GEOCHEMICAL RESULTS FROM A SECTION OF THE BUCK CREEK MAFIC/ ULTRAMAFIC COMPLEX, NC.
CONSTRAINTS ON CURRENT MODELS FOR THE ORIGIN OF THE BUCK CREEK MAFIC/ULTRAMAFIC COMPLEX, WESTERN NC.
STUDY OF THE METAMORPHIC EVOLUTION OF THE BUCK CREEK MAFIC/ULTRAMAFIC COMPLEX, NC.
RARE EARTH ELEMENT SYSTEMATICS OF THE BUCK CREEK MAFIC/ULTRAMAFIC COMPLEX, NC.
GEOCHEMICAL CONSTRAINTS ON AMPHIBOLITES IN THE BUCK CREEK MAFIC-ULTRAMAFIC COMPLEX, NC.
LITHOLOGIES AND STRUCTURE OF THE BUCK CREEK MAFIC/ULTRAMAFIC COMPLEX, NC
STONESIFER K., FREY B., MANOOCH S., THORNBERRY T., BECKER A., COLLINS J., GASKIN P., LUCAS S., O'LEARY J., PALMER J., SIMONS K,, SLUSSER T. (1997 REU Program); V.L. PETERSON (Geosciences, Western Carolina Univ., Cullowhee, NC); M. EMILIO, J.G. RYAN (Dept. Geology, Univ. South Florida, Tampa, FL)
Detailed mapping of part of the Buck Creek mafic/ultramafic complex (BC), western NC, supports interpretation of observed lithologies as part of a layered mafic igneous complex. We identify four major rock types in the Corundum Knob area: dunite, troctolite, edenite-margarite schist (EMS) (hydrated metatroctolite), and hornblende amphibolite. Field contacts appear sharp to gradational; however, chemical, mineralogical and physical gradation is present between all rock units. The following field and contact relations are observed: 1) Layering reflects variations in mineralogy and grain size. 2) The ultramafic body is dominated by dunite with thin interlayers of metatroctolite/EMS and underlain by the mafic hornblende amphibolite. 3) Metatroctolite is gradational to dunite. 4) EMS is found between dunite and metatroctolite and gradational to both. 5) Amphibolite is exposed at the top of Corundum knob within the main ultramafic body. 6) The main mafic/ultramafic contact is a complex zone of interlayered dunite, metatroctolite, EMS and amphibolite. 7) Talc-chlorite veins crosscut the dunites, likely reflecting a late hydrothermal event.
The rocks on Corundum Knob exhibit both ductile and brittle behavior. Pervasive foliation typically strikes N30E and N70W. Crenulations with NE axes are locally developed in micaceous rocks. Dunites have annealed textures and outcrop-scale ductile shear zones are found locally. We recognize some of the numerous late brittle faults mapped by Hadley (1949); however, the discontinuous nature of the troctolite layers could easily result from boudinage due to relatively stiff behavior of these dry, plagioclase-rich rocks earlier in the deformation history. Preliminary hypotheses are suggested to explain the structural complexities observed on Corundum Knob. The mafic-ultramafic sequence may represent an overturned section of oceanic lithosphere. Thrust juxtaposition cannot be ruled out, however chemical similarities suggest a common source for the mafic-ultramafic rocks. Variations in foliation and the discontinuous nature of metatroctolite bodies could be explained by early boudinage or by dome and basin fold interference. [Back to top]
GEOCHEMICAL RESULTS FROM A SECTION OF THE BUCK CREEK MAFIC/ ULTRAMAFIC COMPLEX, NC
COLLINS J., BECKER A., PALMER, J., SIMONS, K., FREY B., GASKIN P., LUCAS S., MANOOCH S., O'LEARY J., SLUSSER T., STONESIFER K., THORNBERRY T. (1997 REU Program); J.G. RYAN, R. BEK, M. EMILIO (Dept. Geology, Univ. South Florida, Tampa, FL); V.L. PETERSON (Geosciences, Western Carolina Univ., Cullowhee, NC)
Ongoing field studies of the Buck Creek mafic-ultramafic complex have demonstrated that it is a complexly deformed and metamorphosed body comprising four discernible lithologies: dunite, metatroctolites, edenite-margarite schist (EMS) and amphibolite. Representative samples encompassing all observable variation were prepared and analyzed for major and trace elements at the University of South Florida. Our results demonstrate that the mappable units at Buck Creek represent a continuous gradation in igneous protolith from cumulate dunite through troctolite into gabbro.
