J. metamorphic Geol., 2003, 21, 65–80 Deep in the Heart of Dixie: Pre-Alleghanian Eclogite and HP Granulite Metamorphism in the Carolina Terrane, South Carolina, USA J. W. SHERVAIS,1 A. J. DENNIS,2 J. J. MCGEE3 AND D. SECOR3 1Department of Geology, Utah State University, Logan UT, 84322, USA ([email protected]) 2Department of Geology and Biology, University of South Carolina, Aiken SC 29801, USA 3Department of Geological Sciences, University of South Carolina, Columbia SC, 29208, USA ABSTRACT The central part of the Carolina terrane in western South Carolina comprises a 30 to 40 km wide zone of high grade gneisses that are distinct from greenschist facies metavolcanic rocks of the Carolina slate belt (to the SE) and amphibolite facies metavolcanic and metaplutonic rocks of the Charlotte belt (to the NW). This region, termed the Silverstreet domain, is characterized by penetratively deformed felsic gneisses, granitic gneisses, and amphibolites. Mineral assemblages and textures suggest that these rocks formed under high-pressure metamorphic conditions, ranging from eclogite facies through high-P granulite to upper amphibolite facies. Mafic rocks occur as amphibolite dykes, as metre-scale blocks of coarse-grained garnet-clinopyroxene amphibolite in felsic gneiss, and as residual boulders in deeply weathered felsic gneiss. Inferred omphacite has been replaced by a vermicular symplectite of sodic plagioclase in diopside, consistent with decompression at moderate to high temperatures and a change from eclogite to granulite facies con- ditions. All samples have been partially or wholly retrograded to amphibolite assemblages. We infer the following P-T-t history: (1) eclogite facies P-T conditions at ‡ 1.4 GPa, 650–730 °C (2) high-P granulite facies P-T conditions at 1.2–1.5 GPa, 700–800 °C (3) retrograde amphibolite facies P-T conditions at 0.9–1.2 GPa and 720–660 °C. This metamorphic evolution must predate intrusion of the 415 Ma Newberry granite and must postdate formation of the Charlotte belt and Slate belt arcs (620 to 550 Ma). Comparison with other medium temperature eclogites and high pressure granulites suggests that these assemblages are most likely to form during collisional orogenesis. Eclogite and high-P granulite facies metamorphism in the Silverstreet domain may coincide with a 570–535 Ma event documented in the western Charlotte belt or to a late Ordovician-early Silurian event. The occurrence of these high-P assemblages within the Carolina terrane implies that, prior to this event, the western Carolina terrane (Charlotte belt) and the eastern Carolina terrane (Carolina Slate belt) formed separate terranes. The collisional event represented by these high-pressure assemblages implies amalgamation of these formerly separate terranes into a single composite terrane prior to its accretion to Laurentia. Key words: amphibolite; Carolina terrane; southern Appalachians; eclogite; HP granulite. during collision (e.g. Carswell, 1990; O’Brien & Ro¨ t- INTRODUCTION zler, 2003). In many areas, these rocks are commonly High-pressure granulites, characterized by the ortho- associated with retrogressed felsic gneisses that were pyroxene-free assemblage Grt + Cpx + Pl ± Qtz, originally cofacial with the enclosed eclogites (e.g. comprise a newly recognized subfacies transitional Cuthbert & Carswell, 1990; Cuthbert et al., 2000; between plagioclase-free eclogites and orthopyroxene- O’Brien et al., 1990). bearing granulites (Pattison, 2003). O’Brien & Ro¨ tzler, The eastern margin of North America in the 2003) distinguished two varieties of high-P granulite: southern and central Appalachians comprises a tec- ultra-high temperature assemblages with melt reaction tonic collage of terranes that formed in exotic locations textures, and medium-T, high-P assemblages (700– during the late Neoproterozoic through early Palaeo- 850 °C, 1.0–1.4 GPa) that overprint former eclogite zoic, and were subsequently accreted to Laurentia facies assemblages. Like medium temperature (MT) during the mid- to late Palaeozoic (Williams & eclogites (Carswell, 1990), the medium-T, high-P Hatcher, 1983; Secor et al., 1983; Horton et al., 1989, granulite subfacies is typically associated with colli- 1991; van Staal et al., 1998). These exotic terranes sional orogens, which form in tectonically thickened evolved independently of Laurentia for much of their arc or continental crust, typically in response to the existence, and preserve evidence of orogenic and attempted subduction of an arc or continental margin magmatic events that are not observed in Laurentia. Ó Blackwell Science Inc., 0263-4929/03/$15.00 65 Journal of Metamorphic Geology, Volume 21, Number 1, 2003 66 J. W. SHERVAIS ET AL. Fig. 1. Regional geology of the southern Appalachians, showing principal