Character of the Alleghanian Orogeny in the Southern Appalachians: Part II

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Character of the Alleghanian orogeny in the southern Appalachians: Part II. Geochronological constraints on the tectonothermal evolution of the eastern Piedmont in South Carolina

R. DAVID DALLMEYER Department of Geology, University of Georgia, Athens, Georgia 30601 JAMES E. WRIGHT Department of Geology, Stanford University, Stanford, California 94305 DONALD T. SECOR, JR. Department of Geology, University of South Carolina, Columbia, South Carolina 29208 ARTHUR W. SNOKE Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071

ABSTRACT Piedmont at ca. 315 Ma (prior to or during GEOLOGIC OVERVIEW the early stages of the Alleghanian orogeny). A nearly concordant U-Pb zircon age of The thermal maximum of Alleghanian re- Early workers subdivided the southeastern 550 ± 4 Ma is interpreted to closely date crys- gional metamorphism (amphibolite facies in Piedmont into several northeast-trending litho- tallization of the epizonal Little Mountain the Kiokee belt; greenschist facies in the tectonic belts (Crickmay, 1952; King, 1955; metatonalite in the southeastern part of the southeastern part of the Carolina slate belt) Hatcher, 1972; see Fig. 3 of Secor and others, Charlotte belt in South Carolina. This con- occurred during ca. 295-315 Ma. During the 1986b). In South Carolina, belts characterized firms field studies which indicate that the late Carboniferous and Early Permian, the by low- to medium-grade regional metamor- Charlotte belt contains a plutonic metaigne- eastern Piedmont experienced differential up- phism (Belair, Carolina slate, Kings Mountain, ous complex that developed as a sub-vol- lift, erosion, and relatively rapid postmeta- Chauga) alternate with medium- to high-grade canic-arc infrastructure, contemporaneous morphic cooling. Isothermal surfaces were belts (Kiokee, Charlotte, Inner Piedmont). Strat- with vulcanism manifested in the Carolina folded into an antiform-synform-antiform igraphic sequences within the Belair, Kiokee, slate belt. Both paleontological and geochron- configuration corresponding to the Kiokee, Carolina slate, and Charlotte belts are generally ological controls indicate that the South Carolina slate, and Charlotte belts, respec- similar (Maher, 1978; Secor and others, 1982; Carolina slate belt is mostly younger than tively. Hauck, 1984; Secor and others, 1986a). Each is 570 Ma (Cambrian?), whereas the slate belt in The geochronological data provide the fol- represented by an association of intermediate to North Carolina and Virginia is mostly of late lowing calibration for the late Paleozoic de- felsic, epizonal, metaplutonic rocks and/or in- Proterozoic age. formational chronology recorded in the Kio- termediate to felsic metavolcanic rocks which is A regionally significant mid-Paleozoic (ca. kee and Carolina slate belts: D2 (Lake overlain by metasedimentary sequences includ- 340-360 Ma) thermal event is suggested by Murray deformation), ca. 295-315 Ma; D3 ing mudstone, siltstone, wacke, and/or feld- discordant ^Ar/^Ar whole-rock age spectra (Clarks Hill deformation), ca. 285-295 Ma; spathic sandstone. In central South Carolina,

of slate/phyllite in the northwestern Carolina and D4 (Irmo deformation), ca. 268-290 field studies suggest that the Charlotte belt in- slate belt and from hornblende in the south- Ma. cludes a sub-volcanic-arc infrastructure, which eastern Charlotte belt. It is uncertain if this likely developed contemporaneously beneath event was associated with deformation in the INTRODUCTION extrusive arc sequences of the Carolina slate belt eastern Piedmont; however, mid-Paleozoic (Secor and others, 1982). Geochronological studies in South Carolina (Butler and Fullagar, deformation has been previously documented The relationship of late Paleozoic deforma- 1975) and Georgia (Carpenter and others, elsewhere in the western Piedmont. tion and metamorphism recorded in the eastern 1982) suggest a Cambrian age for at least some A slightly disconcordant U-Pb zircon age Piedmont (Secor and others, 1986a) with the of these arc sequences. The report of the occur- of 317 ± 4 Ma is interpreted to closely date Alleghanian orogenic development of the west- rence of Atlantic province Middle Cambrian initial crystallization of the deformed Edge- ern Appalachian foreland (Woodward, 1957) trilobites in metasedimentary rocks within the field granite and confirms a record of late has long been uncertain. This report presents Carolina slate belt (Secor and others, 1983; Paleozoic penetrative deformation in the Ki- U-Pb zircon and 40Ar/39Ar geochronological Samson, 1984) suggests that the Belair, Kiokee, okee belt of South Carolina. U-Pb isotope results from the eastern Piedmont in South Car- Carolina slate, and Charlotte belts likely origi- data for the Lake Murray orthogneiss and olina. The thermal evolution suggested by these nated in environments spatially distant from Clouds Creek granite are discordant and sug- data provides controls essential to a more North America and should be considered an ex- gest that the magmas of these plutons were general analysis of the Alleghanian orogeny in otic Appalachian terrane (the Carolina terrane). derived by partial melting of a sialic Precam- the southern Appalachians (discussed by Secor Structural and paleomagnetic evidence suggests brian source and then emplaced in the eastern and others, 1986b).

Additional material for this article (tables and appendices) may be secured free of charge by requesting Supplementary Data 86-25 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 97, p. 1329-1344, 10 figs., 2 tables, November 1986.

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Figure 1. Generalized geologic map of the Piedmont in west-central South Carolina. Modified from Wagener (1977), Secor and others (1982), Halik (1983), Hauck (1984), Kirk (1985), and Secor and others (1986a). Localities sampled for U-Pb zircon geochronological analysis are indicated by numbered stars.

CAROLINA SLATE BEL.T^ KIOKEE BELT BELAIR BELT

that the Carolina cerrane was accreted to North 1983; Dallmeyer and others, 1985b). A predom- related to initial phases of extension during America in the ea rly or middle Paleozoic (Ell- inantly felsic magmatic arc developed within the opening of the present Atlantic Ocean. The wood, 1982; Barton and Brown, 1983; Dooley, eastern Piedmont between ca. 285 and 325 Ma crystalline rocks in the study area are uncon- 1983; Secor and others, 1983). Predominant (Sinha and Zietz, 1982). Northwest of the Kio- formably overlain by Late Cretaceous and Ter- lithologies within, the Kings Mountain belt kee belt, these plutons are not regionally duc- tiary sediments of the Atlantic Coastal Plain. (phyllite, manganiferous schist, amphibolite, tilely deformed; however, within or adjacent to marble, metacongi omerate) and Inner Piedmont the Kiokee belt, the 285- to 325-Ma plutons PREVIOUS GEOCHRONOLOGY AND (sillimanite schist, biotite paragneiss, felsic or- display regionally penetrative ductile deforma- ITS GEOLOGICAL SIGNIFICANCE thogneiss) are unlike those in the Carolina ter- tion fabrics (Snoke and others, 1980). Geologi- rane. The northwest edge of the Carolina terrane cal and geochronological studies (Kish, 1983; During the past 25 yr, there have been nu- is interpreted to be within or adjacent to the Secor and others, 1986a) indicate that the Kio- merous geochronological attempts to determine Kings Mountain belt on the basis of geological kee belt and the southeastern edge of the Caro- initial crystallization or eruption ages of plu- and geophysical evidence (Glover and Sinha, lina slate belt underwent polyphase deforma- tonic, metaplutonic, or metavolcanic rocks in 1973; Hatcher ar d Zietz, 1980; Williams and tion (D2-D4) and a greenschist to amphibolite the eastern Piedmont. The ages reported are Hatcher, 1983). facies metamorphism (M2) during the late Pa- separable into three groups: late Precambrian- The study area of this report (Fig. 1) was leozoic. The rocks in the study area are de- Cambrian1 (520-740 Ma; Fullagar, 1971; penetratively defarmed (Dj) and regionally formed by a series of northeast-trending, Glover, 1971; Glover and Sinha, 1973; Briggs metamorphosed (Mi) to the greenschist and post-D4 brittle faults with small to moderate amphibolite facies in the early and/or middle displacement (see Fig. 2 of Secor and others, 1986a, the folded insert accompanying this Paleozoic (Kish and others, 1979; Fullagar, 'This paper uses the 1983 Decade of North Ameri- 1981; Secor and e thers, 1982; Sutter and others, issue). These are likely of Mesozoic age and can Geology time scale (Palmer, 1983).

