DISTINGUISHING AND CORRELATING MULTIPLE PHASES OF METAMORPHISM ACROSSA MULTIPLY DEFORMED REGION USING THE AXES OF SPIRAL, STAIRCASE AND SIGMOIDAL INCLUSION TRAILS IN GARNET

Schists from the Appalachian Orogen in south-east Vermont have undergo ne multiple phases of garnet growth. These phases can be distinguished by the trend and relative timing of foliation inflexion or intersecti on axes (FIAs) of foliations preserved as inclusion trails in garnet p orphyroblasts. The relative timing of different generations of FIAs is determined from samples containing porphyroblasts with two or three d ifferently trending FIAs developed outwards from core to rim (multi-FI A porphyroblasts). Schists from south-east Vermont show a consistent p attern of relative clockwise rotation of FIA trends from oldest to you ngest. Four populations or sets of FIAs can be distinguished on the ba sis 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 co nsistent trend at all scales across a 35 x 125 km area containing nume rous mesoscopic and macroscopic folds. The FIAs of Set 4 are defined b y inclusion trails that are continuous with matrix foliations, have tr ends subparallel to most folds and are inferred to have developed cont emporaneously with these structures. Conversely, Sets 1 to 3 are obliq ue 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 Or ogeny. The lack of statistically significant differences in the-distri bution of FIA trends across the study area and their consistent relati ve timings in multi-FIA porphyroblasts, despite a complex regional def ormation history involving numerous phases of folding at all scales, s uggest the porphyroblasts have not rotated relative to one another. Th e change in FIA trend with time resulted from rotation of the kinemati c reference frame of bulk flow, possibly as a consequence of the reorg anization of lithospheric plates responsible for Acadian orogenesis. R ecognition of distinct generations of FIAs provides a means of disting uishing different phases of porphyroblast growth. Four periods of garn et porphyroblast growth occurred in the schists of south-east Vermont. This growth was heterogeneously distributed on the cm(2)-m(2) scale. No single porphyroblast records all stages of growth, and adjacent sam ples from the same or dissimilar rock types commonly contain porphyrob lasts that preserve different sequences of growth. Factors that may ha ve been responsible for switching porphyroblast growth on and off at t his scale include: (i) subtle differences in bulk chemical composition ; (ii) oscillating levels of heat: owing to the buffering effect of en dothermic garnet-forming reactions; (iii) channelized infiltration of fluids with localized fluid buffering of bulk composition; and (iv) cy clic controls on the rates of diffusion and material transport of reac tants, either by channelized fluid flow or by a changing pattern of mi crofracturing during foliation development. Consistency in FIA trend a nd relative timing provide a new method for potentially distinguishing and correlating successive metamorphic events, or even phases of meta morphism within a progressive tectonothermal event, along and across o rogens. Using a consistent pattern of core to rim changes in FIA trend , multiple phases of growth of a single porphyroblastic mineral can be quantitatively distinguished, allowing correlation of different phase s of growth around and across macroscopic folds. The relative timing o f growth of different porphyroblastic minerals can also be quantitativ ely determined using FIA data and correlated around and across macrosc opic folds. Conceptually, the paragenetic history preserved in each ge neration of porphyroblast growth, in the form of chemical zoning and t he minerals in inclusion trails, could be combined to produce a more d etailed P-T-t-deformation path than previously determined.