Tschermak Substitution in Low- and Middle-grade Pelitic Schists

In low- and middle-grade pelitic metamorphic rocks, the extentsof Tschermak substitution in muscovite, chlorite and biotitechange regularly with bulk-rock composition and external conditions.This paper gives a theoretical analysis of the changes. From equipotential lines for Al₂O₃ plotted on AFM diagrams,we have derived a series of Thompson-type muscovite compositiondiagrams, which show how the celadonite content of muscovitevaries with the associated ferromagnesian minerals or with thebulk-rock composition under constant external conditions. Thedistribution coefficient of the exchange reaction for Tschermaksubstitution between muscovite and chlorite varies greatly notonly with temperature but also with the extent of this substitutionin the two minerals because of their strong deviation from ideality.Muscovites with a high celadonite content (phengites) occurin low-temperature rocks in any of the high-, medium- and low-pressuretypes of metamorphism, probably because the exchange equilibriumfor Tschermak substitution between muscovite and chlorite isnot sensitive to pressure. When both Mg-Fe and Tschermak substitutionoccur in muscovite and some other silicates, a metapelite containingthree AFM phases together with muscovite and quartz has at leasttwo independent net-transfer reactions that take place sideby side with changing external conditions. The mass balancerequirement in the rock is imposed on a linear combination ofthe two reaction equations, leading to a constraint on the stoichiometricequations among phase components and the progressive compositionalchanges of muscovite, chlorite and biotite. From such a viewpoint,we examine reactions and progressive mineralogical changes inmetapelites, beginning with K-feldspar-bearing low-grade metapeliticrocks, in which biotite appears by reaction of K-feldspar withchlorite at a temperature lower than that of the biotite isograddefined for K-feldspar-free pelitic rocks. When both Mg-Fe and Tschermak substitution occur in muscoviteand some other silicates, a metapelite containing three AFMphases together with muscovite and quartz has at least two independentnet-transfer reactions that take place side by side with changingexternal conditions. The mass balance requirement in the rockis imposed on a linear combination of the two reaction equations,leading to a constraint on the stoichiometric equations amongphase components and the progressive compositional changes ofmuscovite, chlorite and biotite. From such a viewpoint, we examinereactions and progressive mineralogical changes in metapelites,beginning with K-feldspar-bearing low-grade metapelitic rocks,in which biotite appears by reaction of K-feldspar with chloriteat a temperature lower than that of the biotite isoerad definedfor K-feldsoar-free oelitic rocks. The equations for the reactions that produce biotite or biotite? garnet in K-feldspar-free metapelites have been derived. Combinedwith the composition relations of coexisting muscovite, chlorite,biotite and garnet, they lead to the inference that progressof these reactions with rising temperature causes a decreaseof the celadonite content of muscovite, the antigorite contentof chlorite and the phlogopite content of biotite in the metapelitesof the chlorite, biotite and lower garnet zones, and that thistrend may not continue into the staurolite zone. This inferencehas been verified by examination of the analytical data of mineralsin four progressive metamorphic terranes covering the high-,medium- and low-pressure types. The composition of chlorite is buffered by the garnet-producingreaction so that chlorites in garnet-bearing metamorphic rocksdevelop (Mg ? Fe)/Al₂ ratios close to that of almandine at elevatedtemperatures. The maximum possible celadonite content in muscoviteunder given external conditions is realized in K-feldspar-bearingrocks, and decreases with rising temperature.