Aluminum silicates in the Mount Raleigh pendant, British Columbia

In regionally metamorphosed pelites of the Mount Raleigh pendant, the fibrolite isograd occurs 5km downgrade from the sillimanite isograd. Fibrolite formed from the decomposition of biotite, a reaction that probably resulted from the late‐stage influx of acidic volatiles. In contrast, sillimanite formed by the direct,‘volume‐for‐volume’replacement of andalusite. Andalusite and sillimanite coexist in a 3 km‐wide zone above the sillimanite isograd. Electron probe analyses of these phases reveal low minor element contents and yield K D [= X ] values close to unity; the low Fe 2 O 3 contents are compatible with reducing conditions implied by the ubiquity of graphite. Because K D → 1.0, the zone of coexisting andalusite + sillimanite cannot be attributed to multivariancy resulting from partitioning of minor elements between these phases. Rather, the metastable persistence of andalusite into the sillimanite P‐T stability field is suggested. The modal proportions of sillimanite versus andalusite imply that minimal (<5%) and alusitesillimanite reaction occurred in a zone 1.5km above the sillimanite isograd; in contrast, there was a marked increase in reaction progress immediately above this zone. With an estimated thermal gradient (in the plane of exposure) of approximately 20°C/km, the 1.5 km‐wide zone of nil reaction suggests that the andalusite‐sillimanite equilibrium boundary was overstepped by about 30 °C before significant reaction occurred. Inclusion‐rich areas in andalusite provided favourable sites for sillimanite nucleation ; however, the growth of sillimanite may have been impeded by‘pinning’of sillimanite grain boundaries by inclusions.