Transcript LECTURE 20 - Blueschist and Eclogites

Metamorphism of Basic Igneous Rocks
In this Lecture
• Tectonic Environment and Geothermal Gradients
• Changes in Metamorphic Facies
• Mineralogical Characteristics
• Blueschists
• Eclogites
• Blueschist Facies Minerals
Metamorphic Facies
Tectonic Setting - Subduction Zones
Metamorphic Phase Changes in
Subduction Zones
The Facies Classification
• The mineral assemblages of metabasites are less sensitive to
changes in pressure and temperature than those of pelites
except at very low grades
• Therefore the zones that they define represent a broader
range of P-T conditions of formation
• Can be regarded as less useful than pelites
• However, metabasites are more common than true metapelites
and so are more easily correlated over large areas
Mineralogical Changes Defining the Facies
• Changes in the composition of amphibole
– Low temperature – actinolite [Ca- amphibole]
– High temperature – hornblende [K-Ca amphibole]
– High pressure – glaucophane [Na-amphibole]
• Formation of pyroxene under extreme conditions
– High pressure – low temperature - Jadeite-rich clinopyroxene
– High pressure – higher temperature – omphacitic clinopyroxene
• Changes in feldspar composition
– Low temperature – albite
– Higher temperature – plagioclase
– High pressure – albite and no plagioclase
Mineralogical Changes Defining the Facies
• Formation of hydrous Ca-Al silicates
– Low grades - Zeolites, prehnite, pumpellyite
– High pressure / low to med temperatures – lawsonite
– Epidote stable over wide P-T region although progressively
replaced by plagioclase at high temperatures
– Zoisite and clinozoisite typical of relatively high-pressures
and/or high temperatures
• Presence or absence of garnet
– Only present at medium to high pressures
Metamorphic Facies Basic Igneous Rocks
Geothermal Gradients and Examples
Blueschists
• Definition
– Inferred to form under unusual physical conditions, relatively high P
and relatively low T.
– If pressure is supplied by loading, ie burial, then this must be a
transient state or else the temperature would increase
– Lead to the idea that blueschist are formed in subduction zones
where such P-T conditions are possible
• A necessary corollary of the above is that blueschist must also
be unroofed quickly or else they would not be preserved. Hence
exhumation rates are rapid
• Blueschists are most common in the Phanerozoic. Very few
genuine examples of blueschists older than 200Ma exist.
• Mineralogy
– Characteristic glaucophane, lawsonite, albite
– Also may be present actinolite, chlorite, zoisite, clilnozoisite
Eclogites
• Definition
– Essentially a rock of basaltic bulk composition composed of garnet +
omphacite and/or rutile.
– Density about 3.4 g/cm3 (or higher) vs basalt about 2.9 g/cm3 and
amphibolite about 3 g/cm3
• Types
– Type A mostly inclusions in ultramafic rocks under high T conditions
900-1600°C
– Type B associated with high-grade rocks under medium T conditions
of about 550-900°C
– Type C associated with blueschists under low T conditions of about
450-550°C
• Typical Mineralogy
– Garnet, omphacite, kyanite, orthopyroxene, phengite,
paragonite, rutile
Important Reactions
• First appearance of lawsonite
– May form directly from igneous minerals
– Plagioclase + H2O -> lawsonite + albite
– In this case the igneous relicts can influence the way in
which the metamorphic minerals develop
Lawsonite
Important Reactions
• Breakdown of Lawsonite
– Lawsonite + albite = zoisite + paragonite + quartz + H2O
– Or at higher pressures
– Lawsonite + jadeite = zoisite + paragonite + H2O
• Transition between greenschist and blueschist facies
– Albite + chlorite + actinolite = glaucophane + lawsonite
• Upper Temperature Limit of Blueschists
– Zoisite + glaucophane = garnet + omphacite + paragonite + quartz + H2O
• Feldspar stability
–
–
–
–
Albite limited at high pressures by the reaction
Albite = jadeite + quartz
Anorthite breaks down according to the reaction
3 anorthite = grossular garnet + 2 kyanite + quartz
Implications for Reaction Textures
• Reaction textures in blueschists and eclogites often difficult to
interpret
– Relatively low temperature conditions means that
recrystallisation reactions are sluggish and there is often
preservation of too many mineral phases for them all to be
stable
– Complex zoning because of variations in chemical composition
of different minerals
– Blueschists and eclogites can often co-exist because of
differences in bulk composition that often result from seafloor alteration processes
Complex Reaction Textures
Talc – tremolite schist
Coesite
•
This is a photomicrograph (a couple of
mm across) of a garnet with inclusions
of silica. Most silica at the earth's
surface is in the form of quartz. But
under high pressures (equivalent to
depths in the earth in excess of 80
km), the stable form of silica is the
mineral coesite. The garnet crystal has
acted as a protective pressure vessel
so that pieces of coesite have been
preserved to the earth's surface. The
attempted change to quartz has tried
to expand the inclusion - causing radial
cracks in the garnet. This classic image
(provided by Christian Chopin) is from
the Dora Miara internal basement
massif. So this fragment of
continental crust was once over 80 km
down in the earth.
