GEOCHEMISTRY OF CONTAMINATED KOMATIITES FROM THE ARCHAEAN UMBURANAS GREENSTONE BELT, BAHIA STATE, BRAZIL

The late Archaean Umburanas greenstone belt (UGB) is located in the São Francisco Craton, southwest of Bahia State, Brazil. The Lower Unit of UGB comprises of basal komatiite lavas and tholeiitic basalts intercalated with felsic volcanic rocks. The regional crystalline basement rocks, the Gavião block, predominantly comprise of granitic, granodioritic and migmatitic continental crust along with tonalite-trondhjemite-granodiorite (TTG) associations. Petrographic studies of UGB komatiites reveal characteristic spinifex igneous texture although primary mineralogy is rarely preserved. Based on textural relationship, komatiites are divided into: cumulate, spinifex, and massive types. The MgO content vary in the range 31.5-40.4 wt%. A negative MgO-SiO2 correlation in komatiites suggest olivine fractionation trend. The UGB komatiites are Al-undepleted type and characterized with Al2O3/TiO2 (21-48), enrichment in highly incompatible large ion lithophiles relative to moderately incompatible high field strength elements, distinct negative Nb, Sr and Eu anomalies, depletion of light rare earths, convex-downward rare earth patterns typically not observed in komatiites world-wide, and primitive mantle normalized Gd/Yb (1.03-1.23) and La/Sm (2.36-4.99) ratios. The negative Eu anomaly is attributed to the circulation of H2O-rich fluid, whereas the negative Nb and Sr anomalies are attributed to contamination from granitic basement rocks of the Gavião block. The UGB komatiites are most likely derived from adiabatic decompressional melting of a mantle plume. The melting took place at liquidus temperatures in the range 1572-1711oC, which is consistent with mantle-plume origin invoked for several other komatiites in the Archaean greenstone belts elsewhere. The melts were more likely generated at a depth shallower than 100 km (pressure < 2.5 GPa) where garnet was absent in the source mineralogy. Geochemical characteristics suggest contamination of primary melts with granitic basement rocks either during ascent of melt or during emplacement of magma in a continental basin setting. Greenschist to lower amphibolitic facies metamorphism at ~ 2Ga ago may also have played a role in modifying komatiite chemistry.