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 1 Department of Geology, Utah State University, Logan UT, 84322, USA ([email protected]) 2 Department of Geology and Biology, University of South Carolina, Aiken SC 29801, USA 3 Department 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 Inferred omphacite has been replaced by a vermicular symplectite of sodic plagioclase in diopside, consistent with decompression at moderate to high temperatures and a change from eclogite to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 conditions. 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.

INTRODUCTION

High-pressure granulites, characterized by the orthopyroxene-free assemblage Grt + Cpx + Pl ± Qtz, comprise a newly recognized subfacies transitional between plagioclase-free eclogites and orthopyroxenebearing granulites (Pattison, 2003). O’Brien & Ro¨tzler, 2003) distinguished two varieties of high-P granulite: ultra-high temperature assemblages with melt reaction textures, and medium-T, high-P assemblages (700– 850 C, 1.0–1.4 GPa) that overprint former eclogite facies assemblages. Like medium temperature (MT) eclogites (Carswell, 1990), the medium-T, high-P granulite subfacies is typically associated with collisional orogens, which form in tectonically thickened arc or continental crust, typically in response to the attempted subduction of an arc or continental margin  Blackwell Science Inc., 0263-4929/03/$15.00 Journal of Metamorphic Geology, Volume 21, Number 1, 2003

during collision (e.g. Carswell, 1990; O’Brien & Ro¨tzler, 2003). In many areas, these rocks are commonly associated with retrogressed felsic gneisses that were originally cofacial with the enclosed eclogites (e.g. Cuthbert & Carswell, 1990; Cuthbert et al., 2000; O’Brien et al., 1990). The eastern margin of North America in the southern and central Appalachians comprises a tectonic collage of terranes that formed in exotic locations during the late Neoproterozoic through early Palaeozoic, and were subsequently accreted to Laurentia during the mid- to late Palaeozoic (Williams & Hatcher, 1983; Secor et al., 1983; Horton et al., 1989, 1991; van Staal et al., 1998). These exotic terranes evolved independently of Laurentia for much of their existence, and preserve evidence of orogenic and magmatic events that are not observed in Laurentia. 65

66 J. W. SHERVAIS ET AL.

Fig. 1. Regional geology of the southern Appalachians, showing principal sub-divisions, 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 periGondwana terranes is the Carolina terrane, which comprises a large portion of the southern Appalachian orogen east of the Blue Ridge province (Secor et al., 1983; Fig. 1). The Carolina terrane is an exotic Avalonian terrane that originally formed adjacent to Gondwana in the late Neoproterozoic, and was not accreted to Laurentia until the mid- to late Palaeozoic (Secor et al., 1983; Williams & Hatcher, 1983). We have recently re-examined a little known occurrence of high-P granulite and amphibolite, with an inferred MT eclogite precursor, within the central part of the Carolina terrane (Dennis et al., 2000). These rocks, which were originally interpreted as pyroxene-bearing garnet amphibolites, contain relict garnet-pyroxene-plagioclase assemblages that record a previously unrecognized episode of eclogite transitional to medium temperature HP granulite facies metamorphism within the Carolina arc terrane. This event has broad implications for the evolution of the southern Appalachians, and for models of metamorphism and exhumation in accreted arc terranes in general. We present here a first look at these newly discovered high pressure rocks, their inferred P–T–t history and some tectonic implications of their occurrence. ECLOGITES AND GRANULITES OF THE CAROLINA TERRANE Regional Setting The Carolina terrane in the southern Appalachians is a calc-alkaline island arc that is exotic to Laurentia and does not share a common history with North America until the late Palaeozoic Alleghanian orogeny (Fig. 1). It is largely Neoproterozoic in age but includes sections of early to middle Cambrian age (Secor et al., 1983; Samson et al., 1990; Shervais et al., 1996; Dennis & Shervais, 1996; Wortman et al., 2000). Recent field and geochronological studies show that the Carolina terrane formed during two major episodes of arc magmatism at
620 Ma and
550 Ma (Dennis & Wright, 1997; Heatherington et al., 1996).

The Carolina terrane has been divided into three belts with different metamorphic and petrological characteristics: (1) the Kings Mountain belt, which consists of greenschist facies mafic metavolcanic rocks and forms the north-western margin of the Carolina terrane; (2) the Charlotte belt, which consists largely of lower to middle amphibolite facies, dominantly mafic metavolcanic and metaplutonic rocks; and (3) the Carolina Slate belt, which is dominated by low-grade (greenschist to subgreenschist) felsic metavolcanic rocks with subordinate mafic lavas and mudstones (Fig. 1). The Carolina terrane was metamorphosed and ductilely deformed during the latest Neoproterozoic to early Cambrian (Dennis & Wright, 1995, 1997; Hibbard & Samson, 1995; Barker et al., 1998). Metamorphism and ductile deformation resulting from the Alleghanian (
320 Ma) collision of Laurentia and Gondwana is restricted to narrow shear zones which separate broad zones containing older fabric and mineral assemblages (e.g. Secor et al., 1986; Dallmeyer et al., 1986; Horton et al., 1989; Horton & Dicken, 2001). The Charlotte belt was intruded by a suite of undeformed Devonian gabbros and granitoids (
400 Ma; McSween et al., 1991) that crosscut regional foliation and mark the upper age limit of penetrative deformation within most of the terrane. The exotic nature of the Carolina terrane is shown clearly by the occurrence of a diverse Middle Cambrian peri-Gondwanan trilobite fauna in the Carolina Slate belt (Samson et al., 1990). In addition, combined field-geochronological studies have shown that metamorphic fabric in most of the Carolina terrane formed prior to 535 Ma, approximately coeval with the rift-drift transition on the Laurentian margin (Dennis & Wright, 1995, 1997; Hibbard & Samson, 1995; Barker et al., 1998).

