Abstract
The two generations of micas in the Shaw’s Cove minette (mica–potash feldspar lamprophyre) of northern New Brunswick, Canada, are TiO2-rich phlogopites with variable but low silica and potash contents, low soda, and effectively all their aluminum in tetrahedral sites. A majority of the micas have an excess of Y cations and a deficiency of X cations; there are positive correlations between excess Y ions and number of A1 ions in Z sites, between K2O and SiO2 contents, and between K2O and volatile-free totals; yet the micas optically appear fresh, do not respond to hydration or glycolation, and X-ray diffraction patterns show none of the peaks of talc, montmorillonites, vermiculite, or 7- or 14-Å chlorites. This suggests that some normally octahedrally coordinated cations and water or hydronium ions are located in the interlayer X sites. Phenocryst micas are sharply zoned. Cores are paler colored and have high mg [atomic ratio Mg/()] (0.81–0.88), Cr2O3 (x = 0.88 wt.%), and NiO (x = 0.08 wt.%), but relatively low TiO2 (x = 2.93 wt.%), and negligible BaO. Phenocryst rims and groundmass micas are darker reddish brown and have lower mg (0.68–0.79), nil Cr2O3 and NiO, but high TiO2 (x = 5.40 wt.%) and BaO (x = 0.38 wt.%). Micas in minettes from other localities (62 analyses) are frequently richer in silica and potash than the Shaw’s Cove samples but otherwise are generally chemically comparable. Chemical zoning or bimodalism is nearly universal in minette micas, though the boundary between high-mg–low-TiO2 (phenocryst) and low-mg–high-TiO2 (groundmass) groups varies from province to province and even from minette to minette within a swarm.
Comparison of analyses of minette micas with 353 analyses of micas from other mafic (largely potassic) rocks indicates that micas chemically indistinguishable from those of minettes can be found within some other lamprophyres, (ultra)potassic rocks, kimberlites, and high-pressure xenoliths in alkaline volcanic rocks and kimberlites, though each of these groups also contains micas unlike those in minettes. Those kimberlite phlogopites that match minette micas are predominantly secondary rims of phenocrysts (or megacrysts) or groundmass crystals (mainly “Type II”), but some are unzoned pre-fluidization phenocrysts. While primary, primary-metasomatic, and MARiD-suite phlogopites of mantle xenoliths are unlike minette micas (higher mg and SiO2, lower TiO2 and Al2O3), some of the secondary-metasomatic phlogopites in sheared garnetiferous mantle xenoliths are wholly minette-like.
The chemical identity of minette-mica phenocryst cores and some phenocrysts of diverse mantle-derived rocks implies crystallization under similar conditions. By analogy with the chemistry of phlogopites produced experimentally in potassic systems at high pressures and studies of equilibration conditions of natural phlogopites, it is concluded that phenocrystic phlogopites in minettes may form at temperatures and pressures up to at least ~1250°C and ~40 kbar and probably under f02, conditions between the NNO and HM buffers: higher f02, than for primary, primary-metasomatic, and MARID-suite micas in kimberlites and their included xenoliths.