American Mineralogist, Volume 65, pages 654-669, 1980 Textures and mechanismsof metamorphicreactions in the CockeysvilleMarble near Texas, Maryland RtcHnno F. SnNrono' The JohnsHoPkins UniversitY Baltimore, Maryland 2I 218 Abstract The Cockeysville Marble at Texas, Maryland was regionally metamorphosed at about 600.C and 6 kbar. During metamorphismthe reactionsbetween dolomite, qvattz, and micro- cline to producephlogopite, tremolite, and diopsidecaused the formation of mineral segrega- tions and reaction bands. Various textures, including radiating spraysof tremolite mantled by calcite in dolomitic rocks and reaction zones of tremolite + calcite and phlogopite between metadolomiteand calcite marble, can be explained by local metasomaticreactions taking placenear equilibrium by masstransfer down chemicalpotential gradientsbetween local do- mains. Modal mineralogy and reaction texturesare used to infer the relative magnitude of fluxesfor the major components,viz. CaO> MgO > SiO2> KAlSi3O8.Experimental solubil- ity data support the generalization that the flux of a fluid speciesis proportional to its solubil- itv. Introduction prominent dark brown phlogopite bands between white calcite marble and tan metadolomite, and Detailed study of mineral relations in metamor- striking clusters of radiating, bladed tremolite man- phosed carbonate and pelitic rocks has shown that tled by calcite within massive metadolomite. These chemical equilibrium is commonly not achieved on and related textures, formed during the growth of the scaleof a thin section.Thompson (1959)consid- metamorphicminerals, were clear$ causedby chem- ered the role of local equihbrium and transferof ma- ical reaction and transport during metamorphism. in the presenceof activity gradients.The con- terial It will be shown that assemblagesof minerals was successfully employed by Carmichael cept whose grains are in mutual contact reveal the equi- (1969),Fisher (1970), Eugster (1970), Loomis (1976), librium metamorphic conditions of particular sam- Weare et al. (1976),and Foster (1977,1978) to inter- ples, and that the sequenceand modal relations of pret in pelitic rocks;by Vidale (1969),Hew- textures minerals in a set of reaction zonesyield information itt (1973),Vidale and Hewitt (1973),Burt (1974), on the stoichiometryof local metasomaticreaction$ Joesten(1974, 1977),Thompson (1975)' Brady and the nature of the dissolved,transported species' (1977),and Hoersch(1979) to explain calc-silicatere- In most casesmobile and inert components can be bands; and by Brady (1977) and Sanford action distinguished unambiguously from observedreaction (1978)to interpret ultramafic reactionzones. Prelimi- textures. The method of determining relative mobil- nary results of the present study were presented ities consistsof proposing limiting, alternative com- orally by Sanford and Eugster(1974). binations of mobile and immobile components in This paper is a systematicanalysis of the mineral- each reaction and predicting the various possible re- ogy and textures of the Cockeysville Marble at action textures.These predicted textures are clearly Texas, Maryland. The Cockeysville Marble in the dislinguishablefrom one another, and the choice of Flintkote StoneProducts (formerly, Harry T. Camp- which one representsthe actual mechanismis usually bell Sons')quarry at Texas,Maryland, containssev- obvious by comparison with the natural rock. An im- varieties of mineral segregations,including eral portant check on the mass transfer can be obtained potential gradients of in- I Presentaddress: Mail Stop 959, U. S. GeologicalSurvey, Res- by calculating the chemical ton, Yirginia 22092. dividual speciesin the fluid. 0003-004x,/80/0708-0654$02.00 654 SANFORD: M ETAMORPHIC REACTI)N S Geologicsetting Formation under$ing the Cockeysville Marble at Texas and much of Figure I showsthe location of the quarry and of the Wissahickon Formation, which overlies the samples. The Cockeysville Formation occurs the Cockeysville, are composed of kyanite-, near the baseof the Glenarm Series,a group of pre_ staurolite-, and garnet-bearing schists (Fisher, cambrian or Early Paleozoicmetasediments resting l97l). unconformably on the precambrian Baltimore Gneiss(Higgins, 1972;Crowley, 1976).The Cockeys_ Rock types ville Formation is underlain by the SettersForma- The principal rock types found in the Texas tion, which comprises muscovite-microcline quiartz_ Quarry are metadolomite,calcite marble, and calc- ite with subordinate mica gneiss and mica schist. schist(for definitions seeChoquette, 1960). Medium- and overlain by the thick Wissahickon Formation, grained, bluish-gray metadolomite, calcite marble, which is composedmainly of pelitic schists (Higgins and calc-schist,all interlayered on a fine scale and and