Multiple Rodingitization Stages in Alkaline, Tholeiitic, and Calc-Alkaline Basaltic Dikes Intruding Exhumed Serpentinized Tethyan Mantle from Evia Island, Greece

Abstract

Abstract In Pagondas and Kimi areas, central Evia Island, Greece, variably serpentinized ultramafic rocks of an ophiolitic origin are crosscut by rodingite dikes. Based on their protoliths, these rodingites are distinguished into the following: (i) rodingites of island arc tholeiitic dolerites, (ii) rodingites of Mg-rich tholeiitic gabbros, (iii) rodingites of alkaline basalts, and (iv) rodingites of calc-alkaline basalts. Rodingitization evolved in three successive phases (namely, stages 1, 2, and 3) of increasing metasomatic degree that occurred during the exhumation of the mantle wedge serpentinites. Thermodynamic modeling suggests that stage 1 was developed at ~5 kbar and 450-550°C, whereas at stage 2 and stage 3, gradually decreasing P-T conditions prevailed. Stage 1 affected the protoliths of all rodingite types and was characterized by the crystallization of grossular, diopside, chlorite, and prehnite leading to Ca and LOI enrichments. The observed Si and REE depletions are assigned to the dissolution of primary clinopyroxene and amphibole. Stage 2 mainly affected the rodingites that were derived from alkaline and calc-alkaline basalts, promoting the growth of andradite, vesuvianite, and chlorite at the expense of stage 1 rodingitization mineral phases. At this stage, the marginal zones of rodingite dikes from alkaline and calc-alkaline basalts experienced remarkable Mg and REE enrichments. Stage 3 corresponds to derodingitization and restricted carbonation processes of the previously formed rodingites, which are associated with Fe enrichments. During this stage, significant trace elements including REEs were transferred from the marginal zones to the inner cores of rodingite dikes that were derived from metasomatism of calc-alkaline basalts. Calcite δ18Ο-δ13C isotopic data suggest that carbonation processes were developed at shallow depths below the Tethyan ocean floor, through mixing of seawater and serpentinization-derived fluids. Such multiple stages of rodingitization processes that affected four different lithotypes, as well as the unusual case of alkaline basalts’ rodingitization, would render the Evia rodingites as an important and very rare example of metasomatism.