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

Transcription

1 J. metamorphic Geol., 2000, 18, One-dimensional thermal modelling of Acadian metamorphism in southern Vermont, USA T. R. ARMSTRONG1 AND R. J. TRACY2 1US Geological Survey, 926A National Center, Reston, VA 20192, USA 2Department of Geological Sciences, Virginia Tech, Blacksburg, VA , USA 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. Key words: Acadian; metamorphism; tectonic; thermal modelling; Vermont. INTRODUCTION One-dimensional thermal (1DT) modelling is a forward technique and a powerful tool for predicting pressure temperature time (P T t) paths for rocks in the metamorphic cores of orogens. Metamorphic patterns in such orogenic cores may preserve significant information about pressure temperature evolution that can be used to better understand tectonic processes. Calculated P T t paths for a specific region are functions of several physicochemical and thermal input parameters that should be chosen to be representative of specific and appropriate tectonic processes and environments (England & Richardson, 1977; England & Thompson, 1984, 1986; Thompson & England, 1984; Blackwell Science Inc., /00/$15.00 Journal of Metamorphic Geology, Volume 18, Number 6, 2000 Peacock, 1989). The validity of forward-modelled P T t paths for constraining the thermal evolution of a specific area thus depends upon the selection of modelling parameters, which in turn requires: (i) an accurate deformational chronology; (ii) estimates of the durations and magnitudes of crustal loading and deformational events; and (iii) reasonable estimates of strain rates and unroofing rates. This paper compares modelled P T t paths with those estimated from geological and petrological data for rocks in the southern pre-silurian Vermont Sequence, which was mapped by Skehan (1961) and Hepburn et al. (1984), at a scale of , and has recently been remapped at a scale of (Ratcliffe & Armstrong, 1999) as shown in Fig. 1. When 625

2 626 T. R. ARMSTRONG & R. J. TRACY Fig. 1. Geological map of southern Vermont (from Ratcliffe et al., 1992). The dash-dotted line separates the garnet-grade rocks of the western domain (labelled; domains discussed in text) from the staurolite- or kyanite-grade rocks of the eastern domain. The heavy dashed line around the Athens Dome shows the limit of kyanite-bearing rocks within the dome structure. Rocks outward of this line are typically at staurolite-grade. The Sadawga, Rayponda and Athens Domes are Acadian interference-fold (domal) structures cored by middle Proterozoic basement that lie within rocks of the pre-silurian Taconide Zone. integrated with regional geochronological data (Sutter & Hatch, 1985; Sutter et al., 1985; Drake et al., 1989; Harrison et al., 1989; Laird et al., 1991; Armstrong & Hames, 1993) and quantitative thermobarometry (Armstrong et al., 1992; Ratcliffe et al., 1992; Spear, 1992; Armstrong, 1993), the new mapping and structural data provide tectonic boundary constraints on the relative and absolute temporal relationships, dur-

3 THERMAL MODELLING OF ACADIAN METAMORPHISM 627 ations and magnitudes of Acadian deformational and Crank Nicolson finite difference technique in order to metamorphic episodes in this area. This geological solve the equation (Haugerud, 1986). information can then be used to estimate P T t paths, Inherent limits to the accuracy of finite difference from which reasonable values for the thermal and techniques are presented in Carnahan et al. (1969) and tectonic input parameters used in the one-dimensional Smith (1985), and are further discussed by Haugerud thermal model (developed by Haugerud, 1986) can be (1986). The sensitivity of the results from 1DT, that is extracted or refined. P T t paths calculated from the related to the value of the time steps used in the 1DT model can then be compared with those obtained model, is discussed by Haugerud ( 1986). from the petrological methods noted above in a test of the consistency of the thermal model and the structure-based tectonic models and parameters. A note on the use of the model The pressures, temperatures and times calculated The one-dimensional thermal model was regarded as using the one-dimensional thermal model presented in adequate for this study because results from mapping this paper are, to a large extent, independent of the in this area showed no evidence of Acadian synmetamorphic petrological data used to estimate the fundamental thrusting, which might have resulted in lateral parameters for the modelling, because they are derived heat transfer and the necessity for the use of twodimensional using different values and types of thermal variables, finite element calculations (Peacock, e.g. thermal conductivity, heat capacity, heat generation 1989). Furthermore, staurolite zone to sillimanite and heat flux (for quartzo-feldspathic rock types), and orthoclase zone rocks, informally ascribed to the New tectonic variables, e.g. onset and duration of specific Hampshire Sequence, are believed to be responsible tectonic events, unroofing rates and related strain for the loading and metamorphism of rocks of the rates. The modelling has thus produced calculated Vermont Sequence discussed in this paper (e.g. Spear P T t values that can be usefully compared with et al., 1991; Armstrong et al., 1992; Spear, 1992). Spear measurements acquired from standard petrological (1992), however, concluded that the west-directed (thermobarometric and geochronological) techniques. thrust package consisting of New Hampshire Sequence The tectonic (and related thermal) input parameters rocks was fully assembled and had already attained associated with modelled P T t values that best fit the peak metamorphic conditions prior to final emplace- measured P T t values are then further