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Petrology and Geochemistry of Metamorphosed Ultramafic Bodies

Petrology and Geochemistry of Metamorphosed Ultramafic Bodies

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

Petrologyand geochemistryof metamorphosedultramafic bodiesin a portion of the Blue Ridge of North Carolina and Virginia

Devro M. ScorroRD AND JBrpney R. Wrr-llnnasr

Department of Geology Miami University Oxford, Ohio 45056

Abstract

Seventeen metamorphosed ultramafic bodies in the Precambrian Ashe Formation exposedalong a northeast-trendingbelt from near West Jefferson,North Carolina to near Floyd, Virginia in the Blue Ridge, were investigated.The assemblagesin individual samplesconstitute subgroupsdrawn from the larger group consistingofolivine, enstatite, , , , , chlorite, , and . Although the ultramafic bodies occur in the garnet, staurolite, and kyanite zones recognized in the country , no correlation exists betweenthe mineral assemblagesin the and the metamorphic zones. Modal analysesand quantitative chemical analysesfor Mg, Al, Ca, Fe, Ni, Mn, Cu, Co, K, andZn lead to the recognitionof two distinct types of metamorphosedultramafic bodies based on the degree of metasomatic alteration. The metasomaticexchange with the country rock involves the loss of magnesiumand and the introduction of aluminum, calcium, , manganese,and into the ultramafic bodies. The less altered bodies are more massiveand contain relict and rare relict enstatite plus tremolite, anthophyllite, chlorite, and antigorite whereasthe more altered bodies are more strongly foliated with abundant tremolite and chlorite and less common relict olivine. Textural evidence, notably the pseudomorphouspartial replacement of tremolite by serpentine,combined with that provided by the geochemicalstudy provides the basis for understandingthe recrystallization history and the calculation of massbalance equations. A retrograde hydration of the ultramafic protolith produced the tremolite-chlorite-talc anthophyllite assemblagewith relict olivine. In a second stageof this hydration serpentine replaced some of the tremolite and olivine. A later partial dehydration led to the more strongly foliated and, in someplaces, mylonitized tremolite--chlorite-talcschist or phyllon- ite.

Introduction as probably originally ultramafic although they are now largely chlorite-tremolite and Alpine ultramafic bodies occur throughout the and were mapped by Rankin, Espenshade, and length of the Appalachian from euebec Newman (1972)as generally small lens-shapedbod- to Alabama. These typically peridotitic or dunitic ies in the PrecambrianAshe Formation. plutons and their serpentinized equivalents have The bodies investigated are typically elongate in received considerable attention from petrologists map view rangingin sizefrom 0.6 km by 0.2 km to (Misra and Keller, 1978),but a group of metamor- 8.0 km by 1.5 km. They are exposedin a northeast phosed and metasomatized ultramafic rocks ex- trending belt from southwest of West Jefferson in posed in the eastern Blue Ridge of North Carolina Ashe County, North Carolina to near Floyd in and Virginia (Fig. 1) have not been investigated in Floyd County, Virginia, a distanceof 150 km. A any detailed way. These petrographically rather total of fifteen bodies were sampledfor petrograph- uncommonrocks were recognizedby Keith (1903) ic and geochemicalstudy (Table l). The study was designedto answer four principal I Presentaddress: 6709 Hopewell Avenue, Springfield,Virgin- questions. (1) What was the nature of the protolith ia 22151. from which these extensivelv altered rocks were 0003-004)v83/0I 02-0078$02.00 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES 79

[Illuo.n"t Zone llTlr.ouroriteZone [Ilrvonite Zone Fig. l. Location of ultramafic bodies in easternBlue Ridge in North Carolina and Virginia southeastof Fries fault. pee-Elk Park Plutonic Group, pec{rossnor Plutonic Group, pea-Ashe Formation, p€m-Mount Rodgers Formation, ab-Alligator Back Formation, +Cambrian Sediments,Pza-Spruce Pine Plutonic Group. After Rankinet al. (1972).Thearea of kyanite zone shown in the northeast corner of the map was mapped only as " " by Rankin et al, (1972). derived? (2) What relationship exists between the chemical mass transfer between the country rock presentmineralogy of theserocks and the metamor- and the ultramafic intrusives. The mineral assem- phic grade of the country rock which changesalong blagesof individual specimensare subgroupsdrawn the trend of the exposuresof ultramafic rock (Fig. from the larger suite which comprises olivine, en- 1) from the garnet zone through the staurolite zone statite, tremolite, anthophyllite, chlorite, talc, ser- to the kyanite zone basedon the mapping of Rankin pentine, magnetite, and dolomite. et al. (1972). (3) What metasomatic changes have occuned during recrystallization of these rocks Previous investigations based on the present concentrations of the major Evans (1977)provided a valuable review of the elements Mg, Fe, Ca, and Al, and the trace ele- of metamorphosed and ser- mentsMn, Co, Cu, K, Ni, and Zn? (4) What is the pentinites. Trommsdorf and Evans (1974)described recrystallization history of these bodies? the development of mineralogy similar to that ob- The size, shape,and geologicoccurrence ofthese served in this study in ultramafics subjected to bodies in deformed Precambrian metasediments alpine regional and contact , except and volcanics strongly suggests that they were that in their study olivine is the dominant mineral alpine ultramafic intrusives (as defined by Jackson rather than tremolite or chlorite. and Thayer, 1972)and probably emplaced tectoni- The textural relationships in the study by cally in a non-liquid state (see Misra and Keller, Trommsdorf and Evans persuasivelydemonstrate a 1978).What is not clear is whether theserocks were prograde metamorphism from to peri- originally or , common among dotite in response to a regional metamorphic epi- the unaltered ultramafic bodies of the Appala- sode. As is discussedbelow, the texture observed chians, or were another ultramafic rock type. in the ultramafic bodies described here is quite The alteration history of these rocks is undoubt- different and is not interpreted as indicating a single edly complex, involving retrogressiverecrystalliza- prograde metamorphism. tion from the original ultramafic assemblageplus Some recent studies of ultramafic rocks in the SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 1. Division of ultramafic bodies into Todd-type and The rocks in both of these studies have been Edmonds-type interpreted as the result of partial serpentinization during or after the implacement of , but Todd-type bodies Edmonds-typebodies neither has been extensively recrystallized since (T) Todd (ED) Edmonds emplacement.

(W)Warrensville (GR)Grayson (HN) ltigger (EN)Enni ce Petrolog5r (WB) WoodrowBare (B) BrushCreek Generalstatement (L) Little PeakCreek In outcrop these metamorphosedultramafic bod- (SS) ShatleySprings ies are grayish-greenwith a distinct schistosefolia- tion which typically parallelsthat of the amphibolite (P) PhoenixMountain or biotite muscovite gneissesof the Ashe Formation (C) Cranbemy with which they are in contact. The is (RR) RockyRidge expressed microscopically by the parallelism of such platy or prismatic as chlorite, tremo- (T0) Twin 0aks lite, and talc and the elongation of such non-platy (WL)Woodlawn minerals as olivine and dolomite in the foliation plane. (DG)Dugspur In places the foliation planes are deformed into small irregular folds. Larger olivine (up to 5 (PK) ParkwayI mm diameter)and someof the less common ensta- (PK) Parkway2 tite crystals are wrapped by the foliation indicating their relict status. Although all of the rock bodies have a similar megascopicappearance, they can be divided readily SouthernAppalachians are pertinent to this investi- into two principal petrographic groups (Table 1) on gation. Swanson(1980) described dunite, harzbur- the basis of mineral associationand major and trace gite, and orthopyroxenite from two small bodies in element concentrations (Tables 2 and 3). The first the Ashe Formation on Rich Mountain in North group, designatedthe "Todd-type" after the largest Carolina. The metamorphic mineralogy of these body representativeof this group, is characterized rocks included tremolite, talc, and anthophyllite, by high concentrations of tremolite and chlorite chlorite, serpentine, and . Textural rela- with lesser talc and minor amounts of olivine. The tionships indicate the rocks have undergone two, second group is called the "Edmonds-type" be- and possibly three, deformations. Swanson con- causethat large body is typical ofthe group. These cluded that the olivine and in these rocks bodies are characterized by higher concentrations are metamorphic products and not relicts of the of olivine, antigorite, anthophyllite, and talc and primary igneous mineralogy. Although these rocks lesseramounts of tremolite and chlorite. The Todd- occur in the Ashe Formation approximately 15 type bodies characteristically have lower abun- kilometers to the southwestof the belt of ultramafic dances of magnesiumand nickel and higher abun- bodies described here, they are significantly differ- dancesof calcium,aluminum, manganese, and zinc ent mineralogically and texturally. Most notably as compared to the Edmonds-type bodies. Iron is they contain much higher concentrations of olivine somewhatmore abundant in the Todd-type bodies, and pyroxene and only minor amounts of tremolite and the concentration of is not significantly and chlorite, the dominant minerals in the ultramaf- different in the two types of bodies. ic bodies to the northeast. Additionally, unlike the Table 4 contains the point count mineral analysis rocks investigatedin this study, they show little or for each sample with the name of the body from no evidenceof metasomatism.They also apparently which it was obtained and the metamorphicgrade of lack textural evidence indicating relict olivine. the country rock. Figure I is a map showing the Other recent studies of less altered ultramafic name and location of each of the ultramafic bodies bodies in North Carolina are those of Hahn and sampled. Figure 2 is a map showing sample loca- Heimlich (1977) and Alcorn and Carpenter (1977). tions. SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES 8l

