Description and Classification Introduction to Lab MR-1 Nesse Ch. 11, p. 194-200 Lab Manual:Introduction p. 30-49 Lab MR-1 p. 50-52 Metamorphic Rocks 1:

Description and ClassificationIntroduction to Lab MR-1

Nesse Ch. 11, p. 194-200Lab Manual: Introduction p. 30-49

Lab MR-1 p. 50-52

Metamorphic Rocks 1:

Rock-Mineral Associations?

Nesse, Fig. 5.1

mineral stabilityis a function of:

pressure (P)temperature (T)composition (X)

..... any dominantly solid-state process that produces

a change in the physical, mineralogical, textural,

or chemical character of

a pre-existing igneous, sedimentary, or

metamorphic rock, in response to

changes in pressure (P), temperature (T), chemical

environment (X), or stress field ()

What is metamorphism?

(meta = change; morph = form)

Because change is essential to metamorphism,

studying metamorphic rocks always involves 2 questions:

a) What is it? (now)

b) What was it? (before metamorphism)

Because change is essential to metamorphism,

studying metamorphic rocks always involves 2 questions:

a) What is it? (now)

b) What was it? (before metamorphism)

observation, description, classification

observation, interpretation

igneous or sedimentary (or metamorphic) precursor is referred to as the protolith

Metamorphic rocks can be classified according to:

a) protolith

b) bulk composition

c) mineral assemblage

d) texture

The choice of rock name depends on:

- which feature(s) dominate

- what the observer wants to emphasise

i.e., more than one name may be appropriate!


a) protolith

b) bulk composition


d) texture

This has led to considerable inconsistency/confusion in classifying and naming metamorphic rocks

• recently, an IUGS subcommittee has addressed this problem and proposed some possible solutions

• work still in progress, no general consensus (yet)• preliminary results on poster in lab and in handout


a) protolith determined from relict features inherited from protolith and preserved during metamorphism and deformation and/or bulk composition of rock

b) bulk composition


d) texture



b) bulk compositiondetermined by/from minerals present in rock (types,

compositions, modal %) and/or chemical analysis


d) texture





c) mineral assemblagereflects bulk composition and metamorphic grade

(P-T conditions)determined by petrographic observation

d) texture





c) mineral assemblagereflects bulk composition and metamorphic grade

(P-T conditions)determined by petrographic observation

d) texturemay be inherited from protolith and/or developed during

deformation and (re)crystallisationdetermined by petrographic observation


a) protolith

b) bulk composition


d) texture

in order to reduce the number of possible variables to be considered, Labs 4 and 5 will focus on specific bulk compositions (and corresponding protoliths)

limits the range of possible mineral assemblages and textures


a) protolith

b) bulk composition


d) texture

LAB 4: Metabasites (derived from mafic/intermediate igneous rocks)

LAB 5: Pelites (derived from mudstones, shales, siltstones)

LAB 6: Other (derived from felsic and ultramafic igneous rocks; limestones, sandstones)

a) protolith - mafic or intermediate igneous rocks (basalts, gabbros,

andesites, diorites) and their pyroclastic or volcano- clastic equivalents (mafic tuffs, etc.)

b) bulk composition (= metabasite)- relatively rich in Mg, Fe, Ca- relatively poor in Si, Al, K

c) mineral assemblageamphibole + plagioclase ubiquitous except for unusual

P-T conditions; compositions vary with gradegarnet, biotite, quartz, epidote, chlorite, pyroxene (cpx

+/- opx), titanite, ilmenite, apatite (depends on grade)

d) texture- ranges from massive to foliated- relict features may be well preserved

LAB MR-1: METABASITES

a) protolith mafic or intermediate igneous rocks (basalts, gabbros, andesites, diorites) and their pyroclastic or volcano-clastic equivalents (mafic tuffs, volcanogenic sandstones etc.) if protolith obvious, rock name = meta + protolith name


IR-3: gabbro plag laths with interstitial cpx(2.5 mm, PPL) ERTH 2002 Lab IR-2

MR-5: metagabbro relict plag laths with relict cpx

(6.25 mm, PPL) ERTH 2002 Lab MR-1

MR-1: ???? amphibole in foliated matrix


protolith?

