Verde antique
Part of the Taconic Terrane?
This little vase, only 7 cm high, represents a lot of geologic history. It’s mostly serpentine, a metamorphic rock made of magnesium silicate minerals, laced with veins of white calcite. In the artistic stone trade, this is called verde antique (“ancient green”). It is such a dark green that it is nearly black.
Material like this occurs in various parts of the world, and I’m not certain where this carved vase came from, but the most likely location is Vermont where verde antique has been quarried since 1858 around Roxbury and Rochester in the Green Mountains of central Vermont (whose name means “green mountain”).
About 460 million years ago (Middle Ordovician time) a complex island arc was colliding along a more or less north-south zone in what is now New England. “Mainland” North America’s edge was at the east side of the Adirondacks of New York. As the east-to-west pushing collision proceeded, oceanic crust between North America and the island arc became incorporated into slices tens of kilometers long and a few kilometers wide, and the whole works was added to (accreted to) North America in a mountain-building event that lasted at least 60 million years. The action was dominated by thrust faults pushing the slabs of oceanic crust, island arc, and piles of sediment and sedimentary rocks to the west. We call it the Taconic Orogeny, and it was the first really major pulse of tectonic activity in the long-lived deformation that taken together add up to the Appalachian Mountain Systems. The bits of oceanic crust were very mafic (magnesium-iron bearing) rocks like basalt and gabbro, and even ultramafic (“beyond mafic” – even more rich in iron and magnesium), pretty standard oceanic crust.
The Taconic mountains of Late Ordovician time were probably pretty high, but 80 million years of erosion had probably worn them down to moderate hills by Late Devonian time. What is now New England was ready for its next “event,” about 385 to 370 million years ago, when a long, narrow strip of continent called Avalonia began to collide with eastern North America, including along zones in New England pretty near the focus of the older Taconic Orogeny. We call this one the Acadian Orogeny for its development in the Canadian Maritimes, an area called Acadia by the French. Avalonia takes its name from the Avalon Peninsula of Newfoundland, which comprises part of the old continental fragment.
Avalonia probably rifted off the northwestern edge of Gondwana, the part that is northwestern Africa and northern South America today. Such “ribbon continents” are actually pretty common over geologic time. A present-day example might be something like the long narrow strip including Japan, Sakhalin Island, the Kurile Islands, and Kamchatka, a package that includes continental fragments, volcanic island arcs, oceanic crust, and more.
This time, with true continental material encroaching in addition to relatively weak island arcs and oceanic crust, the deformation (folding and faulting) and metamorphism (changing rocks under extreme heat and pressure) was probably greater than in the Taconic Orogeny, and the Acadian collision was also geographically far more extensive, from the Maritime Provinces of Canada to Alabama, although the southern portion in the Carolinas and beyond may have been a discrete microcontinent.
In Vermont, those slices of oceanic crust and the older bits of the Taconic Terrane were metamorphosed into greenstone containing serpentine, a magnesium-bearing rock that is more hydrated than the original basalt. The ongoing collision broke those rocks after they were metamorphosed, forming an angular boxwork (a rock called breccia, from words meaning “break”) of serpentine and related minerals, and in some of the cavities in the boxwork, calcite, probably derived from limestone, was deposited. The intricate lacy calcite veins cross-cutting the dark green serpentine give the unique texture of verde antique that you see in the little carved vase in my photo.
If the vase is from the verde antique quarries near Roxbury, Vermont, the material is probably from the Stowe Formation, with a heritage that may be as old as Cambrian to Paleoproterozoic (530 to 550 million years ago), but added to North America and perhaps metamorphosed during the Taconic Orogeny 460 million years ago, then metamorphosed more strongly during the Acadian Orogeny 385 to 370 million years ago.
Acadian orogeny serpentine also found alongside Loch Lomond (yes the bonnie banks) along the Highland Boundary Fault. I've read suggestion that the streaky calcite may represent direct sequestration of CO2 from atmosphere. What do you think on that one?
Looks very familiar!😊