Green Stone Belt

GREEN STONE BELT Greenstones are ancient metamorphosis rocks unique to Archeans. Earth's crust was formed in an explosion of creation that began 4 billion years ago, just a few million years after the formation of the planet itself. The early crust consisted of a thin layer of basalt embedded with scattered granite blocks called "rockbergs." Granite merges into a stable body of underground rocks, where all other rocks are deposited. Bedrock became the core of the continent and is currently exposed in a vast, low, dome-like dome structure called a shield (Fig. 23). The Precambrian Shield is a vast raised area surrounded by sediment-covered bedrock called a continental platform. It is a broad and shallow depression of a basement complex (crystalline rock) filled with almost flat sedimentary rocks. The most famous areas are the Canadian Shield, which covers most of eastern Canada and extends to Wisconsin and Minnesota in North America, and the Fennoscandian Shield, which covers most of Scandinavia in Europe. More than a third of Australia's territory is the Precambrian shield. Considerable shields exist in Africa, South America, and Asia as well. Many shields are fully exposed where flowing ice sheets eroded their sedimentary cover during the Pleistocene ice age. The exposure of the Canadian Shield from Manitoba to Ontario is associated with crustal uplift by mantle plumes and erosion of sediments in uplifted areas. Some of the oldest known rocks in North America are 2.5-billion-year-old granite from the Canadian Shield. The shield is associated with a belt of greenstone stones (Fig. 24). It is a mixture of metamorphosed (recrystallized) lava flows and sediments that may have come from island arcs (chains of volcanic islands at the edge of deep-sea trenches) trapped between colliding continents. Although no major continents existed during this time, the foundations on which they formed were present as protocontinents. This small landmass is separated by sea basins that accumulate lava and sediments mostly from volcanic rocks which then metamorphose into a belt of greenstone. The greenstone belt occupies the ancient core of the continent. They span several hundred square miles, surrounded by vast expanses of gneiss, which are the metamorphic equivalent of granite and the dominant Archean rock type (Fig.25). Its color comes from chlorite, a greenish mineral, similar to mica. The existence of greenstone belts is evidence that plate tectonics (crustal plate interactions) may have operated as early as the Archean, about 2.7 billion years ago, with small tectonic plates colliding with each other as much as 4 billion years ago. The most famous greenstone belt is the Swaziland order in the Land of Barberton Mountain in southeast Africa. It is more than 3 billion years old and reaches a thickness of nearly 12 miles. Trapped within the Archean greenstone belt are ophiolites, from the Greek word ophis meaning "serpent." They are slices of seafloor pushed into continents by drifting plates and are 3.6 billion years old. Pillow lava (Fig. 26) is a tubular body of basalt extruded under the sea. They also appear in the greenstone belt, signaling that a volcanic eruption occurred on the ocean floor. Thus, these deposits are one of the best evidence that plate tectonics operated in the early Archeans. Indeed, plate tectonics appears to have worked throughout most of Earth's history in the same way it does today. The greenstone belts are of particular interest to geologists and miners because they hold most of the world's gold deposits. Archean age ores are very similar all over the world. Mineralized veins are Archean in age or they invade Archean rocks later in life. Archean-era gold was mined on every continent except Antarctica. In Africa, the best gold deposits are in rocks as old as 3.4 billion years. Most of South Africa's gold mines are in the greenstone belt. India's Kolar greenstone belt, formed when two plates fell together about 2.5 billion years ago, holds the world's richest gold deposits. In North America, the best gold mines are in the Great Slave region of northwestern Canada, where more than 1,000 deposits are known. Most of the gold is in the greenstone belt that is attacked by hot magmatic solutions that come from the intrusion of granite bodies into the greenstone. Gold occurs in blood vessels associated with quartz. Since the greenstone belts have no equivalent in modern geology, the geological conditions under which they formed were very different from those observed today. Active tectonic forces (landform buildings) in the mantle often open up thin Archean crust and inject magma into deep crustal fracture zones. Such large-scale magmatic intrusions along with numerous large meteorite impacts mark the unusual Archean geology. Because the greenstone belts are geologically unique to Archeans, their absence from the geological record about 2.5 billion years ago marks the end of the eon.