Geochemical Signatures and Inclusions in Apatite as Markers of a Hidden Ultrahigh-Pressure Event (Betic Cordillera, Spain)

Diamond inclusions have been found in apatite from a specific type of orthogneiss of a volcanosedimentary sequence from the deepest tectonic complex (Nevado-Filábride) of the Alpine Betic-Rif belt (Spain). This unusual finding led us to make a detailed petrographical and geochemical study, with the aim of characterizing ultrahigh-pressure (UHP) apatite and comparing it with the younger and widespread magmatic and/or metamorphic generations. The UHP apatite is abundant in biotite-rich orthogneisses, whereas it is lacking in the most common biotite-poor orthogneisses, which instead contain abundant magmatic apatite. In addition to diamond, UHP apatite contains abundant biphased inclusions consisting of chlorides and hydrocarbons. The late alteration of fluid inclusions caused oligomerization of hydrocarbons and graphitization of diamond, leading to a characteristic irregular outer brownish zone. The UHP apatite shows bell-shaped rare earth element (REE) profiles, with weak Eu anomalies and Sr and Th contents higher than those of magmatic apatite. The variable degree of modification of REE (mainly increase of light-REE contents) and trace-element composition (mainly decrease in Sr contents) of UHP apatite may be related to alteration during exhumation and partial reequilibration with the melt during the subsequent anatexis. This discovery suggests that apatite preserves some UHP geochemical signatures during anatexis and that it can help in the recognition of UHP events.