Glaucophane
•
Classic blueschists. The slight blue tinge results from the mineral
glaucophane (an amphibole), which here forms the rather stubby
needles. This rock started life as a volcanic rock of basic composition,
part of the old ocean floor of Tethys. Blueschists are comonly thought
to be diagnostic of former subduction zones, because they imply
relatively high pressure conditions relative to the temperature
(compared to normal geothermal gradients).
Glaucophane
• Blue Amphibole
• Colour is lavender blue, blue, dark blue, gray or black.
• Distinct pleochroism: X= colorless, pale blue, yellow; Y=
lavender-blue, bluish green; Z= blue, greenish blue, violet
• Maximum birefringence = 0.006-0.023 and varies with Fecontent
• There is no twinning in glaucophane but normal amphibole
cleavage 56° and 124°.
• Glaucophane also has a parallel extinction when viewed under
cross polars.
Glaucophane in Thin-Section
Plane Light
Crossed Polars
Glaucophane in Thin-Section
Y direction
Z direction
Lawsonite
•
Another example of blueschist metamorphism. Here the small rhombshapes are diagnostic of the mineral lawsonite. This mineral is very
rarely preserved but the shape remains (these types of features are
generally called pseudomorphs).
Lawsonite in Thin-Section
• CaAl2(Si2O7)(OH)2 H2O same composition as anorthite but with
additional (AlO.OH) octahedra
• Tabular or prismatic crystals with polysynthetic twins on {110}
• Perfect cleavage {010} and {001}, Poor cleavage {110}
• Colorless, pale blue to bluish-grey. Vitreous to greacy luster.
Transluscent.
• Pleochroism includes colorless, blue, and yellow.
• If measured against cleavage, extinction angle may become
symmetrical depending on orientation.
• Maximum Birefringence 0.0190-0.0210
• Biaxial (+)
Lawsonite in Thin-Section
Lawsonite in Thin-Section
PPL
XPL
Omphacite / Jadeite / Aegirine
• Omphacite is characteristic of the eclogite facies, and commonly
occurs with garnet, glaucophane, rutile, clinozoisite and
hornblende. It is essentially a solid solution of jadeite
(NaAlSi2O6) and diopside (MgCaSi2O6) with variable aegerine
(NaFe3+Si2O6) and Ca-tschermakite (CaAl2SiO6) components in
the range 0.2<Na/(Na+Ca)<0.8 and [Al]6/([Al]6 + Fe3+) >0.5.
Aegerine-rich omphacite is more intensely coloured than
aegerine-poor omphacite.
• Jadeite has lower indices of refraction than other pyroxenes.
• Omphacite has higher birefringence and aegirine and aegirineaugite have higher birefringence and are more distinctly green.
• Jadeite with anomalous interference colors resembles zoisite
has higher indices of refraction and parallel extinction angle
{010} sections and has typical pyroxene cleavage.
Omphacite
•
•
•
•
•
Color in Hand Sample: green to dark green
Color in PPL: colorless to pale green
Pleochroism: weakly pleochroic, Z=very pale green to blue green
High relief in thin section.
Characteristic pyroxene cleavage
Omphacite with Glaucophane
Omphacite
PPL
XPL
Clinozoisite
PPL
XPL
Actinolite
PPL
XPL