garnet Hornblende Plag garnet Metabasites Metacarbonates Francis 2014
garnet Hornblende Plag garnet Metabasites / Metacarbonates Francis, 2014
Metabasites: Mafic volcanics such as basalts 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.
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 Ca. Al 2 Si 4 O 12. 6 H 2 O Wn. Tm. O 2 m. s. H 2 O W = Na, Ca, K, (Ba, Sr, …) T = Si, Al Na Zeolites Phillipsite Na 3 Al 3 Si 5 O 16. 6 H 2 O Stilbite Ca. Al 2 Si 7 O 18. 7 H 2 O High T Heulandite Ca. Al 2 Si 7 O 18. 6 H 2 O Analcime Na. Al. Si 2 O 6. H 2 O Laumontite Ca. Al 2 Si 4 O 12. 4 H 2 O Natrolite Na 4 Al 4 Si 6 O 20. 4 H 2 O Wairakite Ca. Al 2 Si 4 O 12. 2 H 2 O Albite - Feldspar Na. Al. Si 3 O 8
Zeolites in volcanic amygdules
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 Ca. Al 2 Si 4 O 12. 4 H 2 O + CO 2 Calcite + Kaolinite + Quartz + Water Ca. CO 3 + Al 2 Si 2 O 5(OH)4 + 2 Si. O 2 + 2 H 2 O (XCO 2 > 0. 01)
Prehnite - Pumpellyite sub-Facies of the Greenschist Facies 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 Ca 2 Al(Al. Si 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. 2 H 2 O sorosilicate T = Si, Al Y = Al, Fe 3+, Ti 4+ X = Mn, Fe 2+, Mg, Al, Fe 3+ W = Ca, K, Na meta-pillow basalt
Greenschist Facies epidote & actinolite - in prehnite + qtz + chlorite zoisite + actinolite + water Ca 2 Al(Al. Si 3)O 10(OH)2 + Si. O 2 + (Mg, Fe)3(Al, Si)4 O 10(OH)2 (Mg, Fe)3(OH)6 Ca 2(Fe, Al)3 O(Si. O 4)(Si 2 O 7)(OH) + Ca 2(Mg, Fe)5 Si 8 O 22(OH)2 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(Si. O 4)2. 2 H 2 O + Si. O 2 + (Mg, Fe)3(Al, Si)4 O 10(OH)2 (Mg, Fe)3(OH)6 Ca 2(Fe, Al)3 O(Si. O 4)(Si 2 O 7)(OH) + Ca 2(Mg, Fe)5 Si 8 O 22(OH)
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(Fe, Al)3 O(Si. O 4)(Si 2 O 7)(OH) + Si. O 2 Ca 2(Mg, Fe)5 Si 8 O 22(OH) + (Ca, Na)(Al, Si)4 O 8
Amphibolite Facies Hornblende - in actinolite hornblende Ca 2(Mg, Fe)5(Si 8 O 22)(OH)2 Na. Ca 2(Mg, Fe, Al)5(Al 2 Si 6 O 22)(OH)2 Garnet & Hb - in epidote + chlorite hornblende + garnet + water Ca 2(Fe, Al)3 O(Si. O 4)(Si 2 O 7)(OH) (Mg, Fe)3(Al, Si)4 O 10(OH)2 (Mg, Fe)3(OH)6 Na. Ca 2(Mg, Fe, Al)5(Al 2 Si 6 O 22(OH)2 + (Ca, Fe, Mg)3(Al, Fe 3+)2(Si. O 4)3
Upper Amphibolite Facies Cpx - in hornblende + epidote clinopyroxene + plagioclase + water Na. Ca 2(Mg, Fe, Al)5(Al 2 Si 6 O 22(OH)2 + Ca 2(Fe, Al)3 O(Si. O 4)(Si 2 O 7)(OH) Ca(Mg, Fe)Si 2 O 6 + (Ca, Na)(Al, Si)4 O 8 hornblende can no longer coexist with epidote Opx - in hornblende + garnet Na. Ca 2(Mg, Fe, Al)5(Al 2 Si 6 O 22(OH)2 + (Ca, Fe, Mg)3(Al, Fe 3+)2(Si. O 4)3 orthopyroxene + plagioclase + water (Mg, Fe)2 Si 2 O 6 + (Ca, Na)(Al, Si)4 O 8 hornblende can no longer coexist with garnet
Granulite Facies Hornblende - out hornblende clinopyroxene + plagioclase + opx + water Na. Ca 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 + (Mg. Fe)Si. O 3
Garnet - in anorthite + orthopyroxene Ca. Al 2 Si 2 O 8 5(Fe, Mg)Si. O 3 garnet + (Fe, Mg)3 Al 2(Si. O 4)3 clinopyroxene Ca(Mg, Fe)2 Si 2 O 6
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
Oliv + Plag Opx + Cpx + Spin / Garnet Pyroxenite Plag Oliv Eclogite Oliv Plag
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 The Granulite - Eclogite Transition: and ending with: • disappearance of Plagioclase: ing as re Eclogite inc Garnet Granulite e ur ss e pr 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.
Meta-Plutonic Rocks mafic magma
Plag Metamorphic Coronas Oliv + Plag Opx + Cpx + Spin or Garn Oliv Plag
calcite grossularite diopside Meta-carbonates / Skarns
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 3 Ca. Mg(CO 3)2 + 4 Si. O 2 + H 2 O Dolomite Talc - in Qtz Mg 3 Si 4 O 10(OH)2 + 3 Ca. CO 3 + 3 CO 2 Talc 5 Ca. Mg(CO 3)2 + 8 Si. O 2 + H 2 O Dolomite Greenwood Classification Tremolite - in Calcite Ca 2 Mg 5 Si 8 O 22(OH)2 + 3 Ca. CO 3 +7 CO 2 Qtz Tremolite Calcite #6 Calcite Diopside - in Ca 2 Mg 5 Si 8 O 22(OH)2 + 3 Ca. CO 3 + 2 Si. O 2 5 Ca. Mg. Si 2 O 6 + 3 CO 2 + H 2 O Tremolite #6 #4 Diopside Olivine - in Ca 2 Mg 5 Si 8 O 22(OH)2 + 11 Ca. Mg(CO 3)2 8 Mg 2 Si. O 4 + 13 Ca. CO 3 + 9 CO 2 + H 2 O Tremolite Olivine Dolomite Ca 2 Mg 5 Si 8 O 22(OH)2 + Ca. CO 3 Tremolite Calcite Ca. CO 3 + Si. O 2 Calcite Tremolite-out Qtz Wollastonite-in #4 Calcite Mg 2 Si. O 4 + 3 Ca. Mg. Si 2 O 6 + CO 2 + H 2 O Olivine Ca. Si. O 3 + CO 2 Wollastonite #4 Diopside #3
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