Cordierite-anorthoclase hornfels xenoliths in Stromboli lavas (Aeolian Islands, Sicily): an example of a fast cooled contact aureole

Abstract

High-grade hornfels xenoliths in the 60 ka old calcalkaline basaltic andesite lava flows of Stromboli (Omo lavas; Paleostromboli II period) consist of fine- to medium-grained holocrystalline rocks. Feldspar (mainly anorthoclase) and cordierite represent ca . 85 vol% of the modal mineralogy. Aluminiferous spinel (hercynite), sillimanite, ± corundum, ± ilmenite, ± chlorapatite are also present. Major and trace element compositions, together with Sr, Nd and Pb isotopic data, indicate that the source rocks are metapelites compatible with upper continental crust-derived materials. According to common petrogenetic grids for pelitic rocks ( e.g. NaKFMASH), a pressure range of 2–3.5 kbar and a peak temperature close to 800° C can be inferred for the mineral assemblage. A honeycomb texture and wide, high-T compositional range of the ternary feldspars (Ab 35–70 Or 3–63 An 2–49 ) suggest incipient melting followed by quenching. Accordingly, the xenoliths may well represent contact metamorphosed rocks (contact aureole), compatible with metapelites belonging to the continental crust basement of Stromboli. TEM (Transmission Electron Microscopy) observations of alkali-feldspar showed the presence of a cross-hatched pattern oriented parallel to (010) and (001), and of albite twinning in the orthoclase-enriched and -depleted samples, respectively. Both textures are related to a monoclinic to triclinic phase transition in a highly disordered feldspar. A highly disordered Al-Si configuration is also corroborated by cell parameters. Incipient decomposition textures, but no exsolution lamellae, are present in the intermediate alkali-feldspars. These results support a fast cooling of the hornfels xenoliths, of the order of degrees per hour. The high-T feldspar textures were preserved because the xenoliths, stoped into the uprising basaltic andesite host magma, were rapidly transported to the surface shortly after they reached the contact aureole peak temperature.