M. A. Evans
University of Pittsburgh
Pittsburgh, PA 15260

M. J. Bartholomew
University of South Carolina
Columbia, SC 29208

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Fluid inclusion microthermometric data from veins in the southeastern Piedmont province record the changes in fluid composition and deformation conditions during regional exhumation and cooling related to Late Paleozoic syn- to post-orogenic processes and early Mesozoic rifting. In general, the composition of post-metamorphic fluids that were trapped late during the Alleghanian orogeny and during post-orogenic exhumation are remarkably consistent across the southeastern Piedmont, indicating regional fracture connectivity.

The first fluids were trapped in veins that formed during during the last phases of the Alleghanian. These syn-deformational fluids are CO₂-saturated low salinity brines (salinities of 2.6 to 5.7 wt.% NaCl equivalent) with homogenization temperatures in the range of 200° to 365°C. They were trapped under lithostatic pressures between 240 and 280 MPa, indicating burial depths of 11.2 to 12.7 km. These depths are similar to emplacement depths of post-kinematic plutons, suggesting a period of rapid isobaric cooling.

Low-salinity H₂O inclusions and rare CO₂-rich inclusions are evidence for Early Mesozoic regional decompression as fracturing above the brittle-to-ductile transition allowed regional pore-fluid pressure to drop to hydrostatic levels. Convective circulation of meteoric water resulted in the dilution of 'in-situ' fluids, and ultimately to a system saturated with meteoric water. These fluids continued to be trapped in vein minerals through much of the Mesozoic as rift basins formed during the opening of the Atlantic Ocean.

Late Paleozoic through Mesozoic exhumation rates for the eastern Piedmont province average 0.063 km m.y.^-1 and cooling rates average ~1.9°C m.y.^-1. These low rates may be directly related to thinned crust and lithosphere resulting from delamination processes during the late Alleghanian orogeny.

Metamorphic terranes of orogenic belts may undergo a period of exhumation (as defined by England and Molnar, 1990, p.1173) as a result of syn-orogenic crustal delamination (Sacks and Secor, 1990; Nelson, 1992). However, our understanding of post-metamorphic rock processes during this period of exhumation is limited by mineral instability and sluggishness of retrograde reactions (Craw, 1990). As with all other stages of orogenesis, fluids play a critical role during uplift. Studies of these fluids and their interactions with the rock can yield much information on processes that occur in exhumed terranes (Hollister and others, 1979; Frey and others, 1980; Boullier and others, 1989; Craw and Koons, 1989). The presence of fluids during unroofing and cooling of the metamorphic belt is evidenced by veins that cross-cut the metamorphic rocks. Fluid inclusion studies of fluids trapped in these post-metamorphic veins provide information on fluid evolution and changes in environmental conditions during uplift (Sauniac and Touret, 1983; Craw, 1987; Holm and others 1989; Craw, 1990; Jenkin and others, 1994).

This study examines late syn- to post-orogenic veins in the southeastern Piedmont province of the southern Appalachian orogen. During the late Paleozoic and early Mesozoic, the area experienced a period of regional exhumation with concurrent unroofing and cooling (Dallmeyer and others, 1986). During this time, rocks metamorphosed at amphibolite grade (~600°C) during the Alleghanian orogeny were cooled to <200°C, resulting in changes in environmental conditions. This was followed by continued exhumation during Mesozoic rifting and opening of the Atlantic Ocean. By Late Cretaceous time, the metamorphic rocks were at surface conditions (~20°C).

The goals of this paper are twofold. First, we will characterize the chemistry and evolution of fluids present in Piedmont rocks during late Alleghanian orogenesis and into early Mesozoic rifting. This will be achieved by integrating field observations, microthermometry of fluid inclusions in vein minerals, and vein petrography and mineral paragenesis. Second, we determine environmental conditions during fluid trapping and relate those to the exhumation and cooling history.

Following the usage by Maher and others (1994), the eastern Piedmont of South Carolina and Georgia is divided into multiple terranes, each with a distinct lithotectonic assemblage and geologic history. The Carolina terrane (Fig. 1) is divided into two parts. The western portion of the Carolina terrane consists of Precambrian to Cambrian mafic to intermediate metavolcanic rocks and metasedimentary rocks penetrated by a series of mafic to ultramafic intrusive complexes (Secor, 1987; Dennis and Shervais, 1991, 1996). These rocks were metamophosed to upper greenschist to lower amphibolite facies between 500 and 430 ma (Butler, 1991). The eastern portion of the Carolina terrane consists of Cambrian felsic to intermediate metavolcanic rocks and metasedimentary rocks broadly encompassed by the Persimmon Fork, Richtex, and Asbill Pond Formations (Fig. 1) (Dennis and Shervais, 1991, 1996). These rocks were regionally metamorphosed to chlorite and biotite grade between 480 to 450 Ma (Butler, 1991). The Savannah River terrane (Mahar and others, 1991) consists of migmatitic sillimanite-bearing schists, paragneisses, and granite orthogneisses, and subordinate metamorphosed mafic to ultramafic rocks (Mahar and others, 1994). The Modoc fault zone separates the Carolina and Savannah River terranes. It consists of originally lower grade Carolina terrane rocks that underwent significant granitic sheet intrusion, prograde metamorphism and penetrative strain during the late Paleozoic Alleghanian orogeny (Secor and others, 1986).

