Metamorphic Rocks 1: Description and Classification

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1 Metamorphic Rocks 1: Description and Classification
Introduction to Lab MR-1 Nesse Ch. 11, p Lab Manual: Introduction p Lab MR-1 p

2 Rock-Mineral Associations?
mineral stability is a function of: pressure (P) temperature (T) composition (X) Nesse, Fig. 5.1

3 What is metamorphism? (meta = change; morph = form)
..... 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 (s)

4 Because change is essential to metamorphism,
studying metamorphic rocks always involves 2 questions: a) What is it? (now) b) What was it? (before metamorphism)

5 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

6 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!

7 Metamorphic rocks can be classified according to: a) protolith
b) bulk composition c) mineral assemblage 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

8 Metamorphic rocks can be classified according to: a) protolith
determined from relict features inherited from protolith and preserved during metamorphism and deformation and/or bulk composition of rock b) bulk composition c) mineral assemblage d) texture

9 Metamorphic rocks can be classified according to: a) protolith
determined from relict features inherited from protolith and preserved during metamorphism and deformation and/or bulk composition of rock b) bulk composition determined by/from minerals present in rock (types, compositions, modal %) and/or chemical analysis c) mineral assemblage d) texture

10 Metamorphic rocks can be classified according to: a) protolith
determined from relict features inherited from protolith and preserved during metamorphism and deformation and/or bulk composition of rock b) bulk composition determined by/from minerals present in rock (types, compositions, modal %) and/or chemical analysis c) mineral assemblage reflects bulk composition and metamorphic grade (P-T conditions) determined by petrographic observation d) texture

11 Metamorphic rocks can be classified according to: a) protolith
determined from relict features inherited from protolith and preserved during metamorphism and deformation and/or bulk composition of rock b) bulk composition determined by/from minerals present in rock (types, compositions, modal %) and/or chemical analysis c) mineral assemblage reflects bulk composition and metamorphic grade (P-T conditions) determined by petrographic observation d) texture may be inherited from protolith and/or developed during deformation and (re)crystallisation

12 Metamorphic rocks can be classified according to: a) protolith
b) bulk composition c) mineral assemblage 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

13 Metamorphic rocks can be classified according to: a) protolith
b) bulk composition c) mineral assemblage 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)

14 LAB MR-1: METABASITES 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 assemblage - 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) d) texture - ranges from massive to foliated - relict features may be well preserved

15 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

16 amphibole in foliated matrix
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 protolith? MR-1: ???? amphibole in foliated matrix (6.25 mm, PPL) ERTH 2002 Lab MR-1

17 LAB MR-1: METABASITES b) bulk composition (= metabasite)
relatively rich in Mg, Fe, Ca, relatively poor in Si, Al, K reflected in minerals present in rock (types, compositions, %) classification based on bulk composition: - find appropriate special name - modify textural root term with compositional term

18 LAB MR-1: METABASITES b) bulk composition (= metabasite)
relatively rich in Mg, Fe, Ca, relatively poor in Si, Al, K reflected 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

19 LAB MR-1: METABASITES 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 appropriate special rock name requires knowing mineralogy and texture conversely, special rock names convey information about both mineralogy and texture MR-1: ???? amphibole in foliated matrix (6.25 mm, PPL) ERTH 2002 Lab MR-1

20 LAB MR-1: METABASITES 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: actinolite hornblende: blue-green green brown lower P,T plagioclase: albite (An0-10) oligoclase (An20-30) andesine (An30-50) higher P,T

21 LAB MR-1: METABASITES 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) greenschist facies: chlorite ± epidote ± albite (An0-10) ± actinolite + quartz + titanite amphibolite facies: hornblende + plagioclase (An20-40) + quartz ± biotite ± garnet ± titanite ± ilmenite granulite facies: orthopyroxene + clinopyroxene + plagioclase (An30-50) + quartz + ilmenite ± rutile ± garnet ± hornblende ± biotite lower P,T higher P,T metamorphic facies: a set of mineral assemblages indicative of metamorphic P-T conditions (grade)

22 LAB MR-1: METABASITES c) mineral assemblage
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-1: ???? amph + plag + qtz (2.5 mm, PPL) ERTH 2002 Lab MR-1 MR-5: metagabbro plag + gnt + px + amph (6.25 mm, PPL) ERTH 2002 Lab MR-1

23 LAB MR-1: METABASITES 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.)

24 LAB MR-1: METABASITES d) texture: common textures of metabasites
relict = inherited (from protolith or earlier stage of metamorphism) 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

25 LAB MR-1: METABASITES d) texture: common textures of metabasites
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 parallel alignment of platy minerals (e.g., chlorite, mica) gneissosity – compositional layering in medium- to coarse- grained rocks defined by variable proportions of light and dark minerals

26 amphibole in foliated matrix
LAB MR-1: METABASITES d) texture: common textures of metabasites 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 parallel alignment of platy minerals (e.g., chlorite, mica) gneissosity – compositional layering in medium- to coarse- grained rocks defined by variable proportions of light and dark minerals S1 MR-1: (6.25 mm, PPL) amphibole in foliated matrix ERTH 2002 Lab 4

27 LAB MR-1: METABASITES d) texture: common textures of metabasites
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

28 LAB MR-1: METABASITES d) texture: common textures of metabasites
porphyroblasts = coarser-grained minerals in a finer-grained matrix (cf. phenocrysts in igneous rocks) porphyroblast shapes can be described as: euhedral or idioblastic subhedral or subidioblastic anhedral or xenoblastic because porphyroblasts grow from a solid matrix, they generally contain inclusions that may preserve an earlier stage of the metamorphic history (minerals +/- textures) = poikiloblastic texture MR-1: (6.25 mm, PPL) amphibole porphyroblasts in finer-grained foliated matrix

29 LAB MR-1: METABASITES d) texture: common textures of metabasites
reaction texture = replacement of one (or more) minerals (reactants) by one (or more) new minerals (products) reaction textures include: pseudomorph – replacement by one or more minerals, where shape of 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 MR-5: metagabbro reaction rim (corona) between plagioclase + another mineral (6.25 mm, PPL) ERTH 2002 Lab 4

30 LAB MR-1: METABASITES Classification/nomenclature according to IUGS:
a) protolith – do features inherited from protolith dominate? if so, add meta- to original rock name

31 LAB MR-1: METABASITES 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)

32 LAB MR-1: METABASITES 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)

33 LAB MR-1: METABASITES 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) 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”

34 LAB MR-1: METABASITES Classification/nomenclature of MR-1 and MR-5 based on: MR-1: (6.25 mm, PPL) MR-5: (6.25 mm, PPL) protolith: bulk composition: = metabasite mineral assemblage: texture:

35 LAB MR-1: METABASITES Classification/nomenclature of MR-1 and MR-5 based on: MR-1: (6.25 mm, PPL) MR-5: (6.25 mm, PPL) protolith: ???? metagabbro bulk composition: greenschist metagabbro (=metabasite) mineral assemblage: greenschist gnt-px metagabbro texture: greenschist coronitic metagabbro

36 LAB MR-1: METABASITES Classification/nomenclature of MR-1 and MR-5 based on: MR-1: (6.25 mm, PPL) MR-5: (6.25 mm, PPL) protolith: ???? metagabbro bulk composition: greenschist metagabbro (=metabasite) mineral assemblage: greenschist gnt-px metagabbro texture: greenschist coronitic metagabbro

37 LAB MR-1: METABASITES 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