REE-depleted anatectic marble reveals the primary source of crustal carbonatites

Abstract

Crustal-derived carbonatitic melts are increasingly recognised in high-grade terrains, but their residual source rocks remain largely unexplored. As a result, the crustal origin of carbonatites is generally constrained from geochemical signatures of the crystallised melts without a direct link to their anatectic source. Assessing this problem, and the trace-element redistribution during carbonate melting, requires integrated microstructural and geochemical analyses. Here we show a set of partial melting microstructures preserved in an olivine–spinel dolomite marble from the Variscan lower crust exposed in central Calabria (Italy). The residual anatectic marble contains REE-poor pockets of dolomite crystallised from a residual Mg-rich carbonatitic melt, left behind after extraction and fractionation of the highly mobile carbonatitic melt, whose remnants crystallised as intergranular calcite films. Volatile-bearing olivine and highly porous calcite enclosed in dolomite pockets document an early generation of H2O–F-rich carbonatitic melt formed through fluid-present incongruent melting. Mass-balance calculations indicate that at least 35% of the initial REE budget, with an LREE/HREE ratio of ~9:1, was removed with the extracted melt. Our results demonstrate that granulite-facies metacarbonate rocks can act as an efficient source of carbonatitic melts, implying that crustal carbonate melting may exert first-order control on trace-element signatures in the continental crust of orogens. Granulite-facies metacarbonate melting generates carbonatitic melts with ~35% REE enrichment, highlighting crustal carbonate melting as a key mechanism for rare-earth elements redistribution in orogenic crust, according to analysis of a 15-m thick marble layer exposed in the Serre Massif of Calabria in southern Italy.