Petrogenesis and Geotectonic Setting of Mantle Peridotites from the Baiyunshan Ophiolite in Beishan Orogen

The Baiyunshan ophiolite is one of the most extensive tracts of oceanic crust preserved in Beishan orogen, but its origin has remained controversial. This massif consists of peridotite, gabbro, and volcanic rocks. The peridotites are mostly harzburgite with a small amount of lherzolite. Most of these bodies are suffered various degrees of serpentinization and also have been highly deformed and metamorphosed. 9 peridotites have been collected from the Baiyunshan ophiolite massif for carrying out a study of major element composition and mineral chemistry. The mineral and bulk-rock compositions show that these peridotites belonged to mantle rocks. The compositions of the spinel cores and clinopyroxenes were used to deduce the petrogenesis and tectonic environments for the formation of the Baiyunshan ophiolite massif. These samples contain high Cr^# spinel (46.7-68.1) and low Al₂O₃ (0.78%-2.94%) clinopyroxene. The degree of partial melting of mantle peridotite is 23%-36%, indicating that Baiyunshan mantle peridotite may experience a multi-stage process. The mineralogy and geochemistry of the Baiyunshan peridotites suggest that they were formed originally at a MOR setting and were later modified by SSZ melts.