EM - Escola de Minas

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Ferroan alkalic volcanism associated with Calymmian rifting in the Paramirim aulacogen, São Francisco craton, Brazil : new insights from lithofacies analysis and evidence of mantle-derived alkaline H2O-rich metasomatic fluids affecting ancient crustal materials.
(2020) Santos, Cláudia dos; Danderfer Filho, André; Queiroga, Gláucia Nascimento; Zincone, Stéfano Albino; Castro, Marco Paulo de; Lana, Cristiano de Carvalho
Rift-related volcanism is widely used in models of supercontinent reconstruction, while the associated metasomatic fluids provide information about the mantle-crust process during intracrustal melting events. The Paleoproterozoic/Mesoproterozoic Paramirim aulacogen corresponds to multiple intracratonic rift cycles that register many basin developing events. Intense volcanic activity is recorded in some rift events, but the volcanic lithofacies and their petrogenesis are poorly constrained. Herein, we present lithofaciological characterization, whole-rock and mineral chemistry, and U-Pb and Lu-Hf isotope data from the 1.58 Ga volcanic rocks. Trachytic and trachy-andesitic lava flows and primary to secondary volcaniclastic lithotypes characterize this volcanism. The primary volcaniclastic rocks resulted from direct volcanic activity and can be divided into pyroclastic and hydroclastic lithofacies associations. The pyroclastic lithotypes correspond to air-fall, density currents and blockand-ash flow deposits formed in explosive eruptions; hydroclastic lithotypes are represented by hyaloclastites and peperites, reflecting effusive eruptions and interactions between magma, water and sediments. The secondary volcaniclastic lithotypes resulted from late-stage reworking processes and consist of debris flows, ephemeral streams associated with high energy and gravitational flow deposits. The lava flows show well-preserved primary volcanic features, such as magma flow textures and euhedral phenocrysts. Based on petrographic and chemical characteristics, associated with negative ɛHf(t) values, we infer that high melt rate of an ancient crust, previously metasomatized by H2O-rich alkaline mantle-derived fluids, generated the alkalic ferroan lavas. These fluids promoted potassic and sodic metasomatism that affected all the volcanic rocks, modifying the chemistry of the feldspars to pure orthoclase (Or96-98) and albite (Ab98-100). In some lavas, a higher concentration of Na-rich fluids synthesized aegirine at a minimum temperature of 200 °C and promoted its peralkaline character. The potassic-sodic metasomatism influenced the volcano-sedimentary sequence of the Pajeú Group and the oldest rocks of the basement but did not affect the overlying sedimentary sequences, indicating that fluids ceased with the end of volcanism. We suggest that the H2O-rich alkaline fluids arose by focused degassing in response to a rising asthenospheric mantle during the Calymmian rift of the Paramirim aulacogen.
Stratigraphic and geochronological characterization of the Mato Verde group, Central Espinhaço (Brazil) : an Eocalymmian rifting record in the western domain of the Congo-São Francisco paleocontinent.
(2018) Costa, Alice Fernanda de Oliveira; Danderfer Filho, André; Lana, Cristiano de Carvalho
The Mato Verde Group comprises an important volcano-sedimentary record occurring in the eastern Congo-São Francisco paleocontinent along the western border of the Central Espinhaço. This succession is interpreted as a basin rift related to one of the stages of basin formation in the Espinhaço aulacogen at the northern end of the Araçuaí Orogen. The stratigraphic architecture and U-Pb data from the volcano-sedimentary succession, as well from the overlying succession, provide new insight into the evolution of Mato Verde rifting. The sedimentary pattern of this succession is interpreted as the basin infilling of an intracontinental rift that was compartmentalized in two half-grabens. The basal succession defined as the Panelas Formation contains entirely siliciclastic sedimentation, such as alluvial fan, delta fan and minor fluvial and lacustrine facies associations. Detrital zircon grains extracted from basal conglomerates show mainly Archean and Paleoproterozoic ages. The upper sequence is represented by the Riacho Seco Formation, which consists of volcanic lava beds and volcaniclastic rocks. At the top of the succession, a lapilli tuff is dated at 1586 ± 15 Ma, which is interpreted as the final age of the rift phase. The Vereda da Cruz Formation occurs in stratigraphic discontinuity above the Mato Verde succession and comprises exclusively aeolian sedimentation. The youngest grain defines a maximum sedimentation age of 1616 ± 30 Ma. This basin stage is interpreted as due to a thermal subsidence process that is related to Mato Verde rifting. The equivalent unit to the Mato Verde Group is only found in the Northern Espinhaço and is represented by the Pajeú Group. The basin-fill patterns and Calymmian ages suggest a direct link with the second rifting event within the São Francisco paleocontinent, which was responsible for the development of the Espinhaço basin.

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