Gondwanan/Peri-Gondwanan Origin for the Uchee Terrane, Alabama And

Home , Carolina terrane, Window (geology)

Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and Georgia: Carolina zone or Suwannee terrane(?) and its suture with Grenvillian basement of the Pine Mountain window

Mark G. Steltenpohl Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA Paul M. Mueller Ann L. Heatherington Department of Geology, University of Florida, Gainesville, Florida 32611, USA Thomas B. Hanley Department of Chemistry and Geology, Columbus State University, Columbus, Georgia 31907, USA Joseph L. Wooden U.S. Geological Survey, Menlo Park, California 94025, USA

ABSTRACT Uchee terrane may be correlative with meta- most signifi cant events in Appalachian history morphic basement of the Suwannee terrane. (Hibbard and Sampson, 1995; Hibbard, 2000; The poorly known, suspect, Uchee ter- The ca. 300 Ma overgrowths on zircons are Hibbard et al., 2002). The collage of suspect rane occupies a critical tectonic position with compatible with previously reported 295 to and accreted Gondwanan/peri-Gondwanan regard to how and when peri-Gondwanan 288 Ma 40Ar/39Ar hornblende dates on Uchee terranes within the Carolina zone constitutes (Carolina) and Gondwanan (Suwannee) ter- terrane rocks, which were interpreted to indi- a major part of the Appalachian orogen (Fig. ranes were sutured to Laurentia. It lies sand- cate deep tectonic burial of the Uchee terrane 1). A paucity of reliable isotopic dates within wiched between Laurentian(?) continental contemporaneous with the Alleghanian orog- many of the terranes makes their pre-Appala- basement exposed in the Pine Mountain eny recorded in the foreland. Temperature- chian relations uncertain and, together with window and adjacent buried Gondwanan time paths for the Uchee terrane are similar limited fi eld/structural control, explains why crust of the Suwannee terrane. The Uchee to that of the Pine Mountain terrane, indicat- the time of docking remains a debatable issue. terrane has been proposed as both a septum ing a minimum age of ca. 295 Ma for dock- The Carolina zone comprises heterogeneously of Piedmont rocks that once was continuous ing. In terms of tectono-metamorphic history deformed and metamorphosed Neoproterozoic across the erosionally breached Pine Moun- of the Uchee terrane, it is important to note to Cambrian magmatic arc terranes that formed tain window or part of the Carolina zone. To that no evidence for intermediate “Appala- prior to the Iapetus ocean, likely off the margin help resolve this issue, we conducted U-Pb chian” dates (e.g., Acadian or Taconian) has of Gondwana (Hibbard and Sampson, 1995; (SHRIMP-RG) (sensitive high-resolution ion been reported. This younger history, together Hibbard, 2000; Hibbard et al., 2002). Dock- microprobe–reverse geometry) zircon studies with the ages of metaigneous rocks and evi- ing has been proposed to have occurred during and whole-rock isotopic analyses of principal dence for pre-Grenville basement, suggests the Taconic (Carolina zone subducted beneath metasedimentary and metaplutonic units. the Uchee terrane is likely of Gondwanan Laurentia; Hibbard, 2000), Acadian (Laurentia U-Pb ages for zircons from the Phenix City origin and may be related to Carolina zone beneath Carolina zone; Wortman et al., 1998; Gneiss suggest igneous crystallization at ca. terranes that accreted during the Allegha- Hatcher et al., 1999; Bream et al., 2000, 2004; 620 Ma, inheritance ca. 1000 to ca. 1700 Ma, nian orogeny. Merschat et al., 2005; Hatcher and Merschat, and a ca. 300 Ma (Alleghanian) overprint 2006), or as late as the early Alleghanian (Den- recorded by zircon rims. Zircons from the Keywords: Uchee terrane, Carolina zone, nis and Wright, 1997; West, 1998). metasedimentary/metavolcaniclastic Moffi ts Gondwana, peri-Gondwana, southern Appala- The poorly known Uchee terrane occupies a Mill Schist yield bimodal dates at ca. 620 and chians critical tectonic position with regard to how and 640 Ma. The 620 to 640 Ma dates make these when the Carolina zone was sutured to Lauren- rocks age-equivalent to the oldest parts of the INTRODUCTION tia (Fig. 1). It lies sandwiched between prob- Carolina slate belt (Virgilina and Savannah able Laurentian continental basement exposed River) and strongly suggest a Gondwanan The nature and timing of docking of the in the Pine Mountain window and overlying (Pan-African and/or Trans-Brasiliano) ori- Carolina zone to Laurentia is controversial Gondwanan crust of the Suwannee terrane, gin for the Uchee terrane. Alternatively, the and remains one of the least understood but which is buried beneath sediments of the Gulf

Geosphere; February 2008; v. 4; no. 1; p. 131–144; doi: 10.1130/GES00079.1; 5 ﬁ gures; 3 tables.

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Figure 1. (A) Tectonic map of the southern Appalachians with section line A–A' (modiﬁ ed from: Hatcher, 2004; Morton et al., 1984, 1989; Hibbard et al., 2002, 2006; Steltenpohl, 2005a). Red square is area of detailed geologic map in Figure 2. (B) Cross section A–A' (modiﬁ ed from: Thomas and coworkers as presented in Hatcher et al., 1990; Steltenpohl 2005a).

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Coastal Plain (Fig. 1). In general, two contrast- from Alabama and westernmost Georgia (Bent- be coeval with these orthogneiss sheets. Dates ing schools of thought have held sway for the ley and Neathery, 1970; Hanley, 1983, 1987; of crystallization for neither the Motts Gneiss origin of the Uchee terrane: (1) Laurentian— McRae, 1992; McRae and Steltenpohl, 1993; nor the Hospilika Granite are known. that is, it may be a septum of Piedmont rocks Steltenpohl and Salpas, 1993; Hanley and Fabric relations in some rocks of the Uchee that once was continuous across the now ero- Steltenpohl, 1997; Steltenpohl, 2005a, 2005b). terrane clearly indicate that two amphibolite- sionally breached Pine Mountain anticlinorium These studies indicate that the Uchee is divis- facies events affected them (McRae, 1992; (Hanley et al., 1997); or (2) Exotic—that is, it ible into three parts (Fig. 2): a structurally lower McRae and Steltenpohl, 1993). The dominant

might be part of the peri-Gondwanan Carolina dioritoidal gneiss and amphibolite complex gneissosity/schistosity, S0/S1, is interpreted as a zone (Russell, 1978, 1985; Hooper and Hatcher, (Phenix City Gneiss; Figs. 3A and 3B); an inter- transposition foliation formed under uppermost 1988, 1990; Hanley et al., 1997; Hibbard et al., mediate-level package of relatively well-lay- amphibolite-facies conditions (630–780 °C and

2002; McBride et al., 2005) or the Gondwanan ered metasedimentary and metavolcanic rocks 5.7–10.6 kbar; Chalokwu, 1989) during M1;

