ARE SOME PROTEROZOIC GARNET–BEARING QUARTZITES IN COLORADO REALLY METARHYOLITES? EVIDENCE FROM PETROGRAPHY, GEOCHEMISTRY, AND GEOCHRONOLOGY

Proterozoic garnet–bearing quartzites near Howard, CO (northern Sangre de Cristo Range) have been previously recognized on geologic maps and from field descriptions. Multiple lines of physical and geochemical evidence now suggest that some of these rocks may be metarhyolites. One sample is from a buff-white to tan-colored, layered rock with abundant and clearly visible millimeter-sized garnet grains, collected from a ~5-meter-high outcrop adjacent to Sand Gulch Road (lat 38.475°, long 105.805°).

Thin-section petrography of this sample shows an abundance of granoblastic quartz with plagioclase (albite) and garnet, and minor amounts of biotite and zircon. The biotite is fresh and unaltered, and the garnet contains abundant quartz and minor xenotime and biotite inclusions (poiklitic texture). There are few metamorphic phyllosilicate minerals that would indicate metamorphism of primary clay minerals, typical of sedimentary origin. Electron microprobe analyses of the garnets reveal that they are almandine–rich (81.2%), have an average composition of (Fe_2.48 Mg_0.39 Mn_0.15 Ca_0.03) Al_2.00 Si_2.97 O₁₂ and are comparatively homogeneous in Fe, Mg, and Mn concentrations from core to rim, common to volcanic garnet. This is in contrast to pronounced bell-shaped elemental rim-core-rim zoning commonly found in metamorphic garnets.

Composited garnets and whole-rock (excluding garnet grains) have δ¹⁸O values of 4.7‰ and 7.8‰, respectively. The whole-rock value is typical of felsic volcanic rocks, and atypical of clastic sediments that are commonly isotopically heavier, with δ¹⁸O values ranging from 11 to 20‰. These δ¹⁸O values in a quartz-garnet fractionation equation indicate a plausible crystallization temperature of 661°C.

Zircon grains from this rock are small (max length ~150 µm), pink, and euhedral with no rounding, pitting, or abrading from fluvial or aeolian transport. The zircons contain melt inclusions, a hallmark of volcanic conditions. SHRIMP-RG U-Pb zircon geochronology gives a Paleoproterozoic age of 1745±5 Ma. A distribution plot of ²⁰⁷Pb/²⁰⁶Pb ages illustrates that there is only one zircon age peak in the data set (n=32). If a sedimentary protolith existed before being metamorphosed to a presumed quartzite, multiple age peaks reflecting different detrital zircon sources would likely be expected.