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garnet
Hornblende
Plag
garnet
garnet
Metabasites
Francis, 2014
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.
Zeolite Facies
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:
Ca Zeolites
Low T
Chabazite
CaAl2Si4O12.6H2O
WnTmO2m.sH2O
W = Na, Ca, K, (Ba, Sr,…)
T = Si, Al
Na Zeolites
Phillipsite
Na3Al3Si5O16.6H2O
Stilbite
CaAl2Si7O18.7H2O
High T
Heulandite
CaAl2Si7O18.6H2O
Analcime
NaAlSi2O6.H2O
Laumontite
CaAl2Si4O12.4H2O
Natrolite
Na4Al4Si6O20.4H2O
Wairakite
CaAl2Si4O12.2H2O
Albite - Feldspar
NaAlSi3O8
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
CaAl2Si4O12.4H2O + CO2
Calcite + Kaolinite + Quartz + Water
CaCO3 + Al2Si2O5(OH)4 + 2SiO2 + 2H2O
(XCO2 > 0.01)
Prehnite - Pumpellyite sub-Facies
The development of prehnite and pumpellyite in the both the groundmass and void spaces marks the
beginning of the prehnite - pumpellyite 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
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
Lawsonite
CaAl2(OH)2SiO2.H2O
Greenschist Facies
epidote & actinolite - in
prehnite + qtz + chlorite
zoisite + actinolite + water
Ca2Al(AlSi3)O10(OH)2 + SiO2
+
(Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6
Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH) + Ca2(Mg,Fe)5Si8O22(OH)2
epidote & actinolite - in
pumpellyite + qtz + chlorite
zoisite
+
actinolite + water
Ca4(Mg,Fe)(Al,Fe3+)5O(OH)3(Si2O7)2(SiO4)2.2H2O
+ SiO2 + (Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6
Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH) + Ca2(Mg,Fe)5Si8O22(OH)
Ca-plag - in
chlorite + zoisite + qtz
actinolite + plagioclase + water
(Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6
+ Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH) + SiO2
Ca2(Mg,Fe)5Si8O22(OH) + (Ca,Na)(Al,Si)4O8
Amphibolite Facies
Hornblende - in
actinolite
hornblende
Ca2(Mg,Fe)5(Si8O22)(OH)2
NaCa2(Mg,Fe,Al)5(Al2Si6O22)(OH)2
Garnet - in
epidote + chlorite
hornblende + garnet + water
Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH)
(Mg,Fe)3(Al,Si)4O10(OH)2(Mg,Fe)3(OH)6
NaCa2(Mg,Fe,Al)5(Al2Si6O22(OH)2 +
(Ca,Fe,Mg)3(Al,Fe3+)2(SiO4)3
Upper Amphibolite Facies
Cpx - in
hornblende + epidote
clinopyroxene + plagioclase + water
NaCa2(Mg,Fe,Al)5(Al2Si6O22(OH)2 +
Ca2(Fe,Al)3O(SiO4)(Si2O7)(OH)
Ca(Mg,Fe)Si2O6 + (Ca,Na)(Al,Si)4O8
Opx - in
hornblende + garnet
NaCa2(Mg,Fe,Al)5(Al2Si6O22(OH)2 +
(Ca,Fe,Mg)3(Al,Fe3+)2(SiO4)3
orthopyroxene + plagioclase + water
(Mg,Fe)2Si2O6
+ (Ca,Na)(Al,Si)4O8
Granulite Facies
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
Garnet - in
anorthite + orthopyroxene
CaAl2Si2O8
5(Fe,Mg)SiO3
garnet
+
(Fe,Mg)3Al2(SiO4)3
clinopyroxene
Ca(Mg,Fe)2Si2O6
Summary of Metabasites
brown
green
blue
black
Partial
Melt
Summary of Metabasic Volcanic Rocks
granular
gneissic
brownish
weathered
blackish
greenish
Crystalline
gneissic
fine-grain
schitose if deformed
bluish
un-equilibrated
phyllites
hydrous
partial
melt
Partial Melting of Metabasites:
In the presence of a vapour phase, the partial
melting of metabasite lithologies is typically
controlled by the intersection of the curve for the
upper stability limit of amphibole with the wet
solidus curve for basalt.
