Peraluminous magmatism in the Southernmost Dom Feliciano Belt (Brazil): Magmatic evolution process, sources and tectonic implications

Abstract

This paper presents new petrological and geochronological constraints on the evolution of the Três Figueiras Granite, the main occurrence of peraluminous granitic rocks in the Southernmost Dom Feliciano Belt, Brazil. The main goal of this work is to constrain the time frame and possible crustal melting processes and sources for this magmatism and its tectonic implications for the evolution of the Dom Feliciano Belt during the Ediacaran, based on field and structural data, petrography (optical microscopy), mineral chemistry (electron proble microanalysis), isotope geochemistry (whole-rock Sr–Nd) and geochronology (U–Pb zircon SHRIMP). The granite is composed of quartz, orthoclase (Or91), oligoclase (An14- Ab86), primary muscovite and biotite, with garnet, tourmaline, zircon, monazite and apatite as accessory minerals. This magmatism is synkinematic to the Ayrosa Galvão-Arroio Grande ductile shear zone system, developing a mylonitic fabric with a subvertical (>70°) southeastward dipping tectonic foliation, and a subhorizontal, ENE plunging stretching lineation marked by elongated mica crystals and aggregates of quartz and feldspar, with dextral kinematics. The granite shows intermediate 87Sr/86Sr(i) ratios (0.7096–0.7123), negative εNd(t) values (−2.61 to −6.54) and Nd model ages between 1.47 and 1.78 Ga. We suggest that the main sources for this magmatism include metagreywackes and others rocks from the Arroio Grande Complex, as a result of dehydration melting, in a transpressional regime, which is a common feature in oblique collisional orogens. The oblique collision is possibly related to the closure of the South Adamastor paleo-ocean. The new U–Pb zircon ages reported here, between 609 and 560 Ma, are similar to those of peraluminous granitic rocks associated to the Dorsal de Canguçu shear zone in the central domain of the Dom Feliciano Belt, as well as to those reported for the Otazo-Cerro Amaro shear zone system, which is the Uruguayan counterpart of the Ayrosa Galvão-Arroio Grande shear zone system.