American Mineralogist, Volume 6E, pages 78-94, 1983

Petrology and geochemistry of metamorphosed ultramafic bodies in a portionof the Blue Ridge of North Carolina and Virginia

Devro M. ScorroRD AND JBrpney R. Wrr-llnnasr

Department of GeologyMiami University

Oxford, Ohio 45056

Abstract

Seventeen metamorphosed ultramafic bodies in the Precambrian Ashe Formationexposed along a northeast-trending belt from near West Jefferson, North Carolina to nearFloyd, Virginia in the Blue Ridge, were investigated. The mineral assemblages in individualsamples constitute subgroups drawn from the larger group consisting ofolivine, enstatite,tremolite, anthophyllite, antigorite, talc, chlorite, magnetite, and dolomite. Although theultramafic bodies occur in the garnet, staurolite, and kyanite zones recognized in thecountry rock, no correlation exists between the mineral assemblages in the ultramafic rockand the metamorphic zones. Modal analyses and quantitative chemical analyses for Mg,Al, Ca, Fe, Ni, Mn, Cu, Co, K, andZn lead to the recognition of two distinct types ofmetamorphosed ultramafic bodies based on the degree of metasomatic alteration. Themetasomatic exchange with the country rock involves the loss of magnesium and nickeland the introduction of aluminum, calcium, iron, manganese, and potassium into theultramafic bodies. The less altered bodies are more massive and contain relict olivine andrare relict enstatite plus tremolite, anthophyllite, chlorite, and antigorite whereas the morealtered bodies are more strongly foliated with abundant tremolite and chlorite and lesscommon relict olivine.

Textural evidence, notably the pseudomorphous partial replacement of tremolite byserpentine, combined with that provided by the geochemical study provides the basis forunderstanding the recrystallization history and the calculation of mass balance equations.A retrograde hydration of the ultramafic protolith produced the tremolite-chlorite-talcanthophyllite assemblage with relict olivine. In a second stage of this hydration serpentinereplaced some of the tremolite and olivine. A later partial dehydration led to the morestrongly foliated and, in some places, mylonitized tremolite--chlorite-talc schist or phyllon-ite.

Introduction

Alpine ultramafic bodies occur throughout thelength of the Appalachian Mountains from euebecto Alabama. These typically peridotitic or duniticplutons and their serpentinized equivalents havereceived considerable attention from petrologists(Misra and Keller, 1978), but a group of metamor-phosed and metasomatized ultramafic rocks ex-posed in the eastern Blue Ridge of North Carolinaand Virginia (Fig. 1) have not been investigated inany detailed way. These petrographically ratheruncommon rocks were recognized by Keith (1903)

I Present address: 6709 Hopewell Avenue, Springfield, Virgin-ia 22151.

0003-004)v83/0 I 02-0078$02.00

as probably originally ultramafic although they arenow largely chlorite-tremolite schists and gneissesand were mapped by Rankin, Espenshade, andNewman (1972) as generally small lens-shaped bod-ies in the Precambrian Ashe Formation.

The bodies investigated are typically elongate inmap view ranging in size from 0.6 km by 0.2 km to8.0 km by 1.5 km. They are exposed in a northeasttrending belt from southwest of West Jefferson inAshe County, North Carolina to near Floyd inFloyd County, Virginia, a distance of 150 km. Atotal of fifteen bodies were sampled for petrograph-ic and geochemical study (Table l).

The study was designed to answer four principalquestions. (1) What was the nature of the protolithfrom which these extensivelv altered rocks were

SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

[Illuo.n"t Zone llTlr.ourorite Zone [Ilrvonite ZoneFig. l. Location of ultramafic bodies in eastern Blue Ridge in North Carolina and Virginia southeast of Fries fault. pee-Elk Park

Plutonic Group, pec{rossnor Plutonic Group, pea-Ashe Formation, p€m-Mount Rodgers Formation, ab-Alligator BackFormation, +Cambrian Sediments, Pza-Spruce Pine Plutonic Group. After Rankinet al. (1972).The area of kyanite zone shown inthe northeast corner of the map was mapped only as "Amphibolite facies" by Rankin et al, (1972).

79

derived? (2) What relationship exists between thepresent mineralogy of these rocks and the metamor-phic grade of the country rock which changes alongthe trend of the exposures of ultramafic rock (Fig.1) from the garnet zone through the staurolite zoneto the kyanite zone based on the mapping of Rankinet al. (1972). (3) What metasomatic changes haveoccuned during recrystallization of these rocksbased on the present concentrations of the majorelements Mg, Fe, Ca, and Al, and the trace ele-ments Mn, Co, Cu, K, Ni, and Zn? (4) What is therecrystallization history of these bodies?

The size, shape, and geologic occurrence ofthesebodies in deformed Precambrian metasedimentsand volcanics strongly suggests that they werealpine ultramafic intrusives (as defined by Jacksonand Thayer, 1972) and probably emplaced tectoni-cally in a non-liquid state (see Misra and Keller,1978). What is not clear is whether these rocks wereoriginally harzburgites or dunites, common amongthe unaltered ultramafic bodies of the Appala-chians, or were another ultramafic rock type.

The alteration history of these rocks is undoubt-edly complex, involving retrogressive recrystalliza-tion from the original ultramafic assemblage plus

chemical mass transfer between the country rockand the ultramafic intrusives. The mineral assem-blages of individual specimens are subgroups drawnfrom the larger suite which comprises olivine, en-statite, tremolite, anthophyllite, chlorite, talc, ser-pentine, magnetite, and dolomite.

Previous investigations

Evans (1977) provided a valuable review of thepetrology of metamorphosed peridotites and ser-pentinites. Trommsdorf and Evans (1974) describedthe development of mineralogy similar to that ob-served in this study in ultramafics subjected toalpine regional and contact metamorphism, exceptthat in their study olivine is the dominant mineralrather than tremolite or chlorite.

The textural relationships in the study byTrommsdorf and Evans persuasively demonstrate aprograde metamorphism from serpentinite to peri-dotite in response to a regional metamorphic epi-sode. As is discussed below, the texture observedin the ultramafic bodies described here is quitedifferent and is not interpreted as indicating a singleprograde metamorphism.

Some recent studies of ultramafic rocks in the

SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 1. Division of ultramafic bodies into Todd-type andEdmonds-type

Todd-type bodies Edmonds-type bodies

(T) Todd

(W) Warrensv i l le

(HN) l t igger Mountain

(WB) Woodrow Bare

(L) L i t t le Peak Creek

(SS) Shat ley Spr ings

(P) Phoen ix Mounta in

(C) Cranbemy

(RR) Rocky Ridge

(T0) Twin 0aks

(WL) Woodlawn

(DG) Dugspur

(PK) Parkway I

(PK) Parkway 2

(ED) Edmonds

(GR) Grayson

(EN) Enn i ce

(B) Brush Creek

Southern Appalachians are pertinent to this investi-gation. Swanson (1980) described dunite, harzbur-gite, and orthopyroxenite from two small bodies inthe Ashe Formation on Rich Mountain in NorthCarolina. The metamorphic mineralogy of theserocks included tremolite, talc, and anthophyllite,chlorite, serpentine, and magnesite. Textural rela-tionships indicate the rocks have undergone two,and possibly three, deformations. Swanson con-cluded that the olivine and pyroxene in these rocksare metamorphic products and not relicts of theprimary igneous mineralogy. Although these rocksoccur in the Ashe Formation approximately 15kilometers to the southwest of the belt of ultramaficbodies described here, they are significantly differ-ent mineralogically and texturally. Most notablythey contain much higher concentrations of olivineand pyroxene and only minor amounts of tremoliteand chlorite, the dominant minerals in the ultramaf-ic bodies to the northeast. Additionally, unlike therocks investigated in this study, they show little orno evidence of metasomatism. They also apparentlylack textural evidence indicating relict olivine.

Other recent studies of less altered ultramaficbodies in North Carolina are those of Hahn andHeimlich (1977) and Alcorn and Carpenter (1977).

The rocks in both of these studies have beeninterpreted as the result of partial serpentinizationduring or after the implacement of dunite, butneither has been extensively recrystallized sinceemplacement.

Petrolog5r

General statement

In outcrop these metamorphosed ultramafic bod-ies are grayish-green with a distinct schistose folia-tion which typically parallels that of the amphiboliteor biotite muscovite gneisses of the Ashe Formationwith which they are in contact. The foliation isexpressed microscopically by the parallelism ofsuch platy or prismatic minerals as chlorite, tremo-lite, and talc and the elongation of such non-platyminerals as olivine and dolomite in the foliationplane. In places the foliation planes are deformedinto small irregular folds. Larger olivine (up to 5mm diameter) and some of the less common ensta-tite crystals are wrapped by the foliation indicatingtheir relict status.

