CHEMICAL AND TEXTURAL ANALYSIS OF GRANOPHYRE/GABBRO CONTACT FROM THE DULUTH COMPLEX

The Duluth Layered Series (DLS) is a layered mafic intrusion within the 1.1 Ga Midcontinental Rift System (MRS). This igneous complex was emplaced into the North Shore Volcanic Group (NSVG) and is ~3-5 km thick. In between the DLS and the NSVG occurs the Anorthositic Series, a ~1 km thick body of almost pure plagioclase. The relationship between the anorthosite and the gabbro at the top of the DLS is disputed in literature; however, recent dating shows identical ages. This study reports small scale mineralogical and compositional variations occurring in granophyric pods sandwiched between the Duluth Layered Series (DLS) and the Anorthositic series.

Specifically, I cut 5 1” samples from a hand sample containing both granophyre and uppermost DLS. In some samples, the granophyric layer has a transitional border zone in which the granophyre appears to be intermixed with the surrounding gabbroic rock. The relationship between the granophyre and the gabbro at the top of the DLS is not well understood. Chemical and mineralogical changes in plagioclase, ilmenite, and sphene through a transect from the transitional border zone shows evidence of diffusion fronts which likely led to the formation of the granophyric pods.