KEWEENAWAN VOLCANIC ROCKS OF THE GRANDVIEW-MINONG AREA, NORTHWESTERN WISCONSIN

Petrographic, petrochemical, and paleomagnetic analyses indicate that the volcanic sequence of the Grandview-Minong area is closely related to the Chengwatana Group of east-central Minnesota and the St. Croix Falls area. Both of these sequences probably formed within the same basin of accumulation during Keweenawan time. Eastward the Grandview-Minong volcanic sequence correlates mainly with Cooper's (1973) "unit 3" basalts of the Hurley-Upson area. These Grandview-Minong rocks consist mainly of massive and amygdaloidal basalts, with olivine, plagioclase, clinopyroxene, and iron oxides comprising the main primary minerals. Epidote, chlorite, iron oxides, quartz, calcite, and pumpellyite are the main secondary minerals. Burial of these basalts caused metamorphism of the prehnitepumpellyite facies that occurred at depths ranging from 7.3 to 13.9 km.;Olivine tholeiites form the most abundant type of basalt in the Grandview-Minong area, but quartz tholeiites are also common. Major and trace variations are similar to patterns found in most other continental flood basalts. Enrichment in light rare earth element is characteristic, and some of the least evolved basalts display a positive europium anomaly.;Petrogenetic studies suggest that one specimen (UW 1742/2) with an Mg-value of 0.67 may represent a parental magma for the Grandview-Minong volcanic sequence; this magma was probably generated by partial melting of mantle periodotite. Mixing calculations and a trace element fractionation model indicate that crystal fractionation is a likely explanation for the compositional changes observed within the volcanic sequence.;Limited paleomagnetic measurements indicate that the Grandview-Minong basalts are normally polarized and that the virtual geomagnetic pole is about 30.8(DEGREES)N. latitude, 161.4(DEGREES)E. longitude. This VGP is consistent with other normal polarity Keweenawan paleopole positions. This data, together with the petrochemical analysis, suggest that the studied basalts may correlate with the Portage Lake Lava Group of upper Michigan.;Structurally the Grandview-Minong basalts form a broad open anticlinal nose on the southeastern limb of the Lake Superior syncline. The Lake Owen fault is a major dip-slip reverse fault; its trace can be studied in the Davis Hill area south of Grandview. The trace of Atkins Lake-Crystal Lake fault is inferred to lie between the Middle Precambrian Ironwood Formation and the Keweenawan volcanic rocks.