Identifying source-rocks using trace-elements and garnet discrimination diagrams: Example from Potiguar Basin, equatorial margin, NE Brazil

Abstract

Sedimentary provenance analysis based on the major element composition of detrital garnet and tourmaline is a conventional and widely used method. However, overlapping compositional fields in standard garnet classification diagrams often hinder the accurate identification of source areas. To overcome such ambiguities, this study applies an integrated analytical approach. The methodology involved the compositional analysis of major and trace elements (Zn, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in 1043 garnet grains, major elements in 1493 tourmaline grains, and multivariate statistical analysis to refine the provenance of sandstones from the Pendência, Alagamar and Açu formations (Potiguar Basin, NE Brazil). Initial classification using major elements suggested a significant contribution from felsic igneous sources (Bi-type). However, trace-element geochemistry revealed that a significant portion of these garnets ($\approx 79\text{\%}$ of the original Bi type) shows an affinity with amphibolite and granulite-facies metapelites. This reclassification is corroborated by the predominance of tourmalines derived from metapelites and by the statistical analysis of the major elements of the garnets in the investigated samples. Therefore, the results indicate that the sediment supply for the studied formations has a more significant metasedimentary contribution than suggested by conventional methods alone. The integrated data point to the Seridó Terrane and the Caicó Complex, within the Borborema Province, as the most likely source areas, with a predominance of amphibolite-facies and, secondarily, granulite-facies metasedimentary rocks. The participation of igneous sources was more evident in the pre-rift phase due to precursor and contemporaneous Cretaceous magmatic activity. A mixed supply, with an expressive contribution from igneous and metamorphic basement sources, characterized the rift phase (Pendência Formation). The post-rift (Alagamar Formation) and drift (Açu Formation) phases maintained this mixed pattern, with a predominance of metasedimentary sources from the basement, but with the addition of new magmatic pulses and the reworking of preexisting volcaniclastic materials during the drift phase.