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 sea-floor 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 Fe-

content • 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 rhomb-shapes 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

aegirine-augite 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