Hornblende Plag Metabasites Francis, 2014 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 H 2 O. 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: W n T m O 2m.sH 2 O W = Na, Ca, K, (Ba, Sr,…) T = Si, Al Zeolite Facies Ca Zeolites Na Zeolites Low T ChabazitePhillipsite CaAl 2 Si 4 O 12.6H 2 ONa 3 Al 3 Si 5 O 16.6H 2 O Stilbite CaAl 2 Si 7 O 18.7H2O HeulanditeAnalcime CaAl 2 Si 7 O 18.6H 2 ONaAlSi 2 O 6.H 2 O LaumontiteNatrolite CaAl 2 Si 4 O 12.4H 2 ONa 4 Al 4 Si 6 O 20.4H 2 O High TWairakiteAlbite - Feldspar CaAl 2 Si 4 O 12.2H 2 ONaAlSi 3 O 8

Zeolite assemblages are stable only at relatively low PCO 2. Even at relatively modest levels of CO 2, zeolite mineral assemblages are commonly replaced by carbonate and clay minerals. Laumontite + CO 2 Calcite + Kaolinite + Quartz + Water CaAl 2 Si 4 O 12.4H 2 O + CO 2 CaCO 3 + Al 2 Si 2 O 5 (OH) 4 + 2SiO 2 + 2H 2 O (XCO 2 > 0.01)

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 - Pumpellyite sub-Facies Prehnite Ca 2 Al(AlSi 3 )O 10 (OH) 2 brittle mica Pumpellyite W 4 X(OH,O)Y 5 O(OH) 3 (TO 4 ) 2 (T 2 O 7 ) 2.2H 2 O sorosilicate T = Si, Al Y = Al, Fe 3+, Ti 4+ X = Mn, Fe 2+, Mg, Al, Fe 3+ W = Ca, K, Na Lawsonite CaAl 2 (OH) 2 SiO 2.H 2 O

Greenschist Facies epidote & actinolite - in prehnite + qtz + chlorite zoisite + actinolite + water Ca 2 Al(AlSi 3 )O 10 (OH) 2 + SiO 2 + Ca 2 (Fe,Al) 3 O(SiO 4 )(Si 2 O 7 )(OH) + Ca 2 (Mg,Fe) 5 Si 8 O 22 (OH) 2 (Mg,Fe) 3 (Al,Si) 4 O 10 (OH) 2  (Mg,Fe) 3 (OH) 6 epidote & actinolite - in pumpellyite + qtz + chlorite zoisite + actinolite + water Ca 4 (Mg,Fe)(Al,Fe 3+ ) 5 O(OH) 3 (Si 2 O 7 ) 2 (SiO 4 ) 2.2H 2 O Ca 2 (Fe,Al) 3 O(SiO 4 )(Si 2 O 7 )(OH) + Ca 2 (Mg,Fe) 5 Si 8 O 22 (OH) + SiO 2 + (Mg,Fe) 3 (Al,Si) 4 O 10 (OH) 2  (Mg,Fe) 3 (OH) 6

Ca-plag - in chlorite + zoisite + qtz actinolite + plagioclase + water (Mg,Fe) 3 (Al,Si) 4 O 10 (OH) 2  (Mg,Fe) 3 (OH) 6 Ca 2 (Mg,Fe) 5 Si 8 O 22 (OH) + (Ca,Na)(Al,Si) 4 O 8 + Ca 2 (Fe,Al) 3 O(SiO 4 )(Si 2 O 7 )(OH) + SiO 2

Hornblende - in actinolitehornblende Ca 2 (Mg,Fe) 5 (Si 8 O 22 )(OH) 2 NaCa 2 (Mg,Fe,Al) 5 (Al 2 Si 6 O 22 )(OH) 2 Garnet - in epidote + chloritehornblende + garnet + water Ca 2 (Fe,Al) 3 O(SiO 4 )(Si 2 O 7 )(OH) NaCa 2 (Mg,Fe,Al) 5 (Al 2 Si 6 O 22 (OH) 2 + (Mg,Fe) 3 (Al,Si) 4 O 10 (OH) 2  (Mg,Fe) 3 (OH) 6 (Ca,Fe,Mg) 3 (Al,Fe 3+ ) 2 (SiO 4 ) 3 Amphibolite Facies

Cpx - in hornblende + epidoteclinopyroxene + plagioclase + water NaCa 2 (Mg,Fe,Al) 5 (Al 2 Si 6 O 22 (OH) 2 + Ca(Mg,Fe)Si 2 O 6 + (Ca,Na)(Al,Si) 4 O 8 Ca 2 (Fe,Al) 3 O(SiO 4 )(Si 2 O 7 )(OH) Opx - in hornblende + garnet orthopyroxene + plagioclase + water NaCa 2 (Mg,Fe,Al) 5 (Al 2 Si 6 O 22 (OH) 2 + (Mg,Fe) 2 Si 2 O 6 + (Ca,Na)(Al,Si) 4 O 8 (Ca,Fe,Mg) 3 (Al,Fe 3+ ) 2 (SiO 4 ) 3 Upper Amphibolite Facies

