Petrology and geochemistry of mafic granulite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Abstract

The Lahtojoki kimberlite pipe in Kaavi, eastern Finland contains lower crustal mafic granulite xenoliths with a mineral assemblage cpx−amph−pl±grt±opx±bt. The TWEEQU thermobarometry indicates crystallization at ca. 800–900°C and 0.75–1.25 GPa, which corresponds with crustal depths of ca. 22–38 km. Reaction textures and results of thermobarometry in some xenoliths indicate post-peak decompression with cooling. Chemical composition of the xenoliths suggest that their protoliths crystallized from basaltic melts, with some K-enrichment. Their REE patterns are slightly LREE-enriched, some xenoliths having small negative Eu anomalies. The U-Pb ion probe dating of zircons from two samples yielded variable ages for zircons even in the same xenolith, between ca. 2.6 and 1.7 Ga. These ages correspond with main late Archaean and Palaeoproterozoic orogenic events in the Fennoscandian shield. The Sm–Nd garnet–clinopyroxene isochron age from one dated sample is 1.6 Ga, which represents either a cooling or reheating of the lower crust by rapakivi magmatism. Because the petrology, geochemistry and ages of xenoliths do not correlate well with the nearest exposed Archaean mafic granulites, it is evident that the present lower crust, underlying the Archaean rocks, has a considerable Palaeoproterozoic component.