The Halifax County complex: Oceanic lithosphere in the eastern North Carolina Piedmont

LUCILLE E. KITE* I Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, EDWARD F. STODDARD J North Carolina 27695

ABSTRACT represents the lower portions of a lithospheric 1979) was followed by Farrar's (1980) report section produced at a mid-ocean ridge or in a on the geology of the eastern North Carolina The Halifax County complex, in the Eastern marginal basin, or, perhaps more likely, it was Piedmont. Slate Belt of North Carolina, consists of the low- formed in the early stages of the development of Prior to this investigation, the Halifax com- grade metamorphic equivalents of the following an intraoceanic volcanic arc. We believe the plex had been studied in a preliminary fashion lithologic groups: (1) an ultramafic group com- Halifax complex is representative of the under- by Stoddard and Teseneer (1978), who sug- posed of , , and dunite, lo- pinnings of the Eastern Slate Belt volcanic arc. gested a possible early Paleozoic oceanic origin cally preserving cumulate textures; (2) gabbroids for the complex. The important implications of consisting of leucogabbro, anorthosite, and gab- INTRODUCTION an oceanic origin for the Halifax complex, and bro; (3) local quartz diorite and plagiogranite; the constraints it would place upon tectonic and (4) a relatively large volume of porphyritic The Halifax County mafic-ultramafic com- models for the region, accentuate the signifi- and massive basaltic rocks. Within the complex, plex, located in the extreme eastern Piedmont of cance of examining these rocks in detail. This the lithologies conform approximately, from North Carolina, consists of the greenschist to paper, then, reports on the petrology and geo- west to east, to the sequence: peridotite and lowermost amphibolite-facies metamorphic chemistry of the rocks and primary dunite-pyroxenite-anorthosite and leucogabbro- equivalents of associated mafic and ultramafic within the Halifax County complex. These data, -diorite-basaltic rocks. On its west side, rocks. It is situated within the Eastern Slate Belt, as well as the field relationships observed, are the complex is separated from metavolcanic which is characterized by volcanic and sedimen- employed in an attempt to establish the origin of rocks and metagraywackes of the Eastern Slate tary rocks that were also subjected to low-grade the complex. Belt by the late Paleozoic Hollister mylonite . These slate-belt rocks are sim- zone; to the east, the complex disappears be- ilar to those of the Carolina Slate Belt to the ANALYTICAL TECHNIQUES neath sediments of the Atlantic Coastal Plain. west and are separated from them by the Ra- Relict primary silicate and oxide minerals in- leigh Belt, an amphibolite-facies metamorphic clude of F079, clinopyroxene with com- terrain with an antiformal structure. Most analyses were done in the electron positional range of Wo^.^ En44_49 Fs5_9 in workers believe that the rocks within both the microprobe laboratory at Virginia Polytechnic the ultramafic rocks and W034.49 En42_4g Eastern and Carolina Slate Belts represent vol- Institute and State University; correction factors Fsg_2i in the gabbroids, plagioclase of as much canic and volcanogenic sedimentary material of Albee and Ray (1970) were used. Whole- as A1193 in the leucogabbros, Cr-rich , that originated in an early Paleozoic volcanic arc analyses (except rare earths) were done by and ilmenite. Major- and trace-element analyses environment (Fullagar and Butler, 1977; Whit- X-ray fluorescence (XRF) spectrometry at of samples from each of the rock groups show ney and others, 1978; Hatcher, 1972; Glover, North Carolina State University, using tech- an Fe-enrichment trend extending through the 1976; Spence and Carpenter, 1976; Rankin, niques described by Fabbi (1972). Rare-earth- mafic sequence of gabbroids and basaltic rocks. 1975). Both the Raleigh and Eastern Slate Belts element analyses were made by instrumental The chemical characteristics of the basaltic rocks were intruded by numerous late Paleozoic grani- neutron activation analysis—five by XRAL compare favorably with those of tholeiitic toid plutons (Fullagar and Butler, 1979). Laboratories, Toronto, and the remaining three ocean-floor and still more favorably with using on-campus reactor and facilities at North Studies of the eastern Piedmont of North island-arc tholeiites. Field relationships, plus Carolina State University. Further details con- Carolina, and of the Eastern Slate Belt in par- lithologic, geochemical, and mineralogical evi- cerning procedures and estimated analytical ac- ticular, have been few in number and for the dence, suggest that the complex originated as curacy and precision are presented elsewhere most part of a reconnaissance nature. Parker (early Paleozoic?) oceanic material. Either it (Kite, 1982). (1968) summarized the structural characteris- tics of the Raleigh and Eastern Slate Belts. Stratigraphy and lithologies of some of the vol- PETROGRAPHY OF THE HALIFAX canic and volcaniclastic rocks within the COUNTY COMPLEX Eastern Slate Belt were discussed by Stanley and Cavaroc (1980). Reconnaissance geologic •Present address: South Carolina Geological Survey, Low-grade (greenschist- to lowermost am- Harbison Forest Road, Columbia, South Carolina mapping of Halifax County (Wilson, 1981) and phibolite-facies) metamorphism of pre-late Pale- 29210. adjacent Nash County (Wilson and Spence, ozoic age has affected the lithologies within the

Geological Society of America Bulletin, v. 95, p. 422-432, 9 figs., 4 tables, April 1984.

