ABSTRACT The Appalachian Mountains in northern Vermont host a complex rock record of the tectonic evolution of eastern Laurentia, from the opening of the Iapetus Ocean to the subsequent formation of a convergent Paleozoic margin involving multiple phases of orogenesis. Prior 40 Ar/ 39 Ar studies in Vermont and northern Massachusetts have generally interpreted two major events associated with a dominantly Ordovician Taconic orogeny and a Devonian Acadian orogeny; intermediate ages were considered to reflect Taconic metamorphism and/or deformation that was “partially reset” during the Acadian orogeny. However, recent studies have documented Salinic ages in northern Vermont, aligning with multiple lines of evidence in southern Quebec for an intervening Salinic orogeny during the Silurian. This study reports integrated microstructural and 40 Ar/ 39 Ar geochronological analyses of samples collected across the Green Mountain anticlinorium in northern Vermont. The dominant S 2 and S 3 foliations are defined in thin section by predominantly white mica/quartz microlithons and aligned mica cleavage domains in schist to graphitic schist that formed under greenschist-facies conditions. Correlation of microstructures across the field area and associated 40 Ar/ 39 Ar plateau ages reveal a spatial pattern associated with microstructural development across the anticlinorium. In the eastern limb, the oldest plateau age, 457.6 ± 2.0 Ma (1σ), is interpreted to reflect the timing of formation of S 2 . The youngest plateau age, 419.0 ± 2.4 Ma, comes from the western limb of the anticline near the trace of the Honey Hollow fault, where S 2 is completely transposed by S 3 . Intermediate ages were obtained across the axis of the anticline, where S 3 is a crenulation cleavage. While the Green Mountain anticlinorium has been previously interpreted to have formed in the Devonian during the Acadian orogeny, the typical ca. 386–355 Ma ages are notably absent in the data set, except in locally disturbed spectra. The results of this work are closely aligned with published results of 40 Ar/ 39 Ar dating in southern Quebec that reflect deformation during Taconic and Salinic orogenesis. These new data, together with recently reported ages of west-directed transport on Taconic thrusts along the western Green Mountain front at ca. 420 Ma, suggest a phase of mountain building in the New England Appalachians that has been previously unreported in Vermont. The formation of the Green Mountain anticlinorium coincided with a complex tectonic interval that overlapped temporally with (1) the transition from Salinic thrusting to normal faulting, (2) magmatism attributed to slab breakoff, and (3) syntectonic deposition in the Connecticut Valley–Gaspé Basin.

ABSTRACT Much of the early prograde history in metamorphic rocks is lost due to overprinting at near-peak conditions or through retrograde modification during exhumation. Fortunately, inclusions encapsulated in rigid porphyroblasts may preserve a record of early burial conditions. Quartz inclusions in garnet porphyroblasts from the Strafford Dome, eastern Vermont, have homogeneous Ti concentrations ([Ti]) that differ from matrix quartz, which retains a history of Si-liberating metamorphic reactions and fluid influx. We applied growth-composition models to evaluate potential processes associated with Ti partitioning in quartz before encapsulation in garnet, including a model for constant-volume growth of quartz due to mineral dissolution-transfer processes and growth as a result of Si-liberating diagenetic and metamorphic reactions. Because these processes typically occur at low temperatures, quartz with exceedingly low [Ti] (<<1 ppm) would be formed and cannot account for the homogeneous Ti distribution at concentrations between 2.5 and 5 ppm observed in the sample. This suggests that chemical reequilibration through dynamic recrystallization must have taken place prior to encapsulation in garnet. Analysis of fluid and graphite inclusions with Raman spectroscopy in different microstructural settings allowed the characterization of fluid composition and temperature of microstructure development early in the prograde history. The findings from this study exemplify the utility of garnet hosts to shield inclusion minerals from chemical modification and recrystallization during later events. As such, they provide a window into the early stages of orogenesis and provide insights concerning the mechanisms controlling equilibration of quartz.

Geologic maps and cross-sections effectively summarize the structural geology of a region, but they can be difficult for non-geologists to interpret. Textbooks and interpretive guides commonly integrate maps and cross-sections into static perspective block diagrams to help novices visualize basic concepts in geology. The inherent power of block diagrams, however, is dramatically increased by software such as Google SketchUp, a free downloadable program, which can create interactive 3-D models of a region. The stand-alone models can be Rotated, Panned, and Zoomed by the user and exported for animations. An efficient way to create block diagrams is to combine the individual strengths of dedicated GIS software with SketchUp, and merge the results into a single 3-D model. Effective 3-D block diagrams drape a geologic map on a digital elevation model and show how the map and cross-sections connect at the topographic surface. Creating block diagrams in such a way that portions of the map between cross-section planes are independent segments gives the user flexibility to make portions of the map invisible. By “turning off” parts of the surface, it is possible to sequentially reveal multiple cross-sections. 3-D block diagrams help students and non-specialists visualize geologic structures. Once created, the 3-D block diagrams can be quickly edited by substituting alternate images of geologic maps and cross-sections. Thus they provide an elegant approach for comparing different interpretations of a region. Combined with tools available in SketchUp, they also provide geologists with a valuable resource for assessing the geometric plausibility of geologic cross-sections.