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

Five mafic garnet granulite xenoliths have been obtained from Devonian kimb erlite hosts from the Pachuga field, Arkhangelsk, NW Russia. Whole rock maj or 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 w ith well-developed granoblastic fabrics. Primary mineralogy consists of alm andine-pyrope-rich garnet, diopside, oligoclase plagioclase and scapolite; minor phases include rutile and ilmenite. One sample contains only garnet a nd metasomatic pargasitic amphibole, pseudomorphing primary diopside. All a nalysed samples have sub-alkaline basaltic compositions. REE patterns show variable LREE-enrichment (La-N/Yb-N = 13 to 1.4) and small negative and pos itive Eu anomalies (0.87-1.32), indicating that both plagioclase fractionat ion and accumulation have occurred. Present-day Sr-87/Sr-86 and Nd-143/Nd-1 44 isotope ratios are very low (0.70272-0.70378 and 0.511736-0.512225, resp ectively), 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 degrees C , 1.4-1.6 GPa) indicate equilibration depths of up to 50 km. Calculated com pressional wave velocities range from 6.94 to 7.70 km/s and densities from 3.2 to 3.5 Mg/m(3), assuming P/T conditions of 500 degrees C and 1.4 GPa. T hese values support an origin in the deep continental crust. T-DM model age s are 1.7 to 1.9 Ga and may represent the time of extraction from the mantl e during the mid-Proterozoic. These xenoliths may represent the in situ maf ic lower crust beneath Arkhangelsk, formed as a result of basaltic underpla ting during the mid-Proterozoic. (C) 2000 Published by Elsevier Science B.V . All rights reserved.