422

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complex, and most of the rocks are composed of completely replaced by actinolite and serpen- these rocks contain clinozoisite or epidote, inter- a mixture of primary and metamorphic miner- tine; typically secondary oxide minerals outline stitial to relict clinopyroxene phenocrysts, or ac- als. Primary mineral phases include clinopyrox- the original crystal forms of the primary miner- tinolite pseudomorphs. These epidote-group ene, rare olivine, nearly ubiquitous plagioclase, als. Clinopyroxene, locally found filling inter- minerals are presumed to replace primary plagi- and some oxides. Major secondary mineral stices between and around equant serpentine oclase, and they suggest a protolith of feldspathic phases include actinolite, hornblende, clinozoi- pseudomorphs, suggests a relict cumulate texture pyroxenite cumulate. site, serpentine, and chlorite. Quartz, oxides, and and is an indication of the crystallization of oli- minor amounts of white mica and talc are less vine prior to clinopyroxene (Fig. la). Chlorite is Gabbroids abundant. also a major constituent of these ultramafic On the basis of modal mineralogy and tex- rocks, typically occurring in masses between and Inferred primary lithologies within this group- tures, rocks within the Halifax complex have around the clinopyroxene and olivine grains. Al- ing include anorthosite, leucogabbro, and gab- been classified (IUGS classification) into 13 though some such chlorite masses may be pseu- bro. All are typified by abundant blocky grains rock types (Table 1), which can be subdivided domorphous after primary orthopyroxene, no of primary plagioclase and lesser amounts of fer- into 4 basic groups: ultramafics, gabbroids, felsic relict orthopyroxene remains. romagnesian mineral phases. plutonics, and basaltic rocks. Given that meta- The dunites consist of relict olivine, now al- Anorthosite is found at a few localities in the morphism has affected all lithologies to some most entirely represented by obviously pseu- complex and consists almost entirely of calcic extent, and that in some cases no primary min- domorphous serpentine, with only minor plagioclase (Fig. lb). It is characteristically very eral phases remain, the rock types are designated amounts of clinopyroxene, actinolite, or chlorite. coarse grained and contains only minor amounts according to their inferred primary igneous lith- In some samples, these serpentine pseudo- of actinolite and/or chlorite. The leucogabbros ologies, with the exception of the basaltic group, morphs are equidimensional, rather euhedral are dominated by coarse-grained calcic plagio- where distinctions are made on the basis of the and coarse grained, suggesting slow, even crys- clase with subordinate amounts of pale green effects of metamorphism. The prefix "meta-" tallization (or recrystallization) of the original actinolite and a few occurrences of relict clino- has been dropped but is implicit when referring olivine under conditions of hydrostatic stress, . The primary textural features are still to rocks of the Halifax complex. whereas other dunite samples exhibit serpentine obvious in many cases, and it appears that the pseudomorphs and locally spinels, which are amphibole is interstitial to the plagioclase, indi- Ultramafic Rocks very elongate in shape, indicative of a primary cating that plagioclase crystallized first. tectonite fabric. In the , the primary mineralogy is lim- Inferred primary lithologies within the ultra- Those ultramafic rocks in which there is ited to the occurrence of plagioclase, and the mafic group include peridotite ( and abundant clinopyroxene and/or actinolite and proportion of amphibole to plagioclase is greatly ), dunite, clinopyroxenite, and webster- chlorite, and no serpentine or olivine, are identi- increased. In contrast to the actinolite of the leu- ite. contain both clinopyroxene and fied as metamorphosed equivalents of pyroxen- cogabbros (and a few Fe-poor gabbros), the olivine that in most cases have been partially or ites (clinopyroxenite or websterite). Locally, amphibole of most of the gabbros typically ex- hibits a darker blue-green pleochroism, and there is a tendency for the plagioclase to be re- TABLE I. SUMMARY OF ROCK TYPES AND MINERALOGY OF THE HALIFAX COUNTY COMPLEX placed by highly birefringent epidote rather than clinozoisite. This is consistent with the higher Mineralogy* Fe/Mg ratio of the gabbros. Ilmenite is the Primary Metamorphic common oxide mineral in the gabbros.

8 I Felsic Plutonic Rocks S 3 8- Rock type a a Rocks richer in silica, presumed to be differ-

Ultramafic rocks entiates of the mafic series, are locally found

Dunite with the gabbros and can be considered the met- Wehrlite and olivine amorphic equivalents of quartz diorite and clinopyroxenite Lherzolite tonalite. These rocks contain primary plagio- Websterite Clinopyroxenite clase and abundant quartz. Quartz diorites con- tain metamorphic hornblende and fine-grained

Anorthosite plagioclase as well as minor amounts of secon- Leucogabbro X dary oxides. The rare tonalites are coarser Gabbro x§ Felsic plutonic rocks grained and consist mainly of plagioclase and

Tonalite quartz, lacking any ferromagnesian mineral Quartz diorite phases.

Metabasalt Basaltic Rocks Basaltic amphibolite Amphibolite The metamorphosed basaltic rocks of the Hal- •X: >15 vol %: x: 2% to \5%: tr. <2%. tPredommantly Cr-rich spinel rimmed by magnetite in ultramafic rocks, ilmenite in gabbroids and basaltic rocks; in most cases, a mixture of primary and ifax complex consist of three types: metabasalts, metamorphic phases. basaltic amphibolites, and amphibolites. The §The two amphibole varieties are mutually exclusive in the gabbros; their distribution is a function of rock Fe content. metabasalts have retained an obvious ophitic or

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subophitic texture (Fig. lc). The major primary There are no primary ferromagnesian minerals mineral is again the blue-green amphibole and mineral remaining in these rocks is plagioclase, remaining in the metabasalts, and although pla- the preferred orientation of these grains pro- with only local occurrences of relict oxides. The gioclase is abundant, the dominant mineral is a duces the local foliation. Quartz is quite plagioclase typically forms a groundmass of metamorphic blue-green amphibole. In most of common, yet it is not as abundant as is the pla- smaller lathlike crystals in which are found large the metabasalts, the amphibole appears to be gioclase, which commonly is at least partially euhedral plagioclase phenocrysts that locally interstitial to the plagioclase and presumably is replaced by epidote. occur in large glomerocrysts. Both plagioclase pseudomorphous after primary augite. The amphibolites are strongly foliated and laths and phenocrysts exhibit strong, apparently The basaltic amphibolites have retained more completely recrystallized, tend to be very primary, normal zonation. Locally, phenocrysts enough primary plagioclase to suggest a basaltic fine grained, and consist predominantly of the retain fine igneous oscillatory zonation (Fig. Id). texture, yet they may be foliated. The dominant blue-green amphibole with lesser amounts of

C d

Figure 1. a. Lherzolite, with relict primary clinopyroxene (cpx) and olivine (ol) and secondary serpentine (srp). Ol in lower right is only marginally altered to srp; no ol remains in srp pseudomorphs at upper left. Texture suggests primary crystallization of ol as a cumuius phase followed by intercumulus cpx. Abundant, irregular masses of chlorite elsewhere in the section have completely replaced original orthopyroxene. Plane-polarized light; scale bar is 0.5 mm. b. Interlocking grains of calcic plagioclase (An 92) in leucogabbro. Crossed polars; scale bar is 0.5 mm. c. Relict basaltic texture in groundmass of porphyritic metabasalt. Augite has been replaced by metamorphic amphibole. Plane-polarized light; scale bar is 0.2 mm. d. Primary oscillatory zonation preserved in plagioclase phenocryst in metabasalt. Composition is in labradorite- andesine range; local thin albite rim is metamorphic. Crossed polars; scale bar is 0.5 mm.

