Potassic and Sodic Igneous Rocks from Eastern Paraguay: their Origin from the Lithospheric Mantle and Genetic Relationships with the Associated Parana Flood Tholeiites

Abstract

Eastern Paraguay represents the westernmost fringe of Early Cretaceous Paraná flood tholeiites (Serra Geral Formation, SGF). Besides the SGF, eastern Paraguay has been the site of alkaline magmatism since Mesozoic times; (1) Late Permian-Early Triassic sodic intrusions and lavas; (2) Early Cretaceous potassic igneous rocks and very scarce sodic lavas; (3) Late Cretaceous-Oligocene sodic lavas. Two distinct magmatic events are dominant in the Asunción-Sapucai graben (ASU) oj eastern Paraguay: (1) widespread potassic magmatism and SGF tholeiites (Early Cretaceous); (2) Asunción sodic magmatism (Late Cretaceous-Oligocene). The potassic rocks form a compositional continuum from moderately to strongly potassic. Two potassic suites are proposed, i.e. basanite to phonolite and alkali basalt to trachyte and their intrusive analogues. The sodic rocks include ankaratrites, nephelinites and phonolites. Two similar but distinct parental magmas have been inferred for the potassic suites, both characterized by strongly fractionated REE and negative 'Ta-Nb-Tt anomalies'. Slight positive Ta and M anomalies distinguish the sodic rocks. Sr-Nd isotope data confirm the distinction of the potassic rocks, enriched in radiogenic Sr and low in radiogenic Nd, from the sodic rocks, close to bulk Earth. Crustal contamination does not appear to have been significant in the generation of the investigated rocks, supported by δ18O data. The source of potassic rocks is constrained by high LILE, LREE, Th, U and K, relative to a primitive mantle composition, and a garnet pendotite is favoured as a possible mantle source for the investigated rocks. The close association of potassic and sodic rock suites in the ASU demands that their parental magmas derived from a heterogeneous subcontinental mantle, variously enriched in incompatible elements. Significant H2O, CO2and F are expected in the mantle source(s) for the occurrence of coeval carbonatites. Any genetic hypothesis based on a 'mantle plume' system is constrained by strong lithosphenc mantle characteristics. This does not preclude that thermal perturbations from the asthenosphere may have triggered magmatic activity in the lithospheric mantle. Model ages indicate that two distinct metasomatic events may have occurred during Late and Early-Middle Proterozoic as precursor to the genesis of tholeiitic and alkaline magmatism in the Paraná Basin. These metasomatic processes were chemically distinct, indicated by the strong differences in Ti, LILE and HFSE concentrations found in both alkaline provinces (e.g. potassic rocks from ASU vs Alto Paranáiba Igneous Province) and Paraná tholeiites (low vs high Ti). In general, the relationships between the alkaline rocks from southeastern Brazil, i.e. Alto Paranaíba, Ponta Grossa Arch, Serra do Mar, Loges and the flood basalts of the Paraná Basin, support a common origin in the lithospheric mantle. Sr-Nd isotope and other geochemical data indicate that a significant role was played by a mantle component depleted in incompatible elements and with high Sm/Nd ratio. This component (N-MORB type) would be represented by the depleted portions of a mantle which was variously metasomatized during Proterozoic times. The isotopic and geochemical features of the modem Tristan da Cunha plume are distinctly different from the component depleted in incompatible elements, and its contribution is not apparent in the compositions of the Paraná tholeiites.