examined in ment upon the Vermont Sequence. Because this thrust order to evaluate how these data regarding crustal package would therefore have been close to attain- depths, temperatures and timing of metamorphism and ing a steady-state geothermal gradient, we feel that coeval deformations fit current structural and tectonic it is not necessary to use multidimensional (two- models that have been presented for this area. Results dimensional or three-dimensional) modelling that from the two different geological domains examined in incorporates significant lateral heat transfer. southern Vermont are then compared in order to assess the tectonic factors that may have controlled differences in the thermal evolution of both domains. GEOLOGICAL SETTING To avoid confusion, it should be noted that, in this The geology of southern Vermont is quite complex, paper, we refer to pressure, temperature and time consisting of multiply deformed and polymetamorphic values produced by one-dimensional thermal modelling late Precambrian through Devonian sedimentary, as calculated values, whereas pressure, temperature volcanic and plutonic rocks. A detailed description of and time data derived from geology, from standard the geological setting can be found in Armstrong et al. petrological thermobarometric techniques and from (1997), Ratcliffe & Armstrong (1999) and Ratcliffe geochronology and thermochronology are referred to et al. (1992, 1997a, 1997b). Rocks within this region as measured values. include middle Proterozoic gneiss, quartzite and schist that were first metamorphosed during the middle One-dimensional thermal model Proterozoic Grenville Orogeny, and comprise the Laurentian Craton. Unconformably overlying the The one-dimensional thermal model used during this middle Proterozoic section are a series of highly study is 1DT, a finite difference model developed by deformed and metamorphosed late Precambrian Haugerud (1986) that utilizes the conductive and through Ordovician schists, amphibolites, quartzite advective heat transfer equation: and granofelses that have undergone deformation and dt /dt=kd2t /dz2 UdT /dz+a/rc metamorphism during both the Ordovician Taconian and Devonian Acadian Orogenies (Armstrong et al., where T is the temperature, t the time, K the thermal 1992). A sequence of younger Silurian Devonian rocks, diffusivity, z the depth, U the velocity of the medium including carbonaceous schist and phyllite, quartzite in the z (vertical) direction, A the volumetric heat and quartz-pebble conglomerate, and intermediate to production rate, r the rock density and c the heat mafic volcanics, is found immediately east of the study capacity. In order to solve the equation, it is necessary area. These rocks are less deformed than the pre- to specify initial and boundary conditions. 1DT uses a Silurian rocks and were only affected by metamorphism

4 628 T. R. ARMSTRONG & R. J. TRACY related to the Acadian Orogeny (Armstrong et al., domain range from 388 to 376 Ma (Sutter & Hatch, 1997). 1985; Sutter et al., 1985). Based upon a thermobaro- Figure 1 shows the various lithotectonic belts folded metric estimate for these same rocks of 535 C and into several large doubly plunging, or dome, structures 7.5 kbar (Armstrong, 1993) and a proposed range of (Sadawga, Rayponda and Athens Domes). These dome typical tectonic cooling rates (Sutter et al., 1985), the structures cored by the middle Proterozoic rocks are maximum temperature was probably reached between Acadian in age, and have a composite origin related 395 and 388 Ma (Armstrong et al., 1992). to the development of at least three different Acadian Geochronological and thermochronological data cleavages (Ratcliffe et al., 1992, 1997a). Based upon an (references listed in Table 4, see later), along with eastward relative younging of the cleavages, and their relative age relationships between Acadian metamor- cross-cutting relationships to dated Devonian granite phic mineral assemblages and deformational fabrics, dykes, Armstrong (1993) suggested that the Acadian indicate that Acadian deformation in the western deformations and dome evolution are younger to the domain occurred at least episodically over an age east within the Silurian Devonian rocks than within range of approximately Myr, starting with the pre-silurian rocks to the west. In addition, thrust-related crustal loading at around 400 Ma and thermobarometric estimates of Acadian metamorphic ending with late dome-stage, post-peak metamor- conditions across the region showed that both peak phic deformation at approximately Ma temperatures and related pressures increase eastward (Armstrong, 1992) that accompanied exhumation. Peak across the dome structures, such that the distribution metamorphic assemblages show only minor retrogression, of the isotherms and isobars is not directly related to and this is exclusively related to zones of the Acadian dome structures (Armstrong, 1993). pervasive late dome-stage deformation. This indicates Furthermore, Acadian mineral deformational fabric that initially rapid and relatively high-strain postage relationships indicated that peak temperature metamorphic exhumation and related cooling were conditions were achieved prior to most of the Acadian succeeded by slower, low-strain exhumation (Ratcliffe cleavage formation in the areas of the Rayponda and & Armstrong, 1999). Sadawga Domes (Fig. 1), whereas higher temperature Thermobarometric measurements of peak temperaand pressure conditions farther east (near the Athens ture and associated pressure from garnet-grade and Dome) were achieved during cleavage development staurolite- or kyanite-grade rocks around the Athens (Armstrong, 1993). It is important to note that no Dome in the eastern domain range from 550±25 C Acadian age faults were mapped within the areas to 600±25 C, and 7.5 to 9.2 kbar (Ratcliffe et al., discussed in this paper, and that all of the Acadian 1992; Armstrong, 1993). 