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quartz, epidote-group minerals, and plagioclase. conform approximately, from west to east, to amount of anorthosite in the complex is found at A primary igneous texture is not retained. the sequence: peridotite and dunite-pyroxenite- the base of the transition zone from ultramafic to anorthosite and leucogabbro-gabbro-diorite-ba- gabbroic rock. The leucogabbro crops out only FIELD RELATIONSHIPS AND saltic rocks (Fig. 2). The contact between the locally but it appears to have a consistently close REGIONAL GEOPHYSICS ultramafic and gabbroic rocks appears to be spatial relationship to the ultramafic rocks of the gradational, with pyroxenite and feldspathic transition zone. In the field, the different inferred primary pyroxenite occurring in a transition zone with The type of contact between the gabbroic and lithologies within the mafic-ultramafic complex the gabbroic rocks. Where exposed, the small basaltic rocks in the complex is unknown. In only one instance has been observed in contact with the gabbro, as a small dike cutting 77*55' 77°5d the coarser-grained mafic rock. However, in 36ft 5'+ + many locations, small outcrops of basaltic rock, probable dikes but with unknown orientation, appear to be surrounded by gabbro, but the con- tact between these rock types is not exposed.

Cenozoic COASTAL PLAIN sedimentary cover

Early DIABASE and RHYOLITE PORPHYRY DIKES Mesozoic

Late Paleozoic ROCKY MOUNT PLUTON- unfoliated, K-feldspar porphyritic biotite granitoid

GRANITIC GNEISS-with biotite, locally showing mylonitic ma fabric 1km METASEDIMENTARY and METAVOLCANIC ROCKS - mainly coarse to fine metagraywacke

MAFIC - ULTRAMAFIC COMPLEX

METABASALT-BASALTIC AMPHIBOLITE- primary plagioclase and relict basaltic texture* - Basaltic* o AMPHIBOLITE- foliated amphibole • 36 10"-(- Early Paleozoic quartz • plagioclase rock

TV* AMPHIBOLITE with felsic ORTHOGNEISS

QUARTZ DIORITE

GABBRO-local primary plagioclase a 1° LEUCOGABBRO - primary Gabbroid» 2 » plagioclase and clinopyroxene h E « . a BYTOWNITE ANORTHOSITE •«o CT5 o>® Ì TRANSITION ZONE-feldspathic clino- o pyroxenite with some gabbro

ULTRAMAFICS-includes peridotite, pyroxenite, dunite; ubiquitous primary clinopyroxene; local primary olivine, oxides

Figure 2. Genera) geologic map of the Halifax County com- plex; inset location map also shows major lithotectonic belts of the North Carolina Piedmont. Abbreviations: CSB, Carolina 38105'+ Slate Belt; TB, Triassic basins; RB, Raleigh Belt; ESB, Eastern Slate Belt; CP, Coastal Plain; NCMZ, Nutbush Creek mylonite zone; HMZ, Hollister mylonite zone.

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To the west, all of the units in the complex the Eastern Slate Belt east of the map area. The sidic-augite (Wo44_49 En44_49 Fs5_9; sue Fig. 3) appear to be in contact with the metasedimen- actual eastward extent of the Halifax complex with relatively little variation in composition tary and metavolcanic rocks of the Eastern Slate and its relationship with other rock types be- and cluster at the high-Mg end of the tholeiitic Belt along a fault or shear zone, considered by neath the Coastal Plain sediments is unknown. differentiation curve (Wager and Brown, 1967). Farrar (1980) to be part of the Hollister mylo- However, gravity studies of the area indicate a These clinopyroxenes are comparable to the nite zone (Fig. 2). This zone cuts the Eastern steep gradient with a gravity high to the east range of compositions shown by Coleman Slate Belt into two parts and is the eastern struc- located beneath the Coastal Plain of northeast- (1977) for clinopyroxenes from cumu- tural boundary of the Raleigh block (Farrar, ern North Carolina and a low beneath the cen- late sequences. For example, they are similar in 1980). The Hollister mylonite zone is a major tral Raleigh Belt to the west (Haworth and composition to those analyzed by Himmelberg structural feature of the North Carolina Pied- others, 1980), suggesting the presence of denser and Loney (1980) from clinopyrox:nite and mont and is thought to be the northeastward lithosphere to the east. feldspathic wehrlite metacumulates of the Can- continuation of the Augusta fault zone, which yon Mountain ophiolite (Fig. 3). extends from Georgia to Virginia (Hatcher and ANALYTICAL RESULTS Clinopyroxene from the leucogabbros of the others, 1977). Halifax complex is somewhat more Fe-enriched Along this zone, immediately west of the Hal- Primary Mineral Chemistry and exhibits a greater variation in composition

