Arc-Related Rapakivi Granites from the Ribeira Fold Belt, Se Brazil.*

Revista Brasileira de Geociências 30(1):020-024, março de 2000 ARC-RELATED RAPAKIVI GRANITES FROM THE RIBEIRA FOLD BELT, SE BRAZIL.* EBERHARD WERNICK ABSTRACT Arc-related late orogenic rapakivi granites from the Late Precambrian Ribeira Fold Belt (RFB) are presented and discussed. They occur intimately associated in space and time with Caledonian-type granites and appinites (mainly potassic diorites) and were emplaced during the Post-collisional Crustal Identation and Uplift stage of the RFB. The three rocks groups, together with shoshonitic and ultrapotassic ones, belong to the Pluriserial Ribeira Magmatic System 590 (PRMS-590), with acted between 620 and 570 Ma ago during the RFB back-arc development. Petrographic, geochemical and isotopic data define a high-K alkali-calcic nature and a lower crustal origin for the rapakivi granites. The crustal magmas may underwent or not further compositional changes either by mixing (up to 14 vol. %) with enriched mantle-derived appinitic magmas and/or by assimilation of fluid-rich upper crustal material. ε values for crust- and enriched mantle- derived rocks range between –12 and –16 & –7 and –10, respectively, and T ages range betweenNd(585) 2.4 and 1.8 Ga. & between (fictive) 1.4 and 1.6 Ga., respectively. With increasing participation of the appinitic componentDM during mixing, the rapakivi granites became progressively more alkaline and their ε and T values became less negative and lower, respectively. Nd(585) DM Geochemical and isotopic data show that the rapakivi magmas are derived from calc-alkaline biotite and hornblende- bearing tonalitic/ granodioritic sources by incongruent dehydration melting at low P , high T, and f typical for high-K rocks. The development of the rapakivi texture is controlled mainly by physical conditions during magmat ascent and coolingO2 than by magma composition. Rapakivi plutons were emplaced under decompressional conditions during rapid orogenic uplift as shown by expressive rising and unroofing of their partly only slightly other calc-alkaline host-rocks. Due to their compositional and isotopic similarity, same age and close spatial association, the Ribeira rapakivi and Caledonian granites are considered as products derived from a same source under slightly variable melting conditions. Keywords: Ribeira Fold Belt, Magmatic System, Rapakivi granites, Genesis, Evolution 2 2 INTRODUCTION The Late Precambrian Ribeira Fold Belt km ) and the large (310 km ) polycentred Itu complex, partially hidden (RFB), about 2800 km long and with a mean width of around 200 km, by Paleozoic sediments, comprising four coalescent plutons (Salto, runs parallel to the South American coast with a dominantly NE/SW Cabreúva, Indaiatuba and Itupeva, the latter a Caledonian-type strike from SE Brazil (northerly Vitória, Espírito Santo State) to granite). Northern Argentina (Fig. 1). A synthesis, including structural All major bodies show a rather complex magmatic history that organization, deformation phases, metamorphic events, magmatic starts with a restricted initial, often high-K dioritic, phase actually episodes, was presented by Wernick (1998a). 