Nd and Sr-Isotopic and U-Pb Geochronologic Constraints for Evolution of the Shoshonitic Brasiliano Bom Jardim and Toritama Complexes: Evidence for a Transamazonian Enriched Mantle Under Borborema Tectonic Province, Brazil

Abstract

The Bom Jardim and Toritama complexes form elongated bodies between the Pernambuco-Alagoas massif and Pajeú-Paraíba fold belt and comprise multiphase intrusions of monzonitic and syenitic compositions. Low-angle foliation recorded in both complexes, indicated by alignment of euhedral minerals, runs parallel to the main thrust faults within the region. This suggests that emplacement of the complexes could be related to the thrust faults. The whole-rock geochemistry of major and trace elements, and the associated mineral chemistry, have established a shoshonitic affinity for these complexes.

The mineral chemistry of the essential minerals within both complexes, the presence of magnetite, and the constant Fe/Fe + Mg ratios in biotites and amphiboles with increasing differentiation indicate trends compatible with a fractionating magma under high f(O₂) conditions. The general overlap and similarities in mineral chemistry between the mafic minor facies and the less-evolved members of the major facies indicate that they share the same source. Major and trace-element chemistry coupled with mineral chemistry indicates that the main geochemical trends determined for the complexes are the result of fractional crystallization, with the removal of amphibole plus biotite and plagioclase in the early stage, followed by the removal of K-feldspar within the developing magmas.

The 592 ± 7.2 Ma U/Pb zircon age recorded in both complexes is associated with low-angle foliation and suggests that they probably represent magmatism associated with the end of a Neoproterozoic compressive regime within the Pajeu-Paraiba fold belt. The Bom Jardim and Toritama rocks have high LILE/HFSE ratios; high MgO, Cr, and Ni contents; and lower ⁸⁷Sr/⁸⁶Sr ratios compared to crustal granitoids. However, the complexes also have a value of εNd (−20 at 0.6 Ma) similar to those recorded in 2.0 Ga (U/Pb in zircon) migmatites from the Pajeu-Paraiba fold belt. Thus, the source of these rocks probably is the result of partial melting of a metasomatized Transamazonian (2.0 Ga) mantle, which occurred during the end of the Brasiliano compressive regime.