415 AGE OF THE EASTERN PARAGUAY ALKALINE MAGMATISM Gomes, C.B. 1 , Velázquez, V.F. 2 and Comin-Chiaramonti, P. 3 1. Instituto de Geociências, Universidade de São Paulo. Rua do Lago, 562, São Paulo, Brasil. E-mail: cgomes@usp.br 2. Escola de Artes, Ciências e Humanidades, Universidade de São Paulo. Rua Arlindo Bettio, 1000, São Paulo, Brasil. E- mail: vvf@usp.br 3. Dipartimento di Ingegneria Chimica, dell‘Ambiente e delle Materie Prime, Universitá di Trieste. Piazzale |Europa 1, Trieste, Italia. E-mail: comin@univ.trieste.it Keywords: alkaline magmatism, geochronology Eastern Paraguay lies in an intercratonic region which includes the westernmost side of the Paraná Basin of Brazil. It is bounded by the N-S Paraguay river lineament (Fig. 1), separating the Paraná Basin (east) from the Gran Chaco Basin (west). The basement rocks are mainly Proterozoic to Early Paleozoic granitic intrusions and high- to low-grade metasediments and are represented in two structural highs, the Caapucú, in the south, and the Apa, in the north. During Late Mesozoic times, the whole area was subjected to NE-SW-trending crust extension, probably related to the western Gondwana break-up and the opening of the South Atlantic Ocean, leading to the formation of NW-trending grabens (e.g. ASU, Asunción-Sapucai-Villarrica), fault systems and fault-controlled sedimentary basins. This extensional tectonics, reflecting basement structures reactivated and enhanced by Early Cretaceous events, controlled the emplacement of both the alkaline magmatism in Eastern Paraguay, and of that in the eastern Paraná Basin (Fig. 2). Eastern Paraguay is a region of particular interest considering that: 1) it has been the site of tholeiitic (Serra Geral Formation: flood basalts and dyke swarms, both represented by high-Ti and low-Ti rock-types; 133 Ma, cf. Renne et al., 1992, and Turner et al., 1994) and alkaline magmatism; 2) Early Cretaceous potassic alkaline complexes and dykes occur emplaced before and after the flood tholeiites; 3) potassic and sodic alkaline rocks are closely related in space. On the basis of geological and geophysical evidence and geochronological data (especially 40 more recent high precision Ar/Ar ages), five main taphrogenic events are suggested for the alkaline magmatism in Eastern Paraguay since the end of Paleozoic times, the several occurrences grouped into six different provinces (Fig. 3): 1) Triassic sodic magmatism of the Alto Paraguay Province, I (240-250 Ma, K/Ar and Ar/Ar, cf. Gomes et al., 1996, and Velázquez et al., 1996; 241.5±1.3 Ma, Ar/Ar, cf. Comin-Chiaramonti et al., 2006), widespread at the southernmost side of the Amazon Craton (Fúlfaro, 1996; Comin-Chiaramonti et al., 2005) with the alkaline intrusions forming a narrow N-S belt along the Paraguay river; 2) Early Cretaceous potassic alkaline-carbonatitic complexes and dykes at the northern area (Rio Apa Province, II; Amambay Province, III), predating the tholeiitic basalts of the Serra Geral Formation and showing poorly documented ages of 138.7±0.2 Ma, Ar/Ar, for the first rocks (cf. Comin-Chiaramonti et al., 2006; ~142 Ma, K/Ar, cf. Gibson et al., 1995) and 137.6±0.7 and 139.3±0.5 Ma, Ar/Ar, for the second occurrences (cf. Comin-Chiaramonti et al., 2006; avg. 141 Ma, K/Ar and fission-track, cf. Sonoki and Garda, 1988, and Eby and Mariano, 1992, respectively); 3) Early Cretaceous potassic alkaline complexes and dykes with subordinate silico-carbonatite flows and dykes (Central Province, IV), mainly widespread in the Asunción-Sapucai-Villarrica graben. Age span varies from 115 to 132 Ma, K/Ar and Rb/Sr (cf. Bitschene, 1987; Velázquez, 1992; Comin-Chiaramonti and Gomes, 1996; Comin-Chiaramonti et al., 1997, 1999), with the highest concentration of new Ar/Ar results clustering in the 126-128 Ma interval (Gomes et al., 2003); an average value of 126.4±0.4 Ma is suggested by Comin- Chiaramonti et al.(2006); 4) Early Cretaceous sodic alkaline plugs and dykes of the Misiones Province (V) occurring at the southern area close to the San Juan Bautista town. Similarly to the Asunción occurrences, these rocks contain spinel peridotite mantle xenoliths. A preliminary age of 120±5 Ma (cf. Comin-Chiaramonti et al., 1997) is confirmed by new available Ar/Ar data (Velázquez et al., 2003), placing this magmatism in the 116-120 Ma interval, with an average value of 118.3±1.6 Ma (cf. Comin- Chiaramonti et al., 2006); 5) Paleocene sodic alkaline complexes, plugs and dykes of the Asunción Province (VI) cropping out in the neighborhood of the Paraguayan capital at the western side of the Asunción-Sapucai-Villarrica graben and having as most significant feature the presence of rocks bearing mantle xenoliths. Previous geochronological data, mostly based on K/Ar determinations (Comte and Hasui, 1971; Bitschene, 1987; Comin-Chiaramonti et al., 1991), indicated for this magmatism a large age span of 39-61 Ma, which was redefined by Gomes et al. (2003) using only new Ar/Ar results, with the histogram