Although all of the rock bodies have a similarmegascopic appearance, they can be divided readilyinto two principal petrographic groups (Table 1) onthe basis of mineral association and major and traceelement concentrations (Tables 2 and 3). The firstgroup, designated the "Todd-type" after the largestbody representative of this group, is characterizedby high concentrations of tremolite and chloritewith lesser talc and minor amounts of olivine. Thesecond group is called the "Edmonds-type" be-cause that large body is typical ofthe group. Thesebodies are characterized by higher concentrationsof olivine, antigorite, anthophyllite, and talc andlesser amounts of tremolite and chlorite. The Todd-type bodies characteristically have lower abun-dances of magnesium and nickel and higher abun-dances of calcium, aluminum, manganese, and zincas compared to the Edmonds-type bodies. Iron issomewhat more abundant in the Todd-type bodies,and the concentration of cobalt is not significantlydifferent in the two types of bodies.

Table 4 contains the point count mineral analysisfor each sample with the name of the body fromwhich it was obtained and the metamorphic grade ofthe country rock. Figure I is a map showing thename and location of each of the ultramafic bodiessampled. Figure 2 is a map showing sample loca-tions.

SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 2. Mean values of modal mineral amount by body and type of body

Mi neral

Body Mg Car Antho Talc Antig Lim En

8 l

Tr

Todd-type

ToddN ige r Moun ta i nl l a r r ensv i l l ePhoenix l , lountainL i t t l e Peak C reekl,loodrol BareCranberryShat ley Spr ingsRocky Ri dgeTwin 0aksParkwaytrJoodl awn

50. 759. 154 .964.876.066.?6 0 . 06 1 . 46 1 . 950. 646.444.4

5 6 . 4

68.91 5 . 97 . 7

? 7 . 9l,lean* 37 .6

l 5l 54444J

108I2I

3 3 . 829.73 0 . 627 .620.730.023.929.13 1 . l42.736 .644.6

31.7

8 . 90 . 90 . 12 . 10 . 00 . 6tr4 . 5t r0 . 06 . 50 . 0, ' l

0 . 0o . 20 . 30 . 00 . 00 . 00 . 00 . 20 . 00 . 70 . 00 . 00 . 1

0 . 02 . 80 . 00 . 60 . 00 . 00 . 00 . 01 . 10 . 06 . 50 . 03 . 0

0 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 0

2 . 0 3 . 40 . 9 2 . 21 .4 12 .40 . 6 3 . 20 . 8 0 . 31 . 2 0 . 7t . 7 1 4 . 02 . 0 0 . 32 . 1 3 . 30 . 7 5 . 34 . 7 0 . 5t r 9 . 1

1 . 5 4 . 5

0 . 0 0 . 00 . 5 0 . 00 . 4 0 . 00 . 1 0 . 0t r 0 . 00 . 4 0 . 00 . 3 0 . 00 . I 0 . 00 . 0 0 . r0 . 0 0 . f0 . 0 t F0 . 0 0 . r

0 . 2 t r

0 . 0 0 . 00 . 0 0 . 00 . 0 3 . 30 . ? 0 . 0

Brush Creek IEnni ce 3Grayson 4Edmonds 9

Edmonds-type

2 3 . 5 0 . 5 2 . 729.9 0 .0 2 .02 0 . 9 1 9 . 9 2 . 8?6 .0 2 r .8 3 .3

2 5 . L 1 0 . 6 2 . 7

0 . 0 0 . 0 0 . 0 0 . 0r4 .7 1 .3 24 .6 0 .0r4 .7 12 .7 10 .9 7 .70 . 1 1 . 9 9 . 1 9 . 7

7 . 4 4 . 0 l l . 2 2 . 4 t r 0 . 8

-Mean of the nean values of individual bodies, thus sl ightly dif ferent from the true means of the Todd-

and Edmonds-type samples.

Relationship between mineralogy and metamor-phic grade

One of the purposes of this study was to examinethe relationship between the mineral assemblage inthe metamorphosed ultramafic bodies and the meta-morphic grade of the country rock in which theywere emplaced. Trommsdorf and Evans (1974) de-scribed a series of mineral assemblages in Alpineultramafic rocks which have undergone prograderegional metamorphism from an original serpentinestage. The sequence, applicable to the rocks understudy here, is (1) antigorite + talc * tremolite,equivalent to the staurolite-kyanite and lowerzones of pelitic rocks; (2) forsterite * talc, equiva-lent to metamorphism slightly higher than the kya-nite zone; (3) forsterite * anthophyllite + tremolite,a somewhat higher grade assemblage; and (4) trem-olite + forsterite * enstatite, an assemblage rough-ly equivalent to the sillimanite zone.

Although these assemblages occur in some of theultramafic bodies described here. their distributiondoes not correspond to the metamorphic grade ofthe country rock as indicated by the isogradsmapped (Fig. 1) in the Ashe Formation by Rankin eral. (1972). Assemblage (1) occurs in some samplesof the Nigger Mountain, Ennice, Grayson, and

Edwards bodies; assemblage (2) in some samples ofthe Nigger Mountain, Edmunds, Grayson, andPhoenix Mountain bodies; assemblage (3) occurs insome samples of the Nigger Mountain, Waynes-ville, Grayson, and Edmunds bodies; assemblage(4) occurs in two samples of the Grayson body andone of the Parkway body (Table 4).

Inspection of Figure 1, the regional map of thesebodies, shows no correspondence between the oc-currence of these assemblages and indicated meta-morphic grade of the country rock perhaps becausethe temperature range recorded in the pelitic rockswas too narrow to be reflected in the ultramaficbodies. Moreover, there is no systematic geograph-ic distribution of these assemblages regardless ofthe grade of the country rock and in several casesassemblages representing different grades occur inthe same body. We conclude that the observedmineral distribution does not reflect a prograderegional metamorphism of serpentinite, but a two-stage retrogressive recrystallization of a dunite orharzburgite protolith. Observations which supportthis assertion will be described in subsequent sec-tions of the paper, but critical to this conclusion isthe occurrence of antigorite pseudomorphs aftertremolite (Fig. 3). Whether the tremolite, olivine,and chlorite crystallized simultaneously or sequen-

E2 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 3. Mean values of chemical data by body and type of body

Weight percent

Body N 1.|90 41203 Cgo Fe Ni Zn

ToddNiger l . lountainPhoeni x l.lountai nL i t t l e Peak C reekWoodrow BareCranbemyShat ley Spr i ngsRocky RidgeTw in OaksParkwayWoodl awn

Brush CreekEnni ceGrays onEdmonds

2 2 . r 3 5 . 7 42 2 . 9 6 5 . 8 520.26 7 .1020.35 9 .002 2 . 4 L 6 . 1 22 2 . 5 0 4 . 8 426.72 5 .9722.49 7 .661 6 . 9 3 7 . 5 41 8 . 8 4 5 . 5 524.03 8 .43

Mean* 21.78 6.71

2 3 . 3 8 2 . 7 42 7 . 0 4 3 . 6 14 1 . 3 1 t . 0 23 3 . l 0 3 . 2 6

Mean* 31.22 2.66

Todd-type5 . 5 3 9 . 2 54 . 0 9 9 . 0 06 . 7 9 8 . 4 57 . 9 3 1 0 . 4 03 . 3 5 9 . 5 05 . 3 4 1 0 . 0 42 . 2 7 1 0 . 5 05 . 9 3 9 . 4 2

1 1 . 0 5 8 . 2 76 . 6 8 9 . 1 84 . 9 1 9 . 8 2

5 . 8 1 9 . 4 4

Edmonds-type

0 . 0 8 5 . 3 23 . 2 0 6 . 8 5

.66 7 .203 . 5 1 6 . 3 91 . 8 6 6 . 4 4

t520 10521620 tL291494 9051875 7351800 12451693 8801480 11701458 10101367 10401646 8761470 9451585 1099

697 1910955 t510674 2220

1092 1836

855 1619

87?z2z

41

32

0. 200 . 2 10 . 2 00 . ? 40 . 2 30 . 2 20 . l 90 . l 90 . 1 80 . 2 10 . 2 2o . 2 l

0 .09U . I 5

0.080 . l 2

0 . 1 l

l 1 9

1 1 9t02139tt7t19

124 r07 75Ito 89 6289 33 457

138 38 26?129 60 69143 63 4lt27 100 56114 r25 66tt7 151 r04104 235 518It6 40 45

104120l l 31031481071 1 69495

10178

107

426 l526254

95 160

l0 2224 2639 437t 63

t2L

10

'Mean of the nean values of the indiv idual bodies, thus s l ight ly d i f ferent f rorn the t rue mean of Todd- and

Edmonds-type samples.

tially, it is quite evident that serpentinization fol-lowed rather than preceded their crystallization.The relict nature of the olivine is strongly suggestedwhere small anhedral olivine crystals with opticalcontinuity are separated by interstitial serpentine(Fie. 3).

Mineralogy and textureOlivine. Relict olivine is present in both Ed-

monds- and Todd-type bodies although it is morecommon and occurs as somewhat larger crystals inthe Edmonds-type samples. In the less recrystal-lized Edmonds-type samples individual crystals of

14.1s '#f*:;

,r,\*#r#{ 'iz " NoRrH .AR.LTNA

Ky. .)ilff. Y.9:-e .''