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

Mineral Body Tr Mg Car Antho Talc Antig Lim En

Todd-type Todd l5 50.7 33.8 8.9 2.0 3.4 0.0 0.0 0.0 0.0 0.0 Niger Mountain l5 59.1 29.7 0.9 0.9 2.2 o.2 2.8 0.0 0.5 0.0 llarrensville 4 54.9 30.6 0.1 1.4 12.4 0.3 0.0 0.0 0.4 0.0 Phoenix l,lountain 4 64.8 27.6 2.1 0.6 3.2 0.0 0.6 0.0 0.1 0.0 Little PeakCreek 4 76.0 20.7 0.0 0.8 0.3 0.0 0.0 0.0 tr 0.0 l,loodrol Bare 4 66.? 30.0 0.6 1.2 0.7 0.0 0.0 0.0 0.4 0.0 Cranberry J 60.0 23.9 tr t.7 14.0 0.0 0.0 0.0 0.3 0.0 Shatley Springs 10 61.4 29.1 4.5 2.0 0.3 0.2 0.0 0.0 0.I 0.0 Rocky Ri dge 8 61.9 31.l tr 2.1 3.3 0.0 1.1 0.0 0.0 0.r Twin 0aks I 50.6 42.7 0.0 0.7 5.3 0.7 0.0 0.0 0.0 0.f Parkway 2 46.4 36.6 6.5 4.7 0.5 0.0 6.5 0.0 0.0 tF trJoodlawn I 44.4 44.6 0.0 tr 9.1 0.0 0.0 0.0 0.0 0.r 56.4 31.7 ,'l 1.5 4.5 0.1 3.0 0.0 0.2 tr

Edmonds-type BrushCreek I 68.9 23.5 0.5 2.7 0.0 0.0 0.0 0.0 0.0 0.0 Ennice 3 15.9 29.9 0.0 2.0 r4.7 1.3 24.6 0.0 0.0 0.0 Grayson 4 7.7 20.9 19.9 2.8 r4.7 12.7 10.9 7.7 0.0 3.3 Edmonds 9 ?7.9 ?6.0 2r.8 3.3 0.1 1.9 9.1 9.7 0.? 0.0 l,lean* 37.6 25.L 10.6 2.7 7.4 4.0 ll.2 2.4 tr 0.8 -Mean of the neanvalues of individual bodies, thus slightly different from the true meansof the Todd- and Edmonds-typesamples.

Relationshipbetween mineralogy and metamor- Edwardsbodies; assemblage (2) in somesamples of phic grade the Nigger Mountain, Edmunds, Grayson, and Phoenix Mountain bodies; assemblage(3) occurs in One of the purposesof this study was to examine some samples of the Nigger Mountain, Waynes- the relationship between the mineral assemblagein ville, Grayson, and Edmunds bodies; assemblage the metamorphosedultramafic bodies and the meta- (4) occurs in two samplesof the Grayson body and morphic grade of the country rock in which they one of the Parkway body (Table 4). were emplaced. Trommsdorf and Evans (1974) de- Inspection of Figure 1, the regional map of these scribed a series of mineral assemblagesin Alpine bodies, shows no correspondencebetween the oc- ultramafic rocks which have undergone prograde currence of these assemblagesand indicated meta- regional metamorphismfrom an original serpentine morphic grade of the country rock perhapsbecause stage.The sequence,applicable to the rocks under the temperature range recorded in the pelitic rocks study here, is (1) antigorite + talc * tremolite, was too narrow to be reflected in the ultramafic equivalent to the staurolite-kyanite and lower bodies. Moreover, there is no systematicgeograph- zones of pelitic rocks; (2) * talc, equiva- ic distribution of these assemblagesregardless of lent to metamorphism slightly higher than the kya- the grade of the country rock and in several cases nite zone; (3) forsterite * anthophyllite + tremolite, assemblagesrepresenting different grades occur in a somewhathigher grade assemblage;and (4) trem- the same body. We conclude that the observed olite + forsterite * enstatite, an assemblagerough- mineral distribution does not reflect a prograde ly equivalent to the sillimanite zone. regional metamorphism of serpentinite, but a two- Although these assemblagesoccur in some of the stage retrogressive recrystallization of a dunite or ultramafic bodies described here. their distribution protolith. Observations which support does not correspond to the metamorphic grade of this assertion will be described in subsequentsec- the country rock as indicated by the isograds tions of the paper, but critical to this conclusion is mapped(Fig. 1) in the Ashe Formation by Rankin er the occurrence of antigorite pseudomorphs after al. (1972).Assemblage (1) occurs in some samples tremolite (Fig. 3). Whether the tremolite, olivine, of the Nigger Mountain, Ennice, Grayson, and and chlorite crystallized simultaneouslyor 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 Todd-type Todd 8 22.r3 5.74 5.53 9.25 0.20 124 r07 75 t520 1052 104 Niger l.lountain 7 22.96 5.85 4.09 9.00 0.21 Ito 89 62 1620 tL29 120 Phoenix l.lountain ? 20.26 7.10 6.79 8.45 0.20 89 33 457 1494 905 ll3 Little PeakCreek z 20.35 9.00 7.93 10.40 0.?4 138 38 26? 1875 735 103 WoodrowBare 2 22.4L 6.12 3.35 9.50 0.23 129 60 69 1800 1245 148 Cranbemy z 22.50 4.84 5.34 10.04 0.22 143 63 4l 1693 880 107 Shatley Spri ngs 26.72 5.97 2.27 10.50 0.l9 t27 100 56 1480 1170 116 Rocky Ridge 4 22.49 7.66 5.93 9.42 0.l9 114 r25 66 1458 1010 94 Twin Oaks 1 16.93 7.54 11.0 5 8.27 0.18 tt7 151 r04 1367 1040 95 Parkway 3 18.84 5.55 6.68 9.18 0.21 104 235 518 1646 876 101 Woodlawn 2 24.03 8.43 4.91 9.82 0.22 It6 40 45 1470 945 78 Mean* 21.78 6.71 5.81 9.44 o.2l l19 95 160 1585 1099 107 Edmonds-type Brush Creek t 23.38 2.74 0.08 5.32 0.09 119 l0 22 697 1910 42 Ennice 2 27.04 3.61 3.20 6.85 U. I5 t02 24 26 955 t510 6l Grayson L 41.31 t.02 .66 7.20 0.08 139 39 43 674 2220 52 Edmonds 10 33.l0 3.26 3.51 6.39 0.l2 tt7 7t 63 1092 1836 62 Mean* 31.22 2.66 1.86 6.44 0.1l t19 855 1619 54

'Mean of the nean values of the individual bodies, thus slightly different frorn the true meanof Todd- and Edmonds-typesamples.

tially, it is quite evident that serpentinization fol- Mineralogy and texture lowed rather than preceded their crystallization. Olivine. Relict olivine is present in both Ed- The relict nature of the olivine is strongly suggested monds- and Todd-type bodies although it is more where small anhedral olivine crystals with optical common and occurs as somewhat larger crystals in continuity are separated by interstitial serpentine the Edmonds-typesamples. In the less recrystal- (Fie. 3). lized Edmonds-typesamples individual crystals of

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

rr,'- North Corolino

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

'#f*:; 14.1s

Fig. 2. Map of samplelocations. See Figure I for name and geologicposition of individual bodies. SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES 83

Table 4. Mineralogical data derived from point count analysisof samplesfrom the ultramafic bodies in North Carolina and Virginia