In this area, the Alleghanian orogeny is divided into three deformation events that reflect the continuum of orogenesis (Secor and others, 1986). D₂ is associated with amphibolite facies regional metamorphism and mid-crustal felsic plutonism that occurred between 315 to 295 Ma (Dallmeyer and others 1986; McSween and others, 1991). D₂ reflects dextral strike-slip displacement along the Modoc fault zone (Fig. 1) (West and others, 1995). The D₃ event occurred between 290 to 310 Ma (Dallmeyer and others, 1986; Pray and others, 1997) and folded the Modoc fault zone and high grade gneisses and schists of the Savannah River terrane and juxtaposed them against Carolina terrane rocks. This resulted in retrograde metamorphism from amphibolite to lower greenschist conditions (Mahar, 1987). Much of the study area was overprinted by ductile dextral shearing (Irmo deformation event, D₄ of Secor and others, 1986) that terminated by 275 Ma (Pray and others, 1997).

Syn-orogenic granitoid and gabbroic plutons were emplaced between 327± 21 Ma and 286 Ma, and some as young as 266 Ma (McSween and others, 1991; Speer and others, 1994, D. Secor, personal communication, 2000). Many plutons are syn-kinematic and contain pervasive foliations and lineations. However, plutons discussed in this paper are considered to be post-kinematic because they lack ductile deformation structures, but may contain later brittle features including faults and joints. These plutons are considered to have been emplaced late in the orogeny (between 309± 2 Ma and 286± 3 Ma) (Speer and others, 1991). These late plutons were emplaced as water saturated (P_fluid = 0.5 P_total), relatively hot magmas (650° to 750°C ) at 290 to 330 MPa pressure (11.2 to 12.7 km depth) (Speer and others, 1994).

After the Alleghanian orogeny, during the early Mesozoic, this region was affected by deformation related to rifting that eventually resulted in the formation of the Atlantic Ocean. The initial phase of this deformation is the transition from the D₄ dextral strike-slip event to north-south extension (D₅) (Bartholomew and others, 1997). The second phase of Mesozoic rifting is characterized by northwest-southeast extension (D₆). During this event, circulating hydrothermal fluids produced reactions in the wall-rock of D₆ veins and earlier-formed reopened veins, altering calcium feldspar to K-feldspar (Mauldin and others, 1997). This 'pinking' event is dated at approximately 220 Ma (Kish, 1992). The last deformation event (D₇) associated with rifting is the emplacement of syn-rifting diabase dikes throughout the Piedmont (Burt and others, 1978; Ragland and others, 1983; Bell, 1988; Goldsmith and others, 1988; Withjack and others, 1998). This event is dated at approximately 200 Ma (McHone and Butler, 1984; Sutter, 1988).

The goal of sampling process was to sample only those veins that are post-metamorphic in origin. In order to accomplish this, a set of criteria was established to determine which veins were metamorphic. Veins that were parallel to foliation, boudinaged, folded, or exhibited obvious recrystallization textures in the field were assumed to be pre- to syn-metamorphic and were not sampled. In contrast, veins that cut across foliation (generally at a high angle) or are associated with normal faulting were assumed to be late syn- to post-metamorphic. These veins are generally planar to subplanar, 0.5 cm to 10’s of cm wide, and are vertical to steeply dipping. In general, veins in the post-kinematic granites and in the country rock are 1) the same orientation, 2) contain the same paragenetic sequence, and 3) contains the same fluid inclusion history. Therefore, the veins examined are late Alleghanian or later.

At each structural station (Fig. 1), vein orientations, along with other structural data (joints, faults, bedding, cleavage, fold axes, etc.) were measured. Samples of vein material from each vein set represented at the outcrop were collected for petrologic, and microthermometric analysis.

Five major vein sets are defined based on mineral paragenesis and vein orientation (Fig. 2, Table 1). V₄ veins are found only in the Modoc fault zone and in the northeastern Carolina terrane. They are interpreted to have formed during D₄ deformation (Secor and others, 1986; Pray and others, 1997) as the rock temperatures cooled below the brittle to ductile transition for quartz-feldspar aggregates (~300° to 400°C). The V₄ set is divided into two sub-orthogonal subsets. V_4a veins strike 325° ± 20° , contain the assemblage quartz ± pyrite and are orthogonal to the Modoc fault zone. V_4b veins strike 058° ± 12° , contain the assemblage quartz ± muscovite, and are parallel to the Modoc fault zone (Fig. 2, Table 1). V₅ veins strike 279° ± 21° and are parallel to east-west striking normal faults. They contain the mineral assemblage quartz ± muscovite ± epidote plus late calicte ± zeolite ± siderite (Fig. 2, Table 1). V₅ is the most common vein set, occurring throughout the study area. V₆ veins strike 000° ± 9° and contain the assemblage quartz ± chlorite plus late calicte ± zeolite ± prehnite. V₇ veins (Fig. 2, Table 1) strike 014° ± 24° and are found only in the northeast part of the Carolina terrane. In general, the V₇ set veins are subparallel to the strike of the Jurassic diabase dikes in the Carolinas (Ragland, 1991). V₇ veins contain a distinctive calcite± prehnite± zeolite± quartz assemblage. V₇ veins parallel V₆ veins and in some cases are reopened V₆ veins.

Fluid inclusion compositions and densities were determined by measuring the temperatures of phase transitions within fluid inclusions using a modified U.S. Geological Survey-type heating-freezing stage manufactured by FLUID Inc. The stage was calibrated at 0°C (ice bath), 374.1°C (critical point of water), and -56.6°C (CO₂ triple point) (the latter two standards supplied by SYNFLINC, Inc.). Individual temperature measurements were found to be reproducible to ± 0.2°C at low temperatures (CO₂ melting, ice, clathrate; CO₂ homogenization), and to ± 2°C at high temperatures (aqueous liquid-vapor or aqueous carbonic homogenization).

