioclase as main minerals, and locally clinopyroxene, garnet or cummingtonite. They might correspond with metamorphosed basaltic sills or dykes, although trace element chemical data are lacking. Calcsilicate nodules with diopside, tremolite and andradite are common, and in one drill core dolomitic marble has been encountered. Furthermore intercalations of ferruginous, manganiferous and barium-rich quartzites and anthophyllitic ultramafic rocks have been found (Kroonenberg, Reference Kroonenberg1976).

Fig. 12. U–Pb SHRIMP ages of zircon cores (2053 Ma) and rims (1986 Ma) of sample KG 826, cordierite–muscovite–biotite gneiss, in which cordierite is partly replaced by ferrogedrite, andalusite, sillimanite, staurolite and garnet. Lucie River, sample number 24, 25 in Fig. 4 and Table 4.

Possibly the two phases of metamorphism recorded in the cordierite-bearing pelitic gneisses correspond with the two age groups around 2.08–2.05 Ga and 1.99–1.98 Ga (Table 4), as in the two most thoroughly studied samples the cores of the zircons show the older age and the rims the younger metamorphic age. On the other hand it cannot be excluded that the older age represents detrital grains just like some still older zircon crystals in the same samples (see Fig. 12 and discussion below). Solving this dilemma requires detailed studies of individual zircon crystals, which is beyond the scope of the present paper.

Dome Hill Gneiss

In the area between the Coeroeni and Boven-Corantijn Rivers, culminating in the Dome Hill at 500 m, as well as in the area around the confluence of those two rivers, migmatitic granulite-facies pelitic gneisses with coexisting sillimanite, garnet and cordierite are common, as well as intercalations of orthopyroxene-bearing amphibolites and pyribolites, locally with garnet. In contrast to the Bakhuis Granulite Belt, no orthopyroxene is found in the granulite-facies pelitic gneisses, so that the metamorphic conditions are definitely lower, maximum around 800°C and 6–8 kbar. Replacement of cordierite by higher-pressure assemblages is rarely encountered (Kroonenberg, Reference Kroonenberg1976).

Dalbana Formation

Felsic metavolcanic rocks were designated Dalbana Formation by Bosma (Reference Bosma1971). They have been described extensively by Verhofstad (Reference Verhofstad1971) in the Wilhelmina Mountains in central Suriname. The most common rocks are grey, reddish or black metamorphosed rhyolitic ash-flow tuffs (ignimbrites), macroscopically characterised by fine lamination and fiamme, collapsed pumice fragments up to a few centimetres long (Fig. 13).

Fig. 13. Fiamme (collapsed pumice fragments) in felsic metavolcanics, Sipaliwini River. Scale in centimetres.

Microscopically flattened Y-shaped glass shards can sometimes be distinguished, but usually devitrification and low-grade metamorphic recrystallisation to an extremely fine-grained quartz-feldspar groundmass erased much of the fine microstructure of these rocks. Small alkali feldspar phenocrysts are often broken. Slightly more mafic metarhyodacites have mainly plagioclase and embayed HT quartz phenocrysts. The most mafic metavolcanics are metadacitic to meta-andesitic lavas with amphibole pseudomorphs after pyroxene. Geochemically the metavolcanics show SiO₂ contents between 76% and 52%, with K₂O/Na₂O ratios varying between 0.70 and 2.10 (data from Verhofstad, Reference Verhofstad1971), and present a calcalkaline differentation trend (unpublished data by Bosma & De Roever, Reference Bosma and De Roever1975; De Vletter & Kroonenberg, Reference De Vletter and Kroonenberg1987; De Vletter et al., Reference De Vletter, Aleva and Kroonenberg1998; Delor et al., Reference Delor, de Roever, Lafon, Lahondère, Rossi, Cocherie and Potrel2003b). Only the most acid metavolcanics with SiO₂ > 75% are slightly peraluminous, the others are metaluminous (data from Verhofstad, Reference Verhofstad1971). Close to granite the metavolcanics show a more advanced recrystallisation with a hornfelsic groundmass, testifying to the intrusive character of the granite plutons. The rocks show gentle, open folding, in contrast to the tight to isoclinal folding in the Marowijne Greenstone Belt metavolcanics. Reis et al. (Reference Reis, Dreher, Fraga, Scandolara and Betiollo2009) identified a possible caldera structure in the Serra Tepequém in the northern part of Roraima State, Brazil, which might have been a source of the Surumu-Dalbana felsic volcanics.

