Publication Type:Book Chapter
Source:Goldschmidt 2012 abstract volume, Mineralogical Society, London, United Kingdom, Volume 76, p.1752 (2012)
Keywords:British Columbia, Canada, chemical composition, electron probe data, geologic thermometry, metals, Metamorphic rocks, metapelite, metasedimentary rocks, mineral composition, oxides, pelitic texture, Precambrian, rutile, textures, Western Canada, zirconium
Rutile occurs both as inclusions in garnet crystals and in the matrices of Late Precambrian metapelitic rocks in the Mica Creek area, British Columbia. Zr contents of these rutile crystals have been analyzed by electron microprobe. The Zr detection limit is 25 ppm (20 samples). Metamorphic grade ranges from staurolite-kyanite zone to K-feldspar-sillimanite zone. Temperatures are based upon experimental calibrations. Temperature (T) estimates for rutile inclusions in garnet range from 555 to 705 degrees C (calculated at 7 kbar). T's for groundmass rutile range from <475 to 700 degrees C. There is no apparent correlation of T with the metamorphic grade. In most samples there is no statistical difference between T's from inclusions and T's from matrix rutile. Many of these inclusion T's are simlar to those obtained from garnet-biotite geothermometry. Isopleths for garnet in isochemical phase diagram sections when compared to core garnet compositions yield T's which are much lower than those from Zr geothermometry. The isochemical phase diagram sections also suggest that garnet and rutile are not stable together except at high P-T. T's based upon Zr geothermometry for inclusions appear to be too high for the equilibrium growth of garnet at lower metamorphic grade. A study of Ti in quartz geothermometry for four samples (inclusions in garnet) yielded T's ranging from 417 to 553 degrees C. Zr in rutile has been analyzed on two different microprobes and by laser ablation ICP-MS with consistent results. Other workers have found reasonable correlation between Zr T's and those based upon other methods. For the Mica Creek samples disequilibrium is a possibility but would require metastable growth of rutile with higher Zr contents. Most scenarios would predict T's which are too low rather than too high. Exchange of Zr between garnet and rutile inclusions during subsequent heating does not appear to be plausible. We suggest equilibrium at higher P-T between garnet and rutile inclusions than those suggested from garnet isopleths.
GeoRef, Copyright 2018, American Geological Institute.<br/>2014-008979<br/>Mica Creek