rachels_poster_gussied by niusheng11


									Rare-Earth Element Systematics of Webster-Addie/Willits/Balsam Gap
Rachel Soraruf, Mount Mafic and Ultramafic Rocks
Holyoke College, South
Hadley, MA
Jeff Ryan, University of
South Florida, Tampa, FL                                                                                                                                              Typical Analyses and CIPW Norms of WA-Wi
 and                                                                                                                                                                         WI01RF1A WI01RF1E WA01SP8E WA01RF09 WI01RF08

The 2001 REU Program                                                                                                                                               SiO2 (wt%) 62.13
                                                                                                                                                                   Al2O3       14.64
                                                                                                                                                                   Fe2O3        7.07     7.09     8.73    11.83     9.22
Rare earth element abundances were measured on samples collected                                                                                                   MgO          3.98     4.19     6.86     8.62     6.48
during the 2001 REU summer program from mafic and ultramafic                                                                                                       CaO          4.28     5.48     8.81    11.46    10.13
rock exposures in the Willits area (WI), and in and near the Webster-                                                                                              MnO          0.09     0.09     0.18     0.25     0.15
Addie (WA) and Balsam Gap (BG) ultramafic bodies of the eastern
                                                                                                                                                                   K2O          2.57     1.95     0.88     0.77     0.83
Blue Ridge province of NC. Samples were dissolved at the Univ.
South Florida via a Na2CO3 fluxed fusion method, and analyzed for                                                                                                  Na2O         2.85     3.48     3.78     1.15     3.09
                                                                            Regional Map of NC Blue Ridge, showing mafic-ultramafic rock occurrences
REE abundances by ICP-MS as the University of Boston.                                                                                                              TiO2         1.27     1.11     0.95     1.40     1.17
Amphibolites from Willits and Webster-Addie show LREE-enriched
patterns with 10-50x chondrites HREE abundances. Leucosome-                                                                                                        Sr (ppm)    195      257      268      172      253
melanosome splits from migmatitic amphibolites show                                                                    The amphibolites of the WA-WI-BG area
                                                                                                                       range from lenticular horizons in the       Ba          671      551      136      138      114
complementary patterns, with HREE and Eu enrichments in the
                                                                                                                       country rock, to regionally extensive       Ni           44       49       62       98       75
melanosome. Amphibolitized zones within the Webster-Addie
ultramafic body show lower REE abundances overall (0.5 -10x                                                            occurrences of migmatitic amphibolite.      Sc           11       14       24       43       29
chondrites) and concave-up REE patterns; while a fresh pyroxenite                                                      Amphibolitized zones also occur within      Cr          113      110      180      327      228
from the Balsam Gap body shows a "U" shaped REE pattern at 0.5-                                                        the nearby Webster-Addie ultramafic         V           132      121      156      272      215
1x chondrite levels.                                                                                                   body.                                       Zn           94       96      121      182      121
The WI-WA amphibolites from these two regions display different                                                                                                    Cu           37       61       31       23       36
REE systematics than that of the Buck Creek Complex (BC). Berger                                                                                                              97.61    98.87    97.53    99.96    98.51
and others (2001) found BC samples to be consistently LREE
depleted, and suggested an origin from MORB-like protoliths. REE                                                                                                   CIPW Norm
and bulk chemical systematics of WI-WA amphibolites suggest
                                                                                                                                                                       Q     19.19     14.35      0       2.75     1.87
subduction-related protoliths, which have been intensely reworked by
                                                                                                                                                                    An+Ab    44.41     51.09    56.67    46.51    54.17
metasomatism. The BG pyroxenite shows a similar REE pattern to
pyroxenites from the Moore's Knob and Webster-Addie ultramafic                                                                                                        Or     15.42     11.58     5.32     4.55     4.96
bodies (see Berger et al. 2001), raising the possibility that these units                                                                                              Di     1.52      4.85    16.76    15.93    18.59
have similar origins.                                                                                                                                                 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
                      Major and Trace Element Geochemistry of Willits/Webster-Addie amphibolites

