Thermal State of the Lithospheric Mantle Beneath the Birekte Terrane: Reconstructions Based on the Study of Xenocrysts from Different-Aged Kimberlites of the Yakutian Kimberlite Province

The results of studies of the clinopyroxene xenocrysts collected from the heavy fraction concentrate of kimberlite breccia of the Ivushka pipe (Toluopka field) and tuff-breccia of Anomaly 75/90 (Molodo field), located within the Birekte terrane of the Siberian craton, are presented. The composition of the clinopyroxenes from the Ivushka pipe indicate the presence of high-temperature rocks (a megacryst assemblage or sheared lherzolites) in the lower part of the lithospheric mantle, which is consistent with the results obtained from garnet xenocrysts. In contrast, clinopyroxene xenocrysts that may belong to a megacryst assemblage (or high-temperature lherzolites) are almost absent in the samples from Anomaly 75/90. A preliminary reconstruction of the thermal state of the lithospheric mantle was conducted using clinopyroxene single-crystal thermobarometry. The results demonstrate that the lithospheric mantle was relatively thick across the entire Siberian craton during the Paleozoic kimberlite magmatism. However, a slight decrease in thickness was observed from the central part (Mirny field – 260 km) toward the northern part of the craton (Upper Muna field – 230 km and Toluopka field – 215 km).