a) protolith mafic or intermediate igneous rocks (basalts, gabbros, andesites, diorites) and their pyroclastic or volcano-clastic equivalents (mafic tuffs, volcanogenic sandstones etc.) if protolith obvious, rock name = meta + protolith name


b) bulk composition (= metabasite)relatively rich in Mg, Fe, Ca, relatively poor in Si, Al, Kreflected in minerals present in rock (types, compositions, %)

classification based on bulk composition:

- find appropriate special name - modify textural root term with compositional term


b) bulk composition (= metabasite)relatively rich in Mg, Fe, Ca, relatively poor in Si, Al, Kreflected in minerals present in rock (types, compositions, %)

some special rock names that apply specifically to metabasites:

greenschist – fine- to medium-grained metabasite with a distinct foliation, containing some combination of chlorite +/- actinolite +/- epidote

greenstone – fine- to medium-grained massive metabasite, typically preserving some relict features, containing some combination of chlorite +/- actinolite +/- epidote

amphibolite – medium- to coarse-grained, massive to foliated metabasite consisting largely of hornblende + plagioclase

granulite – medium- to coarse-grained, massive to foliated metabasite containing metamorphic opx + cpx + plag

others (e.g., blueschist, eclogite) will not be encountered this term


MR-1: ???? amphibole in foliated matrix


b) bulk composition (= metabasite)relatively rich in Mg, Fe, Ca, relatively poor in Si, Al, K

reflected in minerals present in rock (types, compositions, %)

greenschist? greenstone? amphibolite? granulite?

picking appropriatespecial rock namerequires knowing

mineralogy and texture

conversely, special rock namesconvey information about

both mineralogy and texture


c) mineral assemblage (reflects bulk composition and grade)

amphibole + plagioclase ubiquitous except for unusual P-T conditions; compositions vary with grade

garnet, biotite, quartz, epidote, chlorite, pyroxene (cpx +/- opx), titanite, ilmenite, apatite (depends on grade)

amphibole:

actinolitehornblende: blue-green green brown

lower P,T

higher P,T

plagioclase:

albite (An0-10)

oligoclase (An20-30)andesine (An30-50)


lower P,T

higher P,T

greenschist facies: chlorite ± epidote ± albite (An0-10) ± actinolite + quartz + titaniteamphibolite facies: hornblende + plagioclase (An20-40) + quartz ± biotite ± garnet ± titanite ± ilmenitegranulite facies: orthopyroxene + clinopyroxene + plagioclase (An30-50) + quartz + ilmenite ± rutile ± garnet ± hornblende ± biotite

metamorphic facies: a set of mineral assemblagesindicative of metamorphic P-T conditions (grade)

c) mineral assemblage (reflects bulk composition and grade)

amphibole + plagioclase ubiquitous except for unusual P-T conditions; compositions vary with grade

garnet, biotite, quartz, epidote, chlorite, pyroxene (cpx +/- opx), titanite, ilmenite, apatite (depends on grade)


MR-5: metagabbroplag + gnt + px + amph + ......


MR-1: ???? amph + plag + qtz + ......


c) mineral assemblageamphibole + plagioclase ubiquitous except for unusual P-T

conditions; compositions vary with gradegarnet, biotite, quartz, epidote, chlorite, pyroxene (cpx +/- opx),

titanite, ilmenite, apatite (depends on grade)


d) texture: common textures of metabasites

- relict = inherited (from protolith or earlier stage of metamorphism)

- foliation = planar fabric (layering, alignment of platy minerals, etc.)

- lineation = linear fabric (stretching, alignment of elongated minerals, etc.)

- porphyroblasts = coarser-grained metamorphic minerals in a finer-grained matrix (cf. phenocryst)

- reaction texture = replacement of one (or more) mineral by another mineral (or minerals) (pseudomorphs, reaction rims, coronas, etc.)


