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Petrology of whiteschists from Mautia Hill, Tanzania:
Fluid infiltration under high-grade metamorphic conditions?
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Staff:
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N. Jöns
V. Schenk
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Funding:
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N.N.
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Publications:
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N. Jöns & V. Schenk (2004): Petrology of whiteschists and
associated rocks at Mautia Hill (Tanzania): Fluid infiltration during
high-grade metamorphism? Journal of Petrology 45,
1959-1981.
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Abstract |
doi:
10.1093/petrology/egh044 |
Conference abstracts (pdf files):
DMG 2002;
DMG 2003;
CAG 2004
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Breakdown of magnesiohornblende, kyanite and Mn-oxides. Yoderite,
manganian andalusite (Vir) and piemontite are formed.
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The talc-kyanite schists (whiteschists) at Mautia Hill (Mozambique Belt,
central Tanzania) are famous for a variety of unusual minerals, e. g. yellow
sapphirine, green yoderite, högbomite, kornerupine, piemontite and
manganian andalusite ("viridine"), and they are still the only known rocks
that contain purple yoderite. These highly oxidised minerals have been
found to occur not only in whiteschists, but also in amphibole-chlorite schists,
which are closely associated with metabasites, metapelites, marbles,
quartzites and migmatitic gneisses of granitic composition. The close
association and common deformation history make it most likely that all
lithological units experienced the same metamorphic evolution. Conventional
geothermobarometry, in addition to equilibria of the uncommon mineral
assemblages and reaction textures have been used to elucidate the
metamorphic P-T path and the P-T-aH2O/fO2
conditions leading to the formation of the rare, oxidised minerals.
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Yoderite and piemontite in a quartz rich talc-kyanite schist (whiteschist)
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Pressures during peak metamorphism have been estimated with the
garnet-kyanite-quartz-plagioclase (GASP; metapelites) and the
garnet-plagioclase-clinopyroxene-quartz equilibria (GADS; metabasites) at
10-11 kbar. This estimate is confirmed by the high-fO2
barometric assemblage hollandite + kyanite + quartz + Mn-andalusite with
up to 19.5 % Mn2SiO5 component. The
maximum temperature is restricted to <720°C by the occurrence of
yoderite + quartz. However, conventional Fe-Mg exchange thermometry
(Grt-Bt, Grt-Cpx, Grt-Hbl) seems to point to somewhat higher temperatures
(730-800°C).
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This is a spinel, which contains lamellae of haematite and
högbomite.
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Prograde kyanite inclusions in metapelitic garnet and late-stage
cordierite reaction rims between yoderite and talc confirm earlier suggestions
of a clockwise P-T path, based only on yoderite reaction rims between talc
and kyanite.
The omnipresence of prograde oxide minerals like haematite,
pseudobrookite and rutile evidences a prograde oxidation process.
Therefore, formation of yoderite + quartz at pressures of 10-11 kbar has to
be attributed to an increase of water activity, which can be explained by the
infiltration of an aqueous fluid. The highly oxidising conditions resulted in
iron and manganese occurring predominantly in the trivalent state and
therefore being mainly restricted to oxide minerals. Coexisting Fe-Mg
silicates are nearly pure Mg end-members in which ferric iron substitutes
for aluminium. The formation of the talc-kyanite-quartz assemblage
instead of yoderite + quartz near peak-metamorphic conditions at 10-11
kbar and c. 700°C implies that the increase of water activity and the
proposed associated fluid infiltration took place near the peak of
metamorphism or during the early stages of isothermal uplift. This was at a
depth of about 30 to 35 km.
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This intergrowth of haematite and rutile is due to breakdown of
pseudobrookite, which is also present as a relic.
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The phase relations and the reaction history of whiteschists and
amphibole-chlorite schists can be described in the chemical systems
MgO-Al2O3-SiO2-H2O-Fe2O3
(MASH-Fe2O3) and
CaO-MgO-Al2O3-SiO2-H2O-CO2-Fe2O3
(CMASH-CO2-Fe2O3),
respectively. The formation of different peak assemblages of
MASH-Fe2O3 whiteschists (Tlc-Ky-Qtz, Tlc-Ky,
Spr-Chl-En) reflect different chlorite/quartz modal ratios in the lower grade
precursor rocks. Near isothermal post-peak decompression in combination
with increase of water activity by fluid infiltration resulted in the breakdown of
the high-pressure assemblage talc + kyanite + haematite +
H2O to purple yoderite + quartz and aluminous
anthophyllite (Tlc+Ky+V=Ath+Qtz). Mn-poor "green" yoderite
forms reaction rims around kyanite and is itself surrounded by cordierite
(Tlc+Qtz+Yod=Crd+Hem +V). A metasomatic rehydration is evidenced by
the reaction En+Spr+V=Chl+Krn in quartz-deficient chlorite-enstatite schists.
In most CMASH-CO2-Fe2O3 rocks,
the peak-metamorphic assemblages contain magnesiohornblende +
kyanite.
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In quartz free schists talk and kyanite are separated from each other by
yoderite and cordierite.
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During late-stage metamorphism, this assemblage broke down to form
yoderite, piemontite, talc and Mn-andalusite
(Hbl+Ky+Mn-Ox+V=Yod+Tlc+Piem and Ky+Qtz+Mn-Ox=Mn-And), which
represents a new yoderite-forming reaction. Magnesiohornblende + kyanite
seems to be a high-pressure assemblage similar to the whiteschist
assemblage talc + kyanite. In other bulk rock chemistries other hornblende
consuming reactions developed (Hbl+V=Dol+Chl+En+SiO2 and
Hbl+Spl+V=Chl/En+Spr+Dol). The spinel in a
CMASH-CO2-Fe2O3 rock contains a
small magnesioferrite component (3%), is slightly birefringent and, like the
coexisting sapphirine, of yellow colour. It exsolves haematite and
högbomite lamellae.
In summary, the formation of whiteschists at Mautia Hill is attributed to
crustal thickening, occurring in the Mozambique Belt during Pan-African
continent collision. During peak metamorphism and the initial stages of
near-isothermal uplift, most likely water infiltration lead to an increase of
H2O that stabilized yoderite and other highly oxidised
silicates at the expense of talc + kyanite and hornblende + kyanite
coexisting with haematite and Fe-Mn oxides.
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These ostentatious rims of yoderite and quartz are formed by consumation
of talc, kyanite and haematite.
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Yoderite and manganian andalusite in a talc-kyanite schist.
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