Abstract
The Morro Redondo Complex is one of the most important occurrences of the Graciosa A-type Province, southern Brazil. It consists of the Papanduva and Quiriri granitic plutons and a contemporaneous bimodal volcanic association. The Papanduva Pluton includes massive and deformed peralkaline alkali-feldspar granites with Na-Ca and Na-amphiboles and clinopyroxenes. The deformed types are the most evolved rocks in the province and carry rare ‘agpaitic’ minerals, some being described for the first time in granites from Brazil. The larger Quiriri Pluton comprises massive, slightly peraluminous, biotite syeno- and monzogranites with rare Ca-amphibole. Biotite compositions are relatively homogeneous, whereas sodic amphiboles and clinopyroxenes show increasing Na and Fe3+ evolving paths. The Morro Redondo granites are ferroan, with high SiO2, alkalis and HFSE contents; the peralkaline types registering the highest fe#. LILE and HFSE abundances increase with the agpaitic index and the most evolved are HHP granites, with radiogenic heat production up to 5.7 µWm–3. Geothermobarometric estimates indicate emplacement under low pressures (∼100 MPa), at temperatures up to 850-800 °C, and relatively reduced (QFM) and oxidized (+1 REPLACE_LT ΔQFM REPLACE_LT +3) environments for the Papanduva and Quiriri Plutons, respectively. In both cases, melts evolved to relatively high oxidation states upon crystallization progress.
Highlights
A-type magmatic provinces are widespread in the Earth’s crust and have been formed since Archean times as a result of geodynamic processes associated to extensional tectonic environments
magnetic susceptibility (MS) measurements were made with the GM Instruments SM-20 Portable Susceptibilimeter, considering a mean of 10 measurements per sample taken in appropriate outcrops during the field work and in rock slabs afterwards
The texture of the main massive facies is charac terized by large tabular crystals of mesoperthitic alkali feldspar with well-developed quartz; amphiboles and pyroxene are interstitial and homogeneously distributed (Fig. 6a)
Summary
A-type magmatic provinces are widespread in the Earth’s crust and have been formed since Archean times as a result of geodynamic processes associated to extensional tectonic environments. The diversity of rock types, with contrasting petrographic and geochemical signatures FREDERICO C.J. VILALVA and SILVIO R.F. VLACH two contrasted petrographic associations, namely alkaline and aluminous (subalkaline), as well as related K-rich dioritic, gabbroic and some volcanic rocks were emplaced during the final stages of the Brasiliano/Pan-African Orogeny, at the end of the Neoproterozoic. VLACH two contrasted petrographic associations, namely alkaline and aluminous (subalkaline), as well as related K-rich dioritic, gabbroic and some volcanic rocks were emplaced during the final stages of the Brasiliano/Pan-African Orogeny, at the end of the Neoproterozoic Such diversity provides an ideal setting to investigate the evolution, composition and geotectonic significance of A-type granitoids and associated rocks (Gualda and Vlach 2007a, b, c, Vlach and Gualda 2007, Vlach et al 2011)
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