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Comment and Reply on "Arc rifting of the Carolina terrane in northwestern South Carolina"
Rodingites from the Southern Appalachian Piedmont, South Carolina, USA
Comments and Replies on "Ocmulgee fault: The Piedmont-Avalon terrane boundary in central Georgia"
Foreword
The Baltimore mafic complex in the central Appalachian Piedmont consists of tectonically emplaced layered ultramafic, mafic cumulate, and volcanic rocks. The general stratigraphy of the layered rocks is basal dunite and chromitite grading upward through lherzolite and websterite to hypersthene gabbro and quartz-gabbro at the top. Cyclicity within the cumulus sequence is shown by the presence of upper-level peridotite and the occurrence of olivine gabbro at higher stratigraphic levels. The cumulus crystallization sequence is olivine plus chrome spinel followed by orthopyroxene, clinopyroxene, and plagioclase. Cryptic variation in the composition of the cumulus minerals is shown by olivine Mg 97–75 , orthopyroxene Mg 90–49 , clinopyroxene Mg 92–59 , and plagioclase An 97–73 . Coexisting pyroxenes define a wide miscibility gap reflecting subsolidus reequilibration. Pre- and post-tectonic emplacement recrystallization histories in the metagabbros are shown by textural relationships and compositions of amphiboles. Post-emplacement metamorphic recrystallization and chemical redistribution occurred after tectonic emplacement into the metasedimentary rocks of the Glenarm Series. Strontium and neodymium isotopic data show that the parent magma was contaminated by a continental component prior to extensive crystallization. Association of the plutonic rocks with metavolcanic gneisses characterized by bimodal chemistry suggests that the complex may have formed in a marginal basin originating inland from a continental margin volcanic arc or as a sub-arc plutonic complex.
Review and classification of ultramafic bodies in the Piedmont of the Carolinas
Ultramafites in the Piedmont of North Carolina and South Carolina occur mainly in four types of bodies. (1) In many parts of the Inner Piedmont, small pods and lenses of Alpine-type altered dunite and peridotite are typically less than 200 m long, and their emplacement predates the main regional deformation and metamorphism. (2) In the south-central Inner Piedmont of South Carolina, more than 100 bodies of potassic ultramafic rocks occur in an area of several hundred square kilometers; the bodies are probably metamorphosed lamproites. (3) In the western Charlotte and Kings Mountain belts, peridotite, clinopyroxenite, and hornblende-bearing ultramafites are gradational into gabbroic rocks in large complexes that extend over tens of square kilometers; the ultramafites are probably cumulates formed in the roots of a calc-alkaline magmatic arc. (4) Several occurrences of ultramafic and related rocks interpreted to be dismembered ophiolites or ophiolitic mélanges are located near the Inner Piedmont–Kings Mountain belt boundary, on the flanks of the Raleigh belt, and in the Kiokee belt.
Primary pods and layers of chromitite occur locally in bodies of metamorphosed rock within the Falls Lake mélange, eastern Piedmont of North Carolina. The chromitite, together with its ultramafic host and the enclosing quartz-mica schist were subjected to Paleozoic regional metamorphism within the kyanite-staurolite zone of the amphibolite facies. The chromitite-bearing ultramafic body is characterized by the assemblage talc + chlorite + actinolite + magnetite. The chromitite displays pull-apart texture with metamorphic minerals occurring within the vein-like interstices between spinel host material. The vein assemblages contain various combinations of unusually Cr-enriched corundum, kyanite, tourmaline, margarite, chlorite, and muscovite (fuchsite). By analogy with skarns, the origin of these minerals is attributed to metasomatic interaction among three distinct chemical reservoirs: the primary Mg-Al-Cr spinels, the ultramafic host rock, and the surrounding schistose mélange matrix.
The Hammett Grove Meta-igneous Suite; A possible ophiolite in the northwestern South Carolina Piedmont
The Hammett Grove Meta-igneous Suite, here named formally, is composed of altered ultramafite (soapstone and serpentinite), metapyroxenite, metagabbro, and metabasalt lithodemes. It is interpreted to represent a dismembered ophiolite that may have formed as fore-arc basement to the Carolina arc terrane. This amphibolite-facies and retrograde greenschist-facies suite of metamorphosed, comagmatic igneous rocks crops out in the Piedmont of northwestern South Carolina near the eastern edge of the Inner Piedmont belt. The northeasternmost part of the suite lies within the Kings Mountain shear zone, which constitutes the Inner Piedmont belt-Kings Mountain belt boundary. The suite is interpreted as a thrust slice or klippe derived from the Kings Mountain belt, implying that the boundary is—at least in part—an overthrust. As ophiolites occurring in ancient orogenic terranes are often related to fundamental boundary tectonics, it is proposed that the Inner Piedmont-Kings Mountain belt boundary represents a terrane suture and marks an accretionary event. Thrusting of the Hammett Grove Suite over rocks of the Inner Piedmont resulted either from orthogonal terrane accretion or from transpression related to wrench faulting.