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TABLE 1. Rb-Sr WHOLE-ROCK AGES FOR SELECTED PLUTONS and others, 1978; Wright and Seiders, 1980; ciated with well-developed contact metamor- IN THE EASTERN PIEDMONT OF SOUTH CAROLINA THAT Black, 1980; Carpenter and others, 1982; phism aureoles. Along the east side of the arc, HAVE EXPERIENCED EPISODES OF ALLEGHANIAN . PENETRATIVE DUCTILE DEFORMATION McConnell and Glover, 1982), Silurian and however, some of the plutons have locally experienced penetrative ductile deformation and Devonian (385-415 Ma; Butler and Fullagar, Name Age (Ma) 1978; Fullagar, 1981; McSween and others, greenschist to amphibolite facies regional meta- 1984), and Carboniferous and Permian (270- morphism (Kish and Fullagar, 1978; Secor Lake Murray gneiss 313 ± 24 Clouds Creek granite 319 ± 27 330 Ma; Fullagar, 1971; Jones and Walker, and Snoke, 1978; Snoke and others, 1980; Edgefield granite 254 ± II 1973; Wright and others, 1975; Kish and Ful- Farrar, 1985; see Fig. 1 of Secor and others, Batesburg augen gneiss 291 ± 4

lagar, 1978; Fullagar and Butler, 1979; Fullagar, 1986b). These deformed plutons are a manifes- Note: data from Snoke and others (1980) ari d Fullagar (1981). 1981). tation of an important episode of Alleghanian The late Precambrian-Cambrian age group penetrative deformation and regional metamor- includes both plutonic and metavolcanic rocks. phism. Petrologic characteristics suggest that the pluton- During the past 30 yr, there have been nu- not obtained from the tuff. Analytical results ics represent epizonal intrusive equivalents of the merous Rb-Sr, K-Ar, and ^Ar/^Ar mineral from the metatonalite (sample 5, Fig. 1) are metavolcanic rocks (Butler and Ragland, 1969a; ages reported from the Piedmont (Pinson and presented subsequently. Briggs and others, 1978; Weisenfluh and Snoke, others, 1957; Long and others, 1959; Kulp and Late Paleozoic Rb-Sr whole-rock isochron 1978; Whitney and others, 1978; McConnell Eckelmann, 1961; Dallmeyer, 1978; Kish and ages were interpreted as initial igneous crystalli- and Glover, 1982). These upper Precambrian others, 1979; Fullagar and Kish, 1981; Sutter zation dates for the Lake Murray gneiss, Clouds and Cambrian rocks have been interpreted to and others, 1983, 1984; Russell and others, Creek granite,3 Edgefield granite, and Batesburg have accumulated in association with one or 1985). Kulp and Eckelmann (1961) interpreted augen gneiss (Snoke and others, 1980; see Table more subduction-related volcanic arcs (Butler mineral age data in the Piedmont to indicate a 1). These provided critical evidence for recogni- and Ragland, 1969b; Whitney and others, 1978; widespread period of regional metamorphism tion of the late Paleozoic age of the penetrative Misra and McSween, 1984). If the beginning at ca. 350 Ma. These workers suggested that ductile deformation and regional metamorphism of the Cambrian is fixed at ca. 570 Ma (Palmer, a second period of regional metamorphism recorded in the eastern Piedmont (Snoke and 1983), the available fossil and geochronological occurred at ca. 250 Ma, along a narrow zone in others, 1980; see Table 1). Samples of each of evidence suggests that the North Carolina slate the central Piedmont extending from what is the South Carolina plutons were collected for belt is mostly of late Precambrian age (Glover now Georgia to Virginia. The subsequent reali- U-Pb zircon analysis to confirm the emplace- and Sinha, 1973; Wright and Seiders, 1980; zation that mineral ages may indicate times of ment ages inferred from Rb-Sr whole-rock data. McConnell and Glover, 1982; Gibson and oth- uplift and cooling long after the thermal peak of A satisfactory zircon separate was not obtained ers, 1984), whereas the South Carolina slate belt regional metamorphism (Hadley, 1964) led from the Batesburg augen gneiss. Analytical re- is mostly Cambrian (Butler and Fullagar, 1975; Dallmeyer (1978) to question the concept of a sults from the Lake Murray gneiss, Clouds Carpenter and others, 1982; Secor and others, distinct late Paleozoic regional metamorphism Creek granite, and Edgefield granite are pre- 1983; Samson, 1984). in the central Piedmont. In central South Caro- sented subsequently (samples 2,3, and 4; Fig. 1). The Silurian-Devonian age group is repre- lina, all published K-Ar and Rb-Sr mineral ages Precise coordinates of the U-Pb zircon sam- sented by a suite of weakly deformed to are younger than ca. 330 Ma (Fullagar and pling sites are listed in Appendix A.4 2 undeformed, mafic to felsic plutonic rocks in the Kish, 1981), indicating late Paleozoic uplift and Charlotte belt in North and South Carolina. In cooling. Substantially older cooling ages (in the U-Pb ANALYTICAL PROCEDURES central South Carolina, the Newberry granite range 396-483 Ma) are reported in the Pied- mont of south-central North Carolina (Worth- (415 ± 9 Ma, Rb-Sr whole-rock isochron crys- Zircon separates were prepared by standard ington and Lutz, 1975; Kish and others, 1979; tallization age; Fullagar, 1981) is not penetra- mineral separation techniques. Sample dissolu- tively deformed, but it contains rotated xenoliths Sutter and others, 1983, 1984). tion and ion exchange chemistry were modified which carry a very strong penetrative deforma- from Krogh (1973). Isotopic data were deter- tion fabric. The xenolith fabric is interpreted to U-Pb SAMPLING RATIONALE mined using a 35-cm, 90°-sector, solid-source be correlated with D] fabrics developed in host mass spectrometer at the University of Califor- metamorphic rocks of the Charlotte belt. De- Detailed field mapping in central South Caro- nia, Santa Barbara, California. Uranium and tailed field mapping along the boundary be- lina indicates that the penetratively deformed, lead concentrations were determined on solution tween the Carolina slate belt and Charlotte belt epizonal Little Mountain metatonalite represents aliquots from each sample and are considered to (Secor and others, 1982; Halik, 1983; Kirk, one of the Charlotte belt subvoclanic plutons be precise to at least 0.25%. Measured 1985) indicates that the boundary largely rep- which was emplaced in association with con- 208pb/206pb and 207pb/206pb were predse tQ resents a metamorphic contrast and that both struction of the Carolina slate belt volcanic arc at least 0.1%, and 206Pb/204Pb ratios were pre- belts contain similar Dj fabrics. Dj (Delmar) (Secor and others, 1982). Samples of both the cise to at least 1%. Total-lead blanks were in the deformation in the Carolina slate belt (Secor Little Mountain pluton and a felsic tuff from the 0.1 to 0.5 ng range. Results of the isotopic anal- and others, 1986a) is therefore interpreted to Carolina slate belt were collected for U-Pb zir- have occurred prior to ca. 415 Ma. con analysis. A satisfactory zircon separate was 3 The Carboniferous-Permian age group is a The Clouds Creek granite is a subunit in the Clouds Creek igneous complex (Secor and others, suite of predominantly granitoid plutons that 2 1986a). occur in a northwestwardly concave arc extend- The Rb-Sr biotite age of 388 ± 5 Ma reported by Fullagar and Kish (1981) for the Bald Rock granite is 4 Appendices A and B and Tables A-H are on file ing from Maryland to Georgia (Sinha and Zietz, incorrect. The Rb-Sr age of biotite from the Bald Rock with the GSA Data Repository. To secure free copies, 1982). In most places, these plutons have not granite is ca. 290 Ma (Paul Fullagar, 1985, personal request Supplementary Data 86-25 from the GSA been penetratively deformed, and they are asso- commun.). Documents Secretary.

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TABLE 2, URANIUM-LEAD ISOTOPIC DATA

Sample Wl. (mg) U 206Pb* Measured ratios Atomic ratios Apparent ages (Ma) (ppm) (ppm) 2 2<*Pb 2<"Pb 208pb 2°6pb. 2°W 207pb. 2«W °7pb« 207 pb*

204p,, 2«¡Pb 206^ 238u 235U 206Pb* 238u 235u 206pb.

SC = 2 >200 20.0 405.6 23.54 3242 0.06175 0.11709 0.06756 0.53340 0.05727 421.4 434.1 502 SC = 2 <200 13.1 467.4 25.40 7279 0.05796 0.09251 0.06301 0.48736 0.05610 393.9 403.1 452 SC = 2 <325 24.1 563.5 29.95 7911 0.05802 0.07342 0.06186 0.47918 0.05618 386.9 397.5 459

SC = 3 >200 19.8 360.7 22.14 528 0.08522 0.17773 0.07145 0.56862 0.05772 444.9 457.1 519 SC = 3 <200 21.8 361.0 21.85 4794 0.06041 0.11119 0.07044 0.55734 0.05738 438.8 449.8 506 SC = 3 <325 11.5 404.8 23.33 9079 0.05865 0.10520 0.06708 0.52886 0.05718 418.6 431.0 498

SC = 4 >100 6.0 4236 157.6 1433 0.06285 0.06030 0.04331 0.31485 0.05272 273.3 277.9 317 SC = 4 <200 13.9 2873 101.7 1255 0.06438 0.08394 0.04113 0.29900 0.05272 259.8 265.6 317

SC = 5 >200 20.7 304.1 22.87 2627 0.06405 0.21471 0.08754 0.70648 0.05853 540.9 542.6 550 SC = 5 <325 18.5 452.6 33.10 5391 0.06123 0.22216 0.08511 0.68698 0.05854 526.6 531.0 550

Note: common Pb correction: 206Pb/204Pb =• 18.6;20'Pb/2MPb = 15.6. decay constants: 235U = 0.9848! i X 10"' yra8U = 0.155125 X ,0-9yr-l. 238^235^ 137.88. •Denotes radiogenic Pb.