Field Occurrence of High-Pressure Rocks The boundary between the Charlotte belt and the Slate belt in central South Carolina comprises a 30-km wide zone of high grade gneisses 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 southeast; Fig. 2). This region, termed the Silverstreet domain, consists of high-grade felsic biotite gneisses, granitic gneisses, and amphibolites that form the SE margin of the Charlotte belt (Secor et al., 1982, 1988; Halik, 1983; Hauck, 1984). The Silverstreet domain is intruded by the undeformed early Devonian Newberry granite (415 ± 9 Ma; Fullagar, 1981; Samson & Secor, 2000), which cross-cuts regional foliation and includes xenoliths of sheared and foliated country rock. The age of this pluton represents an uppermost age limit for formation and deformation of rocks in the Silverstreet domain. The Silverstreet domain is bounded to the north and south by shear zones. A variety of field studies have demonstrated that the

DEEP IN THE HEART OF DIXIE 67

e nt re R e

tmi Whi Clinton

Joanna

B

?

Newberry NW

c. 414 ± 8 Ma (U-Pb z) Newberry granite

E

Whitmire S

Blair

High grade Silverstreet domain: eclogite, high-P granulite and enclosing felsic gneisses

34¡30'N

edmont ear zone er Pi ek sh e n r n E C I r : eave E r a nt

81¡W

Clinton

+ +

+

+ +

c. 295 ± 4 Ma (Rb-Sr w.r.) Winnsboro granite

Salem X-roads

Little Mountain metatonalite orthogneiss

E known eclogite - high P granulite localities

E

Wateree L.

Newberry granite 414 ± 8 Ma U-Pb z. Cross Hill

L.

Bush River

re

en

E

wo

od

E

S t o ne

Pomaria

Newberry E

Newberry W

E

G

Winnsboro granite 295 ± 2 Rb-Sr w.r.

y H i ll

area of Secor and others (1982) investigation outlined in dash

Jenkinsville

ss ognei

orth

Mesozoic brittle faults Orientation of some major structures in Carolina slate belt and Kiokee belt

Little Mtn metatonalite 550 ± 4 Ma U-Pb z.

82¡W

Dyson

Chappells

Silverstreet

Carolina slate belt and

34¡N

Kiokee b

Prosperity

Little Mtn

elt

7.5' quadrangles indicated and named in SE corner. This report focussed on quadrangles shown in bold.

Chapin

Lake Murray

Columbia Good Hope

Saluda N

Denny

Delmar

Fig. 2. Geological map showing location of eclogite ⁄ high pressure granulite-bearing Silverstreet domain of the Charlotte belt relative to the Carolina Slate belt and the Whitmire reentrant of the Inner Piedmont. Capital ÔEÕ shows location of known eclogite ⁄ granulite blocks. boundary between the high-grade gneisses of the Silverstreet domain and the Carolina Slate belt is a fault over much of its length, but its geometry and kinematics are not known (e.g. Dennis et al., 2000; Offield, 1995; Offield & Sutphin, 2000; Secor et al., 1988, 1989). It is inferred to be a normal fault in part because it juxtaposes high-grade Charlotte belt rocks against low-grade Slate belt rocks. Locally, however, it can be demonstrated that most recent ductile motion along the Stony Hill orthogneiss was right lateral, based on composite planar fabric and asymmetric porphyroclasts (Dennis et al., 2000). The northern margin of the Silverstreet domain is a 10-km wide, E-W trending ductile shear zone (Beaver Creek shear zone) with dextral shear sense indicators that separates it from less deformed rocks of the Charlotte belt (West, 1998). Mafic rocks in less deformed parts of the Silverstreet domain form amphibolite dykes up to
20 cm thick that are oriented parallel to regional foliaton. In more deformed areas, isolated metre-scale blocks within felsic gneiss are interpreted to represent boudinaged mafic dykes (Fig. 3). In many areas, these blocks form residual boulders that have weathered out of the felsic gneisses; where these occur in flat upland terrain they are interpreted to be approximately in place. Retrogressed eclogite and high-P granulite assemblages commonly are preserved in the cores of these isolated blocks. Blocks with relict high-pressure assemblages are found within the Beaver Creek shear zone along the north side of the Silverstreet domain and as residual boulders near the centre of the terrane, south of the Newberry granite, and clearly outside of the Beaver Creek shear zone (Fig. 2). Foliation in the shear zone wraps around the eclogite blocks and clearly postdates eclogite formation.

Mineralogy and Mineral Chemistry Mafic rocks that preserve high-pressure assemblages are modally diverse with
20–40% pink garnet, 20–60% green, diopsidic clinopyroxene, 15–45% hornblende, up to 10% plagioclase, and 3–5% ilmenite, with accessory rutile, epidote, apatite, zircon, titanite and calcite (e.g. Libby & Carpenter, 1969). Relict garnet and pyroxene grains are up to 1 cm diameter, but typical grain size for relict phases is
1–3 mm (Fig. 4). In thin section, hornblende, plagioclase and ilmenite replace clinopyroxene, while garnet is replaced along its margins by kelyphitic intergrowths of plagioclase with hornblende

Fig. 3. Field photo of eclogite block in felsic gneiss of Beaver Creek shear zone. Block is about 1.5 m across, with a trapezoidal shape; foliation in the shear zone wraps around the block. This and other blocks were sampled using portable core drill.

