EXPLORING THE GEOCHEMICAL LINKS BETWEEN THE TOPAZ RHYOLITES AND RAPAKIVI GRANITE

Rapakivi textures and the Topaz Rhyolites have previously been linked geochemically. For example when major elements, alkalis vs Si, aluminum saturation vs Si, and FeO/(FeO+MgO) vs Si are plotted similar patterns emerge. The links with Topaz Rhyolite were found using Finnish rapakivi topaz granite. A previous attempt at exploring the scope of the relationship using Alleghenian biotite rapakivi granite from Elberton, Georgia found little similarity between the Topaz Rhyolites of western Utah and this rapakivi textured granite. The major elements tended to not plot within standard deviations of the Topaz Rhyolites, and had different values for aluminum saturation, alkalis, and Fe# plots. The Finnish rapakivi contained topaz, which may explain the relationship.

To determine the relationship between Topaz Rhyolites and rapakivi textures more data is needed. Values for the Topaz Rhyolites were compiled from previous research as well as The Geology and Geochemistry of the Cenozoic Topaz Rhyolites by Christiansen et al. Values for rapakivi granites were compiled from sources as well as our previous research. The data was then plotted on Harker diagrams against the Topaz Rhyolites.

The Topaz Rhyolites are richer in Si than the rapakivi granites (73.0-76.0 wt% compared to 60.0-70.0 wt%). Topaz granite associated with rapakivi textures from the Llano Uplift tend to be closer in Si values to the Topaz Rhyolites. The rapakivi granites tend to plot similarly in value to the Topaz Rhyolites on the major element plots except for the Mg plot, where rapakivi granites plotted higher than the topaz rhyolites. The Alkali vs. Si, Aluminum saturation, and Fe# plots revealed rapakivi granites to be very similar to the Topaz Rhyolites.

Non-rapakivi topaz granites/aplites tended to plot within the Si ranges of the Topaz Rhyolites. On the Harker diagrams they tended to be indistinguishable in value from the Topaz Rhyolites except for the Mn plot, where they tended to plot lower in value than the Topaz Rhyolites. On the Fe#, aluminum saturation, and alkali plots the topaz granites/aplites plotted in the cluster of the Topaz Rhyolites.

While there is a strong geochemical link between the Topaz Rhyolites and rapakivi granites, the stronger relationship tended to be between the non-rapakivi associated topaz granites/aplites and the Topaz Rhyolites.