Suwannee terrane. Neither the strongly meta- (Mofﬁ ts Mill Schist; Fig. 3D); and an upper S0 is interpreted as an earlier primary layering morphosed lithologies nor the existing sparse complex of diverse migmatite, orthogneiss, and (bedding or other type of compositional band-

isotopic age information, however, allows for amphibolite (North Columbus metamorphic ing) transposed parallel to S1 based on inclu-

distinguishing which proposal is correct. Conse- complex; Fig. 3C), all disposed in the core of a sion trails in M1 porphyroblasts, but no other

quently, we conducted U-Pb zircon age determi- gently NE-plunging synform (Lake Oliver syn- evidence for S0 was clearly observed. M2 and

nations to place more deﬁ nitive age control on form; Fig. 2). Migmatites are commonplace but D2 resulted in a mostly weak but locally intense

the times of protolith formation of metaigneous occur randomly at all structural levels in each of schistosity and/or mineral lineation (S2 and/or

and metasedimentary rocks and timing of meta- the three rock packages. Minor ultramaﬁ c rocks L2, respectively) that clearly overprint S0/S1 and morphism of rocks in the Uchee terrane. Our occur together with the maﬁ c units. Major- and all earlier formed structures (Figs. 3B, 3C, and

results indicate that an exotic (non-Laurentian) trace-element geochemistry indicates a volcanic 3F). Retrograde assemblages defi ning S2 and L2 origin for the Uchee terrane is most probable, arc to backarc setting (Chalokwu and Hanley, and symplectic overgrowths formed under mid- thus providing a new puzzle piece helping to 1990; Steltenpohl et al., 2002; Hanley et al., dle-amphibolite-facies conditions (550–580 °C constrain Carolina zone tectonic evolution and 2005). Limited U-Pb dates on multigrain zircon and 6.8–7.6 kbar; Chalokwu, 1989). Steltenpohl its docking with Laurentia. populations and Rb-Sr errorchron dates on some and Kunk (1993) reported 40Ar/39Ar hornblende of the Uchee rocks were previously reported to cooling dates from rocks of the Uchee terrane GEOLOGIC CONTEXT range from Neoproterozoic to Devonian (Rus- that document amphibolite-facies conditions sell, 1978; Maher et al., 1992). (i.e., ~500 °C closure temperature for horn- The Uchee terrane is the most eastern and Two suites of late-stage granitoidal plutonic blende) lasting as late as 288 Ma, recording the internal terrane exposed in the Alabama and rock intrude the Uchee protoliths (Fig. 2). late stages of the Alleghanian event. central Georgia Piedmont (Fig. 1). It is bounded The earlier phase, the Motts Gneiss (Bentley The Pine Mountain terrane (Figs. 1 and 2) on the northwest by fundamental southern and Neathery, 1970; Raymond et al., 1988), comprises multiply folded and faulted Grenville Appalachian mylonite zones. The Bartletts occurs as thick (up to 3 km), extensive (up to (ca. 1.05 Ga) basement gneisses and younger Ferry, Goat Rock, and Box Ankle fault zones 30 km strike length), tabular sheets (Fig. 2). stratigraphic cover exposed in a complex tectonic have juxtaposed the Uchee terrane with Gren- These granodioritic rocks characteristically are window (Schamel et al., 1980; Sears et al., 1981a; ville basement and its stratigraphic cover within strongly lineated L-tectonites (Fig. 3E). The Sears and Cook, 1984; Hooper and Hatcher, the Pine Mountain window (Figs. 1 and 2, this Hospilika Granite is a later phase occurring as 1988). A platformal metasedimentary cover paper; Schamel et al., 1980; Sears et al., 1981a; small (<1 km2) vein-like injections that locally sequence mostly mantles the Grenville gneisses Steltenpohl, 1988; Hooper and Hatcher, 1988; intrude the Motts (Fig. 2; McRae, 1992) but do and is called the Pine Mountain Group (Galpin, West et al., 1995; McBride et al., 2005). North- not carry the same intensely developed elonga- 1915; Adams, 1933; Crickmay, 1933, 1952). The east of the Pine Mountain window, the Modoc tion lineation. The lack of any strongly devel- Pine Mountain Group (Fig. 2) comprises, from fault zone separates the Uchee from the Charlotte oped fabric within the Hospilika led workers to stratigraphic bottom to top, the Hollis Quartz- terrane (Fig. 2), an infrastructural block within hypothesize that they are Carboniferous, late- ite, Chewacla Marble, and Manchester Schist the Carolina zone (Secor et al., 1986; West et al., Alleghanian intrusions (Bentley and Neathery, (Crickmay, 1952; Clarke, 1952; Bentley and 1995; Hibbard et al., 2002). The Box Ankle fault 1970; McRae, 1992; Steltenpohl and Kunk, Neathery, 1970; Raymond et al., 1988), and these is a west-directed thrust (Hooper and Hatcher, 1993). McRae (1992) and Steltenpohl and Kunk units have been suggested to lithologically cor- 1988; West et al., 1995), whereas the others are (1993) point out several geological relationships relate, respectively, with the Chilhowee-Shady/ right-slip fault zones and each formed at various that imply the Motts Gneiss protolith might also Knox-Rome platformal sequence in the foreland times during the Middle to Late Carboniferous have intruded during the Carboniferous. First, (Clarke, 1952; Sears et al., 1981b; Steltenpohl, (Secor et al., 1986; Steltenpohl, 1988; Stelten- the Motts Gneiss is sheared into the right-slip 1992; Yokel et al., 1997). Feldspathic schists pohl et al., 1992). To the east, the Uchee terrane Goat Rock fault zone which formed during the lying between the Hollis Quartzite and the base- is mostly covered beneath sedimentary rocks late Carboniferous (ca. 288 Ma; Steltenpohl et ment gneiss, the Halawaka and Sparks schists of the Gulf and Atlantic Coastal Plains. To the al., 1992). The elongation lineation within both (Clarke, 1952; Schamel et al., 1980; Raymond northeast, however, it is in tectonic contact with the Goat Rock fault zone and the Motts Gneiss et al., 1988), are interpreted as Ocoee rift facies the Milledgeville terrane, a largely unknown is coaxial. Second, because the Motts Gneiss (Clarke, 1952; Bentley and Neathery, 1970; suprastructural block within the Carolina zone is similar in its chemistry, petrology, and fi eld Schamel et al., 1980; Sears et al., 1981a, 1981b; (Higgins et al., 1988; Hibbard et al., 2002). characteristics to Late Carboniferous ortho- Steltenpohl, 1992; Yokel et al., 1997). The Uchee terrane is not well mapped, and gneiss sheets of the Modoc fault zone (ca. 315 Two distinct metamorphic events affected our present understanding is based primarily Ma; Maher et al., 1992), which occur northeast- basement rocks of the Pine Mountain ter- on M.S. thesis and fi eldtrip guidebook reports ward along strike to the Motts (Fig. 2), it may rane (Schamel and Bauer, 1980; Steltenpohl

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Figure 2. (A) Geologic map of the Uchee and adjacent terranes depicting sample localities for our U-Pb zircon studies (modiﬁ ed after: McRae, 1992; McRae and Steltenpohl, 1993; Steltenpohl and Kunk, 1993; Hanley et al., 1997; T.B. Hanley, unpublished information). (B) Cross section A–A' (modiﬁ ed from: Thomas and coworkers as presented in Hatcher et al., 1990; Steltenpohl 2005a).