sub-divi- sions, including the Carolina terrane (pale grey), the Blue Ridge terrane (dark grey), the Inner Piedmont terrane, and the Atlantic Coastal Plain. Rocks of the Inner Piedmont terrane (including the Chauga belt) and the Carolina terrane (including the Charlotte belt, the Carolina Slate belt, and the Kings Mountain belt [KMB]) are all exotic to North America. One of the most extensive of these exotic peri- The Carolina terrane has been divided into three belts with dif- Gondwana terranes is the Carolina terrane, which ferent metamorphic and petrological characteristics: (1) the Kings Mountain belt, which consists of greenschist facies mafic metavol- comprises a large portion of the southern Appalachian canic rocks and forms the north-western margin of the Carolina orogen east of the Blue Ridge province (Secor et al., terrane; (2) the Charlotte belt, which consists largely of lower to 1983; Fig. 1). The Carolina terrane is an exotic middle amphibolite facies, dominantly mafic metavolcanic and meta- Avalonian terrane that originally formed adjacent to plutonic rocks; and (3) the Carolina Slate belt, which is dominated by Gondwana in the late Neoproterozoic, and was not low-grade (greenschist to subgreenschist) felsic metavolcanic rocks with subordinate mafic lavas and mudstones (Fig. 1). accreted to Laurentia until the mid- to late Palaeozoic The Carolina terrane was metamorphosed and ductilely deformed (Secor et al., 1983; Williams & Hatcher, 1983). during the latest Neoproterozoic to early Cambrian (Dennis & We have recently re-examined a little known Wright, 1995, 1997; Hibbard & Samson, 1995; Barker et al., 1998). occurrence of high-P granulite and amphibolite, with Metamorphism and ductile deformation resulting from the Alle- ghanian (320 Ma) collision of Laurentia and Gondwana is an inferred MT eclogite precursor, within the central restricted to narrow shear zones which separate broad zones con- part of the Carolina terrane (Dennis et al., 2000). taining older fabric and mineral assemblages (e.g. Secor et al., 1986; These rocks, which were originally interpreted as Dallmeyer et al., 1986; Horton et al., 1989; Horton & Dicken, 2001). pyroxene-bearing garnet amphibolites, contain relict The Charlotte belt was intruded by a suite of undeformed Devonian garnet-pyroxene-plagioclase assemblages that record a gabbros and granitoids (400 Ma; McSween et al., 1991) that cross- cut regional foliation and mark the upper age limit of penetrative previously unrecognized episode of eclogite trans- deformation within most of the terrane. itional to medium temperature HP granulite facies The exotic nature of the Carolina terrane is shown clearly by the metamorphism within the Carolina arc terrane. This occurrence of a diverse Middle Cambrian peri-Gondwanan trilobite event has broad implications for the evolution of the fauna in the Carolina Slate belt (Samson et al., 1990). In addition, combined field-geochronological studies have shown that metamor- southern Appalachians, and for models of metamor- phic fabric in most of the Carolina terrane formed prior to 535 Ma, phism and exhumation in accreted arc terranes approximately coeval with the rift-drift transition on the Laurentian in general. We present here a first look at these margin (Dennis & Wright, 1995, 1997; Hibbard & Samson, 1995; newly discovered high pressure rocks, their inferred Barker et al., 1998). P–T–t history and some tectonic implications of their occurrence. Field Occurrence of High-Pressure Rocks The boundary between the Charlotte belt and the Slate belt in central ECLOGITES AND GRANULITES OF THE South Carolina comprises a 30-km wide zone of high grade gneisses CAROLINA TERRANE that are distinct from less highly deformed amphibolite facies (dominantly) mafic rocks of the Charlotte belt (to the northwest) and low-grade felsic metavolcanic rocks of the slate belt (to the south- Regional Setting east; Fig. 2). This region, termed the Silverstreet domain, consists of The Carolina terrane in the southern Appalachians is a calc-alkaline high-grade felsic biotite gneisses, granitic gneisses, and amphibolites island arc that is exotic to Laurentia and does not share a common that form the SE margin of the Charlotte belt (Secor et al., 1982, history with North America until the late Palaeozoic Alleghanian 1988; Halik, 1983; Hauck, 1984). The Silverstreet domain is intruded orogeny (Fig. 1). It is largely Neoproterozoic in age but includes by the undeformed early Devonian Newberry granite (415 ± 9 Ma; sections of early to middle Cambrian age (Secor et al., 1983; Samson Fullagar, 1981; Samson & Secor, 2000), which cross-cuts regional et al., 1990; Shervais et al., 1996; Dennis & Shervais, 1996; Wortman foliation and includes xenoliths of sheared and foliated