Fisher,l97l; Crowley,1976).These three forma_ containing the assemblagedolornite * quartz, char- tions make up the Glenarm Series,which mantles a acteiae the rocks in the northwestend of the quarry. group of elongategneiss domes. The Texaseuarry is The main part of the quarry contains coarse-grained, located in the Cockeysville Formation on the west white calcite marble grading into medium- and fine- flank of the Texas Dome. A detailed study of the grained bluish-gray and tan varieties of metadolo- Cockeysville Formation was carried out by Cho_ mite. The tan color is imparted by phlogopite.In this quette(1957, 1960). part of the quarry, tremolite occursin place of the as- The quarry is located on the noseand SW timb of semblagedolomite + quartz. Rare thin (l-2 cm) lay- an anticline plunging gently NW (Choquette, l95Z). ers of microcline-quartz rock also occur. Modes for Compositional layering, interpreted as bedding, typical samplesof each of these rock types are pre- strikes approximatelyNW and dips from 20o to 40o sentedin Table l. SW with a generalsteepening toward the SW. Fisher (1971) has demonstratedthat the-CockeysvilleFor_ Effects of sedimentaryand diageneticprocesses mation exposed at Texas is right-side-up, as in- The Paleozoicorthoquartzite-carbonate sequence, dicated by a bore hole which terminatesin the Set_ of which the CockeysvilleMarble is a part, resting ters Formation. The upper beds of the Setters uncomformably on Precambrian gneisses,is typical CONVEYOBBELT Fig' l' Map of Flintkote StoneProducts quarry at Texas,Maryland with locationsof samplessited in text. SANFORD: METAMORPHIC REACTIONS qvuttz, and feldspar' Table 1. Representative modes of rock types in Cockeysville of calcite, dolomite, phlogopite' Marble at Texas, Maryland Prominent color variations from white to bluish gray to tan on the scale of meters are causedby locally No. phlogopite' respec- Q 0thersx Polnts abundant calcite, dolomite, or tively. Dolomite and calcite marbles very comnonly (1) 14.0 71.3 0.0 6.2 0.0 2.2 0.2 1518 contain thin (l-2 mm thick) partings of calc-schist 12, 16.1 72.9 2.8 7,4 0.0 0.0 2.8 2230 (l) 1r.4 80,8 0.8 6.8 0.0 0. I 0. I 3023 spacedfrom 2 to l0 mrn apart. Phlogopitegives the the calc- (4) 0.0 0.0 0.4 4.8 9.1 0,4 963 prominent tan to dark brown coloration to (5) 94.5 0,0 0.0 4.3 0.4 0.6 0.2 4000 ichist layers,but microcline,q\attz, dolomite (in cal- (6) 11.1 0.0 21.0 19.5 2.1 1.3 967 cite marbles), and opaques are also concentrated in (7) 28.6 l.o 28.1 34.1 5.5 0.5 1.1 11 41 theselayers. (8) 5.t 66.0 19,9 6.2 0.l 0.1 Calc-schist layers are similar to seamsdescribed by (1964),and interpreted by him to be the re- *l4osily pyrite, rare tourmalIne' Barrett (1) Me+adolml+e,C27-20O sult of intrastratal solution moving along planespar- (2) l"le+adolomi+e,Cl-1 99 (f ) Me+adolml+e, TQ?-z and 1Q12-3 allel or subparallel to bedding, removing calcareous (4) Calcite marble C27-200 insoluble, non-car- (5) Calci+e na.ble, fQ2-22 constituents and concentrating (6) Calc-schist, TQB-2 minerals. Vertical spacing between layers, (7) Calc-schlsi, TQ2-4 bonate (8) Microcllne-quar+z rock, Cl-199 broad horizontal extent oflayers, closegeneral paral- lelism, the merging and diverging of layers, and a sim- of sedimentationat the edgeof a stablecraton having structures resembling cross-beddingall suggest Texas differs low relief (Crowley, 1976).Dolomitic rocks' which ilar origin. The Cockeysville Marble at coarser,and are the prirnary object of this study, probably repre- in that partings are thicker, grain sizeis of glauco- sentdeposition in a restrictedintertidal, marine basin phlogopite or microcline is presentinstead to re- or a supertidal sabka environment (Selley, 1970, nite and illite. These effectscan be attributed than thesethin chap. 8). Such conditions would be expectedto pro- gional metamorphism.Less abundant of quartzite and duce evidence of sea-level fluctuations, periodic partings are layers (l-2 cm thick) be relict flooding and evaporation,pore water replacementre- microcline-quartz rock. These layers may possibly meta- actions, and occasional deposition of land-derived sedimentarybeds of quartz and clay, (Bathhurst' clastic material. Many features of the Cockeysville morphosed glauconite-rich hardground due to Marble at Texascan be attributed to thesesedimen- 1975,p.3g44l2), they may be concentrations pressuresolution, or they may be due tary and diageneticProcesses. locally intense regional The minerals calcite, dolomite, quattz, and micro- to chemical differences generated during later' cline are consideredto be premetamorphic.Dolomite metamorphism.The last possibility is discussed probably parallel may have formed by direct precipitation from sea- Taken together,these layers are be- water in a lagoon-like basin