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Figure 3. Collage of some lithologies and fabric relations from the Uchee terrane. (A) Layered orthogneisses and paragneisses of the Phenix

City Gneiss. (B) Outcrop photo of Phenix City Gneiss containing two sets of planar fabrics. S0/S1 is the composite compositional layering

(bedding?; S0) and coplanar upper amphibolite-facies schistosity (S1), and S2 is an amphibolite-facies schistosity axial planar to ptygmatic

F2 folds. (C) Migmatized North Columbus metamorphic complex gneiss, from which sample A-13 was collected. Note blocks of gneiss

containing S0/S1 gneissosity at a high angle to S2 within encapsulating leucosome. (D) Mofﬁ ts Mill Schist at the type locality at Mofﬁ ts Mill, Little Uchee Creek. Sample UB-1-03 was collected from this outcrop. (E) Close-up view of slabbed and polished Motts Gneiss sample with

characteristic L2 mineral stretching lineation. Dime (1.8 cm diameter) for scale. (F) Two sets of foliations developed in a folded and sheared

Mofﬁ ts Mill Schist sample (from McRae, 1992). S0/S1 is the composite compositional layering (bedding?; S0) and coplanar upper amphibo-

lite-facies schistosity (S1). Middle amphibolite-facies foliation (S2) is axial planar to small-scale open folds and parallels the sheared margin of a granitoidal injection along right-hand edge of the sample. Scale is marked in squares with one-centimeter sides.

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and Moore, 1988; Steltenpohl, 1992; Stelten- multiple analysis of the standard JNdi-1. The the melt during anatexis and, therefore, repre- pohl and Kunk, 1993). Mesoproterozoic upper sample localities (Fig. 2A) and analytical results sent the source to some degree. Regardless of amphibolite- to granulite-facies metamorphism, are presented in Tables 1, 2, and 3. the exact method of incorporation, it is impor-

M1, resulted from the Grenville event (Odom et tant to recognize that these data clearly suggest al., 1973, 1985; Steltenpohl and Moore, 1988; Phenix City Gneiss, Uchee Terrane the presence of older crust within the Uchee Stieve and Size, 1988). Paleozoic (Appalachian) terrane. The primary age of the protolith (ca.

M2 metamorphism is recorded in both the base- Sample TH-04 is a quartzofeldspathic 600 Ma), in conjunction with the ages of the ment gneisses and the Pine Mountain Group (hornblende-plagioclase) gneiss collected near older xenocrystic zircons, provides a basis for and ranged from kyanite and sillimanite grade Columbus, Georgia (Fig. 2). U-Pb and Pb-Pb explaining the scatter in the original results of in Georgia to staurolite grade in Alabama (Sears ages of single zircon grains are largely concor- Russell (1985) and for proposing and testing and Cook, 1984; Steltenpohl and Moore, 1988; dant and range from ca. 300 to ca. 1700 Ma, models for correlation of the Uchee terrane to Steltenpohl and Kunk, 1993; Yokel, 1996). Tim- with a strong concentration in the range from other Appalachian terranes, as described in the

ing of M2 is not well constrained because no fos- 590 to 640 Ma (Fig. 4A). Zircons in this age discussion section below. sils are reported and only a few reliable isotopic range have typical igneous Th and U contents dates exist. Tull (1980) used fossil and isotopic (Th/U ratios from 0.5 to 0.8), which suggest the Moffi ts Mill Schist, Uchee Terrane evidence from across the southern Appalachians primary protolith to the gneiss was a ca. 600 Ma to infer a widespread Devonian (Acadian) meta- magmatic (or possibly volcaniclastic) rock. It is Sample UB-1-03 is a quartzofeldspathic schist morphic peak. Wampler et al. (1970) report a diffi cult to provide a more defi nitive age esti- (metagraywacke) collected from the type local- conventional K/Ar mineral cooling date on an mate, however, due to the scatter of ages near ity at Little Uchee Creek near Moffi ts Mill, Ala- actual Pine Mountain rock that was ca. 311 Ma 600 Ma, which may be related to Pb loss asso- bama (Figs. 2 and 3D). The age spectrum shows (Alleghanian). 40Ar/39Ar mineral cooling studies ciated with a strong, late Paleozoic overprint at both a traditional histogram of 206Pb/238U ages corroborated late Pennsylvanian through Perm- ca. 300 Ma or a limited range of magmatic and and the probability distribution derived from ian cooling for the temperature interval between xenocrystic grains. The late Paleozoic overprint these data, which refl ect 29 single-grain zircon ca. 350 and 180 °C (Steltenpohl and Kunk, is evident in the relatively large percentage of analyses (SHRIMP-RG) for a single sample of 1993). If the lithologic correlations proposed grains with high-U overgrowths. In most cases, this unit (Fig. 4B). There is a strong concentra- for the Pine Mountain Group cover units are the uranium contents of the overgrowths were so tion of ages at ca. 600 Ma and a range of younger valid, then the metamorphic “peak” post-dated high that useful measurement was not possible. ages down to ca. 300 Ma; grains older than ca. the Early Ordovician. Steltenpohl et al. (2004) Those that were measurable are characterized 600 Ma were not detected. Overall, concordance reported a ca. 354-Ma lower intercept U-Pb by the low Th/U ratios commonly associated among the <600 Ma grains was less than in A- date on zircons from the Hollis Quartzite, but with hydrothermal growth; all ages <590 Ma 13 and TH-04; U concentrations range up to the date carries a large, ±140 Ma error estimate. have Th/U <0.1. 2000 ppm. The data are interpreted to represent Steltenpohl and Kunk (1993) reported a “dis- A third age component in this rock is evident deposition of an original sedimentary or volca- turbed” 40Ar/39Ar spectrum on hornblende from in zircons with ages in the range of ca. 1000 to niclastic protolith subsequent to ca. 600 Ma, fol- the basement complex, indicating that cooling ca. 1700 Ma. Th/U ratios for these grains range lowed by signifi cant metamorphic overprint(s). through ~500 °C occurred some time after ca. from 0.1 to 1.0, and the grains are interpreted The limited range of Neoproterozoic ages likely 337 Ma. In the next section, we report U-Pb iso- to be largely, if not completely, igneous in ori- refl ects a very limited provenance of the proto- topic analysis of rutile from the Hollis Quartz- gin. Their tectonic/lithologic origin, however, lith (e.g., a graben within a developing arc). The ite in an attempt to further delimit the higher is less certain because of the overall migmatitic overprinting is evident exclusively in younger temperature part of the Pine Mountain cooling character of the Phenix City Gneiss. If we con- overgrowths (285–375 Ma, mean = 334 Ma), path. sider only the relatively homogeneous sample which exhibit the high U/Th ratios (all >20) analyzed here, then the most straightforward characteristic of hydrothermally grown zircon. U-Pb AND Sm-Nd RESULTS AND interpretation is that these >1000 Ma grains are These “Alleghanian” ages were not measured in INTERPRETATIONS xenocrystic and derived from a chronologically any whole grains or cores. The two intermedi- diverse basement into/onto which the protolith ate ages (435 and 483 Ma) exhibit intermediate Analyses of the U-Pb systematics of zircon was emplaced. It is also possible, however, that U/Th ratios and may result from some mixing of grains were carried out by sensitive, high-reso- these older grains were directly entrained into older and younger components during analyses. lution, ion microprobe (SHRIMP), and proce- dures are the same as those reported in Stelten- pohl et al. (2004). For the Nd and Sm analyses, whole-rock powders were dissolved, spiked TABLE 1. SAMPLE LOCALITIES 150 147 with a mixed Nd/ Sm spike, and Nd and Sm Sample Terrane Rock/unit Location were separated via standard chromatographic TH-01 Uchee Motts Gneiss N32 32.797 W84 52.915 methods using laboratories in the Department of TH-02 Uchee Motts Gneiss N33 00.00 W85 9.870 Geology at the University of Florida. Analysis of TH-03 Uchee Motts Gneiss N33 33.113 W85 10.339 Nd and Sm isotopic composition was performed TH-04 Uchee Phenix City Gneiss N32 28.168 W84 59.896 TH-05 Uchee Phenix City Gneiss N32 28.167 W84 59.894 using a Nu MC-ICP-MS (multi-collector–induc- TH-06 Uchee Phenix City Gneiss N32 30.161 W84 56.620 tively coupled plasma–mass spectrometer), fol- A-13 Uchee Moffits Mill Schist N32 30.434 W85 9.963 lowing methods described by Kamenov (2006). UB-1-03 Uchee Moffits Mill Schist N32 33.411 W85 11.087 External precision on measured 143Nd/144Nd UB-2-03 Uchee Moffits Mill Schist N32 33.415 W85 11.901 ratios is ±0.4 epsilon units (2 sigma) based on H-1 Pine Mountain Hollis Quartzite N32 33.16 W85 25.72