               Constraints on the origin of Webster-
                   Addie/Willets amphibolites
                                                                                                                                                                       Summary of Observations and
On most tectonic classification diagrams, the WA-WI amphibolites do
not plot coherently. On the V vs. Ti classification plot of Shervais                                                                                                          Inferences:
(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                                                                                               WA-WI amphibolites occur as lenticular bodies
between the calc-alkaline and tholeiitic fields, and on the V vs. Cr plot                                                                                       in the field, on a variety of scales, and may or
of Pearce (1973), most of the WA-WI samples fall outside the field for                                                                                          may not be in close association with ultramafic
The WA-WI amphibolites differ from calc-alkaline lavas in that they
have high concentrations of TiO2, evidenced in the samples as                                                                                                      Amphibolite fabrics and textures range from
abundant titanite, ilmenite, and rutile. The process of partial melting                                                                                         strongly foliated to migmatitic. "Block in Matrix"
during migmatite formation sequesters Fe-Mg-Ti bearing minerals                                                                                                 structures are commonly observed in the field.
such that on the scale of hand samples, anomalously high (and low) Fe
and Ti concentrations are possible, depending on whether the sample                                                                                               Mineral assemblages are Hbld + Plag + Qz +
included more leucosome vs. melansome.                                                                                                                          Bio ± Gt+ Titanite ± ilmenite ± rutile, with
                                                                                                            TiO2 vs. Fe#   (Serri, 1981)
                                                                                                                                                                chlorite, clinozoisite, and carbonates as
Interestingly, even if one takes the mean values of our amphibolite
samples, they still plot anomalously on classification diagrams.                                                                                                secondary metamorphic phases.
Options for explaining these patterns include the extraction of                           3.0
                                                                                                                                                                   Major-element chemistry of WA-WI
anatectic melt, which would shift the compositions of the rocks (and
                                                                                                                                                                amphibolites points to probable igneous rock
would imply their protoliths were even more felsic), or the WA-WI
                                                                             TiO2, %wt.

amphibolites may not, in fact, represent igneous protoliths.                                                High Ti
                                                                                                                                                                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
                                                                                                                                             Low Ti             of tectonic affinities.

                                                                                                                                                                   In terms of composition and probable protolith,
                                                                                                0.0   0.2        0.4         0.6           0.8        1.0       the WA-WI amphibolites are profoundly different
                                                                                                               tFe/(tFe+Mg)                                     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.
 Sampling and Analytical Methods
                                                                          Standard Reference Material for BHVO-1:
         Samples analyzed for REE abundances were selected from
                                                                            A Comparison of USGS Recommended
 the larger suite of WA/WI/BG mafic and ultramafic rocks that
 were collected and characterized during the 2001 Blue Ridge REU
                                                                                        BU ICP-MS
 Summer Program. To facilitate comparisons to the Buck Creek
 complex (Berger et al 2001) and other mafic rocks, we focused            Element USGS Recommended BU ICP-MS
 primarily on those samples mapped as amphibolites, and on
 amphibolitized zones within the Webster-Addie ultramafic body.
                                                                             La                0.63       0.6
 Where available, fresh pyroxenites from both the Webster-Addie              Ce                1.95       1.9
 and Balsam Gap ultramafic bodies were prepared, so as to                    Pr                0.38       0.4
 facilitate petrogenetic comparisons among these units.                      Nd                2.50       2.4
         All REE samples were digested via an Na2CO3 fluxed                 Sm                 1.10       1.1
 fusion method, following the methods of Berger et al. (2001).
                                                                             Eu                0.54       0.5
 Fusion cakes undergo a water leach to pull of excess Na2CO3 and
 silica, leaving the REE quantitatively in the insoluble fraction.           Tb                1.85       0.4
 These residues were reacted in ultra-pure HNO3, and diluted to             Gd                 0.36       2.0
 1500:1 for analysis.                                                        Dy                2.50       2.6
         ICP-MS analyses for REE abundances were done in the                 Ho                0.57       0.6
 Geological Sciences Department of the University of Boston,                 Er                1.70       1.7
 using a Thermo Elemental PQ ExCell ICP-MS facility maintained
 by Dr. Terry Plank. Sample data were calibrated against USGS
                                                                             Yb                0.26       0.0                     La Ce Pr Nd       Sm Eu Gd Tb Dy Ho Er             Yb Lu

 reference materials BHVO-2 and BIR-1, which were run as                     Lu                1.65       1.7
                                                                            Tm                 0.26       0.3                Rare-earth element patterns of WA-WI amphibolites range from flat to
 unknowns during each run. No internal standards were used, nor
                                                                                                                             distinctively LREE enriched, with relatively flat HREE patterns.
 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%

