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11. The Gondwanide deformation along the southwestern border of the Río de la Plata Craton: Geochemical and geochronological constraints on ductile shear zones from the Ventania System basement, Argentina

Author

Ballivián Justiniano, Carlos A., Oriolo, Sebastián, Basei, Miguel A.S., Lanfranchini, Mabel E., Christiansen, Rodolfo O., Uriz, Norberto J., Vázquez Lucero, Sebastián E., Del Bono, Daniela A., Forster, Marnie A., Etcheverry, Ricardo O., Tassinari, Colombo C.G., Comerio, Marcos A., and Prezzi, Claudia B.

Published
2023
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16. A conceptual model for the evolution of a magmatic system based on U-Pb zircon studies on the Devonian granites of Sierra de San Luis, Argentina

Author

Dahlquist, Juan A., Morales Cámera, Matías M., Moreno Moreno, Juan Antonio, Basei, Miguel A.S., Santos da Cruz, Gilmara, Oliveira, Elson P., Dahlquist, Juan A., Morales Cámera, Matías M., Moreno Moreno, Juan Antonio, Basei, Miguel A.S., Santos da Cruz, Gilmara, and Oliveira, Elson P.

Abstract

It is widely acknowledged that thermal models clearly demonstrate that crustal magma bodies should solidify rapidly upon emplacement. Small plutons can cool below the solidus in thousands of years, while even large plutons require hundreds of thousands of years, but not more than a million years. However, recent U-Pb zircon geochronological data contradict these results, strongly suggesting that magmatic systems are often characterized by protracted events. Therefore, a conceptual framework that reconciles thermal models and the geochronology data is necessary. The Devonian foreland magmatism of the Sierra de San Luis is made up of two distinctive suites, the Monzonite suite (<65 wt.% SiO2) and the Granite suite (>65 wt.% SiO2), both emplaced at ca. 3.7 kbar. Classification of the studied Devonian granitoids is debatable because they have a hybrid I- to A-type granite signature. Based on a robust geochronological U-Pb zircon dataset we corroborate the development of a protracted magmatic activity with three major crystallization events for this Devonian magmatism: 391 ± 1, 384 ± 1, and 379 ± 2 Ma. Considering these geochronological data, we postulate the presence of a deep mush reservoir, where a prolonged magmatic activity, permitted the prolonged crystallization of antecrysts (ca. 395–384 Ma). Migration of the parental magma from the mush reservoir zone occurred near the time of emplacement and culminated in the formation of an ephemeral magma chamber located at shallow levels, where zircon autocrysts crystallized (ca. 379 Ma). Age spectra reported within individual samples support the idea of a massive migration of magma when conditions were favourable (e.g. thermally matured crust). Individual crystallization ages recorded by monazites hosted in two-mica granites are comparable to those obtained from zircons, supporting the presence of a long-lasting hot source. Additional geochronological data indicate that a later thermal event (ca. 353 Ma) could have partiall, Consejo Nacional de Investigaciones Científicas y Técnicas, Fondo para la Investigación Científica y Tecnológica, Fundação de Amparo à Pesquisa do Estado de São Paulo, Secretaria de Ciencia y Tecnología - Universidad Nacional de Córdoba, Depto. de Mineralogía y Petrología, Fac. de Ciencias Geológicas, TRUE, pub

Published
2024

45. PAIRED RESIDUAL DEFORMATION AND POST-OROGENIC EXHUMATION OF THE KAOKO AND DOM FELICIANO BELTS: impacts of the migrating orogenic front during the assembly of western Gondwana

Author

HUECK, MATHIAS, BASEI, MIGUEL A.S., WEMMER, KLAUS, DUNKL, ISTVÁN, TESSER, LUCAS, CAMPOS NETO, MARIO C., FRIMMEL, HARTWIG, and SIEGESMUND, SIEGFRIED

Subjects
shear zone reactivation, ductile-brittle transition, structural inheritance, low-T thermochronology, Brasiliano/Pan-African orogenic cycle
Abstract

The reactivation of shear zones under brittle conditions during exhumation due to extensional collapse of the thickened continental crust is a common feature of post-collisional settings. New illite K-Ar analyses and (U-Th)/He data help establish the timing of exhumation and residual deformation during the final stages of orogenic activity in the Kaoko Belt, NW Namibia. Dating of clay-sized white mica from fractionated samples can be used to estimate late-stage deformation in shear zones and constrain the transition from the ductile to the brittle regime. XRD characterization of the dated material indicates crystallization under epizonal conditions (>300°C), which, together with the lack of correlations with age in fractionated samples are suggestive of protracted mineral growth. This supports the interpretation that K-Ar ages at 510-480 Ma in the analyzed samples set a maximum age for the end of ductile activity in the Kaoko Belt, after which brittle faulting is interpreted to have been the predominant style of deformation. This is in accordance with zircon thermochronological data, which predominantly record fast cooling towards temperatures below 200°C shortly thereafter. When compared with equivalent data in South America, the new results reveal that, while the peak orogenic stage of the Dom Feliciano Belt (650-570) took place many tens of Myr earlier than in the Kaoko Belt (580-550 Ma), the transition from ductile to brittle deformation (535-465 Ma and 510-480 Ma, respectively) and rapid exhumation into upper crust conditions (550-400 Ma and 500-350 Ma, respectively) has much more significant overlaps. This observation suggests that, with the progression of the main deformation front from present- day South America to Africa at the time of the collisional process in the Kaoko Belt, the late-stage deformation and eventual post-orogenic exhumation of the Dom Feliciano Belt may have been controlled by the far-field tectonic influence of the continuing orogenic activity to the West. The late stages of the Kaoko Belt, on its turn, are also influenced by a continuation of the progressive migration of the orogenic front towards the Damara Belt, thus highlighting the interconnected character of deformation and exhumation within the entire orogenic system., 4° Encontro dos Pesquisadores de Pós-Doutorado do IGc/USP, Instituto de Geociências da Universidade de São Paulo, São Paulo, SP, Brasil, 29 e 30 de novembro de 2022.

Published
2022
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