Paleoproterozoic Bimodal Post-collisional Magmatism In The Southwestern Amazonian Craton, Mato Grosso, Brazil: Geochemistry And Isotopic Evidence

摘要

This paper discusses geological and geochemical aspects of a Paleoproterozoic volcano-plutonic association that crops out in southwestern Amazonian Craton, Mato Grosso, Brazil. The study area was divided into undeformed and deformed domains, based on structural and geochronology studies. The unde-formed domain is composed mainly of felsic explosive and effusive flows. Inter-layered mafic flows of basalts and sedimentary rocks are also present. The deformed domain is mainly composed of titanite and hornblende-bearing monzogranite to syenogranite and biotite monzogranite, while granodiorite is less common. U-Pb single zircon analyses yielded ages of 1.8-1.75 Ga in granites and felsic volcanic rocks for both domains. Basalts from the undeformed domain are phaneritic, fine-grained, and are often hydro-thermally altered. They show tholeiitic affinity and are LREE enriched. Their trace element composition resembles those of within-plate associations. The ε_(Nd) (t = 1.75 Ga) for all these rocks are positive, ranging from 0.12 to 1.49, which reflect a juvenile source. The felsic volcanism comprises subalkaline rocks with high K contents and is divided into two groups: crystal enriched ignimbrites and effusive rhyolites. REE patterns of effusive rocks show negative-Eu anomalies and are smooth in the ignimbrites. Trace element patterns similar to those of the effusive rocks and ignimbrites are found in magmatic rocks derived from sources affected by subduction-related metasomatism. Hornblende and biotite granites occur in the deformed felsic plutonic domain. These rocks show evidence of low-temperature metamorphism and deformation, and in some places, of hydrothermal alteration. The LREE/Nb (or Ta) ratios of these rocks are consistent with those observed in granites of post-collisional settings. The ε_(Nd) (t = 1.75 Ga) values are slightly negative on average, with few positive values (-1.41 to +2.96). These data are interpreted as indicative of a magmatism produced during a post-collisional event from mixed sources: a metasoma-tised mantle and a Paleoproterozoic continental crust. An intracontinental shearing with age of 1.7-1.66 Ga created the Teles Pires-Juruena lineament which partially controlled this magmatism.