MEGA-RIEBECKITE CRYSTALS FROM THE APPROXIMATELY 1.5 GAWAUSAU SYENITE, CENTRAL WISCONSIN

The Wausau Syenite Complex (WSC) is a large anorogenic plutonic complex that is roughly 1.48-1.50Ga old. It is associated with the larger plutonic complex known as the Wolf River Batholith (WRB) that is 1.45Ga. A new 400 meter long outcrop within the WSC was recently exposed approximately 10 km west of the WRB on the northern border of the city of Wausau, WI. The outcrop is composed dominantly of quartz syenite, with low-plagioclase granite and xenoliths. The granite occurs as large, irregular crosscutting lens and as defined dikes up to 3 meters wide with sharp contacts.

One of the dikes is approximately 6m long x 1m wide and dominantly pegmatitic. It consists of approximately 40% subhedral orthoclase, 25% mostly anhedral to subhedral quartz and 35% of mostly euhedral amphibole. The amphibole crystals are up to 30 cm in length. X-Ray Diffraction indicated that the composition of the amphibole was riebeckite. Micro-probe analysis of several of the mega-crystals determined that the composition approached end-member sodic riebeckite and contained up to approximately 1.5% fluorine. No zoning rim to core was noted.

The field and laboratory data collected for this study was used to constrain the nature and relationship of the various units of the outcrop, and in particular to speculate on the origin of the unusual riebeckite mega-crystals. Riebeckite is a common mineral that forms from the crystallization of peralkaline magma and the local abundance of riebeckite within some of these granitic pegmatites suggests the original magma was likely peralkaline. No zoning was noted in the mega-crystals, likely the result of rapid crystallization. In addition, we infer from the weight percentages of fluorine in the riebeckite and the texture of these mega-crystals that they formed in a highly volatile system with high water contents and higher than normal amounts of flux elements. When a flux element like fluorine, lithium, or boron collects in high concentrations, it lowers the temperature for crystallization and facilitates diffusion rates, which may promote the growth of large crystals. Future work will examine additional rim to core variations in elements, particularly lithium and boron.