The role of carbon from recycled carbonated metapelites in the origin of ultrapotassic igneous rocks in the Central Mediterranean

The Central Mediterranean region is one of the most important areas on Earth for studying subduction-related potassic and ultrapotassic magmatism, derived from partial melting of the metasomatised lithospheric mantle wedge. In this region, leucite-free (i.e., lamproite) and leucite-bearing (i.e., kamafugite, leucitite, and plagioleucitite) ultrapotassic rocks closely occur, in a time-related progression, linked to the evolution of both the mantle source and the regional tectonic regime. Time- and space-related magmatism migration followed the roll-back of the subducting slab and the anticlockwise drift of the Italian Peninsula. Leucite-free silica-rich lamproites are restricted to the early stage of magmatism and are associated with ultrapotassic shoshonites and high-K calc-alkaline volcanic rocks. Leucite-bearing (i.e., Roman Province) rocks are erupted consistently later than lamproite-like and associated shoshonitic rocks, with post-leucititic volcanism occurring in the late stage of volcanic activity with eruption of alkali-basaltic to latitic and trachytic rocks, often after major caldera-forming events. Present-day ultrapotassic volcanism is restricted to the Neapolitan area. Central Mediterranean potassic and ultrapotassic rocks are extremely enriched in incompatible trace elements with variable fractionation of Ta, Nb, and Ti in comparison to Th and large ion lithophile elements (LILE). They are also variably enriched in radiogenic Sr and Pb and unradiogenic Nd. The main geochemical and isotopic signatures are consistent with sediment recycling within the mantle wedge via subduction. A twofold metasomatism, induced by the recycle of pelitic sediments and dehydration of lawsonite-bearing schists generates the early metasomatic events that enriched the mantle wedge from which leucite-free ultrapotassic rocks (i.e., lamproite) were generated. Recycling of carbonate-rich pelites played an important role in the shift to silica-undersaturated ultrapotassic rocks (kalsilite- and leucite-bearing) of the classic ‘Roman province’

Heavy oxygen recycled into the lithospheric mantle

Magmas in volcanic arcs have geochemical and isotopic signatures that can be related to mantle metasomatism due to fluids and melts released by the down-going oceanic crust and overlying sediments, which modify the chemistry and mineralogy of the mantle wedge. However, the effectiveness of subduction-related metasomatic processes is difficult to evaluate because the composition of arc magmas is often overprinted by interactions with crustal lithologies occurring during magma ascent and emplacement. Here, we show unequivocal evidence for recycling of continental crust components into the mantle. Veined peridotite xenoliths sampled from Tallante monogenetic volcanoes in the Betic Cordillera (southern Spain) provide insights for mantle domains that reacted with Si-rich melts derived by partial melting of subducted crustal material. Felsic veins crosscutting peridotite and the surrounding orthopyroxene-rich metasomatic aureoles show the highest 18O/16O ratios measured to date in upper mantle assemblages worldwide. The anomalously high oxygen isotope compositions, coupled with very high 87Sr/86Sr values, imply the continental crust origin of the injected melts. Isotopic anomalies are progressively attenuated in peridotite away from the veins, showing 18O isotope variations well correlated with the amount of newly formed orthopyroxene. Diffusion may also affect the isotope ratios of mantle rocks undergoing crustal metasomatism due to the relaxation of 18O isotope anomalies to normal mantle values through time. Overall, the data define an O isotope “benchmark” allowing discrimination between mantle sources that attained re-equilibration after metasomatism (>5 Myr) and those affected by more recent subduction-derived enrichment processes

Quartz-bearing rhyolitic melts in the Earth’s mantle

The occurrence of rhyolite melts in the mantle has been predicted by high pressure-high temperature experiments but never observed in nature. Here we report natural quartz-bearing rhyolitic melt inclusions and interstitial glass within peridotite xenoliths. The oxygen isotope composition of quartz crystals shows the unequivocal continental crustal derivation of these melts, which approximate the minimum composition in the quartz-albite-orthoclase system. Thermodynamic modelling suggests rhyolite was originated from partial melting of near-anhydrous garnet-bearing metapelites at temperatures ~1000 °C and interacted with peridotite at pressure ~1 GPa. Reaction of rhyolite with olivine converted lherzolite rocks into orthopyroxene-domains and orthopyroxene + plagioclase veins. The recognition of rhyolitic melts in the mantle provides direct evidence for element cycling through earth's reservoirs, accommodated by dehydration and melting of crustal material, brought into the mantle by subduction, chemically modifying the mantle source, and ultimately returning to surface by arc magmatism

Petrogenesis of Mediterranean lamproites and associated rocks: the role of overprinted metasomatic events in the postcollisional lithospheric upper mantle

High-MgO lamproite and lamproite-like (i.e. lamprophyric) ultrapotassic rocks are recurrent in the Mediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonites and high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmost sector of the Alpine–Himalayan collisional margin, which followed the closure of the Tethys Ocean. Subduc- tion-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Medi- terranean region in the form of shallow level intrusions (e.g. plugs, dykes and laccoliths) and small volume lava flows, with very subordinate pyroclastic rocks, starting from the Oligocene, in the Western Alps (northern Italy), through the Late Miocene in Corsica (southern France) and in Murcia-Almeria (southeastern Spain), to the Plio- Pleistocene in Southern Tuscany and Northern Latium (central Italy), in the Balkan peninsula (Serbia and Mac- edonia) and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latitic lavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamp- roite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively low Al2O3, CaO and Na2O contents, resulting in plagioclase-free parageneses, and consist of abundant K-feldspar, phlogopite, diopsidic clinopyroxene and highly forsteritic olivine. Leucite is generally absent, and it is rarely found only in the groundmasses of Spanish lamproites. Mediterranean lamproites and associated rocks share an extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elements and high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd iso- tope compositions, high 207Pb over 206Pb and high time-integrated 232Th/238U. Their composition requires an originally depleted lithospheric mantle source metasomatized by at least two different agents: (1) a high Th/ La and Sm/La (i.e. SALATHO) component deriving from lawsonite-bearing, ancient crustal domains likely hosted in mélanges formed during the diachronous collision of the northward drifting continental slivers from Gondwana; (2) a K-rich component derived from a recent subduction and recycling of siliciclastic sediments. These metasomatic melts produced a lithospheric mantle source characterized by network of felsic and phlogo- pite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive melt- ing of the different types of veins and the surrounding peridotite generating the entire compositional spectrum of the observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that charac- terize areas that were affected by multiple Wilson cycles, as observed in the Alpine–Himalayan Realm