Sodic alkaline magmatism in Eastern Paraguay revisited: geochemical and petrological implications

Abstract

Associated with subordinate potassic rocks, Mesozoic to Cenozoic sodic alkaline rocks are represented by four distinct provinces in Eastern Paraguay: Alto Paraguay, Central, Misiones, and Asunción; additional Rio Apa and Amambay provinces are exclusively of potassic affinity. Alkaline magmatism is clearly controlled by a tectonic extensional regime that generated NW-SE-trending faults and grabens. It varies in composition, with the intrusive sodic rocks mainly composed of foid syenites and quartz-bearing syenites and the potassic ones of syenites, syenogabbros, and gabbros, all of which are accompanied by fine-grained hypoabissal and volcanic counterparts. In the Central Province, potassic and sodic rocks occur together and are represented by two distinct magmatic series: basanites-phonolites and alkali basalts-trachytes/trachyphonolites. Ultramafic sodic rocks (ankaratrites and nephelinites) are uniquely found in the Misiones and Asunción provinces. In general, the sodic rocks exhibit LILE enrichment, high field strength elements depletion, and Nb-Ta positive anomalies in the mantle's normalized incompatible elements (IE) distribution. Chondrite-normalized rare earth element (REE) diagrams point to strong REE enrichment and high light/heavy rare earth element fractionation in different subparallel patterns. Conversely, the potassic rocks display negative anomalies of Nb-Ta and are fractionated in REE. In the initial Sr_i versus Nd_i diagram, the sodic rocks approach to bulk earth, whereas the potassic ones plot into the Sr-enriched quadrant. Sr-Nd-Pb isotope data suggest that a high U/Pb mantle component played an important role in the genesis of the Late Early Cretaceous and Tertiary sodic magmas, while an enriched mantle component dominated the Early Cretaceous potassic magmas. The close association of potassic and sodic rocks implies that their parental magmas derived from a subcontinental mantle variable in composition enriched in IE.

KEYWORDS:
alkaline magmatism; geochemistry; petrology; Eastern Paraguay