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Petrology and Geochemistry of Metamorphosed Ultramafic Bodies

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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 mineral assemblagesin individual samplesconstitute subgroupsdrawn from the larger group consistingofolivine, enstatite, tremolite, anthophyllite, antigorite, talc, chlorite, magnetite, and dolomite. Although the ultramafic bodies occur in the garnet, staurolite, and kyanite zones recognized in the country rock, no correlation exists betweenthe mineral assemblagesin the ultramafic rock 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 nickel and the introduction of aluminum, calcium, iron, manganese,and potassium into the ultramafic bodies. The less altered bodies are more massiveand contain relict olivine 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 schists and gneisses Alpine ultramafic bodies occur throughout the and were mapped by Rankin, Espenshade, and length of the Appalachian Mountains 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 "Amphibolite facies" 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- petrology of metamorphosed peridotites 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 metamorphism, 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 serpentinite to peri- originally harzburgites or dunites, 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 dunite, but Todd-type bodies Edmonds-typebodies neither has been extensively recrystallized since (T) Todd (ED) Edmonds emplacement. (W)Warrensville (GR)Grayson (HN) ltigger Mountain (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 foliation is (RR) RockyRidge expressed microscopically by the parallelism of such platy or prismatic minerals 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 magnesite. 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 pyroxene 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
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