Alkaline rocks of the eastern part of the Baltic Shield (Kola Peninsula)

Summary The Kola Peninsula contains a remarkable assemblage of alkaline rocks. These can be sub-divided into (a) those of Proterozoic age (1900–1600 Ma), including the Yelet’ozero and Gremiakha-Vyrmes complexes and (b) those of Palaeozoic age (about 365 Ma). The latter include numerous alkaline ultramafic intrusions and the larger agpaitic complexes at Khibina and Lovozero. The Proterozoic activity involves syenites and nepheline syenites in an alkaline gabbroic association. In both the Yelet’ozero and the Gremiakha-Vyrmes complexes the magmas become more salic with time. In the Palaeozoic associations alkaline gabbroic rocks are absent and the assemblages range from ultramafic (olivinites, pyroxenites, melilitites etc.) to highly differentiated agpaitic rocks and also to carbonatites. Low initial 87 Sr/ 86 Sr ratios (0.7035–0.7039) suggest that the magmas were co-genetic and of mantle origin. Experimental studies coupled with investigation of micro-inclusions suggest that the agpaitic magmas crystallized over a wide temperature interval (more than 300°C) under highly reducing conditions. The associated gas phase had a low f H 2 O but relatively high CH 4 , H 2 and H 2 S fugacities. The apatite deposits in the Khibina complex are uniquely large and are associated with ijolite, melteigite, urtite, juvite and malignite. The phosphate deposits are interpreted as apatite cumulates produced by suspension of apatite from the host magma after precipitation of clinopyroxene and nepheline.