File:Intergranular calcite in autobrecciated amygdaloidal tholeiite basalt aa lava flow top (flow B, Two Harbors Basalts, North Shore Volcanic Series, Mesoproterozoic, 1097-1098 Ma; Burlington Bay, Two Harbors, Minnesota, USA) 7.jpg

English: Intergranular calcite in autobrecciated amygdaloidal quartz tholeiite basalt aa lava flow in the Precambrian of Minnesota, USA.

Along the northern shore of western Lake Superior are numerous exposures of a lava flow-dominated succession called the North Shore Volcanic Series. This is equivalent to & the same age as the Portage Lake Volcanic Series of northern Michigan's Keweenaw Peninsula (www.flickr.com/photos/jsjgeology/albums/72157632266738191). The North Shore and Portage Lake successions are ~1.1 billion years old and represent basalt lava flows, plus minor sedimentary rocks, that filled up an ancient rift valley. This old rift is the Lake Superior segment of the Mid-Continent Rift System, a tear in the ancient North American paleocontinent of Laurentia (see: minerals.usgs.gov/science/midcontinent-rift-minerals/imag...). Tectonic rifting started along this tear, exactly like the modern-day East African Rift Valley. Laurentia's Mid-Continent Rift System started and then stopped and was subsequently filled and buried. This ancient failed rift is now exposed on either side of Lake Superior in North America's Great Lakes.

The outcrop shown above is along the shoreline of Burlington Bay at the town of Two Harbors, Minnesota. The rocks are quartz tholeiite basalts - they are part of the Two Harbors Basalts succession. Three lava flows are exposed at this site, designated flow A, flow B, and flow C in the geologic literature. The above photo is a close-up of the upper part of flow B. The basalt has been broken into angular clasts, forming an autobreccia (= dark brown areas). The basalt itself has numerous gas vesicles that have since been filled with minerals, resulting in amygdaloidal basalt. The top of this lava flow is rough, angular, and blocky. The originally-Hawaiian word "aa" is used to refer to lava flows with such surfaces. So, the full term for the rock unit at this outcrop is: autobrecciated amygdaloidal quartz tholeiite basalt aa lava flow.

The spaces between the angular clasts of the basalt autobreccia are filled principally with the salmon-colored mineral laumontite. Laumontite is one of the zeolite minerals. Zeolites are often found filling or partially filling cavities in volcanic rocks. Laumonite is a hydrous calcium aluminosilicate mineral, CaAl2Si4O12·4H2O.

The intergranular spaces also have some coarsely-crystalline calcite (CaCO3 - calcium carbonate), which is the blocky material in the above photo.

Stratigraphy: flow B, Two Harbors Basalts, Upper Southwest Sequence, North Shore Volcanic Series, Keweenawan Supergroup, upper Mesoproterozoic, ~1097-1098 Ma

Locality: shoreline exposure on the western side of Burlington Bay, eastern side of the town of Two Harbors, northeastern Minnesota, USA