Integração de dados radiométricos e petrográficos de plutons ediacaranos para detalhamento de mapeamento geológico no domínio Rio Piranhas-Seridó, extremo NE da Província Borborema

Abstract

Radiometry is a shallow geophysical investigation method based on the emission of natural gamma radiation from rocks, which aids in identifying lithotypes and lithologicalcontacts. The qualitative interpretation of radiometric data allowed for the characterization of the main signatures of the shoshonitic, high-potassium calc-alkaline porphyritic, high-potassium calc-alkaline equigranular, calc-alkaline suites associated with Ediacaran magmatism (635 – 541 Ma) in the northeastern edge of the Borborema Province. Four plutons were highlighted as target areas: i. Acari; ii. Totoró; iii. São Rafael; and iv. Serrada Garganta. Data from the potassium channels (K in %), equivalent thorium (eTh in ppm), equivalent uranium (eU in ppm), and ternary radiometric composition RGB (red, green, and blue) were used to delineate radiometric domains in a GIS environment. Thesedomains represent closed polygons where the distribution pattern of K, eTh, and eU radioelement contents is classified as low (1), medium (2), and high (3). Interpretation maps for each target area were created based on the 27 possible color combinations in the RGB pattern. The mineralogical composition of the granites, which are mainly composed of quartz, feldspars (plagioclase and microcline), and micas (biotite and muscovite), favors generic K-rich signatures. Additionally, the texture of these granites (grain size and crystal orientation) affects the distribution of these elements. A pattern predominantly showing high values for K and eTh is observed in rocks belonging to the high-potassium calc-alkaline porphyritic (Itaporanga type), alkaline (Conceição type), and highpotassium calc-alkaline equigranular (Dona Inês type) suites. However, the texture of these rocks also influences this analysis, as porphyritic granites tend to highlight K values in relation to eTh and eU content due to the size of the K-feldspar crystals. Nevertheless, regions mapped as the Itaporanga Intrusive Suite that exhibit low to medium patterns in radioelements can be explained by the interference of lenses of dioritic rocks, metasedimentary rocks, terrigenous quartzose sediments, as well as water bodies. For the São João do Sabugi Suite, a predominant pattern of low values for the radioelements was observed, attributed to the mineralogical composition of the rocks in this suite, characterized by the low representation of K, eTh, and eU-rich minerals, with a predominance of biotite and amphibole.