Mineral Sample Antho- Antig: Limo- Ensta- ]'leta. n umber Tremolite Chlorite 0livine Magnetite Carbonatephyllite Talc ori te ni te ti te zone T5 42.6 33.I t8.2 3.3 1.8 0.0 0.0 0.0 0.0 0.0 G T6 50.8 35.9 0.0 2.3 0.0 0.0 0.0 0.0 0.0 0.0 G T 13-b 28.9 25.6 38.6 4.5 2.1 0.0 0.0 0.0 0.0 0.0 G T l3-d 35.5 49.7 12.6 1.9 0.3 0.0 0.0 0.0 0.0 0.0 G T 13-1 58.9 33.6 0.0 1.0 0.0 0.0 4.5 0.0 0.0 0.0 G T 14-b 57.9 37.9 0.0 1.6 5.8 0.0 0.0 0.0 0.0 0.0 G T 15-b 6t.2 31.6 0.0 l.l 3.8 0.0 0.0 0.0 0.0 0.0 G T 16-d 46.3 28.3 19.5 l.l 4.0 0.0 0.0 0.0 0.0 0.0 G T 16-a 65.6 29.4 0.0 t.? 0.6 0.0 0.0 0.0 0.0 0.0 G r2 49.4 33.0 0.0 1.0 ll.6 0.0 0.0 0.0 0.0 0.0 G NM2l-b 43.4 37.1 3.1 0.6 2.0 2.0 9.0 0.0 0.0 0.0 s NM2l-c 61.8 36.5 0.0 1.6 0.0 0.0 0.0 0.0 0.1 0.0 s NM21-d 55.3 44.3 0.0 1.0 0.0 0.0 0.0 0.0 ?.3 0.0 s NM2l-e 56.4 28.0 0.0 0.5 4.5 0.0 10.4 0.0 0.0 0.0 s NM21-f 51.9 45.9 0.0 0.3 0.0 0.0 0.0 0.0 0.0 0.0 s NM2l-g 66.5 29.1 2.2 0.6 0.5 0.2 0.0 0.6 0.0 0.0 s NM7 67.6 16.6 0.0 0.? 4.2 6.0 | .? 0.0 2.2 0.0 s NM24 58.7 37.2 0.0 1.8 0.0 0.0 0.0 0.0 1.3 0.0 s NM22 72.2 t7.0 0.0 0.5 10.2 0.0 0.0 0.0 0.0 0.0 s NM23 56.0 28.7 6.1 0.8 0.0 0.0 7.2 0.0 0.7 0.0 s NM25-a 56.6 30.4 2.0 r.2 6.4 0.0 3.4 0.0 0.0 0.0 s NM25-b 61.5 36.9 0.0 1.3 0.0 0.0 0.0 0.0 0.0 0.0 s NM26-b 53.8 31.( 0.0 1.3 0.2 0.0 10.6 0.0 0.0 0.0 K NM26 60.8 35.3 2.0 1.2 0.0 0.0 0.0 0.0 0.7 0.0 s NM9 63.9 28.7 0.0 0.2 4.5 0.0 0.0 0.0 0.0 0.0 K lt 17-xA 66.4 13.6 0.0 1.2 18.7 0.0 0.0 0.0 0.2 0.0 K hl 17-X 32.7 40.l 0.0 1.0 26.2 0.0 0.0 0.0 0.0 0.0 K t^ll7-1 59.9 30.9 0.4 2.2 4.4 1.1 0.0 0.0 1.1 0.0 K w 17-2 60.6 37.7 0.0 1.2 0.1 0.0 0.0 0.0 0.4 0.0 K P l8-a 66.0 31.8 0.0 0.2 0.6 0.0 0.0 0.0 0.0 0.0 K P 18-b 62.1 32.9 0.8 0.8 0.0 0.0 2.4 0.0 0.0 0.0 K P 20-u 86.6 13.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 K P 20-a 44.3 32.L 7.4 r.2 13.2 0.0 0.0 0.0 0.0 0.0 K

olivine are fractured with serpentine occurring in rocks indicate compositions of 90 and 9l molVo the fractures. Anthophyllite and tremolite have forsterite. replacedolivine and are in turn partially replacedby Antigorite. The serpentinemineral is identified as serpentine(Fig. 3). In the Todd-typebodies olivine antigorite on the basis of its X-ray diffraction pat- occurs as smaller fragmented clusters which are tern. No unique lizardite peaks were observed and elongated in the foliation. Here olivine does not four typical antigorite peaks are present (Borg and occur with serpentine but commonly occurs with Smith, 1979).Antigorite has partially replaced oliv- chlorite which is located in the pressure shadows ine in some Edmonds-type rocks and also has formed by the olivine clusters, as well as in the replaced tremolite resulting in partial pseudo- foliation. The presence of chlorite in the pressure morphs of serpentineafter tremolite (Fig. 3). It also shadowsof olivine attests to the secondaryorigin of appearsto have replaced chlorite in a few samples. the chlorite with respect to olivine and the relict It is absentin Todd-type bodies. nature of the olivine. Tremolite. Tremolite is common in both types of Chemicaland optical data obtained by Laskowski ultramafic bodies. In the more massive Edmonds- and Scotford (1980)from an Edmonds-type sample type rocks its prismatic crystals are completely (0-25) indicate that the olivine composition is 88.5 unoriented producing ajack-straw texture. In those molTo forsterite, this value being based on atomic Edmonds-type rocks which have been lightly absorptionanalyses of mineral separates.This same sheared with a relatively widely spaced fracture samplecontains only 737 ppm Ca. The 2V is *88' foliation, the tremolite laths are shredded at their and the np B is 1.672for this specimen.The optical terminations and have been rotated into parallelism data of two samples of olivine from Todd-type with the foliation. In some places where the basal 84 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 4. (continued)

Mineral Sample Antho- Antig- Limo- Ensta- Meta. numDer Tremolite Chlorite 0livine Magnetite Carbonate phylI i te Talc orite nite tite zone L 29-b 99.1 0.0 0.0 0.0 0.0 tr 0.0 0.0 tr 0.0 s 130 74.5 23.9 0.0 u.5 1.3 0.0 0.0 0.0 0.0 0.0 s 110 68.7 ?5.0 0.0 1.4 0.0 0.0 0.0 0.0 0.0 0.0 s 131 6t.7 34.2 0.0 1.3 0.0 0.0 0.0 0.0 0.0 0.0 s !{B 32-c 62.0 33.4 0.0 0.8 0.2 0.0 0.0 0.0 0.0 0.0 q hlB33-a 66.8 28.5 0.0 1.5 2.2 0.0 0.0 0.0 0.0 0.0 s t,lB33-b 66.9 ?6.6 2.4 1.9 o.2 0.0 0.0 0.0 t.7 0.0 s t^JB36 67.9 3l .4 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 S 54.6 36.7 0.0 2.t 6.6 0.0 0.0 0.0 0.0 0.0 \ C 34-a 68.l L?.4 0.0 1.5 l6.9 0.0 0.0 0.0 0.3 0.0 s c 34-b 57.3 22.7 tr 1.5 18.5 0.0 0.0 0.0 0.0 0.0 SS47-a 70.2 21.8 0.0 1.0 0.0 0.0 7.r 0.0 0.0 0.0 K ss 47-b 86.0 9.0 0.0 1.4 0.0 1.0 0.0 0.0 0.0 0.0 K ss 50 63.6 27.5 1.0 0.5 1.1 1.0 0.0 0.0 0.0 0.0 K SS 48 63.9 28.8 2.5 0.5 0.0 0.0 0.0 0.0 0.3 0.0 K SS 48-a 67.4 ?4.4 4.6 1.3 0.0 0.0 0.0 0.0 0.0 0.0 K SS 45-a 66.9 30.8 0.0 0.8 0.0 0.0 0.0 0.0 0.0 0.0 K ss 45-b 5r.9 45.6 0.0 1.0 0.0 0.0 0.0 0.0 1.0 0.0 K SS46 58.8 3t.2 5.5 4.0 0.5 0.0 0.0 0.0 0.0 0.0 K ss 46-d 49.3 29.5 18.0 2.8 0.2 0.0 0.0 0.0 0.0 0.0 K SS69-a 36.1 4?.6 13.4 7.1 1.0 0.0 0.0 0.0 0.0 0.0 K RR43 52.0 34.6 0.0 0.3 r2.8 0.0 0.0 0.0 0.0 0.0 K RR42-b 7t.l 28.1 0.6 0.2 0.0 0.0 0.0 0.0 0.0 0.0 K RR44 66.4 29.0 0.0 0.5 0.1 0.0 0.0 0.0 0.0 0.0 s RR41 65.2 3l .4 0.0 1.1 1.6 0.0 0.0 0.0 0.0 0.0 s RR40-b 59.5 34.9 0.0 L.7 0.0 0.0 0.0 0.0 0.0 0.0 s RR37-a 60.I 27.r 0.0 0.3 t.7 0.0 9.1 0.0 0.0 0.0 s RR39 62.3 36.0 0.0 1.5 0.2 0.0 0.0 0.0 0.0 0.0 s RR38-b 58.5 36.7 0.0 2.7 2.1 0.0 0.0 0.0 0.0 0.0 s T0 11 50.6 42.7 0.0 0.7 5.3 0.7 0.0 0.0 0.0 0.0 s B 12-a 68.9 23.5 0.5 2.7 0.0 0.0 0.0 0.0 0.0 0.0 s EN53-b 42.3 4?.4 0.0 1.9 7.7 0.0 5.8 0.0 0.0 0.0 s EN53-a 41.9 36.6 0.0 3.7 13.1 0.0 4.7 0.0 0.0 0.0 s EN52 0.0 10.9 0.0 0.5 23.2 3.8 62.6 0.0 0.0 0.0 s

