The Roraima Alkaline Province: A cretaceous alkaline province in the Amazonian Craton

The rift-related magmatism that generated the Roraima Alkaline Province (RAP) took place along main faults and shear zones that stretch out for circa 200 km in the central Guyana Shield, northern Amazonian Craton. As such, RAP is the first alkaline province proposed to the Guyana Shield. It encompasses felsic and mafic rock types – the former constituting foid-syenites and phonolites, while the latter including lamprophyre, tephrite and tephriphonolite dikes. In this paper, we examine whether these rocks are cogenetic and construct a genetic model for RAP with coupled petrography, geochemistry, and U-Pb zircon geochronology. Our data show that the RAP rocks are nepheline-normative, metaluminous to peralkaline, and miaskitic to agpaitic. The geochemical signatures indicate that fractional crystallization was the primary mechanism that produced the compositional variability between mafic and felsic magmas. Increasing peralkalinity is attributed to extensive alkali feldspar fractionation. Elemental, isotopic, geochronological and petrographic features show that crustal assimilation also played a role in magma genesis. Both ages obtained in this study and compiled from previous works range from 100 to 116 Ma, indicating that the Cretaceous alkaline magmatism in the Guyana Shield was short-lived. The Sr-Nd isotopic signatures of the least-contaminated samples require mixing of depleted and enriched mantles. Depleted-mantle model ages obtained from the least-contaminated samples fall in relatively short intervals, between 548 and 657 Ma, which coincide with the model ages of the Central Atlantic Magmatic Province (CAMP) tholeiites, suggesting that the same metasomatic event affected both Mesozoic alkaline and tholeiite magmatic occurrences of the Guyana Shield. It is concluded that the lithospheric mantle metasomatism took place shortly before magmatism, triggered by an extensional tectonic event involving the Tacutu rifting and the opening of the Atlantic.