"Geology of Córrego das Campinas Gabbro-Anorthosite Suite and metamorphic rocks of the southern portion of the Araguaia Belt: relationship with the Goiás Magmatic Arc and implication for the geodynamic evolution of the Tocantins Province"
Tocantins Province, Goiás Magmatic Arc, Rodínia, Gondwana, Heat Traps, Anorthosite, Araguaia Belt, Monte do Carmo Formation, high thermal gradients.
The Tocantins Province, located in Central Brazil, comprises rock types that are related to important Neoproterozoic geotectonic events. The evolution history of the province is complex, marked by accretionary and rifting events that led to the Western Gondwana amalgamation. In order to contribute to the understanding of the evolution processes, in the present work, rocks of two distinct geological contexts located in the central part of the Tocantins Province were studied. New possibilities for the geodynamic processes that resulted in the formation of the Tocantins Province are raised from the results of this study. In the first area, the Córrego das Campinas Gabbro-Anorthosite Suite was investigated. It is inserted in the Goiás Magmatic Arc framework, Brasília Belt. The Córrego das Campinas Gabbro-Anortosite Suite is constituted of a Proterozoic massive-type anorthosite body associated to a gabbroic body of similar dimensions and minor intrusions and lenses of tonalite, quartz monzogranite, quartz syenite, albite granite, Fe-diorite dykes, and ilmenite-enriched layers. The interpretation of petrographic, lithogeochemistry and mineral chemistry data indicate that the Córrego das Campinas Gabbro-Anorthosite Suite genesis is in accordance with the traditional petrological model of the massive-type anorthosites related to calc-alkaline basaltic magma segregation, plagioclase accumulation in magmatic chambers and its subsequent diapiric ascent, generating anorthosites, gabbros, and quartz syenites. Crystallization of residual liquids formed the other rock types. Zircon U-Pb dating of different rocks of the suite shows crystallization ages of ca. 661 and 648 Ma. The age of the suite and its positioning indicates a correlation with an important mafic magmatic event in the Brasília Belt at 670–600 Ma. Interpretation of the Brasília Belt geological data, associated with the present study, indicates that the rocks of the suite developed in an active continental margin back-arc environment. The presence of massive type anorthosite in the Brasília Belt context reinforces the thesis that the evolution of this belt is characterized by thermal gradient raising. It is suggested that mantle isolation that resulted from a “heat trap”, heat coming from the Rodinia break-off mantle plume, crustal thickening, and elongated shape of the tectonic plate were the main factors responsible for the thermal gradients in the context of the Brasília Belt formation. The other study area is located to the west, in the southern Araguaia Belt. In this area, NE-SW-oriented layers of mainly immature metasedimentary rocks associated with metamafic and orthogneiss lenses are exposed. This framework was affected by compressive forces forming folds and thrusts, and by the Transbrasiliano Lineament dextral shear zone system. The immature metasedimentary rocks show compositions similar to those of tonalite to granite, suggesting graywacke and lithoarenite as likely protoliths. Detrital zircon provenance study indicates Neoproterozoic sedimentation, with maximum depositional ages at around 600 Ma, and contribution of older Archean to Mesoproterozoic sources. The metamafic rocks are represented mainly by narrow amphibolite lenses interpreted as probable dykes and sills. Lithogeochemical data of the amphibolites show basaltic to andesitic compositions, with OIB and E-MORB signatures developed in a magmatic arc setting. Zircon U-Pb dating of a fine-grained amphibolite yielded a crystallization age of ca. 664 Ma. The association between immature metasedimentary rocks and metamafic lenses indicates an early stage opening or reactivation of basin, in which dyke swarms and high-energy proximal sedimentation are common. The magmatic arc signature of metamafic and paraderived rocks, associated with Neoproterozoic ages, indicates a probable retro-arc basin environment related to the evolution of the Goiás Magmatic Arc, between approximately 660 and 600 Ma. The metasedimentary and mafic rocks can be correlated to the Monte do Carmo Formation rocks, indicating the possible extension of this unit to the southern portion of the Araguaia Belt. Zircon U-Pb dating of orthogneisses associated with the metasedimentary rocks provided crystallization ages of 596±18 Ma and 1985±12 Ma. The youngest rock can be correlated to the Aliança Suite, while the oldest can be associated with the Rio dos Mangues Complex, interpreted as the Araguaia Belt basement. Geochronological data also revealed two metamorphic ages (ca. 580 and 540 Ma), indicative of two collisional events in the context of the Araguaia Belt. From the obtained data, it is discussed the possibility that the Goiás Magmatic Arc had developed also in the Faixa Araguaia context. The new evolutional model would explain some peculiar features of the Araguaia Belt, such as the common presence of textural and mineralogical immature metasedimentary rocks, two metamorphic peaks, and magmatic arc-related rocks. The study area represents two distinct geological contexts of the Tocantins Province evolution, separated by the Porangatu Suture, but displays rocks of similar tectonic settings and ages. This fact indicates that the province evolution is complex and was likely marked by many accretionary fronts of the Goiás Magmatic Arc during similar age periods. Integration of obtained data from this study and the extensive available data of the Tocantins Province allows the establishment of the compartments and geotectonic events of the province and the elaboration of an evolution model. These interpretations indicate that the geotectonic events that formed the Tocantins Province are correlated to the processes that induced the Rodinia fragmentation. In this sense, it is suggested that the continuous raising of thermal gradients inside the Tocantins Province is related to the Rodinia break-off. This condition of elevated temperatures resulted in many lithological constituents and geodynamic processes generating, for example, adakites, charnockites, ultrahigh-temperature metamorphism, extensive mafic magmatism with many layered complexes, and Proterozoic massive-type anorthosites. Corroborating this correlation between the events, many studies show OIB-type magmatism in the evolution context of the province, suggesting the likely presence of a mantle plume on various occasions of its evolution. Further, the rifting processes that took place in the Province are also time-correlated to the divergent settings of Rodinia fragmentation. Thus, it appears that the Rodinia break-off had a significant role in the Western Gondwana generation.