Abstract We investigated a suite of metamorphosed mafic and ultramafic rocks from Matthews Ridge (MR), NW Guyana, to characterize their magmatic history through geochronology and geochemistry. Three petrographic groups were defined: MR-1 (meta-gabbros), MR-2 (meta-ultramafic cumulates), and MR-3 (meta-basalts). U-Pb SIMS dating of zircon and baddeleyite yields ages of 2100 ± 35 Ma, 2125 ± 25 Ma, and 2152 ± 42 Ma for MR-1, and 2222 ± 12 Ma for MR-2. Although undated, MR-3 displays Nd isotopic signatures suggesting coeval formation with MR-2. All petrographic groups exhibit low, uniform Th/La (0.07–0.10) and Sm/La (0.5–1.1), consistent with derivation from a depleted mantle source. Slightly positive initial εNd values indicate interaction between depleted and enriched mantle reservoirs. Comparisons with other Paleoproterozoic greenstone belts in the Guiana Shield show that western domains, including Matthews Ridge, record less crustal or sedimentary input than those to the east. Tectonic discrimination diagrams reveal little arc affinity, except for two MR-1 samples. Instead, geochemical features support an intra-oceanic origin: MR-3 likely formed at a mid-ocean ridge (~2.22 Ga), whereas high-FeTi rocks in MR-1 and MR-3 reflect mantle plume influence around 2.1 Ga. This supports a model where Matthews Ridge magmatism reflects oceanic plateau development, similar to Block B of the El Callao Mining District in Venezuela.
Carvalho et al. (2026) studied this question.