yses are given in Table 2 and are plotted on a age of this pluton, the 550 ± 4-Ma U-Pb age is dates emplacement of the Edgefield granite be- concordia diagram in Figure 2. compatible with field observations indicating cause (1) AT/39 AT data presented below indi- that the Little Mountain metatonalite carries a cate that the Kiokee belt cooled through 300 °C ZIRCON MORPHOLOGY penetrative fabric resulting from the pre-415- during 278-288 Ma. The absence of contact Ma D] deformation (Secor and others, 1982). metamorphism in the metasedimentary rocks The mineral separate obtained from the Little around the Edgefield granite indicates that it was Mountain metatonalite contained a very homog- EDGEFIELD GRANITE emplaced into Kiokee belt country rocks main- enous zircon population characterized by clear, tained above 300 °C prior to 278-288 Ma. (2) equant, euhedral crystals. The Edgefield zircon Isotopic data obtained on two zircon size frac- The Edgefield granite appears to carry a weak to population contains a homogenous euhedral zir- tions separated from the Edgefield granite (SC- moderate S2 foliation. The granite, therefore, con population, but all zircons are dark brown 4) are given in Table 2 and are plotted on a must have been emplaced prior to or during D2, 40 39 and at least partia.ly metamict, consistent with concordia diagram in Figure 2. The isotopic sys- which Ar/ Ar ages bracket to have occurred the very high uranium contents of the two frac- tematics of these two zircon fractions are readily prior to ca. 295 Ma. tions analyzed. Both the Lake Murray gneiss and interpreted in terms of an upper intercept mag- Clouds Creek granite contain distinct brownish, matic age of 317 ± 4 Ma, with subsequent post- LAKE MURRAY GNEISS AND euhedral zircon populations; however, examina- emplacement loss of radiogenic lead (probably CLOUDS CREEK GRANITE tion of these separates under high-index-of- by continuous diffusion processes). The loss of refraction oils revealed the presence of partially small amounts of radiogenic lead from these two Isotopic data obtained on three zircon size resorbed rounded cores within some crystals. zircons fractions is not surprising because of the fractions from the Lake Murray gneiss (SC-2) The zircon populations from these plutons are very high uranium contents. The 317 ± 4-Ma and three zircon size fractions from the Clouds mutually indistinguishable, and, in light of the zircon age is markedly discordant with a pre- Creek granite (SC-3) are given in Table 2 and analytical data discussed below, we interpret the viously reported Rb-Sr isochron whole-rock age are plotted on a concordia diagram in Figure 2. cores as a Precambrian inherited zircon compo- of 254 ± 11 Ma (Snoke and others, 1980). We All of the zircon size fractions from both the nent derived from the source region during pro- suggest that the U-Pb zircon age more closely Lake Murray gneiss and the Clouds Creek gran- duction of the granite magmas.

LITTLE MOUNTAIN METATONALITE 695 Ma Isotopic data obtained on two zircon size fractions separated from the Little Mountain meta- Figure 2. Concordia diagram of zir- tonalite (SC-5) are given in Table 2 and are con size fractions from the Little plotted on a concordia diagram in Figure 2. The Mountain metatonalite, Edgefield gran- isotopic systematic of these two zircon fractions ite, Lake Murray gneiss, and the Gouds Creek granite. Triangles are are most easily interpreted in terms of an upper LITTLE MTN. 5 data from the Little Mountain meta- intercept magmatic age of 550 ± 4 Ma with EDGEFIELD tonalite (SC-5); hexagons, from the subsequent postemplacement loss of a small LAKE MURRAY Edgefield granite (SC-4); circles, from proportion of radiogenic lead (probably by con- CLOUDS CREEK tinuous diffusion processes). Although there are the Lake Murray gneiss (SC-2); and no other data available that bear directly on the squares, from the Clouds Creek gran- 317 Ma ite (SC-3). 207pb./235u

5Errors in U-Pb ages are expressed as 2a. o.?o ! 0.90

ite are strongly discordant on the concordia dia- genie lead, if superimposed upon a mixing line olina slate belt. An eruptive age of ca. 550 ± 4 gram and display isotopic systematica indicative produced by a combination of magmatic zircon Ma is therefore indicated for this felsic tuff of mixing between magmatic and older inherited and an older, inherited zircon component, could (Persimmon Fork Formation) in nearby por- or xenocrystic zircon components. The chord produce scatter in the isotopic data and a non- tions of the Carolina slate belt stratigraphic se- shown on Figure 2 is the best-fit regression line linear data array. Another consideration is that quence. This age is compatible with the 568-Ma for the Lake Murray gneiss.6 The presence of the older zircon component contained within U-Pb zircon and the 554 ± 20-Ma Rb-Sr older inherited or xenocrystic zircon compo- both the Clouds Creek and Lake Murray zircon whole-rock isochron crystallization ages re- nents is consistent with the morphology of the populations might be isotopically heterogenous. ported for the Lincolnton metadacite in western zircon populations (cores with overgrowths) Specifically, the older zircon component might South Carolina and eastern Georgia (Carpenter separated from both plutons. have been of nonuniform age and/or discordant and others, 1982). The Lincolnton metadacite Rigorous error regression analysis of these at the time of incorporation into the Lake Mur- occurs in the lower part of the slate belt strati- zircon analytical data (Ludwig, 1984) suggests a ray and Clouds Creek magmas. Because of the graphic sequence and is interpreted to be later- very approximate magmatic age for the Lake relatively small age difference between the older ally correlative with the Persimmon Fork Murray gneiss of ca. 317 Ma with an older zir- zircon component and the magmatic component Formation (Carpenter and others, 1982). The con component of ca. 695 Ma (Fig. 2). A mag- and because of the relatively large contribution 550 ± 4-Ma zircon age of the Little Mountain matic age of ca. 240 Ma is suggested for the (-25% in the case of SC-3 >200) of lead from metatonalite is also compatible with the 522 ± Clouds Creek granite with an older zircon com- the old zircon component, any isotopic heter- 24-Ma Rb-Sr whole-rock isochron minimum ponent of ca. 580 Ma. Error estimates on the ogeneity in the old zircon component could age reported for felsic volcanic rocks in the slate lower (magmatic age) and upper (inherited- or produce the observed scatter in the data array. belt in northeastern South Carolina (Butler and xenocrystic-component age) concordia inter- The U-Pb zircon age data from the Clouds Fullagar, 1975). The geochronological data cepts of both the Lake Murray gneiss and Creek granite and the Lake Murray gneiss are from the Little Mountain metatonalite are com- Clouds Creek granite, however, are extremely compatible with an igneous crystallization age of patible with tectonic models which interpret the large (-50-75 Ma). The plutons are therefore ca. 315 Ma (estimated from Rb-Sr whole-rock Charlotte belt to be, at least in part, volcanic indistinguishable in terms of both their mag- isotopic data, Table 1), but the zircon analytical infrastructure for the Carolina slate belt (Secor matic ages and the ages of the older zircon com- results do not improve the precision of this esti- and others, 1982) and indicate that the rocks in ponents from the viewpoint of a rigorous mate. If the Lake Murray and Clouds Creek the South Carolina slate belt are mostly or en- statistical analysis of the data. The mean square plutons were emplaced at ca. 315 Ma, as indi- tirely younger than sequences in the North Caro- weight deviations of the chords calculated for cated by Rb-Sr whole-rock isotopic data, the lina slate belt (Cloud and others, 1976; Secor both the Lake Murray and Clouds Creek sam- upper concordia intercept for both the Clouds and others, 1983; Gibson and others, 1984). ples are very high, indicating that analytical er- Creek and Lake Murray zircon fractions can be U-Pb zircon ages of the deformed Lake Mur- rors alone cannot account for the scatter of the no younger than ca. 650 Ma. Although the max- ray gneiss, Clouds Creek granite, and Edgefield data points (that is, their nonlinearity). Some imum age of the upper concordia intercept is granite confirm an Alleghanian age for the pen- combination of geologic and analytical uncer- very poorly constrained, the older included zir- etrative ductile deformation these plutons re- tainties must thus be controlling the scatter of con components must be of Precambrian age. cord. The lake Murray gneiss records Allegha- the isotopic data. Potentially, the older included zircon compo- nian D2 fabric (Secor and others, 1986a) which The small spread in the isotopic ages from the nent could be xenocrystic (incorporated into the must have developed after its intrusion at ca. coarse to fine zircon fractions certainly contrib- Clouds Creek and Lake Murray magmas by as- 315 Ma. Although the Edgefield granite records utes to the large uncertainties calculated for the similation of country rock during magma as- D2 and D4 fabric elements, they are not as error envelopes of the two chords. Small cent) or inherited from the source region of the strongly devloped as in the Lake Murray gneiss. amounts of analytical error thus can produce two plutons. Although field relations and zircon A 317 ± 4-Ma emplacement age for the Edge- large uncertainties in calculated intercepts due to isotopic data alone cannot conclusively differen- field granite indicates (at least in some places) tight clustering of the data points. In addition, tiate between these two alternatives, both of that Alleghanian D2 deformation was less in- two other geologic uncertainties could poten- these plutons have relatively high initial tense inside the Kiokee belt than along its tially explain the scatter in the isotopic data. 87Sr/86Sr ratios (0.7119, Lake Murray gneiss; boundaries. One is possible postemplacement loss of radio- 0.7099, Clouds Creek granite), whereas most of Zircon U-Pb results from the Lake Murray genic lead from the zircon fractions by episodic the other upper Paleozoic plutons in the eastern gneiss and Clouds Creek granite suggest that the and/or continuous diffusion processes. This Piedmont have very low initial ratios (Fullagar, magmas of these two plutons were derived from seems possible because of the demonstrable loss 1981). It is therefore likely that the older zircon a sialic Precambrian source. Although rocks of small amounts of radiogenic lead from the component was inherited from the source older than ca. 570 Ma have not been reported in zircon fractions separated from the Little Moun- region. the South Carolina Piedmont (Fullagar, 1981; tain tonalite (which have uranium concentra- Carpenter and others, 1982), much of the North tions comparable to both the Lake Murray and REGIONAL GEOLOGICAL Carolina slate belt is made up of upper Prot- Clouds Creek zircon populations). Such post- SIGNIFICANCE OF U-Pb DATA erozoic metasedimentary, metavolcanic, and emplacement loss of small amounts of radio- metaplutonic rocks (Glover and Sinha, 1973; Geological studies along the southeastern McConnell and Glover, 1982; Gibson and oth- edge of the Charlotte belt (Secor and others, ers, 1984), and Farrar (1985) has suggested that 6 For a detailed discussion of the interpretation of 1982) suggest that the Little Mountain metaton- the Raleigh belt in North Carolina (see Fig. 1 of concordia mixing lines produced by a combination of Secor and others, 1986b) is cored with middle magmatic and inherited or xenocrystic zircon popula- alite represents an epizonal subvolcanic equiva- tions, see Wright and Haxel (1982) and references lent of quartz-feldspar crystal-lapilli metatuffs Proterozoic basement rocks. The U-Pb zircon therein. mapped within the adjacent portions of the Car- analytical results from the Clouds Creek granite

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and Lake Murray gneiss may indicate that middle or upper Proteiozoic rocks underlay portions of the South Carolina slate and Kiokee belts at the time that the magmas in these two plutons were generated.

