Pegmatites (“Pegmatoids”) in lava flows

In Keweenawan flows pegmatite (or pegmatoid) layers are conspicuous, especially in the thicker flows.  They appear to be analogous to vesicle sheets that are found in most flood basalts, but they may result from more evolution during the longer solidification times.

This collection of beach cobbles shows obvious texture of Keweenawan lava pegmatite--note conspicuous plagioclase laths.  These layers have vesicular texture and are typically mineralized with zeolite facies minerals.

The thickest lava flows in the Keweenawan Portage Lake Volcanics contain horizons called “pegmatites”, “pegmatoids” or “dolerites”.  The following description of these features is from Longo (1984):

Lacroix (1928, 1929) coins the term ''pegmatitoide" to describe the coarse-grained zones considered to represent the final stages of differentiation in basaltic lavas of France. The lavas of Michigan's Copper Country show similar differentiates for which Lane (1893) applies the term "doleritic." Cornwall (1951) adopts the textural term "pegmatite" from the usage of Butler and Burbank (1929). He changed the confusing "doleritic" term to "pegmatitic facies, " and subsequently described such units in the Greenstone flow, Big Trap, and several other large flows within the PLV on the Keweenaw Peninsula. For the present study, the term "pegmatoid zone" from Lindsley et al. (1971) is adopted to encompass the portion of the Greenstone flow with numerous en echelon, lens-shaped pegmatoids, associated granophyric phases, and subophitic layers. Texturally, pegmatoids are coarse grained when compared to ophitic zones. Coarse plagioclase laths dominate with interstitial, subhedral clinopyroxene and abundant interstitial to somewhat poikilitic magnetite and ilmenite. Consequently, the pegmatoids are strongly magnetic compared to ophitic units. This suggests that a higher titaniferous magnetite/ilmenite ratio for magmatoids than for ophites. Visual inspection generally reveals a greater overall opaque (oxide) concentration in the pegmatoids. Subophitic layers are often found hosting the en echelon pegmatoids. These layers, like pegmatoids, are strongly magnetic and very coarse grained stratiform features, but contain less abundant, smaller sized pyroxene. The contacts between pegmatoids and subophitic units are usually sharp, although instances of gradational contacts have been observed. Subophitic layers grade into the ophites and seem to occupy the greatest volume of the pegmatoid zone. They have been observed to pinch out within pegmatoid units and may not be continuous planar features throughout the flow. Perhaps pegmatoid units are not only lens-shaped but also flattened amoeboid-like features interfingering with subophitic layers. The frequency of pegmatoids and subophitic layers increases proportionally with increasing flow thicknesses. Both vary in thickness and shape and typically occur in the upper half of a lava flow. Pegmatoids have also been observed as auto intrusions, such as in the entablature on Isle Royale and the upper ophite on the Keweenaw Peninsula. The stratiform pegmatoids are usually found armoring the tops of cliffs formed of the lower ophite. The extension of weak vertical joint patterns into the pegmatoid (forming crude large columns) suggests that pegmatoids may be part of the colonnade. In most cases pegmatoid zones separate a basal colonnade from an upper colonnade. Pegmatoids are not unique to thick flows of the PLV. Lindsley et at. (1971) assert that three of the thicker flows from the Picture Gorge Basalt contained pegmatoid lenses. Santin (1969) discusses the presence of pegmatoids in horizontal basalts of the Lanzarote and Fuerteventura Islands in the Canarian Archipelago.

--Longo 1984

Pegmatites are found to be especially well developed in thicker flows such as the Greenstone (pg), which can be more than 1200 ft thick. Pegmatite layers (up to 30 ft thick) are found above the flows midpoint at a stratigraphic layer analogous to the vesicle sheets near the top of the core of idealized pahoehoe flows as described by Self et al., 1998 (see Flow Structure). Cornwall shows a Greenstone flow section from the Keweenaw (see below).

In thinner flows pegmatite layers are thin (often a few cm) and resemble vesicle sheets (see figure below).


Columnar section through Greenstone Flow at Delaware, on the Keweenaw, showing Pegmatite layers (black) in upper part of the 1300 ft section (Cornwall, 1951).

Lower Ophite

Upper Ophite


Cornwall, 1951

Pegmatite layer within horizontally fractured section of 20-30 m thick flow on Raspberry Island.  This layer is about 5 m stratigraphically above segregation cylinders.

4 cm

Thin pegmatite or vesicle sheet from 6 m thick lava flow of Lake Shore Traps, Silver Island.

Below: Polished surface of a pegmatite boulder from the Greenstone Flow, Passage Island.