The average dunite is 40.8wt% SiO2 44.2wt% MgO and 11.3wt% Fe2O3*. Some samples which classify as dunites contain significant CaO and Al2O3, suggesting they represent cumulus dunites with minor plagioclase. The meta-troctolites are highly variable in MgO (7.83-34.3wt%) and average 20.6wt% Al2O3 and 11.7wt% CaO. The EMS rocks were the most heterogenous rock type, including gabbros, altered troctolites and peridotites. With a few exceptions, EMS compositions are intermediate between the dunites and troctolites. The amphibolites are divisible into two groups based on TiO2 content; the first group averages 0.19wt% and the second 1.22wt%, with a few intermediate Ti samples. Amphibolite chemistries are comparable to gabbroic and basaltic rocks.
A plot of MgO vs Al2O3 shows a continuous variation in abundances, suggesting a near-continuous gradation in igneous protolith. Whole rock geochemistry points to the association of metamorphic peridotites with ultramafic cumulates, gabbros and basaltic differentiates, indicating an oceanic crust origin. Further study of trace element systematics for Nb, Ta, Y, Zr and REEs may help to confirm this hypothesis, but trace element data could be problematic due to the effects of metamorphism. [Back to top]
CONSTRAINTS ON CURRENT MODELS FOR THE ORIGIN OF THE BUCK CREEK MAFIC/ULTRAMAFIC COMPLEX, WESTERN NC
SLUSSER T., GASKIN P., BECKER, A, LUCAS S., COLLINS J., PALMER J., SIMONS K., FREY B., MANOOCH S., O'LEARY J, STONESIFER K., THORNBERRY T. (1997 REU Program); V.L. PETERSON (Geosciences, Western Carolina Univ., Cullowhee, NC); J.G. RYAN, M. EMILIO (Dept. Geology, Univ. of South Florida, Tampa, FL)
The Buck Creek mafic/ultramafic complex (BC) in the western NC mountains, is one of approximately 250 isolated Appalachian ultramafic bodies. The BC includes fresh to moderately serpentinized dunite, locally corundum-bearing metatroctolite, and amphibolite. Models for the origin of the BC and other southern Appalachian ultramafic bodies include emplacement in a melange (Laccazette & Rast, 1990), as a piece of ocean floor in an ophiolite complex (MacElhaney & McSween, 1983), or as a mantle diapir (Yurkovich, 1973; Carpenter & Phypher; 1968). Detailed mapping and geochemical sampling in the Corundum Knob area of the BCUC provide new constraints on these models.
BC lithologies have related major and trace element chemistries. Metatroctolites are interlayered and chemically, mineralogically, and structurally gradational with dunite and hornblende amphibolite. The mafic-ultramafic contact shows complex interlayering between all BC lithologies. Tenthorey et al. (1996) show BC meta-troctolites to have an early deep-dry metamorphic history. The dry character of BC units is inconsistent with an accretionary wedge melange and traditional models for ophiolite emplacement, though emplacement of the entire BC as a mega-block in a deep, dry fault zone is viable and BC lithologies are comparable to those in Semail-type ophiolites. Troctolite implies early plagioclase crystallization and a relatively dry parent magma in a low-P magma chamber; unlikely in a sub-arc, back-arc, or deep subcrustal setting (i.e., Hartley, 1973; Meen, 1988). Geochemically and mineralogically, Buck Creek rocks show a distinctly crustal character (see also Ranson & Garihan, 1993), suggesting that emplacement as an ultramafic diapir may be inappropriate.