ifax complex, and separating the major portion (\V034_49 En2_48 FS8_21). This rock type has the of the complex from metamudstones and meta- Primary olivine, clinopyroxene, plagioclase, highest calcium and the lowest iron bulk rock graywackes of the Eastern Slate Belt, is a thin and opaque oxides were analyzed by electron content of the Halifax gabbroids. Although the band of granitic gneiss that locally exhibits my- microprobe. Representative mineral composi- Halifax gabbros and basaltic rocks lack relict lonitic fabric. These rocks contain large feldspar tions and structural formulas are summarized in clinopyroxene, these rock types do exhibit a porphyroclasts and a strong foliation delineated Table 2. steady increase in bulk rock iron content (total by biotite. Strong foliation within this unit and Olivine. Olivine was analyzed in one lherzo- Fe as Fe203) and a decrease in calcium content, the ultramafic rocks located alongside it is con- lite sample from the Halifax complex and de- which suggests that their original clinopyroxene sistent with the suggestion that a segment of the termined to contain 78 to 79 mole % was somewhat more Fe-enriched. These clino- Hollister mylonite zone passes between the two (Fo). This composition is compatible with the from Halifax leucogabbros are similar lithologies. A prominent north-south aeromag- compositional range of olivine typically found in in composition to clinopyroxenes from ophio- netic anomaly of regional extent also coincides the ultramafic and mafic cumulates of ophiolite litic and oceanic gabbros (Himmelberg and with this boundary (U.S. Geological Survey, sequences, where olivine commonly exhibits a Loney, 1980; Coleman, 1977; Cann, 1971; Suen 1973). trend of iron enrichment ranging from F093 in and others, 1979; Coish and Church, 1979) and To the east, the complex disappears beneath the ultramafics to Fo68 in olivine gabbros other tholeiitic gabbros (Wager and Brown, Coastal Plain sediments. Local exposures of the (Coleman, 1977; Himmelberg and Loney, 1980; 1967) and basalts (Fodor and others, 1975). bedrock in this area consist of small outcrops of Suen and others, 1979; Jaques, 1981). Plagioclase. Primary plagioclase within the unfoliated granite, and Farrar (1980) reported Clinopyroxene. Clinopyroxenes from the Halifax complex shows a wide range in anor- further exposure of the metavolcanic rocks of Halifax peridotites and are diop- thite content (Fig. 4). Exceedingly calcic plagio-

TABLE 2. REPRESENTATIVE ANALYSES OF PRIMARY MINERALS FROM THE HALIFAX COUNTY COMPLEX

Olivine Clinopyroxene Plagioclase Cr-magnetite I menile

Rock type Lherzolile Leucogabbro Leucogabbro Gabbro Ihenolile Gabbro

SiOj 38.7 54.4 52.5 44.5 54.5 1.5 0.05 TiOj 0.03 0.03 0.14 2.2 44.5 ALO, 0.06 0.37 1.3 5.1 0.0 0.05 0.05 0.26 19.4 0.07 . 35.5t 14.6t FeO 20.0" 3.2* 5.8' .04' 0.19" 32.0 ;.6.2 MnO 0.35 0.10 0.26 .02 0.00 .95 3.5 MgO 41.2 17.3 15.2 .04 0.00 2.6 0.15 CaO .21 24.4 23.3 18.6 11.1 .12 0.12

Na20 .01 0.01 0.25 .59 4.78 K,0 .01 0.07 0.03 .01 .06

99.9 98.6 99.2 119.2

Si .990 1.983 1.959 2.078 2.467 .055 .002 Ti .001 .001 .005 .062 1.714 Al .002 016 .056 1.914 1.523 .219 .000 Ci .001 .001 .564 .002 Fe3" .983 .564 Fe2* .428 .098 .002 .007 .940 1.548 Mn .015 .003 .000 .000 .030 .150 Mg 1.572 .940 .846 .002 .000 .144 .012 Ca .006 .953 .933 .932 .536 .004 .008 Na .000 .001 .018 .054 .420 K .000 .004 .002 .000 .003

Wo 47.9 47.6 An 92.0 55.9 Fe3V{Fe3*+Cr+AI) .557 2 En 47.2 43.2 Ab 8.0 43.8 Mg/(Mg+Fe ') .133 Fs 4.9 9.2 OR 0.0 0.3

•Total iron as FeO. tFe203 and FeO calculated from stotchiometry.

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° HALIFAX COMPLEX ultramafics • HALIFAX COMPLEX gabbroids

« Gabbros from oceanic basement (Cann,1971) * Gabbro and perknite, Betts Cove ophiolite (Coish and Church, 1979) • Gabbro, Bay of Islands ophiolite (Suen and others, 1979) Cumulate ophiolite (Coleman , 1977) Canyon Mountain ophiolite, cumulates (Himmelberg and Loney, 1980)

Figure 3. Compositions of relict clinopyroxenes from ultra- mafic and gabbroic units.

clase (An82_94) is found in leucogabbros and plex have low Mg/(Mg + Fe) and very high Ti02, and MnO with increasing Si02. The anorthosites, whereas plagioclase from other Ha- Fe3+/(Fe3+ + Cr + Al) and thus are not com- FeO*/(FeO* + MgO) ratio also decreases with

lifax gabbroids ranges from An5o to An59. The parable with most primary oxide minerals from Si02, from .725 at 53.75% Si02 to .532 at more felsic plutonic rocks contain much more ultramafic rocks (Irvine, 1974). 65.65% Si02. sodic plagioclase. One quartz diorite contains Basaltic Rocks. The metabasalts, amphibo-

plagioclase of about An34 and a coarse-grained Whole-Rock Chemistry lites, and basaltic amphibolites are not distin- tonalité contains plagioclase of An2) to Ann. guishable one from another on the basis of Phenocrystic plagioclase within the Halifax Average whole-rock analyses of samples from major-element chemical criteria and thus pro- metabasalts typically exhibits strong normal zo- the Halifax complex, determined by XRF, are gressive textural alteration apparently was not nation. One phenocryst analyzed from a meta- given in Table 3. accompanied by significant metasomatism, ex- basalt with a relict ophitic texture has a core of Ultramafic Rocks. The peridotites are greatly cept for notable increases in Rb and especially