40Ar/39Ar hornblende plateaus deformation and related shortening within this region from these rocks range in age from 376 Ma (staurolite- appears to be related to the development of cleavage grade) to 365 Ma ( kyanite-grade) (Drake et al., 1989; and related fold structures (Ratcliffe et al., 1992, Laird et al., 1991). 40Ar/39Ar muscovite cooling ages 1997a, 1997b). of 365 to 340 Ma (Drake et al., 1989; Harrison et al., 1989) are similar to the younger end of hornblende EVALUATION OF MODEL PARAMETERS cooling age range, and indicate that these rocks underwent rapid cooling (and presumably rapid Geological constraints on structural parameters exhumation) immediately after peak temperature was attained. K-feldspar 40Ar/39Ar plateau ages of 310 Ma Two different structural and metamorphic domains in from Silurian Devonian rocks immediately east of the southern Vermont have been examined in this study: Taconide Zone (Spear & Harrison, 1989) indicate garnet-grade pre-silurian rocks along the east flank of slower subsequent exhumation following muscovite the Rayponda and Sadawga Domes (western domain closure (Harrison et al., 1989). These ages, coupled of Fig. 1), and staurolite-grade to kyanite-grade rocks with petrographical observations and associated ther- of the same pre-silurian lithotectonic belt that emerge mobarometric results, yield integrated initial unroofing along the eastern flank of the Athens Dome, approximately rates of 1.4 mm yr 1 (from the time of peak temperature 5 10 km farther east (eastern domain). Both to Ar closure in hornblende, at about 500 C), and a domains lie within the so-called Taconide Zone (see subsequent integrated rate of 0.1 mm yr 1 (from Fig. 1), in which rocks were both deformed and hornblende closure to muscovite closure) for Acadian metamorphosed in the Taconian (Ordovician) orogenic post-peak cooling (Armstrong et al., 1992). episode. Recent geological mapping in this area has Based upon the previously discussed petrological shown that a pre-existing Taconian regional foliation and geochronological data, as well as detailed mapping has been deformed into a series of interference dome and structural analysis, the structural evolution of the and basin structures by multiple synmetamorphic lithotectonic belts comprising both domains can be Acadian (Devonian) deformation ( Fig. 1; Skehan, 1961; summarized as follows: (i) the onset of metamorphism Ratcliffe et al., 1992; Armstrong, 1993). 40Ar/39Ar of the Vermont Sequence, related to c. 400 Ma crustal hornblende plateau ages from garnet-grade rocks on loading by west-directed thrust-nappes that may have the east flank of the Sadawga Dome in the western been as much as 30 km thick and that rooted within

5 THERMAL MODELLING OF ACADIAN METAMORPHISM 629 the previously metamorphosed, high-grade (up to Sil Table 2. Thermal modelling parameters. Kfs zone and migmatitic) rocks, informally referred to (a) Heat capacity sensitivity. here as the New Hampshire Sequence in Fig. 1 Particle path initial depth (after thrusting event) (Armstrong et al., 1992; Spear, 1992); (ii) continued 30 km 35 km 40 km crustal shortening within the Vermont Sequence, Thermal conductivity accommodated by upright fold interference development (W m 1 K 1) T ( C) P (kbar) T ( C) P (kbar) T ( C) P (kbar) (early dome-stage deformation); (iii) rapid exhum ation and cooling; (iv) subsequent lower strain deformation and slower cooling and exhumation ( late dome-stage deformation). Current tectonic models suggest that west-directed movement of rocks along one or more deep-seated thrusts above an east-dipping ramp structure resulted in west to east younging in the Vermont Sequence (the footwall of the regional Heat capacity= JK 1 kg 1. structures) of both the attainment of regional maximum (b) Density sensitivity. metamorphic temperatures and related, vertically directed dome-stage deformation (Armstrong, 1992; Density (kg m 3) T ( C) P (kbar) Spear, 1992) Evaluation of thermal parameters The structural, thermobarometric and geochronolog ical constraints noted above were used to develop an initial set of tectonic input parameters for use in the Heat capacity= JK 1 kg 1. Thermal conductivity=2.75 W m 1 K 1. one-dimensional thermal model (Table 1). This initial (c) Heat generation sensitivity. set of tectonic parameters was held fixed in order to Heat generation observe how changes in the thermal parameters would (W m 3) T ( C) P (kbar) affect the outcome of the model calculations, especially T t and P t curves and thus differences in modelled peak temperature, related pressure, maximum pressure, plausible Ar cooling ages for different minerals and modelled exhumation and cooling rates. Table 2 summarizes the results of modelling using different (d) Variable heat generation (top and bottom of rock column). thermal parameters. All thermal modelling runs used an arbitrary Top Bottom T ( C) P (kbar) 100 Myr total duration, beginning at t=0 with the onset of crustal loading and continuing through Heat capacity= JK 1kg 1. Thermal conductivity=2.75 W m 1 K 1. Density= constrained dome-stage and late dome-stage deforrepresents 2.75 kg m 3. Uses garnet-grade initial tectonic input parameters from Table 1. Bold parameter values that yield the closest approximation to actual measured mation episodes, and finally post-deformational passive P T values. exhumation. In the western (older) domain, this exhumation phase lasts 70 Myr ( Myr in the model), whereas in the eastern domain this unroofing stage is constrained at 65 Myr duration in order to Table 1. Tectonic modelling parameters. account for the observed eastward younging of deformation Event (Table 1). The 100 