40Ar/39Ar SAMPLING RATIONALE

A regionally systematic 40Ar/39Ar age dating program was undertaken to determine the extent, relative intensity, and detailed chronology of the late Paleozoic metamorphism in the eastern Piedmont of South Carolina. Samples were collected along a northwest-southeast transect extending from the edge of the Atlantic Coastal Plain to the Inner Piedmont (Figs. 1 and 3-7) in areas covered by recent detailed field mapping (Secor and others, 1982, 1986a). Samples con- taining biotite and/'or hornblende were collected either in quarries or at fresh creek or roadcut exposures within the Kiokee, Charlotte, Kings Mountain, and Inner Piedmont belts, as well as within the Clouds Creek granite. The letters A or B following a ¡sample number indicate that different samples were collected at the same location for hornblende and biotite separates. Slate or phyllite samples were collected for whole-rock dating within the slate belt. Coordi- nates of 40Ar/39/.r sample locations are listed in Appendix B.

40Ar/39Ar ANALYTICAL TECHNIQUES

The principles of 40Ar/39Ar incremental- release dating have been described by Dal- rymple and Lanphere (1971), Dallmeyer (1979), and Dalrymple and others (1981). The $ 0 20 40 60 80 100 techniques used during analysis of the samples ACCUMULATIVE »»Ar RELEASED generally followed those described in detail by Dallmeyer and Rivers (1983). In the present Figure 3. 40Ar/39Ar incremental-release mineral age spectra from the northeastern Kiokee study, pure mineral concentrates (>99%) were belt in central South Carolina. Sample numbers are indicated at lower left; minerals analyzed, prepared from crushed and sized rock powders at lower right (H = hornblende; B = biotite). All spectra have coordinates as shown in lower left using heavy-liquid and magnetic separation unless otherwise indicated (sample 4). Uncertainties in age (2a) are indicated by vertical width techniques. The slate and phyllite whole-rock of bar. Experimental temperatures increase from left to right. Plateau or total-gas ages samples were crushed and sieved. The rock (parentheses) listed on each of the spectra. Map patterns as in Figure 1. powders (0.15-0.18 mm) were prepared for analysis by leaching in dilute HC1 for 1 hr and thorough washing. Mineral concentrates and step was maintained for 45 min. Measured iso- Dalrymple and Lanphere (1971) which consid- whole-rock powders were wrapped in alumi- topic ratios were corrected for the effects of mass ers variations in inlet-time extrapolations, moni- num-foil packets, encapsulated in sealed quartz discrimination and interfering isotopes produced tor ages, and various irradiation parameters. A vials, and irradiated for 40 hr at 1,000 kW in the during irradiation using factors reported by Dal- 40Ar/39Ar "plateau" is defined as that portion central thimble position of the U.S. Geological rymple and others (1981) for the reactor used in of a spectrum in which contiguous gas fractions Survey TRIG A rsactor in Denver, Colorado. the present study. Apparent 40Ar/39Ar ages (together constituting >50% of all the gas Variations in the flux of neutrons along the were calculated from the corrected isotopic ra- evolved from a sample) have apparent ages that length of the irradiation assembly were moni- tios using the decay constants and isotopic are not different at a 2a level of uncertainty tored using several mineral standards, including abundance ratios listed by Steiger and Jäger (after Fleck and others, 1977). Analyses of the MMhb-1 (Alexander and others, 1978). The (1977). Total interlaboratory uncertainties in MMhb-1 monitor indicate that apparent K/Ca samples were incrementally heated until fusion each apparent age (quoted at 2a) have been ratios may be calculated through the relation- using an RF induction generator. Each heating calculated following the method outlined by ship 0.518 (±0.005) x (39Ar/37Ar) corrected.

internally concordant spectrum with a total-gas age of 288 ± 4 Ma. Biotite. Biotite concentrates were prepared from samples collected at four of the same exposures from which hornblende was analyzed (locations 3, 4, 5A, and 6A). In addition, biotite concentrates were prepared from variably mylonitic samples of Lake Murray gneiss and Lex- ington metagranite collected at locations 1 and 2, respectively. All of the biotite analyses are displayed as incremental-release spectra in Fig- ure 3, and the corresponding analytical data are presented in Table B. All six biotite concentrates yielded internally concordant release spectra which define total-gas or plateau ages which range between 278 and 288 Ma.

Carolina Slate Belt

Ten whole-rock phyllite samples from the Carolina slate belt were analyzed by 40Ar/39Ar incremental-release techniques. Primary mineral constituents include well-crystallized 2M white mica and subordinate chlorite. Also present is minor quartz together with trace amounts of opaque minerals and detrital plagioclase. The protolith of samples 13 and 14 was felsic tuff, whereas the protolith of samples 9-12 and 15-18 was siltstone or mudstone. The 40Ar/ 39Ar analyses are displayed as incremental- release spectra in Figure 4, and the resultant analytical results are presented in Table C. Markedly variable results have been obtained; however, spectra character does not appear to 0 20 40 60 80 100 be related to protolith affinity. Samples 15, 16, ACCUMULATIVE % 39Ar RELEASED and 18 were collected from within 7 km of the Figure 4. Whole-rock slate and phyllite 40Ar/39Ar incremental-release age spectra from the contact with the Kiokee belt, and they display slate belt in central South Carolina. All spectra have coordinates as shown in lower left. Data generally similar release spectra in which initial, plotted as in Figure 3. Map patterns as in Figure 1. low-temperature gas fractions record ages slightly younger than the relatively well defined plateaus defined by the gas released in interme- 40 39 diate- and high-temperature gas fractions. The Ar/ Ar RESULTS cordant patterns in which anomalously older plateau ages range from 280 ± 6 to 291 ± 5 Ma. ages are recorded in low-temperature gas frac- The fusion increment of each of these three Kiokee Belt tions. Intermediate- and high-temperature frac- samples yields a markedly older age than that of tions, however, define plateau ages of 319 ± 6 the corresponding plateau date. Samples 9 and Hornblende. Hornblende concentrates were and 299 ± 6 Ma, respectively. Samples 5A and 17 were also collected within 7 km of the con- prepared from seven samples collected at six ex- 6B yielded more complicated release patterns in tact with the Kiokee belt; however, they contain posures in the northeastern part of the Kiokee which low-temperature gas fractions and high- a significantly greater modal proportion of belt. These were analyzed by 40Ar/39Ar incre- temperature gas fractions define older ages than chlorite than do samples 15, 16, and 18. These mental-release techniques. These analyses are do intermediate-temperature fractions. These two samples yielded markedly discordant, sad- displayed as incremental-release spectra in Fig- samples yield total-gas ages of 294 ± 6 and 324 die-shaped release spectra which record total-gas ure 3, and the resultant analytical data are pre- ± 7 Ma, respectively. Porphyroblastic metamor- ages of 281 ±7 and 292 ± 6 Ma, respectively. sented in Table A. Amphibolite samples contain- phic hornblende was separated from calc-silicate Samples 10-14 were collected >7 km from the ing a variably penetrative S2 foliation were rocks collected at location 7. This concentrate Kiokee belt boundary. These whole-rock sam- collected at locations 4, 5A, 5B, 6B, and 8. yielded an internally concordant release spec- ples display internally discordant release spectra Hornblende concentrates from these samples trum with a total-gas age of 298 ± 4 Ma. Horn- which record total-gas ages ranging from 314 ± yielded variably discordant 40Ar/39Ar release blende was also separated from a late-kinematic 7 to 331 ±6 Ma. Apparent ages recorded by spectra. Sample 8 yielded a nearly concordant to postkinematic, relatively undeformed grano- low-temperature gas fractions evolved from release pattern with a total-gas age of 292 ± 5 diorite which intrudes mylonitic Lake Murray sample 11 are ca. 320 Ma. Intermediate- and Ma. Samples 4 and 5B yielded internally dis- gneiss at location 3. This concentrate yielded an

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high-temperature gits fractions nearly define a CAROLINA plateau of ca. 340 Ma. Sample 12 shows a similar low- to high-temperature increase in appar- SLATE ent age. These range from ca. 330 Ma in low-temperature gas fractions to ca. 340 Ma in BELT high-temperature fractions, with a well-defined intermediate-temperature plateau of 328 ± 6 Ma. In samples 13 and 14, apparent ages in low-temperature gas fractions are younger than in samples 11 and 12, but intermediate- and high-temperature giis fractions define similar ca. 330- to 340-Ma plateau ages. Sample 10 was also collected >7 km from the Kiokee belt boundary; however, it contains a significantly greater modal pro|X)rtion of chlorite than do samples 11-14. It displays a saddle-shaped release spectrum with a minimum age of 309 ± 6 Ma in the 700 °C increment and a maximum age of 337 ± 7 Ma in the fusion increment.