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TABLE 2. U-Pb DATA FOR ZIRCON ANALYSES Sample 207Pb*/235U % error 206Pb*/238U % error 206Pb/238U Absolute 207Pb/206Pb Absolute Discordance (1 ) (1 ) Age error Age error (%) 207Pb*/235U 206Pb*/238U (Ma) (1 ) (Ma) (1 ) Phenix City Gneiss TH-04 SHRIMP data grain.spot TH-04-7.1 .0380 19.9 299.0 19.7 290.6 21.9 –100 TH-04-7.2 0.83 1.5 .1009 0.4 620.4 2.5 618.1 2.8 –6 TH-04-8 .0429 2.7 281.6 4.7 282.4 4.6 –1081 TH-04-9 1.61 2.4 .1695 0.8 1014.3 7.5 1012.3 7.4 –14 TH-04-10 0.81 2.1 .1013 0.6 623.7 3.7 622.8 4.1 –18 TH-04-11 0.79 3.4 .0974 0.7 599.8 4.4 598.0 4.9 –14 TH-04-12 0.79 2.4 .0961 0.6 591.2 3.8 589.9 4.2 –3 TH-04-13 0.81 1.7 .0975 0.5 599.3 2.7 597.7 3.1 1 TH-04-14 0.84 1.4 .0986 0.4 604.8 2.6 603.7 2.9 8 TH-04-15 4.04 2.2 .2929 1.1 1660.1 18.0 1655.8 18.2 –3 TH-04-16 3.06 1.1 .2383 0.5 1368.6 6.9 1365.3 7.7 8 TH-04-17 1.37 2.3 .1382 0.7 829.4 5.7 835.8 6.2 15 TH-04-18 1.67 2.8 .1710 0.5 1019.7 4.8 1025.2 4.6 –7 TH-04-18.2 0.43 70.7 .2148 1.4 1349.8 13.1 1348.2 12.4 –312 TH-04-19 0.38 0.6 .0518 0.1 325.4 0.5 326.0 0.5 –2 TH-04-20 0.86 1.4 .1020 0.4 625.6 2.7 625.0 3.0 0 TH-04-21 0.85 7.4 .0834 1.2 506.5 5.3 2444.5 632.5 90 TH-04-22 0.88 2.2 .1049 0.7 643.3 4.4 644.5 4.6 –5 TH-04-23 0.85 5.9 .0979 0.7 599.5 3.6 600.7 4.1 16 TH-04-24 0.95 7.8 .1000 1.3 607.9 6.7 606.3 7.1 35

Moffits Mill Schist A-13 SHRIMP data grain.spot A-13-1.1 0.74 3.6 .0960 1.1 619.8 7.1 630 65 1 A-13-2.1 0.82 2.8 .1024 0.8 618.3 3.7 674 30 9 A-13-3.1 .0876 3.0 609.4 6.1 633 59 4 A-13-4.1 0.75 3.2 .0951 0.8 593.4 4.7 612 37 3 A-13-5.1 0.86 2.2 .1009 0.8 360.9 10.3 –100 A-13-6.1 0.88 1.5 .1034 0.5 592.7 4.5 524 53 –12 A-13-7.1 0.84 4.4 .1033 1.2 620.7 7.2 703 70 13 A-13-8.1 0.84 2.0 .1014 0.7 295.8 2.1 –982 496 –441 A-13-9.1 0.85 3.2 .1012 1.0 294.1 1.6 377 64 28 A-13-10.1 0.86 1.5 .1008 0.5 383.0 2.2 –150 151 –139 A-13-11.1 0.83 2.9 .0992 0.9 306.8 11.3 –100 A-13-12.1 0.80 1.8 .0969 0.7 301.2 1.4 –77 119 –126 A-13-13.1 .0389 14.3 619.8 7.1 630 65 1 A-13-14.1 0.77 2.5 .0964 0.7 618.3 3.7 674 30 9 A-13-15.1 0.88 3.4 .1013 1.0 609.4 6.1 633 59 4 A-13-16.1 0.20 16.8 .0457 0.9 593.4 4.7 612 37 3 A-13-17.1 0.35 2.9 .0468 0.6 360.9 10.3 –100 A-13-18.1 0.37 6.1 .0619 0.5 592.7 4.5 524 53 –12 A-13-19.1 .0399 10.0 620.7 7.2 703 70 13 A A-13-20.1 0.29 4.9 .0473 0.5 295.8 2.1 –982 496 –441

Moffits Mill Schist UB-1-03 SHRIMP data grain.spot UB-1-03-1 1.86 2.6 0.179 1.9 1060 19 1083 34 2 UB-1-03-2 1.93 2.6 0.191 1.9 1125 20 1030 36 –9 UB-1-03-3 1.87 2.3 0.175 1.9 1037 18 1144 26 9 UB-1-03-4R 1.82 2.9 0.185 2.0 1094 20 964 44 –13 UB-1-03-5 1.86 2.3 0.174 1.9 1036 18 1134 25 9 UB-1-03-6 1.95 2.1 0.183 1.8 1083 18 1129 19 4 UB-1-03-7 1.90 2.4 0.184 1.9 1089 19 1062 28 –3 UB-1-03-8 1.67 3.0 0.169 1.9 1005 18 982 46 –2 UB-1-03-9 1.85 2.3 0.174 1.9 1037 18 1116 27 7 UB-1-03-10R 1.75 2.5 0.172 1.9 1024 18 1032 34 1 UB-1-03-11 1.70 2.2 0.170 1.8 1010 17 1007 22 0 UB-1-03-12 1.75 5.2 0.169 2.0 1005 19 1071 97 6 (continued)

