Discovery of a post-impact rhyolitic volcanism at the northern margin of a granit core near Araguainha (Mato Grosso, Goias, Brasil). Geological reinterpretation of an old SLAR of this ringstructure.

Abstract

Jean-Michel Thery¹, Eva Vetone Akos², Essaid Bilal³ and Alvaro Crosta⁴

¹9 Jardin Gabrieli 37 000Tours;

²Geological institute of Hungary,

³ENSM Saint Etienne

⁴Campinas University, Brazil

Based on recentl aboratory data, we have been able to put forward, in an ovcrall regional geological context, a new geological reinterpretation of the SLAR image provided with the first geological and photogeological studies (Crosta, 1981, 1983).

These different studies have led not only to a confirmation of older important data regarding the impact of the end oh the Permian (Theilen-Willige, 1981; Crosta, 1981; Von Engelhardt,1992, 1998; Deutsch, 1992) but also to place this granite structure, originally formed during the Upper Ordovician (Ashgillian) Ocloyica orogeny (Deutsch,1992), in a over all context of this key arca of SouthAmerica.

We are presently able to analyse the evolution of this zone during a pre-impact period (Cambro-Ordovician tothe end of the Permian in the region of Cuiaba-Araguainha). We also have a better understanding of the order of tectonic and magnetic events, together with the mechanical effect of the impact of the meteorite on the strata in particular at the end of the Upper Perniam and the Lower to Middle Triassic (247-245 +3Ma).

The insertion of the results of new petrographic analyses (normative or on trace elements), carried out by Eva Vetone Akos (Geological Institute at Budapest) and E.Bilal (L’École Nationale Supérieure des Mines at Saint-Etienne), into the diagrams of Le-Bas (1981) and Mac-Donald et al (1992) have led to the discovery of the following three points:

1. A mixture of rhyolitic and dacitic breccia fragments and tuff and even divitrified volcanic material was detected at the northern periphery,outside the impact breccias. The volcanic components are often intermixed with the impact breccias as described by the authors. Fragments of the original granite core were even present, deformed, in some samples from the periphery of the ring of impact breccias.

2. Were able at present to establish an order of events which followed the moment of impact with the propagation of shock waves from the Lower Devonian Furnas Formation up to the top of the Passa Doris Formation which can be observed in the field in the outer rim south of Araguainha (Tatarian-Kazanian?).

3. We have been able to begin the post-impact study on field samples regarding the stratigraphy and paleogeography (route Araguainha-Alto Garças-Montevidiu). The morphological and paleogeological reinterpretation of published data (Crosta, 1981) showed in fact that this structure observed on the SLAR image was initially Ordovician, belonging to an Ordovician trend know in the Cuiabo-Piranhas zone and which continues as far as the NNW side oh the Araguainha structure. It is, in fact, part of the“Paraguaian Fold Belt (“Ceinture Plissée Paraguayenne”, Trompette, 1984). This fold belt isdescribed from the base the Paleozoic and continues into the Siluro-Devonian along a much older continental margin, studied in Argentina and Boliviaas well as at Mato Grosso. This margin was closed by the closure of the Siluro-Devonian rift along the Transbrasilian lineament between the Upper Devonianand Upper Permian. At the end of the Perniam “reddikes”, composed of granite breccias and fragments,were formed in the granite core which was up lifted and recrystallized by the impact (Von Engelhardt,1992).These dikes are without doubt contemporaneous with the acid volcanic breccias and tuffs in the periphery,formed at the end of Permian.

REFERENCES

CROSTA A.P, GASPAR J.C, and FORNONI CANDIA A, 1981 rev,Bras. de Geoc. H3, 130-146

DEUTSCH A, BUHL D, LANGENHORTS F- Tectonophysics 216,205-218

ENGELHARDT (Von) W., MATTHAI S K and WALZBRUCK J; 1981 Meteoritics 27, 442-457

MACDONALD R, SMITH R, and THOMAS J E; 1992 US Geological Survey Professional Paper 1523

LE BAS M J, LE MAITRE R W, STRECKEISEN A, and ZANETTINB;1986- Joun; of Petrology, vol 27 Part 3; pp745-750

THEILEN-WILLIGE B. Cla1usthalerGeowissenschaftlichDissertation HH8, 210 p (unpublished thesis) 1981

TROMPETTE R, 1994 350 p, in Balkema Eds, Amsterdam

Abstract.Thanks to the support of Dr Mario Carvalho of theGeological Survey of Goias, Brasil, we were able toobtain access to new outcrops and field samples chosenby Dr Alvaro Crosta in July 1998.