Abstract

The results of electron-microprobe and optical examination of Ca-poor pyroxenes from anorthositic and intermediate rocks of the Nain complex, Labrador, show a range in host composition from 100Fe/(Fe + Mg) = 34 to 77 and three distinct exsolution types. Type 1 pyroxenes have Bush veld-type exsolution with fine (100) augite lamelle and were primary orthopyroxenes. Type 2 and type 3 pyroxenes are inverted pigeonite with Stillwater-type exsolution. Type 2 varieties display irregular and unoriented lamellae and blebs of augite. Type 3 pyroxenes show complex exsolution features including broad “001” and “100” augite lamellae, multiple sets of pigeonite lamellae exsolving from “001” augite, fine (100) augite lamellae, and patches of uninverted pigeonite. Stacking faults, which are present in orthopyroxene and pigeonite host and “001” augite lamellae, bend 2-4° in a counterclockwise direction as they pass from host into “001” augite.

Exsolution occurred by the mechanism of nucleation and growth for all three pyroxene types. Observations suggest that homogeneous nucleation produced exsolution in type 1 and type 3 pyroxenes but that heterogeneous nucleation is responsible for the irregular exsolution in type 2 pyroxenes. Conventional estimates of crystallization temperatures are unrealistically low for type 1 pyroxenes, probably because of granule exsolution that produced discrete pyroxenes that were not included in microprobe analyses. Type 2 and type 3 pyroxenes crystallized at temperatures between 1100 and 1000°C at approximately 3-kbar pressure and began to exsolve augite. During initial cooling, strain developed along augite-pigeonite interphase boundaries, and stacking faults formed to relieve the strain. Fine stacking faults that propagated into the host from augite lamellae provided the mechanism for the pigeonite to orthopyroxene transformation. Some constrained patches of host pigeonite failed to invert. Coarse augite exsolution continued down to temperatures perhaps as low as 600°C. Pigeonite exsolution in “001” augite likely began before inversion and continued metastably to low but uncertain temperatures.

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