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TABLE 2. U-Pb DATA FOR ZIRCON ANALYSES (continued) Sample 207Pb*/235U % error 206Pb*/238U % error 206Pb/238U Absolute 207Pb/206Pb Absolute Discordance (1? ) (1?) Age error Age error (%) 207Pb*/235U 206Pb*/238U (Ma) (1?) (Ma) (1?) Moffits Mill Schist UB-1-03 SHRIMP data grain.spot (continued) UB-1-03-13R 1.72 2.0 0.170 1.8 1014 17 1027 16 1 UB-1-03-14R 1.76 2.0 0.171 1.8 1016 17 1057 19 4 UB-1-03-15 1.95 2.4 0.183 1.9 1085 19 1124 28 3 UB-1-03-16R 0.39 N.D. .0565 1.6 356.2 4.8 181 267 –49 UB-1-03-17 0.91 3.7 .1055 1.8 645.3 N.D. 697 68 8 UB-1-03-18 0.92 3.6 .1040 1.4 633.8 N.D. 736 71 15 UB-1-03-19 0.88 3.6 .1025 1.5 625.3 N.D. 689 70 10 UB-1-03-20 0.52 9.2 .0778 1.5 481.8 7.3 135 214 –72 UB-1-03-21R 0.46 N.D. .0698 1.4 429.4 6.1 79 242 –82 UB-1-03-22R .0514 5.3 336.1 N.D. –2636 0 –916 UB-1-03-23R 0.29 N.D. .0545 2.6 348.5 6.2 –464 N.D. –236 UB-1-03-24 0.85 2.9 .1009 1.6 619.0 N.D. 635 53 3 UB-1-03-25 0.74 4.9 .0952 1.9 586.7 N.D. 470 99 –20 UB-1-03-26 0.90 2.7 .1056 1.4 644.8 9.1 667 51 3 UB-1-03-27 0.82 1.7 .0999 1.2 609.2 9.0 596 27 –3 UB-1-03-28 0.81 3.5 .0984 1.7 604.4 N.D. 589 65 –3 UB-1-03-29 0.83 1.9 .1018 1.2 626.8 9.8 578 33 –8 UB-1-03-30R 0.90 1.4 .1078 1.1 659.4 7.0 614 18 –7

Sample 207Pb*/235U % error 206Pb*/238U % error 206Pb/238U Absolute error 207Pb/235U Absolute 207Pb/206Pb Absolute Discordance (1?) (1 ?) Age (1 ?) Age error Age error (%) 207Pb*/235U 206Pb*/238U (Ma) (Ma) (1 ?) (Ma) (1 ?) Hollis Quartzite TIMS analyses RUTILES *1 0.3421 1.51 0.04666 0.85 294.0 5.0 298.8 4.5 336.6 27 –14 *2 0.398859 8.3 0.0467086 1.28 294.3 3.8 340.8 28.1 671.8 51 –57 3 0.386842 2.5 0.0461451 0.81 290.8 2.3 332.1 8.2 632.2 14 –54 *4 0.47953 2.7 0.0626766 0.23 391.9 0.9 397.8 1.0 432.0 0.5 –9 Note: N.D.—not detectable. *Air abraded.

Migmatized Moffi ts Mill Schist, Uchee between this sample and TH-04, therefore, is the addition, Sm-Nd analysis of a separate sample Terrane absence of any grains older than ca. 600 Ma in of Motts Gneiss collected at White’s Creek this sample. Nonetheless, a whole-rock, Sm-Nd yielded a Tdm value of 820 Ma. The antiquity U-Pb ages were determined for zircons sepa- depleted, mantle model age (Tdm) of 1000 Ma of this model age compared with the crystalli- rated from the leucocratic portion of the mig- considerably older than the age of the igneous zation age of the Motts Gneiss further supports matitic part of Moffi ts Mill Schist sample A-13 protolith strongly suggests the involvement of a crustal origin for this unit, and may indicate (Figs. 2 and 3C). Analysis yielded a data set older (possibly Grenville age-equivalent) crust a mixture of Mesoproterozoic and Neoprotero- similar to that of Phenix City Gneiss sample in the generation of this rock. zoic crust. TH-04 in terms of a protolith crystallization age of ca. 600 Ma and evidence for late Paleozoic Motts Gneiss, Uchee Terrane Hollis Quartzite, Pine Mountain Terrane overprinting (cf. Figs. 4A and 4C). The nine most concordant and oldest grains within an Sample TH-03 is a sample of Motts Gneiss, a We performed U-Pb analysis of metamor- older group of 13 grains (585–635 Ma) yielded quartzofeldspathic gneiss collected ~1 km west phic rutile from a sample of the Hollis Quartzite an error-weighted mean age of 623 ± 7 Ma (2 of type locality at Stroud, Alabama (Figs. 2 and to provide additional constraints on timing of sigma). This is taken to be the age of emplace- 3E). Precise U-Pb isotopic data were not readily Paleozoic tectonometamorphic development in ment of an igneous protolith of the leucosome. acquired from this sample because of the very rocks of the Pine Mountain terrane. Our objec- Younger ages recorded from overgrowths were high degrees of discordance and high concen- tive is to compare and contrast tectonothermal generally late Paleozoic (300–400 Ma). These trations of common Pb in the zircons. These development between the Pine Mountain ter- younger ages are from grains characterized by problems are attributed to the markedly high U rane (footwall block) and the overlying Uchee lower Th/U ratios than the ca. 600 Ma grains contents (up to ~5000 ppm) that characterize terrane (hanging wall), in the area of Alabama and show a general positive correlation of age new mineral growth as both whole grains and and west Georgia where the Bartletts Ferry/ and Th/U ratio, which suggests that some analy- overgrowths. The imprecise data available do Goat Rock fault zones mark their boundary. ses may have incorporated both overgrowth and suggest, however, that the Motts is a late Paleo- Muscovite and K-feldspar dates of 287–277 original grain. The three analyses with the lowest zoic intrusive rock. Limited data from cores to Ma and ca. 260 Ma, respectively, help to con- Th/U (<0.05) are concordant and have 206Pb/238U these younger grains suggest signifi cant interac- strain the lower temperature parts of the Pine ages of ca. 300 Ma. The primary difference tion with (or melting of) ca. 600 Ma crust. In Mountain cooling path (blocking temperatures

