Petrographic and Geochemical Evidence from the Jatunyacu River Outcrop, Central Abitagua Batholith, Ecuadorian Amazon: Preliminary Constraints on Magmatic Evolution and Arc Affinity

The Abitagua batholith is a 120 km long plutonic body located in the northern sub-Andean zone of Ecuador. Despite its size, previous studies have focused on its northern and southern sectors, leaving the central sector uncharacterized. This study presents the first petrographic and geochemical evidence from a single outcrop exposed along the Jatunyacu River, in the central part of the Abitagua Batholith, in order to understand its magmatic evolution and tectonic affinity. Petrographically, the dominant lithology is an equigranular monzogranite. The fractured zones show localized hydrothermal alteration, including epidote, sericitization of plagioclase, and chloritization of biotite. Subordinate bodies include tonalitic enclaves, felsic dikes, and an andesitic dike. Geochemically, the studied sector shows a calc-alkaline affinity, peraluminous character, and a volcanic arc granite (VAG) signature broadly consistent with I-type granitoids formed in a continental arc related to subduction. Samples from fractured zones show small shifts toward the S-type field in the K₂O vs. Na₂O diagram, attributed to hydrothermal alkali mobility rather than primary magmatic variation, as supported by petrographic evidence. Multi-element normalized diagrams reveal distinct signatures among subordinate bodies: tonalitic enclaves show strong enrichment in mafic components and Nb, suggesting a primitive mafic source; felsic dikes display enrichment in incompatible elements (Nb, Rb) consistent with evolved residual melts; and the andesitic dike exhibits the most primitive composition with apparent minimal interaction with the felsic host. These are interpreted as evidence of a complex magmatic evolution involving mafic recharge, magma mixing, late injection of residual melts, and localized hydrothermal alteration. Comparison with previous studies suggest that the studied outcrop records an arc signature similar to that reported for the northern and southern sectors, although further work is needed to confirm the extent of this affinity across the central sector.