Revista Brasileira de Geociências 30(1):082-086, março de 2000 THE MESOPROTEROZOIC VOLCANO-SEDIMENTARY SERRA DO ITABERABA GROUP OF THE CENTRAL RIBEIRA BELT, SÃO PAULO STATE, BRAZIL: IMPLICATIONS FOR THE AGE OF THE OVERLYING SÃO ROQUE GROUP CAETANO JULIANI 1 , PETER HACKSPACHER 2 , ELTON LUIZ DANTAS 3 , ALLEN HUTCHESON FETTER 2 ABSTRACT One of the fundamental problems to understanding the evolution of volcano-sedimentary sequences in southeastern Brazil is constraining their depositional ages. Brasiliano tectonic and metamorphic overprinting either obscured or destroyed primary features, such as unconformities, as well as other geologic relationships. This problem is exemplified by the Serra do Itaberaba and São Roque groups, where the lack of data about the timing of their deposition has prevented resolution of proposed one- and two-stage geotectonic/depositional models. Recent U-Pb zircon data obtained from metavolcanic rocks in the São Roque Group indicate that it was deposited between 628 and 607 Ma. New U-Pb zircon data of 1395 + 10 Ma for a metandesite in the basal Morro da Pedra Preta Formation (Serra do Itaberaba Group) indicate the maximum age for the beginning of the deposition of the pelites overlying MORB-like basalt. A metarhyolite of the upper unit, the Nhanguçu Formation, contains two zircon populations. One yielded an age of 619 + 3 Ma, which defines the crystallization age of the rock, and the other an age of 1449 + 3 Ma, interpreted as inherited xenocrystal grains from older units of the Serra do Itaberaba Group. The younger metarhyolite was affected only by the S 2 foliation, generated during the Brasiliano orogenesis, whereas the Middle Proterozoic metavolcano-sedimentary sequence records additional metamorphic and deformational events, confirming the presence of two different geotectonic cycles. Keywords: U-Pb geochronology, metandesite, metarhyolite, Serra does Itaberaba Group, São Roque Group 1 - Instituto de Geociências - USP, Rua do Lago 562, Cidade Universitária, CEP 05508-900, São Paulo, SP, FAX 0xx 11 8184248, e-mail cjuliani@usp.br 2 - Instituto de Geociências e Ciências Exatas - UNESP, Av. 24, 1515, CEP 13506-900, Rio Claro, SP, Fax 0xx19 5262831, e-mails: phack@rc.unesp.br and fetter@rc.unesp.br 3 - Instituto de Geociências - UnB, Brasília, DF, e-mail: elton@unb.br INTRODUCTION Previous geochronological studies of the Ribei- ra Belt primarily focused on obtaining the ages of granitic, gneissic and migmatitic rocks, e.g., Almeida et al. (1976) and Hasui et al. (1984), to help establish its geotectonic evolution. Only recently, however, have systematic studies been carried on to constrain deposition ages for the Precambrian supracrustal sequences in the orogen. U-Pb zircon geochronological data obtained from metavolcanic rocks in these sequences provide an important means to solve lithostratigraphic relationships and tectonic problems. The Serra do Itaberaba (SIG) and São Roque (SRG) groups are two major lithostratigraphic units in the State of São Paulo (Fig. 1). The ongoing debate about the stratigraphic relationship of these units has focused on whether or not an age gap separates the deposition of these two groups (Coutinho 1955, Almeida et al. 1981, Hasui 1981, Juliani et al. 1986). According to Juliani et al. (1986) the SIG is composed of a volcano-sedimentary sequence and is more intensely deformed than the metasediments of SRG, usually with S 0 /S 2 fabrics, and recording medium-grade Barrovian metamorphism. In contrast, the SRG contains S 1 /S 0 fabrics without stratigraphic inversions, and abundant sedimentary structures are preserved. Furthermore, the mineral assemblages of the SRG indicate that it was only subjected to greenschist facies metamorphism. Some K-Ar geochronological data from lato sensu SRG lithologies suggest that it was deposited at 1.4 Ga, with metamorphism occurring between 650–600 Ma (Cordani & Bittencourt 1967, Cordani & Teixeira 1979). Subsequent K-Ar data for SIG metamafic rocks supplied an age of 1690 + 157 Ga (Juliani et al. 1986), suggesting that deposition and metamorphism of the volcano-sedimentary sequence occurred prior to the deposition of SRG. Van Schmus et al. (1986) obtained U-Pb zircon age of 1790 + 14 Ma from the Polvilho Hill metarhyodacite that is intercalated in the basal part of the SRG. The age was interpreted as representing magmatic crystallization, indicating that the deposition of SRG began around 1.8 Ga, hence precluding a Neoproterozoic depositional age for the SRG. More detailed mapping and petrography, however, show that the apparent metarhyodacite is actually a meta-arkose that is intercalated with metaconglomerates in the basal portion of the SRG (Juliani et al. 1997). The zircon grains in the arkose are mostly rounded, detrital, which means that they place a maximum depositional age on the SRG, not the actual age of the onset of sedimentation. Based on U-Pb, Rb-Sr and K-Ar data, Tassinari (1988) proposed that the deposition of the volcano-sedimentary sequence of the São Roque/Serra do Itaberaba belt began at 1.8 Ga and was followed by two magmatic-metamorphic events between 1.3–1.0 Ga and 800–720 Figure 1 – Regional geological map. (1) Tertiary and quaternary sediments, (2) Mesozoic alkaline intrusions, (3) Brasiliano granitoids, (4) São Roque Group, (5) Serra do Itaberaba Group, (6) Amparo, Paraíba do Sul and Itapira groups and Igaratá Complex, (7) Embú Complex and (8) Costeiro Complex. Lines represent the main transcurrent shear zones: (9) Itú, (10) Jundiuvira, (11) Taxaquara and (12) Rio Jaguari (modified from Almeida et al. 1981).