, . ' V i rg in io

r r , ' - Nor th Coro l ino

Fig. 2. Map of sample locations. See Figure I for name and geologic position of individual bodies.

SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 4. Mineralogical data derived from point count analysis of samples from the ultramafic bodies in North Carolina and Virginia

Mineral

83

Sampl en umbe r Tremol i te Ch lor i te 0 l i v ine Magnet i te

Antho-Carbonate phy l l i te

Limo- Ensta- ] ' leta.ni te t i te zoneTa l c

Ant i g :ori te

T 5T 6T 13-bT l 3 - dT 1 3 - 1T 14-bT 1 5 - bT 16-dT 1 6 - ar 2NM 2 l -bNM 2 l -cN M 2 1 - dNM 2 l -eN M 2 1 - fN M 2 l - gN M 7NM 24NM 22NM 23NM 25-aNM 25-bNM 26-bNM 26N M 9lt 17-xAh l 17-Xt^l l7-1w 17-2

P l8 -aP 18-bP 20-uP 20-a

0 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 04 . 5 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 09 . 0 0 . 00 . 0 0 . 00 . 0 0 . 0

1 0 . 4 0 . 00 . 0 0 . 00 . 0 0 . 6| . ? 0 . 00 . 0 0 . 00 . 0 0 . 07 . 2 0 . 03 . 4 0 . 00 . 0 0 . 0

1 0 . 6 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 0

0 . 0 0 . 02 . 4 0 . 00 . 0 0 . 00 . 0 0 . 0

0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 K0 . 0 s0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 K

0 . 0 K0 . 0 K0 . 0 K0 . 0 K

42.65 0 . 828.93 5 . 558.95 7 . 96t.24 6 . 365 .649.44 3 . 46 1 . 85 5 . 35 6 . 45 1 . 96 6 . 56 7 . 658. 77 2 . 25 6 . 05 6 . 66 1 . 55 3 . 86 0 . 86 3 . 9

66 .432.759 .96 0 . 6

66 .062.186.64 4 . 3

33 . I35 .925.649.733 .637.93 1 . 628.329.433 .03 7 . 136 .544. 328 .045 .929.116 .637 .2t7 . 028.730 .436 .93 1 . (35 .328.71 3 . 640. l30 .937 .7

3 1 . 832 .91 3 . 432.L

t8.20 . 0

38 .61 2 . 60 . 00 . 00 . 0

19 . 50 . 00 . 03 . 10 . 00 . 00 . 00 . 02 . 20 . 00 . 00 . 06 . 12 . 00 . 00 . 02 . 00 . 00 . 00 . 00 . 40 . 0

0 . 00 . 80 . 07 . 4

3 . 32 . 34 . 51 . 91 . 01 . 6l . ll . lt . ?1 . 00 . 61 . 61 . 00 . 50 . 30 . 60 . ?1 . 80 . 50 . 8r . 21 . 31 . 31 . 20 . 21 . 21 . 02 . 21 . 2

0 . 20 . 80 . 0r . 2

1 . 80 . 02 . 10 . 30 . 05 . 83 . 84 . 00 . 6

l l . 62 . 00 . 00 . 04 . 50 . 00 . 54 . 20 . 0

10. 20 . 06 . 40 . 00 . 20 . 04 . 5

18 .726.24 . 40 . 1

0 . 60 . 00 . 0

13.2

0 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 02 . 00 . 00 . 00 . 00 . 00 . 26 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 01 . 10 . 0

0 . 00 . 00 . 00 . 0

0 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 1? . 30 . 00 . 00 . 02 . 21 . 30 . 00 . 70 . 00 . 00 . 00 . 70 . 00 . 20 . 01 . 10 . 4

0 . 00 . 00 . 00 . 0

olivine are fractured with serpentine occurring inthe fractures. Anthophyllite and tremolite havereplaced olivine and are in turn partially replaced byserpentine (Fig. 3). In the Todd-type bodies olivineoccurs as smaller fragmented clusters which areelongated in the foliation. Here olivine does notoccur with serpentine but commonly occurs withchlorite which is located in the pressure shadowsformed by the olivine clusters, as well as in thefoliation. The presence of chlorite in the pressureshadows of olivine attests to the secondary origin ofthe chlorite with respect to olivine and the relictnature of the olivine.

Chemical and optical data obtained by Laskowskiand Scotford (1980) from an Edmonds-type sample(0-25) indicate that the olivine composition is 88.5molTo forsterite, this value being based on atomicabsorption analyses of mineral separates. This samesample contains only 737 ppm Ca. The 2V is *88'and the np B is 1.672 for this specimen. The opticaldata of two samples of olivine from Todd-type

rocks indicate compositions of 90 and 9l molVoforsterite.

Antigorite. The serpentine mineral is identified asantigorite on the basis of its X-ray diffraction pat-tern. No unique lizardite peaks were observed andfour typical antigorite peaks are present (Borg andSmith, 1979). Antigorite has partially replaced oliv-ine in some Edmonds-type rocks and also hasreplaced tremolite resulting in partial pseudo-morphs of serpentine after tremolite (Fig. 3). It alsoappears to have replaced chlorite in a few samples.It is absent in Todd-type bodies.

Tremolite. Tremolite is common in both types ofultramafic bodies. In the more massive Edmonds-type rocks its prismatic crystals are completelyunoriented producing ajack-straw texture. In thoseEdmonds-type rocks which have been lightlysheared with a relatively widely spaced fracturefoliation, the tremolite laths are shredded at theirterminations and have been rotated into parallelismwith the foliation. In some places where the basal

84 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 4. (continued)

Mi neral

Sampl enumDer Tremol i te Ch lor i te 0 l i v ine Magnet i te

Antho- Antig-Carbonate phyl I i te Talc ori te

Limo- Ensta- Meta.n i te t i te zone

L 29-b1 3 01 1 01 3 1

0 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 0

0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 00 . 00 . 00 . 00 . 00 . 00 . 0

!{B 32-chlB 33-at,lB 33-bt^JB 36

C 34-ac 34-bS S 4 7 - ass 47-bss 50SS 48SS 48 -aSS 45 -ass 45-bSS 46ss 46-dSS 69-aRR 43RR 42-bRR 44RR 41RR 40-bRR 37-aRR 39RR 38-bT0 11B 1 2 - aEN 53-bEN 53-aEN 52

9 9 . 17 4 . 56 8 . 76 t .762.066 .866 .96 7 . 95 4 . 66 8 . l5 7 . 370.28 6 . 06 3 . 66 3 . 96 7 . 46 6 . 95 r . 95 8 . 84 9 . 33 6 . 152.07 t . l66 .465.25 9 . 56 0 . I62.358. 55 0 . 66 8 . 942.34 1 . 90 . 0

0 . 02 3 . 9? 5 . 034.23 3 . 428.5? 6 . 63 l . 436.7L ? . 42 2 . 72 1 . 89 . 0

2 7 . 528 .8? 4 . 43 0 . 84 5 . 63t.229.54 ? . 634 .628.129.03 l . 43 4 . 92 7 . r36 .036.742.723.54 ? . 436 .61 0 . 9

0 . 00 . 00 . 00 . 00 . 00 . 02 . 40 . 00 . 00 . 0t r0 . 00 . 01 . 02 . 54 . 60 . 00 . 05 . 5

18 .01 3 . 40 . 00 . 60 . 00 . 00 . 00 . 00 . 00 . 00 . 0

0 . 0u . 51 . 41 . 30 . 81 . 51 . 90 . 52 . t1 . 51 . 51 . 01 . 40 . 50 . 51 . 30 . 81 . 04 . 02 . 87 . 10 . 30 . 20 . 51 . 1L . 70 . 31 . 52 . 70 . 72 . 71 . 93 . 70 . 5

0 . 01 . 30 . 00 . 00 . 22 . 2o . 20 . 06 . 6

l 6 . 918 .50 . 00 . 01 . 10 . 00 . 00 . 00 . 00 . 50 . 21 . 0

r 2 . 80 . 00 . 11 . 60 . 0t . 70 . 22 . 15 . 30 . 07 . 7

1 3 . 123.2

tr0 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 01 . 01 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 70 . 00 . 00 . 03 . 8

tr0 . 00 . 00 . 00 . 00 . 0t . 70 . 00 . 00 . 30 . 00 . 00 . 00 . 00 . 30 . 00 . 01 . 00 . 00 .00 . 00 . 00 . 00 .00 .00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 0

0 . 0 0 . 00 . 0 0 . 00 . 0 0 . 0

q

ssS\s

0 . 50 . 00 . 00 . 0

7 . r 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 09 . 1 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 00 . 0 0 . 05 . 8 0 . 04 . 7 0 . 0

62.6 0.0

0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 K0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s0 . 0 s

sections are subparallel with the plane of the thinsection, the drag of chlorite and antigorite crystalsat their contacts with the larger tremolite basalsections gives the impression of rotation or "snowball" texture (Fig. a). Tremolite has replaced oliv-ine, rare enstatite, and in a few places showsevidence suggesting it has replaced chlorite (Fig. 5).In Todd-type bodies the tremolite laths are alignedparallel to the foliation plane in many samples. Inothers the tremolite crystals have been shreddedand deformed in response to later shearing produc-ing a phyllonitic texture. In some well-shearedrocks the larger tremolite crystals constitute por-phyroclasts in a matrix of smaller chlorite andtremolite prismatic crystals.