An7] and gradually becomes more sodic (An37) enriched in MgO compared to the pyroxenites Sr. The basaltic rocks as a whole are character- toward the outer margin (Fig. 4). This more and are further distinguishable from the latter ized by high Ti02 (0.85-2.28 wt %) and low

sodic margin, typical of most of the plagioclase group because of the lower CaO and Si02 and alkali content (1.39-3.13 wt % Na20 + K20). phenocrysts, is comparable in composition to higher Fe203' content of the peridotites. The The FeO*/(FeO* + MgO) ratio is relatively interstitial material found in tholeiitic basalts re- pyroxenites also have a somewhat higher A1203 high, averaging .644, whereas the iron content it- ported by Keil and others (1972). Some of the content. self is quite variable (9.35-17.78 wt % Fe203'). plagioclase within the basaltic rocks has thin al- Gabbroids. The gabbroids are CaO-rich and

bitic rims, presumably products of low-grade Ti02-poor with an A1203 content that is quite Rare-Earth Elements metamorphism. variable, about 14 wt % for those gabbros rich in The distribution of plagioclase on the An-Ab- ferromagnesian minerals to nearly 21 wt % for Selected rare-earth elements in two samples Or diagram of Figure 4 illustrates the differentia- the leucogabbros in which plagioclase predomi- of gabbro and six basaltic rocks were analyzed tion that occurred in the plutonic sequence of nates. With only two exceptions, the gabbroids by instrumental neutron activation analysis. the Halifax complex, the An content of the pla- analyzed all contain normative olivine although Chrondrite-normalized patterns (Fig. 5) are flat gioclase gradually decreasing as differentiation no modal olivine was found. or have a slight negative slope; gabbros display continued. Potassium remains consistently low Quartz Diorites. Those quartz diorites anal- lower abundances of all rare-earth elements

throughout the sequence. The occurrence of the yzed show a wide range in Si02 content, with (2-8 x chondritic) than the basaltic rocks most calcic plagioclase within leucogabbros and decreasing amounts of Fe203', MgO, CaO, (10-40*). Among the latter, the thoroughly re- anorthosites may support the idea that these rocks formed at an earlier stage of differentiation Plagioclase from than did the gabbros, which contain less calcic e Leucogabbro • Quartz Diorite plagioclase. o Gabbro • Tonalité Oxides. Primary oxides are rare in the ultra- « Anorthosite • Metabasalt mafic samples from the Halifax complex and are small where present. Those analyzed from pe- Metabasalt ridotites reveal a core of a Cr- and/or Ti-rich Rim- -Core spinel-group mineral containing as much as 25.5

wt % Cr203 and 4.9 wt % Ti02, and rimmed by secondary magnetite. Primary oxides are not common within the gabbroic rocks, but analyses Plutonic Rocks from one gabbro disclosed ilmenite containing 8» COO, as much as 3.9 wt % MnO. Ab An Chromium-rich such as those from the ultramafic rocks of the Halifax com- Figure 4. Compositional variation of plagioclase from plutonic and basaltic rocks.

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TABLE 3. AVERAGE ANALYSES OF WHOLE-ROCK SAMPLES FROM THE HALIFAX COUNTY COMPLEX

PERIDOTITE PYROXENITE ANORTHOSITE LEUCOGABBRO GABBRO n = 3t n - 4 n = I n = 3 n - 12

Si02 38.9 1.5 45.5 2.9 44.0 46.8 2.0 46.0 2.3 Ti02 .14 .04 .21 .05 .06 .15 .0 .27 .17 AI;O3 7.2 3.6 8.9 26.5 20.7 .4 16.0 1.5 Fe!°j' 13.3 1.6 9.1 2.9 4.1 .6 7.3 1.6 .27 .08 .20 .07 .10 .03 .16 .03 MnO 34.1 1.4 21.8 6.8 8.8 1.3 12.2 2.2 MgO 4.75 2.22 11.35 16.08 14.61 .14 13.95 1.71 CaO 0.0 0.0 0.0 1.0 1.0 .1 1.0 .5 Na,0 .03 .01 .04 .40 .08 .06 .34 .36 PKjO .18 .05 .47 .04 .06 .01 .06 .02 A n.d. L.I. 2.40 2.00 1.6 .7 2.16 .76

98.9 99.9 98.04 99.4

Co 17 27 43 12 Ct 80 225 360 279 Cu 101 88 Ni 582 28 271 120 35 52 38 67 35 Rb 2 2 2 3 21 9 4 23 20 Sr 9 4 18 II 181 139 37 122 49 V 28 59 5 111 58 Y Zn 10 10 31 15 Zr 4 3 17 17 17 II

FeO' .022 .03 FeO' + MgO

Note: wt % for oxides; ppm for trace elements. tn= number of analyses; x = mean; a = standard deviation.

crystallized amphibolites are more enriched in 1976; Pallister and Hopson, 1981). The low clinopyroxene-bearing nature of the plutonic

rare earths than are those metabasic rocks that K20 content and the normative mineralogy of rocks of the Halifax complex bear close re- retain relict igneous textures. the leucocratic rocks of the Halifax complex semblance to features of cumulate sequences correlate with those of oceanic plagiogranites found in many (Hopson and others, DISCUSSION (Coleman, 1977). Although they have a close 1975; Pallister and Hopson, 1981; Himmel- spatial relationship with the gabbroic rocks, berg and Loney, 1980; George, 1978; Sinton, Similarities to Ophiolite Sequences the exact nature of the contact between these 1980). The deformational fabric suggested by two rock types is unknown. Quartz diorites or elongate olivine grains in the dunite of the Whole-rock and mineral compositions for other felsic rocks are not found within either Halifax complex is not unusual in cumulate the gabbroids indicate that magmatic differen- the Halifax ultramafic or basaltic units. portions of ophiolite sequences; Thayer tiation occurred during crystallization of the The sequence of units (peridotite and (1980) pointed out that such tectonic fabric plutonic rocks of the Halifax complex. As dunite-pyroxenite-gabbroid-diorite) and the due to subsolidus deformation is frequently pointed out above, the gabbroids are petro- graphically separable: leucogabbros contain light green actinolite and a high plagioclase to amphibole ratio, whereas gabbros contain blue-green amphibole and a lower ratio. These differences in modal mineralogy are mani- fested in the whole-rock compositions with the leucogabbros (Table 3) having generally 1 higher CaO values and lower FejOj and •o c Figure 5. Chondrite- o MgO values than do the gabbros. The close JC normalized (values of o spatial relationship of the leucogabbros to the \ Haskin and others, 1968) « ultramafic rocks, as well as the textural infer- Q. rare-earth-element pat- E ence that plagioclase crystallized prior to py-

