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

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 Kfeldspar. 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 ablationICPMS UPb 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.

💡 Research Summary

The Alpi Apuane metamorphic complex in northern Italy exposes a sizable portion of the pre‑Triassic Paleozoic basement of the Adria plate. While the “Porfiroidi e scisti porfirici” (Porphyroids) rocks have long been recognized as Ordovician‑age felsic metavolcanics, the southern sector of the Alpi Apuane contains massive, tourmaline‑rich, porphyritic bodies that differ markedly from the classic Porphyroids. These bodies have been formally designated the Fornovolasco Metarhyolite Formation (Fornovolasco Metarhyolite Fm).

The authors carried out an integrated study comprising field mapping, petrography, scanning electron microscopy (SEM) with energy‑dispersive spectroscopy, electron microprobe analyses, whole‑rock major and trace element chemistry (ICP‑OES, ICP‑MS), and laser‑ablation inductively coupled plasma mass spectrometry (LA‑ICP‑MS) U‑Pb dating of zircon. Four representative samples (FVb13, FVc1, POL1, SAS1) yielded concordant zircon ages between 292 ± 5 Ma and 271 ± 4 Ma, establishing a Permian crystallization age for the Fornovolasco Metarhyolite. This is the first robust Permian magmatic age recorded in the Northern Apennines’ pre‑Triassic basement.

Petrographically, the rocks are granular to porphyritic, with quartz phenocrysts (often showing magmatic embayment), pseudomorphosed feldspar phenocrysts, and both biotite and muscovite phenocrysts set in a fine‑grained groundmass of quartz, white mica, albite, and K‑feldspar. Tourmaline (schorl‑dravite composition) is abundant, forming cm‑scale spots or aggregates. Chemically, the rocks plot in the rhyolite field of the Total Alkali‑Silica (TAS) diagram and are peraluminous (Alumina Saturation Index 1.3–3.2). Their trace‑element pattern—high K₂O/Na₂O, enriched Rb, Ba, Th, U, and depleted Nb, Ta—is typical of highly evolved, orogenic felsic magmas.

Structurally, the metarhyolite bodies are up to 1.5 km long and 120 m thick, occurring as lens‑shaped bodies within the “Filladi inferiori” metasedimentary schists. They are generally concordant with the regional Alpine D1/D2 foliation but locally cut the foliation, indicating emplacement both before and after the main Alpine deformation. Contact zones are often brecciated and display blackening of the host rock, suggesting mechanical disruption during intrusion.

A notable observation is the spatial association between the tourmaline‑rich metarhyolite and polymetallic ore deposits (Pb‑Zn‑Ag, pyrite, barite, iron oxides) that are common in the same sector. Tourmaline is known to facilitate fluid transport and metal complexation in high‑temperature hydrothermal systems, implying that the Permian felsic magmatism may have provided the heat and metal source for the later hydrothermal ore formation.

From a geodynamic perspective, the Permian felsic magmatism recorded here likely reflects partial melting of a thinned continental lithosphere at the northern margin of Gondwana, now the Adria plate, during the late Paleozoic to early Mesozoic transition. This magmatic episode is distinct from the earlier Variscan (Ordovician‑Silurian) magmatism previously documented in the region and adds a new chapter to the tectonic evolution of the Adria plate’s basement.

In summary, the study demonstrates that: (1) the Fornovolasco Metarhyolite represents a previously undocumented Permian felsic magmatic event in the Northern Apennines; (2) its geochemical signature aligns with highly evolved, orogenic rhyolites; (3) the intrusion is temporally and spatially linked to the region’s polymetallic hydrothermal ore belts, suggesting a genetic relationship; and (4) the findings refine the geochronological framework of the Adria plate’s Paleozoic–Permian evolution, highlighting the importance of integrated petrographic‑geochemical‑geochronological approaches in unraveling complex basement histories.