Quartz and calcite hosted fluid inclusions were first warmed to their homogenization temperatures in order to reduce the risk of inclusion stretching due to ice formation (Lawler and Crawford, 1983; Meunier, 1989). The inclusions were then cooled below -100°C and subsequently warmed rapidly to determine the approximate temperature of ice melting (T_mIce) and homogenization (T_hTot) of constituent phases. Subsequent runs were then performed at slower rates to more accurately determine these temperatures. Depending on the composition of each inclusion, the following temperatures and phase ratios were determined: T_mCO2, percent CO₂ vapor relative to CO₂ liquid at T_mCO2, T_mIce (water ice), T_mcl (clathrate), T_hCO2(CO₂ L-V), percent CO₂ at 35°C, T_hTot (H₂0 L-V), and T_hTot (H₂O-carbonic). Because of the difficulty of viewing phase changes in small inclusions, clathrate and ice melting measurements could not be performed on all inclusions for which T_hTot was determined. In the CO₂-bearing aqueous inclusions, salinity was derived from either 1) the dissociation temperature of the clathrate (T_mcl) in the presence of CO₂-liquid and vapor (Chen, 1972; Bozzo and others, 1973; Diamond, 1992) or 2) from the melting temperature of ice in the presence of CO₂ clathrate and CO₂ vapor, using the experimental data of Bozzo and others (1973).

A fluid inclusion population comprises 'a group of inclusions that formed at the same time from the same parent fluid under identical physicochemical conditions and which have since undergone the same post-entrapment history' (Guscott and Burley, 1993). In this study, the populations are defined by inclusion fluid composition and inclusion homogenization temperature. Four populations of fluid inclusions are found in the veins studied.

H₂O. These inclusions range in size from <2 m m to ~ 25 m m and contain a vapor bubble that occupies from 1 to 2 volume % of the inclusion. Homogenization temperatures (T_hTot) range from 65° to 224°C (Figs. 3 and 4) and always occurs by homogenization to a liquid. Water ice melting temperatures (T_mIce) range from –0.2° to 0.0°C, but are concentrated at 0.0°C and are interpreted to represent meteoric water (Fig. 3, Table 1) Several positive T_mIce values (1.0 to 2.3°C) represent metastable 'superheated' ice (Roedder, 1984). Primary H₂O inclusions occur in calcite as irregular clusters without a planar or other clear geometric distribution. Pseudosecondary H₂O inclusions occur in calcite as planar arrays along discordant, annealed fractures that terminate within the crystal margins. These inclusions are compositionally the same as the primary inclusions, suggesting that they are related through a continuous temporal sequence. In quartz, the H₂O inclusions always occur as planar clusters and appear to be secondary. In cross-cutting planar arrays, the array with the lower average T_hTot is always the latest (Fig. 3a).

Low-Salinity H₂O. These inclusions range in size from <2 m m to ~ 50 m m and contain a vapor bubble that occupies from 1 to 2 volume % of the inclusion. Homogenization temperatures (T_hTot) range from 86° to 181°C (Figs. 3 and 4, Table 1) and always occurs by homogenization to a liquid. Water ice melting temperatures (T_mIce) range from –3.2° to -0.3°C, representing salinities in the range of 0.5 to 5.3 wt. % NaCl equivalent. Low-salinity H₂O inclusions occur only in quartz as isolated inclusions and irregular clusters interpreted as primary and as planar clusters that are interpreted as pseudosecondary or secondary (Fig. 3b). This inclusion population is interpreted as fluids representing formed as a result of mixing of meteoric water with more saline in-situ fluids.

Low XCO₂. These inclusions range in size from <2 m m to over 100 m m and contain two or three phases at room temperature depending upon the T_hCO2 of the CO₂ vapor. When three phases are present, they include a carbonic vapor, a carbonic liquid, and an immiscible aqueous liquid (Figs. 3c and 3d). Upon homogenization of the carbonic vapor between 17.4° to 30.9°C, the two remaining phases are a carbonic bubble that occupies from 20 to 30 volume % of the inclusion and an aqueous liquid. More rarely, inclusions contain a vapor bubble that occupies 80 to 90 volume % of the inclusion. These homogenize to a vapor. Many inclusions were too small to observe T_hCO2, so for these inclusions, only T_hTot is reported (Figs. 3e and 3f). CO₂ melting temperatures range from -56.8° to -56.4°C. Values lower than –56.6°C may be accounted for by small amounts of CH₄. However, melting temperatures above –56.6°C cannot be accounted for by any known phase equilibria, although Burruss (1981) suggested that such high values may be the result of a mixture of CO₂ and SO₂. Total homogenization temperatures (T_hTot) range from 198° to 365°C, and may be to a vapor or liquid (Figs. 3 and 4, Tables 1 and 2). Several inclusions decrepitated before reaching homogenization. These are used to provide a lower limit to the trapping pressure. Water ice melting temperatures (T_mIce) range from -4.2° - 0.9°C and represent salinities in the range of 1.5 – 6.7 wt.% NaCl equivalent (Tables 1 and 2). In larger inclusions, CO₂ clathrate melting may be observed between 7.8 - 9.2°C indicating salinities in the range of (1.7 - 4.3 wt. % NaCl equivalent). Only two eutectic temperature values could be determined, -34°C and -28°C, suggesting the fluid contains a mixture of NaCl-CaCl₂ (T_e = -52°C [Schafer and Lax, 1962]), NaCl-CaCl₂-MgCl₂ (T_e = -57°C [Luzhnaya and Vereshtechetina, 1946]), or NaCl-MgCl₂ (T_e = -35°C [Schafer and Lax, 1962]). CO₂ density was derived from the T_hCO2 measured in the metastable absence of CO₂ clathrate, assuming CO₂ to be pure (Angus and others, 1973). Densities range from 0.37 to 0.97. These inclusions occur as both primary and pseudosecondary inclusions in quartz. The pseudosecondary inclusions have T_hTot values similar to, or lower than the T_hTot values of the primary inclusions, suggesting a coeval or sequential trapping sequence. In intersecting planar arrays, the younger array has lower T_hTot that the older array (Fig. 3f)

