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Humboldt Field Research Institute

Posted March 30, 2000

Mt. Waldo Granite Study Yields New Clues About State's Geological Past

Detailed study of a Maine granite is contributing new evidence to explain how dynamic magma chambers characterized the state's distant geologic past. Daniel Lux, professor in the University of Maine Dept. of Geological Sciences, is collaborating with David Gibson of the Department of Natural Sciences at the University of Maine at Farmington on an analysis of the gray building stone known as Mount Waldo granite.

Lux presented a paper on their research at the Northeast regional meeting of the Geological Society of America in March.

The Mount Waldo granite is exposed at the surface over an area of about 150 square kilometers (58 square miles) west of Bucksport, Maine and extends into the earth to an estimated depth of about seven kilometers (four miles). In the past, the stone was widely used for building material, and abandoned quarries provide Lux and Gibson with a small window through which to understand the processes that created the granite more than 370 million years ago.

At that time, the tectonic plates that make up the Earth's crust were not located as they are today. The North American plate, on which Maine sits, was colliding with the smaller Avalon plate to the east. Remnants of the Avalon plate can still be found in coastal portions of modern day Newfoundland, Nova Scotia and sections of the U.S. East Coast. The collision forced the leading edge of the North American plate beneath the Avalon plate, where elevated temperature and pressure caused the rock to melt. The resulting igneous activity was similar to what is happening today in the Pacific Northwest.

“We know that the granite exposed on Mount Waldo formed at a depth about ten kilometers (six miles) below the surface,” says Lux. “Since then overlying rock has been eroded away to expose the granite. My interest is what occurred within the magma chamber while the Mount Waldo granite was forming.”

“The chamber that was once filled with liquid magma is now solid granite. Through our field and laboratory research, we are attempting to unravel the sequence of the growth and physical sorting of crystals within the magma chamber and to understand what chemical exchanges might have occurred between crystals and liquid.”

One group of geologists who study igneous processes has generally thought that magma, once concentrated in a chamber, simply cools and solidifies. However, recent evidence from other volcanically active areas, such as Yellowstone Park, suggests that some magma chambers are anything but stagnant. Lux and Gibson are adding to the latter view.

They report that crystals of a common rock forming mineral known as plagioclase feldspar, which is observed in the Mount Waldo granite, have distinctive growth histories. Initially, some of the plagioclase crystals grew and then were partially dissolved whereas others just grew. In the later stages of the crystallization process, the crystals were transported within the chamber and deposited together on the chamber floor.

In addition, Lux and Gibson have found distinct mineralogic layers, a feature not commonly observed in granites, which have significant differences in chemical composition. Taken together, these layers suggest that after the Mount Waldo magma was in place and cooling, new magmas with variable compositions were injected into the chamber.

“Mineral crystals within a magma behave in some sense like snow flakes in a storm cloud,” says Lux. “Snow flakes are blown about as they grow. Each snowflake is different and their shape reflects growth in variable microenvironments before they accumulate on the ground. Similarly, crystals in granite start to form in a cooling magma chamber. New injections of hot magma may partially melt some of the growing crystals and transport them to different locations. The variable shapes and textures of the crystal reflect this dynamic environment.”

The question is whether or not repeated injection of hot magmas is a fundamental process common to all magma chambers or restricted to the Yellowstone magmas and a few other examples.

Lux and Gibson have worked together since the mid-1980s when Gibson was a post-doctoral researcher at UMaine.

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