Anthop hyllife. Anthophyllite, distinguished fromcummingtonite by its parallel extinction and lack oftwinning, is present in some Edmonds-type sampleswhere its occurrence is similar to that of tremolite.

It is rare in Todd-type rocks. Some crystals show aprismatic habit whereas others are fibrous.

Chlorite. Chlorite is common in both types ofultramafic bodies. It is colorless or faintly green-tanpleochoric and has a small extinction angle suggest-ing clinochlore. It occurs as a matrix mineral replac-ing olivine in the less recrystallized Edmonds-typebodies or is concentrated in the foliation planes inthose Edmonds-type samples having foliation. In afew samples it is concentrated in small veinlets. Itappears to have a wide range of temperature stabil-ity as it also occurs abundantly in the Todd-typetremolite phyllonites. Chlorite occurs in the pres-sure shadows of relict olivine phenoclasts in someTodd-type samples. A few chlorite crystals arecross cut by tremolite laths suggesting a parageneticsequence: chlorite-tremolite (Fig. 5). However, it isprobable the chlorite was stable throughout therecrystallization and deformation history of these

SCOTFORD AND WILLTAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 4. (continued)

85

Mi neral

Sampl enumDe r fremoli te Chlori te 0l ivine l4agneti te Carbonate

Antho-p h y l l i t e T a l c

Limo- Ensta- Meta.n i te t i te zone

Anti g-ori te

G R 5 l - aG R 5 1 - bG R 5 1 - cGR 68ED 56-aED 56-cE D 5 7 - aED 57-bED SB-aED 59ED 60-bED 66-aED 67-a

PK 62PK 6r -b

t^ll 65

4 . 715.26 . 04 . 8

82.82 . 6

2 t . l50.2

r . b0 . 08 . 8

2 t . 46 2 . 7

5 4 . 03 8 . 8

44.4

32.112.0? 7 . ?1 2 . l1 . 5

t6.76 5 . 44 3 . 69 . 3

25.028.71 3 . I3 0 . 3

3 1 . 94 1 . 2

44.6

18 . I2 .1 ,

23.?36.20 . 0

48. 9L0.21 . 6

30 .75 5 . 41 5 . 733 .60 . 00 . 0

1 3 . 10 . 0

10 . 340 .01 . 07 . 60 . 00 . 60 . 00 . 00 . 00 . ?0 . 00 . 00 . 0

t r0 . 0

2 2 . 88 . 20 . 08 . 00 . 00 . 0

44 . 89 . 11 . 7

23.60 . 00 . 00 . 00 . 0

1 . 20 . 99 . 1Ir

1 . 33 . 22 . 60 . 31 . 02 . 86 . 07 . 13 . 26 . ?tr

2 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 00 . 01 . 30 . 60 . 00 . 00 . 00 . 00 . 0

30 .2 1 .1tr 29.61 0 . 7 0 . 00 . 0 1 3 . 0

1 0 . 0 0 . 04 .2 17 .40 . 0 0 . 00 . 0 2 . 0t r 1 3 . 90 . 8 7 . 2t r 41.22 . 0 t r0 . 0 0 . 00 . 0 l l . 10 . 0 0 . 00 . 0 0 . 0

0 . 0 s0 . 0 sf r S

1 3 . 1 S0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G0 . 0 G

0 . 01 . 09 . 1

0 . 0 Gt r G0 . 0 G

G = G a r n e t ; S = S t a u r o l i t e ' K = K y a n i t e ; T = T o d d ; N H = N i g g e r M o u n t a i n ; W = l ' l a r e n s v i l l e ; P = P h o e n i xM o u n t a i n ; L = L i t t l e P e a k C r e e k ; } | B = W o o d r o w B a r e ; C = C r a n b e r r y ; S S = S h a t l e y S p r i n g s ; R R = R o c k y R ' i d g e ;T 0 = T w i n 0 a k s ; E l , l = E n n i c e ; G R = G r a y s o n ; E D = E d m o n d s ; P K = P a r k w a y - l ; t ' I L = W o o d l a w n ; B = B r u s h C r e e k .

rocks as it clearly replaces olivine but is present inthe fracture foliation of the most sheared Todd-typebodies. Also, its occurrence in the pressure shad-ows around tremolite indicates that in those placesit post-dates that mineral.

Microprobe analyses by William Zhong (pers.comm.) of chlorite samples separated from speci-mens used in this study show a distinct differencebetween the compositions of three chlorite samplestaken from Edmonds-type rocks and eight takenfrom Todd-type rocks. The mean major elementweight percentages of these two types of chlorites

Fig. 3. Prismatic tremolite (Tr) crystal with pseudomorphousreplacement by antigorite (A). Relict olivine (01) partiaflyreplaced by antigorite (A). Bar = 0.025 mm. Edmonds-type.

are as follows: Mg22.19 for Edmonds versus 16.85for Todd; Fe 3.16 for Edmonds versus 8.98 forTodd; Al 6.28 for Edmonds versus 8.41 for Todd; Si14.47 for Edmonds versus 12.51 for Todd. There islittle difference in Cr concentration between the twotypes of samples; 0.48% for Edmonds and 0.43 forTodd.

Talc. Talc is more common in the Edmonds-typerocks but occurs in both types. It is most commonlya matrix mineral not aligned with respect to afoliation or fracture planes.

Dolomite. Dolomite (a : 1.691, e : 1.548) occurswidely but is rarely more than an accessory miner-al. It typically cross-cuts the foliation or appears tohave replaced other minerals, suggesting a latestage crystallization.

Enstatite. Relict enstatite occurs in a few Ed-monds-type samples where it has been replaced byantigorite, anthophyllite, or tremolite. The mea-sured 2V of *82" indicates a composition of aboutEnelFse.

Textural interpretation

Mineral occurrence and textural fabric allow aninterpretation of the crystallization history. Theserocks have undergone two principal stages of re-crystallization which appear to have been approxi-mately synchronous with two deformation epi-sodes. An initial retrograde hydration during ductile

E6 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Fig. 4. "Snowball" basal section of tremolite (Tr) with finerchlorite (Ch) in pressure shadow. Bar = 0.025 mm. Todd-type.

deformation was followed by a progressive dehy-dration during a brittle deformation. The Edmonds-type bodies have been affected principally by thefirst stage whereas the Todd-type bodies show theeffects ofboth stages.

The peridotite protolith has not been preservedbut the least altered of the Edmonds-type rockscontain olivine and enstatite which have been par-tially replaced by chlorite, talc, anthophyllite, anddolomite. Tremolite replaced some of the olivine,and in turn was replaced by antigorite which alsoreplaced some of the remaining olivine along frac-tures (Fig. 3). These rocks were only slightly affect-ed by a ductile deformation which caused a pooralignment of tremolite blades and chlorite plates insome specimens whereas others remain unaffectedand exhibit a jack-straw texture. The retrograderecrystallization began with the peridotite protolithassemblage of olivine and enstatite being partiallyreplaced by tremolite and chlorite and to a lesserextent by talc and dolomite. The final event in thissequence was the partial serpentinization of thetremolite and remaining olivine.

The second stage, a dehydration and cataclasisaccompanying a brittle shearing deformation, isbest seen in the Todd-type samples. There is agradation in the degree of shearing from rocks withwidely spaced shear fractures in which large tremo-lite porphyroclasts were partially shredded androtated into parallelism with the shear planes, tophyllonites composed almost entirely of smallertremolite fragments and chlorite plates stronglyoriented in the fracture foliation. The perfection ofthe preferred orientation of the tremolite and chlo-rite is proportional to the closeness of the spacing of

Fig. 5. Tremolite (Tr) crystal cutting chlorite (Ch) and beingcut by antigorite (A). Bar = 0.05 mm. Todd-type.

the fractures. Typically large porphyroclasts oftremolite remain surrounded by deflected foliationbands of finer tremolite and chlorite. Small relictolivine phenoclasts persist in rnany of these phyl-lonites attesting to the ultramafic protolith of theserocks.

The crystallization sequence in the second stagebegins with the tremolite-rich rocks containingsome chlorite and minor talc, olivine, serpentine,and chlorite. As the tremolite was shredded byshearing deformation, additional chlorite formed inthe foliation and in pressure shadows. Some dolo-mite replaced both of these minerals after theshearing episode. Talc is present but not common inthese rocks and serpentine is absent. Magnetite, aproduct of the olivine serpentinization, persistsfrom the first stage and minor iron oxide has formedin some shear fractures.