CO₂-rich. These inclusions range in size from < 5 m m to over 75 m m. They are two-phase at room temperature and include a carbonic vapor bubble that occupies 10 to 20 volume % of the inclusion and an immiscible carbonic liquid. The presence of up to 4 mole % CH₄ (Table 3) is indicated by the homogenization of a very small vapor bubble at approximately –82.2°C. Homogenization temperatures range from 17.4° to 30.9°C (Table 3). Although no visible aqueous liquid is present, the presence of water (most likely along the inclusion walls) is indicated by CO₂ clathrate melting temperatures of 7.7° to 8.2°C which represents salinities in the range of 3.9 to 4.4 wt. % NaCl equivalent (Table 3). Low-salinity H₂O inclusions are found in the same irregular cluster as the CO₂-rich inclusions and are assumed to be coeval (Fig. 3g). The CO₂-rich inclusions only occur as primary and pseudosecondary inclusions in a single quartz sample from the Savannah River terrane.

Isochores were constructed for the H₂O and Low-salinity H₂O inclusions using the MacFlincor software (Brown and Hagemann, 1994, 1995) and the equations of state for the H₂O-CO₂-NaCl system (Brown and Lamb, 1989). A geothermal gradient range of 20° to 30°C km^-1 and hydrostatic (10 MPa km^-1) pressure gradient was used to provide upper and lower bounds to the trapping pressure (Fig. 5).

For Low XCO₂ inclusions, the salinity, CO₂ density, and homogenization temperatures of the inclusions were used to estimate CO₂ content and the minimum pressure of entrapment at T_hTot using the MacFlincor software (Brown and Hagemann, 1994, 1995) and the procedure outlined in Brown and Lamb (1989). Homogenous fluids could have been trapped at higher temperatures along appropriate isochores for each fluid composition, therefore, the values determined represent a minimum trapping pressure and a minimum trapping temperature (T_hTot). The true entrapment pressure-temperature conditions lie along isochores in the single-phase region for each fluid inclusion composition.

Isochores for CO₂-rich inclusions were constructed using data from Swanenberg (1979). T_hTot values from associated Low-salinity H₂O inclusions were then used to constrain the trapping temperatures.

Modoc fault zone. Two distinct fluids are trapped in V₄ veins in rocks of the northeastern Modoc fault zone. The earlier-trapped fluids are Low XCO₂ (Table 2) with a CO₂ content ranging from 6.5 to 30.1 mole %, a density ranging from 0.36 to 0.97 g cm^-3, and salinities ranging from 3.1 to 4.4 weight % NaCl equivalent. T_hTot values range from 201° to 363°C (Fig. 4a). The calculated minimum pressures of entrapment have a bimodal distribution (Fig. 5a) which is interpreted to be the result of pore-fluid pressure transients accompanying dilatant fracturing in the fault zone (Parry and Bruhn, 1990; Parry and others, 1991). The highest pressure values fall near the lithostatic thermobaric gradient, while the lowest pressures are near the hydrostatic thermobaric gradient. Similar bimodal distributions have been documented along the Dixie Valley fault in Nevada (Parry and Bruhn, 1990; Parry and others, 1991) and in syntectonic veins from the Irish Variscides (Meere, 1992). The pressure transients most likely occurred during periodic reactivation of the Modoc fault during D₃ deformation. The scatter in values may be the result of errors in the T_hTot measurements and pressure estimates, from post-entrapment changes in fluid inclusions, from entrapment of a non-homogenous fluid, or from real temperature and/or fluid pressure variations.

Maximum pressures are in the range of 240 to 280 MPa, indicating burial depths of 11.2 to 12.7 km at lithostatic conditions. The data indicate geothermal gradients of 35°C km^-1 to as low as 20°C km^-1. Minimum pressures are in the range of 98 to 145 MPa under hydrostatic conditions.

The second fluid population consists of Low-salinity H₂O inclusions that represent an influx of meteoric fluids. Maximum pressures are assumed to be hydrostatic, although they could have periodically been higher. Using a thermobaric gradient of 10°C km^-1, trapping of this population began at depths of nearly 10 km.

Northeastern Carolina Terrane. Veins in the Harbison granite, the Columbia granite, and the Asbill Pond Formation also contain two distinct fluid populations. The early Low XCO₂ inclusions have T_hTot values ranging from 202° to 338°C but with a bimodal salinity distribution, 0.9 to 2.8 wt. % NaCl equivalent and 3.2 to 4.0 wt. % NaCl equivalent (Fig. 4b). The higher salinity fluids are found only in the granites. Only one V_4b vein from the Harbison granite contained Low XCO₂ inclusions large enough to determine T_hCO2. In this vein, the CO₂ content is 0.11 to 0.14 mole %, the salinity 1.8 to 3.5 wt. % NaCl equivalent, and the CO₂ density 0.90 to 0.92 g cm^-3 (Table 2). Trapping conditions for the fluids in this vein are 255° to 275°C and 250 to 275 MPa (Fig. 5b). Assuming lithostatic conditions, this represents a burial depth of 9.6 to 10.6 km. There is no control on trapping pressure for the remaining Low XCO₂ inclusions (Fig. 5b).

