Ninety million years of orogenesis, 250 million years of quiescence and further orogenesis with no change in PT: significance for the role of deformation in porphyroblast growth

Shah, A.A., and Bell, T.H. (2012) Ninety million years of orogenesis, 250 million years of quiescence and further orogenesis with no change in PT: significance for the role of deformation in porphyroblast growth. Journal of Earth System Science, 121 (6). pp. 1365-1399.

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Abstract

In situ dating of monazite grains preserved as inclusions within foliations defining FIAs (foliation inflection/intersection axes preserved within porphyroblasts) contained within garnet, staurolite, andalusite and cordierite porphyroblasts provides a chronology of ages that matches the FIA succession for the Big Thompson region of the northern Colorado Rocky Mountains. FIA sets 1, 2 and 3 trending NE–SW, E–W and SE–NW were formed at 1760.5 ± 9.7, 1719.7 ± 6.4 and 1674 ± 11 Ma, respectively. For three samples where garnet first grew during just one of each of these FIAs, the intersection of Ca, Mg, and Fe isopleths in their cores indicate that these rocks never got above 4 kbars throughout the Colorado Orogeny. Furthermore, they remained around approximately the same depth for ~250 million years to the onset of the younger Berthoud Orogeny at 1415 ± 16 Ma when the pressure decreased slightly as porphyroblasts formed with inclusion trails preserving FIA set 4 trending NNE–SSW. No porphyroblast growth occurred during the intervening ~250 million years of quiescence, even though the PT did not change over this period. This confirms microstructural evidence gathered over the past 25 years that crenulation deformation at the scale of a porphyroblast is required for reactions to re-initiate and enable further growth.

Item ID: 25029
Item Type: Article (Research - C1)
ISSN: 0973-774X
Keywords: FIAs, porphyroblast, monazite, garnet, staurolite
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Funders: James Cook University
Date Deposited: 20 Feb 2013 09:16
FoR Codes: 04 EARTH SCIENCES > 0403 Geology > 040312 Structural Geology @ 100%
SEO Codes: 97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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