Blueschists

Blueschists are a class of metamorphic rocks that recrystallize at depths of 10 to 30 kilometers or more in subduction zones. Blueschists are important because they contain minerals indicating that metamorphism occurred under conditions of unusually high confining pressures and low temperatures. Their presence in mountain belts is the primary criterion for recognizing ancient subduction zones.

Mineral Content

Blueschists are a distinctive class of metamorphic rock containing one or more of the minerals lawsonite, aragonite, sodic amphibole (glaucophane), and sodic pyroxene (omphacite and jadeite plus quartz). These minerals indicate that recrystallization occurred in the temperature range of 150 to 450 degrees Celsius and at pressures of 3 to 10 kilobars or more (a kilobar corresponds to roughly 3 kilometers in depth). Blueschists of basaltic composition typically contain abundant glaucophane, a mineral that can give a rock a striking blue color. Other minerals commonly found in blueschists include quartz, mica, chlorite, garnet, pumpellyite, epidote, stilpnomelane, sphene, and rutile. The abundance of these and rarer minerals depends, as it does in all metamorphic rocks, upon rock composition, the exact pressures and temperatures of recrystallization, and the nature of chemically active fluids that have affected the rocks.

Structural Features

Blueschists in one place or another display a remarkably wide variety of structural features. In the field, many are complexly deformed, with intricate folding and refolding of compositional layering at scales of millimeters to tens of meters. Commonly, folded rocks also display a thickening and thinning of the layering, forming an interesting structure known as boudinage. Many blueschists are faulted, some so intensely that they are fragmented rocks known as breccias. When flaky minerals such as mica are lined up at the microscopic scale, a rock has a scaly foliation known as schistosity. A parallel alignment of rod-shaped amphiboles gives the rock a lineation. Blueschists typically have schistosities, and many also have lineations. The development of these features depends upon the extent to which the rocks deform while undergoing metamorphism (a geologic process known as dynamic metamorphism). Although most blueschists were so highly deformed during metamorphic recrystallization that all original features in the rocks were destroyed, some retain features from the rock’s premetamorphic history, such as ripple marks, delicate fossils, or volcanic flow layering. Blueschists bearing these features were recrystallized but not highly deformed. Hence, the diversity of minerals and deformational features of the class of rocks known as blueschists is great. Many are truly blueschists, but some are neither blue in color where glaucophane is lacking nor schistose in texture when deformation was minor. These rocks are grouped with blueschists because of the presence of distinctive minerals, and by association with other blueschist rocks.

Metamorphic Conditions of Formation

Metamorphic recrystallization near 200 degrees Celsius causes anorthite (calcium-rich plagioclase) in combination with water to recrystallize as lawsonite at approximately 3 kilobars. Under the same temperature, calcite transforms into aragonite at 5 kilobars, and albite (sodium-rich plagioclase) recrystallizes as jadeite plus quartz near 7 kilobars. The breakdown of albite is one of the most distinctive indicators of blueschist metamorphism. At higher temperatures, these changes occur at higher pressures. Experiments combined with other measures of metamorphic temperature conditions indicate that most blueschists were metamorphosed at temperatures of 150 to 450 degrees Celsius and minimum confining pressures of 3 to 10 kilobars, respectively. Metamorphic pressures of 3 to 10 kilobars correspond to burial depths for recrystallization of 10 to 30 kilometers. Thirty kilometers is near the base of typical continental crust. Ultra-high-pressure blueschists containing relics of the mineral coesite (a dense mineral having the same composition as quartz that forms at extremely high confining pressures) have been found in small areas of the Alps, Norway, and China. Laboratory experiments indicate that confining pressures of 25 to 30 kilobars are required to transform quartz to coesite. Although coesite-bearing blueschists are rare, their occurrence is very important because they indicate that some blueschists recrystallized at depths of 75 to 90 kilometers—depths in the earth that are very near the base of the lithosphere.

The mineralogy of blueschists indicates that the metamorphic conditions for their formation within the earth would be equivalent to geothermal gradients of 10 to 15 degrees Celsius per kilometer depth or less. Such ratios of temperature to depth do not exist in the interior of normal lithospheric plates, because geothermal gradients are typically 25 to 35 degrees Celsius per kilometer depth. The plate tectonic setting for the generation of blueschists is thus very unusual.