sections are subparallel with the plane of the thin It is rare in Todd-type rocks. Some crystals show a section, the drag of chlorite and antigorite crystals prismatic habit whereas others are fibrous. at their contacts with the larger tremolite basal Chlorite. Chlorite is common in both types of sections gives the impression of rotation or "snow ultramafic bodies. It is colorless or faintly green-tan ball" texture (Fig. a). Tremolite has replaced oliv- pleochoric and has a small extinction angle suggest- ine, rare enstatite, and in a few places shows ing clinochlore. It occurs as a matrix mineral replac- evidencesuggesting it has replacedchlorite (Fig. 5). ing olivine in the less recrystallized Edmonds-type In Todd-type bodies the tremolite laths are aligned bodies or is concentrated in the foliation planes in parallel to the foliation plane in many samples. In those Edmonds-type sampleshaving foliation. In a others the tremolite crystals have been shredded few samplesit is concentrated in small veinlets. It and deformed in responseto later shearingproduc- appearsto have a wide range of temperature stabil- ing a phyllonitic texture. In some well-sheared ity as it also occurs abundantly in the Todd-type rocks the larger tremolite crystals constitute por- tremolite phyllonites. Chlorite occurs in the pres- phyroclasts in a matrix of smaller chlorite and sure shadows of relict olivine phenoclastsin some tremolite prismatic crystals. Todd-type samples. A few chlorite crystals are Anthop hyllife. Anthophyllite, distinguishedfrom cross cut by tremolite laths suggestinga paragenetic by its parallel extinction and lack of sequence:chlorite-tremolite (Fig. 5). However, it is twinning, is presentin someEdmonds-type samples probable the chlorite was stable throughout the where its occurrence is similar to that of tremolite. recrystallization and deformation history of these SCOTFORD AND WILLTAMS: METAMORPHOSED ULTRAMAFIC BODIES 85

Table 4. (continued)

Mineral

Sample Antho- Antig- Limo- Ensta- Meta. numDer fremoli te Chlorite 0l ivine l4agnetite Carbonate phyllite Talc ori te nite ti te zone GR5l-a 4.7 32.1 18.I 1.2 10.3 30.2 1.1 tr 2.0 0.0 s GR51-b 15.2 12.0 2.1, 0.9 40.0 tr 29.6 0.0 0.0 0.0 s GR51-c 6.0 ?7.? 23.? 9.1 1.0 10.7 0.0 22.8 0.0 fr S GR68 4.8 12.l 36.2 Ir 7.6 0.0 13.0 8.2 0.0 13.1 S ED56-a 82.8 1.5 0.0 0.0 10.0 0.0 0.0 0.0 0.0 G ED56-c 2.6 t6.7 48.9 1.3 0.6 4.2 17.4 8.0 0.0 0.0 G ED57-a 2t.l 65.4 L0.2 3.2 0.0 0.0 0.0 0.0 0.0 0.0 G ED57-b 50.2 43.6 1.6 2.6 0.0 0.0 2.0 0.0 0.0 0.0 G EDSB-a r.b 9.3 30.7 0.3 0.0 tr 13.9 44.8 0.0 0.0 G ED59 0.0 25.0 55.4 1.0 0.? 0.8 7.2 9.1 1.3 0.0 G ED60-b 8.8 28.7 15.7 2.8 0.0 tr 41.2 1.7 0.6 0.0 G ED66-a 2t.4 13.I 33.6 6.0 0.0 2.0 tr 23.6 0.0 0.0 G ED67-a 62.7 30.3 0.0 7.1 0.0 0.0 0.0 0.0 0.0 0.0 G PK62 54.0 31.9 0.0 3.2 0.0 0.0 ll.1 0.0 0.0 0.0 G PK6r-b 38.8 41.2 13.1 6.? 1.0 0.0 0.0 0.0 0.0 trG t^ll 65 44.4 44.6 0.0 tr 9.1 0.0 0.0 0.0 0.0 0.0 G

G=Garnet;S=Staurolite'K=Kyanite;T=Todd;NH=NiggerMountain;W=l'larensville;P=Phoenix Mountain;L=LittlePeakCreek;}|B=WoodrowBare;C=Cranberry;SS=ShatleySprings;RR=RockyR'idge; T0=Twin0aks;El,l=Ennice;GR=Grayson;ED=Edmonds;PK=Parkway-l;t'IL=Woodlawn;B=BrushCreek. rocks as it clearly replaces olivine but is present in are as follows:Mg22.19 for Edmondsversus 16.85 the fracture foliation of the most shearedTodd-type for Todd; Fe 3.16 for Edmonds versus 8.98 for bodies. Also, its occurrencein the pressureshad- Todd; Al 6.28for Edmondsversus 8.41 for Todd; Si ows around tremolite indicates that in those places 14.47for Edmondsversus 12.51for Todd. There is it post-datesthat mineral. little differencein Cr concentrationbetween the two Microprobe analyses by William Zhong (pers. types of samples;0.48% for Edmondsand 0.43for comm.) of chlorite samples separatedfrom speci- Todd. mens used in this study show a distinct difference Talc. Talc is more commonin the Edmonds-type between the compositions of three chlorite samples rocks but occursin both types.It is most commonly taken from Edmonds-type rocks and eight taken a matrix mineral not aligned with respect to a from Todd-type rocks. The mean major element foliation or fracture planes. weight percentagesof these two types of chlorites Dolomite. Dolomite(a : 1.691,e : 1.548)occurs widely but is rarely more than an accessoryminer- al. It typically cross-cutsthe foliation or appearsto have replaced other minerals, suggesting a late stagecrystallization. Enstatite. Relict enstatite occurs in a few Ed- monds-type sampleswhere it has been replaced by antigorite, anthophyllite, or tremolite. The mea- sured 2V of *82" indicates a composition of about EnelFse.

Textural interpretation Mineral occurrence and textural fabric allow an interpretation of the crystallization history. These rocks have undergone two principal stages of re- Fig. 3. Prismatic tremolite (Tr) crystal with pseudomorphous crystallization which appear to have been approxi- replacement by antigorite (A). Relict olivine (01) partiafly mately synchronous with two deformation epi- replacedby antigorite (A). Bar = 0.025 mm. Edmonds-type. sodes.An initial retrogradehydration during ductile E6 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Fig. 4. "Snowball" basal section of tremolite (Tr) with finer Fig. 5. Tremolite (Tr) crystal cutting chlorite (Ch) and being chlorite (Ch) in pressureshadow. Bar = 0.025 mm. Todd-type. cut by antigorite (A). Bar = 0.05 mm. Todd-type. deformation was followed by a progressive dehy- the fractures. Typically large porphyroclasts of dration during a brittle deformation. The Edmonds- tremolite remain surrounded by deflected foliation type bodies have been affected principally by the bands of finer tremolite and chlorite. Small relict first stage whereas the Todd-type bodies show the olivine phenoclastspersist in rnany of these phyl- effectsofboth stages. lonites attesting to the ultramafic protolith of these The protolith has not been preserved rocks. but the least altered of the Edmonds-type rocks The crystallization sequencein the second stage contain olivine and enstatite which have been par- begins with the tremolite-rich rocks containing tially replaced by chlorite, talc, anthophyllite, and some chlorite and minor talc, olivine, serpentine, dolomite. Tremolite replaced some of the olivine, and chlorite. As the tremolite was shredded by and in turn was replaced by antigorite which also shearingdeformation, additional chlorite formed in replaced some of the remaining olivine along frac- the foliation and in pressure shadows. Some dolo- tures (Fig. 3). These rocks were only slightly affect- mite replaced both of these minerals after the ed by a ductile deformation which caused a poor shearingepisode. Talc is presentbut not commonin alignment of tremolite blades and chlorite plates in theserocks and serpentineis absent.Magnetite, a some specimenswhereas others remain unaffected product of the olivine serpentinization, persists and exhibit a jack-straw texture. The retrograde from the first stageand minor iron oxide has formed recrystallization began with the peridotite protolith in some shearfractures. assemblageof olivine and enstatite being partially The texture of the rocks under investigation here replaced by tremolite and chlorite and to a lesser contrasts sharply with that describedby Evans and extent by talc and dolomite. The final event in this Trommsdorf (1970) in metamorphosed ultramafic sequence was the partial serpentinization of the rocks in the Alps. The Alpine rocks contain a tremolite and remaining olivine. discerniblefoliation but are characteized by mosa- The second stage, a dehydration and cataclasis ic textures involving olivine, tremolite, anthophyl- accompanying a brittle shearing deformation, is lite, antigorite, and enstatite. These textures are best seen in the Todd-type samples. There is a correctly interpreted as representing equilibrium gradation in the degreeof shearingfrom rocks with assemblagesin a prograde recrystallization of ser- widely spacedshear fractures in which large tremo- pentinite. Such textural arrangementsare not pre- lite porphyroclasts were partially shredded and sent in the rocks under study here, nor are the rotated into parallelism with the shear planes, to overgrowth textures reported by Trommsdorf and phyllonites composed almost entirely of smaller Evans (1974)indicating a prograde origin of tremo- tremolite fragments and chlorite plates strongly lite and olivine present. Rather, the textural rela- oriented in the fracture foliation. The perfection of tionships in the present rocks indicate a secondary the preferred orientation of the tremolite and chlo- origin of the serpentineand relict olivine and ensta- rite is proportional to the closenessof the spacingof tite. SCOTFORD AND WILLIAMS: METAMORPHOSED T]LTRAMAFIC BODIES

