Lower crustal granulite xenoliths from the Arkhangelsk kimberlite pipes: petrological, geochemical and geophysical results

Five mafic garnet granulite xenoliths have been obtained from Devonian kimberlite hosts from the Pachuga field, Arkhangelsk, NW Russia. Whole rock major and trace element data, Sr and Nd isotopic analyses, mineral chemistry, calculated compressional wave velocities and densities of the xenoliths are reported here. The xenoliths are medium grained and are reasonably fresh with well-developed granoblastic fabrics. Primary mineralogy consists of almandine–pyrope-rich garnet, diopside, oligoclase plagioclase and scapolite; minor phases include rutile and ilmenite. One sample contains only garnet and metasomatic pargasitic amphibole, pseudomorphing primary diopside. All analysed samples have sub-alkaline basaltic compositions. REE patterns show variable LREE-enrichment (LaN/YbN=13 to 1.4) and small negative and positive Eu anomalies (0.87–1.32), indicating that both plagioclase fractionation and accumulation have occurred. Present-day 87Sr/86Sr and 143Nd/144Nd isotope ratios are very low (0.70272–0.70378 and 0.511736–0.512225, respectively), suggesting that both assimilation of Archaean continental crust and removal of Rb, possibly related to granulite-facies metamorphism, took place early in the history of these rocks. P/T estimates (670–730°C, 1.4–1.6 GPa) indicate equilibration depths of up to 50 km. Calculated compressional wave velocities range from 6.94 to 7.70 km/s and densities from 3.2 to 3.5 Mg/m3, assuming P/T conditions of 500°C and 1.4 GPa. These values support an origin in the deep continental crust. TDM model ages are 1.7 to 1.9 Ga and may represent the time of extraction from the mantle during the mid-Proterozoic. These xenoliths may represent the in situ mafic lower crust beneath Arkhangelsk, formed as a result of basaltic underplating during the mid-Proterozoic.