Myr time frame is arbitrary, Onset Duration En bloc uplift rates Strain rate (Ma) (Myr) (mm yr 1) (s 1) and was chosen to be sufficiently long for a thorough evaluation of post-acadian cooling history. Western domain (garnet-grade) The careful selection of appropriate values of thermal Thrustinga conductivity, heat capacity and heat generation is Dome-stage crucial to the success of one-dimensional thermal deformation Late dome-stage modelling. These parameters particularly control the uplift heating rate and thus the maximum attainable tempera- Passive uplift n/a Eastern domain (garnet- and staurolite- or kyanite-grade) ture ( peak T ) during a crustal-thickening metamorphic Thrustinga event (England & Richardson, 1977; England & Dome-stage Thompson, 1984, 1986; Thompson & England, 1984). deformation Late dome-stage There is a substantial database related to the mechanuplift ical, thermal and physical properties of many different Passive uplift n/a rock types that allows close approximation of average a Thrusting includes a 30 km thick overburden overriding a 10 km thick lower plate. rock density, thermal conductivity, heat capacity and

6 630 T. R. ARMSTRONG & R. J. TRACY general estimates of radiogenic heat generation in rock input parameters, a range of exhumation and strain types found in collisional orogens ( Lachenbruch, 1970; rate values and associated durations of related defor- Ozisik, 1980; Sass et al., 1983; Stebbins et al., 1984; mational events were incorporated into the different van Schmus, 1984; Kobolev & Kutas, 1985; Carslaw stages of the two initial tectonic models (Tables 1 & 3). & Jaeger, 1986). Data on intrinsic thermal and physical An evaluation of Stage I (the overthrusting event) properties for quartzo-feldspathic rocks metamorphosed includes an analysis concerning the duration of the to amphibolite facies conditions, similar to crustal loading. Two sets of values that yielded many rocks within the area being modelled, were used modelled P T estimates comparable with the values as thermal input parameters for the computer model derived from thermobarometry ( bold values in Table 3) and are tabulated in Table 2. were acquired from the garnet-grade model, both of Modest variation in thermal input parameters which are nearly identical in P T values. Only one set (within reasonable limits for the rock types found in of comparable values (for rocks at 35 and 40 km initial the area) had only minor effects on the calculated depth) was obtained for particle paths associated with P T t paths. Temperature differences, due to changes the tectonic model for staurolite- or kyanite-grade in thermal conductivity and heat capacity input values, rocks. were less than 100 C for any given particle path depth Stage II (crustal residence) represents the time period (the depth of a given rock after initial crustal loading during and following thrusting, but prior to active at t=0) run in the model (30, 35 & 40 km runs in tectonic exhumation and any subsequent deformational Table 2). Using the garnet-grade (western domain) processes. Only one set of values for the duration of tectonic parameters, the model-calculated P T results crustal residence for each model fits the measured P T that best fitted the independently measured thermobar- data, the 40Ar/39Ar hornblende and muscovite cooling ometric data for that domain given in Table 2 (peak ages, and estimates of the timing of attainment of peak temperature of 535 C at 7.5 kbar, shown in bold type temperature (Table 3, bold type). It should be noted in Table 2) were acquired using a thermal conductivity that the pre-exhumation crustal residence of the value of 2.75 W m 1 K 1 with the full range of heat staurolite- or kyanite-grade domain is 5 Myr longer capacity values and for rocks at 35 km crustal depth. than that of the garnet-grade domain, extending until Modest changes in the average rock density or in heat 390 Ma rather than 395 Ma, from a model starting generation input values produced only minimal shifts time of 400 Ma (t=0). of <10 C and 0.2 kbar for the 35 km crustal depth Stage III (initial exhumation and dome-stage deformation) path (Table 2). With regard to heat generation input represents the onset of en bloc uplift rather values, comparison of model results with the measured than the start of deformational fabric development (thermobarometrically derived) P T data suggests that related to dome evolution. This is an important heat generation values ranging from to distinction, because mapping and structural analysis J K-kg 1 (single value for constant basal (Ratcliffe et al., 1992) have shown that dome geometries flux) and diverential heat generation values of and related fold amplitudes are insufficient to account (top of modelled rock column) and for the regional thermobaric surface gradient (see ( bottom of rock column) are all acceptable. Fig. 1; Armstrong, 1992). P T evolution therefore Finally, because both western and eastern domains probably results from large-scale en bloc uplift rather comprise the same lithotectonic units and the estimated than any specific deformational event. As indicated in physical and thermal properties of the rocks should Table 3, one set of values for the duration of Stage III vary little over the range of P T conditions recorded uplift agrees with measured thermobarometric P T in both domains, a single set of thermal input values for the garnet-grade domain: 10 Myr, ending at parameters was selected for use with the different 385 Ma. This duration was calculated for two values tectonic parameters. of exhumation rate, 0.75 and 1.0 mm yr 1. Only one calculated Stage III duration compares well with Evaluation of tectonic parameters measured P T t values for the staurolite- or kyanitegrade domain: 20 Myr, ending at 380 Ma (Table 