In some bulk
compositions, amphibole breaks down to biotite
first in the granulite facies, and then at higher
temperatures biotite breaks down to produce
melting.
amphibole breakdown:
Hb
Cpx + Opx + Plag + H2O
Go
= - RTlnXH2O
(+)
wet melting:
H2O + basalt
Go
Cpx + Opx + silicate melt
= - RTln(1/XH2O)
(-)
Partial Melting of Metabasites:
Melting in metabasite terrains commonly,
however, begins at the granite minimum in
leucosomes of gneisses developed by
metamorphic differentiation.
migmatite
wet-solidus
granite
Subduction Zones
Slab-dehydration
or
Slab-melting
The absence of high P/T metamorphism
in Archean terranes is often cited as
evidence that the temperatures of
subducting slabs were higher in the
Archean. This is consistent with the
proposal that amphibole broke down by
melting in Archean subducting slabs,
rather than by dehydration, as is thought
to be the case today. The melting of
amphibolitized basaltic slabs in the
Archean has been proposed as the
mechanism for the formation of the
voluminous tonalites of which dominate
Archean granite - greenstone terranes.
2 possible scenarios for melting:
In the presence of excess water,
melting begins upon crossing the wet
solidus, but amphibole persists in the
refractory
residue
until
the
amphibole dehydration curve.
The amount of water is sufficiently
small that it is entirely held in the
amphibole (< 2 wt.% water). In this
case, no melting occurs until the
amphibole dehydration curve is
reached.
Oliv + Plag
Opx + Cpx + Spin / Garn
Garnet
Pyroxenite
Plag
Oliv
Oliv
Eclogite
Oliv
Oliv
Plag
The
Basalt - Eclogite
Transition:
Eclogites are high pressure rocks in which the low
pressure plagioclase – pyroxene mineralogy of
basalts and gabbros is converted to an assemblage of
Jadeiite-rich clinopyroxene called omphacite and
pyrope-rich garnet ± kyanite or quartz. Plagioclase is
unstable and cannot be present.
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:
•
eclogite
disappearance of Plagioclase:
Garnet Granulite
Eclogite
Does the basalt – eclogite transition correspond to
the base of the crust (MOHO)?
MOHO
Only two known rock types have the required
density to match that inferred for the mantle
underlying the MOHO.
Eclogite ( = 3.4 - 3.6), a rock composed of
clinopyroxene and garnet which has the same
chemical composition as basalt, but a different
mineralogy because it has crystallized at high
pressure.
Peridotite ( = 3.2 - 3.4), a rock consisting
predominantly of olivine (60 - 80%), with lesser
amounts of orthopyroxene, clinopyroxene, and spinel.
The composition of peridotite is much richer in Mg
and poorer in Al and Si than basalt, thus olivine
(Y2TO4) predominates over pyroxene (YTO3) as the
ferromagnesian mineral, and feldspar is minor or
absent.
The MOHO is too sharp to be caused by the
basalt- eclogite transition, the base of the
crust must represent a compositional change
from basalt to peridotite
MOHO
The base of the crust is defined by the Mohorovicic Discontinuity (MOHO) at which there is an
increase in seismic velocity (Vp = 6.5 --> 8+ km/sec) and, by inference, density ( = 2.9 --> 3.3+
gm/cc). This seismic discontinuity is found virtually everywhere in the world, with the
exceptions of mid-ocean ridges and hotspots, and is very thin (~1km).
The complete transition from granulite to eclogite facies
occurs over a 5 kb pressure range, beginning with the:
•
appearance of Garnet:
Pyroxene Granulite
The
Granulite - Eclogite
Transition:
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 uppermost mantle is
seismically anisotropic
This is more consistent with a peridotite
versus and eclogite upper mantle
Skarns 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.