Gouds Creek Granite

Biotite concentrates were prepared from four relatively undeformed samples (19-22) and two « penetratively mylonitic samples (23, 24) of the 3 340 Clouds Creek granite. These analyses are dis- UJ o 300 268 t 5 played as incremental-release spectra in Figure 5, and the corresponding analytical data are I—I—I—I Cj listed in Table D. Of the undeformed samples, cc 24 19 and 20 display internally discordant release < 220 IL 0 20 40 60 80 100 patterns yielding total-gas ages of 343 ± 6 and < 39 297 ± 6 Ma, respectively. Samples 21 and 22 ACCUMULATIVE % Ar RELEASED yield concordant release patterns defining pla- Figure S. 40Ar/39Ar incremental-release age spectra of biotite concentrates prepared from teau ages of 285 ± 5 and 307 ± 5 Ma. The two samples collected within the Gouds Creek granite in central South Carolina. All spectra have mylonitic samples collected from deformed coordinates as shown in lower left. Data plotted as in Figure 3 (Cgr = granite that has not Clouds Creek granite yield internally concor- experienced penetrative ductile strain; Cdgr = granite that has experienced penetrative ductile dant release patterns with mutually similar pla- strain; Cdgb = gabbro that has a penetrative ductile deformation fabric of variable intensity). teau ages of 268 ± 5 Ma.

Charlotte Belt tions evolved from samples 28 and 29 yielded total-gas age of 285 ± 6 Ma. This age conforms Hornblende. Hornblende concentrates were an anomalously old apparent age. The apparent to the northwestward younging trend defined by prepared from 12 variably foliated amphibolite ages decrease to ca. 310 Ma in the second in- hornblende concentrates prepared from the samples and one nonfoliated sample of the crement evolved from both samples, and appar- metamorphic rocks. Winnsboro granite collected within the Char- ent ages systematically increase through the Biotite. Metamorphic biotite has been sepa- lotte belt. The 40Ar/39Ar release spectra for analyses to nearly define intermediate- and high- rated from 14 samples of schist or gneiss from the these samples are displayed in Figure 6, and the temperature plateau ages of ca. 330-340 Ma. Charlotte belt. In addition, biotite has been ana- analytical data ars presented in Table E. Sam- The release pattern of sample 26 is similar to lyzed from one sample of the Winnsboro granite ples 25A, 26, 28, 29, and 30 were collected that of samples 28 and 29 except that the low- and two samples of the Newberry granite. The adjacent to the Carolina slate belt. These five temperature age minimum is slightly younger 40Ar/39Ar release spectra for the above samples samples display variable discordant release pat- (ca. 260-280 Ma); however, there are large ana- are displayed in Figure 7, and the corresponding terns. Samples 25 A and 30 yielded release spec- lytical uncertainties. The remaining seven horn- analytical data are presented in Table F. All of tra in which apparent ages systematically blende concentrates prepared from metamor- the biotite samples from the Charlotte belt decreased through low- and intermediate-temp- phic rocks in the Charlotte belt (samples 27,32, yielded well-defined plateau ages. These range erature fractions. In sample 30, a plateau age of 33, 36, 37A, 38A, and 42) are characterized by from 292 ± 5 to 243 ± 4 Ma, the older ages 289 ± 5 Ma is defined; however, in sample 25A, internally concordant release spectra. These dis- having been recorded in southeastern portions of apparent ages dscrease throughout even the play a systematic northwestward decrease in the Charlotte belt (Fig. 8). The biotite data high-temperature gas fractions. The nature of plateau ages which range from 310 ± 6 Ma therefore define a northwestward younging spectra discordancy in samples 26,28, and 29 is (southeast) to 278 ± 6 Ma (northwest). Horn- trend similar to that displayed by the hornblende markedly different from that of samples 25A blende from the Winnsboro granite (sample 31) data; however, at most localities where both bio- and 30. The initial, lowest temperature gas frac- yielded a concordant release pattern with a tite and hornblende were analyzed, biotite yields

0 20 40 60 80 100 ACCUMULATIVE % wAr RELEASED

Figure 6.40Ar/39Ar incremental-release age spectra of hornblende concentrates prepared from samples collected in the Charlotte belt, Kings Mountain belt, and Inner Piedmont in central South Carolina. All spectra have coordinates as shown in lower left. Data plotted as in Figure 3. Map patterns as in Figure 1.

a younger plateau age (-20 Ma) than does release spectra in Figure 7, and the resultant South Carolina (samples 48 and 49). The hornblende. analytical data are presented in Table H. Biotite 40Ar/39Ar release spectra are displayed in Fig- within a late-kinematic diorite (sample 46A) ure 6, and the resultant analytical data are pre- Kings Mountain Belt collected at the same location as the Kings sented in Table G. Hornblende from an Mountain belt amphibolite (sample 46B) amphibolite boudin in felsic gneiss (sample 48) Hornblende. Metamorphic hornblende from yielded a concordant release spectrum with a yielded an internally concordant release pattern a foliated amphibolite xenolith within a late- plateau age of 313 ± 6 Ma. Biotite from schist at with a plateau age of 296 ± 6 Ma. A hornblende synkinematic diorite in the Kings Mountain belt location 45 also yielded a concordant release concentrate prepared from biotite-hornblende has been analyzed (sample 46B). The analytical spectrum, but with a 289 ± 5-Ma plateau age gneiss (sample 49) yielded a more complicated data for this sample are presented in Table G. It significantly younger than that of sample 46A. release pattern in which low-temperature gas displays a concordant release spectrum (Fig. 6), fractions yield apparent ages of ca. 320 Ma, yielding a plateau age of 302 ± 6 Ma. Inner Piedmont whereas intermediate- and high-temperature gas Biotite. Biotite has been analyzed from two fractions define a plateau age of 300 ± 6 Ma. locations in the Kings Mountain belt (samples Hornblende. Hornblende has been analyzed Biotite. Biotite concentrates were prepared 45 and 46A). These are displayed as 40Ar/39Ar from two locations in the Inner Piedmont of from amphibolite (sample 48) and gneiss (sam-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/11/1329/3445149/i0016-7606-97-11-1329.pdf by guest on 23 September 2021 Figure 7. 40Ar/39Ar incremental-release age spectra of biotite concentrates prepared from samples collected in the Charlotte belt, Kings Mountain belt, and Inner Piedmont in central South Carolina. All spectra have coordinates as shown in lower left. Data plotted as in Figure 3. Map patterns as in Figure 1.

pies 47 and 49) from the Inner Piedmont. The a 2 release spectra for these samples are shown in 340 H Figure 7, and the resultant analytical data are A Hornblende 0) a Biotite presented in Table H. The three samples yielded 2> 320 CO concordant release spectra defining plateau ages Figure 8. Internally concordant 40 39 4 of 272 ± 5, 259 ± 5, and 271 ± 5 Ma, hornblende and biotite Ar/ Ar ® 300- respectively. total-gas or plateau ages from the B a Charlotte belt in central South Caro- H H lina projected onto profile A-A' INTERPRETATION OF 40Ar/39Ar t + H (Figs. 6 and 7). Uncertainties (2a) + DATA + are indicated.

