THE CARY MOUND GRANITE: A MINERALIZED COLLAPSED CALDERA IN CENTRAL WISCONSIN

We studied the Cary Mound granite to determine if this small pluton (located about 15 km south of Marshfield in Wood County, Wisconsin) is the remains of an early Proterozoic collapsed caldera. Such structures are rarely preserved in rocks of this age. P.K. Sims of the U.S.G.S. published an age of 1.835 Ga for the pluton. The abundance of spectacular granophyric intergrowths of quartz and alkali feldspar is consistent with the pluton being an anorogenic body post-dating the Penokean Orogeny. Extensive chloritization occurs along joint surfaces and in miarolitic cavities. The chlorite is associated with such minerals as fluorite, siderite, Li-bearing muscovite, cassiterite, pyrite and chalcopyrite. This mineral suite is interpreted as the remains of a hydrothermal system percolating through fractures in a collapsed caldera. Geographically associated with the pluton is a rhyolite with elongated spherulites. We interpret this texture as formed by compaction or flow rather than by later tectonic forces because of the presence of undeformed plagioclase phenocrysts in the cores of the spherulites. The granophyric texture, presence of miarolitic cavities and association with rhyolite all support an epizonal origin for the pluton. Strike and dip studies of joints and faults do not show a pattern consistent with caldera collapse, but instead reveal regional trends with maxima at approximately N50^oE and N40^oW. This may be related to Keweenawan stresses or to the 1.815 Ga. Athens shear zone.