The exotic accessory minerals within mantle metasomatic domains: new insights of the sources of orogenic magmas

The petrographic and mineralogical investigation of ultramafic xenoliths is fundamental for our understanding of the nature and evolution of the upper mantle. These xenoliths are commonly brought to surface by the eruption of alkaline basic magmas in intraplate settings. On the contrary, in convergent settings, their occurrence is rare and usually associated with post-collisional anorogenic volcanism, being of paramount importance since they represent the supra-subduction mantle sources. A particular case is represented by the ultramafic xenolith occurrence in the Betic Cordillera of southern Spain, where decimetre-sized metasomatised mantle xenoliths (lherzolites and harzburgites) were brought to the surface by the eruption of post-orogenic Na-alkaline basalts of the Cabezo Negro de Tallante volcano. Interestingly, a few of them are also crossed by felsic veins and veinlets which have been interpreted as the interaction of crustal-derived melts released in the mantle wedge and peridotite during the subduction (Avanzinelli et al., 2020; Dallai et al., 2022). Petrographic and mineral chemistry analyses show that these veins and veinlets are mainly made up of plagioclase, orthopyroxene and quartz, or by phlogopite/amphibole, diorite and gabbronorite. In centimetric veins the parageneses include sporadic pargasite amphibole, whereas the network (apophysis) of millimetric veinlets contain a large variety of accessory minerals such as amphibole, phlogopite, apatite, zircon, rutile, Fe-Ni(Cu-Zn) oxides and sulphides and peculiar mineral phases pertaining to the apatite, monazite, huttonite/thorite and thorianite-uraninite solid solutions. Preliminary investigation with SEM and EMPA revealed that the compositional range of huttonite/thorite is SiO2 15-18 wt%, ThO2 69-85 wt%, P2O5 1-5 wt%, La2O3 0-3 wt%, Ce2O3 2-6 wt%. Uraninite-thorianite mineral phases have UO2 17-46 wt% and ThO2 41-78 wt% contents. Apatite contains Cl up 6 wt%. Fe-Ni sulphides contain NiO up to 75 wt%. The rutile compositional data indicate that this mineral dominates the budget of Ti (TiO2 80-100 wt%) and Nb (Nb2O5 5-13 wt%). Further major and trace elements analyses will be performed to better constrain the composition of these mineral phases. These xenoliths will offer valuable information about the nature and composition of the metasomatic agents in subduction-related mantle sections, as well as the element redistribution in the metasomatic parageneses through the interaction with the mélange mantle wedge. This is of great importance because they represent the unique (or very rare) direct explanation for the observed geochemical signature of orogenic magmas