Greenschists aremetamorphic rocks that formed under the lowest temperatures and pressures usually produced byregional metamorphism, typically 300–450 °C (570–840 °F) and 2–10 kilobars (29,000–145,000 psi).[1] Greenschists commonly have an abundance of green minerals such aschlorite,serpentine, andepidote, andplaty minerals such asmuscovite and platy serpentine.[1] The platiness gives the rockschistosity (a tendency to split into layers). Other common minerals includequartz,orthoclase,talc,carbonate minerals andamphibole (actinolite).[1]
Greenschist is a general fieldpetrologic term formetamorphic or alteredmaficvolcanic rock. In Europe, the termprasinite is sometimes used. Agreenstone is sometimes a greenschist but can also be rock types without any schistosity, especially metabasalt (spilite). However, basalts may remain quite black if primarypyroxene does not revert to chlorite or actinolite. To qualify for the name, a rock must also exhibit schistosity or some foliation or layering. The rock is derived frombasalt,gabbro or similar rocks containingsodium-richplagioclasefeldspar, chlorite, epidote and quartz.[2]
Greenschist is defined by the presence of the mineralschlorite,epidote, oractinolite, which give the rock its green color. Greenschists also have pronouncedschistosity.[3] Schistosity is a thin layering of the rock produced bymetamorphism (afoliation) that permits the rock to easily be split into flakes or slabs less than 5 to 10 millimeters (0.2 to 0.4 in) thick. This arises from the presence of chlorite or other platy minerals that become aligned in layers during metamorphism.[4][5]Greenschist may also containalbite and often has a lepidoblastic, nematoblastic or schistosetexture defined primarily by chlorite and actinolite. Grain size is rarely coarse, due primarily to the mineral assemblage. Chlorite and to a lesser extent actinolite typically exhibit small, flat oracicular crystal habits.
Greenstone is afield term for any massivemafic volcanic rock that has been altered to a greenish color by the formation of the same minerals that give the green color to greenschist, whether or not the rock displays schistosity.[6] The term has also been used to describe any igneous intrusions into theCoal Measures Group of Scotland, to describechamosite-richmudstone ofEarly Jurassic age in Great Britain, or fornephrite or other greenish gemstones.[7]
Greenschist facies is determined by the particular temperature and pressure conditions required to metamorphose basalt to form the typical greenschist facies minerals chlorite, actinolite, and albite. Greenschist facies results from low temperature, moderate pressure metamorphism. Metamorphic conditions which create typical greenschist facies assemblages are called theBarrovian Facies Sequence, and the lower-pressureAbukuma Facies Series. Temperatures of approximately 400 to 500 °C (750 to 930 °F) and depths of about 8 to 50 kilometres (5 to 31 miles) are the typical envelope of greenschist facies rocks.
The equilibrium mineral assemblage of rocks subjected to greenschist facies conditions depends on primary rock composition.[8]
In greater detail the greenschist facies is subdivided into subgreenschist, lower and upper greenschist. Lower temperatures are transitional with and overlap theprehnite-pumpellyite facies and higher temperatures overlap with and include sub-amphibolite facies.
If burial continues along Barrovian Sequence metamorphic trajectories, greenschist facies gives rise toamphibolite facies assemblages, dominated by amphibole and eventually togranulite facies. Lower pressure, normallycontact metamorphism producesalbite-epidotehornfels while higher pressures at great depth produceseclogite.
Oceanic basalts in the vicinity ofmid-ocean ridges typically exhibit sub-greenschist alteration. Thegreenstone belts of the variousArcheancratons are commonly altered to the greenschist facies. These ancient rocks are noted as host rocks for a variety of ore deposits inAustralia,Namibia andCanada.
Greenschist-like rocks can also be formed underblueschist facies conditions if the original rock (protolith) contains enoughmagnesium. This explains the scarcity of blueschist preserved from before theNeoproterozoic Era 1000Ma ago when the Earth'soceanic crust contained more magnesium than today's oceanic crust.[9]
InMinoan Crete, greenschist andblueschist were used to pave streets andcourtyards between 1650 and 1600 BC. These rocks were likely quarried inAgia Pelagia on the north coast of central Crete.[10]
Across Europe, greenschist rocks have been used to make axes. Several sites, includingGreat Langdale in England, have been identified.
A form of chlorite schist was popular in prehistoric Native American communities for the production of axes andcelts, as well as ornamental items. In the MiddleWoodland period, greenschist was one of the many trade items that were part of theHopewell culture exchange network, sometimes transported over thousands of kilometers.[citation needed]
During the time of theMississippian culture, thepolity ofMoundville apparently had some control over the production and distribution of greenschist. The Moundville source has been shown to be from two localities in the Hillabee Formation of central and eastern Alabama.[citation needed]