and minor epidote. Calcite forms irregular veins and patches. Epidote, quartz, plagioclase, hornblende and oxides are also found as inclusions in garnet, with epidote being the dominant inclusion phase. Representative electron microprobe analyses from one sample are presented in Table 1; these data are presented graphically in Fig. 5. Analytical methods are presented in Appendix A: Methods. Where it has been well preserved, diopside is characterized by a vermicular symplectite of sodic plagioclase (An15)22) that we infer represents the breakdown of omphacite; this is clearly shown by both BSE images and high-resolution X-ray composition maps of the symplectites (Fig. 6). The diopside contains about 15% jadeite component, but modal reconstruction (see Appendix A: Methods) suggests that primary omphacite contained 30% jadeite. The reconstructed omphacite is presented in Table 1. The breakdown of omphacite to diopside + plagioclase symplectite is commonly observed in high-P granulites after a medium

68 J. W. SHERVAIS ET AL.

Fig. 4. Probe mount (2.5 cm diameter) of eclogite ⁄ granulite from central Carolina terrane, sample NEW-1–3. Pink ¼ garnet, pale green ¼ pyroxene and pyroxene-plagioclase symplectite, brown ⁄ dark green ¼ hornblende, clear ¼ plagioclase or calcite, black ¼ ilmentite or Fe-oxides. Note plagioclase-rich kelyphite rims on garnet. temperature eclogite assemblage, and is consistent with decompression at moderate to high temperatures (e.g. Elvevold & Gilotti, 2000; O’Brien & Ro¨tzler, 2003). The inferred primary assemblage omphacite + garnet ± rutile is consistent with formation under eclogite facies conditions, whereas the observed assemblage diopside + plagioclase + garnet represents high pressure granulite facies conditions (Galan & Marcos, 2000; Cooke et al., 2000; Pattison, 2003; O’Brien & Ro¨tzler, 2003). The breakdown of diopside and garnet to form amphibole + plagioclase + epidote + ilmenite represents final equilibration under amphibolite facies conditions. Formation of amphibolite occurred in two stages. The first is represented by aluminous pargasite and relatively calcic plagioclase (An26)53), which replace diopside + sodic plagioclase symplectites; the second is represented by magnesian pargasite and more sodic plagioclase (An17)22) which replace both diopside and garnet. Calculated hornblende-plagioclase temperatures (next section) suggest that the aluminous pargasite-calcic plagioclase pairs formed at higher temperatures than the magnesian pargasite-sodic plagioclase pairs, and that they are closely associated with the high-P granulite assemblages. High resolution X-ray composition maps of garnet show two distinct growth zones (Fig. 7). The inner zone is enriched in Mn and Fe, the outer zone is enriched in Ca and Mg. The Ca and Fe X-ray maps show a sharp interface between the inner core and the outer mantle, while Mg and Mn show smooth, continuous zoning profiles (Fig. 7). Note that these garnet are generally not symmetrically zoned: the centre of growth typically lies close to one edge of the grain. In the example shown here, several small spessartine-rich garnet cores (seen as high Mn spots in the X-ray maps) have been subsumed by the garnet mantle as it grew. Compositional profiles selected to traverse from the true core to rim confirm these trends. A 1500-lm traverse of the grain mapped in Fig. 7 shows smooth profiles for pyrope (Prp) and spessartine (Sps), and sharp steps in profiles for grossular (Grs) and almandine (Alm; Fig. 8). Profiles for three additional garnet are shown in Fig. 9, scaled to percentage of total grain radius. Although these three grains vary somewhat in their innermost core compositions (Grs, Sps), they display consistent profiles for all elements, with (a) an

inner zone (0 to 55% of radius) that is low in Grs and Prp, and high in Alm and Sps, and (b) an outer zone (60 to 100%) that is higher in Grs and Prp, and lower in Alm and Sps (Fig. 9). All grains exhibit a sharp increase in Grs and decrease in Alm at the transition (c. 60% of grain radius) that implies an abrupt change in growth history.

RESULTS Geothermobarometry

We infer from the data presented above that the garnet cores formed during prograde metamorphism at greenschist or amphibolite facies conditions, followed by growth of the garnet mantles at eclogite (GrtOmp-Rt) and then high-P granulite facies conditions (Grt-Di-Hbl-Pl-Ilm); retrograde metamorphism in the amphibolite facies resulted in the breakdown of garnet and formation of the late Hbl-Pl-Ilm assemblage. Because these rocks experienced a range of metamorphic conditions, a number of assumptions are made in determining which compositions to use for thermobarometry. We assume that the reconstructed omphacite was in equilibrium with the more Mg-Ca-rich mantles of the garnet, and that the diopside-plagioclase symplectites were in equilibrium with the more Mg-Ca-rich, Fe-poor outermost rims of the garnet (e.g. Fig. 9). For purposes of calculation, three garnet mantle compositions were used: (a) an average of all garnet mantles from the profile shown in Table 1a (b) an average of all garnet mantles in Fig. 9, from 65 to 99% of grain radius, and (c) the garnet mantle farthest from the

Table 1a. Garnet analyses, profile of single large garnet crystal in sample NEW-1. Garnet formulae per 12 oxygen. Distance from Centre lm SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Cr2O3 Sum Si Ti Al Fe2+ Mn Mg Ca Na K Cr Pyrope Almandine Spessartine Grossular

Core 1

Core 51

Core 204

Core 255

Core 306

Core 508

Core 559

Core 610

Core 762

Core 813

Core 863

Core 914

Core 1016

Mantle 1117

Mantle 1168

Mantle 1270

Mantle 1371

Mantle 1472

37.54 0.11 21.37 29.37 1.60 2.41 8.62 0.03 0.00 0.00 101.03 2.968 0.006 1.991 1.942 0.107 0.284 0.730 0.004 0.000 0.000 9.3 63.4 3.49 23.8

