Major element chemistry and inclusion/lamella mineralogy of garnets from the Garnet Ridge in the Colorado Plateau, northern Arizona

Monday, 15 December 2014
Yuto Sato, Itaru Koga and Yoshihide Ogasawara, Waseda University, Tokyo, Japan
Various garnets with diverse features and origins occur in the Garnet Ridge. These were transported from upper mantle to crustal depths underneath the Colorado Plateau by a kimberlitic diatreme (ca. 30 Ma) as xenocrysts and xenoliths. On the basis of major element chemistry, inclusion/lamella mineralogy, color, and host rocks of garnets, the Garnet Ridge garnets were classified into the following ten groups (Table) using 495 analyzed grains: A. Cr and pyrope-rich garnet, B. pyrope-rich reddish brown garnet, C. garnet aggregate, D. garnet megacryst, E. garnet in eclogite, F. garnet in metasomatized eclogite, G. quartz lamellae-bearing garnet, H. garnet in metasomatic rock I, I. garnet in metasomatic rock II, J. almandine-rich garnet.

A and B are of mantle peridotite origins. Both garnets were characterized by Cr-Spl lamellae for A, and Cpx/Amp lamellae for B, respectively. B is subdivided into 2 types by lamellae and inclusions: (Prp 49-66, Grs 16-26 mol%) lamellae of Rt, Ilm, Cpx, Amp, and Chl, and (Prp 47-66, Grs 11-24 mol%) lamellae of Ilm and fluid inclusions.

C and D have similar chemistry and inclusion/lamella mineralogy. The chemistry (Prp 22-53, Grs 11-41, Alm 26-50 mol%) and the wide variation suggest metasomatism at mantle depths.

E includes Rt, Omp, Zrn, Ap, Kfs, and simplectite of Zo + Ab. F contains Rt, Omp, and Ap. Both E and F have chemical zonation from core to rim in Alm component. These garnets are of subducted oceanic slab origins, probably Fallaron plate.

G includes Cpx, Zrn, and fluid, and oriented lamellae of Rt, Ap, and Qtz. Oriented Qtz lamellae characterize this group.

The host rock of H is of metasomatism origin at crustal depths. H has Grs-rich composition and inclusions of Mt, Zo, Ttn, Ap and fluid.

I has lamellae of Rt and crichtonites, and includes Qtz and Zrn. The host rock of I was strongly altered.

J shows chemical zonation from core to rim in Alm component. This garnet includes Pl, Qtz, Ilm with Mt lamellae, and Rt lamella.

The close relations between the major element chemistry and inclusion/lamella mineralogy of the garnets from the Garnet Ridge are determined. Such results lead to constrain the origins of the garnets and their formation environments, such as metasomatism at crustal to mantle depths, past subductions, lithosphere and upwelling asthenosphere underneath the Colorado Plateau.