The behavior of high field-strength elements (HFSE) in F-rich peralkaline systems

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Peralkaline igneous rocks (rocks with a molar (Na+K)/Al ratio > 1) are strongly enriched in halogens, rare earth elements (REE), and high field strength elements (HFSE; Zr, Hf, Nb, Ta) and represent some of the most promising sources for future HFSE and REE supply. This project investigates the behaviour of halogens and HFSE/REE-bearing phases in the texturally and compositionally zoned peralkaline plutonic Monchique complex (Portugal) that evolved from gabbroic to foid syenitic compositions. The special role of sodalite, fluorite and various F-rich Na-Ca-HFSE disilicates (FDC) for the retention of halogens in peralkaline magmas will be investigated and the variable effects of fractional crystallization, fluid exsolution/degassing and fluid-wall rock interaction on halogen systematics and HFSE/REE mobility will be distinguished from each other. The following three working hypotheses will be guiding the workflow of the anticipated project: 1) Halogen abundances and halogen systematics (F/Cl, Cl/Br ratios) in the Monchique rocks reflect the effects of fractional crystallization during magmatic stages and redistribution of halogens caused by fluid exsolution and accompanied (autometasomatic) hydrothermal alteration. 2) The Monchique complex shows vertical and horizontal zonation with respect to the relative abundance of sodalite-FDS assemblages, which may or may not correlate with the HFSE distribution in this tabular body. 3) Locally developed fenites around the Monchique complex are Cl-rich but F-poor and therefore devoid of FDS but zircon- and titanite-bearing, which is mainly controlled by the low F/Cl ratio of the fenitizing fluids. To test these hypotheses, we will combine detailed petrography, whole-rock analysis (XRF, ICP-MC, CIC), mineral chemistry (EPMA, CIC, LA-ICP-MS), thermodynamic modelling, and fluid inclusion investigations (microthermometry, LA-ICP-MS). The expected results of this project will not only decipher halogen and HFSE behavior in such a peralkaline system, but will certainly have general implications on the formation and migration of halogen-bearing fluids that are capable of HFSE transport within a shallow-level plutonic complex. Geochemical monitors (e.g., whole rock halogen data, HFSE data) will be combined with mineralogical observations (e.g., FDS, sodalite, fluorite distribution) and will allow for a detailed view on fluid exsolution/degassing processes in tabular magma bodies.