André Pouclet

Crustal origin of early Proterozoic syenites in the Congo Craton (Ntem complex), South Cameroon

Two petrographically different syenite suites can be distinguished in the mostly Archean Ntem Complex of Cameroon: a suite of two-pyroxene syenites and a suite of clinopyroxene syenites. The syenites occur as discrete intrusions surrounded by Archean gneisses. Based on structural relationships the syenites are of late to post-orogenic origin. The Pb–Pb zircon evaporation ages indicate that the syenites intruded at ca. 2.3 Ga and thus postdate significantly the Archean episode of crust formation and differentiation that generated the surrounding gneiss terrane. The isotope signatures and the trace elements provide unambiguous evidence that the two syenite suites are of crustal origin. The syenite suites do not show simple chemical trends, have highly variable trace element contents and display only a small variation in their major element contents, particularly in their SiO2 contents. The isotope characteristics, including 143Nd/144Nd, 87Sr/86Sr and common Pb, point to a heterogeneous source or multiple sources for the syenites. The Sr isotope compositions calculated for the time of intrusion vary over a wide range from an unrealistically low 87Sr/86Sr ratio of 0.690 to high values of 0.713. In contrast the initial 143Nd/144Nd ratios are more homogenous with eNd(2320) from −5.8 to −7.5. The Pb isotopes from leached K-feldspars indicate that the sources of the syenites had a history of U depletion and Th/U enrichment. The syenites show the same isotope characteristics as the surrounding gneiss terrane, thus the deep continental crust of the Ntem Complex itself is the most likely source for the syenite melts. This conclusion is also supported by the heterogeneity of the trace elements in the syenites that exclude a homogenous source. In addition, inherited zircons from the syenites yield Pb—Pb evaporation ages that are consistent with ages obtained for zircons extracted from the surrounding gneisses. In contrast to many other syenites worldwide, the syenites in the Ntem Complex do not contain juvenile mantle material, but are the sole products of crustal reworking. Thus, the syenites did not contribute to the crustal growth in the Ntem Complex. However, this conclusion may not preclude the possibility that the syenites were generated in the deep crust during a time of addition of mantle material to the continental crust. The cause for the generation of these large syenite bodies in the Ntem Complex is most likely an episode of deep-seated thermal activity that was so far not known from this part of the Congo Craton.

Cretaceous to Cenozoic volcanism in South Korea and in the Sea of Japan: magmatic constraints on the opening of the back-arc basin

Abstract The major element, trace element, and radiogenic isotope compositions of volcanic rocks in the back-arc area of the eastern Eurasian continental margin provide insight into the nature of the mantle wedge and constrain the magmatic evolution of the Japan Sea back-arc basin linked to its tectonic history. Different phases of post-Early Cretaceous volcanic activity are identified along the Korean margin and in the Japan Sea. Volcanic rocks from Korea include (1) Cretaceous and early Cenozoic calc-alkaline lavas of a volcanic arc at an active margin, and (2) Pliocene and Quaternary intraplate flood basalts and volcanic islands of alkaline composition. Japan Sea volcanic rocks consist of (1) early Cenozoic andesite flows of a remnant arc in the Yamato Bank, (2) early Miocene basalts of the Japan Sea basin basement, which share compositional characteristics of island arc tholeiites, continental rift tholeiites and back-arc basin basalts, (3) late Miocene seamounts of tholeiitic and mildly alkaline compositions, and (4) Pliocene and Quaternary alkaline volcanic islands. Geochemically, these rocks belong to three broad magmatic groups: (1) an arc-related, calc-alkaline group of a continental, Andean margin type, which prevailed prior to the opening of the Japan Sea between the Cretaceous and early Miocene, (2) continental rift tholeiites and back-arc basin basalts, formed during the rifting stage in the early Miocene, and (3) an intraplate alkaline group similar to OIB, erupted later during spreading, between late Miocene and Holocene times. Trace element and Sr, Nd and Pb isotopic compositions of selected samples show that the sources of magma Group 1 calc-alkaline lavas and magma Group 2 tholeiitic lavas included varying contributions of two main mantle components: an Indian Ocean MORB-like depleted mantle source (DMM) and an enriched mantle component similar to EM II. The latter component could represent DMM contaminated by subducted oceanic sediments incorporated into the lower lithosphere during the long-lived subduction of west Pacific crust. During the opening of the Japan Sea back-arc basin, the relative proportion of the DMM component dramatically increased between the rifting and spreading stages. It is also necessary to postulate a third component present in the sources of the Group 3, post-opening alkaline lavas, perhaps enriched mantle of EM I composition, which may also have resided in the subcontinental lithospheric mantle.

