HornblendePlag

Metabasites / MetacarbonatesFrancis, 2014

garnet

garnet

garnet

Metabasites:

Mafic volcanics such as basalts and andesites have more complicated and variable compositions than shales, containing significant quantities of Ca and Na, in addition to Si, Al, Mg Fe, K, and H2O. At least 8 components are necessary

to describe such systems, and it is no longer possible to construct a simple projection scheme that is thermodynamically rigorous. The metamorphic mineral assemblages of mafic volcanic rocks are commonly portrayed in an ACF projection, but it should be remembered that, unlike the metapelite AFM diagram, 3 phase regions in the ACF diagram are not strictly invariant and crossing tie lines are not uncommon.

The beginning of metamorphism in volcanic rocks and volcanogenic sediments is marked by the development of zeolites in vesicles and fractures under conditions of shallow burial. In consequence, volcanic rocks change from being vesicular to amygdular. Apart from the filling of vesicules and void spaces with these secondary minerals, rocks in the zeolite facies look essentially unmetamorphosed, although they often appear somewhat weathered in hand specimen: dirty, brownish, and oxidized because the processes of weathering have continued into the zeolite facies. Massive samples may still be quite pristine, however, retaining their original igneous mineralogy, especially if they have been isolated from extensive weathering and are relatively dry.

General Zeolite formula: WnTmO2m.sH2O

W = Na, Ca, K, (Ba, Sr,…) T = Si, Al

Zeolite Facies

Ca Zeolites Na Zeolites

Low T Chabazite Phillipsite

CaAl2Si4O12.6H2O Na3Al3Si5O16.6H2O

Stilbite

CaAl2Si7O18.7H2O

Heulandite Analcime

CaAl2Si7O18.6H2O NaAlSi2O6.H2O

Laumontite Natrolite

CaAl2Si4O12.4H2O Na4Al4Si6O20.4H2O

High T Wairakite Albite - Feldspar

CaAl2Si4O12.2H2O NaAlSi3O8

Zeolites

in

volcanic amygdules

Zeolite assemblages are stable only at relatively low PCO2. Even at relatively modest levels of

CO2, zeolite mineral assemblages are commonly replaced by carbonate and clay minerals.

Laumontite + CO2 Calcite + Kaolinite + Quartz + Water

CaAl2Si4O12.4H2O + CO2 CaCO3 + Al2Si2O5(OH)4 + 2SiO2 + 2H2O (XCO2 > 0.01)

The development of prehnite and pumpellyite in the both the groundmass and void spaces marks the beginning of the prehnite - pumpellyite sub-facies in metavolcanic and metavolcanogenic sedimentary rocks. Volcanic rocks in this metamorphic facies begin to take on a greenish colour, although they typically are relatively unstrained and unrecrystallized, and commonly appear little metamorphosed in hand specimen, except for the development of a greenish colour. The presence of prehnite and pumpellyite are best recognized in thin section.

Prehnite - Pumpellyite sub-Facies of the Greenschist Facies

Prehnite Ca2Al(AlSi3)O10(OH)2

brittle mica

Pumpellyite W4X(OH,O)Y5O(OH)3(TO4)2(T2O7)2.2H2O sorosilicate T = Si, Al Y = Al, Fe3+, Ti4+

X = Mn, Fe2+, Mg, Al, Fe3+

W = Ca, K, Na

meta-pillow basalt

Greenschist Facies

epidote & actinolite - inprehnite + qtz + chlorite zoisite + actinolite + water

Ca2Al(AlSi3)O10(OH)2 + SiO2 + Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH) + Ca2(Mg,Fe)5Si8O22(OH)2

(Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6

epidote & actinolite - inpumpellyite + qtz + chlorite zoisite + actinolite + water

Ca4(Mg,Fe)(Al,Fe3+)5O(OH)3(Si2O7)2(SiO4)2.2H2O Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH) + Ca2(Mg,Fe)5Si8O22(OH)+ SiO2 + (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6

Ca-plag - in

chlorite + zoisite + qtz actinolite + plagioclase + water

(Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6 Ca2(Mg,Fe)5Si8O22(OH) + (Ca,Na)(Al,Si)4O8

+ Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH) + SiO2

Hornblende - inactinolite hornblende

Ca2(Mg,Fe)5(Si8O22)(OH)2 NaCa2(Mg,Fe,Al)5(Al2Si6O22)(OH)2 Garnet & Hb - inepidote + chlorite hornblende + garnet + water

Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH) NaCa2(Mg,Fe,Al)5(Al2Si6O22(OH)2 +(Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6 (Ca,Fe,Mg)3(Al,Fe3+)2(SiO4)3

Amphibolite Facies

Cpx - in hornblende + epidote clinopyroxene + plagioclase + water

NaCa2(Mg,Fe,Al)5(Al2Si6O22(OH)2 + Ca(Mg,Fe)Si2O6 + (Ca,Na)(Al,Si)4O8

Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH)

Opx - in

hornblende + garnet orthopyroxene + plagioclase + water

NaCa2(Mg,Fe,Al)5(Al2Si6O22(OH)2 + (Mg,Fe)2Si2O6 + (Ca,Na)(Al,Si)4O8

(Ca,Fe,Mg)3(Al,Fe3+)2(SiO4)3

Upper Amphibolite Facies

hornblende can no longer coexist with garnet

hornblende can no longer coexist with epidote

Hornblende - out hornblende clinopyroxene + plagioclase + opx + water

NaCa2(Mg,Fe,Al)5(Al2Si6O22(OH)2 Ca(Mg,Fe)Si2O6 + (Ca,Na)(Al,Si)4O8 + (MgFe)SiO3

Granulite Facies

Garnet - in anorthite + orthopyroxene garnet + clinopyroxene

CaAl2Si2O8 5(Fe,Mg)SiO3 (Fe,Mg)3Al2(SiO4)3 Ca(Mg,Fe)2Si2O6

Summary of Metabasic Volcanic Rocks

brownishweathered

bluishun-

equilibratedphyllites

hydrouspartialmelt

greenishfine-grain

schitose if deformed

blackishCrystalline

gneissic

granulargneissic

Eclogite

GarnetPyroxenite

Oliv

Oliv

Plag

PlagOliv

Oliv + Plag

Opx + Cpx + Spin / Garn

Oliv

The complete transition from granulite to eclogite facies occurs over a 5 kb pressure range, beginning with the:

• appearance of Garnet: Pyroxene Granulite Garnet Granulite

and ending with:

• disappearance of Plagioclase: Garnet Granulite Eclogite

The exact position of these reactions is sensitive to bulk composition. The official Eclogite field is defined for quartz-normative bulk compositions. More Fe-rich and alkaline basalts will convert to a garnet pyroxenite mineralogy at lower pressures in the garnet granulite field. Most petrologists refer to such rocks as garnet pyroxenite rather than eclogite.

The Granulite - Eclogite

Transition:

increasing pressure

maficmagma

Meta-Plutonic Rocks

Metamorphic Coronas

Oliv + Plag Opx + Cpx + Spin or Garn

Oliv

Oliv

Plag

Plag

Meta-carbonates / Skarns

calcite

calcite

diopside

grossularite

MetacarbonatesSkarns are rocks rich in calc-silicate minerals that are produced by the contact metamorphism of limestones and dolomites. The silica required for the prograde metamorphic reactions may come from detrital quartz and/or silica sponge spicules, etc. in the original carbonate sediment, but may also be introduced metasomatically by fluids emanating from the igneous intrusion responsible for the contact metamorphism.

Skarns are typically named on the basis of their most characteristic mineral assemblage,

eg.: olivine-diopside skarn

3CaMg(CO3)2 + 4SiO2 + H2O Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2 #6

Dolomite Qtz Talc Calcite

5CaMg(CO3)2 + 8SiO2 + H2O Ca2Mg5Si8O22(OH)2 + 3CaCO3 +7CO2 #6

Dolomite Qtz Tremolite Calcite

Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 2SiO2 5CaMgSi2O6 + 3CO2 + H2O #4

Tremolite Calcite Diopside

Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2 8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O #4

Tremolite Dolomite Olivine Calcite

Ca2Mg5Si8O22(OH)2 + CaCO3 Mg2SiO4 + 3CaMgSi2O6 + CO2 + H2O #4

Tremolite Calcite Olivine Diopside

CaCO3 + SiO2 CaSiO3 + CO2 #3

Calcite Qtz Wollastonite

Prograde Metacarbonate Reactions GreenwoodClassification

Talc - in

Tremolite - in

Diopside - in

Olivine - in

Tremolite-out

Wollastonite-in