LINKING CL SCREENING AND TI-IN-QUARTZ TEMPERATURES: A NEW TOOL FOR TRACKING VOLCANIC, PLUTONIC, AND PEGMATITIC SOURCES IN SEDIMENTS

Understanding magmatic processes in the early Earth’s magmatic history is a major geological challenge, as most of the rock record has been eroded, or reprocessed by metamorphism or weathering. Quartz and zircons in old sedimentary layers may be the only pristine remnants of lithologies that are no longer available on the Earth’s surface. Identifying the contributions of different magmatic lithologies (i.e. volcanic, plutonic and pegmatitic) could give key insights into Earth’s early magmatic evolution, such as the volcanic:plutonic ratio and the role of volatiles in magmas.

In order to understand how different magmatic environments are represented in the sedimentary record, we compare magmatic samples and associated sediments from the Pikes Peak area in Colorado. The Pikes Peak batholith is a 1.1 Ga A-type granite that hosts several pegmatite bodies and the subvolcanic rhyodacitic Keeton Porphyry. We analyze quartz and zircon crystals from these units and from the Cryogenian Tava sandstone, which occurs in intra-granite sedimentary dikes at the Keeton Porphyry location and represents the oldest terrestrial sediments in the Front Ranges of Colorado.

Calibration of grayscale blue cathodoluminescence images of hundreds of sedimentary quartz grains with selected Ti contents analysed by LA-ICPMS allows for fast and statistically meaningful identification of ranges in crystallization temperatures. By comparing these to Ti-in-quartz and in-zircon temperature compilations from both local lithologies in the Pikes Peak area and additional data from other settings worldwide, we can estimate the proportions of volcanic, plutonic and pegmatitic quartz in these sediments. Preliminary results suggest that the Tava sediments are dominated by low-temperature quartz derived from the pegmatites. A small portion of high-Ti quartz, consistent with the Keeton Porphyry range, hints at a possible volcanic contribution. The applied technique can be useful to identify lithologies that were completely eroded, especially in old terrains, providing significant information on the magmatic history that may not be obtained elsewhere.