—1 1 1 1—•—1 1— —I 1 r Kiokee Belt 0 10 20 30 40 50 60 70 80 90 A km A' Hornblende. 1. Metamorphic. Hornblende Northwest Southeast distance along profile A-A' from point A from samples 7 and 8 yields poorly resolved but nearly concordant 40Ar/39Ar age spectra defin- experimental evolution of extraneous-argon required for argon retention in biotite (300 ± 25 ing mutually similar total-gas dates of 298 and components from relatively high-energy lattice °C, Jäger, 1979; Harrison and others, 1985). 292 Ma. These are interpreted to closely date locations as a consequence of strain recovery cooling through temperatures required for intra- during dynamic recrystallization. This interpre- Carolina Slate Belt crystalline retention of argon within hornblende tation may be appropriate for sample 6B be- (-500 ± 25 °C; Harrison, 1981) following the cause it was collected within a major D4 ductile Three whole-rock slate or phyllite samples last regionally significant metamorphism. Horn- shear zone (Irmo shear zone; Secor and others, collected within 7 km of the Kiokee belt bound- blende separated from samples 4,5A, 5B, and 6 1986a). In sample 4, apparent ages systemati- ary (15, 16, and 18) record mutually similar displays more discordant 40Ar/39Ar age spectra. cally decrease from low- through most interme- plateau ages of 280-291 Ma. These are inter- In all of these concentrates, anomalously old ap- diate-temperature gas fractions with a poorly preted to date cooling through argon retention parent ages are recorded in at least the initial, resolved high-temperature plateau of ca. 319 Ma temperatures appropriate for the constituent, lowest temperature gas increments. This type of defined. This is markedly older than the ca. 290- fine-grained white mica (-350 °C; Wagner and low-temperature spectra discordance in horn- to 300-Ma cooling dates recorded by other others, 1977) following the last regionally signif- blende has been interpreted in other metamor- hornblende samples from the Kiokee belt which icant metamorphism. Two whole-rock slate or phic terranes (for example, Dallmeyer, 1975; display less discordant age spectra. It is uncer- phyllite samples collected within 7 km of the Dallmeyer and Rivers, 1983; Dallmeyer and tain, however, if this reflects slightly earlier Kiokee belt boundary (9, 17) exhibit anoma- others, 1985a) to result from the experimental postmetamorphic cooling of this area or if it is lously old ages in low- and some intermediate- evolution of extraneous (excess) argon from rel- related to the extensive excess-argon contamina- temperature gas fractions. These contain appre- atively low-energy lattice positions. A similar tion evident in the low- and intermediate- ciably more chlorite than do samples 15,16, and interpretation is considered appropriate for the temperature gas fractions. 18. This may indicate that the chlorite contains Kiokee belt results. In samples 5A and 5B, the 2. Igneous. Sample 3 was collected within a significant extraneous-argon contamination intermediate- and high-temperature gas fractions late-kinematic to postkinematic granodiorite, which was liberated from relatively nonretentive yield ages which are similar to the 298- to 292- and the hornblende displays an internally con- intracrystalline sites during low-temperature Ma postmetamorphic cooling dates recorded by cordant age spectrum defining a total-gas age of experimental heating. The remaining interme- samples 7 and 8. In samples 4 and 6B, the extent 288 ± 4 Ma. This is interpreted to date postcrys- diate- and high-temperature gas fractions of of excess-argon contamination appears to be tallization cooling through argon retention samples 9 and 17 yield dates which are gen- more significant because apparent ages systemat- temperatures and is slightly younger than the erally similar to the 280- to 290-Ma cooling ages ically decrease throughout all of the low- and cooling dates recorded by hornblende within the recorded by samples 15,16, and 18. some of the intermediate-temperature gas frac- host metamorphic terrane. This suggests that the Four of the whole-rock slate or phyllite sam- tions. The high-temperature portion of sample granodiorite was emplaced following regional ples collected >7 km from the Kiokee belt 6B is poorly resolved but appears to reflect a postmetamorphic cooling of the host terrane boundary (11-14) display internally discordant slight increase in apparent age compared to that through -500 °C. This is consistent with the release spectra. These are mutually similar and of the intermediate-temperature fractions. This late-kinematic to postkinematic character of the characterized by ca. 280- to 300-Ma apparent results in a slightly saddle-shaped release spec- pluton. ages in most low-temperature gas fractions. The trum. No systematic variations in apparent Biotite. The 40Ar/39Ar age spectra of all six apparent ages systematically increase through K/Ca ratios are related to this spectra discord- biotite samples from the Kiokee belt yield well- intermediate- and high-temperature increments ance, suggesting it does not likely reflect any defined plateaus ranging in age from 278 to 288 to approach values of ca. 340-350 Ma in fusion intercrystalline compositional variations. Similar Ma. The concordant nature of these spectra and increments. This type of spectra discordance is high-temperature spectra discordancy has been the consistency of the plateau ages suggest that characteristic of mineral systems which have ex- interpreted in other areas (for example, Dall- they record postmetamorphic cooling of the perienced partial, volume-diffusive loss of radio- meyer and others, 1985a) to result from the northeastern Kiokee belt through temperatures genic argon as a result of a postcrystalline

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thermal overprint (for example, Turner, 1970; tinguish excess- and radiogenic-argon compo- spectra discordancy suggests extensive excess- Dallmeyer and others, 1981; Dallmeyer, 1984). nents within biotite (for example, Pankhurst and argon contamination within the hornblende The character of the discordant slate belt whole- others, 1973; Dallmeyer and Rivers, 1983; Fo- grains. The significance of the anomalously rock spectra suggest (1) that the ca. 280- to 300- land, 1983), and, as a result, well-defined but young date defined by the fusion increment is Ma low-temperature ages approximately date a geologically meaningless plateau dates are uncertain. relatively low-grade thermal overprint and (2) observed. Hornblende from remaining portions of the that the 340- to 353-Ma high-temperature dates Mylonitic Portions. Dynamically recrystal- Charlotte belt displays release spectra of variable indicate cooling through argon retention tem- lized biotite separated from two mylonitic sam- character. That concentrated from samples 30 peratures following an earlier regional meta- ples of Clouds Creek granite within a D4 ductile and 32 records anomalously old low-tempera- morphism. shear zone (Secor and others, 1986a) displays ture ages which likely result from experimental It is likely that argon liberated from whole- internally concordant spectra defining identical evolution of extraneous-argon components. Pla- rock systems in the 280- to 300-Ma overprint plateau dates of 268 Ma. These are younger than teau ages of 289 Ma are defined by the remain- zone served as a reservoir for the extraneous- any of the ages recorded by biotite within either ing temperature fractions evolved from both argon components which appear to contaminate undeformed portions of the granite or the Kio- concentrates. Except for minor excess-argon the low-temperature gas fractions evolved from kee belt. They are also younger than any whole- contamination in initial, low-temperature in- whole-rock samples that were totally reset at rock ages determined from slate/phyllite sam- crements, the remainder of the Charlotte belt 280-290 Ma. Sample 10 was also collected >7 ples in adjacent portions of the Carolina slate hornblende concentrates display internally con- km from the Kiokee belt boundary. Although it belt. The 268-Ma plateau ages are therefore in- cordant release spectra which define plateau shows an intermediate- and high-temperature terpreted to date cooling following dynamic re- ages between 278 and 310 Ma. These are inter- spectra discordancy similar to samples 11-14, crystallization associated with shear-zone de- preted to date postmetamorphic cooling through low-temperature increments record ages mark- velopment. -500 °C. The regional distribution of horn- edly older than the 280- to 300-Ma low- blende dates (Fig. 8) suggests diachronous cool- temperature dates observed in samples 11-14. Charlotte Belt ing within the Charlotte belt. Sample 10 contains more chlorite than do sam- Biotite. All biotite concentrates prepared ples 11-14, and the low-temperature fractions Hornblende. Three hornblende concentrates from Charlotte belt rocks display internally may have been contaminated with extraneous- prepared from rocks collected in southeastern- concordant release spectra which define plateau argon components which were liberated from most portions of the Charlotte belt display inter- ages ranging from 243 to 292 Ma. These are nonretentive, intracrystalline chlorite sites. nally discordant release spectra of mutually interpreted to date postmetamorphic cooling similar character (samples 26, 28, and 29). In through -300 °C. The regional distribution of Gouds Creek Granite all, initial, low-temperature increments record biotite dates (Fig. 8) also suggests that cooling anomalously old apparent ages compared to the was diachronous within the Charlotte belt. Relatively Undeformed Portions. Four bio- next temperature fraction which yields dates of tite concentrates were analyzed from relatively ca. 280-300 Ma. These are followed by a sys- Kings Mountain Belt undeformed portions of the Clouds Creek gran- tematic increase in apparent age throughout ite. Two (19 and 20) display internally discord- intermediate- and high-temperature gas fractions Interpretation of the 40Ar/39Ar results from ant spectra which yield total-gas dates of 343 to the ca. 340- to 350-Ma date recorded by the Kings Mountain belt is uncertain because and 297 Ma. The discordant nature of the spec- fusion increments. The anomalously old low- only a few samples were analyzed. Hornblende tra indicates that these total-gas ages cannot be temperature dates likely reflect experimental from sample 46B displays an internally con- confidently interpreted to date either postcrystal- evolution of extraneous argon components from cordant age spectrum, and the 302-Ma plateau lization or postmetamorphic cooling through relatively low-energy, intracrystalline sites. age likely dates post metamorphic cooling argon retention temperatures for biotite. Excess- Characteristics of the remainder of the three dis- through -500 °C. Biotite should record a sim- argon contamination is also clearly indicated for cordant spectra are typical of hornblende which, ilar or younger date if cooling through -300 °C; sample 19 because the 343-Ma total-gas date is has undergone partial, volume-diffusive loss of however, the concentrate prepared from another older than the ca. 315-Ma emplacement age radiogenic argon as a result of a metamorphism sample collected at location 46 (46A) yields a suggested for the granite by both the U-Pb zir- following an initial cooling through argon reten- plateau date (313 Ma) older than that of horn- con and Rb-Sr whole-rock isochron ages dis- tion temperatures (for example, Turner, 1970; blende. This suggests the biotite contains a small cussed earlier. Bio :ite from samples 21 and 22 Dallmeyer, 1975). The southeasternmost Char- component of extraneous-argon contamination. records plateau agiss of 307 and 285 Ma. Inter- lotte belt data are therefore interpreted to sug- This interpretation is supported by 289-Ma pla- pretation of these results is uncertain. The 285- gest that at ca. 340-350 Ma, the area cooled teau ages defined by the biotite concentrate pre- Ma date is similar to those recorded by the through -500 °C following an earlier high- pared from a Kings Mountain belt sample biotite in the Kiokee belt and by the whole-rock grade metamorphism. This was followed by a collected nearby (location 45). This 289-Ma slate/phyllite within adjacent portions of the lower grade metamorphic overprint at ca. date is tentatively interpreted to date cooling slate belt. The 285 date may therefore relate to 280-300 Ma. Small and variable components of through biotite argon closure temperatures in cooling following a distinct, postemplacement extraneous argon diffused into hornblende this portion of the Kings Mountain belt. metamorphism of the granite. The 307 plateau grains after cooling from the metamorphic overprint. date recorded by sample 21 could indicate Inner Piedmont slightly diachronoits north-south post metamor- Sample 25A was also collected from the phic cooling, or it could be related to postmag- southeastern Charlotte belt. Constituent horn- Hornblende. The hornblende separated from matic cooling. The: 307-Ma plateau date, how- blende displays a very discordant age spectrum a sample collected at location 48 displays an ever, could be geologically meaningless because in which apparent dates systematically decrease internally concordant release spectrum which several workers have demonstrated that incre- defines a plateau age of 296 Ma. This is inter- 40 39 throughout the analysis to the ca. 265-Ma date mental-release Ar/ Ar analyses cannot dis- recorded by the fusion increment. This type of preted to date cooling through argon retention

