Geology, petrography and geochemistry of the A-type granites from the Morro Redondo Complex (PR-SC), southern Brazil, Graciosa Province

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 Fe 3+ evolving paths. The Morro Redondo granites are ferroan, with high SiO 2 , 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 < Δ QFM < +3) environments for the Papanduva and Quiriri Plutons, respectively. In both cases, melts evolved to relatively high oxidation states upon crystallization progress.

NACARENIOBSITE-(Ce) AND BRITHOLITE-(Ce) IN PERALKALINE GRANITES FROM THE MORRO REDONDO COMPLEX, GRACIOSA PROVINCE, SOUTHERN BRAZIL: OCCURRENCE AND COMPOSITIONAL DATA

“…The modal abundances of mafic minerals in the studied samples are up to 10% and, among them, up to 4% are of rare accessories. Vilalva & Vlach 2013). As expected, abundances for most HFSEs show well-defined positive correlations with the Agpaitic Index.…”

“…These granites followed an agpaitic crystallization sequence, where mafic and accessory minerals began to crystallize after the felsic minerals. Several of the mafic and most of the rare accessory minerals were, in fact, formed during late-to post-magmatic crystallization stages (e.g., Vilalva & Vlach 2010, 2013. The modal abundances of mafic minerals in the studied samples are up to 10% and, among them, up to 4% are of rare accessories.…”

“…Besides typical bimodal basic and acid volcanism, the Morro Redondo Complex consists of two main intrusive units: the Papanduva and Quiriri Plutons (Fig. 1, see also Vilalva & Vlach 2010, 2013. The former is smaller and consists of a variety of peralkaline alkalifeldspar granites (Alkaline Association), whereas the latter contains aluminous, subalkalic, mainly biotite syenogranites (Aluminous Association; .…”

“…The former is smaller and consists of a variety of peralkaline alkalifeldspar granites (Alkaline Association), whereas the latter contains aluminous, subalkalic, mainly biotite syenogranites (Aluminous Association; . General background geology and petrography, as well as additional details, in particular relating to the geochemical aspects of the complex, may be found in Vilalva & Vlach (2013).…”

Petrogenesis of the Triassic Bayan-Ulan alkaline granitic pluton in the North Gobi rift of central Mongolia: Implications for the evolution of Early Mesozoic granitoid magmatism in the Central Asian Orogenic Belt

“…Wide ranges in the compositions of the amphiboles can occur owing to evolving T and f O2 under both magmatic and subsolidus conditions (e.g., Hoshino, 1986). In some plutons, Na amphibole supersedes earlier Ca-Na amphibole (e.g., Vilalva and Vlach, 2014), but in the present instance, both amphiboles appear to be approximately coeval or the sodic amphibole somewhat earlier (Fig. 5C).…”