Mineral inclusions in chrome-spinels from the cumulate sequence of the
Dongcaohe ophiolite of north Qilian Mountains, NW China: Their genesis and
petrological significance
Chien-Yuan Tseng and Houng-Yi Yang
Department of Earth Sciences, National Cheng Kung University, Tainan, Taiwan
Chrome-spinels occurring in the cumulate sequence of the Dongcaohe ophiolite
of north Qilian Mountains, NW China, frequently carry mineral inclusions of
anhydrous silicate minerals (clinopyroxene and orthopyroxene), hydrous
high-temperature silicate minerals (sodium phlogopite and pargasitic hornblende),
hydrous low-temperature silicate minerals (chlorite, serpentine, hydrogarnet, and
analcime), Fe-Ti oxide minerals and base-metal sulfides. Clinopyroxene,
orthopyroxene, sodium phlogopite, pargasitic hornblende and Fe-Ti oxide minerals
are considered as primary by their being texturally-isolated grains within host
chrome-spinels and by their diagnostic characteristic mineral composition. Other
inclusions are clearly secondary because they are connected with the serpentinized
matrix by cracks and they were formed in the pressure-temperature range of their
stability field. The secondary hydrothermal products including chlorite, serpentine,
hydrogarnet, and analcime mainly reflect a low-temperature seafloor alteration under
200oC.
The origin of these primary inclusions reflected a closed system evolution after
entrapment of a H2O-bearing primary basaltic melt. By the fractional crystallization in
the closed system, as temperature down it gave rise to volatile-enriched differentiated
melts to crystallize the hydroxyl-sodic phases (sodium phlogopite and/or pargasitic
hornblende). The H2O-bearing primary basaltic melt was probably resulted from the
melt-mantle interaction in uppermost mantle beneath an active spreading ridge.
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