Sipaliwini Leucogranite

In close association with the metavolcanics three subvolcanic granite types are found, which were mapped separately on the 1977 map, but are here taken together: leucogranites, granophyric granites and fine-grained granites. By virtue of the contact metamorphism in the metavolcanics around these bodies they are considered to be slightly younger, but still comagmatic; they might correspond to the deeper substructures of the volcanoes that erupted the ignimbrites. In the Sipaliwini savanna they form conspicuous hills protruding above the plains underlain by the metavolcanics, such as the Vier Gebroeders Mountains (Maas & Van der Lingen, Reference Maas and Van der Lingen1975). They range in composition between alaskite–leucogranite and granite, and locally to granodiorite. Bluish quartz bipyramids are common phenocrysts, and granophyric intergrowths may form the bulk of the rock. Chemically they are slighty peraluminous, plot in the Syntectonic Collision Granite field in the discriminant Rb/(Yb + Ta) diagram by Pearce et al. (Reference Pearce, Harris and Tindle1984) and have higher REE and more pronounced Eu anomalies than the Wonotobo biotite granites described below (De Vletter & Kroonenberg, Reference De Vletter and Kroonenberg1987).

Wonotobo Granite

The Wonotobo Granite is named after the impressive cataract in the Corantijn river where this granite crops out extensively (Bosma, Reference Bosma1969), which is also the site where the first biotite granite from West Suriname was dated by Rb–Sr (Priem et al., Reference Priem, Hebeda, Boelrijk, Verschure and Verdurmen1968). In Guyana this granite was named Wanatoba–Pakani Granite (Barron, Reference Barron1965). This is the most common and widespread granite in western Suriname, characterised by medium to coarse grain size, variegated colour due to bluish quartz, greenish saussuritised plagioclase and pinkish alkali feldspar, the latter often as megacrysts up to 5 cm in size. Biotite is the common mafic mineral, hornblende occurs less frequently. In contrast to the older granites of eastern Suriname, these rocks are homogeneous, apart from the common occurrence of centimetre- to decimetre-sized rounded xenoliths of a variety of rocks, usually deformed and resorbed beyond recognition. These are I-type granitoids of monzogranitic composition.

Coppename Muscovite Granite

Muscovite granites are widespread in the upper Coppename and upper Saramacca areas, the contact zone between the Marowijne Greenstone Belt and the Older Granites in the east and the Younger Intrusives in the west. They are light-coloured, often pinkish S-type intrusive rocks of alkaligranitic to granitic composition with muscovite, usually also biotite, and commonly with aluminous minerals such as fibrolite, andalusite, cordierite or garnet (Verhofstad, Reference Verhofstad1971; Bosma & Lokhorst, Reference Bosma and Lokhorst1975; Bosma et al., Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984). Particularly in the Coppename area they are closely associated with Armina Formation metagreywackes, but also with Dalbana Formation felsic metavolcanics and subvolcanic leucogranites. Along the contacts between metagreywackes and granites hornfelses with andalusite, cordierite and fibrolite as well as staurolite-andalusite schists occur. Furthermore metapelitic, partly migmatitic gneisses occur in the same area, which may represent the higher-grade equivalents of the Armina Formation metagreywackes and metapelites (Arjomandi et al., Reference Arjomandi, Krook, Bosma and de Roever1973; Bosma & Lokhorst, Reference Bosma and Lokhorst1975). From a muscovite granite in southernmost Suriname, as well as on the border of the older and younger granites, a Pb–Pb zircon age of 1974 ± 2 Ma was obtained (Table 5). This suggests that these muscovite granites are unrelated to the >100 Ma older Patamacca muscovite granites within the greenstone belt. Moreover, they do not show evidence of syntectonic intrusion, and have intruded at a higher crustal level than the Patamacca granites in view of the low-pressure mineral associations in the surrounding hornfelses and schists. They are related to the younger intrusives, and may have formed through the assimilation of Armina metasediments by leucogranites and biotite granites.

