Open-system magmatic evolution and crystallization conditions of the Ediacaran shoshonitic rocks from the Sao Joao do Sabugi Pluton, Borborema Province, NE Brazil

The Sao Joao do Sabugi Pluton (SJSP) (similar to 579 Ma) is one of the most expressive occurrences of Ediacaran, syn- to post-collisional shoshonitic rocks in the Rio Piranhas-Serido Domain, Borborema Province, NE Brazil. The petrography, whole rock and mineral composition of this pluton were investigated to characterize its open-system magmatic differentiation processes. The pluton is composed of three rock groups: gabbro-diorite, with clinopyroxene +/- orthopyroxene +/- amphibole +/- biotite; monzodiorite, with amphibole and biotite (+/- pyroxenes); and granodiorite, with biotite as the main mafic mineral. Evidences of open- system processes, including partially resorbed metasediment xenoliths, are widespread. These rocks are metaluminous, alkali-calcic and magnesian, and have shoshonitic affinity. They show relative LILE and LREE enrichment and pronounced negative Nb-Ta anomalies. Major and trace element geochemical modeling favors magma differentiation by fractional crystallization (56-62%) after crustal assimilation of similar to 30% of local paragneisses and schists from the Serido Group in a relatively stationary magma chamber at crustal depth, under pressures of 5-6 kbar, temperatures of similar to 800-1,000 degrees C and relatively oxidizing conditions. The less evolved gabbro-diorite was generated by similar to 18% modal dynamic partial melting of a metasomatized mantle source.

Automated summary

The Sao Joao do Sabugi Pluton in northeastern Brazil is a significant occurrence of Ediacaran syn- to post-collisional shoshonitic rocks. The petrography and geochemical modeling suggest that the pluton underwent open-system magmatic differentiation processes, with evidence of crustal assimilation and fractional crystallization.