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THE SILICATE MINERAL INCLUSIONS IN THE CHROMIAN SPINEL FROM THE DONGCAOHE OPHIOLITE, NORTH QILIAN MOUNTAINS, NORTHWESTERN CHINA: RECORD OF SYNTEXIS OF LOWER OCEANIC CRUST

Chien-Yuan Tseng^1,, Houng-Yi Yang¹, Han-Quan Wu² and Guo-Chao Zuo³

¹ Department of Earth Sciences, National Cheng Kung University, Tainan 701, Taiwan
² Xi’an Institute of Geology and Mineral Resources, Xi’an 710054, People’s Republic of China
³ Gansu Bureau of Geology and Mineral Resources, Lanzhou 730000, People’s Republic of China

Diopside, enstatite, pargasite, aspidolite, analcime, chlorite, serpentine, titanite, andradite, and grossular are found as mineral inclusions in grains of chromian spinel in the troctolites and dunites of the layered cumulate series and discordant layered bodies, Dongcaohe ophiolite, in the North Qilian Mountains, Qinghai Province, northwestern China. The mineral inclusions in the layered cumulate series are remarkably different in species and composition from those in the discordant layered bodies. Diopside, the liquidus phase, was enclosed by the growing crystals of chromian spinel. Enstatite, pargasite and aspidolite were products of the closed-system reaction between the trapped mineral phases and trapped residual melt. Analcime crystallized or precipitated directly from the evolved entrapped melt or hydrothermal solution in the inclusions. Chlorite, serpentine, titanite, andradite, and grossular are of secondary or metamorphic origin and were produced by alteration of olivine, pyroxene, or amphibole during postmagmatic hydrothermal alteration or subsequent seafloor metamorphism of the ophiolite. Syntexis of the lower oceanic crust was involved in the formation of a modified magma, which solidified as the discordant layered bodies scattered within the layered cumulate series of the Dongcaohe ophiolite.

Keywords: ophiolite, mineral inclusions, chromian spinel, layered cumulate series, discordant layered bodies, syntexis, lower oceanic crust, Dongcaohe ophiolite, Qinghai Province, People’s Republic of China.