The origin of carbonatites from the eastern armutlu peninsula (Nw turkey)

Unusual carbonate dykes, which have a thickness of up to 4 m, cross-cut the amphibolites from the high-grade metamorphic rocks in the Armutlu Peninsula (NW Turkey). They are described as carbonatites on the basis of their petrographic, geochemical and isotope-geochemical characteristics. The carbonatites, which commonly show equigranular texture, are composed of calcite and clinopyroxene with other minor phases of plagioclase, mica, garnet, K-feldspar, quartz, epidote, titanite and opaque minerals. They contain abundant xenoliths of pyroxenite and amphibolite. The geochemical characteristics of the carbonatites are significantly different from those of mantle-derived carbonatites. They have remarkably low incompatible element (e.g. Ba, Th, Nb) and total REE (11–91 ppm) contents compared with mantle-derived carbonatites. The high87Sr/86Sr(i) (0.70797–0.70924) and low εNd(t) (−8.08 to −9.57) of the carbonatites confirm that they were derived from the continental crust rather than from a mantle source. Mica from carbonatite was dated by the40Ar/39 Ar method, yielding a Late Jurassic–Early Cretaceous age (148–137 Ma). This is significantly younger than the age of adjacent amphibolites (Upper Triassic). All data from field studies, as well as petrographic, geochemical and geochronological observations, suggest that these carbonatites were formed from anatectic melting of a carbonated source area in the continental crust.