The texture of the rocks under investigation herecontrasts sharply with that described by Evans andTrommsdorf (1970) in metamorphosed ultramaficrocks in the Alps. The Alpine rocks contain adiscernible foliation but are characteized by mosa-ic textures involving olivine, tremolite, anthophyl-lite, antigorite, and enstatite. These textures arecorrectly interpreted as representing equilibriumassemblages in a prograde recrystallization of ser-pentinite. Such textural arrangements are not pre-sent in the rocks under study here, nor are theovergrowth textures reported by Trommsdorf andEvans (1974) indicating a prograde origin of tremo-lite and olivine present. Rather, the textural rela-tionships in the present rocks indicate a secondaryorigin of the serpentine and relict olivine and ensta-tite.

SCOTFORD AND WILLIAMS: METAMORPHOSED T]LTRAMAFIC BODIES

Table 5. Analyses of USGS standard PCC-I

l leight percent

l '190 43.18 42.16 42,49 42.33

A l 2 0 3 0 . 7 4 0 . 7 6 0 . 7 7 O . 7 7

C a o 0 . s 1 0 . 5 2 0 . 5 1 0 . 5 2

Fe203 2 .85 ND ND ND

Feo 5.24 ND ND N0

Tota l Fe as Fe203 8 .35 8 .02 8 .20 8 .31

K20 0.004 0.004 0.004 0.003

Mn, Co, Cu, K, Ni, and Zn. Acctxacy of theanalyses was evaluated by determining elementalcomposition of the USGS standard PCC-I. Withevery run of four to eight rock samples, a standardwas analyzed along with a contamination blank.Before continuing analyses of unknown ultramaficrocks, elemental analysis of the standard PCC-I hadto be within an acceptable range and the contamina-tion blank was required to be low compared tostandard working solutions. Analyses of PCC-I arelisted in Table 5 with the accepted values as statedby Flanagan (1973). Values for Mn, Zn, Ca, Ni, andCo are within three percent (of the amount present)of the accepted value. The trace elements Cu and Khad higher errors, but this was a result of their lowabundances.

The precision of the analyses was determinedthrough duplicate analyses of pairs of samples.Reproducibility of at least five percent of theamount present was obtained for all elements ex-cept K, Ca, and Fe. Commonly 1-3 percent preci-sion was achieved. From the iron analyses it ap-pears that the powders used for analyses were notcompletely homogeneous. This is also the case withthe analyses of calcium. For potassium most valueswere reproducible to 25 percent of the amountpresent. Duplicate analyses of the standard PCC-Ishowed good reproducibility for all elements.

Table 6 shows the chemical analytical resultsobtained by sample and by body.

Ele me ntal dis t ribution

The mean values quoted in this section are themeans of all samples or the means of Todd- orEdmonds-type samples and differ slightly from themean values shown on Table 2 which are means ofthe mean values of individual ultramafic bodies.

Magnesium. The mean magnesia content for allthe rocks analyzed is 25.59 weight percent MgO,with a range from 14.77 to 42.45 percent. TheEdmonds-type samples have higher magnesia con-tent (33.38% mean MgO) than the Todd-type sam-ples (22.38Vo mean MgO). The Todd-type samplesare low in Mg compared to the average for allultramafic rocks as reported by Turekian and We-depohl (1961, Table 1). Based on a literature search,it appears that an average of 38-40Vo MgO isprobable for alpine-type peridotites. Thus, therocks under study are abnormally low in magne-sium when compared to alpine-type peridotites.

Iron. The distribution of iron reflects the divisionof Todd-type and Edmonds-type ultramafic bodies.The Edmonds-type have lower (6.11% mean) but

Fl anagan

(1973) PCC-I this study

43.32 42.24

0 . 8 5

0 . 5 2 0 . 4 8

NO ND

ND ND

L 3 4 8 . 3 9

0.003 0 .004

ppm

Co l I2 113 115 110

Cu I I 17 13 19

l4n 959 975 975 930

Ni 2339 2390 2300 2325

7n 36 38 38 36

108 112

9 1 5

2320 2160

36 35

Comment should be made on the possibility thatthe Edmonds- and Todd-type bodies represent twocontrasting groups of ultramafic occurrences withseparate protoliths and post-emplacement historiesrather than rocks with the same protolith but repre-senting separate stages of the same post-emplace-ment history as assumed in the above discussion.Observations which favor the single protolith his-tory are: (1) both groups of bodies occur in the samestratigraphic unit (the Ashe Formation) and lieapproximately along the same structural trend; (2)both tend to be elongated in the prominent 51foliation trend; (3) a few samples from Edmonds-type bodies are chemically and mineralogically sim-ilar to Todd-type samples; (4) Todd-type samplescommonly contain relict olivine and porphyroclastsof tremolite-minerals characteristic of the lessrecrystallized Edmonds-type rocks.

The cause of the observed difference in degree ofalteration of the two types of bodies is not self-evident but is possibly attributable to differences inpermeability, with the typically smaller moresheared Todd-type bodies being more permeable tohydrothermal fluids.

Geochemistry of the metamorphosed ultramaficbodies

Analytical methods, accuracy, and results

Sixty-one samples were analyzed by atomic ab-sorption spectrophotometry for Mg, Al, Ca, Fe,

SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

more varied iron (4.03-10.20% Fe) concentrationsthan the Todd-type (9.13% mean, 7.93-ll.40Vorange). The mean for all samples is 8.30% Fe whichis slightly lower than the average (9.38Vo Fe) report-ed by Turekian and Wedepohl (1961, Table 1) for allultramafic rocks. Probably the higher concentrationof iron in the Todd-type samples reflects the meta-somatic loss of magnesium from Fe-Mg minerals inthese bodies.

Aluminum. The distribution of alumina showsthat the Edmonds-Type samples have lower meanconcentrations (3.11% AlzO) than the Todd-typesamples (6.28Vo Al2O3) and that the Todd-type havehigher concentrations of aluminum than the averageof peridotites of 3.99% Al2O3 reported by Wedepohl(1e6e).

Most samples analyzed in this study have veryhigh aluminum concentrations compared to alpine-type harzburgites which dominate the alpine ultra-mafics of the Appalachians, although some of theEdmonds-type samples have aluminum concentra-tions that compare to typical alpine harzburgites. Itis thus probable that all of the Todd-type and someof the Edmonds-type samples have been enriched inaluminum. As no garnet pseudomorphs or relicts offeldspar have been observed, the aluminum must beof metasomatic origin derived from the countryrock.

Calcium. The mean calcium content as CaO forall rocks analyzed is 4.55%. The Edmonds-typesamples show a larger variation (0.94-ll.87Vo CaO)and a lower mean (2.86%) than the Todd-typesamples (2.75-11.47% and 5.19%). It is clear thatthe Edmonds-type samples have generally lowerconcentrations of calcium, although three samples(66c, 56a,53a) show much higher calcium than thetypical Todd-type sarnple.

Wedepohl (1969) reported that the average peri-dotite contains3.467o CaO. Compared to this valuethe Todd-type samples appear to represent rocksenriched in calcium and the Edmonds-type thosedepleted in calcium. It must be kept in mind that theaverage peridotite includes garnet-rich varietieswhich would tend to increase the calcium contentcompared to the average alpine-type harzburgite.

Clinopyroxene accounts for most of the calciumin fresh alpine ultramafics. In view of the largeamount of calcium in most of the samples for thisstudy, a wehrlitic peridotite, garnet wehrlite, orpyroxenite composition is necessitated if the cal-cium is considered primary. However, as the rocksstudied are not fresh ultramafics and contain almostno relict pyroxene, a metasomatic source for the

calcium is favored. The negative correlation ofcalcium with magnesium of -0.669 (P : 0.0001) andthe positive correlation with tremolite of 0.576 (P :

0.0002) and the low concentration of calcium in theolivine suggest that calcium was introduced andwas incorporated into the tremolite as magnesiumwas removed.

Nickel. The mean nickel content for all the ultra-mafics analyzed is 1238 ppm. The Todd-type sam-ples range from 540 to 1680 ppm and have a meancontent of 1053 ppm. The Edmonds-type sampleshave a wider range (461 to 2660 ppm) and a highermean (1730 ppm). If the few Edmonds-type sam-ples, probably altered, with the very low nickelconcentrations are not considered the mean wouldbe much higher.

Turekian and Wedepohl (1961) reported an aver-age nickel content for ultramafic rocks of 2000 ppmNi. Goles 0967) revised this estimate to 1500 ppmNi. A positive correlation (0.811, P : 0.0001) foundin the present study between nickel and magne-sium, the positive correlation with olivine (0'570,P : 0.0002), and the negative correlation with iron(-0.348, P : 0.0301) are consistent with observa-tions made by Vogt (1923) and Stueber and Goles(1967).

