The Crossite Content of Ca-Amphibole as a Guide to Pressure of Metamorphism

A correlation between the crossite component (NaM₄) in Ca-amphiboleand pressure of metamorphism has long been recognized (Shido& Miyashiro, 1959), but only recently has the reaction beenidentified which buffers this aspect of amphibole composition(Brown, 1974): Ca-amphibole+iron oxide+albite+chloriteI+H₂O (±stilp,qtz) = crossite+epidote (±muscovite, qtz). The exact stoichiometry of the reaction depends on compositionalvariables in the minerals, especially Fe²⁺/Mg and Fe³⁺/Al. Ca-amphiboleshould have fixed NaM₄, at any given T and P, where it coexistswith iron oxide, albite, and chlorite. Comparison of Ca-amphibole composition with mineral assemblage,in rocks from Otago, N.Z., and elsewhere, supports this hypothesis.In any terrane NaM₄ is nearly constant at a particular metamorphicgrade where amphibole exists in the buffering assemblage, butvaries widely outside of this assemblage. Variations in Fe²⁺/Mgand Fe³⁺/Al in the amphibole have relatively little effect onNaM₄, but in high pressure amphiboles NaM₄ varies inverselywith Al^iv. Ca-amphiboles from high pressure areas have substantially moreNaM₄ (Otago, 0.6 of 2.0) than those from lower pressure areas(Sierra contact aureoles, 0.1). These relations suggest thatin the buffering assemblage, the NaM₄ content of Ca-amphiboleshould be a useful relative barometer for low to medium grademetamorphic rocks.