VESTIGES OF RODINIA: ADIRONDACK AND HUDSON HIGHLANDS

The Adirondack Mountains are a massif (an uplifted area) of old rock. Unlike the Appalachians or Rockies, the Adirondacks are not an elongate mountain belt but are a circular dome caused by then unwarping of continent basement and erosion of overlying younger rocks. The Adirondacks are largely composed of such metamorphic rocks as gneiss (feldspar-, quartz-, and mica-rich rocks with minerals segregated into layers) and minor marble (metamorphosed limestone). These rocks are composed of minerals formed at temperatures and pressures associated with burial to 19-25 miles (30-40 km). Such depths of burial are produced in Himalayan-type continent-continent collisions where one continent overrides another.

The Adirondacks are a small outcrop area of the Grenville orogen (mountain belt). Grenville rocks extend for 3,100 miles (5,000 km) underground from the eastern United States to Mexico and from northern Lake Huron across Ontario to Labrador, southern Greenland, and areas now separated form North America. All of these areas show a date of 1,100 mya determined by radiometric dating of microscopic zircon crystals produced during metamorphism and mountain building. Anorthosite, an unusual light-colored rock composed largely of the feldspar andesine, underlies large areas of the Adirondack High Peaks and may have been injected as liquid rock early in the Grenville orogeny. Gore Mountain garnet, formed with anorthosite metamorphism, is the state gemstone.

Older dates of 1,300 mya are known on zircons in metamorphosed volcanic rocks in the Adirondacks. These dates indicate the time of volcanism and, likely, the age of such associated metamorphosed sedimentary rocks as quartzites (metamorphosed sandstones) and marbles. Marble-rich areas in the Adirondacks indicate deposition of tropical limestones 1,300 mya. Marble is less resistant to erosion than gneiss and forms valleys; the Fulton Chain of Lakes Lowland is marble belt.

Reconstructing the Grenville orogeny is difficult; it is unclear what continents collided at 1,100 mya. Old, rounded zircons in Adirondack metasedimentary rocks have 2,700 mya ages. This is the age of mountain building in the Superior orogen of the northern Midwest and Ontario, and these zircons were eroded from old rock in the heartland of ancestral North America. Reworked zircons with 2,075 mya and 1,500 mya ages are best known in the northern Amazon Basin. These data suggest that Amazonian rocks were eroded to supply the zircons; it is possible that the southwestern Hudson Highlands lay near a collision zone between ancestral North America and a continent that included much of South America. This collision may have produced the Grenville orogeny and the rocks of the Adirondacks and Hudson Highlands. The Grenville orogeny was an early stage in the assembly of the supercontinent Rodinia. No record of the next 550 million years is known in this region, but this time included erosion of the Grenville mountains and the removal of 19-25 miles (30-40 km) of Grenville rock down to about its present level. It is unknown where the sediment eroded from the Adirondack Mountains and Hudson Highlands in the Grenville belt was deposited.


figure 3
Development of Magmatic Arc and Back Arc Rift


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