Lucie Gabbro

Throughout the country small gabbroic to ultramafic plutons have been found, which formerly were all considered to represent a single magmatic unit, called De Goeje Gabbro, after the De Goeje Mountains in the southeast of the country (GMD, 1977; Bosma et al., Reference Bosma, Kroonenberg, Maas and De Roever1983, Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984). However, new geochronological data indicate that there are at least two different groups: the gabbroic–ultramafic intrusions within the greenstone belt, closely associated with the Paramaka metabasalts (Bemau Ultramafitite, see above), and the gabbroic intrusions outside the greenstone belt. The former were dated in French Guyana at 2147–2144 Ma on the Tampok gabbro, the physical continuation of the De Goeje gabbro body across the Marowijne River into that country (Delor et al., Reference Delor, Lahondère, Egal, Lafon, Cocherie, Guerrot and de Avelar2003a). From the gabbro body at the mouth of the Lucie River into the Corantijn River (Kroonenberg, Reference Kroonenberg1976), far outside the greenstone belt, recently a Pb–Pb age of 1985 ± 2 Ma was obtained (Table 5), ranging this suite of mafic plutons within the series of younger intrusives. We therefore abandoned the name De Goeje Gabbro altogether and designated the younger suite as Lucie Gabbro. In the older literature these bodies, although emplaced within granitoid areas, were thought to be older than the granites because of border effects, but geochronologically they are indistinguishable from the granites.

In the Bakhuis Granulite Belt many gabbroic bodies occur, especially in the area underlain by the Kabalebo Charnockites. Two of these bodies, the granoblastic Moi-Moi gabbro and the Charlie leucogabbro with cumulate texture, have recently been dated at 1984 ± 4 Ma and 1971 ± 15 Ma (Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015, Reference Klaver, de Roever, Thijssen, Bleeker, Söderlund, Chamberlain, Ernst, Berndt and Zeh2016), and so are geochronologically indistinguishable from the surrounding charnockites. Comagmatic metagabbro and metadolerite enclaves suggest contemporaneous emplacement of mafic and charnockitic melts. The mafic magmatism might have provided the heat source for charnockite emplacement (Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015, Reference Klaver, de Roever, Thijssen, Bleeker, Söderlund, Chamberlain, Ernst, Berndt and Zeh2016). Also in the area occupied by the Older Granites, especially near Pikien Rio pyroxene granites, there is a great amount of gabbroic intrusions, suggesting an affinity of these magmas to deep-seated environments, but whether the latter belong to Lucie Gabbro or Bemau Ultramafitite is unknown.

Many detailed descriptions of the Lucie Gabbro bodies exist, as they often form pronounced butterfly-shaped aeromagnetic anomalies, and were therefore targeted and drilled for base metal exploration, especially Cu, Ni and Cr (Bosma, Reference Bosma1973b; Bosma & Lokhorst, Reference Bosma and Lokhorst1975; Oosterbaan, Reference Oosterbaan1975). Their composition ranges from peridotite to pyroxenite to often hornblende- and biotite-bearing gabbronorite as a result of crystal fractionation (Bosma et al., Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984), and more felsic varieties occur as well. They are often referred to as metagabbronorite etc., although the metamorphic overprint is rather variable and often barely discernable; igneous cumulate textures usually predominate. A geochemical study of 117 samples from many different bodies (Bosma et al., Reference Bosma, Maas and De Roever1980) showed that gabbronorites from deep-seated environments in the Bakhuis belt and in the Pikien Rio pyroxene granite area are more tholeiïtic in character, those from shallow-depth environments more calcalcaline. Rb/(Yb + Ta) discriminant diagrams for a large number of gabbro bodies show a considerable scatter of values, and do not give reliable clues as to their tectonic environment. REE profiles show variable degrees of LREE enrichment and often positive Eu anomalies (Bosma et al., Reference Bosma, Maas and De Roever1980).

