The geochemistry of the Barra do Itapirapua Ä carbonatite Ponta Grossa Arch, Brazil): a multiple stockwork E. Ruberti a, * , F. Castorina b , P. Censi c , P. Comin-Chiaramonti d , C.B. Gomes a , P. Antonini d , F.R.D. Andrade a a Instituto de Geocie Ãncias, Universidade de Sa Äo Paulo, Rua do Lago 562, 05508-900 Sao Paulo, Brazil b Dipartimento di Scienze della Terra, Universita Á `La Sapienza', Piazzale Aldo Moro 5, 00185 Rome, Italy c Istituto di Mineralogia, Petrogra®a e Geochimica, Universita Á di Palermo, Via Archira® 36, 90100 Palermo, Italy d Dipartimento di Ingegneria Chimica, dell'Ambiente e delle Materie Prime, Universita Á di Trieste, Piazzale Europa 1, 34100 Trieste, Italy Received 1 July 2001; accepted 1 December 2001 Abstract The Early Cretaceous Barra do Itapirapua Ä carbonatite, in southern Brazil, belongs to the initial stages of the alkaline carbonatitic magmatism in the Ponta Grossa Arch Province, as related to the opening of the south Atlantic Ocean. The magmatic evolution of the complex comprises four phases of intrusive carbonatite that are composed of medium- to coarse-grained Mg and Fe carbonatites with small volumes of late ®ne- to very ®ne-grained Mg carbonatites and show evidence of pervasive hydrothermal events. Drill core samples reveal the existence of silicate rocks of syenitic composition of two different origins. Among the silicate rocks, there are syenites of a likely magmatic origin, as well as fenites derived from the granitic protolith, which constitutes the wall rock of the complex. Fenitization around the Barra do Itapirapua Ä complex is mainly potassic, and fenites are Ca- and LOI-enriched in comparison with the magmatic syenites. Furthermore, the two types of silicate rocks differ in terms of REE and other trace elements. Associated syenites and carbonatites have similar Sr±Nd isotopic compositions, close to the Bulk Earth at 120 Ma. Mixing curves indicate that the carbonatite±syenite association may be accounted for by assimilation of up to 20% crustal material, whereas the late carbonatite composition is consistent with less than 10% assimilation of crustal rocks. An origin by liquid immiscibility for the carbonatite±syenite association can be considered, followed by magmatic differentiation through fractional crystallization of calcite and dolomite in the carbonatite magma. C±O stable isotopic data indicate further reequilibration under hydrothermal conditions at temperatures between 375 and 80 8C. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Fenitization; Carbonatite; Syenite 1. Introduction The Barra do Itapirapua Ä carbonatite suite belongs to the Ponta Grossa Arch Province Almeida, 1983) and is spatially associated to other alkaline or alkaline carbonatite complexes of Cretaceous age e.g. Early Cretaceous: Jacupiranga, Juquia Â; Late Cretaceous: Canane Âia, Mato Preto). In the area shown in Fig. 1, the most important occurrences are the neighboring Itapirapua Ä Gomes, 1970), Banhada Äo Ruberti, 1984), and Tunas Gomes et al., 1987) complexes; the latter intrusions show only silicate rock types. The Barra do Itapirapua Ä occurrence differs from the other alkaline carbonatite intrusions in that it is represented in the ®eld by an ellipsoidal network of dykes and veins of Fe, Mg, and Ca carbonatite intruding the Neoproterozoic Tre Ãs Co Ârregos granite 605 Ma, Gimenez Filho et al., 2000). Notably, this granitic mass penetrates the Mesoproterozoic to Neoproterozoic metasediments of the Ac Ëungui Group Gimenez Filho, 1995; Gimenez Filho et al., 2000). The carbonatite dykes cover an area of approximately 2 km 2 and are located 2 km SE of the Itapirapua Ä alkaline carbonatitic intrusion Gomes, 1970). A systematic bore hole investigation con®rms the stock- work-like structure of the carbonatite body, with prevailing millimeter- to centimeter-thick carbonatite veins toward the surface and decimeter- to meter-thick dykes at depth. Fenitized rocks of syenitic af®nity, and more rarely, syenite with magmatic texture, are found at various topographic levels Ruberti, 1998). K/Ar data on the Itapirapua Ä rocks provides a maximum age of 114.7 ^ 9.7 Ma Gomes et al., 1990), whereas a Journal of South American Earth Sciences 15 2002) 215±228 0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0895-981102)00031-7 www.elsevier.com/locate/jsames * Corresponding author. Tel.: 155-11-381-84080; fax: 155-11-381- 84258. E-mail address: exrubert@usp.br E. Ruberti).