Abstract
Abstract Early and Late Cretaceous alkaline and alkaline–carbonatitic complexes from southern Brazil are located along the main tectonic lineaments of the South America Platform. Calcium-, magnesium-, and ferrocarbonatites are well represented and frequently associated even in the same complex. Primary carbonates present significant variations in C–O isotopic compositions, which are mainly due to isotope exchange with H2O–CO2-rich hydrothermal fluids, whereas fractional crystallization or liquid immiscibility probably affects the δ18O and δ13C values by no more than 2δ‰. Our isotope exchange model implies that the most significant isotopic variations took place in a hydrothermal environment, e.g., in the range 400–80°C, involving fluids with the CO2/H2O ratio ranging from 0.8 to 1. Sr–Nd–Pb isotope systematics highlight heterogeneous mixtures between HIMU and EMI mantle components, similar to the associated alkaline rocks and the flood tholeiites from southern Brazil. In spite of the strong variation shown by C–O isotopes, Sr–Nd–Pb–Os isotopic systematics could be related to an isotopically enriched source where the chemical heterogeneities reflect a depleted mantle “metasomatized” by small-volume melts and fluids rich in incompatible elements. These fluids are expected to have promoted crystallization of K-rich phases in the mantle, which produced a veined network variously enriched in LILE and LREE. The newly formed veins (enriched component) and peridotite matrix (depleted component) underwent a different isotopic evolution with time as reflected by the carbonatites. These conclusions may be extended to the whole Paraná–Etendeka system, where isotopically distinct parent magmas were generated following two main enrichment events of the subcontinental lithospheric mantle at 2.0–1.4 and 1.0–0.5 Ga, respectively, as also supported by Re–Os systematics. The mantle sources preserved the isotopic heterogeneities over a long time, suggesting a nonconvective lithospheric mantle beneath different cratons or intercratonic regions. Overall, the data indicate that the alkaline–carbonatitic magmatism originated from a locally heterogeneous subcontinental mantle.
Highlights
Southern Brazil is part of the Paraná–Etendeka Province (PAEP) [1] characterized by the emplacement of Early Cretaceous tholeiitic flood basalts and dyke swarms, 129–133 Ma aged [1,2,3,4,5,6], and alkaline and alkaline–carbonatite complexes of Early Cretaceous to Paleogene age [8,9,10,11,12]
The isotopic data show that magmatism from the southern Brazilian Platform requires heterogeneous mantle sources, in terms of radiogenic isotopes, probably related to metasomatic s.l. processes that occurred between Neoarchean and Neoproterozoic times, as confirmed by Re–Os systematics on mafic-ultramafic rock types associated with the alkaline– carbonatitic complexes
The areal distribution of magmatism suggests that the time-integrated isotopic enrichment of carbonatites and associated alkaline rocks decreases from west to east, concomitantly with the decreasing age of the magmatism (Early to Late Cretaceous)
Summary
Southern Brazil is part of the Paraná–Etendeka Province (PAEP) [1] characterized by the emplacement of Early Cretaceous tholeiitic flood basalts and dyke swarms, 129–133 Ma aged [1,2,3,4,5,6], and alkaline and alkaline–carbonatite complexes of Early Cretaceous to Paleogene age [8,9,10,11,12]. Field evidences, e.g., subcircular or suboval or ring structures of alkaline complexes with carbonatite body occupying the inner part; (2) association with melilite- or nepheline-bearing rocks; (3) calcite with >1 wt% SrO; (4) presence of perovskite, pyrochlore, F-REE-carbonates, and apatite with high silica content (e.g., >2 wt%); (5) enrichment in Sr, Ba, Nb, U, Th, and rareearth elements (with high LREE vs HREE fractionation) and depletion of other incompatible elements (e.g. K, Rb, Zr, Hf, and Ti) relative to mantle and continental crust [15]; (6) 87Sr/86Sr initial ratios similar to those of the associated silicate alkaline rocks [16]; (7) stable isotopes (O–C) in the “primary carbonatitic box” [17,18] or following well-distinct isotopic fractionation trends [19]. Taking into account the above criteria, this article summarizes field and geological evidences and geochemical characteristics in terms of C–O and Sr– Nd–Pb–Re–Os isotopic data of alkaline–carbonatitic complexes and occurrences in and around the Paraná Basin in southern Brazil, in particular, and in the Paraná–Angola–Namibia (PAN) system in general [20]. (3) Occurrences with unusual geometric relationships: a small number of occurrences in the form of small dykes or ocelli in alkaline silicate rocks (e.g., Vallemí, Cerro Canãda, Cerro E Santa Elena, Itanhaém; cf. Speziale et al, this volume)
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