Heterogeneous mantle sources of potassium-rich magmas in central-southern Italy: Melt inclusion evidence from Roccamonfina and Ernici (Mid Latina Valley)

Igor K. Nikogosian*, Manfred J. van Bergen

*Corresponding author for this work

    Research output: Contribution to JournalArticleAcademicpeer-review

    Abstract

    We present a comprehensive set of data on compositions of melt inclusions and earliest crystallized mineral phases from mafic lavas of Roccamonfina and Ernici, situated in a central sector of the string of Pliocene-Quaternary potassic volcanic centres along the Tyrrhenian border of peninsular Italy. Studied samples of mafic lavas (4.4-7wt.% MgO) cover a wide spectrum of potassium levels, and represent magmas considered to be parental to the ultrapotassic leucite-bearing high-K series (HKS, 4-8wt.% K2O) and to shoshonitic (1.5-5%) and subalkaline (<1.5%) series, here collectively referred to as medium-low potassic series (M-LKS).Highly variable compositions of melt inclusions in olivine hosts (Fo=89-91.5) from single lava samples indicate that all parental magmas are composed of diverse collections of primary melts, consistent with extraction from heterogeneous vein-type mantle lithologies. Major and trace-element systematics provide evidence that primitive HKS and M-LKS magmas originated from separate domains and not through changes in the proportion of vein and wall-rock peridotite during progressive melting of a common source. We infer that parental HKS magmas are largely derived from the vein portion of a heterogeneous phlogopite (±amphibole±apatite) wehrlitic mantle. Contrasting major element, volatile and trace-element signatures of melt inclusions from medium-K lavas point to an amphibole-bearing wehrlitic source for shoshonitic magmas, probably with a subordinate role of phlogopite. Finally, a population of silica-undersaturated potassium-poor melt inclusions with extreme CaO/Al2O3 ratios (>1.2) and fluid-depleted signatures suggests that subalkaline magmas originate either from the same source following the exhaustion of amphibole, or from a separate wehrlitic-pyroxenitic (±apatite±carbonate?) assemblage. Our melt inclusion data are consistent with a mixed metasomatic imprint by siliceous potassium-rich and carbonate-rich (carbonatitic) potassium-poor melts. Siliceous melt components dominated in the HKS sources, but a minor group of potassium-poor melt inclusions in HKS samples, with compositions not represented by erupted products, carries trace-element signatures pointing to metasomatism by carbonatite-like melts. The melt inclusions show a general inverse relationship between fO2 and potassium enrichment, suggesting that primitive potassium-poor magmas are the most oxidized (NNO+0.5), whereas primitive HKS magmas have the lowest oxidation state (NNO-1.6).Mineral chemistry of (near-)liquidus assemblages varies systematically according to the signature of the primary melts from which they crystallized. Calcium contents of Mg-rich olivines and clinopyroxenes are unrelated to the calcium content of the melts but are higher in HKS than in M-LKS samples. Forsterite-rich olivines contain Cr-spinel inclusions with Cr/(Cr. +. Al) ratios that tend to be higher in M-LKS (0.58-0.80) than in HKS (0.48-0.67) samples. We infer that this parameter, particularly at the more elevated values, is predominantly controlled by the metasomatic veins, and does not necessarily reflect the ('refractory') nature of the pre-metasomatic peridotite component. We surmise that primitive HKS and M-LKS melts of Roccamonfina-Ernici are derived by lower degrees of partial melting of vein lithologies with a stronger metasomatic imprint of the carbonatitic component and more subordinate involvement of pristine wall-rock peridotite than their equivalents of the northern Roman Province. The distinctive HKS and M-LKS sources point to a layered vein-permeated mantle column. Melting possibly occurred in response to a heat pulse induced by exposure to hot asthenospheric mantle, facilitated by post-collisional slab tearing and shallow break-off.

    Original languageEnglish
    Pages (from-to)279-302
    Number of pages24
    JournalJournal of Volcanology and Geothermal research
    Volume197
    Issue number1-4
    DOIs
    Publication statusPublished - 30 Nov 2010

    Keywords

    • Ernici (Mid Latina Valley)
    • Italian potassic magmas
    • Mantle sources
    • Melt extraction
    • Melt inclusions
    • Metasomatic veins
    • Roccamonfina

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