Transcript Slide 1
garnet Hornblende Plag garnet garnet Metabasites / Metacarbonates 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 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 CaAl2Si4O12.4H2O + CO2 Calcite + Kaolinite + Quartz + Water CaCO3 + Al2Si2O5(OH)4 + 2SiO2 + 2H2O (XCO2 > 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 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 & Hb - 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 hornblende can no longer coexist with epidote 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 hornblende can no longer coexist with garnet 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 Metabasic Volcanic Rocks granular brownish weathered blackish greenish crystalline fine-grain bluish un-equilibrated Partial Melt garnet pyroxenite Eclogite Meta-Plutonic Rocks mafic magma Plag Metamorphic Coronas Oliv Oliv + Plag Opx + Cpx + Spin or Garn Oliv Plag calcite 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 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