37.48 0.10 21.40 28.82 1.49 2.33 9.28 0.03 0.00 0.00 100.94 2.963 0.006 1.995 1.906 0.100 0.275 0.786 0.005 0.000 0.000 9.0 62.1 3.26 25.6

37.64 0.09 21.59 28.86 1.27 2.36 9.34 0.03 0.00 0.00 101.19 2.965 0.005 2.004 1.901 0.085 0.277 0.789 0.005 0.000 0.000 9.1 62.3 2.79 25.9

37.67 0.12 21.37 29.07 1.14 2.39 9.41 0.03 0.00 0.00 101.20 2.969 0.007 1.985 1.916 0.076 0.281 0.795 0.004 0.000 0.000 9.2 62.5 2.48 25.9

37.35 0.12 21.54 29.00 1.07 2.40 9.44 0.04 0.00 0.00 100.94 2.952 0.007 2.006 1.917 0.071 0.282 0.799 0.006 0.000 0.000 9.2 62.5 2.31 26.0

37.42 0.10 21.71 28.93 0.81 2.38 9.73 0.03 0.00 0.00 101.11 2.949 0.006 2.017 1.907 0.054 0.280 0.821 0.004 0.000 0.000 9.1 62.3 1.76 26.8

36.94 0.10 21.46 28.88 0.75 2.39 9.87 0.03 0.00 0.00 100.42 2.937 0.006 2.011 1.920 0.051 0.283 0.841 0.005 0.000 0.000 9.1 62.0 1.65 27.2

36.89 0.07 21.44 28.82 0.68 2.48 9.92 0.00 0.00 0.00 100.30 2.936 0.004 2.011 1.918 0.046 0.294 0.845 0.000 0.000 0.000 9.5 61.8 1.48 27.2

37.28 0.13 21.32 28.97 0.47 2.62 9.88 0.01 0.00 0.00 100.68 2.951 0.007 1.989 1.918 0.031 0.309 0.838 0.002 0.000 0.000 10.0 62.0 1.00 27.1

37.27 0.10 21.25 28.93 0.37 2.68 9.84 0.02 0.00 0.01 100.46 2.956 0.006 1.986 1.918 0.025 0.317 0.836 0.003 0.000 0.000 10.2 62.0 0.81 27.0

37.10 0.12 21.30 29.05 0.38 2.76 9.79 0.04 0.00 0.00 100.54 2.943 0.007 1.991 1.927 0.025 0.326 0.832 0.005 0.000 0.000 10.5 62.0 0.80 26.8

37.54 0.07 21.30 29.17 0.32 2.93 9.64 0.00 0.00 0.00 100.98 2.959 0.004 1.979 1.923 0.021 0.345 0.814 0.000 0.000 0.000 11.1 62.0 0.68 26.2

37.39 0.11 21.27 28.46 0.20 3.09 9.90 0.02 0.00 0.00 100.43 2.957 0.007 1.982 1.882 0.013 0.365 0.839 0.003 0.000 0.000 11.8 60.7 0.42 27.1

37.21 0.07 21.46 27.69 0.12 3.34 10.38 0.05 0.00 0.00 100.31 2.941 0.004 1.998 1.830 0.008 0.394 0.878 0.007 0.000 0.000 12.7 58.8 0.26 28.2

37.24 0.17 21.27 27.27 0.12 3.25 10.73 0.06 0.00 0.00 100.12 2.947 0.010 1.984 1.805 0.008 0.383 0.910 0.010 0.000 0.000 12.3 58.1 0.26 29.3

37.81 0.08 21.49 27.20 0.13 3.33 10.71 0.04 0.00 0.03 100.81 2.964 0.005 1.985 1.783 0.009 0.389 0.899 0.006 0.000 0.002 12.6 57.9 0.29 29.2

37.81 0.09 21.46 26.68 0.10 3.39 11.10 0.02 0.00 0.01 100.65 2.964 0.005 1.983 1.750 0.006 0.396 0.932 0.003 0.000 0.001 12.8 56.7 0.19 30.2

37.86 0.17 21.31 26.53 0.12 3.47 11.10 0.04 0.00 0.00 100.59 2.969 0.010 1.970 1.740 0.008 0.405 0.933 0.005 0.000 0.000 13.1 56.4 0.26 30.2

Table 1b. Pyroxene (6 oxygen), hornblende (23 oxygen), and feldspar (8 oxygen) analyses from eclogite ⁄ granulite sample NEW-1. Di

Omp

Hbl adj

Hbl adj

actinolite

Pl in Di

Pl in Di Pl in Di Pl in Di Pl in Di Pl in Di Pl in Di Pl in Di Pl in Di Interstitial Pl Interstitial Pl Interstitial Pl Interstitial Pl Interstitial Pl Interstitial Pl

50.64 0.32 4.2 11.85 0.1 11.45 19.45 1.42 0.05 0.01 99.43 1.917 0.009 0.187 0.375 0.003 0.646 0.789 0.1042 0.002 0.000

51.66 0.18 3.47 11.03 0.07 10.69 20.27 1.74 0.01 0.00 99.10 1.958 0.005 0.155 0.350 0.002 0.604 0.823 0.128 0.001 0.000

51.50 0.26 9.36 9.61 0.08 9.21 16.51 3.47 0.07 0.01 100.00 1.937 0.007 0.415 0.302 0.003 0.516 0.665 0.253 0.003 0.000

