Geochemistry and Sr, Nd, Pb isotopic composition of the Central Atlantic Magmatic Province (CAMP) in Guyana and Guinea

Abstract

The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, extending more than 5000 km north to south, on both sides of the Atlantic Ocean. Its emplacement occurred about 200 Ma ago, at the Triassic-Jurassic boundary, and is linked to the initial breakup of Pangaea. Two areas of the province are studied here: French Guyana/Surinam (South America) and Guinea (West Africa), in order to document the petrogenesis and geodynamical significance of high-Ti and low-Ti basaltic magmas from the CAMP.

In Guyana, doleritic and gabbroic dykes are located on the edge of the Guiana Shield, and represent limited volumes of magma. They display low SiO2 (47-50%), high TiO2 (2.5-3.5%) and high FeO tholeiitic trends and show variably enriched trace element patterns ((La/Yb)(n)=1.5-5.1). Their isotopic signature and ratios of very incompatible elements (epsilon Nd-i=+5.8 to +4.2, (Sr-87/Sr-86)(i)=0.703-0.705, (Pb-207/Pb-204)(i)=15.46-15.64) match a depleted PREMA (prevalent mantle)like source. Their genesis can be modeled by ca. 15% partial melting of a lherzolite source, and a subsequent limited fractional crystallization (5-10%) or a slight upper crustal assimilation-fractional crystallization (AFC, r=0.1, Proterozoic contaminant). In Guinea, in contrast, huge volumes of CAMP magmas were intruded along the Rockelides suture and the West African craton, forming the Fouta Djalon sills and the Kakoulima laccolith. The laccolith is more than 1000 m thick. These features consist of gabbros, dolerites, diorites and mafic (gabbro) and ultramafic (dunite, wherlite) cumulates. Guinean tholeiites show high SiO2 (51-58%), low TiO2 (0.7-1.2%) and FeO trends, with high LILE/HFSE ratios and slight negative Nb-Ta anomalies. Isotopic signatures (epsilon Nd-i=+0.4 to -5.3, (Sr-87/Sr-86)(in)=0.705-0.710, (Pb-207/Pb-204)(i)=15.57-15.66) indicate a more enriched source than for Guyana as well as a higher rate of magma-upper crust interaction through an AFC process (r=0.3, Birimian crust contaminant) and, probably, an additional upper crustal contamination for the most differentiated sample.

This geochemical study supports the prevalence in Guinea, as for other low-Ti CAMP tholeiites, of a lithospheric mantle source, previously enriched during ancient subduction events, and preferentially reactivated in late Triassic times by edge-driven convection between cratonic and mobile belt domains. A larger contribution from a depleted asthenospheric source is required to generate high-Ti tholeiites in Guyana, which may reflect the development of CAMP rifting towards the initiation of the Central Atlantic oceanic crust.