Subduction

Regional terranes of blueschist extending for hundreds of kilometers in length and tens of kilometers in width are found in California, Alaska, Japan, the Alps, and New Caledonia. Smaller bodies of blueschist that are probably remnants of once-extensive terranes are found at numerous other sites around the world. Blueschists are found as fault-bounded terranes juxtaposed against deposits of unmetamorphosed sediments, igneous batholiths, or sequences of basalt, gabbro, and peridotite thought to be fragments of ocean crust (ophiolites) or other metamorphic terranes. The common feature of all occurrences is that they are regions that were probably the sites of ancient lithospheric plate convergence, a tectonic process commonly known as subduction. Subduction carries surficial rocks into the depths of the earth, where the increase in pressure and temperature causes metamorphism. Plate convergence involves localized shearing action between the descending plate and the overriding plate. As a result, blueschists and associated rocks typically undergo a complex deformational history, sometimes forming chaotic mixtures known as mélanges where deformation was particularly intense. It is of special interest that nearly all extensive blueschist terranes are of Mesozoic age or younger (less than about 250 million years). Why old blueschists are rare is a puzzle. The two most prevalent theories are that older blueschists have been destroyed by erosion and later metamorphism, or that the early earth was too hot to achieve the right combination of high pressure and low temperature.

Sites of plate subduction in the modern world are marked by ocean trenches, great earthquakes, and arcs of andesitic volcanoes. The region between the trench and volcanic arc is known as the arc-trench gap. Typically, a fore-arc basin is on the arc side of the gap and an accretionary prism is on the trench side. The fore-arc basin sits atop the overriding plate and becomes filled largely with basaltic to andesitic volcanic debris generated in the nearby arc. Fore-arc basin deposits are essentially undeformed and unmetamorphosed. In striking contrast, the accretionary prism is directly above the descending plate and consists largely of variously deformed and metamorphosed sediments that were bulldozed off the descending plate during plate convergence. Most typically, blueschists are found in the arcward parts of an accretionary prism, locally faulted directly against fore-arc basin deposits.

The unusual conditions of very low temperatures for a given depth of burial can develop within subduction shear zones because plate convergence at speeds of tens of kilometers per million years (centimeters per year) transports cold lithosphere downward faster than the earth’s interior heat is conducted upward through it. As a result, after a few tens of millions of years of subduction, the front of the overriding plate cools, and the local geothermal gradients become greatly depressed. After fast plate convergence has occurred for a few tens of millions of years, temperatures less than 200 degrees Celsius at depths of 30 kilometers or more can be attained. The subduction zone metamorphism that creates blueschists is also known as high-pressure/low-temperature metamorphism.

Offscraping and Underplating

Because blueschists are found within accretionary prisms, it is important to understand how prisms grow and deform. Subduction accretion occurs by both offscraping and underplating. Offscraping is the process of trenchward growth or widening of the prism by addition at its toe of incoming sediments and seamounts (which are ocean islands such as Hawaii). It occurs by bulldozer-like action that causes the incoming pile of oceanic and trench-axis sediments to be folded and thrust-faulted. Offscraped rocks are weakly metamorphosed with the development of zeolite-group minerals and, at somewhat greater depths, the minerals prehnite and pumpellyite. Underplating is the process of addition of material to the bottom of a prism and, at greater depths, the bottom of the overlying crystalline plate. Underplating thickens and uplifts the overriding block and occurs concurrent with the shearing motions driven by the movement of the descending plate. Blueschists form in the region of underplating.

Underplating appears to be the basic process that drives both the thickening of accretionary prism and the uplift of included masses of blueschist. Underplating by itself, however, does not bring blueschists nearer to the surface. The presence of a steep trench slope (5-10 degrees) causes a prism to thin by downslope spreading, which is driven by gravity and behaves much like a glacier as it flows and thins down a mountain. Prism thinning seems to occur by a combination of normal faulting and rock flowage. Over a period of tens of millions of years, underplating-driven thickening at the base of the prism and gravity-driven thinning near the surface of the prism would slowly uplift a large terrane of blueschist near the edge of the overriding plate. The actual exposure of blueschist bedrock over a substantial area typically occurs only after subduction ceases and the top of the prism has become exposed to erosion.