Neoarchæan crustal evolution in the Congo Craton: evidence from K rich granitoids of the Ntem Complex, southern Cameroon

Abstract The Neoarchaean (2.6 Gal K rich granitoids of the Ebolowa area (Ntem Complex, Congo Craton) in southern Cameroon form small massifs trending north-northwest-south-southeast to north-south and contain xenoliths of the surrounding tonalite-trondhjemite-granodiorite (TTG) series and of the greenstone belt country rocks. The granitoids range in modal composition from granodiorite to granite and leucogranite. The rocks display high SiO2 (69–76.5%), Na2O+K2O (5.5–9.1 %) and K 2 O Na 2 O (0.5–1.9). They are metaluminous to slightly peraluminous (0.9 The formation of these late granitoids in the Ebolowa area is attributed to a large-scale tectonothermal event around 2.6 Ga that led to the formation of similar granitoids throughout the extensive Ntem Complex. The intrusion of these granitoids is probably the terminal tectono thermal episode following the Archaean accretion and differentiation of the northwest Congo Craton.

The French Guyana doleritic dykes: geochemical evidence of three populations and new data for the Jurassic Central Atlantic Magmatic Province

Abstract A petrographic and geochemical study of 15 Early Jurassic and 7 Proterozoic dolerites of French Guyana, and of one Jurassic dolerite from Ivory-Coast were carried out. The Early Jurassic SSW-NNE trending dykes have doleritic aphyric or gabbroic phyric texture. Their chemical compositions, slightly under-saturated to over-saturated, show moderate to low Mg-ratios (63–36), high TiO2 contents (1.85–3.56 wt.%), weak rare earth element fractionation [1.8

The Cambrian volcano-sedimentary formations of the westernmost High Atlas (Morocco): their place in the geodynamic evolution of the West African Palaeo-Gondwana northern margin

Abstract In the westernmost part of the High Atlas, two Palaeozoic formations, rich in mafic volcanic rocks, are distinguished. They belong to different structural blocks created during the Variscan orogeny. New U–Pb dating yields an Early Cambrian age. The basaltic lavas have the composition of continental tholeiites and the magmatic signature of an initial rifting tectonic setting. They are related to the western Moroccan Cambrian rift. Their geodynamical context could be a passive margin initiated from an active rift that aborted in the Middle Cambrian.

Paleomagnetic study of French Guyana Early Jurassic dolerites: hypothesis of a multistage magmatic event

A detailed paleomagnetic and anisotropy of magnetic susceptibility (AMS) study was carried out on 34 sites of Early Jurassic dolerite dykes from French Guyana, which formed during the initial opening of the Central Atlantic Ocean. Four types of AMS fabrics are recognized: (i) ‘Normal’ fabric (21 dykes) defined by clustering of K1^K2 axes on the dyke plane whereas the K3 axis is nearly perpendicular to it. This fabric is interpreted as due to magma flow. The subhorizontal inclination of the K1 axis permitted to suggest that the French Guyana dykes could be fed by horizontal magma fluxes from a distant magma source. (ii) ‘Reversal’ fabric (8 dykes) is characterized by the K2^K3 plane close to the dyke plane and the K1 perpendicular to dyke orientation. Such fabric was attributed to the local shearing stress. (iii) ‘Intermediate’ fabric (1 dyke) is defined by K1^K3 axes close to the dyke plane and K2 axis is perpendicular to this plane. It was interpreted as due to vertical compaction of a static magma column. (iiii) ‘Other’ fabric (4 dykes) does not show any preferential orientation. Scanning electronic microscope and susceptibility versus temperature experiments show that minerals of the titanomagnetite family are main magnetic remanence carriers. Two magnetic components were isolated. Ages of magnetic remanences are estimated at 198.3 ˛ 2.0 Ma to 192.3 ˛ 1.5 Ma. Their virtual geomagnetic poles are calculated, Pole A: V = 73.2‡N, P = 15.3‡E, k = 288.8, A95 = 3.4‡, n = 8, and Pole B: V = 81.6‡N, P = 89.1‡E, k = 69.8, A95 = 4.2‡, n = 18. These two groups probably correspond to two distinct magmatic events which occurred in a short period. This hypothesis is consistent with published 40 Ar^ 39 Ar radiometric ages though with ‘mini plateau’ spectra. These paleomagnetic results suggest the presence of magmatic pulses led to the construction of the Central Atlantic Magmatic Province in French Guyana during the Early Jurassic. fl 2000 Elsevier Science B.V. All rights reserved.

