SHALLOW MAGMATIC PROVENANCE OF QUARTZ: A CATHODOLUMINESCENCE ZONING SIGNATURE

Identifying the origin of detrital quartz is challenging because of its extremely limited compositional variability. Many studies have suggested that subtle compositional variation revealed by cathodoluminescence imaging (CL) can distinguish quartz grains with different histories (e.g. Zinkernagel 1978, Owen & Carrozi 1986, Seyedolali et al 1997, Bernet & Bassett 2005), but zoning is commonly subtle and difficult to interpret. Recent studies of Ti in quartz and related CL imagery are, however, encouraging. Titanium concentration, which correlates with temperature (Wark & Watson 2006), strongly influences CL brightness (Wark et al 2007), and intense, euhedral, sharply-defined growth zoning appears to be characteristic of volcanic and shallow intrusive quartz (e.g. Peppard et al 2001; Wiebe et al 2007). Diffusivity of Ti in quartz leads to relatively rapid blurring of zone boundaries at high T (Cherniak et al 2007; Gualda et al 2012). Thus, we infer that sharply defined magmatic zone boundaries in detrital quartz indicate shallow, initially high-T but rapidly cooled magmatic provenance. This distinguishes it from quartz formed in deep-seated environments (deep plutons and metamorphic rocks).

To test our hypothesis, we have initiated a reconnaissance study of detrital quartz in river sands and sandstones with diverse, reasonably well-known source regions, including the Yellowstone River, WY (primarily rhyolite); Selway River, ID (deep-seated Idaho Batholith); Grizzly Bear Creek, Black Hills, SD (deep Harney Peak granite); Miocene through modern sandstones and sands, Iceland (rhyolite, shallow granite, and basalt); Pennsylvanian Rockcastle sandstone, TN (Appalachian metamorphic rocks); and Cambrian Lamotte sandstone, MO (underlying mid-Proterozoic rhyolite & shallow granite). Preliminary CL observations from the Yellowstone and Iceland samples reveal that most quartz sand grains have bright, sharply defined, euhedral zoning, consistent with high-T, rapidly cooled quartz. In contrast, Selway and Black Hills samples appear to entirely lack such zoning, instead showing much weaker, diffuse, mottled zoning or no zoning at all. Thus, our preliminary data suggest that CL patterns in detrital quartz may be a straightforward way to identify shallow magmatic provenance.