Shah, Afroz, and Ali, Asghar (2011) Distinguishing and correlating multiple phases of metamorphism across multiply deformed regions using the axes of inclusion trails in porphyroblasts. Acta Geologica Sinica, 85 (4). pp. 784-791.

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Abstract

Successions of FIAs (foliation inflection/intersection axes preserved within porphyroblasts) provide a relative time scale for deformation and metamorphism. In-situ dating of monazite grains preserved as inclusions within garnet and staurolite porphyroblasts within the foliations defining each FIA from such successions provides a rigorous approach to grouping ages that formed over extended periods of deformation and metamorphism. Matching age and FIA progressions confirms the suitability of this approach plus provides access to lengthy histories that cannot otherwise be determined. Tectonism in the Big Thompson region of the Colorado Rocky Mountains, USA began about a SW-NE trending axis defined by FIA set 1 at around 1761 ± 13 Ma. Subsequent periods of tectonism occurred around 1712 ± 25 and 1672 ± 21 Ma about W-E and NW-SE trending FIAs 2 and 3, respectively. Tectonism in the Balcooma Metamorphic Group NW of Greenvale in NE Queensland, Australia began around 470 Ma with the growth of garnet porphyroblasts about a W-E trending axis defined by FIA set 1. No monazite grains were preserved as inclusions in this mineral phase but they were in subsequently grown staurolite. Tectonism then occurred around 443.2±3.8, 424 ± 4 and 408.9 ± 8.9 Ma about NNW-SSE, SSW-NNE and W-E trending FIAs 2, 3 and 4, respectively. This test of a FIA based approach to monazite dating in tectonic regimes in different parts of the world and with dramatically different ages suggests it can be used in all orogens containing porphyroblasts preserving inclusion trails.

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