Abstract
Over 200 plutonic xenoliths have been collected from the pyroclastic deposits of Roccamonfina volcano. Twenty-nine representative specimens have been analyzed for major and trace elements, and the main mineral phases probed in 20 of these. The nodules belong to two cumulate fractionation series: a pyroxenite-gabbro- anorthosite-monzonite syenite-foidal syenite series, and an olivine pyroxenite- phlogopite pyroxenite series. The various nodule types are not simply related to the stratigraphic position of their host pyroclastics. Olivine, clinopyroxene, plagioclase, mica (biotite and phlogopite), K -feldspar, magnetite, apatite and sphene, occurring in various proportions and textures, represent the cumulus minerals. Amphibole appears as a secondary product pseudomorphing pyroxene and biotite. Intercumulus nepheline, sodalite-hauyne, garnet and cancrinite(?) appear in foidal syenites, Modal and chemical relations suggest that pyroxenites, gabbros, monzonites, syenites and foidal syenites correspond to accumulative material from Stage I (High-K Series) lavas with respective phenocryst associations cpx ± trace ol, cpx + pl ± trace ol, cpx + pl + trace K -spar, K -spar + cpx + pl and K -spar+ cpx +pl + sod. The olivine pyroxenites and a single phlogopite pyroxenite specimen appear to be related to Stage II (K Series) ol+cpx+pl-bearing quartz-normative trachybasahs. The inferred genetic connections, however, are only partially supported by comparison of phenocryst and cumulus mineral compositions. The discrepancies may reflect: (1) disequilibrium of nodule and individual lava mineral assemblages due to the contrary motion of different crystal species during fractionation, (2) mixing in minor fractionation chambers of crystalline material (nodule) and magmatic fluids during the eruption of the main magmatic system, (3) lower equilibration temperature and higher equilibration pressures of the nodules relative to cogenetic lavas, and (4) nodule fractionation from separate episodes of magmas evolved independently from the parental magmas.
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