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TABLE 3. [Continued) using other schemes, such as the Ti-Zr-Y dia- gram of Pearce and Cann (1973), they would BASALTIC QUARTZ DIORITE METABASALT AMPHIBOLITE AMPHIBOLITE be classified as low-potassium tholeiites n = 3 n = 10 n = 3 n = 5 (Fig. 7b). Their relatively low Cr, Ni, and x o * o X o Ni/Co also make them more similar to is- land-arc tholiites than to other types (Haw- 59.6 6.0 49.1 1.6 50.2 3.7 51.3 2.9 .87 .47 1.28 .37 1.72 .44 1.70 .54 kins, 1977). Due to the observed increase in 13.8 .5 14.5 1.1 13.2 .8 13.3 1.3 9.3 3.7 11.8 1.8 14.3 1.0 14.6 2.4 Sr content with progressive textural altera- .21 .02 .23 .03 .22 .06 .26 .02 tion, Sr is not thought to be a reliable indi- 4.0 .9 1.3 5.9 1.4 6.3 1.3 6.47 1.84 10.91 .84 9.28 .86 9.71 1.28 cator of tectonic setting for the Halifax 3.6 .6 .4 2.0 .2 1.7 .3 .30 .15 .26 .16 .29 .11 .25 .12 basaltic rocks. .34 .10 .36 .17 .33 .20 .34 .11 n.d. 1.35 .46 1.3 .4 1.78 .59 The Halifax complex gabbroids have an

98.5 98.6 98.7 101.2 FeO*/(FeO* + MgO) consistently lower than

54 9 53 2 58 3 do the basaltic rocks and they also contain 36 12 99 45 55 40 134 39 significantly lesser amounts of Si02, suggest- 131 85 137 23 4 3 49 20 18 15 27 12 ing that the gabbroids are compositionally 16 18 17 18 23 14 34 41 197 60 272 164 568 369 846 877 more primitive than the basaltic rocks. A sim- 268 56 367 92 375 118 ilar relationship has been observed in ophiolite 26 11 32 12 32 7 54 30 95 6 93 16 sequences from Newfoundland and elsewhere 76 26 53 26 69 20 66 14 (Norman and Strong, 1975). The basaltic .636 .097 .599 .070 .743 .03 .673 .082 rocks increase in Ti content with increasing fractionation (based on Si02 content), a corre- lation that has been observed to be indicative of tholeiitic (Shido and others, 1971). In addition to Ti, the basaltic rocks also con- sistently increase in Zr, V, and Sr with increas- seen in gabbros and ultramafics of the lower Chemical analyses presented by Pearce ing Si02 (Fig. 8). In contrast, the gabbroids do part of ophiolite cumulate sequences. (1976) for basalts of different types not increase in these trace elements with in- creasing Si02 but have a consistently low con- When compared to average chemical com- indicate that the high Ti02 (avg 1.48 wt %), positions of oceanic tholeiites (Table 4), the MgO (avg 6.66 wt %) CaO (avg 10.3 wt %), tent of these elements throughout their range tholeiitic nature of the basaltic rocks within and low K2O (avg 0.27 wt %) make the com- of Si02- It is also noteworthy that the gab- the complex is demonstrated by their low al- positions of the Halifax basaltic rocks more broids and basaltic rocks differ somewhat in kali contents, high titanium contents, and rela- compatible with ocean-floor basalts (see Table their Rb/Sr ratios. The basaltic rocks show tively high FeO*/(FeO* + MgO). The increas- 4) than with basalts of other geologic envi- consistently low Rb/Sr ratios (.003-.099), ing proportion of Fe through the sequence ronments. The high Ti content of the basaltic averaging .051, whereas the gabbroids show a ultramafic-gabbroic-basaltic rock is typical of rocks of the Halifax complex tends to increase somewhat higher average of .135, but with a the Fe-enrichment trend of a tholeiitic differ- with increasing Zr, and the samples fall domi- wider range in ratio values (.03-.42). The entiation curve (Irvine and Baragar, 1971) nantly within the field for ocean-floor basalts and similar to that reported from ophiolite se- on many geochemical discrimination plots, TABLE 4. AVERAGE ANALYSES OF OCEANIC THOLEIITIC quences (see Fig. 6). such as the Ti-Zr plot of Figure 7a. However, BASALTS FOR COMPARATIVE PURPOSES

(a)' (b)t

'Average of 75 analyses of ocean-floor basalts (Pearce. 1976). tAverage of 161 analyses of oceanic tholeiites (Hyndman, 1972). §Average composition of oceanic tholeiitic basalt (Engel and others, 1965).

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Figure 7. Trace-element "discrimination" plots for Halifax metabasalts and amphibolites. Of the 18 basaltic rocks analyzed, the range of CaO + MgO is 12.7% to 21.6%; 17 analyses fall within the 10% to 20% "screen." a. Ti-Zr plot; field boundaries from Garcia (1979). b. Ti-Zr-Y plot; field boundaries from Pearce and Cann (1973).