Our data supports origin of the BC as a basal cumulate within a MOR magma chamber that was partially subducted. Subduction polarity reversal (as in the Bismarck and Solomon arcs) may have left the crustal segment containing the BC preserved as a hanging slab. Later collision with a continental or island arc landmass led to a double thickness of crust with emplacement of BC rocks at 30 km depths.[Back to top]
STUDY OF THE METAMORPHIC EVOLUTION OF THE BUCK CREEK MAFIC/ULTRAMAFIC COMPLEX, NC
EMILIO, M.C., J.G. RYAN (Dept. Geology, Univ. South Florida, 4202 E. Fowler Ave., Tampa, FL) V.L.PETERSON (Geosciences, Western Carolina Univ. Cullowhee, NC)
The Buck Creek complex (BC) is one of a series of ultramafic bodies located in the high-grade metamorphic belt of the Blue Ridge Mountains. Models for its origin (and for the tectonic history of these bodies) are critically dependent on its metamorphic record and the history of the P-T-Ph20 changes recorded in its mineral assemblages. BC rocks are well suited to such studies as they include troctolite, amphibolite, and dunite, which preserve both dry and wet metamorphic assemblages over a range of grades and protoliths.
Results on associated troctolite-EMS (edenite-margarite schist) interlayers in BC suggest two main metamorphic assemblages 1) opx + cpx/sp (+/- sappahirine +/- amp) coronal symplectite, indicating hot dry metamorphism (Tenthorey et al, 1996), and 2) a hydrated assemblage of amp + an + marg +/- cor +/- zo +/- ky +/- trem and fine grain alteration products. At least two stages of fluid influx are indicated: 1) a High P-T input during a regional metamorphic event, and 2) and a localized, retrograde alteration. The primary hydrated assemblage (amp + marg + an) is foliated and even creunulated in places suggesting that this metamorphic signature was associated with regional deformation. The secondary event produces fine grained alteration rims around corundum, alteration along cleavage traces in anorthite, and amphibole and bladed crystals which randomly overgrow the foliation. Sources for fluids may have been fluid released by devolitization within BC or derived from the surrounding metasediments.
According to the petrogenic grid of Perkins et al (1986) the reaction of marg <=> cor + zo + ky + H2O will occur over a range of 14-10 kb and 550-700 deg. C. This would indicate that the hydrated EMS assemblages are of a high P-T regime and similar to the hot dry assemblages of the troctolites (Tenthorey et al,1996). The presence of kyanite in the EMS allows for further placement of constraints on the stability of sappahirine in the BC by excluding the portion of the sappahirine stability field which lies in the sillimanite field. A new range of 800 deg. C at 10kb to 800-900 deg. C at 11.5kb in the BC troctolites is proposed.[Back to top]
A RARE EARTH ELEMENT SYSTEMATICS OF THE BUCK CREEK MAFIC/ULTRAMAFIC COMPLEX, NC
SIMONS, K. (Dept. Geol. Sci, Univ. Miami, Coral Gables, FL 33124; (305) 284-4253; email: Kyla.K.Simons@students.Miami.EDU); J.G. RYAN, B. FREY (Dept. Geology, Univ South Florida, Tampa, FL) V.L. PETERSON (Geosciences, Western Carolina Univ, Cullowhee, NC)
The Buck Creek mafic/ultramafic complex (BC), Clay Co., NC, consists of several interleaved and metamorphosed rock units: dunite, metatroctolite, edenite schist, and amphibolite. Geochemical results suggest the protoliths of these units are ultramafic and mafic cumulates (dunites, troctolites and gabbros) such as might be produced in a mafic magma chamber. Extensive, high-grade metamorphism has altered these rocks significantly in their mineralogy, and has modified much of their trace element geochemistry. In order to establish genetic relationships between the BC rocks and determine their igneous origins, we must examine trace elements which are resistant to metamorphic effects. Rare earth element systematics (REE's) offers such a tool, as these elements are relatively immobile during metamorphism, and REE patterns can be used to constrain igneous relationships and define the conditions of magma formation.