The middle Neoproterozoic Sidi Flah Group (Anti-Atlas, Morocco); synrift deposition in a Pan-African continent/ ocean transition zone

The Middle Neoproterozoic (Cryogenian) Sidi Flah Group rocks are located in the Saghro inlier of the Eastern Anti-Atlas and consists of siliciclastic detrital sediment, interbedded basaltic lavas and small ultramafic bodies. Sediment deposition occurred in three turbiditic formations of a deep-sea fan environment and was controlled by synsedimentary collapses. The composition of sandstones and typological study of zircons indicate that detrital material came from the gneisses and granites of a proximal craton. The lavas are synsedimentary subaqueous flows. They show chemical signatures of initial rift tholeiites and of plume-related alkaline intraplate basalts. The ultramafic rocks are serpentinized peridotites that were emplaced along N160° synsedimentary faults as numerous bodies 20–50 m in size. Their petrographical (Cr-spinel signature) and chemical features correspond to intracontinental ultramafic cumulates. The emplacement of the ultramafic rocks was associated with hydrothermal activity that generated calcareous and siliceous rocks such as ophicalcites and jaspers. All the features of the sediments, the lavas and the ultramafic bodies strongly suggest a continent–ocean transition geotectonic context, in an advanced stage of continental rifting that we attribute to the pre-Pan-African ocean passive margin extension.

First paleomagnetic and 40Ar/39Ar study of Paleoproterozoic rocks from the French Guyana (Camopi and Oyapok rivers), northeastern Guyana Shield

In order to understand the Paleoproterozoic geographic evolution of the Guyana Shield, paleomagnetic and 40Ar/39Ar investigations were carried out on granitoids and volcano-sedimentary rocks from the Oyapok and Camopi rivers (French Guyana–Brazil frontier). Scanning electronic microscope, thermomagnetic and isothermomagnetic experiments show that magnetite is the main magnetic remanent carrier in most of the samples. The metavolcano-sedimentary rocks (Paramaca) show a weak magnetization and scattered magnetic directions. Therefore, no reliable magnetic component could be isolated from these samples. Samples taken from tonalite and meta-ultrabasite rocks yield a characteristic magnetic direction, carried by subautomorphous magnetite, that is well defined and distinct from that of the present Earth field and that of nearby Jurassic dikes. A virtual geomagnetic pole (VGP) deduced from this probable primary remanence was calculated, namely pole OYA, λ=28.0°S, φ=346.0°E, N=5, k=31.9 and A95=13.8°. Four 40Ar/39Ar ages, ranging from 2052 to 1973 Ma, were obtained from amphiboles and biotites of tonalite rocks, showing a relatively slow cooling rate of ca 4.8+2.6/−2.1°C Ma−1. The linear extrapolation of this cooling rate to the magnetite unblocking temperature (540 to 580°C) yields a magnetization age of 2036±14 Ma for pole OYA. Pole OYA differs significantly from available paleomagnetic results from Venezuela of the West Guyana Shield dated at 2000±10 Ma. This difference may indicate an important latitudinal movement of the Guyana Shield between 2036 and 2000 Ma with a velocity of 9±7 cm/year.

The Pre-Pan-African rifting of Saghro (Anti-Atlas, Morocco): example of the middle Neoproterozoic Basin of Boumalne

In the Anti-Atlas, the Boumalne basin includes 3,000 m of Middle Neoproterozoic sediments. It consists of turbiditic deposits folded during the major Pan-African event ca 685 Ma. A syn-sedimentary basaltic pile of lava flows is interbedded in the upper part of the lower formation. These lavas show an initial rift tholeiite (IRT) chemical signature. Petrographical analysis of sediments and typology of detrital zircons indicate a continental margin sedimentation, without any volcano-sedimentary supply from a close volcanic arc. It is concluded that the Boumalne Basin formed in a continental passive margin evolving from an intracontinental rift. This interpretation differs clearly from that of a back-arc basin which is commonly accepted. Hence, the opening of this basin is related to the pre-Pan-African Saghro rift synchronous to the Central Anti-Atlas oceanization, and not to the demise of this oceanic domain along an active margin.

Le volcanisme fissural néoprotérozoïque des séries du Dja inférieur, de Yokadouma (Cameroun) et de Nola (RCA) — Signification géotectonique

The basic intrusions in the lower Dja, Yokadouma and Nola Neoproterozoic Series, around 1 Ga, display a continental tholeiitic composition. The magmas are generated from a spinel lherzolite source sharing lithospheric and asthenospheric components, suggesting a lithospheric thinning. This magmatism has to be related to an extensional phase which favored the pre-Pan-African trough formation in the northern part of the Congo Craton.