         Rare Earth Element Concentrations of Webster-Addie, Willits and
                           Balsam Gap Rocks, (ppm)
Sample Name                 La         Ce     Pr     Nd     Sm     Eu     Tb     Gd     Dy     Ho       Er   Yb     Lu
WI01AS10                   32.7      66.6    7.7    31.1    6.7    2.0    0.9    6.5    5.2    1.0    2.4    2.1    0.3
WA01RF009                   7.2      13.8    2.6    11.8    3.6    1.2    0.8    4.7    5.2    1.1    2.9    2.9    0.4
WA01SP8E                    8.8      20.1    3.0    12.8    3.5    1.1    0.7    4.4    4.7    1.0    2.6    2.7    0.4
W101RF03                   35.5      84.1    11.4   52.3    12.7   2.8    2.1    13.4   11.9   2.5    6.4    6.2    0.9
WI01RF008                   5.9      14.2    2.2    10.3    3.2    1.1    0.7    4.3    4.8    1.0    2.7    2.7    0.4
WI01RF002C                  4.1       9.3    1.5     7.1    2.4    1.0    0.6    3.4    3.9    0.8    2.2    2.2    0.3             La Ce Pr Nd        Sm Eu Gd Tb Dy Ho Er            Yb Lu
WI01RF1C                   9.03      19.15   2.68   11.98   3.46   1.05   0.77   4.62   5.22   1.09   2.87   2.82   0.42
WI01AS7A                    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     The migmatization that many of these rocks have undergone does impact
         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   0.43    the REE patterns of samples. Here, we have examined the leucosome and
WI01AS5A                    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     melanosome of a coarsely migmatitic amphibolite from the Willits area.
         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   0.45    The amphibole and biotite-rich melanosome exhibits a strongly LREE
WI01RF005                 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    enriched pattern, while the feldspathic leucosome shows a “U” shaped
         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   0.49    pattern. The average of these two patterns is very comparable to the
WI01RF1A 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    0.3     typical REE signatures that the WA-WI amphibolites preserve overall.
WI01RF1A 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    0.7
Amphibolitized ultramafic rocks
WA01JH1B-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 0.09
WA01JH1B-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 0.02
WA01CB15A                  0.68      2.70 0.49 2.65 1.00 0.15 0.13 1.00 0.66 0.11 0.27 0.24 0.04
BG01CL002                  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

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

                                                                                                                           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
                              La Ce Pr Nd          Sm Eu Gd Tb Dy Ho Er           Yb Lu                                    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).
                                                Comparisons to other Units and Rocks

                                                                                                                                                                       Balsam Gap

            Type 1 Amphibolites
                                                                                                                                                                                    Moore’s Knob

                                                                                                                                                           Franklin, NC

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

                                                                                           Lake Chatuge

                                                                                              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
            Type 2 Amphibolites
                                                                                              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
        La Ce Pr Nd        Sm Eu Gd Tb Dy Ho Er             Yb Lu                             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.
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.
                                                                                                               1) Amphibolites from the Webster-Addie and Willits areas
                                                                                                                  display LREE enriched patterns at 10-30 x chondrites HREE
                                                                                                               2) 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)
                                                                                                               3) 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.
                                                                                                               4) 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
             La Ce Pr Nd           Sm Eu Gd Tb Dy Ho Er                   Yb Lu
                                                                                                                  ultramafic pyroxenites are consistent with a supra-subduction
                                                                                                                  zone setting. These observations, along with the block-in-
    Here, the WA-WI amphibolite suites are compared to both depleted and enriched                                 matrix outcrop pattern of these units, strongly suggest that the
    MORB lavas. Again, the WA-WI rocks are distinct in their REE patterns, with                                   formations of which they are part represent a paleo-
    La/Sm ratios that lie between those of typical enriched and depleted MORBs.
                                                                                                                  accretionary complex sequence.
    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.
               Field Areas for the 2001 Blue Ridge REU Site Program

Virginia Peterson, Steven Yurkovich, Jon Burr
(Western Carolina University)
Jeff Ryan, Sarah Kruse (University of South
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)

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