42.17 1.11 11.95 18.90 0.07 9.46 11.53 1.93 0.36 0.00 97.48 6.400 0.127 2.137 2.399 0.009 2.140 1.874 0.568 0.070 0.000

41.52 1.11 12.72 18.87 0.08 9.34 11.26 2.08 0.43 0.01 97.41 6.311 0.127 2.278 2.398 0.010 2.116 1.834 0.613 0.083 0.001

48.42 0.73 6.20 15.63 0.06 13.01 11.89 1.01 0.19 0.02 97.14 7.172 0.081 1.083 1.936 0.008 2.871 1.887 0.290 0.035 0.003

62.16 0.51 22.55 0.33 0.00 0.01 4.25 8.24 0.08 0.00 98.14 2.796 0.017 1.196 0.013 0.000 0.001 0.205 0.719 0.005 0.000

63.60 0.02 22.36 0.23 0.02 0.01 3.19 9.10 0.09 0.00 98.61 2.838 0.001 1.176 0.009 0.001 0.001 0.153 0.787 0.005 0.000

64.75 0.00 22.41 0.25 0.00 0.00 3.14 9.00 0.12 0.00 99.66 2.854 0.000 1.165 0.009 0.000 0.000 0.148 0.769 0.007 0.000

63.57 0.02 23.09 0.38 0.00 0.00 3.63 8.87 0.11 0.00 99.67 2.812 0.001 1.203 0.014 0.000 0.000 0.172 0.761 0.006 0.000

63.85 0.00 22.50 0.25 0.00 0.00 3.23 9.06 0.07 0.01 98.97 2.838 0.000 1.179 0.009 0.000 0.000 0.154 0.781 0.004 0.001

63.78 0.02 23.06 0.26 0.00 0.00 3.77 8.83 0.09 0.00 99.80 2.815 0.001 1.200 0.009 0.000 0.000 0.179 0.755 0.005 0.000

64.11 0.01 22.68 0.30 0.00 0.00 3.29 8.89 0.13 0.00 99.43 2.836 0.001 1.183 0.011 0.000 0.000 0.156 0.763 0.007 0.000

64.65 0.03 22.41 0.24 0.00 0.00 2.94 9.29 0.10 0.00 99.65 2.851 0.001 1.165 0.009 0.000 0.000 0.139 0.794 0.005 0.000

63.81 0.08 22.63 0.28 0.00 0.00 3.43 8.91 0.10 0.00 99.24 2.830 0.003 1.183 0.010 0.000 0.000 0.163 0.766 0.006 0.000

59.76 0.04 25.27 0.18 0.00 0.00 6.46 7.39 0.03 0.00 99.13 2.678 0.001 1.335 0.007 0.000 0.000 0.311 0.642 0.002 0.000

58.52 0.06 26.32 0.21 0.00 0.00 7.51 6.87 0.05 0.00 99.54 2.621 0.002 1.390 0.008 0.000 0.000 0.361 0.596 0.003 0.000

60.83 0.00 25.55 0.26 0.00 0.00 5.81 6.92 0.07 0.00 99.43 2.703 0.000 1.338 0.010 0.000 0.000 0.277 0.597 0.004 0.000

60.06 0.00 25.19 0.09 0.00 0.00 5.89 6.75 0.04 0.00 98.02 2.705 0.000 1.337 0.004 0.000 0.000 0.284 0.590 0.002 0.000

58.82 0.00 26.31 0.16 0.00 0.00 7.35 6.42 0.04 0.00 99.09 2.638 0.000 1.390 0.006 0.000 0.000 0.353 0.559 0.002 0.000

54.44 0.00 28.75 0.28 0.00 0.00 10.34 5.22 0.01 0.00 99.04 2.473 0.000 1.539 0.011 0.000 0.000 0.503 0.460 0.001 0.000

DEEP IN THE HEART OF DIXIE 69

SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Cr2O3 Sum Si Ti Al Fe2+ Mn Mg Ca Na K Cr

Di

70 J. W. SHERVAIS ET AL.

Table 1c. Hornblende-plagioclase pairs from retrograded eclogite ⁄ granulite sample NEW-1. Sample#

Hb-1a

Hb-2a

Hb-2b

Hb-3a

Hb-4a

Hb-5b

Hb-6a

Hb-6c

Hb-8

Hb-8a

Hb-9

Hb-10

Hb-11

Hb-12

SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O H2O Total Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K

39.97 1.28 13.58 0.02 6.01 12.56 0.04 9.27 11.61 2.09 0.55 2.00 98.98 6.053 0.146 2.425 0.002 0.685 1.592 0.006 2.092 1.885 0.614 0.106

37.91 1.28 16.39 0.01 6.19 11.83 0.05 8.64 11.72 2.22 0.66 2.00 98.90 5.749 0.146 2.929 0.001 0.707 1.501 0.006 1.952 1.904 0.654 0.128

39.84 1.45 14.33 0.01 5.56 11.86 0.05 9.65 11.65 2.17 0.54 2.00 99.11 5.998 0.164 2.544 0.002 0.630 1.493 0.007 2.165 1.879 0.633 0.103

42.00 0.87 11.69 0.01 6.26 12.34 0.04 10.01 11.72 1.85 0.38 2.00 99.17 6.316 0.098 2.074 0.001 0.709 1.552 0.004 2.243 1.889 0.540 0.073

40.88 1.31 12.54 0.02 6.30 12.97 0.03 9.42 11.55 2.04 0.45 2.00 99.51 6.162 0.149 2.228 0.002 0.715 1.635 0.004 2.115 1.866 0.597 0.087

