Sources and settings of Ediacaran post-collisional syenite-monzonite-diorite shoshonitic magmatism from southernmost Brazil

Abstract

The Arroio do Silva Pluton comprises quartz-rich syenite-monzonite-diorite rocks, with subordinate lamprophyre as enclaves. Compositional and textural variations lead to the identification of three main varieties: (i) quartz monzonite to syenite, (ii) quartz monzonite to monzodiorite and (iii) quartz diorite. The transition between all rock types is gradational and marked mostly by the increase of mafic mineral phases, especially biotite, and by the K-feldspar content, which is characteristically euhedral in the quartz monzonite to syenite variety and becomes subhedral to anhedral towards diorite compositions. Geochemical features corroborate field relations and both point to magma mingling as the main differentiation process. Syenite is considered to be the felsic endmember, whilst the mafic one has dioritic composition. Different degrees of interaction between these magmas result in hybrid varieties represented by quartz monzonite to monzodiorite. A third magma type is observed as ultrapotassic lamprophyric enclaves, but its influence in the mixing processes seems to have been minor. Both mafic and felsic magmas have K2O/Na2O ratios between 1 and 2, compatible with their shoshonitic affinity, whilst ratios slightly above 2 characterize lamprophyric enclave magmas as ultrapotassic. The source of shoshonitic and ultrapotassic magmas, according to trace element and isotope evidence, is probably a phlogopite-bearing mantle which was affected by previous subduction of Paleoproterozoic or Neoproterozoic age. The variation of ultrapotassic to shoshonitic magmas is probably controlled by variation of phlogopite contents in the mantle source. UPb zircon crystallization age of 578 Ma (LA-MC-ICP-MS) determined for the ASP quartz monzonite to syenite variety is in agreement with the Neoproterozoic post-collisional period of the Dom Feliciano Belt. This age value is also in agreement with that of other syenitic associations from south Brazil, such as the Piquiri Syenite Massive, that records similar chemical signatures. Therefore, these syenitic associations are taken to represent previously metasomatized mantle sources in late post-collisional setting from the Brasiliano/Pan-African Cycle in southermnmost Brazil.