Amygdaloidal basalt from the Precambrian of northern Michigan, USA.
The Portage Lake Volcanic Series is an extremely thick, Precambrian-aged, flood basalt deposit that fills up an ancient continental rift valley. This rift valley, analogous to the present-day East African Rift Valley, extends from Kansas to Minnesota to the Lake Superior area to southern Michigan. Unlike many flood basalts (e.g., Deccan Traps, Siberian Traps, Columbia River), the Portage Lake only filled up the rift valley. The unit is exposed throughout Michigan's Keweenaw Peninsula, in the vicinity of the towns of Houghton & Hancock.
The Portage Lake succession thickens northward through the Keweenaw, up to 5.5+ kilometers of section in places. The dominant rock type is basalt - vesicular basalts, for the most part. Most of the original vesicles (gas bubbles) have since been filled up with a wide variety of different minerals. A vesicular basalt with its vesicles subsequently filled with secondary minerals is called an amygdaloidal basalt. Keweenaw amygdaloidal basalts have long had significant economic importance because native copper (Cu) is one of the more common vesicle-filling and fracture-filling minerals. Keweenaw has (had) the highest concentration of native copper anywhere on Earth. Numerous Keweenaw-area copper mines have exploited these cupriferous amygdaloidal basalts. The copper mines are no longer operating.
Basalt is the not the only lithology in the Portage Lake succession - coarse-grained siliciclastics (conglomerates, sandstones) were occasionally deposited atop the basalts between lava flow events. These beds are fairly similar to the coarse-grained siliciclastics in the overyling Copper Harbor Conglomerate.
The rock seen here is a typical Portage Lake amygdaloidal basalt. The dark-colored material is basalt. The lighter- and greenish-colored spots and masses are various minerals that have filled up former vesicles in the lava.
Stratigraphy: Portage Lake Volcanic Series, Bergland Group, middle Keweenawan Supergroup, upper Mesoproterozoic, 1.093 to 1.097 Ga
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