On most tectonic classification diagrams, the WA-WI amphibolites do not plot coherently. On the V vs. Ti classification plot of Shervais (1982) and the

Spider Diagram of trace elements for WI/WA, BC, & CK

from Pearce (1983); Sc and Cr from Pearce (1982); Ni from Bevins et al. (1984)

0

2

4

6

8

10

12

14

Ba K Sr Ti Sc Cr Ni

Trace Element

On most tectonic classification diagrams, the WA-WI amphibolites do not plot coherently. On the V vs. Ti classification plot of Shervais (1982) and the TiO2 vs. Fe# plot of Serri (1981), the WI-WA rocks plot largely in the oceanic basalt fields. On the V vs. Fe/Mg classification diagram of Desmos (1980), WA-WI rocks fall largely between the calc-alkaline and tholeiitic fields, and on the V vs. Cr plot of Pearce (1973), most of the WA-WI samples fall outside the field for MORBs.

The WA-WI amphibolites differ from calc-alkaline lavas in that they have high concentrations of TiO2, evidenced in the samples as abundant titanite, ilmenite, and rutile. The process of partial melting during migmatite formation sequesters Fe-Mg-Ti bearing minerals such that on the scale of hand samples, anomalously high (and low) Fe and Ti concentrations are possible, depending on whether the sample included more leucosome vs. melansome.

Interestingly, even if one takes the mean values of our amphibolite samples, they still plot anomalously on classification diagrams. Options for explaining these patterns include the extraction of anatectic melt, which would shift the compositions of the rocks (and would imply their protoliths were even more felsic), or the WA-WI amphibolites may not, in fact, represent igneous protoliths.

Constraints on the origin of Webster-Addie/Willets amphibolites

WA-WI amphibolites occur as lenticular bodies in the field, on a variety of scales, and may or may not be in close association with ultramafic rocks.

Amphibolite fabrics and textures range from strongly foliated to migmatitic. "Block in Matrix" structures are commonly observed in the field.

Mineral assemblages are Hbld + Plag + Qz + Bio ± Gt+ Titanite ± ilmenite ± rutile, with chlorite, clinozoisite, and carbonates as secondary metamorphic phases.

Major-element chemistry of WA-WI amphibolites points to probable igneous rock protoliths of intermediate, calc-alkaline compositions. The processes of metamorphism, migmatization, and melt extraction have extensively redistributed even "immobile" trace elements, preventing more detailed assessments of tectonic affinities.

In terms of composition and probable protolith, the WA-WI amphibolites are profoundly different from those associated with the Buck Creek and Carroll Knob complexes to the SW. The differences in the amphibolites may indicate different origins and histories for the rocks in these regions of the Blue Ridge.

WI01RF1A WI01RF1E WA01SP8E WA01RF09 WI01RF08

SiO2 (wt%) 62.13 60.92 52.53 49.58 52.55

Al2O3 14.64 15.67 15.76 16.30 16.06

Fe2O3 7.07 7.09 8.73 11.83 9.22

MgO 3.98 4.19 6.86 8.62 6.48

CaO 4.28 5.48 8.81 11.46 10.13

MnO 0.09 0.09 0.18 0.25 0.15

K2O 2.57 1.95 0.88 0.77 0.83

Na2O 2.85 3.48 3.78 1.15 3.09

TiO2 1.27 1.11 0.95 1.40 1.17

Sr (ppm) 195 257 268 172 253

Ba 671 551 136 138 114

Ni 44 49 62 98 75

Sc 11 14 24 43 29

Cr 113 110 180 327 228

V 132 121 156 272 215

Zn 94 96 121 182 121

Cu 37 61 31 23 3697.61 98.87 97.53 99.96 98.51

CIPW NormQ 19.19 14.35 0 2.75 1.87

An+Ab 44.41 51.09 56.67 46.51 54.17

Or 15.42 11.58 5.32 4.55 4.96

Di 1.52 4.85 16.76 15.93 18.59

Hy 13.9 12.94 13.79 22.5 14.13

Ol 0 0 1.76 0 0

Ilm 2.47 2.13 1.84 2.66 2.24

Mg 3.12 3.1 3.89 5.12 4.05

Chr 0.03 0.03 0.04 0.07 0.04

Typical Analyses and CIPW Norms of WA-Wi Amphibolites

Rare-Earth Element Systematics of Webster-Addie/Willits/Balsam Gap Mafic and Ultramafic RocksRachel Soraruf, Mount

