KIMZEYITE, ZR-BEARING GARNET FROM CHIHUAHUA, MEXICO

Ti-Fe-rich garnets containing up to 30 wt % ZrO₂ and 0.6 wt % HfO₂ have been identified from a shallow, early Oligocene vent zone complex in the Sueco Quadrangle, north-central Chihuahua, Mexico. In thin section, the Zr-bearing garnets are deep yellow brown. Zr contents are highest in central portions of the grains and decrease outward to much lower or negligible amounts at grain edges. The Zr-bearing garnets are not uniquely identified by appearance; other yellow brown garnets from the same area contain negligible Zr.

The kimzeyite, coarse-grained wollastonite, other Ti-Fe garnets, and rare perovskite, occur as residual clots embedded in finer-grained, intergrown, acicular to bladed, wollastonite, vesuvianite, and cuspidine. Other mineral assemblages include massive granoblastic cuspidine, thomsonite-prehnite-pectolite-apophyllite-aegerine-hi-K sanidine, vesuvianite-magnetite-hydrogrossular (hibschite)-diaspore, and Ca- and CaAl-silicate hydrates. A few, tiny calcite grains were identified; quartz is not present.

Contact metamorphism and extensive metasomatism in a shallow vent zone converted in-situ, coarsely glomerocrystic andesite wall rock and blocks of rhyolitic welded tuff and silica-bearing limestone into the calc-silicate assemblages. The Zr was probably derived locally from the wall rock and carried into the vent zone in a F-rich fluid phase along with Ti, Fe, Al, and Si. Cuspidine in the vent-zone rocks, F-bearing biotite and hornblende in the andesite, and strongly corroded zircons in the andesite support this proposed mechanism.

Tetrahedral ions were assigned in the preferential order Si > Al > Fe³⁺ > Ti; Zr and Hf were assigned to the Y site; and the analysis was constrained to show the Y-site sum equal to 2.00. In the garnet with 30 wt % ZrO₂, Ca essentially fills the X-site, Zr and Ti fill the Y-site, and Fe³⁺ and Ti fill 44 % of the tetrahedral sites. All iron as Fe³⁺ gives site sums of X, 2.97; Y, 2.00; and Z, 3.02. Assigning 0.03 of the total iron to the X-site as Fe²⁺ gives X, 3.00, Y, 2.00, and Z, 2.99.