One-dimensional thermal modelling of Acadian metamorphism southern vermont, USA

One-dimensional thermal (1DT) modelling of an Acadian (Devonian) tectonothe rmal regime in southern Vermont, USA, used measured metamorphic pressures a nd temperatures and estimated metamorphic cooling ages based on published t hermobarometric and geochronological studies to constrain thermal and tecto nic input parameters. The area modelled lies within the Vermont Sequence of the Acadian orogen and includes: (i) a western domain containing garnet-gr ade pre-Silurian metasedimentary and metavolcanic rocks from the eastern fl ank of an Acadian composite dome structure (Rayponda-Sadawga Dome); and (ii ) an eastern domain containing similar, but staurolite- or kyanite-grade, r ocks from the western flank of a second dome structure (Athens Dome), appro ximately 10 km farther east. Using reasonable input parameters based on reg ional geological, petrological and geochronological constraints, the therma l modelling produced plausible P-T paths, and temperature-time (T-t) and pr essure-time (P-t) curves. Information extracted from P-T-t modelling includ es values of maximum temperature and pressure on the P-T paths, pressure at maximum temperature, predicted Ar closure ages for hornblende, muscovite a nd 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 re gional metamorphism in southern Vermont. Modelling results provide some imp ortant bounding limits on the physical conditions during regional metamorph ism, and indicate that the pressure contemporaneous with the attainment of peak temperature was probably as-much as 2.5 kbar lower than the actual max imum pressure experienced by rocks along various particle paths. In additio n, differences in peak metamorphic grade (garnet-grade versus staurolite-gr ade or kyanite-grade) and peak temperature for rocks initially loaded to si milar crustal depths, differences in calculated exhumation rates, and diffe rences in Ar-40/Ar-39 closure ages are likely to have been consequences of variations in the duration of isobaric heating (or 'crustal residence perio ds') and tectonic unroofing rates. Modelling results are consistent with a regional structural model that suggests west to east younging of specific A cadian deformational events, and therefore diachroneity of attainment of pe ak metamorphic conditions and subsequent Ar-40/Ar-39 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 d epths to which rocks were loaded by an eastward-thickening thrust-nappe pil e rooted to the east (New Hampshire Sequence), as well as by diachronous st ructural processes within the lower plate rocks of the Vermont Sequence.