CALCITE STRAIN ANALYSIS ON LAMPROPHYRE DIKES: STRESS-STRAIN ORIENTATIONS DURING EMPLACEMENT

Oriented samples were taken from two perpendicularly oriented ultramafic lamprophyre dikes in Little Presque Isle near Marquette, Michigan. The dikes intrude Archean metamorphic rocks and are presumed to be of Keweenawan age. Primary calcite ocelli are present within a mineral assemblage of phlogopite, amphibole, and Fe-rich oxides (magnetite and ilmenite). The oldest dike is an east-west dike crosscut by a north-south striking dike. Eleven oriented samples (4 E-W, 7 N-S) were taken and 29 thin section were analyzed using the calcite strain-gauge technique. Analysis of the E-W dike (3 samples, 6 thin sections, 109 grains, 15% NEVs) revealed a horizontal strain fabric at a 45^o angle to the dike (N45^oE, 0^o). Analyses from the N-S dike (3 samples, 6 thin sections, 107 grains, 22% NEVs) reveal a horizontal and dike-parallel strain fabric (N7^oE, 6^o). Dike-parallel calcite veins were also analyzed for each lamprophyre. Inferred differential stresses for the dikes and veins are ~50MPa. AMS studies, as a proxy for dike intrusion flow, are underway. These results demonstrate a unique stress-strain fabric for each dike, with no secondary strain overprint (low NEVs), and a strain fabric that is unique to regional Keweenawan strains, allowing for an interpretation of the mechanical stresses caused by dike emplacement as preserved by the primary igneous calcite.