Raymond Capdevila

The Pyrenees: Subduction and collision?

This paper presents a new interpretation of the Pyrenees. After Early Cretaceous rifting, two stable Atlantic-type continental margins formed during the Late Cretaceous. Flysch-type sediments were then deposited in a deep east-west basin. During the latest Cretaceous and Early Eocene, this basin underwent subduction towards the south. We propose that the present-day Pyrenees result from collision between a northern stable margin (Aquitaine Basin and Sub-Pyrenean Belt) and an active southern margin (Pyrenees proper). According to this interpretation, the pre-Mesozoic basement of the North Pyrenean Zone may represent the intermediate continental crust of the Cretaceous stable margin.

Pan-African, post-collisional, ferro-potassic granite and quartz–monzonite plutons of Eastern Nigeria

Abstract Three Pan-African hypersthene-bearing monzogranitic and quartz–monzonitic plutons from the Eastern terrane of Nigeria have been investigated in detail. New major, trace and REE data, used to constrain their origin and nature, indicate that they display chemical features of ferro-potassic trans-alkaline affinity. Further trace element discrimination suggests (i) production of calc-alkaline medium-K diorite magmas by partial melting of fluid-metasomatised mantle wedge possibly combined with melts from the dehydration partial melting of altered oceanic crust; (ii) simultaneously production of the granite–quartz–monzonite ferro-potassic magmas from partial melting of hornblende-bearing granodioritic crustal sources; (iii) mixing of the two magmas. Sr initial ratios of 0.707 to 0.711 witness that the source of the granite magmas is the lower crust. Ages of the lower crustal granulitic protoliths is bracketed by Nd model ages between 1.9 and 2.2 Ga. Pb evaporation ages on single zircons constrain the emplacement of the three plutons around 580 Ma. 40 Ar / 39 Ar ages of amphiboles at about 560 Ma suggest cooling rates around 15°C/Ma. Extensive field work has established that pluton emplacement occurred during a regional north–south dextral strike-slip tectonics following the 630–610 Ma stage of oblique continent–continent collision in this part of west Africa.

Rb-Sr and Sm-Nd geochronology of lower Proterozoic granite-greenstone terrains in French Guiana, South America

Abstract Nine samples of metavolcanic rock from the lower parts of greenstone belts in central French Guiana (the Paramaca series) and 14 granitic samples from the intrusive gneisses (the Degrad Roche and Arawa gneisses) were selected for Sm—Nd and Rb—Sr analysis. The Sm—Nd results from the metavolcanic series (including two tholeiites, five peridotitic komatiites and two andesites) yield an isochron age of 2.11±0.09 (2 σ) Ga with an initial 143 Nd 144 Nd ratio (INd) of 0.51002±9 (2 σ), corresponding to ϵNd(T) = + 2.1 ± 1.8. This isochron is interpreted as representing the age of initial volcanism of the Paramaca series. Acid intrusives were dated by the Rb—Sr method. A whole rock Rb—Sr isochron, including data points from both the Degrad Roche and Arawa gneisses, yields an age of 2.00±0.07 (2 σ) Ga with initial 87 Sr 86 Sr ratio (ISr value) of 0.7019±4 (2 σ). This result is considered to be the time of emplacement of the orthogneiss protoliths. The positive eNd value (+ 2.1 ± 1.8) obtained from the metavolcanic rocks of French Guiana suggests that their mantle sources have evolved in reservoirs slightly depleted in Light Rare Earth Elements (LREE). This result confirms the possible existence of ancient LREE-depleted reservoirs within the lower Proterozoic mantle. Moreover, the high eNd(T) value for these rocks excludes any significant crustal contamination during magma genesis. The French Guianese orthogneisses yield a low ISr value (0.7019±4 (2 σ)) which, together with geochemical considerations, suggests that their granitic protoliths could have originated by partial melting of short-lived crustal precursors of basaltic to granodioritic composition. The present geochronological and isotopic study suggests that the Guiana Shield may represent a major continental accretion event during the lower Proterozoic.

