The geochemistry and petrology of Hamilton Mounds, Wisconsin

Bibliography: pages 73-74.The Hamilton Mounds porphyritic granite is petrologically and chemically typical of what would be expected for a granite which has crystallized as an eutectic, two-feldspar granite. Some of the field relationships suggest that the granite may have been emplaced along fractures in the quartzite, which overlies the granite. Rb-Sr isotopic data indicate a whole-rock errorchron age of 1941 ±402 million years. The Rb-Sr mineral isochron records a homogenization of the strontium isotopes on the mineral scale at approximately 1582 million years ago. The event responsible for this homogenization is probably the major cause of the large uncertainty of the whole-rock errorchron. Many of the whole-rock points appear to have been rotated toward the younger age. A U-Pb zircon age obtained by Van Schmus (pers. comm., 1983) indicates a primary crystallization age of 1764 ±4 million years. An attempt at regional correlation indicates, in terms of age, that the Hamilton Mounds granite is post-Penokean. However, chemical comparisons with both Penokean and post-Penokean rocks indicate that the granites are chemically similar to the Penokean rocks. This raises some doubt as to the validity of distinguishing between the two periods on the basis of their chemical characteristics.M.S. (Master of Science