Holyoke College, South Hadley, MA Jeff Ryan, University of South Florida, Tampa, FL and The 2001 REU Program

Rare earth element abundances were measured on samples collected during the 2001 REU summer program from mafic and ultramafic rock exposures in the Willits area (WI), and in and near the Webster-Addie (WA) and Balsam Gap (BG) ultramafic bodies of the eastern Blue Ridge province of NC. Samples were dissolved at the Univ. South Florida via a Na2CO3 fluxed fusion method, and analyzed for REE abundances by ICP-MS as the University of Boston. Amphibolites from Willits and Webster-Addie show LREE-enriched patterns with 10-50x chondrites HREE abundances. Leucosome-melanosome splits from migmatitic amphibolites show complementary patterns, with HREE and Eu enrichments in the melanosome. Amphibolitized zones within the Webster-Addie ultramafic body show lower REE abundances overall (0.5 -10x chondrites) and concave-up REE patterns; while a fresh pyroxenite from the Balsam Gap body shows a "U" shaped REE pattern at 0.5-1x chondrite levels. The WI-WA amphibolites from these two regions display different REE systematics than that of the Buck Creek Complex (BC). Berger and others (2001) found BC samples to be consistently LREE depleted, and suggested an origin from MORB-like protoliths. REE and bulk chemical systematics of WI-WA amphibolites suggest subduction-related protoliths, which have been intensely reworked by metasomatism. The BG pyroxenite shows a similar REE pattern to pyroxenites from the Moore's Knob and Webster-Addie ultramafic bodies (see Berger et al. 2001), raising the possibility that these units have similar origins.

Major and Trace Element Geochemistry of Willits/Webster-Addie amphibolites

Summary of Observations and Inferences:

Regional Map of NC Blue Ridge, showing mafic-ultramafic rock occurrences

The amphibolites of the WA-WI-BG area range from lenticular horizons in the country rock, to regionally extensive occurrences of migmatitic amphibolite. Amphibolitized zones also occur within the nearby Webster-Addie ultramafic body.

mailto:WA-@I-BG

Standard Reference Material for BHVO-1: A Comparison of USGS Recommended

and BU ICP-MS

Sampling and Analytical Methods

Samples analyzed for REE abundances were selected from the larger suite of WA/WI/BG mafic and ultramafic rocks that were collected and characterized during the 2001 Blue Ridge REU Summer Program. To facilitate comparisons to the Buck Creek complex (Berger et al 2001) and other mafic rocks, we focused primarily on those samples mapped as amphibolites, and on amphibolitized zones within the Webster-Addie ultramafic body. Where available, fresh pyroxenites from both the Webster-Addie and Balsam Gap ultramafic bodies were prepared, so as to facilitate petrogenetic comparisons among these units.

All REE samples were digested via an Na2CO3 fluxed fusion method, following the methods of Berger et al. (2001). Fusion cakes undergo a water leach to pull of excess Na2CO3 and silica, leaving the REE quantitatively in the insoluble fraction. These residues were reacted in ultra-pure HNO3, and diluted to 1500:1 for analysis.

ICP-MS analyses for REE abundances were done in the Geological Sciences Department of the University of Boston, using a Thermo Elemental PQ ExCell ICP-MS facility maintained by Dr. Terry Plank. Sample data were calibrated against USGS reference materials BHVO-2 and BIR-1, which were run as unknowns during each run. No internal standards were used, nor were any necessary, as the signal vs. intensity relationship on the ExCell instrument is essentially constant over the mass range of the REEs.

Based on reproduction of reference material concentrations, and on analyses of replicate sample solutions, the precision of our method for the REEs is on the order of ±10%

Element USGS Recommended BU ICP-MSLa 0.63 0.6Ce 1.95 1.9Pr 0.38 0.4Nd 2.50 2.4Sm 1.10 1.1Eu 0.54 0.5Tb 1.85 0.4Gd 0.36 2.0Dy 2.50 2.6Ho 0.57 0.6Er 1.70 1.7Yb 0.26 0.0Lu 1.65 1.7Tm 0.26 0.3 Rare-earth element patterns of WA-WI amphibolites range from flat

to distinctively LREE enriched, with relatively flat HREE patterns.

