Acta Petrologica et Mineralogica

The North Qilian is characterized by the early Paleozoic ophiolite and high-pressure/low-temperature metamorphic rocks, such as serpentinite, eclogite, blueschist and metasedimentary rocks. In this study, a detailed petrographic, mineral-chemical, and geochemical analysis is presented on the piemontite-bearing metacherts in Qingshuigou, North Qilian. The piemontite-bearing metacherts are mainly composed of quartz, phengite, piemontite, garnet, glaucophane, clinopyroxene, ardennite, and hematite. Based on the mineral assemblage, mineral chemistry, and the p-T conditions of the country rocks, the piemontite-bearing metacherts may have undergone low-temperature and high-pressure eclogite-facies metamorphism. Whole-rock geochemistry suggested that the protolith of the piemontite-bearing metacherts was argilliferous cherts deposited in the oceanic environment with the participation of hydrothermal activities which resulted in the deposition of Fe and Mn. They were involved in the subduction zone with the materials from the continental active margin or continental island arc, and then experienced low-temperature and high-pressure metamorphism. The piemontite, ardennite, spessartite, and many inclusions of hematite in garnet indicated that the piemontite-bearing metacherts experienced a condition of high oxygen fugacity, which also recorded by the decreases of the Fe³⁺ from core to rim of the garnet. Oxygen released in this process played a significant role in the exploration of oxygen fugacity in the lithosphere mantle, the generation of island arc magma, and the oxygen cycle in a subduction zone.

piemontite; Mn-rich metachert; high oxygen fugacity; eclogite-facies metamorphism; North Qilian