peak metamorphic conditions from 4. Apatite fission ages of ca. 130-150 Ma mineral assemblage in pelitic rocks have been reported by Zimmerman (1979) for in the Lake Murray spillway the northeastern Kiokee belt. These likely date O 600 - retention of argon cooling through -100-125 °C (Naeser and f within hornblende M. Cebula, 1978). ILI er intrusion of late granodiorite The time-temperature curve indicates that the 3 400 Kiokee belt cooled from above -500 °C to \ Figure 9. Late Paleo- below -300 °C between ca. 295 and 285 Ma. < retention of argon er T * within biotite zoic-Mesozoic thermal This relatively rapid cooling could not have tu maintenance of evolution of the north- been accomplished if the Kiokee belt had re- I 200 H \ fission tracks in apatite > eastern Kiokee belt. See mained at mid-crustal depths following M2 re- 111 text for discussion. gional metamorphism. The time-temperature curve is therefore interpreted to indicate a period o of rapid uplift and erosion in the latest Carbonif- 1— T i erous and Early Permian. If an average geo- 350 300 250 200 150 100 thermal gradient of 30 °C/km (conservatively c. TIME (Ma) chosen as the maximum gradient compatible

with staurolite-kyanite stability during M2; for M r I >K Pe>|cTr>|< — J >|* Q example, Turner, 1981) is assumed, the period of rapid cooling during 285-295 Ma corre- temperatures. Hornblende from Inner Piedmont Ma when the northeastern Kiokee belt cooled sponds to an average uplift rate of-0.6 mm/yr. sample 49 is characterized by a slightly discord- through the -500 °C required for intracrystal- This is comparable to uplift rates estimated for ant age spectrum in which low-temperature gas line retention of argon in hornblende. young collisional orogens such as the Alps or the fractions yield slightly older apparent ages com- 2. Biotite from the Kiokee belt records Himalayas (Zeuner, 1970; Clark, 1979). pared to a 300-Ma plateau defined by interme- 40Ar/39Ar plateau ages of 278-288 Ma which The Alleghanian deformational chronology diate- and high-temperature gas fractions. The date cooling through -300 °C. of the Kiokee belt is well constrained by the plateau age is similar to that of sample 48 and 3. Mutually similar 40Ar/39Ar plateau ages geochronological data presented here (Fig. 10). also likely reflects cooling through -500 °C. of ca. 285 Ma are recorded by both igneous D2 deformation fabrics (Lake Murray deforma- The older low-temperature ages probably reflect hornblende and biotite from a relatively unde- tion) in metasedimentary rocks within the Kio- minor excess-argon contamination in low-ener- formed, late-kinematic granodiorite. This sug- kee belt are a result of strain and dynamic gy, intracrystalline hornblende sites. gests emplacement of the pluton after the recrystallization under conditions of amphibolite Biotite. Biotite concentrates prepared from country-rock terrane had already cooled below facies regional metamorphism. D2 and M2, locations 47, 48, and 49 within the Inner Pied- temperatures required for argon retention in therefore, are interpreted to have been broadly mont all display internally concordant age spec- hornblende. contemporaneous events. The Lake Murray or- tra which define mutually similar plateau ages of 259,271, and 272 Ma. This consistency and the older hornblende dates suggest that postmetamorphic cooling through -300 °C in this por- Lake Clarks Irmo tion of the Inner Piedmont likely occurred Murray Hill deformation deformation deformation between ca. 260 and 270 Ma.

REGIONAL TECTONIC peak metamorphic conditions from SIGNIFICANCE OF THE mineral assemblage in pelitic rocks f in the Lake Murray spillway GEOCHRONOLOGIC RESULTS 600 - 1 retention of argon within hornblende Kiokee Belt » ill intrusion of late granodiorite The late Paleozoic and Mesozoic thermal evo- 5 400 lution of the Kiokee belt may be evaluated by JO retention of argon < tr . within biotite integrating the results presented in this report cc with previously published radiometric ages. The HI CL 200 - time-temperature history is constrained by the 2 following controls (Fig. 9). LU 1. Northeastern portions of the Kiokee belt underwent D2 ductile deformation in associa- I I I I I tion with attainment of maximum conditions of 320 310 300 290 280 270 260 M2 amphibolite facies regional metamorphism (-4.5 kb and at least 530 °C; Secor and others, c. TIME (Ma) Pennsylvanian __ , ^ Permian 1986a). This occurred after emplacement of the I«- Lake Murray orthogneiss protolith at ca. 315 Ma (both Rb-Sr whole-rock isochron and U-Pb Figure 10. Late Paleozoic deformational and thermal evolution of the northeastern Kiokee zircon crystallization ages) and prior to ca. 295 belt. See text for discussion.

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40 39 thogneiss appears to record all D2 fabric ele- cordant Ar/ Ar age spectra of the five sam- tiform, southeastern portions of the Clouds

ments; therefore, D2 and M2 must have ples collected >7 km from the Kiokee belt Creek pluton were penetratively deformed by a occurred following emplacement of the Lake suggest that this portion of the slate belt initially northeast-trending, D4, dextral ductile shear Murray gneiss protolith at ca. 315 Ma. cooled through argon-retention temperatures at zone (Secor and others, 1986a). Two identical 40Ar/39Ar geochronological data indicate that ca. 340-350 Ma following an earlier metamor- 40Ar/39Ar plateau ages of biotite dynamically

M2 metamorphic conditions had begun to dissi- phism. The whole-rock slate/phyllite ages were recrystallized within the shear zone are inter-

pate by ca. 295 Ma. D2 and the peak of M2 incompletely reset by a subsequent, lower grade preted to date cessation of this strain at or below

regional metamorphism, therefore, are inter- reheating during Alleghanian M2 regional 300 °C at ca. 268 Ma. This is significantly preted to have occurred during ca. 295-315 Ma. metamorphism. younger than the estimated age for D4 in the Overprinting relationships in the Kiokee belt Secor and others (1982, 1986a) argued that Kiokee belt and may indicate that localized, dia-

indicate that D2 was followed by D3 (Clarks D[ (Delmar deformation) fabrics developed chronous episodes of D4 strain occurred Hill deformation), during which numerous mes- synchronously in both the Carolina slate and throughout the late Carboniferous and Early oscopic to macroscopic, horizontal to gently Charlotte belts between ca. 415 and 520 Ma. Permian. plunging F3 folds farmed (including the regional Maximum metamorphic temperatures estimated Kiokee antiform; Secor and others, 1986a). for greenschist facies mineral assemblages in the Charlotte Belt Timing of D3 is indirectly determined by chro- northwestern part of the Carolina slate belt were nologic constraints on the preceding D2 and likely not significantly greater than the -350 °C The Charlotte and the Carolina slate belts subsequent D4 evsnts. These data suggest that argon-retention temperature appropriate for contain a regionally penetrative D) fabric which D3 was contemporaneous with the inferred pe- constituent fine-grained white micas. The 340- is interpreted to have developed synchronously riod of rapid uplift and erosion between ca. 285 Ma ages recorded in the northwestern slate belt in both belts (the Delmar deformation of Secor and 295 Ma. are therefore interpreted to date cooling follow- and others, 1986a). In the southeastern Char- The timing of D4 (Irmo deformation) in the ing a Late Devonian or very early Carboniferous lotte belt, the undeformed Newberry granite re- Kiokee belt is constrained by the following. (340-350 Ma) thermal event which occurred cords a 415 ± 9-Ma Rb-Sr whole-rock isochron 1. The Lexington metagranite, Batesburg after Dj deformation and the associated low- age which has been interpreted to closely date augen gneiss, and Batesburg lineated gneiss do grade metamorphism. Studies in the Inner pluton emplacement (Fullagar, 1981). Because Piedmont and Chauga belts (Dallmeyer, 1978; not record D2 fabric elements but were variably this pluton contains xenoliths which display D| deformed during D4 (Secor and others, 1986a). Dallmeyer and Hatcher, 1985) have docu- fabric elements, D[ has been interpreted to have These units record Rb-Sr whole-rock isochron mented significant mid-Paleozoic tectonother- occurred prior to 415 Ma (Secor and others, igneous crystallization ages of 292 ± 15, 291 ± mal activity which may be correlative with the 1982). 4, and 284 ± 17 Ma, respectively (Snoke and ca. 340- to 350-Ma metamorphism apparently Hornblende separated from three 40Ar/39Ar others, 1980; FulJagar, 1981). If these reflect recorded in the Carolina slate belt exposed in the samples collected along the southeastern edge of present study area. The mid-Paleozoic thermal crystallization ages, D4 must have occurred after the Charlotte belt appears to have cooled ca. 291 Ma. event may be contemporaneous with the F2 through -500 °C at ca. 340-350 Ma. These 2. All of the 43Ar/39Ar biotite plateau ages folding episode described in the northwestern samples are of lowest amphibolite grade and in the Kiokee belt are concordant at ca. slate belt and Charlotte belt (Secor and others, likely experienced maximum metamorphic 278-288 Ma. This concordancy suggests that 1982; Hauck, 1984). A progressive northwest- temperatures not substantially greater than those D4 occurred before these samples cooled ward decrease in the conditions of maximum required for argon retention in hornblende. Rel- Alleghanian M2 regional metamorphism is indi- through the -300 °C argon-retention tempera- 40 39 atively rapid cooling following this event is indi- ture for biotite. cated by both petrologic and Ar/ Ar age cated by the concordance of hornblende ages in data. These indicate that M isothermal surfaces 3. The late-kinematic to postkinematic gran- 2 the Charlotte belt with high-temperature ages are inclined to the northwest, likely a conse- 40 39 odiorite (location 3) does not carry D4 fabric displayed in Ar/ Ar release spectra of whole- quence of rotation along the northwest limb of elements, suggesting that D4 occurred prior to its rock slate/phyllite samples collected in adjacent the Kiokee antiform during D3 deformation at emplacement and rapid postcrystallization cool- portions of the Carolina slate belt. The 340- to ca. 285-295 Ma. ing at ca. 285 Ma (inferred from coincident 350-Ma hornblende ages from the southeastern 40Ar/39Ar hornblende and biotite ages). Charlotte belt, therefore, are interpreted to date Considering the analytical uncertainties in the Clouds Creek Pluton cooling followed a Late Devonian or very early geochronological data, the constraints listed Carboniferous (340-360 Ma) thermal event. This event is interpreted to be the same as that above suggest that D4 occurred at ca. 285-290 The Clouds Creek pluton was emplaced prior Ma in the Kiokee belt. to D3 and must have undergone northwestward which affected at least northwestern segments of rotation during this deformational event (along the Carolina slate belt. Carolina Slate llelt with host rocks of the Carolina slate belt). Ex- All of the biotite 40Ar/39Ar ages from the cluding sample 19 (which is obviously contami- Charlotte belt, as well as hornblende ages from The 40Ar/39A.r whole-rock slate/phyllite age nated with excess argon), older 40Ar/39Ar the central and northwestern Charlotte belt, are data in the Carolina slate belt indicate that the biotite ages occur along the northwestern edge interpreted to date diachronous cooling through

