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Evolution of Plio-Pleistocene shallow-level granites and associated rhyolites in the Tuscan Magmatic Province (Italy).

Federico Farina 1, Andrea Dini2

  • Affiliations: 1Dipartimento di Scienze della Terra "Ardito Desio", Università degli Studi di Milano, Italy  2Istituto di Geoscienze e Georisorse, CNR, Pisa, Italy

  • Presentation type: Talk

  • Presentation time: Tuesday 15:00 - 15:15, Room R380

  • Programme No: 1.3.4

  • Theme 1 > Session 3

Abstract

Understanding the connection between granitic plutons and silicic volcanic rocks is crucial to shed light on the processes triggering volcanic eruptions. A critical limitation to progress in this quest is the overall scarcity of well-preserved plutonic-volcanic pairs. Peraluminous cordierite-bearing granites and rhyolites, representing pure crustal anatectic magmas, occur in the Tuscan Magmatic Province (Italy). High precision U-Pb dating of magmatic zircon crystals from the Larderello-Travale granites yielded ages ranging from 4.5 to 1.6 Ma, suggesting four pulses of magmatic activity. One of these pulses has zircon with ages spreading ca. 400 kyr, from 2.6 to 3.0 Ma. These ages are identical to those determined from zircon in rhyolitic lava flows and domes (Roccastrada rhyolites) cropping out 10 km to the south of the intrusions. Granites and rhyolites have similar whole rock major, trace element and isotope compositions. Individual samples are characterized by a significant variability in zircon oxygen isotope composition and by a large spread in plagioclase 87Sr/86Sr (Δ87Sr/86Sr > 1*10-3), with several crystals exhibiting intra-grain isotope variability. Some of the plagioclases in the granites have cores with high anorthite contents (80 mol%), displaying crustal Sr isotope signatures (> 0.7145). Mineral-scale isotope data support the involvement of multiple crustal sources in the genesis of granites and rhyolites. The large age span recorded by the zircon suggest that crystallization started in the middle crust in isotopically-diverse magma batches that were successively assembled into shallow-level intrusions or erupted. These granites and rhyolites represent different outcomes stemming from a composite long-lasting magma reservoir.