40.82 0.54 13.80 0.01 6.72 12.95 0.08 8.88 11.63 2.00 0.49 2.00 99.92 6.124 0.061 2.441 0.001 0.759 1.625 0.010 1.987 1.869 0.581 0.093

40.65 0.88 13.16 0.02 6.84 13.96 0.08 8.48 11.67 2.03 0.46 2.00 100.23 6.120 0.100 2.335 0.002 0.775 1.757 0.010 1.902 1.883 0.592 0.088

42.44 1.32 10.33 0.02 5.77 14.21 0.06 9.50 11.42 1.89 0.37 2.00 99.33 6.419 0.151 1.842 0.002 0.656 1.798 0.008 2.141 1.851 0.555 0.072

42.57 1.41 11.03 0.00 5.13 11.81 0.03 11.03 11.44 1.93 0.55 2.00 98.93 6.384 0.159 1.951 0.000 0.579 1.481 0.004 2.465 1.839 0.560 0.105

43.46 1.33 10.22 0.00 5.05 11.42 0.01 11.52 11.83 1.75 0.36 2.00 98.95 6.490 0.149 1.800 0.000 0.567 1.426 0.001 2.563 1.893 0.507 0.069

39.38 0.88 14.98 0.00 7.02 12.28 0.05 8.74 11.54 2.15 0.51 2.00 99.53 5.935 0.100 2.661 0.000 0.796 1.548 0.006 1.964 1.863 0.627 0.097

40.76 0.96 13.43 0.00 6.75 11.44 0.00 10.01 11.97 1.99 0.48 2.00 99.79 6.092 0.108 2.367 0.000 0.760 1.430 0.000 2.229 1.917 0.577 0.092

42.99 1.11 11.04 0.00 5.27 12.31 0.04 10.68 11.79 1.80 0.42 2.00 99.45 6.419 0.125 1.944 0.000 0.592 1.537 0.005 2.377 1.887 0.522 0.080

40.97 1.00 12.97 0.00 6.52 11.93 0.02 9.87 11.85 1.99 0.46 2.00 99.58 6.145 0.112 2.293 0.000 0.736 1.497 0.003 2.206 1.905 0.577 0.089

Plagioclase SiO2 Al2O3 CaO Fe2O3 Na2O K2O Total Si Al Ca Fe3+ Na K Xalbite

Pl-1a 64.89 23.30 3.03 0.21 7.61 0.06 99.10 2.855 1.209 0.143 0.008 0.649 0.004 0.82

Pl-2a 54.44 28.75 10.34 0.28 5.22 0.01 99.04 2.473 1.540 0.503 0.011 0.460 0.001 0.48

Pl-2b 60.06 25.19 5.89 0.09 6.75 0.04 98.02 2.705 1.337 0.284 0.004 0.590 0.002 0.67

Pl-3a 64.71 23.60 3.23 0.13 7.46 0.07 99.19 2.845 1.223 0.152 0.005 0.636 0.004 0.80

Pl-4a 58.82 26.31 7.35 0.16 6.42 0.04 99.09 2.638 1.391 0.353 0.006 0.559 0.002 0.61

Pl-5b 57.81 26.85 7.99 0.22 6.41 0.04 99.31 2.597 1.422 0.385 0.008 0.558 0.002 0.59

Pl-6a 58.09 27.44 8.50 0.18 6.24 0.03 100.47 2.582 1.437 0.405 0.007 0.538 0.001 0.57

Pl-6c 64.30 23.35 3.29 0.25 8.68 0.09 99.95 2.825 1.209 0.155 0.009 0.739 0.005 0.82

Pl-8 64.70 23.46 2.75 0.13 7.46 0.09 98.56 2.856 1.221 0.130 0.005 0.639 0.005 0.83

Pl-8a 65.71 23.49 2.70 0.15 7.42 0.10 99.56 2.869 1.209 0.126 0.005 0.628 0.005 0.83

Pl-9 62.40 24.69 4.84 0.18 7.14 0.06 99.30 2.7620 1.288 0.230 0.007 0.613 0.003 0.73

Pl-10 63.95 23.88 3.53 0.22 7.40 0.07 99.04 2.822 1.242 0.167 0.008 0.633 0.004 0.79

Pl-11 65.38 24.52 2.98 0.20 7.26 0.10 100.46 2.834 1.253 0.139 0.007 0.610 0.006 0.81

Pl-12 60.83 25.55 5.81 0.26 6.92 0.07 99.43 2.703 1.338 0.277 0.010 0.597 0.004 0.68

Wo 50

Diopside Jadeite

600 °C 800 °C 1000 °C

B 1200 °C

1000 °C

Enstatite Reconstructed Omphacite

A

Interstitial Plagioclase

Exsolved Diopside DiHd

Plagioclase Symplectite in Pyroxene to EnFs

Fig. 5. Mineral data from eclogite ⁄ granulite sample NEW-2: (a) Jadeite-high Ca Px-Low Ca Px ternary plot, showing analyzed pyroxene (squares) and reconstructed omphacite (X); (b) pyroxene quadrilateral plot showing analyzed pyroxene (squares), along with temperature contours of Lindsley & Anderson (1983); (c) albite corner of the feldspar ternary, showing compositions of plagioclase symplectite in diopside (open) and interstitial plagioclase associated with hornblende (closed).

