Geochemistry of accessory chrome-spinellides from rocks of the Ergakskii chrome-bearing hyperbasite massif (western Sayan) and conditions of its formation

Abstract

The first data obtained on the distribution of REE in Csp from the Ergakskii hyperbasite massif rocks testified that the REE contents in individual grains of this mineral varied greatly. Chondrite-normalized contents of light REE in all cases exceeded the same parameter of heavy REE that was formerly observed in Csp specimens from rocks of several other hyperbasite massifs. It is suggested that along with very poor amounts of REE as an isomorphic impurity in the analyzed Csp grains, they contain varying concentrations of REE in the form of nonstructure impurities, presented mainly by light rare earth elements. Significant variations of Cr 2 O 3 and Al 2 O 3 concentrations and parameter Cr# values were established in the Csp from rocks of the massif. The Cr# parameter values were used for estimation of the degree of partial melting of mantle sources during formation of ultramafic restites from the Ergakskii massif and some other similar massifs. As was determined, the rocks of the Ergakskii massif were formed under a wide interval of degrees of partial melting of the mantle sources. As a whole, this is in agreement with the data obtained from Csp specimens belonging to several other hyperbasite massif rocks. The results examined give grounds to consider that, during formation of the Ergakskii massif as well as other similar hyperbasite massifs, mantle matter underwent uneven partial melting. These data correspond to the spatial adjacence regularly observed in such massifs and complicated alternation of lherzolites, harzburgites, and dunites, the formation of which appeared to be realized under different degrees of partial melting of mantle sources. Due to the significant lowering of detection limits of rare earth elements (REE) during mineral analyses reached with the help of the LA ICP-MS method (mass