3). Model results calculated using the initial set of thermal The choice of different strain rate values for Stage parameters discussed above were then used to evaluate III exhumation (staurolite- or kyanite-grade) produced these thermal input parameters critically and to run small to moderate shifts in calculated temperature and an iterative model that, in turn, allowed the evaluation pressure from 545 to 550 C and from 7.3 to 8.1 kbar of the suitability of the initial tectonic parameters. and slightly larger shifts in predicted 40Ar/39Ar Although this iteration is ultimately dependent upon muscovite closure ages from 366 to 380 Ma for the initial tectonic parameters selected ( because the staurolite-grade rocks and from 330 to 377 Ma for thermal parameters selected in the last section are kyanite-grade rocks. Only one strain rate value themselves dependent upon the initial tectonic values), ( s 1) produced agreement in calculated this analysis is in fact a critical evaluation of the and measured P T values and 40Ar/39Ar muscovite sensitivity of the thermal model to a range of new cooling ages. 40Ar/39Ar hornblende cooling ages show tectonic parameters. Using the selected set of thermal little variation since Stage III exhumation occurs

7 THERMAL MODELLING OF ACADIAN METAMORPHISM 631 Table 3. Tectonic modelling parameters. (a) Stage I: thrusting. Eastern domain Western domain Metamorphic grade Garnet Staurolite Kyanite Particle path depth (km) Time at end of event T ( C) P (kbar) T ( C) P (kbar) T ( C) P (kbar) Ma Ma Ma Ma (b) Stage II: crustal residence: (i) temperatures and pressures. Eastern domain Western domain Metamorphic grade Garnet Garnet Staurolite Kyanite Particle path depth (km) Time at end of event T ( C) P (kbar) T ( C) P(kbar) T ( C) P (kbar) T ( C) P (kbar) 399 Ma Ma Ma Ma Ma Ma (b) Stage II: crustal residence: (ii) peak temperatures and ages. Western domain Metamorphic grade Staurolite Kyanite Particle path depth (km) Estimated ages (Ma) Estimated ages (Ma) Time at end of event Peak T 40Ar/39Ar Hbl 40Ar/39Ar Ms Peak T 40Ar/39Ar Hbl 40Ar/39Ar Ms 399 Ma Ma Ma Ma Ma Ma (c) Stage III: dome-stage uplift: (i) western domain. Metamorphic grade Garnet Particle path depth (km) 35 Exhumation rate 1.0 mm yr 1 Estimated ages (Ma) T ( C) P (kbar) Peak T 40Ar/39Ar Hbl 40Ar/39Ar Ms Time at end of event 390 Ma Ma Ma Ma Exhumation rate (0.5 mm yr 1) Ma Ma Exhumation rate (mm yr 1) approximately during the attainment of peak temperature conditions and close to hornblende closure for Ar (about 500 C). Stage IV exhumation represents late-stage en bloc uplift and erosion following the post-peak temperature, late-stage deformation. There is also some deformation associated with this phase a late crenulation or pressure solution cleavage in rocks that lie between

8 632 T. R. ARMSTRONG & R. J. TRACY Table 3. (Cont.) (c) Stage III: dome-stage uplift: (ii) eastern domain: staurolite. Metamorphic grade Staurolite Particle path depth (km) 35 Exhumation rate 1.0 mm yr 1 Estimated ages (Ma) T ( C) P (kbar) Peak T 40Ar/39Ar Hbl 40Ar/39Ar Ms Time at end of event 390 Ma < Ma Ma Ma Ma Exhumation rate (mm yr 1) Strain rate (s 1) (c) Stage III: dome-stage uplift: (iii) eastern domain: kyanite. Metamorphic grade Kyanite Particle path depth (km) 40 Exhumation rate 1.0 mm yr 1 Estimated ages (Ma) T ( C) P (kbar) Peak T 40Ar/39Ar Hbl 40Ar/39Ar Ms Time at end of event 390 Ma < Ma Ma Ma Ma Exhumation rate (mm yr 1) Strain rate (s 1) (d) Stage IV: continued dome-stage uplift: (i) western domain. Metamorphic grade Garnet Particle path depth (km) 35 Estimated ages (Ma) Exhumation rate (mm yr 1) T ( C) P (kbar) Peak T 40Ar/39Ar Hbl 40Ar/39Ar Ms the western (Rayponda/Sadawga Domes) and eastern garnet-grade rocks and 380 Ma for staurolite- or (Athens Dome) domains, and moderate cleavage kyanite-grade rocks). development within the eastern domain itself. Because The only viable way to constrain reasonable modelled Stage IV features significantly post-date the attainment exhumation rates using geological data is to of peak temperature and most 40Ar/39Ar hornblende compare modelled and calculated 40Ar/39Ar muscovite closure ages, but precede 40Ar/39Ar muscovite closure closure ages. Input exhumation rates of 0.20 and ( largely due to the presence of new muscovite within 0.30 mm yr 1 produce modelled muscovite closure late-stage deformation planes), this event must therefore temperature and time values that match well with post-date Stage III (modelled ages of 385 Ma for measured muscovite closure ages for garnet-grade and

9 THERMAL MODELLING OF ACADIAN METAMORPHISM 633 Table 3. (Cont.) (d) Stage IV: continued dome-stage uplift: (ii) eastern domain: staurolite. Metamorphic grade Staurolite Particle path depth (km) 35 Estimated ages (Ma) Exhumation rate (mm yr 1) T ( C) P (kbar) Peak T 40Ar/39Ar Hbl 40Ar/39Ar Ms (d) Stage IV: continued dome-stage uplift: (iii) eastern domain: kyanite. Metamorphic grade Kyanite Particle path depth (km) 40 Estimated ages (Ma) Exhumation rate (mm yr 1) T ( C) P (kbar) Peak T 40Ar/39Ar Hbl 40Ar/39Ar Ms staurolite- or kyanite-grade domains, respectively generated from calculations using the iterated thermal (Table 3). and tectonic input parameters from the garnet-grade (western) and staurolite- or kyanite-grade (eastern) RESULTS AND DISCUSSION domains. In both domains, the model assumes a rapid loading that is related to the onset of crustal thickening The refined estimates of the various tectonic parameters at c. 400 Ma and is believed to be the result of westdiscussed above, including the number and duration directed fold- and thrust-nappe development (Spear of deformational events, exhumation and associated et al., 1991; Armstrong et al., 1992; Spear, 1992). The cooling rates, domain P T conditions, modelled Ar order-of-magnitude