centre of the grain in Table 1a (but not the rim), corresponding to the analysis at 1371 lm. For the garnet rim compositions, we used the garnet rim from Table 1a (at 1472 lm) and the average rims of the three garnet shown in Fig. 9. Garnet-pyroxene temperatures were calculated using the calibrations of Ellis & Green (1979) and Powell (1985). Because there is no indication of primary plagioclase in equilibrium with omphacite, only minimum pressures are estimated for the inferred eclogite assemblage using the plagioclase in diopside symplectite as a proxy, using the diopside-plagioclase-garnet-quartz (Newton & Perkins, 1982; Powell & Holland, 1988; Moecher et al., 1988), and albite-jadeite-quartz geobarometers (Holland, 1980). For the high-P granulite assemblage, we used diopside-plagioclase-garnetquartz (Newton & Perkins, 1982; Powell & Holland, 1988; Moecher et al., 1988) to estimate pressure, and the jadeite content of clinopyroxene geobarometer of Carswell & Harley (1990), to give a minimum pressure for the eclogite assemblage. For hornblende-bearing assemblages, the garnethornblende (Graham & Powell, 1984) thermometer and garnet-plagioclase-hornblende-quartz geobarometer (Kohn & Spear, 1989, 1990), were used, taking only the

Fig. 6. X-ray composition maps of diopside-sodic plagioclase symplectites (¼ former omphacite) surrounded by hornblende, plagioclase, garnet, and calcite. (A) Mg map, New-1; (B) Ca map, New-1; (C) Al map, 3080E (D) Al map, 3080E. (A, B) Field of view ¼ 5 mm, hotter colours equal higher concentrations. Note shapes of the diopsideplagioclase-hornblende aggregates, which seem to pseudomorph the primary omphacite. (C, D) Field of view 2.5 mm. Lighter shades ¼ higher concentrations.

A

B

C

D

DEEP IN THE HEART OF DIXIE 71

Fig. 7. X-ray maps of zoned garnet surrounded by hornblende with minor plagioclase and ilmenite. A ¼ Fe, B ¼ Mn, C ¼ Mg, D ¼ Ca. Garnet has high Mn and Fe in core, with higher Mg and Ca in mantle. Note the sharp contact between the inner garnet core and the outer garnet mantle seen clearly in the Fe and Ca X-ray maps. Note also the small garnet cores (high Mn and Fe, low Ca and Mg) that have been subsumed by the garnet mantle. Hotter colours equal higher concentration in Fe, Mn, and Mg; darker blue equals higher Ca. Black line in A is approximate location of line profile (Table 1a). Field of view is 5 mm in all maps.

72 J. W. SHERVAIS ET AL.

Fig. 8. Profile of single large garnet grain, from centre to rim. Distance from rim in lm. Note sharp increase in Grs (CaO) and decrease in Alm (FeO) at around 1150 lm. Pyr (MgO) shows less precipitous increase, Sps (MnO) decays exponentially toward rim.

outermost rim analysis of the garnet in Table 1a and selected hornblende-plagioclase pairs from Table 1c. Hornblende-plagioclase temperatures (Holland & Blundy, 1994) were also calculated using the data from Table 1c, which represent adjacent hornblende-plagioclase pairs from a range of textural associations. We also used GRIPS (garnet-rutile-ilmenite-plagioclasesilica, Bohlen & Liotta, 1986) to estimate pressure, since ilmenite is associated with the hornblende-forming reactions. Calculation of equilibrium T-P conditions was carried out using the program GTB of Spear & Kohn (2001), and the hornblende-plagioclase program of Holland & Blundy (1994). The pyroxene quadrilateral thermometer of Lindsley & Anderson (1983) is used for diopside (but not reconstructed omphacite) because its nonquadrilateral components are less than 20%

(Fig. 5). The results from this graphical solvus thermometer
700–800 C, or up to
50 C higher than results from the garnet rim-diopside thermometers (700–750 C). Garnet-clinopyroxene temperatures were calculated using stoichiometry to partition total Fe between Fe2+ and Fe3+; calculation of Fe3+ stoichiometrically is strongly dependent on analytical precision and generally overestimates Fe3+ in pyroxene due to vacancies in the pyroxene lattice (e.g. Robinson, 1980). Calculation of garnet-diopside temperatures assuming total Fe as Fe2+ results in temperatures 50–80 C higher than those using calculated Fe3+ (Table 2). The c. 50 C difference between the pyroxene solvus temperatures and garnet-diopside temperatures calculated here probably results from the overestimation of Fe3+ using stoichiometry. Variations in hornblende-plagioclase temperatures correlate with texture and composition, as noted earlier. Aluminous pargasite and calcic plagioclase that replace diopside-plagioclase symplectites formed at higher temperatures than the magnesian pargasite and sodic plagioclase that replace garnet and form the groundmass in highly amphibolitized samples. Figure 10 shows hornblende-plagioclase temperatures as a function of plagioclase composition at 1.2 GPa, which is the mean pressure calculated for these assemblages. The high temperature hornblende-plagioclase assemblage clearly replaces pre-existing diopside-plagioclase symplectites, but formed at similar temperatures and pressures, probably in response to increased PH2O during the thermal peak; we speculate that this water must have come from dehydration reactions in the enclosing felsic gneisses. Using the data and methods described above, the following equilibration conditions are suggested for each stage of metamorphism, with each diagnostic assemblage shown in parentheses: (1) Eclogite facies metamorphism of a mafic protolith (garnet mantle + omphacite + rutile) at 650–730 C and ‡ 1.4 GPa. (2) HP granulite facies conditions during decompression (garnet rim + diopside + pargasite I + plagioclase) at 700–800 C and 1.2–1.5 GPa. (3) Amphibolite facies conditions (pargasite II + plagioclase + ilmenite ± epidote) at 660 C to 720 C and 0.9–1.2 GPa. These data are summarized in Table 2 and Fig. 11, which depicts the calculated equilibria for various mineral associations and facies with ellipses that overlie the intersection of geothermometer and geobarometer equilibria for each stage of metamorphism. In the absence of primary plagioclase, the pressure estimated for the eclogite assemblage (P ¼ 1.4 GPa) is a minimum equilibration pressure; this may be lowered somewhat based on the high Fe content of the sample, but may also be considerably higher (Carswell & Harley, 1990). The equilibration pressures of the HP granulite assemblage (1.2–1.5 GPa) are well

DEEP IN THE HEART OF DIXIE 73

3.5

2.5

1.5

0.5

Fig. 9. Combined profiles of three garnet grains in sample NEW-1, scaled to percent radius of grain. Note sharp increase in Grs (CaO) and decrease in Alm (FeO) at around 60% of total radius. Pyr (MgO) shows less precipitous increase, Sps (MnO) decays exponentially toward rim.

