Contrib Mineral Petrol (1991) 107 : 180-201 Contributions to Mineralogy and Petrology 9 Springer-Verlag1991 Glaucophane chloritoid-bearing assemblages from NE Oman: petrologic significance and a petrogenetic grid for high P metapelites A.K. El-Shazly and J.G. Liou Geology Department, Stanford University, Stanford, CA 94305, USA Received December 13, 1989 / Accepted August 22, 1990 Abstract, Pelitic layers and lenses interbeddcd with blueschists and eclogites in Saih Hatat, NE Oman con- tain chloritoid- and sodic amphibole-bearing mineral as- semblages that are useful for reconstructing the P-T history of the area. Textural and mineral chemical rela- tions 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+HzO. 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), H20 and phengite (Ph) on the (A1203+F%Oa)-(FeO+MgO)-Na20 plane in the system NFMASH is best suited for the representation of the phase relations in high P meta- pelites. Petrogenetic grids for the model systems NMASH and NFASH were calculated using program GEO-CALC (Berman et al. 1987) and its database (Ber- man 1988) after the retrieval of S ~ and AH~ 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 Offprint requests to: A.K. E1-Shazly Abbreviations. Ab = Albite; Act = Actinolite; Alto = Almandinc; ai=activity of component "i"; Cal=Calcite; Car=Carpholite; Chl = Chlorite; Co = Corundum; Coes = Coesite; Cpx = Clinopy- roxene; Cr = Crossite; Ctd= Chloritoid; Czo = Clinozoisite; Dap = Daphnite; Ep = Epidote; Fgl = Ferroglaucophane; A G~' = Standard state Gibbs free energy of formation of phase "i"; Gln= Glauco- phane; Grph = Graphite; Gt = Garnet; A/-Ff' i = Standard state en- thalpy of formation of phase "i"; Hem = Hematite; Ilm = Ilmenite; Jd=Jadeite; Ky=Kyanite; Lws=Lawsonite; Mgt=Magnetite; Omph = Omphacite; Pg = Paragonite; Ph = Phengite; Prl = Pyro- phyllite; Qz = Quartz; Rt = Rutile; S[' = Third law entropy of phase "i"; St = Staurolite; To-- Talc; Tr = Tremolite; Ttn= Titanite (Sphene); V= H20; WM = white mica; Zo = Zoisite P>6.5 kbar and T<525 ~ C, whereas the assemblage: Ct-Gln-Pg is stable between 435 and 630~ and P>6 kbar. This grid is consistent with the P-T esti- mates for high P metapelites from Oman, New Caledon- ia, Seward Peninsula, Ile de Groix, Sifnos and Pelopon- nese, where Gln+ Ctd bearing units are interbedded with cofacial mafic blueschists and eclogites. The grid also explains the observed textural relations in the metape- lites 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 stabili- ty 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 west- ern Alps. Introduction In the past few decades, petrologic studies of high P metamorphism have focused mainly on rocks of mafic composition characterized by mineral assemblages that yield considerable information about their P-T condi- tions of formation. Quartzofeldspathic and pelitic rocks interbedded with marie blueschists and eclogites have attracted attention only recently (e.g. Koons 1984; Thur- ston 1985; Ghent etal. 1987; Vuichard and BallSvre 1988; Schliestedt and Okrusch 1988), after Chopin (1981), Chopin and Schreyer (1983) and Koons and Thompson (1985) pointed out the sensitivity of such compositions to high PT metamorphism. Pelitic rocks with coexisting glaucophane and chlori-