difference between strain rates and closure ages, and calculated P t and T t curves, can rates of thermal conduction indicate that emplacement now be directly compared with the tectonic model of the tectonic load was effectively instantaneous based upon measured thermobarometric, geochronological relative to any conductive or advective heating, due to and structural data. The thermal model is a reasonable assumed lack of any significant lateral obviously based upon tectonic and thermal input heat transfer (Peacock, 1989). parameters that were, at least in part, assessed using Calculated temperature time (T t) and pressure time assumptions based on the measured data, resulting in (P t) curves (Fig. 3a,b) for the western some circularity. The results, therefore, do not test the domain around the Sadawga Dome indicate the validity of assumptions involved in formulating the attainment of maximum temperature (535 C) and original structural model, but do provide significant associated pressure (7.5 kbar, reflecting about 26.5 km insight into the validity of the assumptions behind the depth) at 391 Ma. These values agree well with the one-dimensional modelling and provide a useful test independently measured petrological and geochronological of the proposed tectonothermal processes that took data (Table 4). It should be noted that the place during the orogeny. maximum pressure, which was attained immediately Following iteration, model P T t values for both after the 400 Ma fold- and thrust-nappe loading event, garnet-grade and staurolite- or kyanite-grade domains does not correspond to the pressure at maximum are internally consistent with measured pressure and temperature as in Fig. 2(a,b). Because the geothermo- temperature from both domains, and with published barometers applied here are based upon net transfer 40Ar/39Ar hornblende and muscovite closure ages (see equilibria that will probably continue to operate until Table 4). Specific pressure temperature (P T ), press- maximum temperature is achieved (and possibly ure time (P t) and temperature time (T t) curves beyond) due to kinetic reasons (Thompson & England, derived from the model, together with the thermal 1984), estimates of pressure at maximum temperature parameters discussed above, are presented and discussed should typically be lower than absolute maximum below, and model results are compared with pressure along a clockwise P T path by an amount measured regional P T t estimates. dependent upon the exhumation rate (i.e. decom- Results from the one-dimensional model include two pression rate) during the last stage of prograde heating sets of modelled P T paths shown in Fig. 2(a,b), (England & Thompson, 1984, 1986; Thompson &

10 634 T. R. ARMSTRONG & R. J. TRACY Table 4. Measured and modelled values. Measured values Modelled values (this paper) Western domain (garnet-grade) P T conditions Maximum T 535 C± C P (at maximum T ) kbar 7.5 kbar Maximum P > 7.3 kbar (Gibbs method) 9.8 kbar Peak T age Ma (estimated) 391 Ma 40Ar/39Ar Hbl closure age Maa,b 385 Ma 40Ar/39Ar Ms closure age 365 Maa 367 Ma 40Ar/39Ar Kfs closure age No ages reported 332 Ma Exhumation rates Peak T to 40Ar/39Ar Hbl closure 1.4 mm yr 1 c 1.8 mm yr 1 40Ar/39Ar Hbl to 40Ar/39Ar Ms closure 0.1 mm yr 1 c 0.19 mm yr 1 40Ar/39Ar Ms to 40Ar/39Ar Kfs closure Not determined 0.10 mm yr 1 Cooling rates Peak T to 40Ar/39Ar Hbl closure 13.0 C Myr 1 c 7.0 C Myr 1 40Ar/39Ar Hbl to 40Ar/39Ar Ms closure 9.0 C Myr 1 c 8.3 C Myr 1 40Ar/39Ar Ms to 40Ar/39Ar Kfs closure Not determined 4.3 C Myr 1 Eastern domain (staurolite-grade) P T conditions Maximum T 550 C± C P (at maximum T ) kbar 7.6 kbar Maximum P kbar (Gibbs method) 9.8 kbar Peak T age Ma (estimated) 386 Ma 40Ar/39Ar Hbl closure age Mab 380 Ma 40Ar/39Ar Ms closure age Maa,d 366 Ma 40Ar/39Ar Kfs closure age No ages reported 340 Ma Exhumation rates Peak T to 40Ar/39Ar Hbl closure 1.4 mm yr 1 c 1.7 mm yr 1 40Ar/39Ar Hbl to 40Ar/39Ar Ms closure 0.1 mm yr 1 c 0.25 mm yr 1 40Ar/39Ar Ms to 40Ar/39Ar Kfs closure Not determined 0.11 mm yr 1 Cooling rates Peak T to 40Ar/39Ar Hbl closure 13.0 C Myr 1 c 8.3 C Myr 1 40Ar/39Ar Hbl to 40Ar/39Ar Ms closure 9.0 C Myr 1 c 10.7 C Myr 1 40Ar/39Ar Ms to 40Ar/39Ar Kfs closure Not determined 3.8 C Myr 1 Eastern domain (kyanite-grade) P T conditions Maximum T 600 C± C P (at maximum T ) kbar 9.0 kbar Maximum P kbar (Gibbs method) 11.2 kbar Peak T age Ma (estimated) 384 Ma 40Ar/39Ar Hbl closure age Mab 372 Ma 40Ar/39Ar Ms closure age c. 340 Mad 347 Ma 40Ar/39Ar Kfs closure age c. 310 Mad 308 Ma Exhumation rates Peak T to 40Ar/39Ar Hbl closure 1.4 mm yr 1 c 1.1 mm yr 1 40Ar/39Ar Hbl to 40Ar/39Ar Ms closure 0.1 mm yr 1 c 0.12 mm yr 1 40Ar/39Ar Ms to 40Ar/39Ar Kfs closure Not determined 0.10 mm yr 1 Cooling rates Peak T to 40Ar/39Ar Hbl closure 13.0 C Myr 1 c 8.3 C Myr 1 40Ar/39Ar Hbl to 40Ar/39Ar Ms closure 9.0 C Myr 1 c 6.0 C Myr 1 40Ar/39Ar Ms to 40Ar/39Ar Kfs closure Not determined 2.6 C Myr 1 Sources: a Sutter et al. (1985); b Laird et al. (1991) and Drake et al. (1989); c Armstrong et al. (1992); d Harrison et al. (1989). England, 1984; Peacock, 1989). Figure 3( b) shows the 1985; Sutter et al., 1985; Harrison et al., 1989; Burton calculated maximum pressure along the modelled P t et al., 1991; Laird et al., 1991; Spear, 1992). A 40Ar/39Ar curve at 9.8 kbar (35 km depth). Although at present K-feldspar age of 323 Ma from Jamaica, Vermont, there is no independent measure of maximum pressure, reported by Burton et al. (1991), is in reasonable a calculated value of 9.8 kbar is consistent with agreement with the western domain model calculation independent petrological calculations based on the of a 332 Ma age for a temperature of 250 C. Gibbs method which suggest that the maximum Integrated exhumation rates predicted from the P t pressure was 1 kbar or more greater than that curve in Fig. 3( b), with closure ages extracted from the calculated from thermobarometry (Armstrong & T t curve, include an early phase of rapid exhumation Tracy, 1991). (1.6 mm yr 1) from the time of maximum temperature Figure 3(a,b) and Table 4 show that the predicted to hornblende Ar closure at 500 C, followed by two Ar closure ages for hornblende and muscovite based subsequent phases of slower exhumation: 0.19 mm yr 1 on the thermal model, assuming respective closure from hornblende Ar closure to muscovite Ar closure temperatures of 500 and 350 C (Harrison et al., 1989), at 350 C, and 0.10 mm yr 1 from muscovite Ar are in good agreement with published 40Ar/39Ar closure to K-feldspar Ar plateau closure at 250 C. geochronological data for this region (Sutter & Hatch, These values are in close agreement with independently

