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This study reports the complex textural and chemical features of K-feldspar megacrysts (up to 5 cm long) hosted in trachydacitic lava flows, lava domes, and coulees from Mt. Amiata volcano (Tuscan Magmatic Province, Central Italy). Backscattering and cathodoluminescence imaging, coupled with core to rim major and trace elements patterns, reveal complex zoning, and resorption surfaces associated with sharp chemical variations (e.g., Sr and Ba). These zoning patterns originated by disequilibrium and re-equilibration events, related to the repeated influx of mafic magmas or convective motions in the trachydacitic magma reservoir. Multiple mafic magma refilling occurences are also supported by the field record of abundant microgranular magmatic enclaves in the studied products. Our results highlight that the abnormal dimension of the studied K-feldspars originates by the interplay between petrological and kinetic processes involving: (i) extensive dissolution; (ii) heterogeneous nucleation; (iii) alternation of spasmodic growth events in disequilibrium and near-equilibrium crystallization. The repeated influx of hotter magmas and reheating can determine the thermal condition to the growth of few, large K-feldspar megacrysts. Also, the strong textural and chemical similarities observed in the K-feldspar megacrysts from Mt. Amiata volcanic rocks and Mt. Capanne monzogranite (Elba Island, Central Italy) support the hypothesis of a phenocrystic origin of intrusive K-feldspar megacrysts.

International audience; At Pantelleria, peralkaline silicic magmas were erupted across a range of eruptive typologies and magnitudes: pyroclastic flows, Plinian to strombolian pumice fallout and lava flows. In this paper we focus on the intermediate cycle of eruptive activity which is bracketed by ignimbrite units slightly older than the two caldera collapses which marked the volcanological activity of the island. This age interval (180-85 ka) was punctuated by six ignimbrite-forming eruptions (silicic and variably peralkaline) for a cumulative erupted magma volume of approximately 6 km3 dense rock equivalent. Based on new 40Ar/39Ar (Na,K)-feldspar ages and petrographic data, we propose an updated volcanostratigraphic scheme for these welded and rheomorphic ignimbrites that can be summarised as follows: (i) the age of the old ('La Vecchia') caldera collapse is now tightly constrained between 140 and 146 ka and the caldera-forming eruption can be traced to a lithic-rich welded tuff breccia that outcrops in two opposite sectors of the island (south-west and north-east); (ii) four ignimbrite units previously considered unrelated are now merged in two distinct eruptive paroxysmal events at 107 and 85 ka. In particular, the 85 ka eruptive event is comparable in magnitude to the younger (caldera forming) Green Tuff Plinian eruption; (iii) the recurrence patterns of the 107 and 85 ka eruptions, compared to the Green Tuff, allow us to qualitatively assess that the climax in production of low-temperature silicic and peralkaline melt was focused in the age interval 85-45 ka.

The island of Pantelleria (Sicily Strait), the type locality for pantellerite, has been the locus of major caldera-forming eruptions that culminated, ca. 50 ka ago, in the formation of the Cinque Denti caldera produced by the Green Tuff eruption. The post-caldera silicic activity since that time has been mostly confined inside the caldera and consists of smaller-energy eruptions represented by more than twenty coalescing pantelleritic centers structurally controlled by resurgence and trapdoor faulting of the caldera floor. A high-resolution 40Ar/39Ar study was conducted on key units spanning the recent (post-20 ka) intracaldera activity to better characterize the present-day status (and forecast the short-term behavior of) the system based on the temporal evolution of the latest eruptions. The new 40Ar/39Ar data capture a long-term (> 15 ka) decline in eruption frequency with a shift in eruptive pace from 3.5 ka−1 to 0.8 ka−1 associated with a prominent paleosol horizon marking the only recognizable volcanic stasis around 12–14 ka. This shift in extraction frequency occurs without major changes in eruptive style, and is paralleled by a subtle trend of decreasing melt differentiation index. We speculate that this decline probably occurred (i) without short-term variations in melt production/differentiation rate in a steady-state compositionally-zoned silicic reservoir progressively tapped deeper through the sequence, and (ii) that it was possibly modulated by outboard eustatic forcing due to the 140 m sea level rise over the past 21 ka. The intracaldera system is experiencing a protracted stasis since 7 ka. Coupled with recent geodetic evidence of deflation and subsidence of the caldera floor, the system appears today to be on a wane with no temporal evidence for a short-term silicic eruption.