Prolonged Acadian orogenesis: Revelations from foliation intersection axis (FIA) controlled monazite dating of foliations in porphyroblasts and matrix

Numerous phases of garnet growth are revealed by detailed studies of foliation inflection/intersection axes preserved in porphyroblasts (FIAs) in Acadian metamorphic rocks in the Chester Dome region of Vermont, in the United States of America. A regionally consistent succession of four different FIA trends in garnet porphyroblasts has been dated by analyzing monazite inclusions with an electron microprobe. These monazite inclusions, which lie within the various foliations that define the FIAs, provide absolute ages for multiple periods of deformation and episodic phases of garnet growth. The monazite inclusions reveal a progression in foliation ages from 431 +/- 2 to 349 +/- 3 million years within porphyroblasts and from 366 +/- 3 to 327 +/- 5 million years in pervasive matrix foliations. Three samples of schist reveal ages ranging through 75 to 70 million years from the cores of porphyroblasts, through the medians to the rims and then into the matrix. Ages determined from monazite grains within garnet porphyroblasts link directly to periods of multiple deformation and episodic garnet growth defined by the FLAS. The four FIA sets began forming prior to 424 +/- 3, 405 +/- 6, 386 +/- 6 and 366 +/- 4 million years ago, respectively. Thus multiple stages of garnet growth occurred throughout Acadian deformation and metamorphism in Vermont, and orogenesis was far more prolonged than previously thought. Invariably, garnet growth occurred early in the accompanying deformation event. Dating of monazite inclusions without careful separation of phases of garnet growth by FIA studies will lead to a spread of ages that will confuse rather than elucidate the metamorphic and structural history. Analysis of monazite grains in the matrix alone will likely only present ages for the youngest events. These ages can he amalgamated from grain to grain to yield apparently precise ages, hut such ages reveal nothing about the deformation and metamorphic processes operating during orogenesis, or the overall continuity of the deformation and metamorphism that accompanies plate motion. FIA and microstructural studies of inclusion trail successions allow numerous phases of garnet growth to be distinguished and then dated using monazite that has dissolved and regrown many times as multiple successive foliations developed. This in turn allows very long and involved metamorphic histories to be fully integrated with similarly complex structural histories.