New U-Pb baddeleyite ages and geochemistry of mafic rocks of Matthews Ridge, NW Guyana: Extension of the Paleoproterozoic Avanavero Large Igneous Province

Abstract

Avanavero mafic dykes and sills are widespread in the Amazonian craton, South America. Although the complete extent of Avanavero magmatism is currently unclear, similar intrusions have been found in places where it had not been previously mapped such as Matthews Ridge, NW Guyana, the focus of this study. We present new geochemical, U-Pb geochronology, and Sm-Nd isotopic data for Matthews Ridge dykes and sills, and a comparison with Avanavero samples from the literature. Our new in-situ U-Pb analysis on baddeleyite grains by SHRIMP yielded an age of 1787 ± 12 Ma, which overlaps within error both the SHRIMP and the ID-TIMS ages reported in the literature and confirms that the unmetamorphosed dolerites from NW Guyana are part of the Avanavero large igneous province (LIP) event. Geography and geochemistry indicate the presence of two groups of Avanavero mafic intrusions. Group 1 includes occurrences in the north of the Guiana Shield (including the units in Matthews Ridge) and are characterized by mostly low Ti concentrations; Group 2 is limited to occurrences in southern Guiana Shield and have high Ti concentrations. Fractional crystallization pathways differ between the groups, with olivine playing a significant role for Group 1, and olivine + clinopyroxene + plagioclase for Group 2. Distinct Ca/Al ratios may suggest different parental magma and varying degrees of crustal contribution within Group 1. The isotopic data suggest that Group 1 and Group 2 are cogenetic and represent different magma batches that came from the same source by potentially different fractionation paths. The low negative to low positive ɛNd values, together with low Nb/Yb, indicate that the samples from both groups are consistent with a continental intraplate setting where primary magmas were contaminated by the lithospheric mantle previously metasomatized during an earlier subduction event.