MATRIX GLASS AND MELT INCLUSIONS RECORD EXTREME FRACTIONATION IN PANTELLERITIC MAGMAS BEYOND THE EXPERIMENTAL MINIMUM IN THE SYSTEM Q-OR-AB

Pantellerite is a strongly peralkaline (P.I. = [Na+K]/Al > 1.0), iron-rich (FeO* > [Al₂O₃-4.4]/1.33) rhyolite that occurs most frequently as the felsic end member of alkaline suites in continental rifts and oceanic islands. The island of Pantelleria (Italy), situated in the Strait of Sicily rift zone and the type locality for pantellerite, has recorded numerous cycles of peralkaline magma evolution in its ~324 ka subaerially exposed outcrop, and although these trachyte-pantellerite series clearly do not belong to the same liquid line of descent (LLOD) they demonstrate consistently similar compositional and mineralogical trends. The LLOD from trachyte to pantellerite is controlled primarily by fractional crystallization of alkali feldspar, following a path along the “thermal valley” in the peralkaline haplogranite system Q-Ab-Or (P_H2O = 98 MPa, with 8.3% each aegirine and soldium metasilicate added; Carmichael and MacKenzie, 1963; AJS 261, 382-396), terminating at the minimum on the feldspar-quartz cotectic (Q_40.5Or_34.5Ab₂₅) at FeO* ≈ 9 wt% and P.I. ≈ 2.0. Samples from the ~159 ka Altura and Gadir volcanic centers, the 46 ka Green Tuff (Cinque Denti caldera), and the 7 ka Cuddia Mida volcanic center have been analyzed and compared. Although the bulk rock compositions for each of these series terminate near this minimum, matrix glass and melt inclusions in phenocrysts analyzed from these highly evolved rocks record extreme differentiation along two different paths beyond this: (1) to Q₄₄Or₂₃Ab₃₃, FeO* ≈ 6.0 wt%, P.I. ≈ 1.6, controlled primarily by aenigmatite crystallization and observed most frequently; and (2) to Q₅₆Or₄₄Ab₀, FeO* ≈ 13.0 wt%, at P.I. ≈ 3.5, which only occurs in the rare suites with amphibole (ferrorichterite) and represents some of the most extreme fractionation observed in peralkaline rocks.