PETROLOGY AND GEOCHEMISTRY OF FLUORESCENT SYENITE COBBLES FROM AGATE BEACH, MI, USA

In 2018, a new find of fluorescent syenite cobbles (yooperlites) was reported on the southern shores of Lake Superior, MI. The presence of fluorescent minerals in these cobbles generated sufficient interest to lead to brief mineralogic analyses, but whole rock analyses were neglected. In this research effort, we present whole rock analyses to classify and constrain the petrogenesis of the samples, as well as to assist in determining the cobbles’ chemical constitution. SiO2 values range from 53.81-57.93 wt. %. Total alkali contents (Na2O+K2O) range from 12.77-13.47 wt. %. On total alkali-silica (TAS) diagrams, the samples plot inside the phonolite field. The samples are depleted in MgO, exhibiting a range from 0.01-0.76 wt. %. Al2O3 ranges from 20.07-20.85 wt. %. Primitive mantle normalized trace element abundances are generally elevated, with LREE and LILE displaying higher degrees of enrichment compared to HFSE (with the exception of Nb analyses, which yield primitive mantle normalized values of 222-280) and HREE. Negative strontium anomalies and positive lead anomalies are present. No Eu anomalies are present. PM-normalized Ba ranges from 226-240, and Th ranges from 228-247. U ranges from 212-220, and Rb ranges from 278-284. La/Sm ratios range from 7.38-7.59, and La/Yb ratios range from 27.19-27.71. The samples plot in the “within-plate-granite” field on tectonic discrimination diagrams and are of metaluminous composition. The samples’ average modal mineralogy comprises (in wt%) 59.3% alkali feldspar, 19.1% nepheline, 10.9% ferroan amphiboles, 5.1% fluorescent minerals, 1% ilmenite, 1.8% FeS2, and 2.8% apatite, with trace amounts of mica. Analyses of fluorescent minerals yield moderate levels of calcium (3.55 mol%), possibly indicating that the fluorescent minerals present may include members of the Nosean-Huayne solid solution series.