Anorthitic plagioclase and pargasitic amphibole in mantle peridotites from the Yungbwa ophiolite (southwestern Tibetan Plateau) formed by hydrous melt metasomatism

摘要

Plagioclase crystals with an anorthite content up to An_(99) have been discovered in mantle peridotites from the Yungbwa ophiolite, which crops out along the Indus-Yarlung Zangbo Suture, southwestern Tibetan Plateau. Amphiboles of pargasitic composition also occur in these plagioclase peridotites. Microtextures support the view that both plagioclase and amphibole were formed metasomatically by hydrous melts entering the Yungbwa peridotites, during which pyroxene was dissolved and olivine was precipitated. The extreme Ca-rich nature of the plagioclase indicates that the metasomatic melts had high Ca/Na ratios, whereas low Ti contents in accompanying spinel and amphibole indicate severe depletion of Ti in the hydrous melts. This suggests that the metasomatic melts were derived from a refractory mantle source. Clinopyroxene trace element data indicate that the metasomatic melts were also enriched in light rare earth elements relative to both middle and heavy rare earth elements. Therefore, the melts show geochemical characteristics similar to arc magmas. This implies that the mantle peridotites in the Yungbwa ophiolite experienced a two-stage evolution; they were formed at a mid-ocean ridge and subsequently entered a subduction zone setting. Their history thus records the opening and closing of the Neo-Tethys Ocean.