138 Geosphere, February 2008

TABLE 3. Sm-Nd ANALYTICAL RESULTS terrane, however, lies between the Uchee and Sample Rock unit 143Nd/144Nd Sm Nd 147Sm/144Nd Nd TDM Savannah River terranes. Unfortunately, the (ppm) (ppm) (Ga) Milledgeville terrane has not been mapped in TH-03 Motts Gneiss 0.512570 1.53 7.50 0.123186 –1.4 0.814 detail, and we are not aware of any structural or UB-1-03 Moffits Mill Schist 0.512486 4.74 23.2 0.123793 –2.9 0.946 geochronological data from it (see also Hibbard UB-2-03 Moffits Mill Schist 0.512528 3.71 17.9 0.125450 –2.1 0.897 et al., 2002). Geologic maps of Georgia (Crick- may, 1933; Pickering, 1976) only indicate that rocks underlying the area of the Milledgeville of ~350–250 °C, respectively), but problems terrane had occupied middle-crustal levels dur- terrane comprise upper greenschist and/or with extraneous argon in hornblendes (blocking the Alleghanian event and that it was joined lower amphibolite-facies phyllite, schist, and ing temperature of ~500 °C) prohibited glean- with the Uchee terrane either prior to or during quartzite. ing meaningful thermochronologic information this event before being uplifted and cooled as a If the Uchee and Savannah River terranes are on the higher temperature parts of the trajectory somewhat coherent block. correlative, then the Milledgeville might be its (Steltenpohl and Kunk, 1993). Rutile, an acces- stratigraphic cover, and, together, they would sory phase in the Hollis Quartzite, has a U-Pb DISCUSSION compose a major terrane internal to the Carolina closure temperature of ~400–450 °C (Mezger zone. Its boundary with the Pine Mountain and et al., 1989), making it useful for this purpose. Peri-Gondwanan or Gondwanan origin for Charlotte terranes, that is, the Bartletts Ferry/ Red rutile grains were separated from sample the Uchee Terrane Goat Rock and Modoc fault zones, respectively, H-1 (Fig. 2). Three rutile grains analyzed for might mark a suture between two volcanic arcs U-Pb isotopes via TIMS (thermal ionization Our 620 to 640 Ma dates are clearly correla- of different ages. This is conjectural, of course, mass spectrometer) exhibited very low Pb con- tive with Pan-African and/or Trans-Brasiliano without geological and geochronological infor- tents (<10 ppm) and yielded nearly concordant elements and strongly suggest an exotic peri- mation that better constrains the evolution of 206Pb/238U ages of 291–294 Ma (Fig. 4D). One Gondwanan or Gondwanan origin for the Uchee the Milledgeville and its boundary zones with of these is essentially concordant at 294 ± 5 Ma terrane protoliths. The Uchee terrane, therefore, adjacent terranes. The age range for Uchee ter- (2 sigma). One rutile grain had a higher Pb con- is likely to be another arc infrastructure terrane rane zircons (620–640 Ma) may alternatively tent (~290 ppm) and an older 206Pb/238U age of component of the Carolina zone of Hibbard et al. suggest a link to the Gondwanan Suwannee 392 Ma. Attempts to date sphene grains from (2002). The plutonic, volcanic, and volcaniclas- terrane (Figs. 1 and 5). Granodiorites intruded the same Hollis sample, and thereby to obtain tic protoliths imply that the Uchee terrane may the Suwannee terrane between 600 and 625 Ma a metamorphic age corresponding to the closure comprise parts of both the plutonic root and the (Heatherington et al., 1993), but little is known temperature of sphene (~600 °C, Tucker et al., cogenetic supracrustal ediﬁ ce to this arc. Because about the country rocks for these plutons 1987; Heaman and Parrish, 1991), were unsuc- many of the zircons we dated are schistosity- or (Guthrie and Raymond, 1992; Steltenpohl et

cessful, because all Pb in the sphene was found gneissosity-forming minerals (i.e., S0/S1 in Figs. al., 1995). In addition, the Suwannee contains to be common (zero-age) Pb. 3B, 3C, and 3F), the upper amphibolite-facies a suite of ca. 550 Ma plutons and coeval fel-

These schistosity-forming metamorphic metamorphic imprint, M1, occurred some time sic to basaltic-andesitic volcanics (Dallmeyer, rutiles are interpreted to record isotopic closure after the Neoproterozoic (see Fig. 5). The Neo- 1987; Heatherington et al., 1996), which have through the ~450 °C isotherm at ca. 295 Ma proterozoic relics in the Uchee terrane likely not yet been detected within the Uchee terrane. while cooling from an earlier Paleozoic amphib- had developed outboard of Laurentia, across the The Paleozoic portion of the T-t trajectory as olite-facies metamorphic event. The rutile date Iapetus ocean, along the Gondwanan margin or presently constrained for the Suwannee terrane is consistent with the 287 to 277 Ma 40Ar/39Ar one of its peripheral island arcs, prior to forma- is ~30 m.y. older than that for its Uchee foot- muscovite dates (Steltenpohl and Kunk, 1993), tion of the eastern Laurentian, early Paleozoic, wall terrane for cooling below biotite closure thus placing constraints on the higher tempera- passive margin (Fig. 5). (~300 °C) (Fig. 5; Steltenpohl et al., 1995). ture part of the Pine Mountain terrane tempera- Compared to other ca. 600 Ma infrastruc- This is consistent with subsequent extension ture-time (T-t) path (Fig. 5). The precise timing tural terranes within the Carolina zone, the and southward down-dropping of the Suwan- of the metamorphic peak, unfortunately, remains Uchee terrane is among some of the oldest and nee terrane from higher crustal levels as is doc- elusive. Deposition of the Hollis Quartzite had to appears most similar to the Savannah River ter- umented by major Mesozoic rift basins along postdate 831 Ma based on detrital zircons within rane (Maher et al., 1991; Dennis et al., 2004). the northern edge of the terrane (see Chowns it (Steltenpohl et al., 2004) and, as described The Savannah River terrane mainly comprises and Williams, 1983, and Guthrie and Raymond, above, the suggested Cambro-Ordovician age migmatitic gneiss, paragneiss, and schist with 1992, and references therein). for the Pine Mountain Group is speculative. pods of maﬁ c and ultramaﬁ c rock in the core of Steltenpohl et al. (2004) report a 354 ± 140 Ma a broad foliation warp. It was constructed and Paleozoic (Appalachian) Evolution of the lower intercept U-Pb age on detrital zircons metamorphosed to upper amphibolite facies at Uchee Terrane from the Hollis Quartzite, which is consistent 620 Ma, distinctly earlier than all other Caro- with fossil evidence for post-early Mississippian lina zone terranes. It also escaped any other Taken as a whole, our U-Pb zircon data (Fig. metamorphism in the nearby Talladega slate belt metamorphic overprint until the Alleghanian, 4E) indicate two principal age populations in (Fig. 2; Gastaldo et al., 1993; McClellan et al., when it, too, was intruded by granitoidal plu- Uchee terrane rocks—one is Neoproterozoic (ca. 2007) and a <337 Ma 40Ar/39Ar date suggested tons (Maher et al., 1991). The Savannah River 620 Ma) and the other is Carboniferous (ca. 300 for hornblende from the Pine Mountain window terrane is geographically close to the Uchee Ma). Although more dates are needed to verify (Steltenpohl and Kunk, 1993). Nonetheless, the terrane and is similarly bounded partly by the this “end-member” age distribution, the lack of T-t path corroborates the suggestion of Stelten- major Alleghanian Modoc shear zone (Maher intermediate “Appalachian” ages (e.g., Acadian pohl and Kunk (1993) that the Pine Mountain et al., 1991). The suprastructural Milledgeville or Taconian) is conspicuous, and combined with

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Figure 4. Collage of U-Pb isotopic results; all error envelopes are 2 sigma. (A) Tera-Wasserburg plot of U-Pb data obtained from individual zircons from Phenix City Gneiss sample TH-04 using the SHRIMP-RG. (B) Combination histogram/relative probability plot of U-Pb ages of individual zircons from the Mofﬁ ts Mill Schist sample UB-1-03 using the SHRIMP-RG. (C) Tera-Wasserburg plot of U-Pb data from individual zircons extracted from the leucocratic phase of a migmatitic part of Mofﬁ ts Mill Schist sample A-13 using the SHRIMP-RG. (D) Tera-Wasserburg concordia plot of U-Pb TIMS data from four rutile grains extracted from Hollis Quartzite sample H-1. (E) Histogram plotting U-Pb ages determined for all Paleozoic zircons, mostly rims, from Uchee terrane samples reported herein.

