MULTI-STAGE ZIRCON GROWTH RECORDING COMPLEX CRYSTALLIZATION AND EMPLACEMENT PROCESSES – A STUDY FROM THE ZONED CIBORRO – ALDEIA DA SERRA PLUTONIC SUITE, SW PORTUGAL

The propensity of zircon to survive melting and metamorphic events can lead to the coexistence of multiple generations of zircon formed at different stages of the emplacement process. Constraining the age of distinct growth stages by precise ID-TIMS analyses can then result in complex data patterns that can be difficult to interpret. Questions concerning the significance of specific zircon growth stages with respect to the evolution of a magmatic system are crucial.

The Ciborro – Aldeia da Serra plutonic suite comprises rocks ranging from biotite > amphibole tonalite to biotite > muscovite granite. They are cut by rhyolite porphyry, microgranite, aplite-pegmatite and pegmatite veins. Twenty samples encompassing the range of compositional diversity were dated by ID-TIMS. The zircons were selected among grains most likely to be free of cores, and were mechanically and, more frequently, chemically abraded. Most samples also contain monazite and/or titanite whose data provide valuable assistance in interpreting the zircon results. The combined data define a range of ages from 340 to 313 Ma, but most populations record multi-stage zircon growth. The data from the tonalite and the two-mica granite exemplify some of the complexities. The zircon data from the tonalite define a main cluster at 328 to 325 Ma but with concordant outliers at 331 and 319 Ma. A microgranular enclave hosted in this rock yields a concordant age of 328 Ma, indicating that the 331 Ma age must reflects inheritance. Conversely, the porphyry, microgranite and aplite-pegmatite veins that crosscut the tonalite are older than 319 Ma. Therefore, this age is interpreted as a late hydrothermal zircon generation, corresponding to local overgrowths visible on some of the zircons and also to late veins elsewhere in the pluton. The two-mica granite also yields a range of concordant zircon age, the older one corresponding to the age of 324.5 Ma for a crosscutting aplite-pegmatite vein, the younger ones (down to 313 Ma) reflecting secondary crystallization. Ages of 319–317 Ma age are very frequent in this plutonic suite, recording the late magmatic stages.