GARNETS FROM THE GARNET RIDGE: A WINDOW TO THE CHEMICAL HETEROGENEITY OF THE MANTLE UNDERNEATH THE COLORADO PLATEAU

Garnet has a diverse chemistry that suggests their formations at wide range of depths and chemical environments. The Garnet Ridge, northern Arizona, located at the center of the Colorado Plateau, is a famous locality of garnets carried by a kimberlitic diatreme of 30 Ma, and is the best place to decode the metasomatism and chemical heterogeneity of the mantle using garnet. These garnets indicate their formations at the depths from shallow crust to deep mantle. We divided these into 11 groups, A to K, in Grs-Prp-Alm triangle with inclusion and lamella mineralogy. A to D are of mantle origin, E to G of subducted oceanic crust origin, H to J of continental crust origin, and K has rare compositions of Ca:Mg = 1:1 with low Alm. Group A, “Navajo Ruby”, is the primitive mantle garnet and contains 0.8-6.3 wt.% of Cr₂O₃ with inclusions of Ol, Cpx, Opx, Ti-Chu/Chn and carbonates, indicating carbonated garnet lherzolites. This rarely contains group K as inclusions. Group B, Prp-rich reddish-brown garnet, is subdivided into 4 (B₁ to B₄). They have a trend to Alm-rich side from group A due to the metasomatism. Sub-group B₁, amphibole lamella type, shows exsolved Na amphibole and inclusions of Rt, Ilm, srilankaite, and Ap. Its precursor is hydrous Na garnet with excess Si suggesting high-P, 6-8 GPa before the Farallon plate subduction, and formed by the metasomatism of A. Group C is megacrysts, 2-10 cm across, with crystal faces and is plotted in the center of Grs-Prp-Alm triangle. Group D is aggregates with similar chemistry of C. Fluid inclusions in C and D suggest the crystallization from fluid during the metasomatism. E and F occurred in eclogite and metasomatized xenoliths, respectively. Both are probably of the Fallaron Plate origin. Group G is Qtz-lamella type, relatively high Na₂O (0.03-0.06 wt.%) with inclusions of Omp and Zrn and lamellae of Qtz; this suggests its eclogitic origin deeper than E and F. Group K shows strange compositions Ca:Mg = 1:1 with low Alm. Such garnet compositions have been known only in the Kokchetav diamond-bearing UHP dolomite marble, and in pyrope from the Garnet Ridge. Above features on various garnets suggest the multi-stage metasomatisms by the subductions of Farallon plate and by the ancient oceanic plates, and the chemical heterogeneities of the mantle underneath the Colorado Plateau.