Isotopic dating of minerals from sample no 27, quartz-diorite, SE1/4, NE1/4 sec 18, T38N, R20E, Marinette Co, WI (outcrop on Chicago-Milwaukee RR grade, south of County Road N). Biotite: K-Ar 1,370 m.y.; Rb-Sr 1,350 m.y. Hornblende: K-Ar 1,580 m.y. Zircon: Th232-Pb208 1,680 m.y.; U238-Pb206 1,600 m.y.; U235-Pb206 1,720 m.y.; Pb207-Pb206 1,870 m.y. Diffusion age: U-Pb 1,930 m.y. Range of dates demonstrates variable resistance to post-formation thermal events shown by various minerals and decay systems tested. Assigned to Middle Precambrian.

Source: GNU records (USGS DDS-6; Reston GNULEX).

Named for Marinette Co, WI in the Lake Superior region where it forms a pluton about 12 sq mi in extent and is considered part of the Wisconsin batholith. Called biotite-hornblende-quartz diorite in earlier reports. Good exposures occur south of Niagara, WI in NW1/4 sec 22 and SW1/4 sec 15, T38N, R20E along the Chicago, Milwaukee, St. Paul, and Pacific RR in S1/2 NE1/4 sec 18, T38N, R20E. No type locality specifically stated. Composed of medium-grained, massive or slightly foliated quartz diorite. Consists of 40-50 percent oligoclase, 0-20 percent potassium feldspar, 10-30 percent quartz, and 20-30 percent prochloride, biotite, or hornblende. Is hypidiomorphic to allotriomorphic granular in texture consisting of large subhedral to anhedral plates of plagioclase, and flakes of biotite or chlorite, and finer grained interstitial grains of quartz and potassium feldspar. Oligoclase shows normal zoning, and has cores as calcic as andesine. Plagioclase is partly altered to sericite or epidote. Age not known with certainty. Intrusive relations show the Marinette to be younger than Quinnesec Formation of early Precambrian age. Intruded by Hoskin Lake Granite. Assigned to the Precambrian.

Source: GNU records (USGS DDS-6; Reston GNULEX).