PETROLOGY, GEOCHEMISTRY AND Sr-Nd ISOTOPES OF THE PALEOPROTEROZOIC MAFIC DYKES FROM THE GOIÁS-CRIXÁS ARCHEAN BLOCK, GOIÁS STATE, BRAZIL

Abstract

Paleoproterozoic mafic dyke swarm that intruded the Goias-Crixas Archean Block comprises two groups of dykes: the high Ti (HTi - TiO2 >1.5%) and the low Ti (TiO2 < 1.5%) tholeiites. HTi and LTi have mg# values ranging from 0.49 to 0.31 and from 0.33 to 0.18 respectively. HTi is enriched in LILE (Rb, Ba, K) and LREE (La, Ce) with respect to LTi. HTi incompatible trace elements (IE) patterns and ratios are broadly intermediate between E-MORB and OIB, whereas LTi is closer to E-MORB, but exhibits some N-MORB features. However Nb anomalies are always negative. The studies on the origin of HTi and BTI reveled that mantle heterogeneity is the only possible explanation. The initial 87Sr/86Sr (Sri) ranges from 0.700 to 0.705 (eSr –21.89 to + 46.52) and the initial 143Nd/144Nd (Nd i) varies from 0.5092 to 0.5120 (eNd – 4.84 to + 0.53) at 2.49 Ga. Isotopic and geochemical data suggest a mixed parental source constituted by DMM and EMI components. It is suggested that the EMI signature originates from an oceanic slab containing eclogites and sediments, subducted during Paleoproterozoic or Archean times. Metasomatic fluids and/or melts from the eclogites account for greater and lesser IE mantle enrichment, giving rise to HTi and LTi respectively. The Nb depletion relative to LILE and LREE is due to retention by rutile of eclogites, and characterizes continental and IAB tholeiites. The positive spikes of Ba in HTi can be attributed to pelagic sediments of the slab. The similarity between geochemical and isotopic data of the Goias dykes and others from the Sao Francisco and Amazonic cratons suggest a similar continental environment. Keywords: mafic dykes, Goias, mantle, metasomatism.