Discussion

Now that the data has been collected, reduced, and analyzed it can be effectively evaluated to test our hypothetical growth mechanisms for the K-feldspar megacryst. The theoretical growth mechanisms for this crystal are; continued growth in the magma chamber through magma mixing or chamber recharge, or growth influenced by magmatic fluids interacting with the crystal after cooling to stimulate continued growth. To test these hypotheses the primary analytes of significance are Ba, Sr, Rb, (Słaby et al., 2011, Davidson et al., 2008; Gagnevin et al., 2005) and the multiple rare earth elements and high field strength elements (Chambers et al., 2020; Słaby et al., 2012). From our data, the rare earth elements and the majority of the high field strength elements were below the limit of detection and therefore cannot be considered to evaluate our hypothesis regarding fluid interactions. This means the primary analytes of significance for our conclusion are Ba, Sr, and Rb.

Barium, strontium, and rubidium are commonly analyzed to study the history of intrusive magmatic bodies because they are commonly enriched or depleted in K-feldspar depending on a multitude of factors. In our sample, there was a decrease in Sr and Ba toward the rim of the megacryst and an increase in Rb. This will frame how our analysis progresses.

Below is a color gradient indication of the distribution of Ba, Sr, and Rb concentrations. The upper gradient for each analyte is the East-West transect and the lower gradient is the North-South transect. For the East-West transect the gradient moves from East to West and for the North-South transect the gradient moves from the South to the North.

Explanations for the cause of this zoning are numerous in current research regarding K-feldspar megacrysts. Gagnevin et al. (2005) and Słaby et al. (2011) both describe a process involving the migration of K-feldspar crystals from a cooling felsic magmatic body into highly developed magmatic enclaves. In the area of the Cathedral Peak granodiorite, enclaves such as this are present and some megacrysts have been found to gather within these enclaves (Chambers et al., 2020; Titus et al., 2005). In a felsic magmatic body, rubidium very quickly homogenizes in the melt while barium and strontium take longer to do so (Słaby et al., 2012). This could explain the large fluctuations in Sr and Ba concentrations within the crystal while Rb remains very consistent. Further, within these enclaves, rubidium could potentially become enriched, while barium and strontium become depleted, explaining the increase in Rb and the decrease in Ba and Sr. The slight increase in magnesium and decrease in potassium could also potentially be explained by these enclaves of depleted magma. Magnesium might become enriched due to the low quantity of felsic minerals that it is compatible with, while potassium could become depleted due to the significant influx of megacrysts absorbing the available ions.

Another explanation posed by Gagnevin et al. (2005) is that the outer rim of the megacryst crystallized simultaneously with the K-feldspar present in the groundmass during late the late stages of crystalization. This is proven through further analysis of the bulk composition of the rock which has not been done for our megacryst. The crystal would have likely grown within a magma chamber where it would consistently encounter magma of different compositions whether through migration or recharge until the magma was completely cooled at which time the final rim would crystalize out of the melt.

The depleted magma enclave and late-stage fractionation theories are just two potential explanations, but it is difficult to draw complete conclusions without further analysis of multiple megacrysts as well as additional trace elements. Future research should focus on diverse sample collection as well as thorough trace element analysis through multiple means. Mineral inclusions within the K-feldspar megacryst should also be analyzed as they may further indicate the composition of the magma chamber at various points throughout the crystallization of megacrysts. LA-ICP-MS is a very powerful tool in the analysis of these megacrysts, and once combined with other analytical techniques a more complete picture can be drawn of the crystalization process of K-feldspar megacrysts.