Evidence of Permian magmatism in the Alpi Apuane metamorphic complex (Northern Apennines, Italy): New hints for the geological evolution of the basement of the Adria plate

摘要

The occurrence of metavolcanic rocks within the Paleozoic basement of the Alpi Apuane metamorphic complex has been known since long time. Among them, some massive porphyritic tourmaline-bearing rocks cropping out in the southern sector of the Alpi Apuane present some distinctive and peculiar features, differing from the better known middle Ordovician metarhyolites of the “Porfiroidi e scisti porfirici” Fm. The porphyritic tourmaline-bearing rocks belong to the recently proposed Fornovolasco Metarhyolite Fm. They are granular to porphyritic, with phenocrysts of quartz (often with magmatic embayment), pseudomorphosed feldspars, and mica (both biotite and muscovite), in a groundmass formed by quartz, white mica, albite, and K-feldspar. Tourmaline (schorl-dravite in composition) is an abundant accessory mineral, in some cases forming cm-sized spots. The studied rocks plot into the rhyolite field of the Total Alkali vs Silica classification diagram. They show a peraluminous nature, having an Alumina Saturation Index ranging from 1.3 and 3.2. Their trace-element signature is that typical of highly evolved orogenic magmas. Laser ablation-ICP-MS U—Pb datings on zircon suggest a Permian crystallization age (weighted average ages of the four samples ranging from 292 and 271?Ma), thus relating these rocks to a post-Variscan magmatism. This new dating represents the very first evidence of a Permian magmatism in the pre-Triassic basement of the Northern Apennines. The potential relationships between Permian felsic magmatism and the ore genesis in the Alpi Apuane metamorphic complex are also discussed.