Kabalebo Charnockite

In the southwestern part of the Bakhuis Granulite Belt a large intrusive body of charnockites occurs, ranging in composition from tonalite to granodiorite and granite (Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015). In the field they are homogeneous magmatic rocks, without compositional banding. The mafic minerals are ortho- and clinopyroxene and biotite. In several places the charnockites show many xenoliths of mafic granulite and metadolerite. In the same area there are a large number of Lucie-type (meta)gabbro intrusions (Moi-Moi metagabbro), and contacts of charnockite with metagabbro have been observed in the field (Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015, Reference Klaver, de Roever, Thijssen, Bleeker, Söderlund, Chamberlain, Ernst, Berndt and Zeh2016). Chemically the charnockites are magnesian, calc-alkaline, metaluminous rocks with 56–74 wt% SiO₂, depleted in the mobile elements Cs, Rb, U and Th, probably as a result of dehydration, and with high TiO₂, P₂O₅, K₂O, Ba, REE and Zr contents compared to average granite (Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015). Their chemistry suggests that they are the product of melting of the intermediate Bakhuis granulites (Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015). Melting did not occur during or slightly after UHT metamorphism at 2.07–2.05 Ga, as the charnockites show ages of 1993–1984 Ma, leaving a time gap of 60 Ma between UHT metamorphism and melting. There are no indications that UHT conditions continued during that interval. Intrusion of (meta)gabbro around 1.99–1.98 Ga produced a second pulse of extra heat-flow, leading to granulite melting (Klaver et al., Reference Klaver, de Roever, Thijssen, Bleeker, Söderlund, Chamberlain, Ernst, Berndt and Zeh2016). The charnockites are coeval with the widespread felsic magmatism surrounding the Bakhuis Granulite Belt. However, the charnockites have a juvenile composition whereas the felsic metavolcanics may contain inherited Archean zircons (Nadeau et al., Reference Nadeau, Chen, Reece, Lachhman, Ault, Faraco, Fraga, Reis and Betiollo2013) and show rather high Nd T_DM model ages of 2.43–2.44 Ga (De Roever et al., Reference De Roever, Lafon, Delor and Guerrot2015), which may indicate that they were derived from a different source (Klaver et al., Reference Klaver, De Roever, Nanne, Mason and Davies2015).

The Lucie type Moi-Moi metagabbro was dated at 1984 ± 4 Ma, but another type of gabbro, the Charlie Gabbro, and dated at 1970 ± 17 Ma, also occurs in the southwestpart of the BGB (Klaver et al., Reference Klaver, de Roever, Thijssen, Bleeker, Söderlund, Chamberlain, Ernst, Berndt and Zeh2016). A rather large anorthosite body (Mozeskreek anorthosite) occurs in the centre of the BGB. It was dated at 1980 ± 5 Ma (De Roever et al., Reference De Roever, Lafon, Delor, Rossi, Cocherie, Guerrot and Potrel2003a).

Tafelberg Formation

The Tafelberg Formation is the Surinam part of the Roraima Supergroup of Reis et al. (Reference Reis, Pinheiro, Costi and Costa1990), Gibbs & Barron (Reference Gibbs and Barron1993) and Santos et al. (Reference Santos, Potter, Reis, Hartmann, Fletcher and McNaughton2003). The Tafelberg is a horizontal sandstone plateau up to 1026 m in central Suriname and the easternmost outlier of the huge Roraima province that covers over 73,000 km² in Venezuela, Guyana and Brazil (Gibbs & Barron, Reference Gibbs and Barron1993). In Suriname it consists of about 700 m of quartzarenites and quartz-rich well-sorted conglomerates, with at various levels thin red jasper-like indurated ash-fall tuffs (Bisschops, Reference Bisschops1969). Sedimentary structures suggest deposition in an arid fluviodeltaic environment (Bisschops, Reference Bisschops1969; Bosma et al., Reference Bosma, Kroonenberg, Maas and De Roever1983, Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984; De Vletter et al., Reference De Vletter, Aleva and Kroonenberg1998), but no detailed sedimentological study has been made. A smaller occurrence is in the Emma Range west of the Tafelberg. The sandstones unconformably overlie Wonotobo-type biotite granite and Sipaliwini Leucogranites and is intruded by Avanavero Dolerite dykes. The basal contact is marked by a reddish paleosol (Fig. 14; Kroonenberg, Reference Kroonenberg1978). Priem et al. (Reference Priem, Boelrijk, Hebeda, Verdurmen and Verschure1973) obtained a Rb–Sr isochron age of 1655 ± 18 Ma for the volcanic ashes. Modern zircon U–Pb data from volcanic ashes from the Pakaraima Plateau in Brazil give a Paleoproterozoic age of 1873 ± 3 Ma, while baddeleyite and zircon from two intruding Avanavero sills gave 1782 ± 3 Ma (Santos et al., Reference Santos, Potter, Reis, Hartmann, Fletcher and McNaughton2003). However, the presence of multiple ash falls with a range of ages cannot be excluded, and therefore new geochronological data from Tafelberg are needed.

