Different-depth gabbro–ultrabasite associations in the Sikhote-Alin ophiolites (Russian Far East)

Based on the new geological, mineralogical, and geochemical data on the crystalline rocks of the Paleozoic ophiolite associations in the Sikhote-Alin (Russian southern Far East), we have reconstructed the tectonic setting of their formation. Two ophiolite associations have been recognized: peridotite–troctolite and peridotite–gabbronorite, differing mainly in the structure of the cumulative part of their sections. In the peridotite–troctolite association, the base of the cumulative unit is formed mostly by olivine–plagioclase rocks (troctolites and olivine gabbro), and in the peridotite–gabbronorite association, by essentially pyroxene rocks (pyroxenites and wehrlites). We have established that the ophiolite rocks crystallized at different pressures: troctolites— < 5 kbar (judging from the stability of the olivine–plagioclase paragenesis), hercynite gabbronorites—5–12 kbar, and garnet gabbro— > 12 kbar. The crystalline rocks form a single geochemical series, with the accumulation of lithophile elements and LREE in more differentiated varieties. The REE patterns of rocks are in good correlation with their mineral composition.We assume that the Sikhote-Alin ophiolites formed at the basement of an oceanic plateau growing as a result of mantle plume intrusion.