Abstract

The definition of a lopolith as a large planar-convex (downward) intrusion was coined by F.F. Grout one hundred years ago for the proposed shape of the Duluth gabbro. Subsequent research has challenged the concept that it is a single body, and that it has the lateral extent (under Lake Superior) and shape originally proposed. Other large basic intrusions have shapes, especially for their lower contacts, that are difficult to constrain, and none can be convincingly shown to be of the proposed shape of a lopolith. Their inferred or proposed shapes range from wedge to funnel to planar shaped, with a rarely exposed vertical feeder, and with angles to the sides that vary from minimal to very steep, and variably contorted surface plan. If no intrusions fit the definition, should the term lopolith be discontinued?

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