Inclusion/lamella mineralogy and chemical characteristics of garnets from the Garnet Ridge in the Colorado Plateau, northern Arizona

A wide variety of garnets as xenocrysts and those in xenoliths, come from kimberlitic diatreme (Smith et al. 2004), occurs at the Garnet Ridge. Koga and Ogasawara (2012) classified these garnets into 9 groups: (a) Cr and pyrope-rich garnet, (b) pyrope-rich reddish brown garnet, (c) reddish brown garnet, (d) garnet in eclogite, (e) garnet in metasomatized eclogite, (f) garnet aggregate, (g) garnet megacryst, (h) garnet in metasomatic rock I, (i) garnet in metasomatic rock II. They divided genetically these groups into four: mantle peridotite (a, b), subducted oceanic crust (d, e), high-pressure metasomatism (c, f, g), low-pressure metasomatism (h, i).In this study, the following 4 groups (a, b, f, g) were chose for inclusion mineralogy by laser Raman spectroscopy. Groups (a) and (b): pyrope-rich garnets (a: 45-82, b: 61-80 Prp mol%) both Cr-rich and Cr-poor (a: 1.0-5.9, b: 0.0-1.0 wt.% Cr₂O₃) are Ca-poor (1.5-7.0 wt.% CaO) and single-crystals of 5-15 mm in diameter. Group (a) is identical to chrome-pyrope based on the classification of kimberlitic garnets by Dawson and Stephens (1975). CaO-Cr₂O₃ ratio of (a, b) indicates lherzorite origin (Turkin and Sobolev 2009). Wang et al. (1999) have reported the detailed inclusion and lamella mineralogy of pyrope-rich garnets from the Garnet Ridge. We identified inclusions of Chl (OH: 3450, 3582, 3679 cm^-1), Amp (OH: 3685, 3711 cm^-1), Ol, Opx, Cpx, Rt (OH: 3295 cm^-1), Mgs, Dol, Cal, sulfides, fluid (OH: 3445 cm^-1) and spherical composite inclusions of Amp, Ap, Dol, Mgs, Rt and sulfides, and oriented lamellae (presumable exsolution) of Qz, Ol, Opx, Cpx, Amp, Chl, Rt, Ilm, crichtonite (6-7 Peaks at 120-820 cm^-1), carmichaelite (710-782 cm^-1, OH: 3340 cm^-1), Ap (OH: 3570 cm^-1) and Ti-Chn (OH: 3404, 3527, 3564 cm^-1) adjacent to the oriented Ol. The mineral assemblages of the inclusion and lamella show a correlation with the host garnet compositions; inclusions: (a, b) Ol + Opx + Cpx × composite, (b, low Mg) Opx + Cpx + Amp, oriented lamellae: (a) Rt + Cr-bearing titanates + Ol, (b) Rt + titanates + Px or Ol, (b, low Mg) Rt + Ilm + Amp + Chl. Groups (f) and (g): aggregate (f) and megacryst (g) are rich in Alm, Grs and Pyp; almost 1:1:1. The chemistries of the garnets fall into pyrope-grossular almandine by Dawson and Stephens (1975). Both garnets contain Cpx, Zrn, sulfides, and oriented Ap and Rt rods. Chl, Cpx, Rt and Ilm grains occur near grain boundaries of (f). A significant variation of hydroxyls was detected in the garnets from the Garnet Ridge (Ogasawara et al. 2012); hydroxyls in garnets (c, f, g) and (a, b), but no hydroxyls in garnet (d).Detected hydroxyls in the host garnet and in identified inclusion/lamella and their assemblages may lead to clarify the volatile environments of the metasomatic processes in peridotites and subducted materials underneath the Colorado Plateau. References Turkin, A.I., Sobolev, N.V. (2009) Russ. J. Geol. Geophys., 50, 1169-1182. Smith, D. et al. (2004) Geochem. Geophys. Geosyst., 5, 4. Dawson, J. B., Stephens, W. E. (1975) J. Geol., 83, 589-607. Koga, I., Ogasawara, Y. (2012) AGU Fall Meeting, V43A-2824. Wang, L. et al., (1999) Contrib. Mineral. Petrol., 135, 164-178. Ogasawara, Y., Sakamaki, K. and Koga, I. (2012) AGU Fall Meeting, V43A-2821.