Age and genesis of quartz-porphyry near White Pine, Michigan

Paleomagnetism of some Lake Superior Keweenawan rocks

“…Figure 22 shows the mean direction for this quartz porphyry before (A) and after (A") a 35° correction for dip of the underlying lava flows .. The proximity of A" to the average direction for the Portage Lake Lava Series and Copper Harbor Conglomerate is consistent with the interpretation of Brooks and Garbutt (1969), but not inconsistent with intrusion during Portage Lake or Copper Harbor time before much tilting had occurred. The paleomagnetic data are not, however, consistent with the Rb-Sr (rubidium-strontium) age of 978±40 ;m.y.…”

“…Dip of the lower contact is approximately 35° NW. (Brooks and Garbutt, 1969), and data in figure 8 and basalts, but they are lithologically distinct (Hubbard, 1968, p. 35) from the middle Keweenawan basalts farther north. In the Marenisco, Mich., area the South Trap Range basalts lie unconformably upon the Animikie (Van Hise and Leith, 1911, p. 234;Fritts, 1965) and near Ironwood they dip 60° to 85° northward.…”

“…A large body of quartz porphyry within the lava series north of Lake Gogebic, Mich. ( fig. 1), has long been regarded as an intrusion, but Brooks and Garbutt (1969) have recently presented evidence that the body is extrusive and separated from overlying mafic lava flows by an unconformity. Figure 22 shows the mean direction for this quartz porphyry before (A) and after (A") a 35° correction for dip of the underlying lava flows ..…”

Preliminary report on the bedrock geology and copper deposits of the Matchwood quadrangle, Ontonagon County, Michigan

“…Houghton, Keweenaw County). Brooks and Garbutt (1969) present even more convincing evidence that the upper contact is an erosional surface in the Bergland quadrangle. A second argument for an extrusive origin is based on the thickness of the unnamed formation: if the porphyritic felsite is assumed to be extrusive and to form the basal unit of the formation, the thickness of the formation increases regularly from about U,000 feet at the Victoria section to about 8,000 feet at the Onondaga section; if the felsite is intrusive, on the other hand, the thickness of the unnamed formation increases from U,000 to 6,000 feet and then decreases to 3,000 feet in this same distance.…”