Table 5. Analyses of USGS standard PCC-I Mn, Co, Cu, K, Ni, and Zn. Acctxacy of the analyses was evaluated by determining elemental Fl anagan composition of the USGS standardPCC-I. With (1973) PCC-I this study every run of four to eight rock samples,a standard was lleight percent analyzed along with a contamination blank. Before continuing analyses of unknown ultramafic l'190 43.18 42.16 42,49 42.33 43.32 42.24 rocks, elementalanalysis of the standardPCC-I had Al 0.74 0.76 0.77 O.77 0.85 203 to be within an acceptablerange and the contamina- Cao 0.s1 0.52 0.51 0.52 0.52 0.48 tion blank was required to be low compared to Fe203 2.85 ND ND ND NO ND standardworking solutions. Analyses of PCC-I are Feo 5.24 ND ND N0 ND ND listed in Table 5 with the acceptedvalues as stated Total Fe as Fe203 8.35 8.02 8.20 8.31 L34 8.39 by Flanagan(1973). Values for Mn, Zn, Ca, Ni, and K20 0.004 0.004 0.004 0.003 0.003 0.004 Co are within three percent (of the amount present) of the acceptedvalue. The trace elementsCu and K ppm had higher errors, but this was a result of their low Co lI2 113 115 110 108 112 abundances. Cu II 17 13 19 915 The precision of the analyses was determined l4n 959 975 975 930 through duplicate analyses of pairs of samples. Ni 2339 2390 2300 2325 2320 2160 Reproducibility of at least five percent of the 7n 36 38 38 36 36 35 amount present was obtained for all elements ex- cept K, Ca, and Fe. Commonly 1-3 percentpreci- sion was achieved. From the iron analyses it ap- Comment should be made on the possibility that pears that the powders used for analyseswere not the Edmonds-and Todd-typebodies represent two completelyhomogeneous. This is alsothe casewith contrasting groups of ultramafic occurrences with the analysesof calcium. For potassiummost values separateprotoliths and post-emplacementhistories were reproducible to 25 percent of the amount rather than rocks with the sameprotolith but repre- present.Duplicate analysesof the standardPCC-I senting separate stagesof the same post-emplace- showed good reproducibility for all elements. ment history as assumedin the above discussion. Table 6 shows the chemical analytical results Observations which favor the single protolith his- obtainedby sampleand by body. tory are: (1) both groups of bodies occur in the same Eleme ntal dist ribution stratigraphic unit (the Ashe Formation) and lie approximately along the same structural trend; (2) The mean values quoted in this section are the both tend to be elongated in the prominent 51 means of all samples or the means of Todd- or foliation trend; (3) a few samplesfrom Edmonds- Edmonds-type samplesand differ slightly from the type bodies are chemically and mineralogically sim- mean values shown on Table 2 which are meansof ilar to Todd-type samples; (4) Todd-type samples the mean values of individual ultramafic bodies. commonly contain relict olivine and porphyroclasts . The mean magnesiacontent for all of tremolite-minerals characteristic of the less the rocks analyzed is 25.59 weight percent MgO, recrystallizedEdmonds-type rocks. with a range from 14.77 to 42.45 percent. The The causeof the observed differencein degreeof Edmonds-typesamples have higher magnesiacon- alteration of the two types of bodies is not self- tent (33.38%mean MgO) than the Todd-type sam- evident but is possibly attributable to differencesin ples (22.38Vomean MgO). The Todd-type samples permeability, with the typically smaller more are low in Mg compared to the average for all shearedTodd-type bodies being more permeableto ultramafic rocks as reported by Turekian and We- hydrothermal fluids. depohl(1961, Table 1). Basedon a literaturesearch, it appears that an average of 38-40Vo MgO is Geochemistryof the metamorphosedultramafic probable for alpine-type peridotites. Thus, the bodies rocks under study are abnormally low in magne- sium when compared to alpine-type peridotites. Analytical methods,accuracy, and results Iron. The distribution of iron reflects the division Sixty-one samples were analyzed by atomic ab- of Todd-type and Edmonds-type ultramafic bodies. sorption spectrophotometryfor Mg, Al, Ca, Fe, The Edmonds-typehave lower (6.11% mean) but SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES more varied iron (4.03-10.20%Fe) concentrations calcium is favored. The negative correlation of than the Todd-type (9.13% mean, 7.93-ll.40Vo calciumwith magnesiumof -0.669 (P : 0.0001)and range).The meanfor all samplesis 8.30%Fe which the positivecorrelation with tremoliteof 0.576(P : is slightly lower than the average(9.38Vo Fe) report- 0.0002)and the low concentration of calcium in the ed by Turekianand Wedepohl(1961, Table 1)for all olivine suggest that calcium was introduced and ultramafic rocks. Probably the higher concentration was incorporated into the tremolite as magnesium of iron in the Todd-type samplesreflects the meta- was removed. somatic loss of magnesiumfrom Fe-Mg minerals in Nickel. The mean nickel content for all the ultra- thesebodies. mafics analyzed is 1238ppm. The Todd-type sam- Aluminum. The distribution of alumina shows ples range from 540 to 1680ppm and have a mean that the Edmonds-Typesamples have lower mean content of 1053ppm. The Edmonds-typesamples concentrations(3.11% AlzO) than the Todd-type have a wider range (461 to 2660ppm) and a higher samples(6.28Vo Al2O3) and that the Todd-typehave mean (1730ppm). If the few Edmonds-typesam- higher concentrationsof aluminum than the average ples, probably altered, with the very low nickel of peridotitesof 3.99%Al2O3 reported by Wedepohl concentrationsare not consideredthe mean would (1e6e). be much higher. Most samplesanalyzed in this study have very Turekian and Wedepohl(1961) reported an aver- high aluminum concentrations compared to alpine- age nickel content for ultramafic rocks of 2000ppm type harzburgites which dominate the alpine ultra- Ni. Goles 0967) revisedthis estimateto 1500ppm mafics of the Appalachians, although some of the Ni. A positivecorrelation (0.811, P : 0.0001)found Edmonds-typesamples have aluminum concentra- in the present study between nickel and magne- tions that compare to typical alpine harzburgites. It sium, the positive correlation with olivine (0'570, is thus probable that all of the Todd-type and some P : 0.0002),and the negativecorrelation with iron of the Edmonds-typesamples have been enriched in (-0.348, P : 0.0301)are consistentwith observa- aluminum.As no garnetpseudomorphs or relicts of tions made by Vogt (1923)and Stueberand Goles feldsparhave been observed, the aluminummust be (1967). of metasomatic origin derived from the country The Todd-type ultramafics have lower nickel rock. contents than peridotites and dunites, but higher Calcium. The mean calcium content as CaO for concentrationsthan . Most of the Ed- all rocks analyzed is 4.55%. The Edmonds-type monds-type samples have nickel concentrations samplesshow a larger variation (0.94-ll.87Vo CaO) similar to dunites and typical alpine peridotites or and a lower mean (2.86%) than the Todd-type harzburgites. This distribution can best be ex- samples(2.75-11.47% and 5.19%). It is clear that plained by a metasomaticmodel where nickel tends the Edmonds-type sampleshave generally lower to follow magnesium as it is removed from the concentrations of calcium, although three samples ultramafic body. This type of variation in nickel (66c, 56a,53a)show much higher calcium than the concentrations in metasomatic environments for typical Todd-type sarnple. ultramafic rocks was demonstrated by Curtis and Wedepohl (1969)reported that the average peri- Brown (1971). dotite contains3.467oCaO. Comparedto this value Cobalt. No easily observabledifferences exist the Todd-type samplesappear to representrocks betweenthe Todd-typeand Edmonds-typesamples enriched in calcium and the Edmonds-typethose with respect to the distributions of cobalt, although depletedin calcium. It must be kept in mind that the it is possiblethat contaminationwith cobalt in the average peridotite includes garnet-rich varieties tungsten carbide grinding elementshas affected the which would tend to increase the calcium content values obtained. The mean cobalt content for all comparedto the averagealpine-type harzburgite. samplesis 118ppm, as it is separatelyfor the Todd- Clinopyroxene accounts for most of the calcium type and Edmonds-typesamples. This valueis very in fresh alpine ultramafics. In view of the large near the 110ppm averagereported by Goles (1967) amount of calcium in most of the samplesfor this for ultramafic rocks. study, a wehrlitic peridotite, garnet , or Manganese. Manganeseconcentrations largely composition is necessitatedif the cal- separate the rocks analyzed into Todd- and Ed- cium is consideredprimary. However, as the rocks monds-types.The mean concentrationis 1358ppm studied are not fresh ultramafics and contain almost for all analyzed samples.The Todd-type samples no relict pyroxene, a metasomaticsource for the range from 973 to 1960ppm Mn and have a 1566 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES 89

