GEOCHEMISTRY AND PETROLOGY OF GARNET SILLIMANITE MIGMATITE AND RESTITE FROM THE COAST MOUNTAINS OF BRITISH COLUMBIA

Garnet-sillimanite gneiss is common within the early Tertiary high-grade core of the Coast Orogen of British Columbia. Trace element and whole rock chemistry of the garnet-sillimanite gneiss are consistent with the rocks being the metamorphic equivalents of volcanoclastic sedimentary rocks and/or shales. The mineralogy of the garnet-sillimanite gneiss includes sillimanite+garnet+plagioclase+ilmenite+biotite+/-quartz+/-cordierite+/-graphite. The garnet in the gneiss commonly contains inclusions of sillimanite, biotite, quartz, plagioclase, kyanite, muscovite and staurolite. The most likely garnet producing reactions based on inclusions are staurolite + muscovite + quartz = garnet + Al₂SiO₅+ biotite + vapor, and biotite + sillimanite = garnet + melt. Assuming the rocks crossed the kyanite-sillimanite boundary after staurolite breakdown, the rocks experienced peak metamorphic conditions of 7-8 kbar and temperatures >750 ^O C. Cordierite coronas on garnet and sillimanite track high temperature decompression during biotite dehydration melting, with the P-T path ending in the andalusite stability field.

Geochemically, some of the gneisses plot near unity on REE diagrams normalized to volcanogenic sedimentary rocks and shales. Others show enrichment of the HREE. The rocks with largest HREE enrichment are depleted in biotite and contain more sillimanite when compared with rocks that show no HREE enrichment. The sample with the largest HREE enrichment also has a small negative Eu anomaly consistent with removal of plagioclase. Because garnet preferentially retains the HREE with respect to melts, the trace element chemistry and mineralogy are consistent with biotite dehydration melting and variable degrees of melt extraction. Garnet-sillimanite gneiss with HREE enrichment is interpreted as restite produced by melt extraction, whereas the samples without HREE enrichment are interpreted as migmatite that retained melt. The melts that escaped the migmatites would have been enriched in K, Na and Si and depleted in HREEs. Garnet-sillimanite gneiss, such as described here, are likely to be a wide-spread lithology in the lower continental crust.