Blueschist (/ˈbluːʃɪst/), also calledglaucophane schist, is ametavolcanic rock[1] that forms by themetamorphism ofbasalt and rocks with similar composition at highpressures and lowtemperatures (200–500 °C (392–932 °F)), approximately corresponding to a depth of 15–30 km (9.3–18.6 mi). The blue color of the rock comes from the presence of the predominant mineralsglaucophane andlawsonite.
Blueschists areschists typically found withinorogenic belts asterranes oflithology infaulted contact withgreenschist or rarelyeclogitefacies rocks.
Blueschist, as a rock type, is defined by the presence of the mineralsglaucophane + (lawsonite orepidote ) +/-jadeite +/-albite orchlorite +/-garnet +/-muscovite in a rock of roughly basaltic composition.
Blueschist often has alepidoblastic,nematoblastic orschistoserock microstructure defined primarily bychlorite, phengitic whitemica, glaucophane, and other minerals with an elongate or platy shape.
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 facies is determined by the particular temperature and pressure conditions required to metamorphose basalt to form blueschist.Felsic rocks andpelitic sediments which are subjected to blueschist facies conditions will form different mineral assemblages than metamorphosed basalt. Thereby, these rocks do not appear blue overall in color.
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 >0.6GPa, 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 theFranciscan facies series, after the west coast of the United States where these rocks are exposed. Well-exposed blueschists also occur inGreece,Turkey,Japan,New Zealand andNew Caledonia.
Continuedsubduction of blueschist faciesoceanic crust will produceeclogite 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 entergreenschist oreclogite 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 typicalgeothermal gradient.
Blueschists and other high-pressure subduction zone rocks are thought to be exhumed rapidly by flow and/or faulting inaccretionary 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 theNeoproterozoic Era indicates that currentlyexhumed rocks never reached blueschist facies atsubduction zones before 1,000 million years ago. This assertion is arguably wrong because the earliestoceanic crust would have contained moremagnesium than today's crust and, therefore, would have formed greenschist-like rocks at blueschist facies.[2]
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InMinoan Crete blueschist andgreenschist 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.[3]
In 1962,Edgar Bailey of theU.S. Geological Survey, introduced the concept of "blueschist" into the subject of metamorphicgeology. His carefully constructed definition established the pressure and temperature conditions which produce this type of metamorphism.
