首页 | 本学科首页   官方微博 | 高级检索  
     


Phase Relations and Crystallization Sequence in a Contact-Metamorphosed Rock from the Gunflint Iron Formation, Minnesota
Authors:SIMMONS, E. C.   LINDSLEY, D. H.   PAPIKE, J. J.
Affiliation:Department of Earth and Space Sciences
State University of New York
Stony Brook, New York 11794
Abstract:Portions of the Gunflint Iron Formation, originally a ferruginoussediment, were metamorphosed by the intrusion of the DuluthComplex to assemblages containing: pigeonite (Wo10En24Fs66)+olivine(Fo13Fa37)+Fe-Ti oxide (Mt62Usp34Hc4)+plagioclase (An94Ab6)+vapor+augite (Wo40En20Fs40) or cummingtonite Fe/(Fe+Mg) {smalltilde} 0.69; quartz was present but probably was not in equilibriumwith olivine. Comparison with synthetic phase-equilibrium studiesindicate conditions of initial recrystallization of T≥ 800 °C,Ptotal ≥ 2kb, fo2 slightly below that of the pure fayalite-magnetite-quartzassemblage, and PH2O < Ptotal. During the slow cooling process following initial recrystallization,the phases present underwent a complex series of exsolution,inversion, oxidation, and hydration reactions. Pigeonite initiallyexsolved augite along (001), then inverted to orthopyroxene,which then exsolved augite along (100). The augite exsolvedonly pigeonite on (001) during its cooling history. The Fe-Tioxide for the most part oxidized to an intergrowth of magnetiteand ilmenite, although unoxidized portions later exsolved ulvöspinel.Cummingtonite exsolved actinolite, forming irregular patchesof the latter. Olivine, orthopyroxene, and augite reacted withplagioclase to form retrograde amphiboles. Orthopyroxene had difficulty nucleating during this slow coolingprocess, forming only at widely spaced points in mosaics ofprimary pigeonite grains, and never nucleating within primaryaugite grains. The resulting orthopyroxene grains are much largerthan the original pigeonite grains.
Keywords:
本文献已被 Oxford 等数据库收录!
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号