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The Halifax County Complex: Oceanic Lithosphere in the Eastern North Carolina Piedmont

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The Halifax County Complex: Oceanic Lithosphere in the Eastern North Carolina Piedmont 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 peridotite, pyroxenite, 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 minerals dunite-pyroxenite-anorthosite and leucogabbro- equivalents of associated mafic and ultramafic within the Halifax County complex. These data, gabbro-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 metamorphism. 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 olivine of F079, clinopyroxene with com- terrain with an antiformal structure. Most Mineral 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 magnetite, that originated in an early Paleozoic volcanic arc rock 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 basalts 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/4/422/3419352/i0016-7606-95-4-422.pdf by guest on 29 September 2021 OCEANIC LITHOSPHERE, NORTH CAROLINA PIEDMONT 423 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, pyroxene. 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 gabbros, 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 (lherzolite and abundant clinopyroxene
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