Fig. 14. Paleosol near base of Tafelberg Formation, Elfriedeval, Tafelberg. Core diameter 4 cm.

Reis et al. (Reference Reis, Pinheiro, Costi and Costa1990) give a synthesis of the stratigraphy and depositional environments of the Roraima Supergroup in Brazil, and Beyer et al. (Reference Beyer, Hiatt, Kyser, Drever and Marlatt2015) in Guyana. Paleocurrent analyses of Roraima rocks in Brazil show provenance from the north, and most detrital zircons show ages between 2171 and 1958 Ma, suggesting that Trans-Amazonian rocks formed the main source of the sediments (Santos et al., Reference Santos, Potter, Reis, Hartmann, Fletcher and McNaughton2003). The Roraima Supergroup therefore can be considered as the molasse of the Trans-Amazonian Orogeny. The source of the ash-fall tuffs is unknown, but in view of the very large area in which they are dispersed they must represent tremendous eruptions. Along the southern border of the Guiana Shield in Brazil the Iricoumé volcanics and associated granitic intrusions of 1.89–1.81 Ga (Valério et al., 2012; Marques et al., Reference Marques, Souza, Dantas, Valério and Do Nascimento2014; Barreto et al., Reference Barreto, Lafon, Da Rosa-Costa and De Lima2013; Fig. 2) have the right age and composition to produce the Roraima ash layers, and caldera-like structures have been documented from the Pitinga mining district (Pierosan et al., Reference Pierosan, Lima, Nardi, DeCampo, Neto, Ferron and Prado2011). From Tafelberg no new geochronological data are available.

Avanavero Dolerite

Conspicuous mafic dykes of 50 m to 1 km in thickness cut all previously described geological units, and often stand out topographically as laterite-capped elongated ridges over 100 km long, such as the Van Asch van Wijck Mountains (GMD, 1977; Bosma et al., Reference Bosma, Kroonenberg, Maas and De Roever1983, Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984). Many dykes have a NE–SW orientation, but other bodies form irregular masses or sills. They also intrude the Roraima Supergroup at Tafelberg and elsewhere in Guyana, Venezuela and Brazil. They were named Avanavero Dolerite, after the Avanavero Falls in the Kabalebo River (Loemban Tobing, Reference Loemban Tobing1969), and this name has been adopted in all the neighbouring countries (Reis et al., Reference Reis, Teixeira, Hamilton, Bispo-Santos, Almeida and D'Agrella-Filho2013). They are characterised by the presence of two pyroxenes, Ca-rich augite and Ca-poor inverted pigeonite, the latter expressed as orthopyroxene with exsolution lamellae of clinopyroxene.

Chemically they are mainly tholeiïtic basalts with high Cs, Rb, Ba and K and low Nb contents, suggesting a lithospheric mantle source, modified by crustal contamination and/or by variable enrichment with slab fluids or melt from oceanic lithosphere (De Roever et al., Reference De Roever, Kroonenberg, Delor and Phillips2003b; Reis et al., Reference Reis, Teixeira, Hamilton, Bispo-Santos, Almeida and D'Agrella-Filho2013). The REE patterns are generally LREE-enriched (~20–70× relative to chondrites), compared to the HREE (7–17×), with no or only slight negative Eu anomaly (Reis et al., Reference Reis, Teixeira, Hamilton, Bispo-Santos, Almeida and D'Agrella-Filho2013). The most precise age obtained so far is 1794.5 ± 1.6 Ma (MSWD = 0.2) on baddeleyite from the Pedra Preta Sill in Roraima State, Brazil (Reis et al., Reference Reis, Teixeira, Hamilton, Bispo-Santos, Almeida and D'Agrella-Filho2013), which closely matches earlier values from the Omai mine in Guyana (Norcross et al., Reference Norcross, Davis, Spooner and Rust2000) and the Cipó sill in Brazil (Santos et al., Reference Santos, Potter, Reis, Hartmann, Fletcher and McNaughton2003; Reis et al., Reference Reis, Teixeira, Hamilton, Bispo-Santos, Almeida and D'Agrella-Filho2013). The Avanavero Dolerite is suggested to form part of a mafic Large Igneous Province (LIP) of continental scale, stretching over 300,000 km² and representing a magma volume of at least 30,000 km³ (Gibbs & Barron, Reference Gibbs and Barron1993; Reis et al., Reference Reis, Teixeira, Hamilton, Bispo-Santos, Almeida and D'Agrella-Filho2013). The preferential NE–SW orientation suggests an episode of continental rifting, although its precise geotectonic significance is not clear.

