CRUSTAL MELTING INVOLVING PHENGITIC WHITE MICA: OBSERVATION AND THEORY

White mica with a significant Al-celadonite component (phengite) is a likely phase involved in the initial stages of crustal melting at P>=10 kbar. Evidence of such melting is generally lacking in mid- to lower crustal migmatites because either the bulk compositions were not appropriate or later melting reactions involving biotite and garnet consumed the earlier-formed melt products. Muscovite-free migmatites in the Wilson Lake area of Labrador are exceptional in this regard because of an unusual event in their near isobaric cooling history. Discordant and undeformed quartzo-feldspathic veins were emplaced and crystallized within the stability field of both kyanite and phengite at about P=10 kbar and T= 750-850oC. These conditions are well below the peak metamorphic temperatures in the host migmatite where the assemblages sapphirine + quartz and orthopyroxene + sillimanite + quartz are stable. Phengite crystallized from the veins ranges from 3.1 to 3.2 Si/11 oxygen formula unit. Phengite (+ quartz) subsequently melted by decompression in response to relatively rapid uplift of the entire terrane. The melting products are now pseudomorphs comprised of kyanite, K-feldspar and phlogopite. Evidence of the peraluminous melt produced by this reaction still remains in the vein matrix as interstitial domains enriched in fine-grained K-feldspar. These results are consistent with experimental melting in the KFMASH system under fluid-absent conditions.

The compositions of octahedral and trioctahedral micas coexisting with melt behave in a regular and predictable manner. Vein phenocrysts of phlogopite have compositions enriched in the same "eastonite" component as phlogopite in the pseudomorphs. Because these micas share the same SiMgAl-2 exchange, less eastonitic phlogopite is associated with more phengitic white mica as a function of bulk composition and P-T conditions. These data provide a useful calibration for phengite melting under more extreme conditions.