Araguainha impact structure, Brazil: New insights into the geology of the central uplift

Extensive, new outcrops along the MT-100 state road in the northern part of the central uplift of the 40-km diameter, 252–259 Ma old Araguainha impact structure, Central Brazil, have become available for investigation. They offer new insight into the contact relationships between the different lithologies and the genesis of different types of impact-related rocks, as well as the current level of erosion of the structure. Three types of impact melt rock (IMR) with different field relationships and compositions can now be distinguished: (1) Type-I of granitic composition and occurring mainly as veins and dikes, besides a few larger pods, in the central alkali granite core of the central uplift; (2) Type-II in the form of plastically deformed clasts of mainly highly silicious compositions in polymict impact breccia; and (3) Type-III, derived from partially melted conglomerate or sandstone precursors, and that occurs at selected sites in (meta)sedimentary strata of the basement in the immediate environs of the alkali granite core. Both polymict lithic and melt-bearing (suevitic) impact breccias are recognized in the 110-m thick integrated section through impact breccia directly overlying the crater floor. This crater floor is composed of (meta)-sedimentary basement strata with granite injections and, locally, sandstones of the Devonian sedimentary Furnas Formation of the Paraná Basin. Main breccia components are (meta)-pelites and (meta)sandstones of the basement that is currently favored to be related to the regional Paraguay Belt and to the lower sequence of the Paraná Basin sedimentary strata. Locally, breccia contains clasts of IMR Type-II, and only very rarely are granitic fragments observed. Clasts of IMR Type-I have never been observed in the breccia deposits. These new observations preclude significant involvement of alkali granite in the formation of the polymict breccia or in the production of shock melts. They also reveal the major role of the (meta)sedimentary precursors in the production of IMR by shock melting and provide essential information for better understanding the cratering processes involved in the formation of an impact structure in a sedimentary target, of the size of the Araguainha impact structure.