Mineralogy and geochemistry of the kimberlite xenocrysts from the Anabar region, Yakutia, Russia

Kimberlitic xenocrysts: garnets, pyroxenes, ilmenites, spinels from Anabar region were analyzed by EPMA and LAICPMS.

Reconstructed mantle sections of t Anabar region published (Ashchepkov et al., 2001; 2016; 2019; 2022 show in general, the relative rarity of sub-Ca pyropes  in lower part of the section and the frequency of wehrlite associations in the upper part. Pyroxenite -eclogite lens from 3 to 5 GPa is widely represented in most sections. Ilmenite trends are not long. In several pipes amphiboles from Cr-hornblendes to Cr-richterites were detected.

In the Anabar region (Khardakh and Staro-Rechensky fiedls) The REE patterns of  garnets are characterized by high variations in the spectra, with elevated LREE and HFSE minima, peaks of U, Pb, associated with subduction fluid flows. Pyroxenes are characterized by inclined REE spectra of La/Ybn ~15 to 20 and with varying HREE with subduction-related Ba, U, Pb peaks. But pyroxenes with plume related spectrums of with smooth spider diagrams (SD) are often found in pyroxenite lens.

Garnets from the Kuranakh field (Malokuonamskaya, Losi, Trudovaya, Universitetskaya etc) are divided into dunite-garburgite with low REE and LREE enrichment and wehrlite with convex REE maximum Gd, Eu and high concentrations of HREE often enriched in Th , U varying Ta-Nb and always low Zr-Hf.  Clinopyroxenes from the Malokuonamskaya pipe often have a local minimum of HREE and variations in the slope and enrichment of the REE spectra. On the CD, the peaks of Ba are varying, they show Ta, Nb enrichment and minima in Zr-Hf.

The garnets from the Universitetskaya pipe are mainly lherzolite-harzburgite with signs of subduction genesis (U peaks), and wide variations of HFSE sometime with Zr–Hf enrichment, due aqueous metasomatism. Clinopyroxenes are generally more diverse in REE spectra, Nb peaks with wide variations of Ba, Th-U and HFSE are common on CD

The Losi dike has a high content of perovskites, with highly enriched spectra with a slope with a decrease in highly charged and Pb and ilmenites with high and inclined REE spectra due to fractionation of proto-kimberlite (essentially carbonatite) melt. The eclogitic minerals with Eu anomalies and peaks of U, Ba and low HFSE are common.

In most mantle sections, ancient subduction sings are recorded in pyrope garnets, an partially adakite metasomatism in pyroxenes. Later they were modified by the action of plume carbonatite melts. The middle pyroxenite-eclogite lens originated as the boundary of the crust in ancient Archaic times. The upper wehrlitic part of the section arose during the melting of the pyroxene lens in the middle part and the migration of melts to the upper part. Protokimberlite metasomatism is not very pronounced. No signs of the supposed delamination of the lithosphere (Griffin et al, 2005) were found in the sections, which was also proved by Opx-Gar thermobarometry under the Duken field (Ashchepkov, 2003). The lower part of the section is depleted  so garnet and pyroxenes are rare and reflect low-temperature geotherm.

 

Grant RBRF 19-05-00788.