Vein mineral assemblages and mineral phase equilibria provide additional support for the pressure-temperature conditions determined by fluid inclusions. V₅ veins in the Harbison granite contain the assemblage quartz ± epidote ± chlorite which is stable in the temperature interval of approximately 250° to 400°C (Fig. 5b).

Nearly pure H₂O inclusions constitute the second fluid population (Fig. 4b, Table 1). These inclusions are found in all vein sets, but are the only type of fluid inclusions in the late calcite ± zeolite bearing V₇ veins (Table 1). Pressure corrected trapping temperatures range from 70° to 225°C and pressures range from 20 to 95 MPa, corresponding to burial depths of up to 9.5 km (assuming a hydrostatic thermobaric gradient). The upper part of the temperature range is within the laumantite ± prehnite stability field (approximately 140° to 240°C) (Fig. 5b) and the presence of prehnite restricts pressures to less than 150 MPa. The presence of zeolite is related to regional heating and fluid flow associated with Mesozoic rifting and emplacement of Jurassic age dikes (Butler, 1977)

Southwest Carolina Terrane. In the southwestern portion of the Carolina terrane, both V₅ and V₆ quartz veins contain two distinct fluid inclusion populations. The earlier trapped fluids are Low XCO₂ and have homogenization temperatures ranging from 200° to 318°C and clathrate melting temperatures in the range of 7.8° to 8.2°C, corresponding to salinities of 3.6 to 4.3 Weight % NaCl equivalent (Fig. 4c, Table 2). For most inclusions, a separate carbonic liquid bubble could not be discerned. Either the inclusions were too small to make out the small bubble, or the carbonic vapor bubble was nearly the same size as the carbonic liquid ‘bubble’. However, the presence of clathrate and the rare carbonic vapor homogenization attest to the presence of CO₂ in the inclusions. The two inclusions used for estimating environmental conditions (Table 2) give minimum trapping temperature of approximately 250° C and pressures of 283 MPa corresponding to a depth of 10.9 km.

Low-salinity H₂O inclusions are uncommon, occurring as pseudosecondary inclusions in the Persimmon Fork Formation, while veins in the Cuffytown Creek granite and the Persimmon Fork Formation contain common secondary H₂O inclusions. Pressure corrected trapping temperatures range from 80° to 320°C and pressures range from 25 to 148 MPa, corresponding to maximum burial depths of 8.8 to 14.8 km (assuming a hydrostatic thermobaric gradient).

Savannah River Terrane. In this area, V_4a and V_4b veins contain Low XCO₂ inclusions with salinities ranging from 2.2 to 6.2 weight % NaCl equivalent, and the Low-salinity H₂O inclusions range from 0.6 to 5.7 weight % NaCl equivalent (Fig. 4d). This overlap suggests that there is a continuum in mixing of 'in-situ' fluids with meteoric fluids, but unlike the other areas there is not apparently a wholesale meteoric influx. In inclusions large enough to see definitive phase transitions, the CO₂ content ranges from 0.10 to 0.16 mole %, the salinity 2.2 to 3.3 weight % NaCl equivalent, and the CO₂ density 0.78 to 0.88 g cm^-3 (Table 2). Trapping conditions for the fluids in this vein are 237° to 308°C and 210 to 249 MPa (Fig. 5d). Assuming lithostatic conditions, this represents a burial depth of 8.1 to 9.6 km. Again, the vein mineral assemblage quartz± epidote± chlorite indicates a temperature interval of approximately 250° to 400°C (Fig. 5b).

Rare CO₂-rich inclusions contain up to 4% CH₄ and have densities ranging from 0.53 to 0.76 (Table 3). Isochore calculated for CO₂-rich and coeval Low-salinity H₂O inclusions give pressure-corrected trapping conditions of 170° to 275°C and 48 to 108 MPa indicating maximum burial depths of up to 10.8 km under hydrostatic conditions.

For the Low-salinity H₂O and H₂O inclusions, pressure corrected trapping temperatures range from 100° to 250°C and pressures range from 25 to 115 MPa, corresponding to maximum burial depths of 7.5 to 11.5 km (assuming a hydrostatic thermobaric gradient) (Fig. 5d). The upper portion of the temperature range is supported by the presence of laumantite in V_4a veins. The presence of late stilbite reflects temperatures lower than 140°C.

The composition of post-metamorphic fluids trapped late during the Alleghanian orogeny and during post-orogenic uplift are remarkably consistent across the southeastern Piedmont. The earliest-trapped fluids are represented by Low-XCO₂ inclusions and these have the same general trapping temperatures and composition in all tectonic and structural settings examined, including post-deformation plutons and host country rock. This suggests that a pervasive interconnected brittle fracture network developed after the rocks cooled to temperatures below the brittle-to-ductile transition for quartzo-feldspathic rocks. Similar regionally homogeneous fluid compositions are found in the Lachlan fold belt of southeastern Australia (Gray and others, 1991), the Ominica belt in western Canada (Nesbitt and Muehlenbachs, 1995), and the central Appalachian Valley and Ridge province (Evans and Battles, 1999).

