Crustal contamination and genesis of transitional alkaline-tholeiitic intrusions: Insights from the José Fernandes Suite, Paraná Magmatic Province, Brazil

Abstract

Abstract The Jose Fernandes suite, located in the Ponta Grossa Arch (PGA) region of southern Brazil, is an approximately 3\u202fkm2 composite intrusion associated with the Early Cretaceous (134\u202fMa) intrusive magmatism that formed the Parana-Etendeka Magmatic Province. The intrusion is emplaced in Precambrian metasedimentary rocks and composed of gabbroic rocks (including a marginal quartz-bearing variety) intruded by synplutonic alkaline dikes. Petrographic and geochemical characteristics indicate the important influence of crustal contamination on the evolution of the suite. Disequilibrium textures and mineral chemical zoning are widespread in the rocks composing the pluton and are evidence of open-system processes. Orthopyroxene (Fe-enstatite) (Mg# 49-68) occurs in reaction rims of iron-rich olivine (Mg# 42-61) and symplectic intergrowths, together with magnetite and ilmenite. The presence of Fe-enstatite can be related to partial melting of host rocks resulting in non isochemical reactions of olivine with fluids under oxidizing conditions. The positive correlation of SiO2 and Sr—Pb isotope initial ratios and negative correlation with eNd indicate different levels of crustal contamination. The alkaline dikes that cut the pluton have basanite to basaltic trachyandesite compositions and vary from primitive to contaminated (87Sr/86Sri\u202f=\u202f0.7049 to 0.7070) dikes, confirming that the intrusion formed from several batches of alkaline magma with distinct crustal contributions. Given the enrichment in K2O over Na2O in samples with high initial 87Sr/86Sr (>0.707) and evidence of a low time-integrated Th/U component typical of the upper crust, metapelitic country rocks are considered potential contaminants in the upper crustal chamber environment. A pole of contamination with low 143Nd/144Nd signatures indicates contamination by gneissic basement with Archean crustal residence at lower crustal levels. Isotopic modeling is consistent with an evolution in which several batches of basanite magma evolved to more radiogenic compositions by assimilation of different proportions (up to ~35%) of partial melts from different crustal contaminants. These contaminants have older crustal residence times than those involved in the generation of silicic magmas associated with the tholeiitic basalts of the Parana-Etendeka Province.