Hornblende - out hornblende clinopyroxene + plagioclase + opx + water NaCa 2 (Mg,Fe,Al) 5 (Al 2 Si 6 O 22 (OH) 2 Ca(Mg,Fe)Si 2 O 6 + (Ca,Na)(Al,Si) 4 O 8 + (MgFe)SiO 3 Granulite Facies

Garnet - in anorthite + orthopyroxene garnet + clinopyroxene CaAl 2 Si 2 O 8 5(Fe,Mg)SiO 3 (Fe,Mg) 3 Al2(SiO4) 3 Ca(Mg,Fe) 2 Si 2 O 6

Summary of Metabasites brown green black blue Partial Melt

Summary of Metabasic Volcanic Rocks brownish weathered bluish un-equilibrated phyllites hydrous partial melt greenish fine-grain schitose if deformed blackish Crystalline gneissic granular gneissic

Hb Cpx + Opx + Plag + H 2 O H 2 O + basaltCpx + Opx + silicate melt amphibole breakdown: wet melting:  G o = - RTlnX H2O (+)  G o = - RTln(1/X H2O ) (-) 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.

migmatite Partial Melting of Metabasites: Melting in metabasite terrains commonly, however, begins at the granite minimum in leucosomes of gneisses developed by metamorphic differentiation. wet-solidus granite

Slab-dehydration or Slab-melting Subduction Zones 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.

Eclogite Garnet Pyroxenite Oliv Plag Oliv 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 Basalt - Eclogite Transition: eclogite 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. Does the basalt – eclogite transition correspond to the base of the crust (MOHO)?

Only two known rock types have the required density to match that inferred for the mantle underlying the MOHO. Eclogite (  = ), 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 (  = ), a rock consisting predominantly of olivine ( %), 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 (Y 2 TO 4 ) predominates over pyroxene (YTO 3 ) as the ferromagnesian mineral, and feldspar is minor or absent. MOHO

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 geotherm

The base of the crust is defined by the Mohorovicic Discontinuity (MOHO) at which there is an increase in seismic velocity (Vp = > 8+ km/sec) and, by inference, density (  = > 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). MOHO

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

The uppermost mantle is seismically anisotropic This is more consistent with a peridotite versus and eclogite upper mantle

Metacarbonates 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. Skarns are typically named on the basis of their most characteristic mineral assemblage, eg.: olivine-diopside skarn

3CaMg(CO 3 ) 2 + 4SiO 2 + H 2 O Mg 3 Si 4 O 10 (OH) 2 + 3CaCO 3 + 3CO 2 # 6 Dolomite Qtz Talc Calcite 5CaMg(CO 3 ) 2 + 8SiO 2 + H 2 O Ca 2 Mg 5 Si 8 O 22 (OH) 2 + 3CaCO 3 +7CO 2 # 6 Dolomite Qtz Tremolite Calcite Ca 2 Mg 5 Si 8 O 22 (OH) 2 + 3CaCO 3 + 2SiO 2 5CaMgSi 2 O 6 + 3CO 2 + H 2 O # 4 Tremolite CalciteDiopside Ca 2 Mg 5 Si 8 O 22 (OH) CaMg(CO 3 ) 2 8Mg 2 SiO CaCO 3 + 9CO 2 + H 2 O #4 Tremolite Dolomite Olivine Calcite Ca 2 Mg 5 Si 8 O 22 (OH) 2 + CaCO 3 Mg 2 SiO 4 + 3CaMgSi 2 O 6 + CO 2 + H 2 O # 4 Tremolite Calcite Olivine Diopside CaCO 3 + SiO 2 CaSiO 3 + CO 2 # 3 Calcite Qtz Wollastonite Prograde Metacarbonate Reactions Greenwood Classification Talc - in Tremolite - in Diopside - in Olivine - in Tremolite-out Wollastonite-in

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: 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: Buffering 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. Magnesite + H 2 O Brucite + CO 2 MgCO 3 H 2 O Mg(OH) 2 CO molecular wt. So that, in a system with 99 gms magnesite and 1 gm fluid (aCO 2 = XCO 2 = 0.02), If at approximately 550 o C: 1 gm magnesite gms H 2 O gms brucite gms CO 2 Then the amount of fluid would increase to 1.31 gms and the activity of CO 2 would be: XCO 2 = / 1.31 = 0.42 Consider the system MgO – H 2 O – CO 2

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 XCO 2 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 XCO 2 will decrease with increasing temperature. 1 gm magnesite gms H 2 O gms brucite gms CO 2 The amount of fluid increases to 1.31 gms and the activity of CO 2 would be: XCO 2 = / 1.31 = 0.42