thermal effects of Alleghanian M2 regional meta- of the pluton. Although this would be expected argon-retention temperatures during the late morphism extended across the Modoz zone if paleoisothermal surfaces had been tilted Carboniferous and Early Permian. These cool- into the slate belt. ^Ar/^Ar age spectra of five northwestward subsequent to cooling, the inter- ing ages are, in part, contemporaneous with the samples from within 7 km of the Kiokee belt are pretation is equivocal because it is not possible Alleghanian deformation documented in the

interpreted to elate 280- to 290-Ma post-M2 to clearly resolve the amount of excess-argon Kiokee belt. The last episode of regionally pene- metamorphic cooling through the -350 °C contamination in biotite from undeformed por- trative deformation in the southeastern Charlotte argon-retention temperatures appropriate for the tions of the pluton. belt, however, must have occurred prior to em-

constituent, fine-grained white micas. The dis- Following development of the D3 Kiokee an- placement of the Newberry granite at ca. 415

Ma. Moreover, the Winnsboro granite (Fig. 6) interior parts of the Charlotte belt (Fig. 8) may cies regional metamorphism (M2) occurred in records a 295 ± 4-Ma Rb-Sr, whole-rock iso- reflect folding and/or faulting following regional the Kiokee belt during ca. 295-315 Ma. chron age interpreted by Fullagar (1981) to cooling below argon-closure temperatures. 8. An episode of uplift, erosion, and rapid closely date emplacement. The pluton is not cooling during the late Carboniferous and Early penetratively deformed; however, it carries xe- Kings Mountain Belt and Inner Piedmont Permian is indicated by 10- to 20-Ma differ- noliths which contain both deformational phases ences between hornblende and biotite 40Ar/39Ar recognizable in the surrounding host-rock ter- 40Ar/39Ar plateau ages of hornblende sam- cooling ages in both the Kiokee and Charlotte rane. This suggests a pre-295-Ma age for the ples increase from ca. 280 Ma in the northwest- belts. various deformational events. On the basis of ern Charlotte belt to ca. 300 Ma in the Kings 9. Systematic regional variations in 40AT/ these plutonic relationships, the late Paleozoic Mountain belt and Inner Piedmont. Biotite pla- 39Ar mineral cooling ages indicate differential 40Ar/39Ar cooling ages recorded within the teau ages in the northwestern Charlotte belt and uplift and rotation of isothermal surfaces during Charlotte belt could date either (1) cooling fol- Inner Piedmont vary between 259 and 275 Ma; the late Paleozoic. The Carolina slate belt was lowing a distinct, late Paleozoic thermal event however, these variations are not clearly related uplifted less than were flanking portions of the which was unaccompanied by penetrative strain to any belt boundary. These preliminary data Kiokee and Charlotte belts. Isothermal surfaces (during which most of the 40Ar/39Ar mineral are tentatively interpreted to indicate faulting or are interpreted to have been regionally folded to ages were reset) or (2) cooling following pro- northwestward tilting of isothermal surfaces be- produce the present contrast in metamorphic longed maintenance at relatively deep crustal tween ca. 275 and 300 Ma. Horton and Stern grade observed in the Kiokee, Carolina slate, levels following an early or mid-Paleozoic high- (1983) have presented evidence for Alleghanian and Charlotte belts. grade regional metamorphism. Regardless of deformation and metamorphism of the High 10. Geochronological data provide the fol- prior thermal history, the cooling ages recorded Shoals gneiss in the Kings Mountain belt -60 lowing constraints for the late Paleozoic defor- by minerals in the Charlotte belt indicate an km northeast of the present study area. Addi- mational chronology within the Kiokee and episode of uplift and erosion during the late Pa- tional field and geochronological studies are southeastern slate belts (Secor and others, leozoic. At most locations where coexisting presently underway in the northwestern Char- 1986a): D2 (Lake Murray deformation), ca. hornblende and biotite have been analyzed, bio- lotte belt, Kings Mountain belt, and Inner Pied- 295-315 Ma; D3 (Clarks Hill deformation), ca. tite records an ~20-Ma younger cooling age. mont in order to determine the regional 285-295 Ma; and D4 (Irmo deformation), ca. Assuming hornblende and biotite argon-reten- significance of Alleghanian tectonothermal ac- 268-290 Ma. tion temperatures of 500 and 300 °C, respec- tivity in this part of the Piedmont. tively, and assuming an average geothermal ACKNOWLEDGMENTS gradient of 40 °C/km (calculated using an in- CONCLUSIONS ferred emplacement pressure of-3.25 kb for the The U-Pb 'isotopic data were determined in Winnsboro granite; J. A. Speer, 1985, personal 1. A U-Pb zircon age of 550 ± 4 Ma for the the laboratories of J. M. Mattinson and G. R. commun.), the difference between the horn- epizonal Little Mountain metatonalite is com- Tilton at the University of California, Santa blende and biotite ages corresponds to an aver- parable to U-Pb zircon and Rb-Sr whole-rock Barbara, California, and Wright gratefully ac- age uplift rate of -0.25 mm/yr. This rate is less isochron crystallization ages previously reported knowledges their generous and continuing sup- than half that estimated previously for the Kio- for felsic metavolcanic rocks in the South Caro- port. This paper benefited from critical reviews kee belt. lina slate belt. by Stewart Farrar and Gail Russell. J. Alex- The occurrence of greenschist fades rocks in 2. The Charlotte belt is, at least in part, a ander Speer contributed unpublished data on the slate belt and amphibolite facies rocks in the sub-volcanic-arc infrastructure which devel- the emplacement depth of the Winnsboro gran- Charlotte belt at the same present topographic oped contemporaneously with and beneath the ite. This work was supported by the Division of level suggests relatively more extensive late Pa- Carolina slate belt. Earth Sciences, National Science Foundation leozoic uplift of the Charlotte belt. The general 3. The exposed South Carolina slate belt is Grants EAR 76-22323 (AWS and DTS, Jr.); northwestward decrease in hornblende and bio- mostly or entirely younger than 570 Ma (Cam- EAR 80-20474, EAR 82-17743, and EAR 85- tite 40Ar/39Ar plateau ages in the southeastern brian?). This is in contrast to the North Carolina 08123 (DTS, Jr.); and EAR 80-20469 (RDD). Charlotte belt (Fig. 8) indicates that late Paleo- slate belt which is mostly of late Proterozoic age. zoic isothermal surfaces are presently inclined 4. A mid-Paleozoic (ca. 340-360 Ma) ther- REFERENCES CITED 40 39 southeastward. The diachronous Ar/ Ar mal event of regional importance is indicated by Alexander, E. C., Jr., Michetson, G. M., and Lanphere, M. A., 1978, A new ages indicate greater relative uplift of the Char- 40 39 *°Ar/39Ar dating standard, in Zartman, R. 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F., and Cormier, MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 8,1986 mont of Virginia and North Carolina: American Journal of Science, R. F., 1957, Age study of some crystalline rocks of the Georgia Pied- REVISED MANUSCRIPT RECEIVED APRIL 28,1986 v. 273-A, p. 234-251. mont [Abs.]: Geological Society of America Bulletin, v. 68, p. 1781. MANUSCRIPT ACCEPTED MAY 9,1986

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