wide range in values suggests that Rb/Sr was geochemical properties in Newfoundland elongate serpentine pseudomorphs) is pre- affected by postmagmatic alteration. ophiolites; Flower and others (1977) used the served locally in the dunites, a well-defined The relative rare-earth enrichment of the same sort of argument to account for chemical mantle tectonite unit of metamorphic harz- Halifax basaltic rocks compared to the gab- variability of mafic rocks from the Mid- burgite apparently is absent from the Halifax bros (Fig. 6) is similar to the relationship be- Atlantic Ridge. Such a model could be applied complex, as the stratigraphically higher ultra- tween basalts and gabbros of the Troodos to the Halifax complex as well. mafic and gabbroic units are in contact to the ophiolite (Kay and Senechal, 1976) and this west with a fault or shear zone. Again, how- relationship further emphasizes the more Missing Ophiolite Features ever, the absence of this unit is not unique to primitive (less differentiated) nature of the the Halifax complex. The incorporation of gabbros. However, the Halifax basaltic rocks When the Halifax complex is compared to ophiolites into orogenic zones has, in other do not display depletion of the light rare-earth an idealized ophiolite sequence and the sec- cases, caused their disruption and fragmenta- elements generally characteristic of mid- tion of ocean floor it represents (Fig. 9), the tion, resulting in discrete fault-bounded bodies ocean-ridge tholeiites (Schilling, 1975). Their similarities in lithologic sequence as discussed of ophiolitic material, or an ophiolitic mélange flat patterns and La/Sm are more akin to above become obvious. However, it is neces- (Hopson and others, 1975; Saleeby, 1978; those of island-arc tholeiites (Jakes and Gill, sary to note the absence from the Halifax Drake and Morgan, 1981). 1970). complex of several significant typical ophiolite If an ocean-floor origin for the basaltic features. Although the pillow lavas of the TECTONIC IMPLICATIONS rocks is acceptable, then it implies that they idealized ophiolite are paralleled in the Hali- are the result of partial melting of mantle fax complex by large amounts of metabasalt, The age of the Halifax complex is unknown, material. The mechanism responsible must no pillow structures have been observed. This but it is deformed by and therefore must be explain the distinct geochemical characteris- may be due to metamorphism and foliation of older than the late Paleozoic Hollister mylonite tics of the basaltic and gabbroic rocks: the rel- the metabasalts, as well as to limited exposure zone (Farrar, 1980); the rocks of the complex ative enrichment in incompatible elements of and small outcrops of these rocks. Sheeted have also suffered Paleozoic regional aietamor- the basaltic rocks; the higher Rb/Sr ratios, as dikes have not been found but, again, poor phism. No age dates exist that bear on the re-

well as extremely high Al203/Ti02 and CaO/ exposure or deformation of the rocks of the crystallized pre-late Paleozoic rocks of the Ti02 ratios, of the gabbroids; the more primi- complex may be responsible. Some of the me- Eastern Slate Belt, but they are probably correla- tive composition of the gabbroids relative to tabasalts have retained a relict diabasic tex- tive with the better-known late Precainbrian to the basaltic rocks; and the lack of gabbroic ture, and at least one gabbro is cut by a early Paleozoic rocks of the Carolina Slate Belt rocks that are equivalent in mineralogy and basaltic dike. In any event, sheeted dike com- to the west. Such a correlation would strongly rock chemistry to the overlying basaltic rocks. plexes are not universally found in all ophio- suggest that the Halifax complex is itself of sim- A system of isolated, small magma reservoirs lite sequences (Davies, 1968; Moores, 1.970; ilar age, because the Halifax complex and the fractionating separately was suggested by Montigny and others, 1973). surrounding metasedimentary rocks have shared Strong and Malpas (1975) to explain similar Although a tectonite fabric (indicated by a common tectonothermal history.

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Gabbroids Oceanic Idealized Halifax County Ultramafics Basaltics Diorites l0lite 10,000 Seismic Layers Mafic-Ultramafic Complex ppm - Layer 1 Sediments Eastern Slate Belt

Pillow Layer 2 lavas Metabasalts and 1000- Amphibolite

Sheeted Diabase Dikes Layer 3 Gabbroic

Mafic Cumulates 100 Cumulates Moho Transition zone Pyroxenite Pyroxenite Ultramafic Ultramafic Cumulate

Cumulates Eastern Slate Layer 4 Belt Rocks

Tectonized

Peridotite

Figure 9. Comparison of Halifax County complex to an ideal- ized section of oceanic lithosphere. HMZ: Hollister mylonite 45 50 55 60 65 70 zone. Ave. wt. % Si02

Figure 8. Average minor- and trace-element content versus by present constraints might involve plate inter-

average Si02 for ultramafic, gabbroid, basaltic, and dioritic actions among a number of small plates (see also groups from the Halifax complex. Zen, 1981), including a microcontinental frag- ment, the Raleigh Belt, and adjacent volcanic The lithologic, chemical, and mineralogical arc. In a study of rocks from the Tongan Islands, arcs, the two slate belts. However, further geo- evidence indicates that the Halifax complex rep- Ewart and Bryan (1972) concluded that they logic study of the North Carolina Piedmont is resents oceanic material. It may represent the were produced in the earliest stages of oceanic needed before a completely successful tectonic ultramafic and mafic cumulates and basaltic plate convergence, and the authors also pointed model can be proposed. units of an ophiolite sequence generated at a out the great similarities the Tongan rocks bear mid-ocean ridge, with nearby metasedimentary to ophiolitic sequences produced at divergent CONCLUSIONS rocks of the Eastern Slate Belt possibly consis- plate margins. Although, at present, insufficient tent with the sedimentary layer 1 of the oceanic information is available to permit a choice from The Halifax County mafic-ultramafic com- crust. In this case, the complex apparently has among the three possible origins, the oceanic plex has sufficient features in common with become detached and separated from most of its affinity of the Halifax complex is evident. well-studied ophiolite sequences that we con- basal mantle material. Alternative explanations Hatcher and Zeitz (1980) considered the clude it also constitutes a fragment of oceanic consistent with much of the available data are Eastern Slate Belt to be underlain by continental lithosphere. Whether it originated at a divergent possible. One is that the rocks of the Halifax material. However, we believe that the oceanic plate margin (mid-ocean ridge or marginal complex were formed in a marginal basin. Crus- identity of the Halifax complex, together with basin), or in the initial stages of the formation of tal sections produced in marginal oceanic basins the well-defined gravity high to the east, indi- an oceanic volcanic arc, is still questionable, but may not contain certain components of ridge- cates that this part of the eastern Piedmont has we believe that it is representative of the under- generated ophiolites, such as sheeted dikes and oceanic underpinnings. West of the Eastern pinnings of the Eastern Slate Belt volcanic arc. pelagic sediments, and because it is at least Slate Belt, the Raleigh Belt has more continental Its position near the boundary between crustal doubtful that these are present in the Halifax characteristics (Hatcher and Zeitz, 1980; Stod- blocks with distinctive gravity signatures, and complex, perhaps the marginal-basin compari- dard and others, 1982). The occurrence of oce- marked by a linear aeromagnetic anomaly, may son is better. However, basalts recovered from anic material near the boundary between be the surface expression of a Paleozoic plate modern marginal basins (Hawkins, 1977) are oceanic and continental terranes suggests the suture. chemically nearly identical to mid-ocean-ridge possibility that this marks the site of a Paleozoic basalts, so the geochemical match would not be plate suture. The Hollister mylonite zone may ACKNOWLEDGMENTS improved. A still better model might be one in represent the surface along which the suture was which the complex originated during initial reactivated during the late Paleozoic. The research reported herein was supported stages in the development of an oceanic volcanic Several possible tectonic scenarios permitted by grants from the National Science Foundation

