Coastal Maine Magmatism

My trip to coastal Maine started in the Deer Isle granite complex. This Devonian granite complex exhibits a complex and highly diverse set of igneous textures that are the result of new magma injection and mixing. The base of the plutonic complex is dominated by cumulates of large alkali feldspars and abundant mafic enclaves. Many of the alkali feldspars are mantled by plagioclase (rapakivi texture). These grains settled to the base of the magma chamber. The rapakivi texture is generally attributed to a relatively isothermal decompression reaction (i.e. as the pressure decreases, the pluton cross the reaction line between alkali feldspar and plagioclase.
Plagioclase mantling alkali feldspar

Also seen throughout the Deer Isle granite is significant mingling of felsic and mafic magmas and composite dikes. These composite dikes are most interesting in that they preserve different generations of dike generation often alternating between felsic and mafic melts and although this intrusion suite was emplaced in a relatively short time frame (thousands of years?) these dikes have relatively sharp edges implying the viscosity of the surrounding magma was high enough not to allow significant mixing.

Small composite dike Geology in Maine is only to be had along the coast
Larger complex composite dike Magma mingling and mixing of mantled feldspars

Moving up the pluton, the large rapakivi feldspars and mafic enclaves disappear as we leave the cumulate pile. Replacing them are abundant ladder dike schlieren (layers of elongate concentrations of mafic minerals) and pods of quartz syenite. Fe content of the alkali feldspars also decreases as you move up in the pluton (indicated by the paling upwards in the color of the feldspar — red to pink/white).

Classic geologist pose (Austin Boles) with
ladder dike schlieren
Close-up of schlieren
The Deer Isle complex is a prime example of an open-system within a magma chamber. The systematic ‘up-pluton’ relationship reflects the presence of compositional and thermal zoning that underwent crystal settling during emplacement and injection of magmas during various stages of crystallization.No trip to the great state of Maine would be complete without a visit to the local lobster house. Hoping to make the best of the situation I asked the waitress for the biggest lobster they had and to my surprise she brought out a lobster much bigger than I was expecting — in fact bigger than I had ever seen.

Larry the 4 pound Lobster

The next stop on my trip was to the Gouldsboro Pluton. The Gouldsboro pluton is a Devonian age bimodal system that is best exposed on the Schoodic Peninsula northeast of Mount Desert Island. The Gouldsboro pluton can be internally divided into several major units based on both composition and textural features. Similar to the Deer Isle granite, the Gouldsboro pluton shows clear evidence of mafic-felsic interaction near base and transitions to miarolitic cavity rich and more evolved granite near the top. Also at the base of the pluton is a large “scatter zone” of the country rocks the Gouldsboro intuded.

Miarolitic cavities occur throughout the Gouldsboro Another clastic geologist pose (Mike Dorais and
Tara Allen) on the Gouldsboro pluton
(note: white spots are bird poop and not geology)
Shatter zone at the base of the Gouldsboro pluton Schist xenolith with granite intruding along foliations

While at the Gouldsboro pluton, my fellow field trippers also stopped to look at the the dikes associated with the Central Atlantic Magmatic Province. The Central Atlantic Magmatic Province is a large group of basalt dikes, sills, and lava flows formed at the beginning of the breakup of Pangaea.

Contact between a dike from the Central Atlantic
Magmatic Province (left) and Gouldsboro Pluton (right)
Although the CAMP dikes post-dated the Gouldsboro
pluton by 220 million years, there is still evidence for
brittle and ductile deformation

The CAMP is a very important province in the debate of whether the breakup of Pangea was controlled by a emplacement of a large mantle plume. Some researchers have presented geochemical data suggesting that magmas generated in the CAMP province display signatures indicative of a deep mantle source (i.e. mantle plume). This deep mantle source of CAMP is at stark contrast to that which resembles oldest mid-ocean ridge basalts in the Atlantic Ocean. These researchers show the plume-signature of the CAMP magmas is found near the center of CAMP magmatism and decreases away from the center of paleo-plume.

Map of CAMP (from Auburn University)

This however, is not the only viewpoint. The folks over at MantlePlumes.org claim there is a significant absence of features presumably required by a deep-mantle plumes. There is no circular uplift associated with the emplacement of a plume, nor evidence of a plume tail of Jurassic alkaline volcanic seamounts as commonly seen along other “mantle plumes” (i.e. Tristan de Cunha and Hawaii). Furthermore, they claim the supposed deep mantle signature presumes that mantle reservoirs are only found in the deep mantle. They site several recent studies that have presented convincing arguments to support upper mantle sources for similar components. Although this ‘camp’ is does not give universally accepted explanations for the New England seamounts (a supposed hotspot track), nor the radiating pattern of the  dikes surrounding the presumed plume center.

It is because of places like CAMP that debates of plumophiles and plumoclasts will never end. Good times.

Sunrise from Mount Desert (the first place in the United States to see the sun)

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