Raman elastic geobarometry from garnet amphibolites in the eastern margin of the Santa Quitéria continental magmatic arc, NE Brazil: New insights into the evolution of Western Gondwana

Raman elastic geobarometry is a physical technique used to estimate pressure ( P ) and temperature ( T ) conditions by analyzing the residual pressure in mineral inclusions entrapped within a host. In this study, we present μ-Raman spectroscopic data for quartz inclusions within garnet hosts from garnet amphibolites located at the eastern margin of the Santa Quitéria continental magmatic arc, NW Borborema Province, and evaluate potential evidence for ultrahigh-pressure (UHP) metamorphism in this segment of Western Gondwana based on previously published μ-Raman spectroscopy data from these rocks. Elastic modeling for quartz inclusions in garnet (QuiG) indicates residual pressure (Pinc) values between -0.18(3) and 0.07(3) GPa, indicating that the inclusions are under tensile to compressive stresses. Calculated isomekes using Equations of State (EoS) for almandine garnet suggest an entrapment pressure (Ptrap) value of 0.81(1) GPa at 700 °C (α-quartz stability, amphibolite facies) for a clinopyroxene-garnet amphibolite sample, and 1.19(1) GPa at 869 ºC (high-pressure (HP) granulite facies) for a garnet amphibolite sample, consistent with available geothermobarometric data. No relict coesite has been identified through μ-Raman spectroscopy. In our analyses, the polished sections prepared without silicon carbide (SiC) grit do not exhibit quartz inclusions with peaks at ~521 cm⁻¹ (characteristic of silicon), and we identified diagnostic Raman peaks of anatase and rutile within fully encapsulated quartz inclusions in garnet. Moreover, elastic modeling suggests that the rocks described as retrograded eclogites along the eastern margin did not reach eclogite-facies conditions, but rather amphibolite-to-granulite-facies conditions. Therefore, we re-interpret the evolution of retrograded eclogite from the western margin, and granulite and amphibolite from the eastern margin as a result of metamorphism at the base of a thickened continental crust during continental collision, followed by channel flow toward shallower crustal levels, rather than representing a subduction-related accretionary system at deep mantle depths. Since subduction is incompatible with geothermal gradients of ca. 460, 730, and 860 °C/GPa, such conditions are more likely to be achieved in a collisional tectonic setting. Conversely, intra-oceanic arc magmatism marks the formation and consumption of oceanic crust at the eastern margin of the Santa Quitéria continental magmatic arc. We propose an alternative geodynamic model for the evolution of the NW Borborema Province.