Oriented growth of sillimanite in andalusite, Placitas-Juan Tabo area, New Mexico, U.S.A

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Abstract

In metapelitic schists near the Sandia granite in this area of New Mexico, nucleation sites for sillimanite are very variable, but growth of sillimanite in andalusite is common, either as bundles of fibrous sillimanite crystals or as coarser-grained parallel prisms or dendritic crystals of sillimanite. Optical relationships indicate a coaxial relationship between andalusite and sillimanite but with a and b axes interchanged. Stacking faults formed by dissociated screw locations in the andalusite may have provided nucleation sites for sillimanite. Some large sillimanite grains appear to have formed by coarsening of fibrous sillimanite, but others show no evidence of such coarsening. Some fibrous sillimanite occurs in folia in zones of inferred non-coaxial strain accumulation, and some occurs in radiating or crystallographically controlled aggregrates in low-strain zones between sillimanite folia. These aggregates do not necessarily indicate post-deformation growth, except on a very small scale, and many of the fibrous sillimanite aggregates have been deformed. Hence, the sillimanite is a product of syn-deformational rather than post-deformational metamorphism, although regional metamorphism and granite emplacement may have been broadly synchronous. The andalusite-sillimanite microstructural relationships indicate a prograde P-T-time path similar to those in other low-P (andalusite-sillimanite) metamorphic areas.-P.Br.

Original languageEnglish
Pages (from-to)580-590
Number of pages11
JournalCanadian journal of earth sciences
Volume24
Issue number3
Publication statusPublished - 1987

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