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

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 (4.4–7 wt.% MgO) cover a wide spectrum of potassium levels, and represent magmas considered to be parental to the ultrapotassic leucite-bearing high-K series (HKS, 4–8 wt.% K 2O) 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/Al 2O 3 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 fO 2 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.