IR-3: gabbro plag laths with interstitial cpx(2.5 mm, PPL) ERTH 2002 Lab IR-2

MR-5: metagabbro relict plag laths with relict cpx



relict = inherited (from protolith or earlier stage of metamorphism)



foliation = planar fabric (layering, alignment of platy minerals, etc.)

special types of foliation include:

cleavage – tendency for rock to split along closely spaced,parallel planes

schistosity – defined by parallelalignment of platy minerals(e.g., chlorite, mica)

gneissosity – compositionallayering in medium- to coarse-grained rocks defined by variable proportions of light and dark minerals



foliation = planar fabric (layering, alignment of platy minerals, etc.)

special types of foliation include:

cleavage – tendency for rock to split along closely spaced,parallel planes

schistosity – defined by parallelalignment of platy minerals(e.g., chlorite, mica)

gneissosity – compositionallayering in medium- to coarse-grained rocks defined by variable proportions of light and dark minerals

MR-1: (6.25 mm, PPL) amphibole in foliated matrix

ERTH 2002 Lab 4

S1



lineation = linear fabric (stretching, alignment of elongated minerals, etc.)

special types of lineation include:

stretching lineation – defined by stretched minerals or mineral aggregates

mineral lineation – defined by parallel alignment of long axes of elongated minerals

intersection lineation – defined by intersection of two planar fabric elements



porphyroblasts = coarser-grained minerals in a finer-grained matrix (cf. phenocrysts in igneous rocks)

MR-1: (6.25 mm, PPL)amphibole porphyroblasts in finer-grained foliated matrix

porphyroblast shapes can bedescribed as:

euhedral or idioblasticsubhedral or subidioblasticanhedral or xenoblastic

because porphyroblasts growfrom a solid matrix, they generally contain inclusionsthat may preserve an earlier stage of the metamorphichistory (minerals +/- textures) = poikiloblastic texture



reaction texture = replacement of one (or more) minerals (reactants) by one (or more) new minerals (products)

MR-5: metagabbroreaction rim (corona) betweenplagioclase + another mineral (6.25 mm, PPL) ERTH 2002 Lab 4

reaction textures include:

pseudomorph – replacement byone or more minerals, where shapeof the original grain is preserved

reaction rim – reactant rimmed by one or more product minerals

corona – reaction rim where reaction products form concentric zones around/between original (reactant) minerals


Classification/nomenclature according to IUGS:

a) protolith – do features inherited from protolith dominate?if so, add meta- to original rock name




b) bulk composition – is there a special rock name that fits this bulk composition? if so, use it!

(alternatively, modify textural root name with compositional term)






c) mineral assemblage – are one or two minerals in the rock particularly important? if so, add mineral name(s) to root name based on texture (see below)






c) mineral assemblage – are one or two minerals in the rock particularly important? if so, add mineral name(s) to root name based on texture (see below)

d) texture – is there a special rock name that fits this texture?

if so, use it! if not....is the rock foliated? if not, use term “granofels”does the rock have a foliation defined by alignment of

sheet silicates? if so, use term “schist”is the rock medium- to coarse-grained, with pronounced cm-scale compositional banding? if so, use term “gneiss”


Classification/nomenclature of MR-1 and MR-5 based on:

MR-5: (6.25 mm, PPL)MR-1: (6.25 mm, PPL)

protolith:

bulk composition: = metabasite

mineral assemblage:

texture:



MR-5: (6.25 mm, PPL)MR-1: (6.25 mm, PPL)

protolith: ???? metagabbro

bulk composition: greenschist metagabbro (=metabasite)mineral assemblage: greenschist gnt-px metagabbro

texture: greenschist coronitic metagabbro


MR-5: (6.25 mm, PPL)MR-1: (6.25 mm, PPL)


protolith: ???? metagabbro

bulk composition: greenschist metagabbro (=metabasite)mineral assemblage: greenschist gnt-px metagabbro

texture: greenschist coronitic metagabbro


1. Fill in mineral compositions on p.50 of lab manual

2. Work through MR-1, MR-5 with partner using

petrographic description sheets

3. Fill in checklist for all 5 samples

have this checked before you leave!

4. Fill out petrographic description sheets for

any 1 of the other 3 samples

hand this in with your checklist at the

beginning of the next lab


Documents

Description and Classification Introduction to Lab MR-1 Nesse Ch. 11, p. 194-200 Lab Manual:Introduction p. 30-49 Lab MR-1 p. 50-52 Metamorphic Rocks 1: