123 Petrology, Igneous

Monday, 19 October 2009
1:30 PM - 5:30 PM
E145

TRACE ELEMENTS AND TEMPERATURE VARIATION IN QUARTZ AND TITANITE IN THE 36 MA HARRISON PASS PLUTON, RUBY MOUNTAINS NORTH-EAST NEVADA

Paper number: 123-10
Program: Technical Sessions
Time: 3:45 PM – 4:00 PM

Authors

Abstract

The Harrison Pass pluton is composed of four units separated into early and late stages. The early stage consists of the Toyn Creek hornblende biotite granodiorite and the Corral Creek biotite monzogranite. The late stage is defined by sheets of leucocratic two-mica monzogranite and the Green Mountain Creek two-mica monzogranite. Quartz and titanite provide a record of magma processes and temperature estimates. The Toyn Creek unit has 3-mm-wide quartz phenocrysts and in all units is present as 1–3 mm diameter interstitial, anhedral grains. SEM-CL zoning is absent, so cores were assumed to be geometric centers, and rims were chosen on proximity to mafic silicates. Titanite in the Toyn Creek unit is euhedral and ranges from ~1–3 mm. Using LA-ICP-MS, trace elements in quartz and titanite were measured. Quartz from Toyn Creek, Corral Creek, and Green Mountain Creek units can be distinguished on the basis of trace element abundances, with higher abundances of B, Sr, K, Fe and lower Li, Al in the Toyn Creek unit compared to the others. Quartz from the Corral Creek unit has high Al, Li and low Fe, Sr, and quartz from the Green Mountain Creek unit has low Ti, Al, B, and K. The core, mantle, and rim compositions of quartz from individual samples are indistinguishable chemically. Some individual quartz crystals are chemically unique.

The TitaniQ thermometer for Ti in quartz and the Zr in titanite thermobarometer were used to estimate temperatures of crystallization. Quartz and titanite from the Toyn Creek unit are reversely zoned in terms of temperature, with average crystallization temperatures in quartz of 709oC in the cores to 719oC in the mantles and 691oC in the rims and average temperatures of 716oC (cores) to 722oC (mantles) and 704oC (rims) in titanite. Quartz from the Corral Creek and Green Mountain Creek units are normally zoned with average crystallization temperatures of 733oC (core) and 698oC (rim) and 681oC (core) to 660oC (rim) respectively. This study shows chemical variation in individual crystals, which is easily overlooked at the whole rock scale. The temperature variations in quartz and titanite suggest magma mixing occurred in the Toyn Creek unit, whereas temperature zoning in quartz from the Corral Creek and Green Mountain Creek units are consistent with fractional crystallization.