The chemical composition of the fluids is similar to that found in other metamorphic settings (e.g. Hollister and Burruss, 1976; Sisson and Hollister, 1990). The origin of the fluids is most likely re-equilibration reactions and retrograde breakdown of metamorphic minerals. There may also be a component of the fluids associated with the intrusion of the late syn-post kinematic plutons. The limited scope of this study precludes a more detailed investigation into the origin of the fluids.

The systematic nature of the fracture/vein networks attest to their origin in relation to a tectonic stress. Although the V₄ vein sets are limited to rocks within and near the Modoc fault zone, the V₅ and V₆ sets are regionally pervasive. These fractures would have aided in the regional circulation of fluid by advective cycling of fluid resulting from episodic dilatancy ‘pumping’ in the fracture networks associated with localized faulting (Sibson and others, 1975; Sibson 1989; Gray and others, 1991). The fluids were trapped in V₄ veins at depth of 11.2 to 12.6 km in the Modoc fault zone, between 9.6 to 10.9 km in V₅ and V₆ veins in the Carolina terrane, and between 8.1 to 9.6 km in V₅ and V₆ veins in the Savannah River terrane. These depths are close to the 11.0 to 14.3 km depth expected for the brittle-to-ductile transition for quartzo-feldspathic rocks (300° to 400°C) using an average geothermal gradients of 30°C/km and a 20°C surface temperature.

With regional uplift, denudation, and cooling, connecting fracture networks reached the paleosurface and allowed meteoric derived surface fluids to penetrate deep into the section where thermal convective circulation aided in distribution and mixing (Fig. 6). Convective circulation can only involve surface fluids with pressures near hydrostatic, since the fluid cannot flow up the pressure gradient toward lithostatically pressured intervals.

The regional pore-fluid pressure drop is evidenced by the presence of Low-salinity H₂O inclusions and CO₂-rich inclusions. These fluids formed as a result of unmixing of the original CO₂-H₂O-NaCl fluid present in the rocks upon decompression into a CO₂-rich phase and a H₂O-NaCl component (Conevey, 1981; Bowers, 1991; Boullier and Robert, 1992; Perry, 1998). Additionally, the fluid pressure reduction would have resulted in the precipitation of quartz and the phase separation of CO₂ (Sibson and others, 1988). As large quantities of meteoric water circulated through this portion of the orogenic belt, any metamorphic fluid that was present was diluted as has been observed in other metamorphic belts (Nesbitt and Muehlenbachs, 1989, 1991, 1995; Botrell and others, 1990; Jenkin and others, 1992, 1994). Similar records of deep meteoric fluid penetration have been reported from the Canadian Cordillera (Nesbitt and others, 1986; Nesbitt and Muehlenbachs, 1991), the Pyrenees (Wickham and Taylor, 1987), and the Alpine Schist metamorphic belt (Jenkin and others, 1994).

Finally, the H₂O inclusions indicate the regional influx and circulation of meteoric waters nearly to the brittle-to-ductile transition zone. Veins in the Savannah River Terrane, however, do not contain pure H₂O inclusions. The lack of pure H₂O inclusions in this area may be attributed to minimal meteoric influx resulting from 1) the presence of foliated rocks in the Modoc fault zone that were structurally above the Savannah River terrane (Fig. 6) thereby creating a permeability barrier to surface derived fluids; or 2) the low inherent permeability of the migmatitic rocks of the Savannah River terrane restricting fluid circulation.

The eastern Carolina terrane and Modoc fault zone have been studied extensively, and a large amount of geologic and radiometric age data is available (Secor and others, 1986, Dallmeyer and others, 1986; Pray and others, 1997). Therefore, the following discussion of the regional exhumation history will center on this area, although a similar history may be applied to the remainder of the study area.

Based on aluminum-in-hornblende barometry (Vyhnal and McSween, 1990), emplacement pressures for local late-synorogenic plutons range from 290± 50 MPa (11.2± 1.9 km) for the Harbison granite, 315± 15 MPa (12.1± 0.6 km)for the Winnsboro granite (Speer and others, 1994), and 320± 50 MPa (12.3± 1.9 km) for the Graniteville pluton. These values, along with the average emplacement temperature of 650° to 750°C, set a starting point for examining the late Paleozoic to early Mesozoic cooling and exhumation history (Figs.7 and 8). The rather shallow emplacement depth for these plutons may be attributed to their emplacement as 'collisional' granites during Alleghanian crustal thinning resulting from rapid delamination (Sacks and Secor, 1990; Nelson, 1992) or crustal arching related to dextral transpression (Gates and others, 1988).

The reminder of the cooling curve is constructed from radiogenic age data, fission track data, and stratigraphic data. Hornblendes from the northeastern Modoc fault zone yield ⁴⁰Ar/³⁹Ar age dates of 292 and 298 Ma (Dallmeyer and others, 1986). Muscovite from the Modoc fault zone and the Savannah River terrane give ⁴⁰Ar/³⁹Ar age dates ranging from 272 to 278 Ma with an average of 275 Ma (Maher and others, 1994; Pray and others, 1997). The closer temperature of argon retention in amphiboles and muscovite is dependent on rate of cooling and pressure (Baldwin and others, 1993). However, for the relatively slow cooling rates of less than 10°C/My encountered in this study, and the relatively low pressures of ~300 MPa, the closure temperatures or these minerals is close to that determined by the initial diffusion equations. Therefore, we will use 500± 25°C for the closure temperature for argon retention in hornblende (Harrison, 1981), and 375± 25°C for the closure temperature for argon retention in muscovite (Brown and Dallmeyer, 1996, based on the preliminary data of Robbins [1972] and the diffusion equations of Dodson [1973]) (Fig. 7). Apatite fission track ages of 140± 10 Ma have been reported by Zimmerman (1979) for the eastern Carolina terrane. These indicate cooling through ~125°C (Naeser and Cebula, 1978). Finally, by at least Santonian time (~87.5 Ma) the rocks of the inner Piedmont are exposed at the Paleosurface (Nystrom and others, 1992) (Fig. 7).