Samples selected for analysis come from the BC sample suite collected during the 1997 REU Summer program, and from samples analyzed by Tenthorey et al (1996). Samples were selected to include compositions ranging from olivine-rich trotolites to rocks with a gabbroic character (edenite schists and amphibolites). As well, pyroxenitic samples from the Lake Chatuge and Webster Addie mafic/ultramafic complexes, and mafic granulites from Winding Stair Gap will be analyzed to compare BC to like associations regionally. REE analyses are conducted via ICP-MS (at USF-St. Petersburg) on solutions prepared via Na2CO3 fusion and digestion of the H2O-insoluble fusion cake in dilute HNO3. Results on USGS standards BHVO-1 and BIR-1 suggests quantitative retention of REE's in solution, and precision of +/- 10-15% at 1 ppm levels in the rock. Preliminary results indicate that all BC samples save the dunites contain high enough REE abundances for determination without a column step.
Preliminary data on
magnesian troctolite BC-1 (approx 35% plagioclase), indicates LREE
abundances of 1-3 x chondrites and HREE at < 1.0 x chondrites.
Calculated REE abundances for a melt in equilibrium with BC-1 indicate
[La/Sm]N of 2.5 +/- 1.0, suggesting slight LREE enrichment. Data on
more evolved rocks will place further constraints on the tectonic
setting of BC formation..
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GEOCHEMICAL CONSTRAINTS ON AMPHIBOLITES IN THE BUCK CREEK MAFIC-ULTRAMAFIC COMPLEX, NC
SLUSSER, T. (Geosciences, Western Carolina Univ., Cullowhee, NC 28723; tomjen@wcu. campus.mci.net); J.G. RYAN (Dept. Geology, Univ. South Florida Tampa, FL); V.L. PETERSON (Geosciences, Western Carolina Univ.)
In the Buck Creek mafic-ultramafic Complex (BC) (Clay Co., NC), ultramafic units (dunite, locally corundum-bearing metatroctolite, edenite-margarite schist) are surrounded by extensive exposures of amphibolites. Minor outcrops of amphibolite also occur within the ultramafic body, crosscutting both dunites and troctolites. Models for the petrogenesis of the BC amphibolites have been critical to past petrologic and tectonic interpretations of the complex (McElhaney and McSween, 1983; Lacazette and Rast, 1990). To assess past models for the BC, and to test ideas from studies of BC ultramafic rocks during the REU summer program, we are conducting a geochemical survey of amphibolites from across the Buck Creek complex. Amphibolite samples were collected in the Type 1 and Type 2 outcrop areas outlined by McElhaney and McSween (1983). As well, amphibolite outcrops within the dunite (analyzed as part of the REU program) are included, along with samples previously collected by Tenthorey (1994) and Willse and Ryan (1995) All are dominantly amph (hornblende)+plag, with minor mag/ilm+neph+spin(?), and local cpx and opx.
Amphibolite SiO2 content range from 46-50% wt; MgO from 7-14%; Al203 from 13.-23%; CaO from 10-15%. The major oxides are consistent with a mafic igneous protolith, and show no clear geographic patterns across BC. However, TiO2 and Ni change with distance from the BC dunites and troctolites: 0.1% and 916 ppm, resp., near dunite-amphibolite contacts, to 1.8% and 189 ppm away from the contacts. Amphibolite outcrops within the BC dunite are also high Ti. The gradations in Ti and Ni, and the relatively high Ni and Cr (251-607 ppm) contents point to a cumulate gabbroic protolith for BC amphibolites Preliminary data for Zr and Y contents (30-40 ppm Zr, 20-30 ppm Y) are conistent with a cumulate origin, but may imply high contents of Zr in the parent magma. Further geochemical work will attempt to identify differences between the Type 1 and Type 2 amphibolites, and to place constraints on the tectonic environment of the BC from its parent magma composition.[Back to top]
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