140 Geosphere, February 2008

these late-stage intrusives, they appear to support the assertion that high-grade Alleghanian metamorphism and associated felsic plutonism extended well into the southernmost exposures of the Appalachians in Alabama. The Uchee terrane, thus, contains a rich and diverse history of Alleghanian high-grade metamorphism, deformation, and plutonism that we are only begin- ning to understand.

Suturing of the Pine Mountain and Uchee Terrane

Confi rmation of peri-Gondwanan rocks of the Carolina zone in direct contact with Gren- ville basement and its stratigraphic cover of the Pine Mountain window in Alabama and Georgia (Figs. 1 and 2) is signifi cant. The Bartletts Ferry/ Goat Rock fault zone marks this juxtaposition, and it appears to be the only place in the southern Appalachians where Carolina zone rocks contact Laurentian crust rather than demonstra- bly “suspect” terranes. Hatcher and Zeitz (1980) referred to the position of the tectonic boundary between Laurentia and the peri-Gondwa- Figure 5. Summary diagram comparing Temperature-time (T-t) paths for the Pine Moun- nan terranes as the “central Piedmont suture,” 40 39 tain, Uchee, and Suwannee terranes in Alabama and westernmost Georgia. Ar/ Ar min- recognizing that the original suture locally was eral cooling dates are summarized from Steltenpohl and Kunk (1993) and Steltenpohl et overprinted by later Paleozoic tectonothermal al. (1992) and combined with U-Pb zircon and rutile dates from this report. Analytical age affects. West (1998) interpreted this boundary to errors and dates between the multiple samples lie within the size of the symbols plotted. be the late Paleozoic suture, whereas Hibbard et al. (2002) argued that the original suture formed earlier and was excised by later shearing. reported fi eld and fabric observations, suggests barometrically constrained average metamor- Our results demonstrate that the central a two-stage evolution: stage one—Neoprotero- phic fi eld gradient of 35 °C/km (Chalokwu Piedmont suture in Alabama and west Georgia zoic construction of the Uchee arc; and stage and Steltenpohl, 1989) yields an unrealistically is the Bartletts Ferry/Goat Rock fault zone. In two—Carboniferous accretion, “Alleghanian” high uplift rate (5.9 cm/yr). The signifi cance central Georgia, we suggest that the Bartletts amphibolite-facies metamorphism, deforma- of this rapid uplift is not yet clear. Steltenpohl Ferry/Goat Rock fault zone likely terminates or tion, and plutonism. and Kunk (1993) interpreted petrologic, mineral merges with the Modoc fault zone, and/or per- The time of “peak” metamorphism associ- cooling, and structural data to indicate that fol- haps other faults (e.g., Dean Creek fault; Hooper ated with stage two is recorded by the ca. 300 lowing tectonic thickening, the once structur- and Hatcher, 1988), as we depict in Figures 1 and Ma overgrowths on zircons in all rocks we ally higher Inner Piedmont terrane was brought 2. Figure 5 compares the temperature-time (T-t) have analyzed from the Uchee terrane (Fig. 5). down along oblique, right- and normal-slip evolution of the Uchee (hanging wall block) and This timing is compatible with 295 to 288 Ma faults fl anking the northwest margin of the com- Pine Mountain (footwall block) terranes across 40Ar/39Ar hornblende cooling dates and docu- bined Pine Mountain and Uchee terranes (Kish, this fault zone along the Chattahoochee River, ments upper amphibolite-facies metamorphism 1988; Schamel et al., 1980; Sears et al., 1981a; which marks the state line between Georgia and and deep tectonic burial (~35 km; Chalokwu, Steltenpohl 1988, 1990; Keefer, 1992; Babaie et Alabama in Figures 1 and 2. The ca. 295 Ma 1989) of the Uchee terrane contemporaneous al., 1991; Hadizadeh et al., 1991), tectonically U-Pb date on rutile from the Hollis Quartzite with the Alleghanian orogeny recorded in the unroofi ng the younger, underlying Alleghanian corroborates Steltenpohl and Kunk’s (1993) foreland (Steltenpohl and Kunk, 1993). A host metamorphic core. This interpretation is con- suggestion that, like the Uchee terrane, the Pine of Uchee terrane structures, including the Goat sistent with either late Alleghanian (Permian) Mountain terrane had occupied middle-crustal Rock/Bartletts Ferry fault zone mylonites and gravitational collapse (Maher, 1987; Steltenpohl levels (amphibolite-facies conditions; see Fig.

S2 and L2 fabrics (Figs. 3 and 5), formed within et al., 1992) or early Mesozoic rifting. 5) during the Alleghanian event. The rutile dates this time span (Steltenpohl and Kunk, 1993; Although a precise U-Pb date on zircons also are consistent with reported ﬁ eld (Stelten- Steltenpohl et al., 1992). Subsequent to meta- from the Motts Gneiss was not determined, the pohl, 1988) and isotopic data (Steltenpohl et al., morphism, rocks of the Uchee terrane cooled suggestion of a late Paleozoic age is consistent 1992; Steltenpohl and Kunk, 1993) indicating from ~780 to ~300 °C (zircon U-Pb and biotite with ﬁ eld relations and existing mineral cooling synmetamorphic juxtaposition of the Uchee 40Ar/39Ar blocking temperatures, respectively; dates supporting that it and the younger suite of and Pine Mountain terranes along the Bartletts Mezger and Krogstad, 1997; Harrison et al., Hospilika Granite likely are Alleghanian intru- Ferry/Goat Rock fault zone. P-T-t trajectories 1985) between ca. 300 and 276 Ma, implying sives. Although additional studies are needed to for the two terranes merge, within analytical a very high rate of uplift. Using the thermo- more tightly constrain the emplacement age of uncertainty, at ca. 295 Ma, and thereafter they

