Fenitisation associated with alkaline-silicate complexes. Implications for HFSE mobility in late-stage fluids, Gardar Rift, SW Greenland

Alkali metasomatism associated with carbonatitic-alkaline magmatism (or fenitisation) is of interest for our understanding of chemical behaviour of economically important high-field-strength (HFSE) and rare-earth (REE) elements during late-stage magmatic-hydrothermal activity. In this project, we explore the expression of alteration and the physico-chemical factors controlling fenitisation across the Proterozoic Gardar Province of Southern Greenland. We present our findings of bulk-rock (XRF/ICP-AES) and microbeam (BSE-SEM-EDAX/EPMA) analyses. Our main case study, the Illerfissalik centre, comprises a series of nested syenitic intrusions. Fenites are exposed at its western margin, where a feldspar-poor Eriksfjord Formation quartz arenite protolith experienced intense metasomatism and deformation. Detailed petrography demonstrates interstitial K-feldspar (commonly perthitic), mafic minerals including pyroxene or Na-amphibole, and titanite (often LREE-bearing) in the proximal high-grade fenite zone. The absence of feldspar in unaltered arenite (<5% mode further than 1.5 km distance from the contact zone) suggests that K-feldspar was introduced by early-stage fenitisation, along with the relatively Na-poor pyroxene. The presence of HFSE-rich phases i.e. Sr-chevkinite also attests to variable mobility of numerous alkali, alkali-earth metals and HFSE in fenitising fluids. Bulk-rock geochemical trends away from the contact are also presented. Further work including isotope systematics will constrain the geochemistry of fenitising fluids.