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

Abstract: High-MgO lamproite and lamproite-like (i.e. lamprophyric) ultrapotassic rocks are recurrent in theMediterranean and surrounding regions. They are associated in space and time with ultrapotassic shoshonitesand high-K calc-alkaline rocks. This magmatism is linked with the geodynamic evolution of the westernmostsector of the Alpine-Himalayan collisional margin, which followed the closure of the Tethys Ocean. Subduction-related lamproites, lamprophyres, shoshonites and high-K calc-alkaline suites were emplaced in the Mediterraneanregion in the form of shallow level intrusions (e.g. plugs, dykes and laccoliths) and small volume lavaflows, 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 Macedonia)and in the Western Anatolia (Turkey). The ultrapotassic rocks are mostly lamprophyric, but olivine latiticlavas with a clear lamproitic affinity are also found, as well as dacitic to trachytic differentiated products. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated composition, have relatively lowAl2O3, 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 rarelyfound only in the groundmasses of Spanish lamproites. Mediterranean lamproites and associated rocks sharean extreme enrichment in many incompatible trace elements and depletion in High Field Strength Elementsand high, and positively correlated Th/La and Sm/La ratios. They have radiogenic Sr and unradiogenic Nd isotopecompositions, high 207Pb over 206Pb and high time-integrated 232Th/238U. Their composition requires anoriginally 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 likelyhosted in mélanges formed during the diachronous collision of the northward drifting continental slivers fromGondwana; (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 phlogopite-rich veins, respectively. Geothermal readjustment during post-collisional events induced progressive meltingof the different types of veins and the surrounding peridotite generating the entire compositional spectrum ofthe observed magmas. In this complex scenario, orogenic Mediterranean lamproites represent rocks that characterizeareas that were affected by multiple Wilson cycles, as observed in the Alpine-Himalayan Realm.