Ultramafic chlorite-tremolite-olivine schists: Three bodies from the Inner Piedmont belt, South Carolina
An ultramafic schist body near Walhalla, South Carolina, contains abundant chlorite, tremolite, and olivine. The olivine is of metamorphic origin and typically occurs as centimeter-sized, sieve-textured porphyroblasts. Accessory minerals include anthophyllite, serpentine, dolomite, ilmenite, and pyrite. Bulk chemical analyses indicate that the ultramafic rocks contain 39.5 ± 1.5 wt percent SiO 2 , generally about 30 wt percent MgO, and AI 2 O 3 in excess of 5 wt percent. Texturally, mineralogically, and chemically, the Walhalla ultramafic body is very similar to ultramafic schist bodies at Clemson and Seneca, South Carolina. All three bodies occur in the Inner Piedmont belt and were subjected to middle amphibolite facies metamorphism. The peak metamorphic assemblage, chlorite + tremolite + olivine ± anthophyllite, is common to the three localities. The occurrence of dolomite in many samples indicates that the fluid phase present during prograde metamorphism contained CO 2 . The ultramafic rocks were affected by a retrograde metamorphic event (lower amphibolite facies?), as evidenced by late-stage growth of serpentine or talc. Development of a mosaic equigranular olivine texture, and microfolding of chlorite laths, are localized features that also originated after the peak of prograde metamorphism. Although regional metamorphism effected a thorough recrystallization of the ultramafic protolith, it may have been principally isochemical. If so, the ultramafic protolith was itself relatively aluminous, with a general chemical affinity to plagioclase (or spinel or garnet) peridotite. Possibly, the bodies represent dismembered ultramafic cumulates torn from an ophiolite sequence. However, rare-earth element (REE) abundances are higher, and the chondrite-normalized patterns are flatter in the Piedmont ultramafic rocks than in corresponding ophiolitic cumulates.
The Burks Mountain complex, Kiokee belt, southern Appalachian Piedmont of South Carolina and Georgia
A region of migmatitic felsic paragneiss and pelitic schist containing concordant pods of serpentinite, talc schist, talc-amphibole schist, amphibolite, and metagabbro—here called the Burks Mountain complex—occurs in the southeastern limb of the Kiokee antiform, an Alleghanian D 3 structure in the eastern Piedmont of Georgia and South Carolina. A similar region of migmatitic felsic paragneiss containing small pods of metamorphosed mafic and ultramafic rocks, which occurs in the northwestern limb of the Kiokee antiform, is inferred to be a continuation of the Burks Mountain complex across the crest of the antiform. The composition of the felsic paragneiss and pelitic schist of the Burks Mountain complex suggests derivation from graywacke and shale, respectively. Relict textures preserved in mafic and ultramafic rocks of the complex, along with preliminary whole-rock geochemistry, indicate derivation from ultramafic tectonite (harzburgite), olivine-pyroxene cumulate (wehrlite, olivine pyroxenite), pyroxene-plagioclase cumulate (gabbro, anorthositic gabbro), and mafic volcanic rock. These protoliths suggest an origin as part of a cumulate mafic and ultramafic intrusive and extrusive complex with some associated mantle tectonite. The complex was disrupted prior to or during amphibolite facies regional metamorphism. The mechanism of disruption is not clear. Possible modes of origin include accumulation as an olistostrome, accumulation in a subduction-related accretionary complex, and formation as extreme boudinage during a regional deformation event. The crystalline rocks in the core of the Kiokee belt (including the Burks Mountain complex) are contained in the footwall of a major ductile shear zone (the Modoc zone) that experienced oblique, down to the north-northeast displacement during the early part of the Alleghanian orogeny. Prior to the Alleghanian orogeny, the Burks Mountain complex was located beneath rocks of the Carolina slate belt that are presently exposed north of the Modoc zone. The Burks Mountain complex may have been derived from a unit within the Carolina slate belt or from a Precambrian basement(?) unit beneath the Carolina slate belt. Alternatively, in view of a unique lithostratigraphy, the Burks Mountain complex may be included in a terrane, exotic with respect to both North America and the Carolina slate belt, which was tectonically incorporated into the Appalachians prior to the Alleghanian orogeny.
The Berner mafic complex, part of the Avalonian arc system in central Georgia, comprises a series of layered metavolcanic gneisses, of mafic to felsic compositions, that have been intruded by a series of plutonic rocks. Ultramafic rocks occur either as thin layers within mafic plutonic rocks, or as layers and pods within layered mafic gneisses. All ultramafic rocks within the complex are metamorphosed, highly altered, and penetratively deformed. Consequently, their origin is somewhat problematic, although most are considered to be comagmatic with their mafic host rocks. The current spatial distribution of mafic-ultramafic bodies within the layered-series gneisses of the Berner mafic complex is considered to result from the heterogeneous, postemplacement tectonic disruption of a series of mafic-ultramafic plutonic rocks that were intruded at various times into a developing arc complex.
Base and precious metal stream-sediment geochemistry related to mafic-ultramafic bodies in the Alabama and adjacent Georgia Inner Piedmont
The Alabama and adjacent Georgia Inner Piedmont consists of a complex suite of metasedimentary and metavolcanic units that could have associated syndepositional hydrothermal systems. To evaluate the potential presence of such systems, a preliminary reconnaissance (silt and heavy minerals stream sediment) geochemical survey, consisting of 553 sites, was conducted. Silts were analyzed for Cu, Pb, and Zn; panned concentrates, for Au. Measured geochemical averages and the corresponding standard deviations for the samples are 16 ± 10 ppm Cu, 9 ± 6 ppm Pb, 37 ± 17 ppm Zn, 61 ± 29 ppm combined Cu-Pb-Zn, and 0.128 ± 0.751 ppm Au. Moderate correlation is shown among Cu, Pb, and Zn, but Au is statistically independent of the base metals. Streams draining the well-documented mafic-ultramafic bodies of the Doss Mountain, Slaughters, and Easton areas exhibit low base and precious metal values. However, the generally similar areas of the Boyds Creek synform and the West Point mélange have anomalously high Cu-Pb-Zn and Au signatures that coincide locally with zones of tourmalinite, iron formation, gossan, and altered ultramafic bodies. These data suggest that hydrothermal systems were associated with the development of the ophiolitic suites of the Boyds Creek synform and nearby West Point mélange, and that stream-sediment geochemical anomalies within these areas may locally reflect mineralization related to such metal-rich systems.