Sub-volcanic infiltration and syn-eruptive quenching of liquids in cumulate wall-rocks: the example of the gabbroic nodules of Stromboli (Aeolian Islands, Italy)

Fine- to coarse-grained plutonic nodules within the Petrazza pyroclastics (Paleo-Stromboli I period) consist of gabbroic rocks with variable amounts of interstitial material. They are characterised by cumulate textures and low pressure modal mineralogy formed by plagioclase (An(96-87)) + clinopyroxene (Mg-v 82-94) + olivine (Fo(83-74)) +/- amphibole +/- opaque minerals; the interstitial material consists of newly crystallised microlites (quenching) of plagioclase (An(73-55)) + amphibole + clinopyroxene +/- olivine +/- biotite +/- opaques and highly variable amounts of residual glasses that range in composition from shoshonite and high-K basaltic andesite to high-K andesite and latite. The interstitial material has a relatively high but variable degree of vesicularity. The whole rock incompatible element abundances are lower than - but the patterns are typical of - in subduction related magmas and the incompatible trace-elements are well correlated with the amount of the interstitial material. The Sr, Ph and Nd isotopic ratios resemble those of the extrusive rocks of Stromboli older series and the mineral chemistry of the gabbros is similar to that of the HKCA Paleo-Stromboli lavas. Modal mineralogy, mineral chemistry and chemical-isotopic whole rock compositions suggest that the cumulus portions of the gabbroic nodules crystallised from basaltic magmas compositionally compatible with those erupted by Stromboli volcano. The interstitial material does not represent the residual liquid after in situ crystallisation of the gabbros; it is also distinct from the juvenile host andesite magma. Textural evidence, Fe-Mg mineral/liquid partioning and mass balance calculations indicate that the interstitial material (quench crystals and vesicular glass) derived from infiltrated hydrous basaltic liquid undercooling and vesiculation of which occurred during the eruption of the Petrazza pyroclastics.