Inversion of primary magma composition for Jinbaoshan ultramafic intrusion, Yunnan.
云南金宝山超镁铁岩原始岩浆成分反演

The Jinbaoshan intrusion, situated in Midu county, Yunnan, host the largest PGE deposits of our country.This small ultramafic intrusion is composed of dunite, Iherzolite as well as pyroxenite, intruded in Jinbaoshan group at Devonian. No accumulated or zoned petrographical features are recongnized, but the mineralization and REE distribution patterns show that it is a cumulate of basic magma rather than ultramafic one. Compositional relationship in terms of molecular ratio shows olivine controlled rock's composition variations, it can be presumed that the intrusion was formed by a mixture of melt and olivine. Most part of magma experienced 11% olivine crystallization and 22% plagioclass, plagioclass was kneaded out from magma chamber; then the residual melt was mixed with crystallized olivine intruded to be the ultra-mafic intrution. Based on the inversion of magma evolution, the primary magma composition is calculated. It is a kind of low-Ti tholeiite magma containing 12. 93% MgO. According to REE contents the primary magma was originated from partial melting of garnet Iherzolite by diapiric uprise in mantle.