The fluid inclusion data can be used to construct a pressure-temperature curve that corresponds to the late Paleozoic to Mesozoic portion of the cooling curve (Fig. 8). The first fluids trapped in the rocks record temperatures up to 365°C and depths on the order of 11.0 to 12.7 km. These depths are nearly the same as the granite emplacement depths and suggest a period of rapid, near isobaric cooling (Fig. 8). Such rapid cooling could result from the crustal delamination exposing warm midcrustal rocks to near surface temperatures and the influx of cool meteoric waters into the recently uplifted rock (Fig. 6). As pointed out by Nesbitt and Meuhlenbachs (1989), the convection of meteoric water in the crust should have a significant impact of heat flow and cooling in the crust. For example, regional geothermal gradients must have rapidly been modified, from 60° to 70°C km^-1 during plutonism to 20° to 40°C km^-1 by the time Low XCO₂ inclusions were trapped (Fig. 8).

The evidence for fluid pressure cycling in the Modoc zone suggests that these rocks were exhumed to above the brittle-to-ductile transition zone prior to, or during D₄ deformation (Fig. 7). A regional drop in pore-fluid pressure to hydrostatic levels most likely occurred by late in D₆ deformation as regional temperatures dropped to the 200° to 250°C range (Figs. 7 and 8).This event is recorded by the regional influx of Low salinity H₂O and H₂O fluids.

Fluid trapping continued after the Alleghanian orogeny and into the early Mesozoic. By this time, the upper crust was saturated with meteoric fluids. Veins containing these meteoric derived fluids formed during the later phase of Mesozoic rifting when the rift-to-shift transition set up a new stress field in the crust (Withjack and others, 1998).

By extrapolating fluid temperatures down the exhumation curve, fluid inclusions with trapping temperatures of 80° to 125°C may indicate trapping as late as 120 to 140 Ma (Fig. 7). These late trapped fluids might be related to the formation of microfractures resulting from crustal stresses associated with inception of the present-day compressive stress field (Zoback and Zoback, 1989).

In summary, the above data yield a two-stage cooling curve for the eastern portion of the study area. Between the time of emplacement of the Harbison pluton (the earliest of the local plutons), and the muscovite argon retention age, the regional cooling rate is ~ 9.5°C m.y.^-1. Then, the regional cooling rate falls to ~1.9°C m.y.^-1 throughout the Mesozoic (Fig. 7). Because there is a rapid decrease in the geothermal gradient from the time of pluton emplacement to the muscovite cooling age, we cannot determine an exhumation rate for this time interval. But, we can say that exhumation was minimal, as pore-fluid pressure was relatively constant. However, we can calculate an approximate exhumation rate for the late Paleozoic through Mesozoic. Here, we assume 1) an initial starting depth of 11.9 km at 275 Ma, 2) surface conditions at 87 Ma, and 3) a relatively constant geothermal gradient. The value determined is 0.063 km m.y.^-1. This relatively slow exhumation rate is nearly 2 orders of magnitude less than those recorded in the Alps and Himalayas (e.g. Zeitler, 1985; and Copeland and others, 1987; Vance and Onions, 1992; Marshall and others, 1997). This may be due to the thickened crust in the active orogens being isostatically compensated for by rapid erosion, whereas in the Piedmont, the thinned crust and lithosphere resulting from the delamination process is already near isostatic equilibrium. Therefore, regional exhumation is slow as crust and lithosphere thickness only increases by cooling during the Mesozoic.

Fluid inclusion microthermometric data from veins in the southeastern Piedmont province record the changes in fluid composition and deformation conditions during regional exhumation and cooling related to Late Paleozoic syn- to post-orogenic processes and Triassic rifting. In general, the composition of post-metamorphic fluids trapped late during the Alleghanian orogeny and during post-orogenic uplift are remarkably consistent across the southeastern Piedmont, indicating regional fracture connectivity.

Syn-deformational fluids were trapped in veins that formed during the last phases of the Alleghanian orogeny. These fluids were CO₂-saturated and had a temperature range of 200° to 365°C and salinities of 2.6 to 5.7 wt.% NaCl equivalent. They were trapped under lithostatic pressures between 240 and 280 MPa indicating burial depths of 11.2 to 12.7 km. These depths are similar to emplacement depths of post-kinematic plutons, suggesting a period of rapid isobaric cooling.

Low-salinity H₂O inclusions and rare CO₂-rich inclusions are evidence for regional decompression as fracturing above the brittle-to-ductile transition allowed the regional pore-fluid pressure to drop to hydrostatic levels. Convective circulation of meteoric water resulted in the dilution of 'in-situ' fluids, and ultimately to a pure-H₂O saturated system. These fluids continued to be trapped in vein minerals through the early Cretaceous.

Late Paleozoic through Mesozoic exhumation rates for the eastern Piedmont province average 0.063 km m.y.^-1 and cooling rates average ~1.9°C m.y.^-1. These low rates may be directly related to thinned crust and lithosphere resulting from delamination processes during the late Alleghanian orogeny.

This work was partially supported through South Carolina University Research and Educational Task 269, Westinghouse Savannah River Company. We thank D. Secor, Jr. for providing many helpful suggestions for improvement of the manuscript.

