High-pressure ultramafics in the lower crustal rocks of the Pekul'ney complex, central Chukchi Peninsula. 1. Petrography and mineralogy

Dunites, peridotites, olivine and spinel pyroxenites, and metagabbroids have been described in the tectonic blocks of the Pekul'ney complex of the central Chukchi Peninsula together with garnet-hornblende-clinopyroxene and zoisite (clinozoisite)-garnet-hornblende rocks, which are indicative of high-pressure complexes. However, the interpretations of previous researchers on the composition, structure, setting, and processes of formation of this rock association are highly controversial. The petrographic and mineralogical results reported in this paper indicate that the blocks of the complex host bodies of cumulate ultramafics among metamorphic rocks. These relationships were supported by the finding of xenoliths and xenocrysts of metamorphic rocks in the ultramafics. The metamorphic country rocks are lower crustal amphibolites and schists with peak metamorphic parameters corresponding to the high-pressure portion of the epidoteamphibolite facies (610-680A degrees C and 9-14 kbar). All the varieties of ultramafic rocks studied in the blocks of the complex are assigned to a single cumulate series (from dunite to clinozoisite-garnet hornblendite), and the compositions of their primary minerals show regular correlations similar to crystallization differentiation trends. Specific features of the ultramafics of the Pekul'ney complex are the early crystallization of hornblende (which is present already in peridotites), wide range of garnet crystallization (associating with clinopyroxene, ceylonite, and hornblende), presence of magmatic clinozoisite in the most evolved assemblages (with garnet, hornblende, and clinopyroxene), and absence of evidence for plagioclase crystallization. Clinopyroxene from the most evolved ultramafic rocks contains more than 15 wt % Al2O3. The classification of the rocks of the complex provides a basis for the interpretation of geological relations between them and the elucidation of the characteristics of the internal structure of the blocks of the complex and bodies of cumulate ultramafic rocks in them.