GEOCHEMISTRY AND ORIGIN OF LEUCOCRATIC ROCKS OF THE EPILCHIK ZONED MAFIC-ULTRAMAFIC MASSIF (SOUTHEASTERN KORYAKIA)

Abstract

New data arc discussed on the major and trace element chemistry of vein albitites and quartz albitites of the zoned Alaskan-type Epilchik dunite-clinopyroxenite-gabbro massif. The vein leucocratic rocks of the massif are magmatic, similar to high-Al granitoids of the tonalite-trondhjemite series, and comparable with subduction-related (island-arc) granitoids of M- and I-types. The studied albitites have extremely high Na2O (7.25-10.80 wt%) and Sr (130-2170 ppm) and low Rb (<20 ppm) concentrations with a consequently low Rb/Sr ratio (0.003-0.028). They also have low Ta, Nb, Y, Zr, and Hf (HFSE) contents and fractionated REE patterns with (Ce/Yb)N = 7-27. We suggest that the albitites originated by the partial melting of an amphibolite protolith owing to local isochemical anatexis occurring in inclusions under the effect of conductive heating induced by calc-alkaline basaltic magma. In mass-balance calculations for REE, the best correlation between the model and natural REE distribution patterns is achieved at 15-20% partial melting of the initial rocks with plagioclase, clinopyroxene, amphibole, and biotite in residue.