Origin of the Archean leucogranodiorite-granite suites: Evidence from the Rio Maria terrane and implications for granite magmatism in the Archean

摘要

In the Mesoarchean Rio Maria granite-greenstone terrane, the leucogranodiorite-granite (LGdG) suites are mostly represented by the 2.87 Ga Guaranta suite which includes the Trairao and Azulona granodiorite and the Guaranta granite plutons. These granites originated around 50 m. y. after the 2.93 ±0.1 Ga major TTG magmatic event and were approximately coeval with the sanukitoid suites (～2.87 Ga), potassic leucogranites (～2.86 Ga) and the younger TTGs (Agua Fria trondhjemite; ～2.86 Ga). The rocks of the LGdG suites are characterised by strongly fractionated REE patterns, with either no Eu anomaly or slight negative Eu anomalies, and show enrichment in Ba and, to a lesser degree, in Sr compared with the other Archean granitoids of the RMGGT. The LGdG have strong geochemical affinity with the high-Ca granites or Transitional TTG of the Yilgarn craton. The different models proposed to explain the origin of these granites are still controversial and apparently are not able to explain the genetic processes and evolution of the LGdG of the RMGGT. The dual geochemical character of the LGdG suites that share some features typical of the TTGs with others more commonly observed in the sanukitoid suites, suggest a complex evolution for these granites and a possible genetic link between them and the mentioned suites. On the basis of modelling and geochemical data we deduced that the LGdG suites could be possibly derived from mixing between a Ba- and Sr-enriched granite magma and trondhjemitic liquids. The granite magmas participating in the mixture originated by fractional crystallization of 35% of a sanukitoid magma of granodioritic composition leaving a residue consisting of plagioclase (46.72%), hornblende (39.05%), clinopyroxene (10.36%), magnetite (3.12%), ilmenite (0.7%) and allanite (0.06%). The large compositional variations observed in the Guaranta suite can be apparently explained by mixing in different proportions between the granite and trondhjemitic liquids. This large spectrum of variation could be augmented if it is assumed that fractional crystallization played a significant role in the evolution of the liquids formed by mixing processes.