Corona Structure From Arc Mafic-ultramafic Cumulates: The Role And Chemical Characteristics Of Late-magmatic Hydrous Liquids

摘要

The Genina Gharbia mafic-ultramafic complex consisting of peridotite, pyroxenite, and gabbronoritc represents a remnant of a hydrous magma chamber that was crystallized at the mantle-crust boundary. The hydrous nature of the magma is expressed in the high modal (up to 60%) of amphibole in cumulates and is reflected in variable textural and chemical features of the complex. One of the important manifestations of the hydrous nature is the development of a corona structure between olivine and plagioclase. The corona texture and mineralogy varied according to the activity of the interstitial liquid, temperature, and P_(H_2O). The corona in peridotite is limited to thin (<100 microns) orthopyroxene and/or amphibole shells. Thick orthopyroxene-(orthopyroxene-spinel symplectites)-clinopyroxene-amphibole coronas are common in norite. Extensive analysis of different minerals of various textures by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) revealed distinct chemical difference between the interstitial and cumulus phases of the corona. The corona pyroxenes and amphibole are depleted in V, Ti, Zr, and Nb and enriched in LREE, reflecting the LI-LE-rich and HFSE-depleted nature of the evolved hydrous liquid. The textural and chemical features of the corona are consistent with the formation of corona minerals at the late-magmatic stage. The orthopyroxene corona in peridotite crystallized directly from the evolved melt, while the amphibole corona developed at the expense of plagioclase. Variable corona minerals in norite are the result of direct crystallization from progressively evolved magma during cooling and continuous increase in P_(H_2O).