Distinguishing and correlating multiple phases of metamorphism across a multiply deformed region using the axes of spiral, staircase and sigmoidal inclusion trails in garnet

Abstract

Schists from the Appalachian Orogen in south-east Vermont have undergone multiple phases of garnet growth. These phases can be distinguished by the trend and relative timing of foliation inflexion or intersection axes (FIAs) of foliations preserved as inclusion trails in garnet porphyroblasts. The relative timing of different generations of FIAs is determined from samples containing porphyroblasts with two or three differently trending FIAs developed outwards from core to rim (multi-FIA porphyroblasts). Schists from south-east Vermont show a consistent pattern of relative clockwise rotation of FIA trends from oldest to youngest. Four populations or sets of FIAs can be distinguished on the basis of their relative timings and trends. From oldest to youngest, the four sets have modal peaks trending SW-NE, W-E, NNW-SSE and SSW-NNE. These peaks show that each of the four FIA sets has a statistically consistent trend at all scales across a 35×125 km area containing numerous mesoscopic and macroscopic folds. The FIAs of Set 4 are defined by inclusion trails that are continuous with matrix foliations, have trends subparallel to most folds and are inferred to have developed contemporaneously with these structures. Conversely, Sets 1 to 3 are oblique to and pre-date most matrix foliations and folds. All four FIA sets occur in Siluro-Devonian rocks and must have formed in the Acadian Orogeny. The lack of statistically significant differences in the distribution of FIA trends across the study area and their consistent relative timings in multi-FIA porphyroblasts, despite a complex regional deformation history involving numerous phases of folding at all scales, suggest the porphyroblasts have not rotated relative to one another. The change in FIA trend with time resulted from rotation of the kinematic reference frame of bulk flow, possibly as a consequence of the reorganization of lithospheric plates responsible for Acadian orogenesis.Recognition of distinct generations of FIAs provides a means of distinguishing different phases of porphyroblast growth. Four periods of garnet porphyroblast growth occurred in the schists of south-east Vermont. This growth was heterogeneously distributed on the cm2-m2 scale. No single porphyroblast records all stages of growth, and adjacent samples from the same or dissimilar rock types commonly contain porphyroblasts that preserve different sequences of growth. Factors that may have been responsible for switching porphyroblast growth on and off at this scale include: (i) subtle differences in bulk chemical composition; (ii) oscillating levels of heat, owing to the buffering effect of endothermic garnetforming reactions; (iii) channelized infiltration of fluids with localized fluid buffering of bulk composition; and (iv) cyclic controls on the rates of diffusion and material transport of reactants, either by channelized fluid flow or by a changing pattern of microfracturing during foliation development.Consistency in FIA trend and relative timing provide a new method for potentially distinguish...

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

“…A number of areas recorded an entire transition in metamorphic grade from the chlorite zone to the onset of migmatization during the Colorado orogeny (Braddock and Cole 1979;Selverstone et al 1997). Hayward (1990) and Bell et al (1995Bell et al ( , 1998 described a technique for analysing the geometries of inclusion trails within porphyroblasts. It involves measurement of the FIA, which is achieved by cutting a minimum of eight vertically oriented thin sections around the compass from each rock sample to locate the switch in inclusion trail asymmetry (clockwise or anticlockwise) within the porphyroblasts ( figure 3a and b).…”

“…It involves measurement of the FIA, which is achieved by cutting a minimum of eight vertically oriented thin sections around the compass from each rock sample to locate the switch in inclusion trail asymmetry (clockwise or anticlockwise) within the porphyroblasts ( figure 3a and b). Where the FIA trends vary from the core to the rim of the porphyroblasts, a relative timing and thus an FIA succession can be established (Bell et al 1998). The accumulated error associated with determining the trend of the FIA in each rock is random, and is estimated to be ±8 in both situations when one uses a COCLAR compass (see Bell et al 1998).…”

“…Where the FIA trends vary from the core to the rim of the porphyroblasts, a relative timing and thus an FIA succession can be established (Bell et al 1998). The accumulated error associated with determining the trend of the FIA in each rock is random, and is estimated to be ±8 in both situations when one uses a COCLAR compass (see Bell et al 1998).…”

“…800 oriented thin sections were prepared and a total of 138 FIA and pseudo-FIA trends were determined (table 1, figure 2). These measurements were achieved by cutting a minimum of eight vertically oriented thin sections around the compass from each rock sample (figure 3) and then locating the switch in inclusion trail asymmetry (clockwise or anticlockwise) within the porphyroblasts (e.g., Bell et al 1998). Bell et al (1995Bell et al ( , 1998 by which the trend of an FIA is measured.…”

“…More than 10 years of research and data have already been published using this technique from tectonically complex regions around the world (e.g., Bell et al 1998Bell et al , 2003Bell et al , 2005Bell and Chen 2002;Cihan 2004;Kim and Bell 2005;Sayab 2005Sayab , 2006Bell and Bruce 2007;Sanislav 2010;Sanislav and Shah 2010;Ali 2010;Sanislav and Bell 2011).…”