Dating brazilian impact structures using LA-ICP-MS zircon fission track and (U-Th)/He

Author(s)

Mariana Velcic Maziviero

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Geociências
• Defense date: 2016-12-12

Examining board members

Alvaro Penteado Crósta; Claudio Riccomini; Emilson Pereira Leite; Peter Christian Hackspacher; Wagner da Silva Amaral

• Advisor: Alvaro Penteado Crósta; Natalia Hauser; Ana Maria Goes

Abstract

Currently, 190 meteorite impact structures have been identified on Earth. In the Brazilian territory, seven structures of impact origin are known so far: Araguainha (MT-GO), Vargeão (SC), Vista Alegre (PR), Cerro do Jarau (RS), Serra da Cangalha (TO), Riachão (MA), and Santa Marta (PI). Among them, Araguainha and Vargeão are the only ones with ages established by isotopic methods: 254.7 ± 2.5 Ma (U-Th-Pb in zircon and monazite grains) and 123.0 ± 1.4 Ma (U-Pb in zircon grains), respectively. The others have only poorly constrained ages estimated by stratigraphic correlation. U-Pb and 40Ar-39Ar are the isotopic methods conventionally used to date impact melt rocks and neo-formed minerals. However, in deeply eroded impact structures these materials are usually scarce or absent, precluding the application of such geochronological dating methods. In order to estimate the impact age, we applied low temperature thermochronological methods in Araguainha, Serra da Cangalha, and Riachão structures, corresponding to fission track and (U-Th)/He in zircon. Thermochronological methods in impact structures are rare in the literature, and still nonexistent in the case of zircon fission track. All impact structures studied are of complex type, are situated in Paraná and Parnaíba sedimentary basins ¿ in the case of Araguainha, rocks from the crystalline basement occur in the central uplift -, the diameter range from 4 to 40 km and show different degrees of erosion. For Araguainha, a pooled age (U-Th)/He of 223 ± 17 Ma has been obtained likely related to the impact event. The zircon fission track analysis of impact breccias from Araguainha provided central ages of 236 ± 49 Ma and 231 ± 13 Ma, obtained by binomial peak fitting method, and that are also likely related to the impact. Sedimentary rocks that outcrop in the other morphostructural zones of Araguainha and one sample collected outside of Araguainha impact structure, show ages between ~500-600 Ma, mostly related in the Brasilian orogeny (850-490 Ma). Serra da Cangalha and Riachão impact structures have few grains with individual ages < 270 Ma. These zircon fission track ages are the provenance age or related to thermal events in the basin before the impact. The majority are aged between ~400 ¿ 600 Ma. The results indicate that the most promising rocks to apply thermochronological methods are the impactites in the Araguainha central uplift. The other rocks that outcrop in the other impact structures may not have reached temperatures high enough for the total annealing of the spontaneous fission track in zircon grains (~200 to 320 ºC). In this work, besides the ages obtained by (U-Th)/He and zircon fission track for the Brazilian impact structures, contributions are made on stratigraphic issues, mineral characterization of an Araguainha impactite, and the assessment of the limitations of thermochronological methods for dating eroded impact structures (AU)

• FAPESP's process: 11/22864-1 - Application of low-temperature thermochronology to brazilian impact structure
• Grantee: Mariana Velcic Maziviero
• Support Opportunities: Scholarships in Brazil - Doctorate