CONTRASTING ZIRCON GROWTH PATTERNS IN NEOPROTEROZOIC GRANITES OF SOUTHERN BRAZIL REVEALED BY SHRIMP U–Pb ANALYSES AND SEM IMAGING: CONSEQUENCES FOR THE DISCRIMINATION OF EMPLACEMENT AND INHERITANCE AGES

INTRODUCTION The Florianopolis Batholith is a major component of northern portion of the Neoproterozoic (Brasiliano Cycle) Dom Feliciano Belt/Pelotas Orogen in Santa Catarina state, southern Brazil. It is exposed over an area of about 12,000 km (200 km long and about 60 km wide), along the Atlantic Ocean coast, extending continuously from 26 45’ S to 28 45’ S. Despite of recent U-Pb studies (Silva et al. 2002a, Basei 2000), many uncertainties persist regarding the relationships between magma generation and tectonic evolution within the batholith. We explore these chronological problems by means of the combined use of the U–Pb SHRIMP analyses and backscattered electron (BSE) and cathodoluminescence (CL) imaging of zircons from four critical plutons: Paulo Lopes (Sample 1), Valsungana II (Sample 2), Guabiruba (Sample 3) and Tabuleiro (Sample 4). A complete version of this abstract, including Sm-Nd data, analytical tables, concordia diagrams, SEM images and analytical procedures may be found elsewhere (Silva et al., 2003). No previous SHRIMP U/Pb zircon ages were available for the targeted plutons. The Paulo Lopes Suite yielded an imprecise Pb/Pb single crystal zircon evaporation age of 642 ± 46 Ma (Silva et al., 1997) and a U/Pb crystallization age of 644± 20 Ma (Basei 2000). Basei (2000) obtained a conventional U/Pb crystallisation age of 638 ± 32 Ma for an early phase of the Valsungana Suite, distinct from the Valsungana II Granite dated herein (sample 2). Finally, a facies from the Guabiruba Granite yielded an imprecise age of 573 ± 44 (Basei 2000).