The type of high-pressure/low-temperature metamorphism varies with depth. At the shallower depths of offscraping, temperatures and pressures are low, and only zeolites, prehnite, and pumpellyite develop. Surficial rocks subducted to depths of 10 to 30 kilometers and temperatures of 150 to 350 degrees Celsius are continuously metamorphosed into blueschists. At depths of 30 to 40 kilometers or more and at higher temperatures, the blueschists turn into the class of rock known as eclogite.

Prograde and Retrograde Metamorphism

The sequence of change from the zeolite to prehnite-pumpellyite to blueschist and finally to eclogite mineral assemblages is known as prograde metamorphism. Rocks in accretionary prisms commonly show all gradations of the progressive sequence. Overall, prograde metamorphism causes a general decrease in rock water content, destruction of the original minerals by recrystallization, increase in rock density, and increase in size of recrystallized crystals. At depths where the basalts and gabbros in the ocean crust (or ophiolite) at the top of the descending plate change from blueschist into eclogite, there is a large increase in the bulk density of the descending plate. This transformation decreases the buoyancy of the descending plate to such an extent that it may be the primary driving force of plate subduction and mantle convection. Most metamorphism is prograde because the loss of water or carbon dioxide prevents minerals from reverting to their original composition.

When the descending plate reaches depths of 100 to 125 kilometers, magmas are generated near its upper surface. They rise to the surface to form a volcanic arc of basaltic to andesitic composition. The presence of ultra-high-pressure blueschists directly confirms that some sediments are actually dragged down to (and returned from) the typical depths of arc magma origin. The intrusion of hot arc magmas near the surface and the eruption of volcanoes cause heating of the wall rocks, creating metamorphic rocks known as greenschists and amphibolites. This near-surface prograde metamorphism is of a low-pressure/high-temperature type. As a result, many ancient subduction zones are delineated on a regional scale by parallel belts of high-pressure/low-temperature and low-pressure/high-temperature metamorphic belts, a distinctive association known as paired metamorphic belts.

Plate convergence stops either when the relative motions between the descending and overriding plates become such that the margin becomes a transform plate margin, or when a buoyant continent or island arc is moved into a trench and “plugs up” the subduction zone. Transform plate motion occurs largely by horizontal movement along steep faults, a type of movement known as strike-slip faulting. In the process, some fault blocks rise and blueschists are eroded while others subside and blueschists become buried, reheated, and remetamorphosed as more normal geothermal conditions are reattained (a process known as retrograde metamorphism). The postsubduction destruction of blueschists by either erosion or retrograde metamorphism is the probable explanation for why most extensive terranes of blueschist are of Mesozoic age or younger.

Geological Mapping of Blueschists

Geologists study blueschists in the field, in the confines of the laboratory, and with computer modeling. Fieldwork involves going to the sites where blueschists are exposed in rock outcrops. Geological maps are made to show the field relations between blueschists and associated rocks. The first stage of geological mapping is recording on a topographic map the distribution of the major types of rocks, the orientation of bedding, and the locations and orientations of major faults and folds. Representative rock samples are collected for later laboratory study. The second stage of mapping is typically of much smaller areas. These detailed maps delineate additional variations in the types of rocks, the orientation of minor faults and folds, and associated schistosities and lineations. This stage of analysis usually provides the basis for determining the detailed movement patterns of the blueschists during subduction-zone deformation.

Study of Component Minerals

Laboratory studies of blueschists include the analysis of thin sections of the rock samples collected in the field, geochemical studies of mineral compositions, and experiments to determine the stability limits of minerals under different conditions of pressure, temperature, and fluid composition. Thin sections of the rocks are examined under polarized light with a petrographic microscope. Different minerals display different colors and other optical properties that enable their identification.

Minerals, particularly finely crystalline ones, are also identified by X-ray diffraction. The analysis of the scattering pattern of a beam of X-rays focused upon the sample enables the researcher to identify minerals and—for minerals such as feldspar, pyroxene, or chlorite—to estimate their elemental composition. The elemental composition of powdered rock samples is commonly determined using X-ray fluorescence. A focused X-ray beam causes atoms in a powder to emit other X rays whose type and intensity depend upon the types and amounts of atoms in the powder. The elemental composition of individual mineral grains is determined by analysis with the electron microprobe. A beam of high-energy electrons is focused on a 100-square-micron portion of a crystal in a highly polished thin section. As for X-ray fluorescence, the type and intensity of emitted X rays depend upon the types and amounts of atoms in a small spot in the crystal. Measurement of the composition of many spots in a traverse across a crystal enables determination of the variation in mineral composition from its core to rim, a variation known as compositional zoning. Zoning is a sensitive measure of the pressure and temperature history of the growing minerals and, hence, of both their prograde and retrograde metamorphic history.

