Oxygen and hydrogen isotope study of minerals from metapelitic rocks, staurolite to sillimanite zones, Mica Creek, British Columbia

Oxygen and hydrogen isotope analyses have been made of coexisting quartz, ilmenite, muscovite, and biotite from Late Precambrian metapelitic rocks, staurolite‐kyanite to K‐feldspar‐muscovite‐sillimanite zones, from Mica Creek, British Columbia. The δ18O and †D values of these minerals are generally uniform and do not decrease significantly with increasing metamorphic grade. This implies that there has not been significant infiltration of deep crustal, possibly magmatic, fluids into the metapelites that has been suggested for other high‐grade metamorphic terranes. The uniformity of oxygen isotope compositions of the Mica Creek metapelite rocks may reflect isotopic uniformity in the sedimentary protolith rather than widespread exchange with an isotopically homogeneous metamorphic pore fluid. Temperature estimates based upon 18O exchange thermometry for samples below the sillimanite zone are in reasonable agreement with the results of garnet‐biotite Fe–Mg exchange thermometry. In the higher grade rocks, the oxygen isotope and garnet‐biotite thermometry yield results which disagree by about 100°C. The highest temperatures recorded by oxygen isotope thermometry, 595°C, are at least 60°C below the minimum temperatures required by phase equilibria. These discrepancies appear to result from pervasive equilibrium retrograde exchange of oxygen isotopes between coexisting minerals. In addition, there are problems with calibration of garnet‐biotite thermometry at higher temperatures. Retrograde oxygen isotope exchange may be a general characteristic of high‐grade metamorphic rocks and oxygen isotope thermometry may not usually record peak metamorphic temperatures if they significantly exceed 600°C.