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu


The migmatization that many of these rocks have undergone does impact the REE patterns of samples. Here, we have examined the leucosome and melanosome of a coarsely migmatitic amphibolite from the Willits area. The amphibole and biotite-rich melanosome exhibits a strongly LREE enriched pattern, while the feldspathic leucosome shows a “U” shaped pattern. The average of these two patterns is very comparable to the typical REE signatures that the WA-WI amphibolites preserve overall.


The amphibolitized rocks of the Webster-Addie ultramafic body preserve a very different REE signature. Overall REE contents in these rocks are 1-2 orders of magnitude lower than in the amphibolites, and they exhibit fractionated REE patterns that are distinctly different from those of the WA-WI amphibolite suite (field on diagram). The shapes of these patterns are similar to those of igneous amphiboles (see McKay, 1989) indicating that amphibole is probably the only REE-bearing phase in these rocks, and that this signature is being added to a very REE-depleted substrate.


We have thus far analyzed only one pyroxenite, sample BG01CL002, an orthopyroxenite from the Balsam Gap ultramafic body. Here, we compare this sample to three others, an orthopyroxenite from the Moores Knob ultramafic body to the SW, a websterite from the Webster-Addie body, and a clinopyroxenite from the Buck Creek ultramafic complex (all analyzed by Berger et al (2001)). There is a marked similarity between the REE pattern of our Balsam gap pyroxenite and those from Webster-Addie and Moores Knob. All of these bodies lie within a zone of migmatitic, olistostromal rocks which first appear east of Franklin, NC, and extends to the NE (see Raymond et al. 1989). By contrast, the pyroxenite from Buck Creek shows both much higher overall REE concentrations, and a LREE-depleted pattern, consistent with an origin as part of an oceanic crustal section (Berger et al. 2001, Peterson and Ryan, in press).

Sample Name La Ce Pr Nd Sm Eu Tb Gd Dy Ho Er Yb LuAmphibolitesWI01AS10 32.7 66.6 7.7 31.1 6.7 2.0 0.9 6.5 5.2 1.0 2.4 2.1 0.3WA01RF009 7.2 13.8 2.6 11.8 3.6 1.2 0.8 4.7 5.2 1.1 2.9 2.9 0.4WA01SP8E 8.8 20.1 3.0 12.8 3.5 1.1 0.7 4.4 4.7 1.0 2.6 2.7 0.4W101RF03 35.5 84.1 11.4 52.3 12.7 2.8 2.1 13.4 11.9 2.5 6.4 6.2 0.9WI01RF008 5.9 14.2 2.2 10.3 3.2 1.1 0.7 4.3 4.8 1.0 2.7 2.7 0.4WI01RF002C 4.1 9.3 1.5 7.1 2.4 1.0 0.6 3.4 3.9 0.8 2.2 2.2 0.3WI01RF1C 9.03 19.15 2.68 11.98 3.46 1.05 0.77 4.62 5.22 1.09 2.87 2.82 0.42WI01AS7A 8.1 19.9 2.8 12.7 3.6 1.4 0.8 4.6 5.0 1.1 2.8 2.8 0.4

replicate 8.91 19.60 2.91 12.78 3.65 1.41 0.75 4.69 5.00 1.06 2.81 2.85 0.43WI01AS5A 9.6 21.0 2.9 12.5 3.6 1.0 0.8 4.5 5.0 1.0 2.7 2.9 0.4

replicate 9.94 19.50 2.83 12.37 3.50 1.02 0.76 4.55 5.05 1.04 2.78 3.01 0.45WI01RF005 10.50 22.64 3.09 14.16 4.14 1.48 0.86 5.46 5.60 1.16 3.12 3.20 0.48

replicate 11.28 23.33 3.24 14.66 4.27 1.50 0.89 5.57 5.69 1.19 3.17 3.30 0.49WI01RF1A melanosome 71.2 117.5 13.1 47.7 7.5 1.3 0.8 6.1 3.7 0.7 1.9 1.8 0.3WI01RF1A leucosome 16.1 31.9 3.5 12.4 2.0 1.0 0.4 2.1 3.0 0.9 2.9 4.1 0.7Amphibolitized ultramafic rocksWA01JH1B-1 4.16 14.74 2.84 15.05 4.87 0.60 0.55 4.41 2.41 0.37 0.79 0.61 0.09WA01JH1B-2 5.87 8.92 1.08 3.79 0.52 0.04 0.04 0.34 0.13 0.02 0.08 0.09 0.02WA01CB15A 0.68 2.70 0.49 2.65 1.00 0.15 0.13 1.00 0.66 0.11 0.27 0.24 0.04PyroxeniteBG01CL002 0.34 0.68 0.09 0.37 0.09 0.03 0.02 0.13 0.17 0.04 0.14 0.21 0.04

