The newly discovered Jurassic Tikiusaaq carbonatite-aillikite occurrence, West Greenland, and some remarks on carbonatite-kimberlite relationships

摘要

We discuss mineral chemistry data and report ten high-precision U-Pb (zircon, baddeleyite, pyrochlore, and perovskite) and Rb-Sr phlogopite ages for the newly discovered Tikiusaaq carbonatite intrusion and associated ultramafic dykes from the North Atlantic craton, West Greenland. At Tikiusaaq, massive dolomite-calcite carbonatite sheets intruded an 2 x 3 km area along a ductile shear zone between ca. 158 and 155 Ma. The accompanying carbonatite and carbonate-rich ultramafic silicate dykes were emplaced between ca. 165 and 157 Ma in close proximity to this carbonatite centre utilizing pre-existing brittle fractures. The deep volatile-rich magmatism at Tikiusaaq forms part of a larger Jurassic alkaline province in southern West Greenland and represents the earliest manifestation of rifting processes related to the opening of the Mesozoic-Cenozoic Labrador Sea Basin.Although the ultramafic silicate dykes macroscopically resemble hypabyssal kimberlites, they are identified as kimzeyite-bearing monticellite aillikites (carbonate-rich ultramafic lamprophyres) using a modern miner-alogical-genetic classification. The overlapping emplacement ages of the carbonatite sheets and aillikite dykes, along with the carbonate-rich nature of the latter, suggest a genetic relationship between these magma types. The aillikites carry garnet peridotite xenoliths and have mineralogical characteristics of primitive magmas such as highly forsteritic olivine (up to Fo_(90) mol%) and Cr-rich spinel (up to 46 wt.% Cr2O3) microphenocrysts; whereas the carbonatite sheets reveal a higher degree of differentiation such as Fe-rich dolomite compositions (up to 9 wt.% FeO). The initial findings reported here from Tikiusaaq suggest that a link between these magma types by an increasing degree of partial melting of a common carbonated upper mantle peridotite source region, as commonly envisaged for the compositionally similar Sarfartoq complex, is untenable. Rather, proto-aillikite magma may be parental to the dolomitic carbonatite sheets, but the nature of the carbonate separation mechanism(s) is presently not understood.