Table 6. Analvses of ultramafic rocks

l'|eight percent ppm Sample number Mgo 41203 CaO Fe t'tn0 Co Cu l'1n Ni Zn T2 25.43 5.19 5.93 7.93 0.17 LL? 146 26 1347 1l4s 93 T l6-a 24.40 5.70 3.19 9.60 0.19 r32 230 42 1460 1190 95 T l3-d t4.77 4.92 7.3? 8.29 0.18 133 7L 73 1390 990 120 T 13-b 26.73 5.37 4.34 9.48 0.20 146 ll0 70 1576 1170 137 T l4-b 14.94 8.23 6.86 8.56 0.21 100 8l 223 1603 680 r57 T 15-b t5.77 5.89 7.06 8.91 0.19 ll0 95 59 1520 ll40 r2r T5 23.L3 s.i6 3.82 9.86 0.21 143 50 53 1637 1070 105 T6 3l .58 6.18 5.75 11.40 0.21 tLz 69 54 1624 1030 r22 NM7 25.58 4.88 4. 35 8.79 0.22 ll4 27 26 1580 1430 tL7 NM9 23-57 5.86 4.57 8.80 0.19 104 98 68 1460 l0B0 132 NM21-e 23.85 5.85 5.57 9.36 0.20 Lrz t79 72 1513 1100 110 Nlrl21-g ?1.69 6.16 3.t7 9.18 0.25 98 57 106 1947 900 133 M'22 16.77 4.73 4.48 8.17 0.20 107 58 67 1550 1100 84 NM25-a 26.17 5.st 3.10 9.42 0.l9 r20 87 33 1510 1260 t47 NM26 23.09 7.9? 3.36 9.28 0.22 118 109 65 1583 1030 114 P 18-b 23.96 4.87 3.48 8.74 0.l9 105 39 42 1458 t270 r22 P l9-a 24.62 5.43 3.91 8.69 0.18 110 76 44 1387 1230 114 P 20 16.56 9.33 r0.l0 8.15 0.20 75 27 857 1530 s40 103 ss 45-b 23.7r 7.4r 3.11 lt.3t 0.21 118 73 54 1637 ll30 ND ss 46 25.19 5.56 4.75 8.03 0.20 t32 86 60 l5l5 1040 106 ss 47-b 22.44 5.03 4.52 8.60 0.24 103 r29 59 1890 880 t20 SS69-a ?8.61 5.79 0.45 9.46 0.17 143 91 ND t323 1038 100 ss 69-b 27.86 4.70 3.24 10.73 0.19 120 135 58 t479 1343 131 RR3B-b 22.49 7.24 6.?L 8.96 0.16 t23 305 31 t247 1350 8l RR39 20.42 10.21 6.23 9.45 0.23 105 131 67 1750 860 104 RR41 26.37 4.87 2.9t 10.31 0.24 LTz 8I L7 1828 t285 ND RR42-b 23.24 7.32 6.76 8.r2 0.19 106 1l toz 1450 930 85 RR43 23.82 5.88 4.51 11.16 0.18 t?L 5l 64 1383 900 104 ED56-a 22.99 0.88 1l.57 4.03 0.14 61 5 111 1084 580 29 ED57-a 33.62 3.09 2.33 10.20 0.21 180 133 30 t624 2030 92 ED58-a 40.09 0.99 0.10 5.63 o.12 119 14 27 904 2340 55 ED59 38.60 I .87 0.04 6.88 0.13 177 L2 34 1044 2350 67 ED60-b 33.86 2.61 t.97 5.47 0.13 lzL 35 32 983 1800 59 ED66-a 39.55 1.86 t.t7 6.07 0.l3 ll5 7l 2l 973 2125 45 ppm mean. The Edmonds-type samplesrange from Zinc. The means for zinc content of the Ed- 397 to 1624ppm Mn and have a mean of 950 ppm. monds- and Todd-type samples show a distinct Most of the Edmonds-type samples have manga- difference between these two groups. The average nese contents similar to the average of ultramafics zinc content for all ultramafics analyzedis 96 ppm. as reported by Goles (1967), but most Todd-type The meanfor 38 Todd-typesamples is 111ppm with samplesshow concentrations higher than this aver- a range of 45 to 157 ppm Zn. The mean for the age value of 1040ppm. Edmonds-typesamples is 57 ppm and the rangeis It has been reported by Livingston(1976) and by 29 to ll0 ppm. This mean is comparableto that for Curtiss and Brown (1971)that manganeseconcen- ultramafic rocks of 50 ppmZn (Turekian and Wede- trations increaseduring metasomatismof ultramafic pohl, 1961).Vinogradov (1962)reported 30 ppm as bodies, which is consistent with observed manga- an averagefor ultramafic rocks. It thus appearsthat nesedistribution in the rocks investigatedhere. The the Todd-type ultramafic rocks have been some- metasomatized Todd-type samples show higher what enriched in zinc compared to the Edmonds- manganese concentrations than the less altered type. Edmonds-typesamples. This transfer of manganese Copper. The division of samplesinto two groups into the ultramafics is supported by a positive basedon copper content is not as straightforward as correlation between manganeseand the principal it is for the other elements.There is a large overlap mineralogical product of metasomatism, tremolite in copper concentrations between Todd- and Ed- (correlationcoefficient 0.605, P : 0.0001).Micro- monds-type samples although the latter tend to probe analysesshow no Mn present in the chlorite have somewhat lower copper concentrations. The (William Zhong, pers. comm.). mean copper content for all samplesis 81 ppm. The 90 SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES

Table 6. (continued)

}'leight percent ppm Sample Ca0 Fe Mn0 Co Cu Itln Ni 7n number Mgo Al 203 ED66-b 39.l0 1.46 2.03 5.65 0.l0 109 L37 20 760 1985 37 ED66-c 2?.36 4.22 11.87 4.41 0.t2 59 19 260 965 550 34 ED67-a 33.40 3.42 3.90 4.31 0.14 103 202 33 1062 2360 110 ED67-b 31.51 12.18 0.18 5.94 0.t? r23 25 19 899 461 58 110 18.87 10.92 rr.47 10.40 0.25 158 32 451 r960 590 102 130 21.83 5.47 4.38 10.40 0.23 118 44 72 1790 880 104 T0 11 16.93 7.s4 11.05 8.27 0.18 Lr7 151 104 1367 1040 95 B l2-a 23.38 2.74 0.08 5.32 0.09 119 l0 22 697 1910 42 EN52 29.47 I .85 0.11 4.24 0.05 9?515 397 1970 30 EN53-a 24.7r s.37 6.30 9.13 0.20 Lrz 43 37 l513 1050 ND WB32-c 20.75 5.81 3.95 9.r7 0.21 126 6 91 1650 970 152 }lB 33-b 24.07 6.42 2.75 9.84 0.25 133 u4 46 1950 1520 143 c 34-b 23.96 4.81 6.74 10.20 0.24 159 30 40 1865 770 111 c35 21.03 4.87 3.93 9.88 0.20 t26 95 4r 1520 1060 103 GR5l-a 40.17 t.02 0.31 9.30 0.09 173 20 4t 727 2660 6l GR68 41.45 ND 1.00 5.08 0.08 104 58 4t 620 1780 43 |'lL 65 24.01 7.7r 5.09 9.79 0.19 ll3 55 45 973 1200 45 blL 64 24.05 9.16 4.74 9.84 0.25 lr8 25 ND t967 690 111 DG63-b 23.34 5.59 6.31 9.7t 0.16 108 93 45 1253 890 100 PK62 23.7| ND 5.07 8.40 0.l5 83 194 35 1200 7?5 89 PK61-b 24.18 5.5s 4.51 9.69 0.23 143 428 ND 1767 1680 101 PK61-c 8.62 ND 10.45 9.45 0.25 84 84 1000 1970 225 ll3

