Banca de QUALIFICAÇÃO: Daniela Alexandra Guerrero Gutiérrez

Uma banca de QUALIFICAÇÃO de DOUTORADO foi cadastrada pelo programa.
STUDENT : Daniela Alexandra Guerrero Gutiérrez
DATE: 06/02/2023
TIME: 14:00
LOCAL: Sala de videoconferência do IGD e Teams
TITLE:

Petrography, geochemistry and isotopic geochemistry of the basement and the impact melt breccias of the impact structure of Rochechouart, NW of Massif Central, France

KEY WORDS:

Rochechouart impact structure, French Massif Central, Geochemistry, Geochronology, Lu–Hf isotopes, Provenance analysis.

PAGES: 97
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

The 24 km diameter medium-sized Rochechouart impact structure is located in the northwestern part of the crystalline basement of the French Massif Central (FMC). This structure offers a great opportunity to study the crater floor, the shocked target rocks, and a wide variety of crater-fill deposits, between them impact melt rocks. The FMC was formed during the amalgamation of Pangea in the Variscan orogeny between the Late Silurian and Early Permian. This project is focused on a comprehensive study of the relationship between the impact melt rock samples and the crystalline basement rocks in the area of the Rochechouart impact structure. This involves petrography including mineral chemistry, whole-rock geochemical analysis, Sr and Nd isotope analysis of whole-rock samples by TIMS, U-Pb, Lu-Hf analysis on zircon by LA-MC-ICP-MS, and U-Pb analysis on zircon by SIMS. In the area of the impact structure, the basement includes gneisses, amphibolites, serpentinites and granitic bodies, and minor intrusions of mafic and felsic dikes. This work has been developed with the purposes of (1) characterize the regional geological, and the shock metamorphic features of the basement rocks, (2) investigate the dynamics of impactite formation in terms of the P-T conditions achieved during the impact event and the contributions from different basement lithologies, (3) place the basement isotopic and chronological results into the regional context of the evolution of the FMC, and (4) study the effects of possible post-impact processes. Microscopic observations show that most of the collected basement samples display low degree of deformation. Only a micro quartz-diorite sample collected near the center of the impact structure registers unequivocal evidence of shock metamorphism and most of the impactites display a mixture of unshocked and shocked clasts. Furthermore, some level of hydrothermal alteration is common for all samples. First U-Pb zircon results on one orthogneiss, one paragneiss, two amphibolite samples, and two impact melt rock samples are presented. Geochemical resultsindicate peraluminous and metaluminous character for the plutonic rocks of the FMC basement. Lu-Hf data on zircon indicate an overall positive εHf (t) values and TDM mainly distributed from 1 to 1.2 Ga, which indicates a major period of crustal formation at that time. Zircons with Neoproterozoic to Carboniferous ages are interpreted as inherited zircons. The amphibolites register Devonian ages, coeval with major stages of migmatization and continental collision of the Variscan orogeny. For many elements, the impact melt rocks display intermediate compositions between the basement lithologies analyzed here; however, the impact melt rocks are characterized by higher contents of K2O and lower contents of Na2O and CaO, attributed previously by other authorsto post-impact alteration. Furthermore, the impactites register sources with younger and older TDM model ages, in comparison with the results obtained for the basement until now. U-Pb geochronology on zircon from the impactites gave a significant diversity of ages, ranging from the Neoproterozoic to the Jurassic. Ages of ~204 Ma obtained in both impact melt rock samples are very similar to the currently most accepted impact age. In addition, the Montoume impact melt rock yielded ~196 Ma ages, which might reflect a to date unconstrained post-impact thermal/hydrothermal event. Radiation damage calculations for these zircons suggest that none suffered significant metamictization. Numerous ages in between the latest stages of the evolution of the FMC (c. 300 Ma) and prior to the impact event were also obtained in zircon from the impactites, including ages which might reflect a magmatic(?) event at 280-225 Ma, previously not recognized in the NW part of the FMC. Remaining results from the first samples and all results from the rest of the samples are in process. The results obtained in this project will be presented in three papers. The first one will be focused on a provenance analysis, the second on U-Pb geochronology results of more impact melt samples and on a detailed characterization of zircon and quartz textures, and the third one on a comparison of the full geochemical/zircon provenance results from the basement lithologies and the impact melt rocks. A draft of the first paper entitled “U-Pb on zircon isotopy and first provenance analysis on impact melt rocks from the Rochechouart impact structure” is presented in this document.

BANKING MEMBERS:
Externo à Instituição - ALISSON LOPES OLIVEIRA
Externa ao Programa - 3000894 - ELIZA INEZ NUNES PEIXOTO
Interno - 1178074 - GUILHERME DE OLIVEIRA GONCALVES
Interno - 2370895 - LUIS GUSTAVO FERREIRA VIEGAS
Presidente - 3043135 - WOLF UWE REIMOLD