VOLCANISM AND VOLCANIC ROCKS

In this area, the beginning of active volcanism, estimated to be about 1109 Ma (Davis and Sutcliffe, 1985), is marked by high-alumina basalts of the Siemens Creek Volcanics of the Powder Mill Group, which spilled out directly from fissures onto the wet sands of the Bessemer Quartzite. The first 50 to 100 m of the Siemens Creek Volcanics are pillowed, but all following volcanic eruptions of the rift were subaerial.

Above the Siemens Creek Volcanics is a thick section of mafic to felsic rocks, stretching more than 100 km along strike from central Wisconsin into Michigan. These are the Kallander Creek Volcanics, made up of basalts, andesites, and rhyolites which are postulated to be the remnants of a central volcano that erupted between 1108 and 1099 Ma (Cannon and others, 1993a; Zartman, unpublished to date).

The Mellon Complex intrudes into the Kallander Creek Volcanics in north central Wisconsin.

This complex consists of several layered gabbroic intrusions and other related intrusions, including a cross-cutting granitic body, a series of gabbro and granophyre sills, and a small peridotitic body. UPb zircon dates from a granophyre and the Mellen Granite are indistinguishable from one another at 11022 Ma (Cannon and others, 1993a).

The Kallander Creek Volcanics are overlain by a voluminous section of olivine tholeiite flood basalts that erupted within the rapidly subsiding rift. These basalts, now beneath Lake Superior, are called the Chengwatana Volcanics in western Wisconsin and Minnesota and the Portage Lake Volcanics in eastern Wisconsin and Michigan. Where exposed on the Keweenaw Peninsula of Michigan, the Portage Lake Volcanics have a thickness of about 5 km, although the bottom of the section is cut off by the Keweenaw fault. Near the rift axis the Portage Lake Volcanics and older basalts are estimated to be more than 20 km thick based on seismic reflection profiles (Cannon and others, 1989). High precision UPb dates on thick flows near the top and bottom of the Portage Lake Volcanics section indicate that most of the section was erupted between about 1096 and 1094 Ma (Davis and Paces, 1990). Interflow sediments of coarse immature conglomerates and sands deposited by streams flowing towards the rift center record hiatuses in volcanic activity (Merk and Jirsa, 1982). There are 22 prominent interflow sedimentary layers interspersed among the Portage Lake basalt flows. Several of these sedimentary layers are marker beds that may be traced more than 100 km along strike (Butler and Burbank, 1929).

The Porcupine Volcanics, a 5 km thick sequence of basalt, andesite, felsite, and rhyolite, erupted from a large shield volcano that became active late in the volcanic history of the rift (Nicholson et al., 1991; Cannon and others, 1992). This eruptive activity marked the end of major volcanism in the region; later events in the rift were dominated by fluvial and lacustrine sedimentation.