Orthopyroxene-bearing tholeiites of the Iblean Plateau (Sicily): constraints on magma origin and evolution from glass inclusions in olivine and orthopyroxene

Abstract

The widespread Pliocene (ca. 2 Ma) orthopyroxene-bearing tholeiites represent by far the most voluminous but short-lived magmatic phase of the Cenozoic Iblean volcanic province. We have studied major, volatile (H 2O, S, and Cl), and light lithophile (Li, Be, and B) elements and 7Li/ 6Li and 11B/ 10B isotopic ratios of olivine- and orthopyroxene-hosted glass inclusions and their host pillow-rim glasses using EPMA and SIMS. Most variations of major elements, H 2O (0.2–0.5 wt.%), Cl (100–350 ppm), and especially of Li (4.6–5.8 ppm), Be (0.5–0.8 ppm) and B (0.6–1.1 ppm) are believed to reflect heterogeneity of the Iblean magma source. Crystallization of these magmas occurred at shallow crustal depths in the range of 1250–1140 °C and oxygen redox conditions corresponding to FMQ–NNO buffers. Olivine [75.8–83.8 mol% Fo], orthopyroxene [Mg#=80.3–85.6], plagioclase [54.2–69.9 mol% An] and spinel [Mg#=40.9–49.6 and Cr#=59.6–63.8] are common liquidus phases. Variable sulfur concentrations in glass inclusions (280–1330 ppm S) and host matrix glasses (160–670 ppm S) reflect degassing of rising and erupting magma. The abundances of SO 4 2− dissolved in the melt (9–38% of total S) fit well to the redox conditions deduced from Fe 2+/Fe 3+ratios of the coexisting spinel. Glass inclusions scatter significantly in 7Li/ 6Li and 11B/ 10B isotopic ratios even within single specimens ( δ 7Li=−3.4‰ to +1.2‰ and δ 11B=−17.1‰ to −3.1‰). The lowest δ 7Li (−3.4‰ to +1.2‰) and δ 11B (−13.9‰ to −16.1‰) probably reflect isotopically light mantle domains beneath the Iblean Plateau. The shift of δ 11B toward more positive values (up to −3.1‰) at nearly the same low δ 7Li (−3.4‰ to 0.9‰) is explained by assimilation of <2 wt.% altered basaltic rocks.