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Five lengthy periods involving multiple phases of cordierite and andalusite growth were revealed by detailed studies of foliation inflection/intersection axes(FIA) preserved in porphyroblasts in schists from the Arkansas River region in Colorado,USA.The regionally consistent character of the succession of five different FIA trends enabled the relative timing of each FIA with respect to the next to be determined.The FIA succession from first to last is:FIA 1 trending W-E,FIA 2 trending SSW-NNE, FIA 3 trending NNW-SSE,FIA 4 trending NW-SE and FIA 5 trending SW-NE.For four of the FIA sets,samples were found containing monazite grains preserved as inclusions.These were dated on an electron microprobe.The ages obtained concur exactly with the FIA succession,with FIA 1 at 1506±15 Ma,FIA 2 at 1467±23 Ma,FIA 3 at 1425±18 Ma,FIA 4 not dated and FIA 5 at 1366±20 Ma. These ages are directly reflected in a succession of plutons in the surrounding region dated by other isotopic approaches,suggesting that deformation,metamorphism and pluton emplacement occurred together episodically,but effectively continuously,for some 140 Ma.
Five lengthy periods involving multiple phases of cordierite and andalusite growth were revealed by detailed studies of foliation inflection / intersection axes (FIA) preserved in porphyroblasts in schists from the Arkansas River region in Colorado, USA. Regionally consistent character of the succession of five different FIA trends enabled the relative timing of each FIA with respect to the next to be determined. The FIA succession from first to last is: FIA 1 trending WE, FIA 2 trending SSW-NNE, FIA 3 trending NNW-SSE, FIA 4 trending NW -SE and FIA 5 trending SW-NE. For four of the FIA sets, samples were found containing monazite grains preserved as inclusions. These were dated on an electron microprobe. The resulting obtained exactly from the FIA succession, with FIA 1 at 1506 ± 15 Ma, FIA 2 at 1467 ± 23 Ma, FIA 3 at 1425 ± 18 Ma, FIA 4 not dated and FIA 5 at 1366 ± 20 Ma. These ages are directly reflected in a succession of plutons in the surrounding region dated by other isotopic approaches, sugge sting that deformation, metamorphism and pluton emplacement occurred together episodically, but effectively continuously, for some 140 Ma.