The petrology and geochemistry of the Montagna Grande-Monte Gibele trachytic shield volcano (Pantelleria, Italy)

Abstract

The 45.7 ± 1.0 ka (2σ) eruption of the compositionally zoned (comenditic trachyte to pantellerite) Green Tuff ignimbrite on the volcanic island of Pantelleria, Italy, was followed by the extrusion of a series of post-caldera metaluminous to mildly peralkaline trachyte lavas between 26.2 ± 2.0 and 22.3 ± 2.9 ka (2σ). This study uses whole-rock major- and trace-element compositions, mineral compositions, geothermobarometry, and petrogenetic models to examine the role of trachytes in the evolution of the Pantelleria magma system. Although previous studies have suggested that all of these units are consanguineous and form a liquid line of descent (LLOD) from metaluminous trachyte to peralkaline trachyte and pantellerite, mineralogical and geochemical evidence, as well as the results of modelling, suggest a much more complex origin. The crystallization of alkali feldspar from metaluminous trachyte liquids (descended from alkali to transitional basaltic magma via fractional crystallization) drives the composition of the residual melt to increasingly peralkaline compositions: pantelleritc trachyte first and then, after >65 % crystallization, to pantellerite. This also results in the formation of a syenitic cumulate. Later intrusion of new mafic melt into the cumulate initiates partial melting, which produces comenditic trachyte melts characterized by low concentrations of incompatible trace elements (Rb, Zr, Nb, Th), high concentrations of Ba and Sr, and strong positive Eu anomalies. Entrainment of trachytic alkali feldspars into these melts as antecrysts further enhances these characteristics in whole-rock compositions. The origin of compositional zoning in the Green Tuff can be attributed to fractional crystallization from metaluminous trachyte within the lower pantelleritic portion and to partial melting of the syenitic cumulate for the upper comenditic trachyte portion.