The Todd-type ultramafics have lower nickelcontents than peridotites and dunites, but higherconcentrations than pyroxenites. Most of the Ed-monds-type samples have nickel concentrationssimilar to dunites and typical alpine peridotites orharzburgites. This distribution can best be ex-plained by a metasomatic model where nickel tendsto follow magnesium as it is removed from theultramafic body. This type of variation in nickelconcentrations in metasomatic environments forultramafic rocks was demonstrated by Curtis andBrown (1971).

Cobalt. No easily observable differences existbetween the Todd-type and Edmonds-type sampleswith respect to the distributions of cobalt, althoughit is possible that contamination with cobalt in thetungsten carbide grinding elements has affected thevalues obtained. The mean cobalt content for allsamples is 118 ppm, as it is separately for the Todd-type and Edmonds-type samples. This value is verynear the 110 ppm average reported by Goles (1967)for ultramafic rocks.

Manganese. Manganese concentrations largelyseparate the rocks analyzed into Todd- and Ed-monds-types. The mean concentration is 1358 ppmfor all analyzed samples. The Todd-type samplesrange from 973 to 1960 ppm Mn and have a 1566

SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 6. Analvses of ultramafic rocks

l ' |eight percent

89

Sampl enumber Mgo 41203 CaO Fe t'tn0

ppm

ZnNil'1nCuCo

T 2T l6-aT l3-dT 13-bT l4-bT 15-bT 5T 6

26427370

223595354266872

1066733654244

857546059ND583167L7

toz64

1 1 1302734322 l

LL? 146r32 230133 7L146 l l0100 8ll l0 95143 50tLz 69

l l4 27104 98Lrz t7998 57

107 58r20 87118 109105 39110 7675 27

N M 7N M 9NM 21-eNlrl 21-gM ' 2 2NM 25-aNM 26

25.4324.40t4.7726.7314. 94t5.7723.L33l .5825. 5823-5723 .85?1 .6916.7726.1723.09

5 . 195 .704 . 9 25 . 3 78 . 2 35 .89s . i 66 . 1 84 .885 .865 .856 . 164 . 7 35 . s t7 . 9?4 .875. 439 . 3 37 . 4 r5. 565 .035 . 7 94 . 7 07 .24

10 .214 .877 . 3 25 .880.883 .090.99I .872 . 6 11 .86

5 .93 7 .933 . 19 9 .607 . 3 ? 8 . 2 94 .34 9 .486 .86 8 .567 . 0 6 8 . 9 13 .82 9 .865 .75 11 .404 . 35 8.794 .57 8 .805 . 5 7 9 . 3 63 . t 7 9 . 1 84 .48 8 .173 . 1 0 9 . 4 23 .36 9 .283 .48 8 .743 .91 8 .69

r 0 . l 0 8 . 1 53 . 1 1 l t . 3 t4 .75 8 .034 .52 8 .600 .45 9 .463 .24 10 .736 .?L 8 .966 . 2 3 9 . 4 52 .9 t 10 . 316 . 7 6 8 . r 24 . 5 1 1 1 . 1 6

1 l . 5 7 4 . 0 32 . 3 3 1 0 . 2 00 . 1 0 5 . 6 30 .04 6 .88t .97 5 .47t . t 7 6 . 0 7

118 73t32 86103 r29143 91120 135t23 305105 131LTz 8I106 1 lt?L 5l61 5

180 133119 14177 L2lzL 35l l 5 7 l

1347 1l4s 931460 1190 951390 990 1201576 1170 1371603 680 r571520 ll40 r2r1637 1070 1051624 1030 r221580 1430 tL71460 l0B0 1321513 1100 1101947 900 1331550 1100 841510 1260 t471583 1030 1141458 t270 r221387 1230 1141530 s40 1031637 ll30 NDl5l5 1040 1061890 880 t20t323 1038 100t479 1343 131t247 1350 8l1750 860 1041828 t285 ND1450 930 851383 900 1041084 580 29t624 2030 92904 2340 55

1044 2350 67983 1800 59973 2125 45

0 . 1 70 . 1 90 . 180 .200 . 2 10 . 190 . 2 10 . 2 10 . 2 20 . 1 90 .200 . 250 .200 . l 90 .220 . l 90 . 180 . 2 00 . 2 10 .200 . 2 40 . 1 70 . 190 . 160 . 2 30 .240 . 190 . 180. 140 . 2 1o . 1 20 . 1 30 . 1 30 . l 3

P 18-b 23 .96P l9-a 24.62P 20 16 .56

ss 45-b 23.7rss 46 25.19ss 47-b 22.44SS 69-a ?8.61ss 69-b 27.86RR 3B-b 22.49RR 39 20.42RR 41 26.37RR 42-b 23.24RR 43 23.82ED 56-a 22.99ED 57-a 33.62ED 58-a 40.09ED 59 38.60ED 60-b 33.86ED 66 -a 39 .55

ppm mean. The Edmonds-type samples range from397 to 1624 ppm Mn and have a mean of 950 ppm.Most of the Edmonds-type samples have manga-nese contents similar to the average of ultramaficsas reported by Goles (1967), but most Todd-typesamples show concentrations higher than this aver-age value of 1040 ppm.

It has been reported by Livingston(1976) and byCurtiss and Brown (1971) that manganese concen-trations increase during metasomatism of ultramaficbodies, which is consistent with observed manga-nese distribution in the rocks investigated here. Themetasomatized Todd-type samples show highermanganese concentrations than the less alteredEdmonds-type samples. This transfer of manganeseinto the ultramafics is supported by a positivecorrelation between manganese and the principalmineralogical product of metasomatism, tremolite(correlation coefficient 0.605, P : 0.0001). Micro-probe analyses show no Mn present in the chlorite(William Zhong, pers. comm.).

Zinc. The means for zinc content of the Ed-monds- and Todd-type samples show a distinctdifference between these two groups. The averagezinc content for all ultramafics analyzed is 96 ppm.The mean for 38 Todd-type samples is 111 ppm witha range of 45 to 157 ppm Zn. The mean for theEdmonds-type samples is 57 ppm and the range is29 to ll0 ppm. This mean is comparable to that forultramafic rocks of 50 ppmZn (Turekian and Wede-pohl, 1961). Vinogradov (1962) reported 30 ppm asan average for ultramafic rocks. It thus appears thatthe Todd-type ultramafic rocks have been some-what enriched in zinc compared to the Edmonds-type.

Copper. The division of samples into two groupsbased on copper content is not as straightforward asit is for the other elements. There is a large overlapin copper concentrations between Todd- and Ed-monds-type samples although the latter tend tohave somewhat lower copper concentrations. Themean copper content for all samples is 81 ppm. The

90 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 6. (continued)

} ' leight percent ppm

Sampl enumber Mgo Al 203 Ca0 7nN iItlnCuCoMn0Fe

ED 66-bED 66-cED 67-aED 67-b

1 1 01 3 0

T 0 1 1

B l 2 - a

EN 52EN 53-a

WB 32-c}lB 33-b

c 34-bc 3 5GR 5 l -aGR 68

|'lL 65blL 64

DG 63-b

PK 62PK 61-bP K 6 1 - c

39. l02? .3633 .403 1 . 5 118.872 1 . 8 316 .9323 .3829.4724.7r20.7524.0723 .962 1 . 0 340.1741 .4524.0124.0523 .3423.7 |24.188 . 6 2

1 . 4 64 . 2 23 .42

1 2 . 1 810. 925 .477 . s42 . 7 4I .85s . 3 75 . 8 16 . 4 24 .814 .87t .02ND7 . 7 r9 . 165 .59ND5. 5sND

0 . l 00 . t 20 . 140 . t ?0 . 250 . 230 . 1 80.090 . 0 50 .200 . 2 10 . 250 . 2 40. 200 .090 .080 . 190 .250 . 16

0 . l 50 . 2 30 . 2 5

760965

1062899

r96017901367697397

l5131650195018651520727620973

t9671253120017671970

2 .03 5 .651 1 . 8 7 4 . 4 13 .90 4 .310 .18 5 .94

rr .47 10.404 .38 10 .40

1 1 . 0 5 8 . 2 70 .08 5 .320 . 1 1 4 . 2 46 . 3 0 9 . 1 33 . 9 5 9 . r 72 . 7 5 9 . 8 46 .74 10 .203 .93 9 .880 . 3 1 9 . 3 01 .00 5 .085 .09 9 . 794 . 7 4 9 . 8 46 . 3 1 9 . 7 t5 .07 8 .404 . 5 1 9 . 6 9

10 .45 9 .45

109 L37 2059 19 260

103 202 33r23 25 19158 32 451118 44 72Lr7 151 104119 l0 229 ? 5 1 5

Lrz 43 37126 6 91133 u4 46159 30 40t26 95 4r173 20 4t104 58 4t

l l3 55 45l r8 25 ND108 93 45

83 194 35143 428 ND84 84 1000

1985 37550 34

2360 110461 58590 102880 104

1040 951910 421970 301050 ND970 152

1520 143770 111

1060 1032660 6l1780 431200 45690 111890 1007?5 89

1680 101225 l l3

T = T o d d ; N M = N . i g g e r M o u n t a i n ; P = P h o e n i x l 4 o u n t a i n ; L = L i t t l e P e a k C r e e k ; l ' l B = H o o d r o w B a r e ; C =Cranberry; SS'= Shatley-springs; RR = Rocky Ridge; T0 = Twin 0aks; EN = Ennice; GR = Grayson; ED = Edmonds;PK = Parkway-l; ! . |L = Woodlawn; B = Brushy Creek; DG = Dugspur.

mean copper content for the Todd-type samples is92 ppm with a range of 6 to 230 ppm. The range ofthe Edmonds-type samples is similar to the Todd-type (5 to202 ppm), but that mean copper content islower (53 ppm). All of these values are high com-pared with averages for ultramafic rocks of 10, 20,and 30 ppm as reported by Turekian and Wedepohl(1961), Vinogradov (1962), and Goles (1967) respec-tively.