The Archaean Greenstone Belts of Karelia (Eastern Finland) and their Komatiitic and Tholeiitic Series

Abstract The geological and geochemical features of three greenstone belts of Eastern Finland (Suomussalmi, Kuhmo and Tipasjarvi) have been studied. We have analysed about 70 metavolcanic rocks from the low greenstone sequence for their major and trace element compositions. The field relationships between various volcanic rocks are rather obscure; but the chemical data allow us to distinguish two general magmatic series, namely, the komatiitic and the tholeiitic series. We have concluded from this preliminary geochemical study that most volcanic rocks in each may have been derived from fractional crystallization of some parental magma. The REE data, mainly presented for the rocks from the Tipasjarvi belt, provide a strong evidence for a “depleted” mantle source, a feature very similar to that of Abitibi, Canada. The REE data also suggest that not all rocks are formed by fractional crystallization; a mechanism of different degree of partial melting is called to account for some rock types. Although the Baltic Shield is one of the first regions in which the Archaean rocks were described (Sederholm, 1897, 1932), it is only recently that the existence of greenstone belts has been clearly demonstrated (Blais et al., 1976, 1977 and in print a and b; Bowes, 1976; Gaal et al., 1976 and in print; Lobach-Zhuckenko et al., 1976; Mutanen, 1976; Suslova, 1976). In this contribution, the principal characteristics of the belts, which we have studied in Finland, are briefly described and we expose the present state of our petrological and geochemical research in this context.

Major and transition trace element abundances in the khondalite suite of the granulite belt of lapland (fennoscandia): Evidence for an early proterozoic flysch belt

Abstract The Granulite belt of Lapland comprises two main units. The more extensive unit, the khondalite suite, is formed of approximately equal proportions of sillimanite-garnet gneisses and garnet gneisses which include impersistent layers of quartz-rich gneisses, calc-silicate rocks and metavolcanics. The less extensive unit, the charnockite complex, corresponds to several tholeiitic and calc-alkaline series, each of them ranging from gabbro to tonalite. An attempt has been made to elucidate the primary features of the khondalite suite and its depositional environment. Major and transition trace element variations suggest that it has an overall sedimentary parentage and that it corresponded to a graywackeshale sequence (turbidites). Original clastic sediments were mainly derived from an eroded continental crust and, in lesser amount, from contemporary volcanism. The geochemical trends for alkalies seem to indicate that this crust was relatively silicic and that it had undergone previous granite-forming events. Taking into consideration the geological setting, the geometry, and the facies of these sedimentary formations, we propose to reinterpret the granulite belt of Lapland as a flysch belt located at a plate junction and pre-dating a continent-continent collision with subducted oceanic crust.

Subduction and tectonics on the continental margin off northern Spain: observations with the submersible Cyana

Summary Short-term latest Mesozoic-early Cenozoic convergence between the Iberian and European plates transformed the passive north Spanish margin into an active margin with southward subduction of the oceanic crust of the Bay of Biscay. This margin became passive again when subduction ceased in late Eocene or Oligocene time. Nine dives with the submersible Cyana allowed us to sample the rocks outcropping on the Le Danois Bank slope. A Mesozoic sequence includes shelf limestones of the Dogger to the Lower Cretaceous, and Lower Cretaceous detrital sediments (sandstones, conglomerates, pelites); it overlies a typical Palaeozoic Asturian basement. The main objective of the diving campaign was to study the structure of these strata in order to elucidate deformation of the continental wall of an active margin during its early stages of evolution, before the formation of an accretionary prism. The occurrence in Lower Cretaceous pelites of a weak schistosity in a reversed fold limb gives evidence of compressive structures, probably northward-verging overfolds. Additional evidence of compressive tectonics is the dislocation of quartz grains and dolomite crystals in Lower Cretaceous sandstones and Jurassic dolomites. These observations suggest that the continental slope in the Le Danois Bank area has a predominantly imbricate structure. Deformation and uplift of the northern Spanish margin appear to be related to the early stages of subduction of the European plate under Iberia.