GEOCHEMISTRY AND ORIGIN OF THE THOLEIITIC MAFIC METAVOLCANIC ROCKS FROM THE RIACHO DE SANTANA GREENSTONE BELT, BAHIA, BRAZIL

The post-Archaean Riacho de Santana greenstone belt (GBRS) is located in the southwestern of Bahia State, Brazil, and occurs as discontinue swaths with approximately 84 km long and 12 km wide. The GBRS has trend north-south and overlain Archaean (granitoid-gneiss) basement rocks. Three distinct units are identified: (i) Lower Unit, comprising of basic metavolcanics that occur as intercalations in ultabasic rocks and associated meta-sediments, (ii) Intermediate Unit, comprising of basic metavolcanics, aluminous-schists, graphite-schists and metacherts, and (iii) Upper Unit, comprising of carbonate sequence and associated meta-basalts and meta-tuffs. The Lower Unit mostly contains amphibolite facies, whereas greenschist facies are exclusively present in other two units. In this study, we report geochemistry (petrography, and major and trace element composition) of the basic metavolcanic rocks of the Intermediate Unit. Petrographic studies reveal granonematoblastic texture with an assemblage of hornblende, plagioclase feldspar (often sericitised), epidote, sphene, chlorite, and opaque minerals. In general, whole-rock SiO2 (45.54-51.37 wt%), CaO (6.42-12.32 wt%) and FeOt (9.73-12.51 wt%) increase with decrease in MgO (7.53-3.08 wt%). Whereas Al2O3 (14.78-12.26 wt%) and K2O (0.4-0.16 wt%) decrease with decrease in MgO. In the total alkali (Na2O+K2O) vs. SiO2 plot, all of the samples plot on the tholeiitic field. LREE patterns are slight enrichment and coupled with near flat HREE [(La/Sm)n=1-3.12; (Gd/Yb)n=1.26-1.84]. Primitive-mantle normalized REE patterns (and trace element abundances) are similar to that observed in mafic metavolcanics of other greenstone belts, e.g., the Abitibi and Yellowknife (Canadian Shield), the Olondo (Aldan Shield), and Bastar (Central India). The REE patterns are also similar to the patterns observed in N-MORB and E-MORB, and suggests that either binary mixing of the above two MORB types or assimilation of N-MORB source with the crustal components may give rise to these Lower Unit basalts. Future work on Sr and Nd isotope systematics may reveal the effect of crustal assimilation.