SHOCK FEATURES IN ROCKS FROM THE VISTA ALEGRE IMPACT STRUCTURE (BRAZIL)

Although impact craters in basalts might provide an analog for impact craters on terrestrial planets with basaltic crusts, such as Mars, they are rare on Earth, the only known are the Lonar crater in India (1.8 km in diameter) and the Logancha crater in East Siberia, Russia (14 km in diameter). The recently discovered and confirmed Vista Alegre complex impact crater, together with similar possible impact structures in southern Brazil, provides new information about shock effects on basaltic target. The 9.5 km Vista Alegre crater lies within the large flood basalt province of the Paranà Basin, which is temporally related to the opening of the South Atlantic Ocean (dated at about 133-132 Ma) and belongs to the Serra Geral Formation. The target rocks involved are tholeiitic basalts from the lower unit of the Serra Geral Formation and quartzose sandstones from the Piramboia/Botucatu Formations. Such sandstones are normally 700-800 m below the volcanic sequence, but now they are exposed at the central uplift. The main shock features are shatter cones and shocked minerals within polymict breccia and suevite. The shatter cones are developed in almost unshocked basalts, mainly composed of plagioclase (An_55-75), pyroxene (Fe-augite and pigeonite) and magnetite. The striated surfaces are locally covered by a melt film. The polymict breccia contains angular to rounded clasts of different basaltic volcanic rocks, quartzose sandstones, and single calcite grains, suspended in a fine-grained cataclastic matrix. Clasts of partially devitrified glass are also included, but their origin is not clearly impact related. Some clasts of quartzose sandstone show shock features, with the development of at least two sets of PDFs per grain; their orientation measurements allow the estimate of a minimum shock pressure of ~20 GPa. Bomb-like samples of basaltic composition were collected around the crater, showing different levels of shock metamorphism, from unshocked basalt at the core to the complete transformation of plagioclase into maskelynite in the outer part. At the meeting we will present results of the first geochemical and petrographic investigations, focusing in detail on the nature of the different types of melt encountered.