Acadia National Park – Paleozoic rocks 2

In the Devonian a whole series of intrusive igneous rocks formed in what is now Acadia (intrusive rocks are igneous rocks that cooled below ground, without erupting at the surface).

The oldest of these intrusive rocks is a gabbro-diorite exposed mostly in the northeast part of Mount Desert Island north of Bar Harbor. These are dark-colored rocks, rich in iron and relatively low in silica (the light-colored streaks are later veins of high-silica minerals):

After the gabbro is a series of three apparently independent plutons (intrusive rock bodies), all from the late Devonian. The oldest of these plutons is an unnamed unit mostly exposed along the coast in close proximity to the Cranberry Island volcanics:

This is a very small-grained granite (that is, the crystals in the rock are small). As the contact between this granite and the older Carnberry Island rocks was only about 50 meters away, I wonder if the grain size is so small because this is a chilled margin of the pluton.

The second pluton is also the largest, the Cadillac Mountain Granite:

There are occasional xenoliths in the Cadillac Mountain (xenoliths are chunks of the older “country” rock that the pluton intruded, which broke off into the plutonic magma before it hardened:

One of the most fascinating geologic features in Acadia is the “shattered zone”. This is an area surrounding the Cadillac Mountain Granite in which the country rock is broken into a breccia, with stringers of granite filling the spaces between the broken pieces of rock:

The shattered zone is the very edge of the magma chamber. As the Cadillac Mountain magma forced its way into the older country rock, it fractured the older rock and filled in the cracks. On the northeast edge of the pluton the shattered zone is made up of pieces of the gabbro; on the east and southeast sides it is made up of Bar Harbor Formation.

The third pluton is the Somesville Granite, found on the western part of the island:

The Somesville is finer grained than the Cadillac Mountain, and apparently has a somewhat different composition (it certainly seems to have less biotite). I’m not sure how it’s related to the Cadillac Mountain, and I wonder if there could be some type of fractionation going on (so that they’re both derived from the same magma).

There is one other igneous event that post-dates the Cadillac Mountain Granite. There are several diabase dikes cutting across the center of the pluton (a large one is visible just to the right of Brett):

Up close, a change in grain size is visible across the thickness of this dike. The crystals are tiny at the margins, where it was chilled by the granite, and get coarser toward the center where it cooled more slowly (the granite is on the right of this picture):

There is also a band of slight contact metamorphism in the granite (causing a color change), where it was heated by the magma in the dike (the dike is on the left):

This complex Paleozoic sequence has been interpreted (as described in Gilman et al.) as resulting from the early Paleozoic subduction of the Iapetus Ocean, followed by a collision between Laurentia (proto-North America) and another continent. This continent, Avalonia, apparently began as a volcanic arc in the Proterozoic, and then collided with Baltica (proto-Europe) before colliding with Laurenta in the Devonian.

The Ellsworth was apparently deposited off the coast of Avalonia and metamorphosed prior to the deposition of the Bar Harbor (since the Bar Harbor doesn’t show the same degree of metamorphism). Extrusive felsic rocks like those in the Cranberry Island Series are typical of volcanoes formed along subduction zones; rocks of this type are found all along the Appalachians beginning in the Ordovician as the Iapetus was subducted. During subduction and collision, magmas are produced at depth and can intrude the older rocks as plutons.

Avalonian rocks pop up in various coastal areas from Massachusetts to Newfoundland, as well as in Britain and the Iberian Peninsula (the rocks were split when the Atlantic Ocean formed). These rocks were identified originally based on trilobite faunas. The Cambrian faunas on each side of the Atlantic are the same in the Avalonian rocks, but differ in from the adjacent North American and European faunas. There are no fossils in the Bar Harbor and Ellsworth (except possible trace fossils in the Bar Harbor), so I assume they’re identified as Avalonian rocks based on lithology and geography.

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