Mass spectrometers are used to determine the isotopic ratios of the component minerals of blueschists for the calculation of the age of metamorphism. Mass spectrometers sort atoms by mass. A small amount of vaporized mineral is accelerated through a magnetic field, which deflects the atoms. More massive atoms are deflected less. Isotopic ratios of neodymium-143 to neodymium-144 and strontium-87 to strontium-86 are indicators of the geologic setting in which igneous rocks were erupted. Measurements of the ratios of oxygen-18 to oxygen-16 in coexisting minerals are indicators of the temperature of metamorphic recrystallization. The age of metamorphism for blueschists is determined from the analysis of radioactive isotopes and their daughter decay products in certain crystals. Examples are potassium-40, which decays into argon-40; rubidium-87, which decays into strontium-87; and uranium-238, which decays into lead-206. The measurement of the ratio of parent to daughter elements in either the whole rock or component minerals can be used to calculate the metamorphic age or ages of the rocks.

Experiments are conducted under controlled conditions in the laboratory to determine the stability limits and compositional relations for minerals at different pressures, temperatures, and fluid compositions. The goal is to simulate physical conditions deep in the earth. Experimental studies are also performed to determine how the ratios of oxygen isotopes in quartz and other minerals vary with different temperatures and oxygen pressure conditions. Laboratory calibration of elemental and isotopic compositions of minerals under controlled laboratory conditions is the basis for estimating the pressures and temperatures of metamorphism.

Computer simulations of the temperature conditions within subduction zones give an understanding of how temperatures change with time. Computer models that employ the principles of continuum mechanics are used to simulate the long-term tectonic deformation of an accretionary prism and the uplift of blueschist terranes. Geochemical computer models employ the principles of thermodynamics and are used to calculate the types of minerals that should develop during prograde and retrograde subduction-zone metamorphism.

Indicators of Geologic Processes

The study of blueschists is important because they are direct indicators of the geologic processes that occur deep within subduction zones that become mountain belts. Their creation indicates that abnormally cold geothermal conditions develop arcward (eastward) of the ocean trenches where rapid plate convergence occurs for tens of millions of years. Their preservation indicates that tectonic movements by faulting, folding, and rock flowage can be such that they become uplifted to near the surface while geothermal conditions remain very cold. Understanding the deformational history of blueschists is important because many of the world’s largest and most destructive earthquakes occur at subduction zones at the very depths where blueschists are forming today. An understanding of how they deform and recrystallize during their downward and upward paths in ancient subduction zones will eventually provide new understanding of how destructive subduction-zone earthquakes are nucleated and, therefore, better earthquake prediction.

Subduction zones are the sites where ocean-floor-capped lithosphere plunges back into the earth to be recycled. Blueschists are direct indicators that some of the sediment on top of the descending plate has also been dragged to near the base of the lithosphere. Their presence in paired metamorphic belts is the primary way that geologists recognize ancient subduction zones. Blueschists are a key part of the geologic story of how continents grow by the addition of accretionary prisms along their edges.

Principal Terms

accretionary prism: the complexly deformed rocks in a subduction zone that are scraped off the descending plate or eroded off the overriding plate

geotherm: a curve on a temperature-depth graph that describes how temperature changes in the subsurface

lithosphere: the outer rigid shell of the earth that forms the tectonic plates, whose movement causes earthquakes, volcanoes, and mountain building

metamorphism: the alteration of the mineralogy and texture of rocks because of changes in pressure and temperature conditions or chemically active fluids

prograde: metamorphic changes that occur primarily because of increasing temperature conditions

recrystallization: the formation of new crystalline grains in a rock

retrograde: metamorphic changes that occur primarily because of decreasing temperature conditions

subduction: the process of sinking of a tectonic plate into the interior of the earth

tectonism: the formation of mountains because of the deformation of the crust of the earth on a large scale

trench: a long and narrow deep trough on the sea floor that forms where the ocean floor is pulled downward because of plate subduction

volcanic arc: a linear or arcuate belt of volcanoes that forms at a subduction zone because of rock melting near the top of the descending plate

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