Product Code Database
Blueschist (), also called glaucophane schist, is a metavolcanic rock that forms by the metamorphism of basalt and similar rocks at relatively low () but very high pressure corresponding to a depth of . The blue color of the rock comes from the presence of the predominant minerals glaucophane and lawsonite. This combination of low temperature occurring at significant depth can only be explained in the context of plate subduction, followed by exhumation, which accounts for the rarity of this rock.
Blueschists are typically found within orogeny as of lithology in faulted contact with greenschist or rarely eclogite facies rocks.
Grain size is rarely coarse, as mineral growth is retarded by the swiftness of the rock's metamorphic trajectory and perhaps more importantly, the low temperatures of metamorphism and in many cases the anhydrous state of the basalts. However, porphyritic varieties do occur. Blueschists may appear blue, black, gray, or blue-green in outcrop.
Blueschist mineralogy varies by rock composition, but the classic equilibrium assemblages of blueschist facies are:
Blueschist facies generally is considered to form under pressures of >0.6 gigapascal, equivalent to depth of burial in excess of 15–18 km, and at temperatures of between 200 and 500 °C. This is a 'low temperature, high pressure' prograde metamorphic path and is also known as the Franciscan facies series, after the west coast of the United States where these rocks are exposed. Well-exposed blueschists also occur in Greece, Turkey, Japan, New Zealand and New Caledonia. Continued subduction of blueschist facies oceanic crust will produce eclogite facies assemblages in metamorphosed basalt (garnet + omphacitic clinopyroxene). Rocks which have been subjected to blueschist conditions during a prograde trajectory will gain heat by conduction with hotter lower crustal rocks if they remain at the 15–18 km depth. Blueschist which heats up to greater than 500 °C via this fashion will enter greenschist or eclogite facies temperature-pressure conditions, and the mineral assemblages will metamorphose to reflect the new facies conditions.
Thus in order for blueschist facies assemblages to be seen at the Earth's surface, the rock must be exhumed swiftly enough to prevent total thermal equilibration of the rocks which are under blueschist facies conditions with the typical geothermal gradient.
Blueschists and other high-pressure subduction zone rocks are thought to be exhumed rapidly by flow and/or faulting in accretionary wedges or the upper parts of subducted crust, or may return to the Earth's surface in part owing to buoyancy if the metabasaltic rocks are associated with low-density continental crust (marble, metapelite, and other rocks of continental margins).
It has been held that the absence of blueschist dating to before the Neoproterozoic indicates that currently exhumed rocks never reached blueschist facies at subduction zones before 1,000 million years ago. This assertion is arguably wrong because the earliest oceanic crust would have contained more magnesium than today's crust and, therefore, would have formed greenschist-like rocks at blueschist facies.
In 1962, Edgar Bailey of the U.S. Geological Survey, introduced the concept of "blueschist" into the subject of metamorphic geology. His carefully constructed definition established the pressure and temperature conditions which produce this type of metamorphism.
|
|
