CONSTRAINING THE ONSET OF CRETACEOUS PERALUMINOUS MAGMATISM IN THE RUBY MOUNTAINS, NEVADA

The Ruby Mountain – East Humboldt Range (RM – EHR) in northeast Nevada exposes at least three generations of peraluminous granites that are Cretaceous or younger. On the basis of field relations, an equigranular banded gneiss is proposed to be the oldest; the equigranular granitic gneiss of Lee et al. (2003). Monazite from a single sample has been dated to 94 – 92 Ma. However, the complexity of the magmatic system, the field relationships, and evidence for post-emplacement deformation, suggest that this should be viewed as a minimum age only. New samples have been prepared for petrography and zircon geochronology to better constrain the emplacement age and source of this lithology.

The accessory mineral budget of all samples is dominated by monazite that is subhedral to rounded in shape and ranges in size from 100-200 μm and appears to be compositionally homogenous. Zircon is rarer and dominated by three morphologies: a 200 μm long needle-like form with a length to width ratio of ~8:1 and prismatic terminations; ~150 μm subhedral prismatically terminated zircon with length to width ratios of 2:1; and, ~100 μm gemmy rounded (soccer ball-like) grains. The first two populations commonly display oscillatory or sector zoned cores, surrounded by oscillatory zoned rims. The rounded population contains oscillatory zoned euhedral cores overgrown by rounded, oscillatory zoned, rims. Grains from all populations sometimes display a <10 μm outermost rim that has bright cathodoluminescence, which is particularly prevalent at prismatic terminations. This is interpreted to be a third stage of zircon growth. Preliminary U-Pb dating of the oscillatory zoned rims indicate ages of 85, 88.5 and 90.5 Ma. This age is younger than the monazite age, but the data shows a significant spread suggesting that the magmatic evolution, emplacement, and post-crystallization history of the lithology may be more complicated than originally proposed.