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Abstracts with Programs, v. 9, p. 137. Pallister, J. S., and Hopson, C. A., 1981, Samail ophiolite plutor ic suite: Field (EAR7919735) and from the North Carolina 1979,32S to 265 m.y.-old granite plutons in the Piedmont of the south- relations, phase variation, cryptic variation and layering and a model State University Faculty Research and Profes- eastern Appalachians: American Journal of Science, v 279, ofa spreading ridge magma chamber: Journal of Geophysical Research, p. 161-185, v. 86, p. 2593-2644. sional Development Fund (no. 00737) to Stod- Garcia, M. O., 1979, Petrology of the Rogue and Galice formations, Klamath Parker, J. M., Ill, 1968, Structure of easternmost North Carol na Piedmont: dard; it is the subject of the M.S. thesis of Kite. Mountains, Oregon: Identification of a Jurassic island arc sequence: Southeastern Geology, v. 9, p. 117-131. Journal of Geology, v. 86, p. 29-41. Pearce, J. A., 1976, Statistical analysis of major element patterns in basalts: We thank T. N. Solberg (Virginia Polytechnic George, R. P., Jr., 1978, Structural petrology of the Olympus ultramafic com- Journal of Petrology, v. 17, p. 15-43. plex in the Troodos ophiolite, Cyprus: Geological Society of America Pearce, J. A., and Cann, J. R., 1973, Tectonic setting of basic /olcanic rocks Institute and State University) for help with the Bulletin, v. 89, p. 845-865. determined using trace element analyses: Earth and Pla letary Science microprobe and M. M. Kimberley and the En- Glover, L., Ill, 1976, Tectonics of the Piedmont in North Carolina and Virginia: Letters, v. 19, p. 290-300. Review and speculation: Geological Society of America Abstracts with Rankin, D. W., 1975, The continental margin of eastern North Américain the gineering Research Services Division (North Programs, v. 8, p. 181. southern Appalachians: The opening and closing of the proto-Atlamic Haskin, L. A., Haskin, M. A., Frey, F. A., and Wildeman, T. R., 1968, Relative Ocean: American Journal of Science, v. 275-A, p. 298-."l36. Carolina State University) for INAA analyses. and absolute terrestrial abundances of the rare earths, in Ahrens, L. H., Saleeby, J., 1978, Kings River ophiolite, southwest Sierra Nevada foothills, This study benefited from the advice and assis- ed.. Origin and distribution of the elements: New York, Pergamon California: Geological Society of America Bulletin, v. 89, p. 617-636. Press, p. 889-912. Schilling, J.-G., 1975, Rare-earth variations across "normal segments" of the tance of R. D. McDaniel, R. V. Fodor, W. R. Hatcher, R. D., Jr., 1972, Developmental model for the southern Appakichians: Reykjanes Ridge, 60°-53°N, Mid-Atlantic Ridge, 2<>°S, and East Geological Society of America Bulletin, v. 83, p. 2735-2760. Pacific Rise 2°-19°S, and evidence on the composition of the under- Boltin, and R. J. Moye. The manuscript was Hatcher, R. D., Jr., and Zeitz, 1., 1980, Tectonic implications of regional lying low-velocity layer: Journal of Geophysical Research, v. 80, improved after helpful scrutiny by H. Y. aeromagnetic and gravity data from the southern Appalachians, in p. 1459-1473. Wones, D. R„ ed., Proceedings "The Caledonides in the U.S.A.", Shido, F., Miyashiro, A., and Ewing, M., 1971, Crystallization of abyssal tho- McSween, Jr., R. D. Hatcher, Jr., and an anon- I.G.C.P. Project 27, Virginia Polytechnic Institute and State Uriversity leiites: Contributions to Mineralogy and Petrology, v. 31, p. 251-266. Memoir no. 2, p. 235-244. Sinton, J. M., 1980, Petrology and evolution of the Red Mountain ophiolite ymous reviewer. We are grateful to each of these Hatcher, R. D., Jr., Howell, D. E, and Talwani, P., 1977, Eastern Piedmont complex, New Zealand: American Journal of Science, v. 280-A, institutions and individuals. fault system: Speculation on its extent: Geology, v. 5, p. 636-640. p. 296-328. Hyndman, D. 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J., 1973, Trace North Carolina. 36-37°N: Contributions to Mineralogy and Petrology, v. 64, element geochemistry and genesis of the Pindos ophiolite suite: Geo- Zen, E-An, 1981, An alternative model for the development of the alloch- p. 167-195. chimica et Cosmochimica Acta, v. 37, p. 2135-2147. thonous southern Appalachian Piedmont: American Journal of Sci- Fodor, R. V., Keil, K„ and Bunch, T. E„ 1975, Contributions to the mineral Moores, E. M., 1970, Petrology and structure of the Vourinous cphiolitic ence, v. 281,p. 1153-1163. chemistry of Hawaiian rocks-IV, Pyroxenes in rocks from Haleakala complex, northern Greece: Geological Society of America Special Paper and West Maui volcanoes, Maui, Hawaii: Contributions to Mineralogy 118, 74 p. and Petrology, v. 50, p. 173-195. Norman, R. E., and Strong, D. F., 1975, The geology and geochemistry of MANUSCRIPT RECEIVED BY THE SOCIETY MAY 10,1982 Fullagar, P. D., and Butler, J. R., 1977, Carolina corridor geotraverse and the ophiolitic rocks exposed at Ming's Bight, Newfoundland: (Canadian REVISED MANUSCRIPT RECEIVED APRIL 1, 1983 evolution of the southern Appalachians: Geological Society of America Journal of Earth Sciences, v. 12, p. 777-797. MANUSCRIPT ACCEPTED APRIL 12, 1983

Piinted in U .S.A.

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