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followed similar paths (Fig. 5). Thus, the two Georgia leaves an incomplete picture of its full ment. Toward this end, new analysis of aero- terranes were joined either prior to or during the geographic extent. magnetic and gravity data penetrating the thin ca. 300 Ma metamorphic event and then were There is no obvious suggestion in the U-Pb veneer of Coastal Plain units in the southeast- uplifted and cooled as a more or less coherent data for pre-Alleghanian, Appalachian events ern USA indicates an anastomosing network of block (Steltenpohl and Kunk, 1993). (i.e., Taconian or Acadian) having affected mylonite zones in the subsurface that extends to It now is clear that the Pine Mountain and rocks of the Uchee terrane. Combined with ini- and appears to merge with the Suwannee suture Uchee terranes in Alabama and west Georgia are tial fabric studies, this information allows for the imaged only 30 km south of the onlap bound- part of a potentially large, amphibolite-facies, following scenario: (a) Neoproterozoic relics ary (Hatcher et al., 2006). The aggregate width Alleghanian tectonothermal zone. The western in the Uchee terrane developed prior to forma- of the Carolina zone along the South Carolina margin of this Alleghanian tectonothermal zone tion of the eastern Laurentian, early Paleozoic, and North Carolina state line is roughly 450 km, appears to coincide with the western margin of passive margin across the Iapetus ocean either but it drastically narrows to less than 30 km in the Pine Mountain window, where the Towa- along the Gondwanan margin or peripheral the area where we are working in Alabama and liga and associated faults have down-dropped to it; (b) the Uchee was rafted across Iapetus Georgia. Furthermore, the new aeromagnetic Piedmont rocks toward the west (Steltenpohl while the Taconic and Acadian orogenies were maps indicate complete excision of the Carolina and Kunk, 1993; see cross sections in Figs. 1 taking place along the Laurentian margin; and zone only a few tens of kilometers to the west and 2). It appears likely that this same Allegha- (c) it fi nally docked with Laurentia during the (Fig. 1). Future work in this area, therefore, is nian thermal and deformational zone continues Alleghanian event (prior to ca. 295 Ma). On the crucial for establishing a lithosphere-scale cross southward beneath the Coastal Plain at least 30 other hand, the four samples we analyzed are section relating the surface faults to both the km where the Alleghanian Suwannee suture is not a robust statistical sampling, and more work Carolina zone and Suwannee sutures and to the interpreted to occur (Higgins and Zeitz, 1983; may be needed to recognize evidence for middle master décollement as well. Horton et al., 1984; Guthrie and Raymond, Paleozoic Appalachian activity. 1992; Hatcher et al., 2006; see Fig. 1). We are Our report brings up many new questions and REFERENCES CITED not aware of any modern thermochronological emphasizes the fact that much work remains to Adams, G.I., 1933, General geology of the crystallines of data constraining the along-strike boundary to be done on the Uchee and Pine Mountain ter- Alabama: The Journal of Geology, v. 41, p. 159–173. this Alleghanian tectonothermal zone northeast ranes. The Uchee terrane needs to be systemati- Babaie, H.A., Hadizadeh, J., Babaie, A., and Ghazi, A.M., of the Alabama-Georgia state line. It may merge cally mapped to determine how relations docu- 1991, Timing and temperature of cataclastic deformation along segments of the Towaliga fault, western Geor- with the well-documented Alleghanian zone in mented in Alabama and westernmost Georgia gia, U.S.A: Journal of Structural Geology, v. 13, no. 5, the east Georgia and South Carolina Piedmont carry out into its northeastern extent as well as p. 579–586, doi: 10.1016/0191-8141(91)90044-J. Bentley, R.D., and Neathery, T.L., 1970, Geology of the (i.e., Savannah River and Dreher Shoals ter- its adjacent terranes. It is particularly critical to Brevard Fault Zone and related rocks of the Inner Pied- ranes), which projects roughly along strike of understand how the Uchee terrane relates to the mont of Alabama: Alabama Geological Society, 8th the Uchee trend within the hanging-wall block Milledgeville and Savannah River terranes and Annual Field Trip Guidebook, 119 p. Bream, B., Hatcher, R.D., Jr., Miller, C., and Fullagar, P., to the Modoc fault zone (Dallmeyer et al., 1986; how they all fi t into the larger picture of Caro- 2000, Paragneiss geochemistry and preliminary ion Secor et al., 1986; West et al., 1995). lina zone evolution. The time of the “peak” of microprobe geochronology of detrital zircons from metamorphism in the Pine Mountain cover rocks the southern Appalachian crystalline core: Geological Society of America Abstracts with Programs, v. 32, CONCLUSIONS AND DIRECTIONS FOR remains largely unconstrained, although it is key no. 7, p. A-31. FUTURE STUDIES to placing a maximum age on docking of the Car- Bream, B., Hatcher, R., Miller, C., and Fullagar, P., 2004, 40 39 Detrital zircon ages and Nd isotopic data from the south- olina zone. High-precision, U-Pb and Ar/ Ar ern Appalachian crystalline core, Georgia, South Caro- The Uchee terrane of Alabama and west thermochronological data also are sorely needed lina, North Carolina, and Tennessee: New provenance Georgia contains 620 to 640 Ma zircons that from rocks marking the northeast terminus of constraints for part of the Laurentian margin. Geological Society of America Memoir 197, p. 459–475. indicate incorporation of Pan-African and/or the Pine Mountain window to establish timing Chalokwu, C.I., 1989, Epidote-amphibolite to amphibolite Trans-Brasiliano elements and support an exotic of formation of major Appalachian fault zones facies transition in the southern Appalachian Piedmont: peri-Gondwanan or Gondwanan origin. This (i.e., Towaliga, Goat Rock, and Modoc zone) P-T conditions across the garnet and calc-silicate iso- grads: Geology, v. 17, p. 491–494, doi: 10.1130/0091- Neoproterozoic crust constitutes a sizable area and their interaction with the Box Ankle fault 7613(1989)017<0491:EATAFT>2.3.CO;2. in the southernmost exposed part of the Carolina zone. In addition to being a suspect for the Caro- Chalokwu, C.I., and Hanley, T.B., 1990, Geochemistry, petrogenesis, and tectonic setting of amphibolites from the zone, and it is among the oldest infrastructural lina zone suture, the Box Ankle fault zone has southernmost exposure of the Appalachian Piedmont: terranes within the zone (i.e., Savannah River). also been interpreted as an exposed segment of The Journal of Geology, v. 98, p. 725–738. The U-Pb data document amphibolite-facies the “Appalachian décollement” (Schamel et al., Chalokwu, C.I., and Steltenpohl, M.G., 1989, Thermo- barometry and thermochronology of the southern metamorphism and mid-crustal–level tectonic 1980; Sears et al., 1981a; Nelson et al., 1987; Appalachian Piedmont: Geological Society of America burial of the combined Uchee (hanging wall) Higgins et al., 1988; Steltenpohl and Kunk, Abstracts with Programs, v. 21, no. 6, p. A142. and Pine Mountain (footwall) terranes con- 1993; Steltenpohl and Moore, 1988; Steltenpohl Chowns, T.M., and Williams, C.T., 1983, Pre-Cretaceous rocks beneath the Georgia Coastal Plain—Regional temporaneous with the Alleghanian orogeny et al., 1992; West et al., 1995) that had passed implications, in Gohn, G.S., ed., Studies related to the recorded in the foreland. The two terranes were above an autochthonous/parautochthonous Charleston, South Carolina, earthquake of 1886—Tec- tonics and seismicity: U.S. Geological Survey Profes- joined by ca. 295 Ma and then were uplifted Pine Mountain terrane. McBride et al. (2005) sional Paper 1313-L, 42 p. and cooled as a somewhat coherent block. recently resynthesized and reinterpreted seis- Clarke, J.W., 1952, Geology and mineral resources of the The northwest margin of this “high-grade,” mic data to provide evidence for scattered and Thomaston quadrangle, Georgia: Georgia Geological Survey, Bulletin 59, 99 p. Alleghanian tectonothermal zone partly coin- weak subhorizontal refl ectors beneath the Pine Crickmay, G.W., 1933, The occurrence of mylonites in the cides with the Towaliga fault along the west Mountain window, but the controversy remains crystalline rocks of Georgia: American Journal of Sci- fl ank of the Pine Mountain window, but the unresolved. It is critical to understand how sur- ence, v.26, p.161–177. Crickmay, G.W., 1952, Geology of the crystalline rocks 40 39 lack of Ar/ Ar mineral cooling data from the face faults exposed around the Pine Mountain of Georgia: Georgia Geological Survey Bulletin 46, northeast terminus of the window in central window relate to the suture and the décolle- p. 32–36.

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