Revisiting the ages of the Poços de Caldas Alkaline Massif: 40Ar/39Ar dating of key samples to solve a conundrum

• Grant number: 22/07167-7
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• Start date: August 01, 2022
• End date: May 31, 2023
• Field of knowledge: Physical Sciences and Mathematics - Geosciences - Geology
• Principal Investigator: Silvio Roberto Farias Vlach
• Grantee: Allan Silva Gomes
• Host Institution: Instituto de Geociências (IGC). Universidade de São Paulo (USP). São Paulo , SP, Brazil
• Associated research grant: 19/22084-8 - The Paraná Magmatic Province: petrogenesis, chronology and environmental impact of Cretaceous tholeiitic, alkaline and silicic magmatism in the Brazilian Platform, AP.TEM

Abstract

The work plan aims obtaining 40Ar/39Ar data in minerals concentrates of representative key-samples of alkaline rocks from the Poços de Caldas Alkaline Massif (MG/SP) to determine their crystallization/emplacement ages with precision. This massif is the largest Brazilian (ca. 800 km2) and one of over a hundred alkaline occurrences that cut across the Brazilian platform, emplaced along the Northern and Eastern margins of the volcano-sedimentary Paraná Basin. It is largely made up by felsic volcanic, subvolcanic and plutonic SiO2-undersaturated felsic, intermediate-miaskitic and agpaitic rocks, which are associated with minor coeval mafic to ultramafic rocks ("ankaratrites") as well as irregular distributed silico-carbonatitic and lamprophyric rocks from distinct genetic lineage. A significant amount of low precision K/Ar and Rb/Sr and a few high precision 40Ar/39Ar and U/Pb dating results are available for a variety of rocks, however they do not allow put the main magmatic episodes precisely into the time scale, as most of these previous results are self-inconsistent due to sampling, analytical and/or particularly hydrothermal overprint effect issues. For instance, the few recent 40Ar/39Ar dating in phlogopite and one U/Pb data for hydrothermal zircon suggest older ages for the main magmatism (e 84 Ma) than the previously accepted values, around 79 Ma. Importantly, geological evidences point to the coeval character of most felsic and mafic-ultramafic rocks, while geomagnetic data suggest possible crystallization ages between 79.1 and 83.0 Ma for the felsic rocks, with reverse polarization, while the mafic-ultramafic varieties, with normal polarization, should present ages in between 73.6 and 79.1 Ma. These information result in a conundrum still to be solved.This proposal aims to solve such conundrum through a detailed dating program by applying the 40Ar/39Ar high precision systematic on selected mineral concentrates (e.g., sanidine, orthoclase, flogopite) from geologically and structurally well-controlled key samples of both subvolcanic and plutonic felsic and ultramafic rocks. This is the best dating method (precise to 0.3-0.5 Ma) currently available, as primary, zircon or baddeleyite are absent in the magmatic rocks, prohibiting the application of U-Pb systematic. The obtained results will allow define the massif's main magmatic periods and its emplacement time interval with the desired precision and, along with the available information for the regional alkaline magmatism, a better understanding of the periodicity of this magmatism in the Brazilian platform and its petrological and geodynamic implications, one of the main themes of the associated thematic project. (AU)