Table 2. Summary of T-P data for high-P mafic lithology from Silverstreet domain. Numbers in brackets refer to references listed below. Assemblage

Facies

Themometer

1

Eclogite

Grt-Cpx

650–730 C @1.4 GPa [1,2]

Grt-Cpx-Pl-Qtz Jd-Ab-Qtz

2

High P Granulite

Grt-Cpx Grt-Cpx Cpx Solvus Grt-Hbl Hbl-Pl Grt-Hbl Hbl-Pl

700–750 C @1.2–1.5 GPa [1,2] 760–780 C @1.2–1.5 GPa [1,2]* 700–800 C [3] 770–820 C @1.2 GPa [4] 760–830 C @1.2 GPa [5] 660–775 C @1.2 GPa [4] 690–740 C @1.2 GPa [5]

Grt-Cpx-Pl-Qtz

3

Amphibolite

Temperature

Barometer

Grt-Hbl-Pl-Qtz GRIPS Grt-Cpx-Pl-Qtz GRIPS

Pressure > 1.3 GPa @700 C [6] > 1.5 GPa @700 C [7,8] > 1.4 Gpa @700 C [9] 1.2 GPa @800 C [6,7] 1.5 GPa @800 C [7,8] 1.2–1.5 1.2–1.5 1.0–1.2 1.0–1.2

GPa GPa GPa GPa

@800 @800 @700 @675

C C C C

[10] [11] [10] [11]

References [1] Ellis & Green (1979); [2] Powell (1985); [3] Lindsley & Anderson (1983); [4] Graham & Powell (1984); [5] Holland & Blundy (1994); [6] Newton & Perkins (1982); [7] Powell & Holland (1988); [8] Moecher et al. (1988); [9] Holland (1980); [10] Kohn & Spear (1989); [11] Bohlen & Liotta (1986). All temperatures with Fe3+ correction except *.

constrained since plagioclase and quartz are present in both assemblages. The conditions calculated for these assemblages imply a clockwise pressure-temperature-time (P-T-t) path (Fig. 11). The clockwise P-T-t configuration is consistent with models that involve collision of large continental or arc blocks, where one block is thrust beneath another (high pressure at relative low temperatures) and then rebounds to an equilibrium geothermal gradient when the block is exhumed during uplift (England & Thompson, 1984). Based on the preservation of primary zoning profiles in the garnet at temperatures up to 800 C, we suggest that uplift and cooling must have been relatively rapid after peak metamorphic conditions were reached (O’Brien, 1997; Cooke et al., 2000).

Protolith of mafic boudins and layers

Field relations suggest that protoliths of the mafic boudins and layers were originally mafic dykes intruded into the more abundant felsic gneisses that compose the country rock of the Silverstreet domain. Whole rock analyses presented here (Table 3) and elsewhere (Dennis & Shervais, 1991, 1996) show that the felsic gneisses were derived from arc-related felsic to intermediate composition metavolcanic and metaplutonic rocks of Charlotte belt affinity. Most of the mafic boudins and dykes are basaltic in composition, with SiO2
50%, MgO
6–8%, FeO*
10% and TiO2
1–3% (Fig. 12). These compositions are typical of oceanic basalts, but are too high in TiO2 to represent arc-related high-alumina

74 J. W. SHERVAIS ET AL.

840 Hbl-Pl Temperatures

820

[email protected] GPa

800 780 760 740 720 700 680 0.40

0.50

0.60

0.70

0.80

0.90

1.00

Albite Fig. 10. Plot of hornblende-plagioclase temperatures at 1.0 GPa pressure, using Holland & Blundy (1994), as a function of plagioclase composition. The strong correlation of calculated temperature with composition is consistent with observed changes in assemblage and texture.

2

Combined P-T path for Silverstreet domain Eclogite

1.6

1 High-P

2 Granulite P GPa

1.2

3 Amphibolite 0.8

1 = Eclogite Gt-Omp 2 = Granulite Gt-Cpx-Hb-Pl 3 = Amphibolite Gt-Hb-Pl-Ilm

0.4

0 400

500

600

700

800

900

T ˚C Fig. 11. P-T-t plot showing clockwise path, with isobaric heating from granulite to hornblende granulite assemblages. 1 ¼ inferred eclogite (reconstructed omphacite-plagioclase-garnet mantles; minimum pressures only); 2 ¼ high-pressure granulite (diopside-pargasite-plagioclase-garnet rims-ilmenite); 3 ¼ amphibolite (pargasite II-plagioclase-ilmenite-epidote).

basalts. Two samples are somewhat unusual ferrobasalts, with SiO2
41%, MgO
6%, FeO*
15–18%, and TiO2
2.5–3.5% (Fig. 12). They are chemically equivalent to evolved tholeiitic basalts, with very low Cr and Ni (