Käyser Dolerite

A separate suite of NW–SE oriented narrow dolerite dykes was discovered in southwest Suriname, stretching over a distance of 300 km from the Sipaliwini area through the Käyser Mountains to the Kabalebo and Corantijn areas (Bosma et al., Reference Bosma, Kroonenberg, Maas and De Roever1983, Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984; De Roever et al., Reference De Roever, Kroonenberg, Delor and Phillips2003b). It differs strongly from the Avanavero and Apatoe dolerites in mineralogy, chemistry and age. It is characterised by abundant fresh olivine, titaniferous augite, biotite and minor kaersutite as mafic minerals. Chemically it is alkalibasaltic in composition instead of tholeiïtic as the other suites, with Na₂O + K₂O up to 4.5–6.5% at SiO₂ contents of 43–50%, and also in various geochemical discriminant diagrams it plots in the alkalibasalt field (De Roever et al., Reference De Roever, Kroonenberg, Delor and Phillips2003b). REE patterns show a marked fractionation, with La_N/Yb_N values of 6.9–8.2. Ar–Ar dating of biotites of a sample from the Westrivier (point 47 in Fig. 4) show plateau ages of 1501 ± 5 Ma, which is taken as the age of their crystallisation (De Roever et al., Reference De Roever, Kroonenberg, Delor and Phillips2003b). This might reflect an event of continental extension in the Guiana Shield, although the nearest coeval rocks are the anorogenic Mucajaí and Surucucus rapakivi granites in Roraima State in Brazil, the Parguaza rapakivi granites in Venezuela and the Mitú granites in the Colombian Amazones (De Roever et al., Reference De Roever, Kroonenberg, Delor and Phillips2003b; Kroonenberg & Reeves, Reference Kroonenberg and Reeves2012; Reis et al., Reference Reis, Fraga, de Faria and Almeida2003; Bonilla-Pérez et al., Reference Bonilla-Pérez, Frantz, Charão-Marques, Cramer, Franco-Victoria, Mulocher and Amaya-Perea2013 and references cited there).

Nickerie Mylonite

Shearing and mylonitisation along major NE–SW fault zones affected large areas of the western Surinam basement, as well along the Imataca Granulite Belt in Venezuela. Mylonites and pseudotachylites are particularly prominent along the border faults of the Bakhuis horst in western Suriname. Low-grade metamorphic recrystallisation up to the pumpellyite-prehnite or greenschist facies is common. At the same time, many Trans-Amazonian rocks of different units in the western part of the country show thermal resetting of their Rb–Sr and K–Ar mineral ages in the range of 1100–1300 Ma; the combined effects were termed Nickerie Metamorphic Episode by Priem et al. (Reference Priem, Hebeda, Boelrijk, Verschure and Verdurmen1968, Reference Priem, Boelrijk, Hebeda, Verdurmen and Verschure1971). In Guyana it has been defined by Barron (Reference Barron1969) as K'Mudku Episode, a term adopted also by Brazilian geologists (Cordani et al., Reference Cordani, Fraga, Reis, Tassinari and Brito-Neves2010), although Gibbs & Barron (Reference Gibbs and Barron1993) prefer Nickerie. Mineral age resetting is restricted to western Suriname in the (former) Nickerie District, in the eastern part of the country mineral ages are Trans-Amazonian. These effects have been attributed to the Grenvillian Laurentia–Amazonia collision around 1100–1000 Ma along the western border of the Guiana Shield (Kroonenberg, Reference Kroonenberg1982, Reference Kroonenberg1994; Cordani et al., Reference Cordani, Fraga, Reis, Tassinari and Brito-Neves2010).

