Impact-induced hydrothermal systems: the Manicouagan example

Abstract

The Manicouagan impact structure is one of the best-preserved impact structures on Earth with a rim-to-rim diameter of approximately 80 km. It was generated at ∼214±1 Ma in Grenvillian (∼1 Ga) metamorphic gneisses. The structure is mostly exposed on a 50 km diameter island covered by impact melt (an average thickness of 200-300 m), which overlies basement rock, and which abuts an off-centre central peak. This study has investigated post-impact hydrothermal activity via the analysis of samples collected at the site and drill core samples from the collection at the Planetary and Space Science Centre. Interpretation of petrography, mineralogy, and chemistry of impact-generated lithologies and basement rocks has revealed five main hydrothermal regimes: (1) intramelt sheet, (2) basal-melt sheet, (3) melt-bearing breccia, (4) basement, and (5) central uplift. The most intense hydrothermal alteration is developed within the melt-bearing breccia regime. Results indicate that zeolitization and metasomatic processes (e.g., Na, K, Ca) dominate the hydrothermal activities developed within the structure. Mineral assemblages and fluid inclusion study suggests that the impact-induced hydrothermal system were formed at T <400 °C, and evolve to ambient temperature. Oxygen isotope determination performed on hydrothermally formed quartz (and amethyst) indicate hydrothermal fluids with magmatic, metamorphic, and meteoric origins were involved in the intra-melt sheet, basal-melt sheet, and melt-bearing breccia hydrothermal regimes, respectively, with potential mixing of fluids in some areas. The hydrothermal fluids mainly circulated via fracture systems, and indicate a low permeability of the melt sheet. This study also indicates low water activity within the melt sheet. The impact-induced hydrothermal system at Manicouagan is similar to that of other terrestrial impact structures. However, low water-rock interaction indicate that there was limited surficial H2O available at the time of impact. This corresponds to an equatorial, semi-arid (continental) setting for the impact event.