0 700 600 500 During the Postcollisional Crustal Identation and Uplift (PCIU) 0 0 evolutional stage of the RFB, between 620 and 570 Ma ago, the 0 Pluriserial Ribeira Magmatic System-590 (PRMS-590) (Wernick 1998a, b) was developed comprising plenty isolated or clustered plutons beside minor flows and volcanoclastic deposits. Their A emplacement is controlled either (subordinately) by arched structures linked to the last regional, open-waved, RFB-folding phase or C (dominantly) by deep crustal weakness zones developed under I transpressive and later on repeatedly reactivated under transtensional and tensional stress regimes during the post-collisional escape II tectonics (Hackspacher and Godoy 1999). The transcurrent fault phase B promotes intensive horizontal crustal domains displacements as well as 200 local and regional horst and graben structures. All rock types from the 200 PRMS-590, independently of their origins and compositions, share D same general features. These include a complex magmatic architecture comprising several magmatic phases and/or cycles. This aspect reflects III several drainage and replenishments of magma chambers by successive reactivation of crustal weakness zones that control the dynamic of magma supply and magma chamber evolution. This processes also IV allows an intensive interaction between crust and mantle-derived VITÓ RIA RIO DE JANEIRO magmas, magma mixing during chamber replenishments, strong SÃO PAULO fractionation and crustal assimilation (Wernick et al. 1993) as well as MONTEVIDEO E an important fluid role (Wernick and Teuppenhayn 1999, Wernick et F A 1- Andean Foldbelt al. 2000). 400 (A) with major B basement area (B) The PRMS-590 comprises 7 high-K (calc-alkaline, shoshonitic, BUENOS AIRES potassic, ultrapotassic) mantle- and crust- derived rock groups, all with 2- Patagonian Platform with arc-related chemical signatures (Wernick et al. 1997a, b, Wernick Paleozoic basement 1998a, b 1999, Wernick and Menezes 2000a, Wernick et al. 2000). 3- Phanerozoic The aim of this paper is to present and discuss the PRMS-590 rapakivi sediments granites based on data from the Itu Province, State of São Paulo, SE South 4- Late Precambrian Brazil. Fig. 2 shows its location in the frame of the Ribeira Magmatic B Arc (RMA). American A foldbelts (A) and Platform basemente inliers (B) THE ITU PROVINCE The Late Precambrian Itu Rapakivi Province (Wernick 1992), surrounding the E/SE border of the Paraná 5- Cratons Basin in the São Paulo State, SE Brazil, comprises about a dozen high- 800 600 400 level emplaced intrusions from which the best known are shown in Fig. 3. Figure 1-Simplified tectonic outline of South America. South America Platform: Larger Phanerozoic basins with depocentres: Amazonas (I), The most outstanding intrusions are the “S”-shaped Sorocaba body Maranhão (II), Alto Xingú (III) and Paraná (IV); Cratons: Guyana (A), 2 (150 km ), the drop-like NE-SW trending São Francisco pluton (130 Guaporé (B), São Luís (C), São Francisco (D), Luis Alves (E) and Plata (F). Departamento de Petrologia e Metalogenia – DPM, Instituto de Geociências e Ciências Exatas – IGCE, Universidade Estadual Paulista – UNESP, PO Box 178, 13506-900 Rio Claro, SP, Brazil E-mail [email protected] * This paper is a contribution to IGC Project 426 “Granite Systems and Proterozoic Lithospheric Processes” ( RJ ) O R 21 I E + N A 0 J + 23 S ++ 0 + + E + 22 S D + + + 0 0 + + O I + + 40 W R 40 W + + + + + + + + + + + + + + + + + + + + + + + + 0 0 + + + + + + + + 45 S 45 W + + + + + + ) + + + + G ++ ++ + + M + + ( + + + + + + É S I T A + A ++ B R U ++ Phanerozoic CoverPhanerozoic Massif Guaxupé São Francisco Craton SãoFrancisco E Brasília Mobile