Muri Alkaline Complex

The Muri Alkaline Complex occupies a small isolated mountain range in the extreme southwest of the area, straddling the boundary between Suriname and Brazil between the headwaters of the Coeroeni and Boven Corantijn Rivers. It was discovered during the Radambrasil campaign and then called Sienito Mutum (Issler et al., Reference Issler, De Lima and Montalvão1975), and it was explored in the 1980s by the UN Revolving Fund for Natural Resources Exploration (Fozzard, Reference Fozzard1986). It consists of two adjacent nepheline syenite and tinguaite bodies intruding into biotite granodiorite. A conical hill next to the northeastern body, Twareitau, is believed to be underlain by carbonatite. The laterite cap on top is high in Nb and Sr, the soil below it shows a high radioactive anomaly and is rich in REE-phosphates, and the rivers draining it have a pH of 8.5. Fenitisation (Na metasomatism) affected rocks in the surroundings (Barron, Reference Barron1981; Premoli & Kroonenberg, Reference Premoli and Kroonenberg1984; Fozzard, Reference Fozzard1986; Gibbs & Barron, Reference Gibbs and Barron1993). Issler et al. (Reference Issler, De Lima and Montalvão1975) provided a K–Ar age on perthite of 1026 ± 28 Ma, while Nadeau (Reference Nadeau2014) gives an age of 1090 Ma. Cordani et al. (Reference Cordani, Fraga, Reis, Tassinari and Brito-Neves2010) suggest that the alkaline magmatism took advantage of the K'Mudku/Nickerie shear zones caused by the Grenvillian orogeny. Bosma et al. (Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984) report a 1.5 m wide, undeformed analcime–nepheline-bearing monchiquite dyke in western Suriname, without further details.

Apatoe Dolerite

A third generation of dyke swarms of Early Jurassic age with a general N–S orientation transects the basement in large parts of the country, called Apatoe after the Apatou village on the French Guiana bank of the Marowijne River, where a dyke swarm crosses the river. They are usually not more than 50 m wide, but nevertheless are often expressed in the topography as parallel narrow laterite-capped ridges and can be followed for over 300 km. They consist of pigeonite dolerite with only rarely olivine and no orthopyroxene, and are completely fresh. Nomade et al. (Reference Nomade, Théveniaut, Chen, Pouclet and Rigollet2000) report a bulk plagioclase K–Ar age of 196.0 ± 1.7 Ma for the Apatoe dolerite at the type locality, and Nomade et al. (Reference Nomade, Knight, Beutel, Renne, Verati, Féraud, Marzoli, Youbi and Bertrand2007) show a major peak of mafic dyke emplacement in the eastern Guiana Shield around 198 Ma.

Chemically they are Fe-enriched tholeiïtes and have higher Zr than either Avanavero or Käyser dolerites, whereas their Sr and Ti contents are intermediate between those (Bosma et al., Reference Bosma, Kroonenberg, van Lissa, Maas and de Roever1984; De Roever et al., Reference De Roever, Kroonenberg, Delor and Phillips2003b; Deckart et al., Reference Deckart, Bertrand and Liégeois2005). REE diagrams show slight enrichment of LREE (La_N/Yb_N 1.49–5.09) and virtually no Eu anomalies, spider diagrams show positive to negative Sr anomalies and a slight positive Ti anomaly. The Nd–Sr isotope signatures point to depleted mantle sources with little crustal contamination (Deckart et al., Reference Deckart, Bertrand and Liégeois2005). The Apatoe Dolerite forms part of the Jurassic Central Atlantic Magmatic Province, which heralds the separation of the American continents from Africa and Eurasia and the origin of the Atlantic Ocean as parts of the breakup of Gondwana (Deckart et al., Reference Deckart, Bertrand and Liégeois2005; Nomade et al., Reference Nomade, Knight, Beutel, Renne, Verati, Féraud, Marzoli, Youbi and Bertrand2007).