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TABLE 1. Data Summary by Area

Table 2. Summary of Data on H₂O-NaCl-CO₂ Fluid Inclusions

Abbreviations as in Table 2. *Density is assuming inclusion is 100% CO₂., n.d. = no data.

Figure 1. Geologic map of the study area (modified from Secor and others [1986] and Bramlett and others [1982]) showing sample localities (large filled circles). Also shown are post-kinematic granites: Appling (AP), Columbia (CO), Cuffytown Creek (CF), Graniteville (GV), Harbison (HB), and Winnsboro (WN).

Figure 2. Lower hemisphere equal-area stereonets of poles to veins. a) Modoc fault zone, b) northeast Carolina terrane, c) southwest Carolina terane, d) Savanah River terrane. Symbol code indicates the minerals present in each vein. V_3a, V_3b, V_4a, V_4b, and V₅ are vein sets that correspond to deformation events discussed in text.

Figure 3. Fluid inclusion assemblage maps showing relationship between inclusion populations. a) Two intersecting planar clusters of pseudosecondary H₂O inclusions. Note that the cluster with the lower average T_h is clearly later. b) primary low-XCO₂ inclusions and pseudosecondary planar clusters of low-salinity H₂O inclusions. c) primary (right) and pseudosecondary (left) low-XCO₂ inclusions. d) Primary low-XCO₂ inclusions. e) primary low-salinity H₂O inclusions. f) two intersections planar clusters of low-salinity H₂O inclusions. Note that the cluster with the lower average T_h is later. g) primary and pseudosecondary CO₂-rich inclusions. Numbers are total homogenization temperatures for the aqueous phase and carbonic phase (values < 30.9°C). Maps are traced from microscope projections of polished thick sections. Maps a, b, c, and g are traced at 25°C, maps d, e, and f are traced at 15°C.

Figure 4. Summary of fluid inclusion data for each area. a) Modoc fault zone, b)northeast Carolina terrane, c) southwest carolina terrane, d) Savannah River terrane. T_mIce is ice melting temperature, T_mcl is CO₂ clathrate melting temperature. Qtz is quartz vein material, cc is calcite vein material. Some inclusions accounted for in the histograms do not have corresponding T_mIce values.

Figure 5. P-T diagram showing trapping conditions for fluid inclusions in each area studied. Symbols are values derived from individual fluid inclusions (Table 2). Light shaded regions are range of trapping conditions for all Low XCO₂ inclusions in each area. Dark shaded region is trapping conditions determined from CO₂-rich inclusions (Table 3). Hatchured regions are trapping conditions determined for Low Salinity H₂O and H₂O inclusions. Also shown are lithostatic (L) and hydrostatic (H) thermobaric gradients for various geothermal gradients. Light shaded lines labeled 1 through 8 are mineral equilibria: 1. Prehnite = laumantite + kaolinite + quartz + water (Bird and Helgeson, 1981), 2. Stilbite = heulandite + water (Cho and others (1987), 3. Heulandite = laumantite + quartz + water (Cho and others, 1987), 4. Laumantite + prehnite + chlorite = pumpellyite (Liou and others, 1987), 5. Stilbite = laumantite + quartz + water (Liou, 1971), 6. Laumantite + prehnite = epidote + quartz + water (Liou and others, 1987), 7. Pumpellyite + laumantite = epidote + chlorite (Liou and others , 1987), 8. Epidote + mica + quartz = fledspar + water (Parray and Bruhn, 1986). Light shaded lines 9 and 10 are boiling curves for 0 wt. % and 5 wt. % NaCl equivalent salinity respectively (based on data from Haas, 1976).

Figure 6. Schematic cross-section across the study area (based on cross-section in Mahar, 1987). Section is restored to the late Alleghanian orogeny using pressures determined from fluid inclusion data and geobarometry of granites (Vyhnal and McSween, 1990). Arrows represent inferred fluid circulation paths.

Figure 7. Post-metamorphic thermal evolution of the northeastern Carolina terrane and Modoc fault zone. Shows a two-stage cooling history from the late Paleozoic through the late Mesozoic. Also shown is the range of trapping temperatures for inclusion types examined in this study. Emplacement temperatures of granites from Vyhnal and McSween (1990) and granite emplacement ages are: Appling (A) dated at 301± 3 Ma (D. Secor, personal communication, 2000), Columbia (C) dated at 286± 3 Ma (Fullagar and Butler, 1979), Harbison (H) dated at 309± 3 Ma (Fullagar and Kish, 1981), and Winnsboro (W) dated at 295± 2Ma (Fullagar, 1981). Retention of argon in Hornblende from Dallmeyer and others (1986). Retention of argon in muscovite from Maher and others (1994) and Pray and others (1997). Age of the potassium alteration (the 'Pinking event') from Kish (1992). Mesozoic dike emplacement age from Sutter (1988). Maintenance of apatite fission tracks from Zimmerman (1979), Earliest late Cretaceous surface from oldest Cretaceous sediments in the eastern Piedmont (Nystrom and others, 1992). Age range for Mesozoic rifting from Withjack and others (1999).

Figure 8. Evolution of pressure–temperature conditions for the northeastern Carolina terrane and Modoc fault zone. Note the similarity of granite emplacement pressures and the trapping of Low XCO₂ inclusions at significantly lower temperatures reflecting near isobaric regional cooling. Dark shaded region is average conditions of granite emplacement from Vyhnal and McSween (1990) and Speer and others (1994). Light shaded area is range of trapping conditions determined from Low XCO₂ fluid inclusions. Hatchured region is range of trapping conditions for Low Salinity H₂O and H₂O fluid inclusions.