Structure, geochemistry and geochronology of a Penokean Lamprophyre Dike Swarm, Archean Wawa Terrane, Little Presque Isle, Michigan, USA

Abstract

Two perpendicular ultramafic lamprophyre dikes on Little Presque Isle near Marquette, Michigan were studied to compare their intrusive magmatic fabric using anisotropy of magnetic susceptibility (AMS) with the strains preserved by mechanical twins in dike magmatic calcite ( δ 18O = −21.93, δ 13C = −4.53) and dike-margin hydrothermal calcite ( δ 18O = −9.72, δ 13C = −3.76) veins. Phlogopite from the lamprophyres yield an 40Ar– 39Ar plateau age of 1877 Ma, and the lamprophyres intrude metadiabase dikes and the Compeau Creek gneiss (2691 ± 25 Ma; LA-ICPMS U–Pb on zircon), all of which is cross-cut by Keweenaw basaltic dikes and unconformably overlain by the undeformed Jacobsville Sandstone (Meso- to Neoproterozoic). The lamprophyre dikes strike east–west and north–south in this small (3 km 2), complex outcrop. Analysis of mechanically twinned calcite in the E–W dike ( n = 89 grains, 26.97% NEVs) reveals a sub-horizontal dike-acute shortening strain axis (N38°E, 15°). Strain analysis of the N–S dike ( n = 107 grains, 22.43% NEVs) reveals a sub-horizontal and dike-parallel shortening strain axis (N17°E, 14°). Calcite veins from the margins of both lamprophyre dikes reveal a NE–SW sub-horizontal shortening, and both the lamprophyres and vein calcite preserve a vertical shortening strain overprint. Differential stress magnitudes for twinning the lamprophyre calcite and adjacent veins was −35 MPa. AMS analyses, used as a proxy for the primary magmatic flow direction, reveal a sub-horizontal and dike-parallel K max, with a dike-normal K min for the E–W dike. The N–S dike preserves a vertical K max and a dike-normal K min. Hysteresis studies on both dikes show that this anisotropy is carried by multi-domain ferromagnetic minerals. These lamprophyres preserve a complex, local intrusive fabric (AMS) that was unaltered by Penokean sub-horizontal shortening (∼SW–NE), or the younger, sub-vertical shortening strain overprint ∼200 km inboard of the unrestored Penokean margin. The twinning strain results are consistent with other Penokean finite strains but not with the strain fabrics of younger orogens (e.g., Yavapai, Mazatzal, Keweenaw-Grenville).