Glaucophane chloritoid-bearing assemblages from NE Oman: petrologic significance and a petrogenetic grid for high P metapelites |
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| Authors: | A K El-Shazly J G Liou |
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| Institution: | (1) Geology Department, Stanford University, 94305 Stanford, CA, USA |
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| Abstract: | Pelitic layers and lenses interbedded with blueschists and eclogites in Saih Hatat, NE Oman contain chloritoid- and sodic amphibole-bearing mineral assemblages that are useful for reconstructing the P-T history of the area. Textural and mineral chemical relations suggest that coexisting glaucophane (Gln) and chloritoid (Ctd) formed at the expense of chlorite (Chl)+paragonite (Pg) and later broke down to garnet (Gt)+Pg during prograde metamorphism according to the reaction: Gln+Ctd+Qz=Gt+Pg+H2O. During retrogression, Gln and Chl first formed at the expense of Gt and Pg, followed by the breakdown of Ctd and Gt to Chl. The final stages of retrogression are marked by the breakdown of Gln to an aggregate of Chl+albite (Ab). A projection from quartz (Qz), H2O and phengite (Ph) on the (Al2O3+Fe2O3)-(FeO+MgO)-Na2O plane in the system NFMASH is best suited for the representation of the phase relations in high P metapelites. Petrogenetic grids for the model systems NMASH and NFASH were calculated using program GEO-CALC (Berman et al. 1987) and its database (Berman 1988) after the retrieval of S
i
o
and  H
f
o
for Gln and Ctd by mathematical programming and calculating all possible reactions among Gln, Ctd, Chl, jadeite (Jd), Ab, Gt, Pg, talc (Tc), pyrophyllite (Prl) and kyanite (Ky). The calculated petrogenetic grid for the system NFASH shows that Fe-Ctd and ferroglaucophane coexist at P>6.5 kbar and T<525°C, whereas the assemblage: Ct-Gln-Pg is stable between 435 and 630°C and P>6 kbar. This grid is consistent with the P-T estimates for high P metapelites from Oman, New Caledonia, Seward Peninsula, Ile de Groix, Sifnos and Peloponnese, where Gln+Ctd bearing units are interbedded with cofacial mafic blueschists and eclogites. The grid also explains the observed textural relations in the metapelites of Oman, and is consistent with the  clockwise P-T path proposed for this area, but differs from the grids of Guiraud et al. (1990) in showing a larger stability field for Gln+Ctd that overlaps with the stability fields of Jd+Qz as well as Ab. The petrogenetic grid calculated for the system NMASH shows that Mg-Ctd+Gln-bearing assemblages require unusually low geothermal gradients to form in metapelites. It also shows that Mg-rich Ctd+Tc coexist at high pressures over a wider P-T range than predicted by Guiraud et al. (1990). This grid can therefore be applied to high P metamorphic assemblages from the eastern and western Alps.Abbreviations
Ab
Albite
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Act
Actinolite
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Alm
Almandinc
-
a
i
activity of component i
-
Cal
Calcite
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Car
Carpholite
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Chl
Chlorite
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Co
Corundum
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Coes
Coesite
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Cpx
Clinopyroxene
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Cr
Crossite
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Ctd
Chloritoid
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Czo
Clinozoisite
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Dap
Daphnite
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Ep
Epidote
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Fgl
Ferroglaucophane
- G
f
o
Standard state Gibbs free energy of formation of phase i
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Gln
Glaucophane
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Grph
Graphite
-
Gt
Garnet
- H
f,i
m
Standard state enthalpy of formation of phase i
-
Hem
Hematite
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Ilm
Ilmenite
-
Jd
Jadeite
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Ky
Kyanite
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Lws
Lawsonite
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Mgt
Magnetite
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Omph
Omphacite
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Pg
Paragonite
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Ph
Phengite
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Prl
Pyrophyllite
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Qz
Quartz
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Rt
Rutile
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S
i
a
Third law entropy of phase i
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St
Staurolite
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Tc
Talc
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Tr
Tremolite
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Ttn
Titanite (Sphene)
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V
H2O
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WM
white mica
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Zo
Zoisite |
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| Keywords: | |
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