Potassium. There is no clear distribution of po-tassium between the Edmonds- and Todd-type sam-ples although most samples with low potassium areof the Edmonds-type. The mean potassium concen-tration for all ultramafics analyzed is 81 ppm. TheTodd-type samples have a mean of 94 ppm and arange of 17 to857 ppm. The Edmonds-type sampleshave a mean of 50 ppm and a range of 15 to 260ppm. As potassium may enter ultramafic rocksduring metasomatism it is interesting to note thatthe samples containing more than 200 ppm K (14b,20, 10, and 66c) on the basis of major elementconcentrations are among those altered metaso-matically to the greatest extent. It is also important

to note that sample 20, which contains the highestpotassium concentrations (857 ppm), was locatedwithin one foot of the contact with country rock.

Carswell et al. (1974) and Curtis and Brown(1969) demonstrated that potassium enters ultra-mafic bodies from solutions derived from the coun-try rock in quantities which are a function ofcountry rock composition, but in amounts smallerthan those of calcium or aluminum. Thus if potassi-um enters the ultramafic along with aluminum andcalcium, these elements should show positive cor-relations. Conversely, as potassium enters the ul-tramafics, magnesium moves toward the countryrock, and therefore correlation between potassiumand magnesium should show a negative correlation.These correlation coemcients are indeed observed:K vs. Al : 0.510 (P : 0.0009), K vs. Ca : 0.553(p : 0.0003), and K vs. Mg : -0.369 (p : 0.0207).

Mineral reactions and metasomatism

A sequence of mineral reactions and metasomaticexchanges which describes the evolution of thesebodies from the protolith ultramafic rock to the

SCOTFORD AND WILLIAMS: METAMORPHOSED (ILTRAMAFIC BODIES

extensively altered tremolite-chlorite schists of theTodd-type can be stated based on the observedtextural relationships, mineral assemblages, and thegeochemical analyses. The sequence begins with adunite with a small concentration of enstatite, eitherfresh or already partially altered, which was tecton-ically emplaced into the Ashe Formation as a num-ber of slivers. The first reactions were retrogradehydrations of this originally high temperature-pres-sure assemblage in response to the lower gradeenvironment of regional metamorphism. A tremo-lite-anthophyllite-chloritFtalc assemblage result-ed, although some relict olivine and rare enstatitecrystals are preserved. This reaction can be written:

3OMg2SiOa + 10Al + 5Si + 2Ca + 24H2O +Forsterite

Ca2Mg5(Sfu O z)(OH)z + MgleAl5(Sir r AI5O40XOH)32Tremolite Chlorite

+ Mg6(Si8OzoXOH)+ + Mg7(SfuOz)(OH)zTalc Anthophyllite

+ 23Mg + 8H*.

The addition of Si, Al, and Ca is necessitated bythe observed mineral assemblage in these leastaltered samples and by the comparison of theconcentration of Al and Ca in the analyzed leastaltered and most altered specimens. The loss of Mgin the reaction is based on the same criteria. Theaddition of Si is further indicated by a 2.3 percenthigher concentration of Si in Todd-type samplesbased on XRF analyses of three specimens.

A second hydration stage involves the serpentinization of olivine and tremolite and is evidenced bythe petrographic observation that serpentine hasreplaced tremolite pseudomorphically and extendsinto fractures in olivine in Edmonds-type rocks(Fig. 3). Thus the crystallization of tremolite whichpartially replaced olivine must have occurred atleast in part before serpentinization. This reactionmay be stated as:

Mg2SiOa * Ca2Mg5(Si8O2r(OH)2 + Si + 8MgForsterite Tremolite

+ l7H2O + 5Mg:(SizO5XOH)4 + 2Ca + l6OH+.Antigorite

The final reaction in the sequence is a dehydra-tion accompanied by shearing and is indicated bythe comparison of the chemical composition, miner-alogy, and textures of the Edmonds- and Todd-typeultramafic bodies. The more altered Todd-type

specimens contain no serpentine but more abundanttremolite, higher concentrations of Mg, and less Ca.The Todd-type rocks are strongly foliated, princi-pally through the alignment of shredded tremoliteand strongly oriented chlorite. This reaction may bewritten as:

3Mg3(Si2O5)(OH)+ + 2Ca + 2Si +Antigorite

Ca2Mg5(SfuOz)(OH)z + 4Mg + 3H2O + 4H+.Tremolite

The above reactions are based on mass transfercalculations, taking into account the observed min-eralogical and chemical changes. The simplest stoi-chiometric formulae are used for each mineral andFe is ignored. The geochemical study suggests anenrichment in Fe in the more altered rocks probablythrough removal of Mg from Fe-Mg silicates, prin-cipally olivine; however, the znalyzed olivines con-tain only about 10 percent mol fraction offayalite soFe is not an important element in the reactionsstudied.

Chlorite occurs in somewhat higher concentra-tion in the more altered Todd-type bodies than inthe Edmonds-type and could be considered to be aproduct of the antigorite reaction which producedadditional tremolite, however, there is no textureevidence of this and the low abundance of antigoritein the Edmonds-type bodies would not generate asignificant amount of chlorite.

Structural relationships

The relationship between the recrystallizationsequence of these ultramafic bodies and the tecton-ic and metamorphic history of the Blue Ridge is afascinating, but at present a rather speculative,problem. The low Rb concentration in alpine ultra-mafic rocks and their susceptibility to Sr metasoma-tism (e.9., Menzies and Murthy, 1976) make deter-mination of crystallization ages by Rb/Sr methodsof even fresh alpine bodies unsuccessful. However,the common elongation of these bodies parallel withprominent foliation of the Ashe Formation and thepresence of that foliation in even the least alteredEdmonds-type bodies restricts their emplacementto a time between the deposition of the AsheFormation in late Precambrian and the developmentof the foliation. Most workers (e.9., Rankin et al.,1973) have called this foliation S1 and assigned it tothe Taconic deformation (450-rm.y.). Rankin er a/.(1973) place the peak of metamorphism in the

SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

eastern Blue Ridge as post-kinematic, thus some-what later than the 51 foliation.

Although a pre-emplacement age for the firststage retrograde hydration of the ultramafic proto-lith cannot be ruled out, the presence of the 51foliation in even the Edmonds-type bodies, which isexpressed by the preferred orientation of chloriteand tremolite, strongly suggests that these reactionsare post-emplacement and reasonably represent apartial reestablishment of equilibrium in the pres-sure, temperature, and water environment of re-gional metamorphism in comparison to that of thehigher intensity environment of the upper mantle.The second stage hydration reaction causing thepartial serpentinization of olivine and tremolite istentatively assignable to the metamorphic regimewhich followed the peak of taconic metamorphism(Hatcher, 1978, Fig. C). The dehydration stage ofthe sequence which was accompanied by shearingis then possibly correlative with the Acadian(?)(375-rm.y.; greenschist metamorphism and brittledeformation associated with the F3 folding (Hatch-er. 1978).

As pointed out by Rankin, Espenshade, andShaw (1973), these ultramafic bodies typically oc-cur on the northwest side of the amphibolite layersin the Ashe Formation although some occur on thesoutheast side and some are independent of theamphibolite outcrops. This association suggests agenetic relationship and perhaps an ophiolitic asso-ciation. Whether or not this is valid, a tectonicemplacement of these ultramafic bodies seems mostlikely. Hatcher (1978) theorized that thrusts initiat-ed in the unstable sea floor brought oceanic crustand mantle, including the mafic and ultramaficrocks of the eastern Blue Ridge, into the sedimenta-ry pile during the closing of a marginal sea in Middleto Late Cambrian.

The nature and history of the protolith before itsprobable tectonic emplacement is an important butdifficult question. It may have progressed through aprograde deserpentinization cycle before the retro-grade events described in this study. Whether ornot an earlier deserpentinization has occurred, wefavor a history involving emplacement of enstatite-bearing dunite from an ophiolite source. The pro-jected major element chemistry of the protolithbased on this study is characterizedby low calciumconcentration. This is typical ofdunite and harzbur-gite from ophiolite metamorphic peridotite com-plexes according to chemical data provided byColeman (1977 , Tables l, 2, 3, and 4) and contrasts

with the high calcium abundances characteristic oflherzolites or cumulate ultramafic rocks fromophiolites. This suggests a source from the base ofthe ophiolite sequence below the cumulate perido-tite and mafic members of the sequence.