Belt A Brasiliano Granitoids (Ribeira M agmatic Arc) RibeiraM obile Belt + T G + + + 0 + + + + ~ + + + 42 W + ~ ++ TERTIARY TO RECENT TO TERTIARY DEPOSITS SEDIMENTARY BASIN PARANÁ PALEOZOIC GRANITES RAPAKIVI PRECAMBRIAN LATE 1 - ITU 2 - SO ROCAB A 3 - SÃ O FR ANCISC LOPES DOS SERRA - O4 DO SUL PILAR - 5 ARCANJO MIGUEL SÃO - 6 DO VEADO CAMPINA - 7 - 8 SGUA RIO/CORREAS GRANITOIDS PRECAMBRIAN OTHERS BASEMENT METAMORPHIC PRECAMBRIAN MAIN FAULTS + 0 ~ + + 24 S ~ ~ + + + ~ + ~~ + ++ 0 ~~ + 0 + + 46 W + + 46 W + + + + +++ + S + N + + A ++ IN + A + M + + + E + + + + + + + C 0 + + O + + + + + 24 S ++ +++ + + + PAULO S. 0 + + + 0 + + 44 W + + + + + 44 W + + + 100 km ++ ++ + + + + + + ++ + C + ++ I 0 0 + T + + 47 W + 47 W + N +++ + + +++ ++ A + + L + 3 + T 200 km + + + A + ++ 1 + + + 40 60 80 + 0 + + + + + 25 W ~ 2 ++ + ~ + + 20 ~ ~ ~ + ++ 0 0 + 100 ~ ~~~ 4 ~ 0 N 23 S I + 46 W ~~~ ~ ~ + S 0 46 W + A ~ ~ ~ ~~ ~ + 5 + SÃO PAULO CITY PAULO SÃO B + ~ ~ ~ ~ ~ ++ + + 0 0 ~ + ~ ~ ~ + ~~ ~ ~ + 48 W 48 W ~ ~ + + + ~~~ ~ + + ~ ~~~ 6 + + + ~ ~ ~ ~ +++ + 0 0 ~ + + ~ + + + ~ ~ + ~ + + 26 S ~ ~ + Á + ++ + N ~ ++ + ~ + + ) A + + + 0 + + + C S R + ++ + + ( + 26 S + + + + A 0 0 10 20 S 30 + PA N + I 0 ++ R + + + A + ++ 48 S + + T + + + + , Volume 30, 2000 , Volume + + A 0 + + + + + C 7 + + + ++ + + 0 8 ++ 0 22 S + + + + 49 W + + + + 49 W + + Fig. 3 Fig. + RJ + + A ) + + T MG + + + P + + N S + ( + CURITIBA A SC + + O + S + + + + L + + SP U A P +++ + + ) + + PR + R + + + O P + + + Ã ( + S Á + + ++ N + + A + + + + + + 0 R A + + + P + + 50 S + + + + 0 0 + + 0 0 + + 50 W 26 S 0 24 S 50 W 26 S 70 60 50 W 40 Figure 3-The Itu Rapakivi Province, São Paulo State, SE Brazil. After Wernick et al. 1997. São Paulo State, SE Brazil. After Wernick 3-The Itu Rapakivi Province, Figure Figure 2-The Brasiliano (Late Precambrian) Ribeira Magmatic Arc in S/SE Brazil with location of the Itu Province (Fig. 3). in S/SE Brazil with location of the Itu Province Ribeira Magmatic Arc 2-The Brasiliano (Late Precambrian) Figure Revista Brasileira de Geociências 22 Revista Brasileira de Geociências, Volume 30, 2000 preserved as enclaves, mega-enclaves or even major continuous areas xenotime, allanite, pyrite and fluorite are conspicuous accessories. (as in the core of the Cabreúva and Itupeva plutons). It follows a main Wernick et al. (1997a), Galembeck (1997) and Galembeck et al. intrusive phase comprising several pulses of variable petrography (1997) discuss geochemical data for rocks and minerals. (magma composition), textures (thermal history) and structures (Rb/Sr)WR and (U/Pb)z data from Tassinari (1988) and Töpfner (magma rheology). The pulses are of small volume as observed in the (1996) for the Cabreúva intrusion (Itu complex) result in ages of 586 87 86 “tectonic staircase” structure of the Sorocaba granite. The structure ± 10 and 583 ± 20 Ma, ( Sr/ Sr)i values of 0.7066 and 0.7081, comprises three tectonic segments uplifted progressively from SE to respectively, and a concordia age of 582 ± 6.4 Ma. For the same rocks δ18 ε NW along normal faults, thus allowing the observation of increasing Zielenski (1993) got high O values (+ 9.5 ‰ ± 0.2). Nd(585) values lower levels of the pluton. The emplacements of stocks and bosses (Wernick et al., in preparation) ranges between –16.6 and –7.45 and correspond to the third phase and the fourth (final) one is represented TDM ages between 2.1 and 1.56 Ga.

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