11 THERMAL MODELLING OF ACADIAN METAMORPHISM 635 Depth (km) (a) Western Domain 395 Peak P 391 Peak T Hbl closure 332 Ms closure Kfs closure (b) Eastern Domain 388 Peak P ca Ma Ma thrusting event Peak T Pressure (kbar) Kfs closure Hbl closure Ms closure Temperature ( C) Fig. 3. (a) Model temperature time (T t) path for garnet-grade rocks of the western domain, calculated from the one- dimensional thermal model. The curve is subdivided by the open circles that reflect the timing of attainment of theoretical mineral Ar closure temperatures. Assumed theoretical blocking temperatures are c. 500 C for hornblende (Hbl), 350 C for muscovite (Ms) and 225 C for K-feldspar (Kfs). Open circles bound segments of the T t curve that yield the different cooling rates shown for each segment (given in C Myr 1). Modelled closure ages are indicated next to each circle. The modelled age for the attainment of maximum temperature is indicated as 391 Ma. (b) Pressure (depth) time (P t) path calculated for garnet-grade rocks of the western domain. Open circles bracket P t segments used for the calculation of the exhumation rates shown next to each segment and given in mm Myr 1. Boxes show the modelled depth and associated pressure at the time of attainment of each modelled P T benchmark, including maximum P, P at peak T, P at Hbl Ar isotopic closure, P at Ms Ar closure and P at Kfs Ar closure. Fig. 2. (a) Pressure temperature time (P T t) path derived from one-dimensional thermal (1DT) modelling. Path begins at the onset of crustal loading (c. 400 Ma) of New Hampshire Sequence rocks upon pre-silurian Vermont Sequence rocks of the western domain. Note that the path includes the attainment of peak pressure prior to the attainment of peak temperature and its associated pressure which are commonly measured by most thermobarometric techniques. Hbl closure, Ms closure and Kfs closure, shown as open circles, refer to the modelled hornblende, muscovite and K-feldspar Ar cooling ages calculated in this study (see text). (b) Pressure temperature time (P T t) paths for kyanite-grade rocks (open circles) and staurolite-grade rocks (filled circles) of the eastern domain, derived from one-dimensional thermal modelling. Similarly to rocks of the western domain, eastern domain rocks attain peak pressure prior to the attainment of peak temperature and its associated pressure. Open circles indicating closure ages are the same as in (a). Note that staurolite-grade rocks are loaded to the same depth as garnet-grade rocks of the western domain (compare with path in a) but, because of the longer duration of nearly isobaric heating (referred to as crustal residence period ), staurolite-grade rocks attain somewhat higher maximum temperature than garnet-grade rocks upon initiation of uplift/exhumation. calculated integrated exhumation rates in the same regional terrane of Hames et al. (1989) for western rocks from the staurolite- or kyanite-grade eastern Connecticut, which indicated rapid initial exhumation domain (Fig. 4). The curves with filled circles (predicted of 1.4 mm yr 1 from peak temperature to hornblende Ar closure ages) represent staurolite-grade rocks on Ar closure, followed by a protracted period of slower the south-western side of the Athens Dome and just exhumation of 0.1 mm yr 1 down to muscovite east of (slightly above in temperature) the staurolite-in Ar closure. isograd (see Fig. 1), where thermobarometric measure- Two sets of P t and T t curves were calculated for ments range from 550 C at 7.5 kbar to 575 C at

12 636 T. R. ARMSTRONG & R. J. TRACY Fig. 4. (a) Temperature time (T t) paths for staurolite-grade rocks (filled circles) and kyanite-grade rocks (open circles) of the eastern domain. Each pair of adjacent circles on either path brackets T t segments used to calculate cooling rates (in C Myr 1) shown adjacent to each segment. Model Ar closure ages for hornblende (Hbl) (at 500 C), muscovite (Ms) (at 350 C) and K-feldspar (Kfs) (at 225 C) are shown. 386 Ma and 384 Ma are the model ages for attainment of maximum temperature for staurolite-grade and kyanite-grade paths, respectively. (b) Pressure (depth) time (P t) paths calculated for staurolite-grade rocks (filled circles) and kyanite-grade rocks (open circles) of the eastern domain. Each pair of adjacent circles on either path brackets specific P t segments used to calculate uplift rates (given in mm Myr 1) and shown adjacent to each segment. Exhumation rate calculations were performed for parts of paths between (i) P at peak T and Hbl Ar isotopic closure; (ii) Hbl Ar closure and Ms Ar closure; and (iii) Ms Ar closure and Kfs Ar closure. pressure (550 C at 7.6 kbar), and a predicted hornblende Ar closure age of 380 Ma from the calculated staurolite-grade curves in Fig. 4. It should be noted that the time of attainment of maximum temperature (550 C at 386 Ma), in conjunction with the hornblende closure age (500 C at 380 Ma), indicates rapid exhumation ( 1.7 mm yr 1) and cooling ( 8.3 C yr 1) through the time of hornblende Ar closure, as shown in Fig. 4(a). The maximum pressure, corresponding to the crustal depth attained immediately following initial crustal loading, is estimated to have been 9.8 kbar (corresponding to 35 km depth) as shown in Fig. 4(b). This calculated result is the same as that obtained for the garnet-grade domain to the west shown in Fig. 3( b), and in fact both of these lithotectonic belts probably were at similar crustal depths following initial loading at 400 Ma, but the garnet-grade rocks probably had shorter residence periods at these deeper crustal levels because of documented west-to-east diachroneity in subsequent dome-related deformation and associated vertical movement. This is reflected in the T t curves (Figs 3a & 4a) and P t curves (Figs 3b & 4b) which show a longer isobaric heating phase for the staurolitegrade rocks ( Ma in Fig. 2b) than for the garnet-grade rocks ( Ma in Fig. 2a). This confirms the interpretation that diachroneity of deformation and subsequent uplift were major factors in determining specific peak temperatures and associated pressures and Ar cooling ages throughout this region. The P t and T t curves for kyanite-grade rocks from the southern closure of the Athens Dome (eastern domain), represented by the open circles in Fig. 4 at C and kbar, are also in close agreement with measured values (Table 4, Fig. 4). It should be noted that the estimated timing of the maximum temperature is similar to that from the staurolite-grade rocks 384 versus 386 Ma (Fig. 4a, filled circles) but subsequent predicted cooling ages show progressive discrepancy, with model predictions of K-feldspar Ar closure ages differing by more than 30 Myr: 340 versus 308 Ma (Fig. 4). This discrepancy is produced both by initial differences in maximum temperature (605 versus 550 C) and by the subsequent differences in durations and rates of cooling with respect to the associated exhumation histories. This is best illustrated by using the P t curves in Fig. 4( b) to show the differences in pressure at maximum temperature and at hornblende Ar closure for staurolite- and kyanite-grade rocks. These pressure differences result in substantial variation in the calculated integrated exhumation rates of 1.7 mm yr 1 for staurolite-grade rocks and 1.1 mm yr 1 for kyanite-grade rocks, as shown in Fig. 4(a), and in initial post-peak temperature cooling rates of 10.7 C Myr 1 for staurolite-grade rocks and 7.8 kbar (Table 4). Laird et al. (1991) reported a 40Ar/39Ar hornblende plateau age of 376 Ma from the 6.0 C Myr 1 for kyanite-grade rocks (Fig. 4a). same locality that yielded the higher P T estimate Contrasts in the thermal evolution of rocks metamor- ( 575 C at 7.8 kbar). These data are consistent with phosed at different crustal levels are again attributable model values of maximum temperature and associated to the residence periods for these rocks during initial

13 THERMAL MODELLING OF ACADIAN METAMORPHISM 637 isobaric heating immediately following the onset of petrological and geochronological techniques, and 400 Ma crustal loading. which are necessary for deciphering the diachronous For the low-temperature part of the kyanite-grade development of tectonic processes that control thermal cooling path as constrained by muscovite Ar closure evolution in complex orogens. and K-feldspar Ar closure (Table 4), it should be noted that the predicted muscovite and K-feldspar 40Ar/39Ar closure ages are in very good agreement with the ACKNOWLEDGEMENTS measured ages from Silurian Devonian rocks immedi- The authors thank Drs O. Vanderhaeghe and D. Lux ately east of the study area given in Table 4 (Harrison for very thoughtful and constructive reviews and Prof. et al., 1989) and with the regional estimates of uplift and cooling rates for this stage of Acadian thermal evolution (Armstrong et al., 1992). M. Brown for editorial help above and beyond the ordinary. G. Blackmer, B. Burton, N. Ratcliffe, J. Sutter and D. Stewart provided reviews of an earlier version of the manuscript. Support for this work was provided through National Science Foundation Grant CONCLUSIONS EAR to RJT. Thermobarometric measurements of pressure and temperature, measured ages of attainment of maximum REFERENCES temperature and subsequent Ar closure, and estimates of regional uplift and cooling rates agree well with Armstrong, T. R., Progressive evolution of Acadian predictions from one-dimensional thermal modelling. dynamothermal events in southern Vermont: evidence for the transgressive dome development. Geological Society of America Results from thermal modelling provide several import- Abstracts with Programs, 24, 4. ant refinements to the tectonothermal models for Armstrong, T. R., Structural and Petrologic Evolution of this area. Acadian Gneiss Domes in Southern Vermont. PhD 1 Estimates from geothermobarometry typically yield Dissertation, Virginia Polytechnic Institute and State University, Blacksburg, VA. pressures coincident with maximum temperatures Armstrong, T. R. & Hames, W. E., Integrated structural, (Spear, 1990); actual values of maximum pressure for petrologic and geochronologic constraints on Acadian dome rocks in this area may have been as much as 2.5 kbar evolution in Vermont. Geological Society of America Abstracts greater. This is consistent with petrological obser- with Programs, 25, A-423. Armstrong, T. R. & Tracy, R. J., Origin of Acadian domes vations from this region and with typical behaviour in in southern Vermont. Geological Society of America Abstracts decompressional orogens, which predict decompression with Programs, 23, 4. prior to the attainment of peak temperature along a Armstrong, T. R., Tracy, R. J. & Hames, W. 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