USGS Science⌕ Search
Home/Archive/USGS

USGS · 70022113

One-dimensional thermal modelling of Acadian metamorphism in southern Vermont, USA

T. R. Armstrong·R.J. Tracy
View original source ↗Export metadata ↓

Abstract

One-dimensional thermal (1DT) modelling of an Acadian (Devonian) tectonothermal regime in southern Vermont, USA, used measured metamorphic pressures and temperatures and estimated metamorphic cooling ages based on published thermobarometric and geochronological studies to constrain thermal and tectonic input parameters. The area modelled lies within the Vermont Sequence of the Acadian orogen and includes: (i) a western domain containing garnet-grade pre-Silurian metasedimentary and metavolcanic rocks from the eastern flank of an Acadian composite dome structure (Rayponda-Sadawga Dome); and (ii) an eastern domain containing similar, but staurolite- or kyanite-grade, rocks from the western flank of a second dome structure (Athens Dome), approximately 10 km farther east. Using reasonable input parameters based on regional geological, petrological and geochronological constraints, the thermal modelling produced plausible P-T paths, and temperature-time (T-t) and pressure-time (P-t) curves. Information extracted from P-T-t modelling includes values of maximum temperature and pressure on the P-T paths, pressure at maximum temperature, predicted Ar closure ages for hornblende, muscovite and K-feldspar, and integrated exhumation and cooling rates for segments of the cooling history. The results from thermal modelling are consistent with independently obtained pressure, temperature and Ar cooling age data on regional metamorphism in southern Vermont. Modelling results provide some important bounding limits on the physical conditions during regional metamorphism, and indicate that the pressure contemporaneous with the attainment of peak temperature was probably as much as 2.5 kbar lower than the actual maximum pressure experienced by rocks along various particle paths. In addition, differences in peak metamorphic grade (garnet-grade versus staurolite-grade or kyanite-grade) and peak temperature for rocks initially loaded to similar crustal depths, differences in calculated exhumation rates, and differences in 40Ar/39Ar closure ages are likely to have been consequences of variations in the duration of isobaric heating (or 'crustal residence periods') and tectonic unroofing rates. Modelling results are consistent with a regional structural model that suggests west to east younging of specific Acadian deformational events, and therefore diachroneity of attainment of peak metamorphic conditions and subsequent 40Ar/39Ar closure during cooling. Modelling is consistent with the proposition that regional variations in timing and peak conditions of metamorphism are the result of the variable depths to which rocks were loaded by an eastward-thickening thrust-nappe pile rooted to the east (New Hampshire Sequence), as well as by diachronous structural processes within the lower plate rocks of the Vermont Sequence.

Explore related subjects

Journal of Metamorphic GeologyArticle

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

T. R. Armstrong, R.J. Tracy. 2008-07-07. One-dimensional thermal modelling of Acadian metamorphism in southern Vermont, USA. https://doi.org/10.1046/j.1525-1314.2000.00284.x

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2024

Deformation-induced graphitization and muscovite recrystallization in a ductile fault zone

A suite of slate samples collected along a 2 km transect crossing the Lishan fault in central Taiwan were evaluated to assess the role of ductile deformation in natural graphitization at lower greenschist facies metamorphic conditions. The process of natural aromatization, or graphitization, of an organic precursor is well established as a thermally driven process; however, experimental studies have shown that the energy provided by deformation can substantially reduce the activation energy required for graphitization. This study provides a natural example of deformation-induced graphitization. A strain gradient approaching the Lishan fault was established by scanning electron microscope imaging and X-ray diffraction analysis of phyllosilicates and quartz that showed an increase in the strength of slaty cleavage development via dissolution-precipitation processes. Thermal conditions were constrained to be near isothermal using calcite-dolomite geothermometry. Raman spectroscopic results from carbonaceous material, including D1-full width-at-half-maximum (FWHM), G-FWHM, Raman band separation (RBS), and a lesser-known vibrational mode B 2g -FWHM, showed robust linear trends across the same sampling transect. However, the G-FWHM parameter showed a trend opposite of that expected from thermally driven graphitization. The Raman results are interpreted to reflect a strain-driven reduction in graphite crystallite size (decrease in G-FWHM) but improvement in structural ordering in individual coherent domains. A multiple linear regression with an R 2 value of 0.92 predicts the graphite D1-FWHM values from the XRD-derived ratio of muscovite populations and muscovite microstrain, demonstrating the concomitant recrystallization of silicates and carbonaceous material across the strain gradient, despite the disparate processes accommodating the deformation. This study demonstrates the role of deformation in natural graphitization and provides a new perspective on the use of graphite as a geothermometer in strongly deformed greenschist facies rocks.

Journal of Metamorphic Geology↗
USGS2016

Reaction softening by dissolution–precipitation creep in a retrograde greenschist facies ductile shear zone, New Hampshire, USA

We describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host schist of the Littleton Formation near Claremont, New Hampshire. Here, Na-poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone is approached. Across the transect, higher grade fabric-forming Na-rich muscovite is also progressively replaced by fabric-forming Na-poor muscovite. The mineralogy of the new phyllonitic fault-rock produced is dominated by Na-poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric-forming Na-rich muscovite is selectively replaced at high-strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high-grade porphyroblasts by weaker Na-poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na-rich muscovite-defined foliation destroys not only the metastable high-grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution–precipitation creep, activated at considerably lower stresses than might be predicted in quartzofeldspathic rocks at the same lower greenschist facies conditions.

New Hampshire↗
USGS2009

The timing of eclogite facies metamorphism and migmatization in the Orlica–Śnieżnik complex, Bohemian Massif: Constraints from a multimethod geochronological study

The Orlica–Śnieżnik complex (OSC) is a key geological element of the eastern Variscides and mainly consists of amphibolite facies orthogneisses and metasedimentary rocks. Sporadic occurrences of eclogites and granulites record high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic conditions. A multimethod geochronological approach ( 40 Ar– 39 Ar, Rb–Sr, Sm–Nd, U–Pb) has been used to gain further insights into the polymetamorphic evolution of eclogites and associated country rocks. Special attention was given to the unresolved significance of a 370- to 360 Ma age group that was repeatedly described in previous studies. Efforts to verify the accuracy of c. 370 Ma K–Ar phengite and biotite dates reported for an eclogite and associated country-rock gneiss from the location Nowa Wieś suggest that these dates are meaningless, due to contamination with extraneous Ar. Extraneous Ar is also considered to be responsible for a significantly older 40 Ar– 39 Ar phengite date of c. 455 Ma for an eclogite from the location Wojtowka. Attempts to further substantiate the importance of 370–360 Ma zircon dates as an indicator for a melt-forming high-temperature (HT) episode did not provide evidence in support of anatectic processes at this time. Instead, SHRIMP U–Pb zircon dating of leucosomes and leucocratic veins within both orthogneisses and (U)HP granulites revealed two age populations (490–450 and 345–330 Ma respectively) that correspond to protolith ages of the magmatic precursors and late Variscan anatexis. The results of this study further underline the importance of Late Carboniferous metamorphic processes for the evolution of the OSC that comprise the waning stages of HP metamorphism and lower pressure HT overprinting with partial melting. Eclogites and their country rocks provided no chronometric evidence for an UHP and ultrahigh-temperature episode at 387–360 Ma, as recently suggested for granulites from the OSC, based on Lu–Hf garnet ages ( Anczkiewicz et al. , 2007 ).

Orlica-Snieznik complex↗