T=Todd;NM=N.iggerMountain;P=Phoenixl4ountain;L=LittlePeakCreek;l'lB=HoodrowBare;C= Cranberry; SS'= Shatley-springs; RR= RockyRidge; 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 samplesis to note that sample 20, which contains the highest 92 ppm with a range of 6 to 230 ppm. The range of potassium concentrations (857 ppm), was located the Edmonds-type samples is similar to the Todd- within one foot of the contact with country rock. type (5 to202 ppm), but that mean copper content is Carswell et al. (1974) and Curtis and Brown lower (53 ppm). All of these values are high com- (1969) demonstrated that potassium enters ultra- pared with averagesfor ultramafic rocks of 10, 20, bodies from solutions derived from the coun- and 30 ppm as reported by Turekian and Wedepohl try rock in quantities which are a function of (1961),Vinogradov (1962), and Goles(1967) respec- country rock composition, but in amounts smaller tively. than those of calcium or aluminum. Thus if potassi- Potassium.There is no clear distribution of po- um enters the ultramafic along with aluminum and tassiumbetween the Edmonds- and Todd-type sam- calcium, these elementsshould show positive cor- ples although most sampleswith low potassium are relations. Conversely,as potassiumenters the ul- of the Edmonds-type.The meanpotassium concen- tramafics, magnesium moves toward the country tration for all ultramafics analyzed is 81 ppm. The rock, and therefore correlation between potassium Todd-type samples have a mean of 94 ppm and a and magnesiumshould show a negativecorrelation. rangeof 17to857 ppm. The Edmonds-typesamples These correlation coemcients are indeed observed: have a mean of 50 ppm and a range of 15 to 260 K vs. Al : 0.510(P : 0.0009),K vs. Ca : 0.553 ppm. As potassium may enter ultramafic rocks (p : 0.0003),and K vs. Mg : -0.369 (p : 0.0207). during metasomatism it is interesting to note that reactionsand metasomatism the samplescontaining more than 200ppm K (14b, Mineral 20, 10, and 66c) on the basis of major element A sequenceof mineral reactions and metasomatic concentrations are among those altered metaso- exchangeswhich describesthe evolution of these maticallyto the greatestextent. It is also important bodies from the protolith ultramafic rock to the SCOTFORD AND WILLIAMS: METAMORPHOSED (ILTRAMAFIC BODIES extensively altered tremolite-chlorite schists of the specimenscontain no serpentinebut more abundant Todd-type can be stated based on the observed tremolite,higher concentrations of Mg, and lessCa. textural relationships,mineral assemblages,and the The Todd-type rocks are strongly foliated, princi- geochemicalanalyses. The sequencebegins with a pally through the alignment of shredded tremolite dunite with a small concentrationof enstatite, either and strongly oriented chlorite. This reaction may be fresh or already partially altered, which was tecton- written as: ically emplacedinto the Ashe Formation as a num- 3Mg3(Si2O5)(OH)++ 2Ca + 2Si + ber of slivers. The first reactions were retrograde Antigorite hydrations of this originally high temperature-pres- sure assemblagein response to the lower grade Ca2Mg5(SfuOz)(OH)z+ 4Mg + 3H2O + 4H+. environment of regional metamorphism. A tremo- Tremolite lite-anthophyllite-chloritFtalc assemblageresult- The above reactions are based on mass transfer ed, although some relict olivine and rare enstatite calculations, taking into account the observed crystalsare preserved. This reactioncan be written: min- eralogical and chemical changes.The simplest stoi- 3OMg2SiOa+ 10Al + 5Si + 2Ca + 24H2O+ chiometric formulae are used for each mineral and Forsterite Fe is ignored. The geochemical study suggestsan enrichment in Fe in the more altered rocks probably Ca2Mg5(SfuOz)(OH)z + MgleAl5(SirrAI5O40XOH)32 through removal of Mg from Fe-Mg silicates, prin- Tremolite Chlorite cipally olivine; however, the znalyzedolivines con- + Mg6(Si8OzoXOH)++ Mg7(SfuOz)(OH)z tain only about 10percent mol fraction offayalite so Talc Anthophyllite Fe is not an important element in the reactions studied. + 23Mg + 8H*. Chlorite occurs in somewhat higher concentra- The addition of Si, Al, and Ca is necessitatedby tion in the more altered Todd-type bodies than in the observed mineral assemblage in these least the Edmonds-typeand could be consideredto be a altered samples and by the comparison of the product of the antigorite reaction which produced concentration of Al and Ca in the analyzed least additional tremolite, however, there is no texture alteredand most alteredspecimens. The lossof Mg evidenceof this and the low abundanceof antigorite in the reaction is based on the same criteria. The in the Edmonds-type bodies would not generate a addition of Si is further indicated by a 2.3 percent significant amount of chlorite. higher concentration of Si in Todd-type samples based on XRF analysesof three specimens. Structural relationships A secondhydration stageinvolves the serpentini The relationship between the recrystallization zation of olivine and tremolite and is evidenced by sequenceof these ultramafic bodies and the tecton- the petrographic observation that serpentine has ic and metamorphic history of the Blue Ridge is a replaced tremolite pseudomorphically and extends fascinating, but at present a rather speculative, into fractures in olivine in Edmonds-type rocks problem. The low Rb concentration in alpine ultra- (Fig. 3). Thus the crystallization of tremolite which maficrocks and their susceptibilityto Sr metasoma- partially replaced olivine must have occurred at tism (e.9., Menziesand Murthy, 1976)make deter- least in part before serpentinization. This reaction mination of crystallization ages by Rb/Sr methods may be stated as: of evenfresh alpinebodies unsuccessful. However, Mg2SiOa * Ca2Mg5(Si8O2r(OH)2+ Si + 8Mg the common elongationof these bodiesparallel with Forsterite Tremolite prominent foliation of the Ashe Formation and the presenceof that foliation in even the least altered + l7H2O + 5Mg:(SizO5XOH)4+ 2Ca + l6OH+. Edmonds-type bodies restricts their emplacement Antigorite to a time between the deposition of the Ashe The final reaction in the sequenceis a dehydra- Formation in late Precambrianand the development tion accompaniedby shearing and is indicated by of the foliation. Most workers (e.9., Rankin et al., the comparisonof the chemical composition, miner- 1973)have called this foliation S1and assignedit to alogy, and textures of the Edmonds- and Todd-type the Taconic deformation(450-rm.y.). Rankin er a/. ultramafic bodies. The more altered Todd-type (1973) place the peak of metamorphism in the SCOTFORD AND WILLIAMS: METAMORPHOSED ULTRAMAFIC BODIES easternBlue Ridge as post-kinematic,thus some- with the high calcium abundancescharacteristic of what later than the 51 foliation. or cumulate ultramafic rocks from Although a pre-emplacement age for the first . This suggestsa source from the base of stageretrograde hydration of the ultramafic proto- the sequencebelow the cumulate perido- lith cannot be ruled out, the presence of the 51 tite and mafic members of the sequence. foliationin eventhe Edmonds-typebodies, which is expressedby the preferred orientation of chlorite Conclusions and tremolite, strongly suggeststhat thesereactions Analysis of the mineralogical and geochemical are post-emplacementand reasonably represent a data, together with textural information, indicates partial reestablishmentof equilibrium in the pres- that the ultramafic bodies investigatedcan be divid- sure, temperature, and environment of re- ed into two distinct groups, the more recrystallized gional metamorphism in comparison to that of the and metasomatizedTodd-type and the less altered higher intensity environment of the upper . Edmonds-type. The contrasting character of these The second stage hydration reaction causing the two types of bodies reflects differing degrees of partial serpentinization of olivine and tremolite is alteration of the original protolith, probably a dunite tentatively assignable to the metamorphic regime which contained a small amount of enstatite. which followed the peak of taconic metamorphism Textural evidence is important in understanding (Hatcher, 1978,Fig. C). The dehydration stageof the sequenceof mineral reactions which converted the sequencewhich was accompaniedby shearing the protolith to the tremolite-chlorite-talc of is then possibly correlative with the Acadian(?) the Todd-typebodies. In the lessaltered Edmonds- (375-rm.y.; greenschistmetamorphism and brittle type rocks primary olivine and uncommon primary deformation associatedwith the F3 folding (Hatch- enstatitehave been replacedby tremolite, chlorite, er. 1978). talc, and anthophyllite. In a secondhydration stage, As pointed out by Rankin, Espenshade,and there was pseudomorphousreplacement of some of Shaw (1973),these ultramafic bodies typically oc- the tremolite by antigorite, which also replaced cur on the northwest side of the amphibolite layers some olivine (Fig. 3). These observationssupport in the Ashe Formation although some occur on the the conclusion that the recrystallization sequence southeastside and some are independentof the did not begin with serpentinization as has been amphiboliteoutcrops. This associationsuggests a reported in other areas.The lack of antigorite in the genetic relationship and perhaps an ophiolitic asso- Todd-type samples and the mylonitic texture of ciation. Whether or not this is valid, a tectonic many of these samples indicate a second major emplacementof theseultramafic bodies seems most stage in the recrystallization was one of partial likely. Hatcher (1978)theorized that thrusts initiat- dehydrationand cataclasis. ed in the unstable sea floor brought An examination of the abundance data for Mg, and mantle, including the mafic and ultramafic Al, Ca, Fe, Ni, Mn, Cu, Co, K, and Zn obtained rocks of the easternBlue Ridge, into the sedimenta- from analysesof these rocks allows conclusions to ry pile during the closing of a marginal seain Middle be drawn concerning the movement of these ele- to Late Cambrian. ments during metasomatism.Assuming, for reasons The nature and history of the protolith before its detailed earlier, that the two types of ultramafic probable tectonic emplacementis an important but bodies investigated represent different degrees of difficult question. It may have progressedthrough a metasomatic alteration of the protolith, the ex- prograde deserpentinizationcycle before the retro- change of these elements between the ultramafic grade events describedin this study. Whether or bodies and the country rock can be described. not an earlier deserpentinizationhas occurred, we There is strongevidence that Mg and Ni have been favor a history involving emplacementof enstatite- removed from the ultramafic rocks and that Al has bearing dunite from an ophiolite source. The pro- been added. It is probable, though less well demon- jected major element of the protolith strated, that Ca, Fe, Mn, and K entered the ultra- basedon this study is characterizedby low calcium mafic bodies and that Co was removed. The move- concentration.This is typical ofdunite and harzbur- ment, if any, of Zn and Cu could not be determined. gite from ophiolite metamorphic peridotite com- A statement of the recrystallization sequencein plexes according to chemical data provided by terms of calculated mass balance reactions can be Coleman(1977 , Tables l, 2, 3, and 4) and contrasts made using constraints provided by the geochemi- SCOTFORD AND WILLIAMS: METAMORPHOSED T]LTRAMAFIC BODIES 93

