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Abstract:
Granulite containing prismatine (Prm), (vac,Mg,Fe)(Al,Mg,Fe)9(Si,Al,B)5O21(OH,F), is a migmatite consisting of ~75 vol% sodic Pl and ~25 vol% Grt, Crn, Ky, Sil, sapphirine and Bt. Prm occurs most abundantly in weakly foliated fine-grained leucocratic Pl domains both as prisms 1-4 cm long and as small grains.
Our re-investigation of the Prm granulite did not confirm the recent reports of coesite and other UHP phases, but instead shows that Prm, Grt and Tur contain polyphase inclusions with primary distribution, well developed negative crystal shape and size ≤50 μm. Using MicroRaman spectroscopy (MRS) we identified systematic differences between mineral assemblages of inclusions from the different hosts. In Prm, inclusions mainly contain kumdykolite (orthorhombic NaAlSi3O8, Kdy) + cristobalite (Crs)/tridymite (Trd) or (rarely) Qz + kokchetavite (hexagonal KAlSi3O8, Kct) + Bt/Phl + white mica ± CO2. In garnet inclusions contain instead Kdy + Bt + Qz/Crs ± Na-K-Ca aluminosilicate phase 430. In Tur inclusions can either be crystallized to Crs/Qz + Kdy + white mica ± Kct or glassy with a shrinkage bubble.
The phase assemblage found in the polyphase inclusions, i.e., Qz + Pl + Kfs (or their polymorphs) + OH-bearing phases, testifies for their former nature of melt inclusions (MI), now crystallized to nanogranitoids. This constrains the growth of the MI-bearing minerals at supra-solidus conditions.
Moreover, MRS results suggests the presence of different melts in Prm and Grt. MI in Prm contain a granitic and hydrous melt, consistently with the finding of both Kfs and Ab polymorphs besides two OH-bearing phases, whereas Kct and white mica are notably absent in MIs in Grt, pointing toward a melt poorer in both K and H2O. Combining MI data and petrological results on the host rock, melt in Grt formed and was trapped at HP conditions, whereas the melt in Prm, results from a second melting event, possibly at lower P. Tur MIs differ from those in Grt and Prm thus a relative timing of entrapment is not easy to determine.
The present study is a clear example of how an investigation using MicroRaman spectroscopy of polyphase inclusions in metamorphic minerals is a fast and reliable approach to provide crucial insights in the supra-solidus evolution of high-grade metamorphic terrains.
