Subduction-related melt refertilisation and alkaline metasomatism in the Eastern Transylvanian Basin lithospheric mantle: Evidence from mineral chemistry and noble gases in fluid inclusions

• Published in: Lithos Vol. 364-365; p. 105516
• Main Authors: Faccini, Barbara, Rizzo, Andrea Luca, Bonadiman, Costanza, Ntaflos, Theodoros, Seghedi, Ioan, Grégoire, Michel, Ferretti, Giacomo, Coltorti, Massimo
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.07.2020; Elsevier
• Subjects: Eastern Transylvanian Basin; Mantle refertilisation; Noble gases; Post-collisional; Sciences of the Universe; Subduction-related metasomatism; Noble gases; Post-collisional; Subduction-related metasomatism; Mantle refertilisation; Eastern Transylvanian Basin
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/j.lithos.2020.105516

Calc-alkaline and alkaline magmatic activity is generally separated in space and/or in time. The Eastern Transylvanian Basin in Romania is one of the few places where, during Pleistocene, alkaline eruptions occurred contemporaneously with the calc-alkaline activity. Mantle xenoliths entrained in Perşani Mts. alkaline volcanic products have been studied in order to investigate the interaction of metasomatic agents of different magmatic affinities with the mantle wedge. Based on mineral major and trace element and noble gases in fluid inclusions, two main events have been recognized. The first was a pervasive, complete re-fertilization of a previously depleted mantle by a calc-alkaline subduction-related melt, causing the formation of very fertile, amphibole-bearing lithotypes. This is shown by the a) increased amounts of modal clinopyroxene up to 21.9 % with Al2O3 contents up to 8.16 wt%, higher than what is expected for clinopyroxene in Primordial Mantle; b) 4He/40Ar* ratios up to 1.2, within the reported range for mantle production; c) 3He/4He in olivine, opx and cpx of 5.8 ± 0.2 Ra, among the most radiogenic values of European mantle, below the typical MORB mantle value (8 ± 1 Ra), reflecting recycling of crustal material in the local lithosphere. The second event is related to later interaction with an alkaline metasomatic agent similar to the host basalts that caused slight LREE enrichment in pyroxenes and disseminated amphiboles and precipitation of vein amphiboles with a composition similar to amphiboles megacrysts also found in the Perşani Mts. volcanic deposits. This is highlighted by the 4He/40Ar* and 3He/4He values found in some opx and cpx, up to 2.5 and 6.6 Ra, respectively, more typical of magmatic fluids. •Two different kinds of metasomatism in Eastern Transylvania lithospheric mantle.•The mantle wedge was refertilized by a calc-alkaline melt.•Refertilization was followed by alkaline metasomatism, forming amphibole veins.•He-Ne-Ar systematics indicate SCLM refertilization by subduction-related melts.