Metacarbonates
Skarns are typically named on the basis of their most characteristic mineral assemblage,
eg.: olivine-diopside skarn
Prograde Metacarbonate Reactions
3CaMg(CO3)2 + 4SiO2 + H2O
Dolomite
Talc - in
Qtz
Greenwood
Classification
Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
Talc
#6
Calcite
Tremolite - in
5CaMg(CO3)2 + 8SiO2 + H2O
Dolomite
Ca2Mg5Si8O22(OH)2 + 3CaCO3 +7CO2
Qtz
Tremolite
Calcite
Diopside - in
Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 2SiO2
5CaMgSi2O6 + 3CO2 + H2O
Tremolite
Calcite
#6
#4
Diopside
Olivine - in
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2
8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O
Tremolite
Olivine
Dolomite
Ca2Mg5Si8O22(OH)2 + CaCO3
Tremolite
Calcite
CaCO3 + SiO2
Calcite
Tremolite-out
Qtz
Wollastonite-in
#4
Calcite
Mg2SiO4 + 3CaMgSi2O6 + CO2 + H2O
Olivine
CaSiO3 + CO2
Wollastonite
#4
Diopside
#3
Buffering
The metamorphic mineral assemblages developed in rocks are sensitive to the degree to
which rocks behave as closed systems. Two end member situations may be imagined:
•
Internally Buffered Systems
The rock behaves as a closed system, in which the activities of all components
within the system are buffered by the bulk composition. In such systems, the
compositions of the fluid phase may vary greatly as metamorphic reactions release
additional volatiles.
•
Externally Buffered Systems
The rock behaves as a completely open system with respect to fluids such that the
activity of water (and other volatiles) is buffered or controlled outside the system.
In extreme cases, the composition of the fluid phase may be fixed, and control the
mineral assemblage in the rock. In general, a degree of freedom is lost for each
component that is externally buffered, which is typically reflected in a reduction in
the number of phases in stable mineral assemblages. The phase rule becomes:
F = C–P + N
where C = only the number of internally
buffered chemical components, and N = 2
(P, T) + the number of externally buffered
chemical components.
In reality, neither of these two end member cases is typical, but something intermediate prevails. The
key parameter is the fluid / rock ratio, the total weight of fluid that has passed through a unit weight of
rock. In systems in which the fluid / rock ratio is low, the bulk composition of the rock will control the
composition of the fluid phase. However, systems that experience a high through put of fluids from the
outside will be transformed so as to equilibrate with the fluid. Rocks that reflect such situations are
usually characterized by phase assemblages that have apparently high variance (ie. a low number of
phases for the number of apparent components). Extreme examples are mono-mineralic chlorite
alteration assemblages in the fluid pathways responsible for exhalative ore deposits.
Consider the system MgO – H2O – CO2
Magnesite
+
H2O
Brucite
MgCO3
H2O
Mg(OH)2
84
18
58
+
CO2
CO2
44
molecular wt.
So that, in a system with 99 gms magnesite and 1 gm fluid (aCO2 = XCO2 = 0.02),
If at approximately 550oC:
1 gm magnesite + 0.214 gms H2O
0.691 gms brucite + 0.524 gms CO2
Then the amount of fluid would increase to 1.31 gms and the activity of CO2 would be:
XCO2 = 0.544 / 1.31 = 0.42
1 gm magnesite + 0.214 gms H2O
0.691 gms brucite + 0.524 gms CO2
The amount of fluid increases to 1.31 gms and the activity of CO2 would be:
XCO2 = 0.544 / 1.31 = 0.42
But this would terminate the reaction at this
temperature.
In a closed system, as
temperature increases the fluid composition
will follow the curve A – MBP, with magnesite
being slowly consumed to form brucite. At
MBP, both brucite and magnesite would react to
form periclase, and the system would be
invariant until either brucite or magnesite
disappeared. This decision will be determined
by the fluid to rock ratio. When the rock
dominates, brucite will disappear and the XCO2
of the vapour will increase with increasing
temperature. If the weight of initial fluid is
sufficiently large with respect to that of the
rock, or the system is open system with a high
through put of fluid, magnesite will disappear
and the XCO2 will decrease with increasing
temperature.