Conclusions

Analysis of the mineralogical and geochemicaldata, together with textural information, indicatesthat the ultramafic bodies investigated can be divid-ed into two distinct groups, the more recrystallizedand metasomatized Todd-type and the less alteredEdmonds-type. The contrasting character of thesetwo types of bodies reflects differing degrees ofalteration of the original protolith, probably a dunitewhich contained a small amount of enstatite.

Textural evidence is important in understandingthe sequence of mineral reactions which convertedthe protolith to the tremolite-chlorite-talc schist ofthe Todd-type bodies. In the less altered Edmonds-type rocks primary olivine and uncommon primaryenstatite have been replaced by tremolite, chlorite,talc, and anthophyllite. In a second hydration stage,there was pseudomorphous replacement of some ofthe tremolite by antigorite, which also replacedsome olivine (Fig. 3). These observations supportthe conclusion that the recrystallization sequencedid not begin with serpentinization as has beenreported in other areas. The lack of antigorite in theTodd-type samples and the mylonitic texture ofmany of these samples indicate a second majorstage in the recrystallization was one of partialdehydration and cataclasis.

An examination of the abundance data for Mg,Al, Ca, Fe, Ni, Mn, Cu, Co, K, and Zn obtainedfrom analyses of these rocks allows conclusions tobe drawn concerning the movement of these ele-ments during metasomatism. Assuming, for reasonsdetailed earlier, that the two types of ultramaficbodies investigated represent different degrees ofmetasomatic alteration of the protolith, the ex-change of these elements between the ultramaficbodies and the country rock can be described.There is strong evidence that Mg and Ni have beenremoved from the ultramafic rocks and that Al hasbeen added. It is probable, though less well demon-strated, that Ca, Fe, Mn, and K entered the ultra-mafic bodies and that Co was removed. The move-ment, if any , of Zn and Cu could not be determined.

A statement of the recrystallization sequence interms of calculated mass balance reactions can bemade using constraints provided by the geochemi-

SCOTFORD AND WILLIAMS: METAMORPHOSED T]LTRAMAFIC BODIES 93

cal, mineralogical, and textural information. Aninitial retrograde hydration converted the protolith,probably a dunite with some enstatite, into alteredrocks of the Edmonds-type by the hydration ofolivine and enstatite producing tremolite, chlorite,talc, and anthophyllite with the addition of Si, Al,Ca, and the loss of Mg leaving some relict olivineand enstatite. In a later stage of this hydration,serpentine (antigorite) was the product of the hydra-tion of some of the olivine and tremolite, with theaddition of Mg and loss of Ca.

The final step in the recrystallization sequencewas a dehydration accompanied by brittle deforma-tion which destroyed the serpentine and increasedthe concentration of tremolite, with the addition ofCa and loss of Mg. The net chemical change be-tween the protolith of the more altered Todd-typerocks was one of loss of Mg and addition of Al andCa as indicated by the geochemical study.

The possible relationship between this recrystalli-zation sequence and the tectonic and metamorphichistory of the eastern Blue Ridge is subject tospeculation. If the reasonable assumption is madethat these ultramafic bodies were emplaced tectoni-cally into the Ashe Formation, retrograde hydrationcould have occurred during the Taconic metamor-phism as a non-equilibrium adjustment to the lowerP-Z conditions of regional metamorphism. Thelater dehydration may be associated with the350+m.y. old (Acadian?) greenschist facies meta-morphism and brittle deformation.

AcknowledgmentsThe authors wish to thank Ann H. Glaser who did most of the

point-count analyses, Thomas E. Laskowski who made a num-ber ofindex ofrefraction determinations ofolivine and dolomite,and Frederick R. Voner who did additional petrographic work.The manuscript was substantially improved by the critical re-views provided by Kula C. Misra and James A. Whitney.

ReferencesAlcom, S. R., and Carpenter, J. R. (1977) Serpentinization of the

Holcome Branch Dunite, North Carolina. Southeastern Geol-ogy,17,243-256.

Borg, I. Y., and Smith, D. K. (1969) Calculated X-ray powderpattems for silicate minerals: Geological Society of AmericaMemoir 172.

Carswell, D. A., Curtis, C. D., and Kanaris-Sotiriou, R. (1974)Vein metasomatism in peridotite at Kalskaret near Tafiord,South Norway. Journal of Petrology, 15,383-402.

Coleman, R. G. (1977) Ophiolites: Springer-Verlag, New york.Curtis, C., and Brown, P. E. (1969) The metasomatic develop-

ment of zoned ultrabasic bodies in Unst, Shetland. Contribu-

tions to Mineralogy and Petrology, 24,27r-292.Curtis, C. and Brown P. E. (1971) Trace element behavior in

zoned metasomatic bodies in Unst, Shetland. Contributions toMineralogy and Petrology, 31, 87-93.

Evans, B. W. (1977) Metamorphism of alpine peridotite andserpentinite. Annual Review Earth and Planetary Science, 5,397447.

Evans, B. W., and Trommsdorf, V. (1970) Regional metamor-phism of ultramafic rocks in the Central Alps: parageneses inthe system CaO-M8O-SiO2-H2O. Schweizerische Mineralo-gische und Petrographische Mitteilungen, 50, 481492.

Flanagan, F. J. (1973) 1972 values for international geochemicalchemical reference samples. Geochemica et CosmochemicaActa.37. l189-1200.

Goles, G. G. (1967) Trace elements in ultramafic rocks. In P. J.Wyllie, Ed., Ultramafic and related rocks, p. 352-162. IohnWiley and Sons, Inc.

Hahn, K. R., and Heimlich, R. A. (1977) Petrology of the duniteexposed at the Mincey Mine, Macon County, North Carolina.Southeastern Geology, 19, 39-53.

Hatcher, R. D., Jr. (1978) Tectonics of the western Piedmont andBlue Ridge southern Appalachians: review and speculation.American Journal of Science, 278,276-3M.

Jackson, E. D., and Thayer, T. P. (1972) Some criteria fordistinguishing between stratiform, concentric, and alpine peri-dotite-gabbro complexes : International Geological Congress,24th, Montreal, sec. 2, 289-296.

Keith, A. (1903) Description of the Cranberry quadrangle, NorthCarolina-Tennessee: U. S. Geological Survey Geologic Atlas,Folio 90, p. 9.

Laskowski, T. E., and Scotford, D. M. (1980) Rapid determina-tion of olivine compositions in thin section using dispersionstaining methodology. American Mineralogist, 65, 401-403.

Livingston, A. (1976) A metamorphosed layered alpine-typeperidotite in the Langarat Valley, South Harris, Outer Hebri-des. Mineralogical Magazine, 40, 493499.

Menzies, M. A., and Murthy, R. V. (1976) Sr-isotope composi-tions of clinopyroxenes from some Mediterranean alpine lher-zolites. Geochemica et Cosmochemica Acta,44, 1577-l5El.

Misra, K. G., and Keller, F. B. (1978) Ultramafic bodies in thesouthern Appalachians: a review. American Journal of Science. 278. 389-41E.

Rankin, D. W., Espenshade, G., and Neuman, R.(1972) Geolog-ic map of the western half of the Winston-Salem Quadrangle,North Carolina, Virginia, Tennessee: Department of Interior,U.S. Geological Survey Miscellaneous Investigations, Map I-709-A.

Rankin, D. W., Espanshade, G., and Shaw (1973) Stratigraphyand structure of the metamorphic belt in northwestern NorthCarolina and southwestern Virginia. American Journal ofScience, Cooper Volume, 273-A. 140.

Stueber, A. M., and Goles, G. C. (1967) Abundances of Na, Mn,Cr. Sc. Co in ultramafic rocks. Geochemica et CosmochemicaActa, 3l , 75-93.

Swanson, S. E. (1980) Petrology of the Rich Mountain ultramaficbodies and associated rocks, Watauga County, North Caroli-na. Southeastern Geology, 21, 209-225.

Trommsdorf, V., and Evans, B. W. (1974) Alpine metamorphismof peridotitic rocks. Schweizerische Mineralogische und Pe-trographische Mitteilungen, 54, 333-352.

Turekian, K. K., and Wedepohl, K. H. (1961) Distribution of theelements in some major units of the Earth's crust. Geological

94 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Society of America Bulletin, 72, 175-192. igneous rocks. In K. H. Wedepohl, Ed.' Handbook of Gee

Vinogradov, A. P. (1962) Average contents of chemical elements chemistry, p. 227-271. Springer, New York.

in the principal types of igneous rocks of the Earth's crust.Geochemistry (traqsl. of Geokhimiya), 641-64. Manuscript received November 23, I9EI ;

Wedepohl, K. H. (1969) Composition and abundance of common accepted for publication, September 7 ' 1982 .