Low-temperature metamorphism and secondary components in the Portage Lake Volcanics: a reassessment

Abstract

We studied the effects of hydrothermal alteration on magnetic properties of the magnetite and hematite components in the 1.1 Ga Portage Lake Volcanics, Upper Peninsula, Michigan. Results show the importance of the intensity and direction of the hematite magnetization in the detection of secondary magnetite overprints in basalts. We collected samples from 20 flows in two stratigraphic sections, one from outcrops and the other from drill core. The remanence unblocked below 590 °C during thermal demagnetization is considered to be due to magnetite, and that unblocked between 610–690 °C is considered to be due to hematite. Hematite remanent intensities in the flow interiors are an order of magnitude less intense than those of magnetite, and correlate with magnetite intensities from the top of the volcanics to the lower prehnite–pumpellyite zone (5.5 km depth) in the outcrop section and to the epidote–pumpellyite zone (7.0 km depth) in the core section. These magnetic properties are very similar to those of Recent Icelandic basalt flows. Below these depths, magnetite intensities decrease until they are equal to hematite intensities. Hematite remanent intensities do not consistently increase with depth, suggesting that secondary hematite produced during hydrothermal alteration is a minor contributor to hematite intensities. The magnetite directions become distinct from the hematite directions in the prehnite–pumpellyite zone (6.2 km depth). This difference in directions indicates an unresolved secondary overprint formed during hydrothermal alteration. We conclude that the magnetite component carries a secondary overprint; a conclusion that contrasts with that of a previous study. Thermo-viscous remanent magnetization obtained over a time period of 1–10 Ma near 300 °C would be enough to account for much of the secondary component.