The Alkaline Intrusion of Cananéia on the South Coast of S o Paulo State: Geology and Geochronology

New radiometric ages (Ar-Ar and Rb-Sr) are available for the alkaline rocks from the Cananéia massif in the so-called Ribeira Valley, which is represented by two small bodies (Morro de S o Jo o, 1.8 km2, and Morrete, 0.4 km2). The rocks are syenitic in composition, dominantly intrusive, covered by recent sediments and emplaced into the Precambrian basement of the A ungui Group. From the tectonic point of view, they are associated with the Guapiara Alignment at the northern limit of the Ponta Grossa Arch and intruded into the homonimous province that includes a great number of alkaline centers. Ar-Ar radiometric determinations in biotite indicate an average age of 83.6 Ma for the syenites, not much different of that given by a Rb-Sr reference isochron on whole rock for different lithologies which yielded an age of 85.6 ± 2.7 Ma, and an initial 87Sr/86Sr ratio of 0.70667 ± 0.00014 (MSWD = 1.14). The data confirm the inclusion of the Cananéia massif within the age group of the Late Cretaceous alkaline occurrences.