BHVO-2 15.56 35.47 5.34 24.62 6.08 1.97 0.93 6.60 5.42 1.02 2.44 2.07 0.30

Rare Earth Element Concentrations of Webster-Addie, Willits and Balsam Gap Rocks, (ppm)



Type 2 Amphibolites

Type 1 Amphibolites

N-MORB

E-MORB


Moore’s Knob

Balsam Gap

Comparisons to other Units and Rocks

In the diagrams above, the WA-WI amphibolites are compared to the two classes of Buck Creek amphibolites identified in Berger et al (2001): the “magmatic” Type 1 amphibolites, and the “cumulate” Type 2 amphibolites. The REE patterns of WA-WI rocks are distinct from either of these amphibolite classes, and REE abundances are much higher, consistent with the WA-WI amphibolites representing more evolved lavas.

Here, the WA-WI amphibolite suites are compared to both depleted and enriched MORB lavas. Again, the WA-WI rocks are distinct in their REE patterns, with La/Sm ratios that lie between those of typical enriched and depleted MORBs. The WA-WI rocks are most similar in their REE abundances and patterns to basaltic andesites and andesites from modern volcanic arcs. This inference is consistent with the distinctly calc-alkaline evolution trend these lavas follow, and their positions on the total alkalies vs. SiO2 discrimination diagram.

What does this new REE data tell us? It confirms the existence of a fundamental compositional difference between the mafic and ultramafic rocks which occur west versus east of Franklin, NC. This divide is broadly consistent with the position of the Dahlonega thrust fault, but in fact the ultramafic and amphibolitic rocks on the east side of this boundary occur on a number of thrust sheets. Mafic-ultramafic rocks to the west of this divide have an oceanic character, and are larger, multi-lithologic units. Mafic-ultramafic occurrences on the west side of the divide, in the Buck Creek, Carroll Knob, and Lake Chatuge complexes, preserve an oceanic signature, and appear to be fragments of an ophiolite. Those on the eastern side of the divide are blocks within a regional olistostromal terrane, and preserve arc-like geochemical signatures, consistent with the kinds of rocks one would expect in a subduction-related accretionary sequence.

Franklin, NC

Lake Chatuge

Conclusions:4) Amphibolites from the Webster-Addie and Willits areas

display LREE enriched patterns at 10-30 x chondrites HREE abundances.

5) Amphibolitized rocks from the Webster-Addie complex have lower REE abundances than the WA-WI amphibolites, and possess amphibole-dominated REE patterns, consistent with metasomatic inputs of REEs (and other elements)

6) Pyroxenites from Balsam Gap, Webster-Addie, and the Moore’s Knob ultramafic bodies show similar REE abundances and similar, U-shaped REE patterns, indicating similar protoliths and origins.

7) The REE systematics of the WA-WI amphibolites and pyroxenites are distinct from those of the Buck Creek complex and the other large mafic-ultramafic associations to the SW, pointing to a compositional divide in the eastern Blue Ridge, defined by the mafic-ultramafic constituents of these regional rock units.

5) The likely protoliths of WA-WI amphibolites are rocks of andesitic composition, and the U-shaped REE patterns of the ultramafic pyroxenites are consistent with a supra-subduction zone setting. These observations, along with the block-in-matrix outcrop pattern of these units, strongly suggest that the formations of which they are part represent a paleo-accretionary complex sequence.

Field Areas for the 2001 Blue Ridge REU Site Program

Faculty:

Virginia Peterson, Steven Yurkovich, Jon Burr (Western Carolina University)

Jeff Ryan, Sarah Kruse (University of South Florida)

REU Participants:

Brian Allison (WCU), Susan Akers (College of the Pacific), Christina Bruinsma (WCU), David Doughty (WCU), Judy Harden (USF), Clayton Loehn (Ashland Univ.), Rory McIlmoil (Furman Univ.) John Newby (USF), Skylar Primm (University of New Orleans), Annie Scherer (San Fransisco State Univ.), Rachel Shannon (Univ. Colorado), Rachel Soraruf (Mt. Hoyloke College)

Documents

On most tectonic classification diagrams, the WA-WI amphibolites do not plot coherently. On the V vs. Ti classification plot of Shervais (1982) and the