cal, mineralogical, and textural information. An tions to Mineralogy and Petrology, 24,27r-292. initial retrograde hydration converted the protolith, Curtis, C. and Brown P. E. (1971) Trace element behavior in probably a dunite with some enstatite, into altered zoned metasomaticbodies in Unst, .Contributions to Mineralogy and Petrology, 31, 87-93. rocks of the Edmonds-type by the hydration of Evans, B. W. (1977) Metamorphism of alpine peridotite and olivine and enstatite producing tremolite, chlorite, serpentinite.Annual Review Earth and Planetary Science,5, talc, and anthophyllite with the addition of Si, Al, 397447. Ca, and the loss of Mg leaving some relict olivine Evans, B. W., and Trommsdorf, V. (1970)Regional metamor- phism and enstatite. In a later stage of this hydration, of ultramafic rocks in the Central Alps: paragenesesin (antigorite) the system CaO-M8O-SiO2-H2O. SchweizerischeMineralo- serpentine was the product of the hydra- gische und PetrographischeMitteilungen, 50, 481492. tion of some of the olivine and tremolite, with the Flanagan,F. J. (1973)1972 values for international geochemical addition of Mg and loss of Ca. chemical reference samples. Geochemica et Cosmochemica The final step in the recrystallization sequence Acta.37. l189-1200. (1967) was a dehydration accompaniedby brittle deforma- Goles, G. G. Trace elementsin ultramafic rocks. In P. J. Wyllie, p. tion Ed., Ultramafic and related rocks, 352-162. Iohn which destroyed the serpentine and increased Wiley and Sons, Inc. the concentration of tremolite, with the addition of Hahn, K. R., and Heimlich, R. A. (1977)Petrology of the dunite Ca and loss of Mg. The net chemical changebe- exposedat the Mincey Mine, Macon County, North Carolina. tween the protolith of the more altered Todd-type SoutheasternGeology, 19, 39-53. (1978) rocks was one of loss of Mg and addition of Al and Hatcher, R. D., Jr. Tectonics of the westernPiedmont and Blue Ridge Ca as geochemical southern Appalachians: review and speculation. indicatedby the study. American Journal of Science, 278,276-3M. The possiblerelationship between this recrystalli- Jackson, E. D., and Thayer, T. P. (1972) Some criteria for zation sequenceand the tectonic and metamorphic distinguishingbetween stratiform, concentric, and alpine peri- history of the eastern Blue Ridge is subject to dotite- complexes: International Geological Congress, speculation. If the reasonableassumption is made 24th, Montreal, sec. 2, 289-296. Keith, A. (1903)Description of the Cranberry quadrangle, that these ultramafic North bodies were emplacedtectoni- Carolina-Tennessee:U. S. Geological Survey Geologic Atlas, cally into the Ashe Formation, retrogradehydration Folio 90, p. 9. could have occurred during the Taconic metamor- Laskowski, T. E., and Scotford, D. M. (1980)Rapid determina- phism as a non-equilibrium adjustment to the lower tion of olivine compositions in thin section using dispersion P-Z conditions of regional metamorphism. The staining methodology. American Mineralogist, 65, 401-403. Livingston, A. (1976) A later dehydration metamorphosed layered alpine-type may be associated with the peridotite in the Langarat Valley, South Harris, Outer Hebri- 350+m.y. old (Acadian?) facies meta- des. Mineralogical Magazine, 40, 493499. morphism and brittle deformation. Menzies,M. A., and Murthy, R. V. (1976)Sr-isotope composi- tions of clinopyroxenesfrom some Mediterraneanalpine lher- zolites.Geochemica et CosmochemicaActa,44, 1577-l5El. Acknowledgments Misra, K. G., and Keller, F. B. (1978)Ultramafic bodies in the southern Appalachians: a review. American Journal of Sci The authors wish to thank Ann H. Glaser who did most of the ence.278. 389-41E. point-count analyses,Thomas E. Laskowski who made a num- Rankin,D. W., Espenshade,G., and Neuman,R.(1972) Geolog- ber ofindex ofrefraction determinationsofolivine and dolomite, ic map of the western half of the Winston-SalemQuadrangle, and Frederick R. Voner who did additional petrographic work. North Carolina, Virginia, Tennessee:Department of Interior, The manuscript was substantially improved by the critical re- U.S. Geological Survey MiscellaneousInvestigations, Map I- views provided by Kula C. Misra and James A. Whitney. 709-A. Rankin, D. W., Espanshade,G., and Shaw (1973)Stratigraphy References and structure of the metamorphic belt in northwestern North Carolina and southwestern Virginia. American Journal of Alcom, S. R., and Carpenter,J. R. (1977)Serpentinization of the Science,Cooper Volume, 273-A. 140. Holcome Branch Dunite, North Carolina. SoutheasternGeol- Stueber,A. M., and Goles,G. C. (1967)Abundances of Na, Mn, ogy,17,243-256. Cr. Sc. Co in ultramafic rocks. Geochemicaet Cosmochemica Borg, I. Y., and Smith, D. K. (1969)Calculated X-ray powder Acta, 3l , 75-93. pattems for silicate minerals: Geological Society of America Swanson,S. E. (1980)Petrology of the Rich Mountain ultramafic Memoir 172. bodies and associatedrocks, Watauga County, North Caroli- Carswell,D. A., Curtis, C. D., and Kanaris-Sotiriou,R. (1974) na. SoutheasternGeology, 21, 209-225. Vein metasomatismin peridotite at Kalskaret near Tafiord, Trommsdorf, V., and Evans, B. W. (1974)Alpine metamorphism South Norway. Journal of Petrology, 15,383-402. of peridotitic rocks. SchweizerischeMineralogische und Pe- Coleman, R. G. (1977)Ophiolites: Springer-Verlag,New york. trographischeMitteilungen, 54, 333-352. Curtis, C., and Brown, P. E. (1969)The metasomaticdevelop- Turekian,K. K., and Wedepohl,K. H. (1961)Distribution of the ment of zoned ultrabasic bodies in Unst, Shetland. Contribu- elementsin 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 contentsof chemicalelements chemistry, p. 227-271.Springer, New York. in the principal types of igneous rocks of the Earth's crust. (traqsl. of Geokhimiya), 641-64. Manuscript received November 23, I9EI ; Wedepohl,K. H. (1969)Composition and abundanceof common acceptedfor publication, September7 ' 1982.

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