MAGMA MIXING IN A COMPOSITIONALLY AND THERMALLY STRATIFIED MAGMA CHAMBER: EVIDENCE FROM COMENDITIC TRACHYTE ENCLAVES IN THE KHAGGAIR LAVA FLOW, PANTELLERIA (ITALY)

Felsic volcanism on the island of Pantelleria (Sicily Channel, Italy) includes lavas and tuffs that range in composition continuously from metaluminous trachyte (SiO₂ ≈ 62 wt%, P.I. [mol Na+K/Al] ≈ 0.9, Zr ≈ 550 ppm) to pantellerite (SiO₂ ≈ 72 wt%, P.I. ≈ 2.0, Zr ≈ 2500 ppm). Between these end member compositions on Pantelleria are the comenditic trachytes (SiO₂ ≈ 65-66 wt%, P.I. ≈ 1.00 - 1.12, Zr ≈ 600-700 ppm). Unlike the trachytes and pantellerites, which comprise the dominant volume of eruptive products on the island, comendtic trachytes erupted only as part of the crystal-rich (accumulate) top of the Green Tuff (the 45.7 ka caldera-forming ignimbrite); otherwise, comenditic trachyte occurs only as magmatic enclaves in post-caldera pantellerite lavas. These enclaves are most abundant in the Khaggiar lava flow, which represents the youngest (~8 ka) episode of felsic volcanism on Pantelleria. Khaggiar lavas consist entirely of pantellerite ranging in composition from SiO₂ ≈ 67-72 wt%, P.I. ≈ 1.24-1.95 , and Zr ≈ 800-2400 ppm with alkali feldspar-glass temperatures between ~800-730°C. This suggests that the Khaggiar magma chamber may have been compositionally zoned, similar to the Green Tuff magma chamber. We present new microanalyses of glass and mineral from a comenditic trachyte enclave that suggests that, unlike the Green Tuff comenditic trachytes, these are not part of the liquid line of decent for the pantelleritic magmas. Bimodal distribution of melt inclusion compositions and mixed populations of glomerophenocrysts supports the interpretation that these enclaves are the result of syn-eruptive magma mixing rather than fractional crystallization. Mineral assemblages in various glomerophenocrysts in the enclaves provide two-oxide temperatures of ~870-830°C and 780-730°C, and are interpreted to suggest the existence of a steep thermal gradient at the base of a compositionally and thermally zoned pantelleritic magma chamber.