The Greenschist-Amphibolite Transition in the CFM Projection

American Mineralogist, Volume 69, pages 250-251, 1984

The greenschist-amphibolitetransition in the CFM projection

Rtcua,Ro N. Annorr, Jn. Department of Geology Appalachian State University, Boone, North Carolina 28608

Abstract

The CFM projection(C = CaO+Na2O+K2O-AlzOr, F : FeO-FezO:, M = MgO) was originally designedfor illustrating mineral assemblagesin upper amphibolite and granulite facies quartz-feldspar-magnetite-bearingmetabasites. By introducing chlorite, the projection can also be used to illustrate mineral assemblagesin greenschistfacies quartz- feldspar-magnetite-bearing metabasites. The greenschist-amphibolite facies transition takes place in the context of Abbott's (1982)lowest gradeCFM topology.

In metabasites,the change from the greenschistfacies by the addition of chlorite. Actinolites and hornblendes to the amphibolite facies commonly proceeds with no plot in the same part of the CFM diagram and since the changein the mineralassemblage other than (l) a change changefrom the former to the latter is, in many instances, in the compositionsof the mineralsand (2)a changein the continuous, this change has virtually no visible affect on modalproportions of the minerals(Laird, 1980;Thomp- the CFM topologies.Also, becauseplagioclase does not son et al., 1982;Laird in Robinson et al., 1982).The appear in the CFM projection, the changefrom albite to common assemblageis amphibole (Amp) + chlorite (Chl) oligoclase has no visible affect on the CFM topologies. + plagioclase(Pla) + epidote(Epi) + quartz (Qtz)+Fe3+ The purpose of this note is to show that by introducing oxide (hematite or magnetite : MCt)tTi-mineral (ilmen- chlorite in the CFM projection the petrogenetic grid, ite or sphene)-+K-mica(muscovite of biotite = Bio)tcar- which I originally designedfor amphibolite and granulite bonate (calcite or ankerite)rgarnet (Gar). In the green- facies rocks, applies just as well to lower grades where schist facies, the amphibole is actinolite (Act) and the typical greenschistassemblages are possible. plagioclaseis albite. In the amphibolite facies, the amphi- By several indications(Winkler, 1979;Turner, 1968; bole is hornblende(Hnb) and the plagioclaseis oligoclase Dobretsovet al., 1972)Chl+Epi+Qtz+Mgt+albite is an or a more calcic plagioclase. The whole-rock reaction important greenschist assemblageat temperatures well relating the two facies is polyvariant and has been treated above the first appearanceofamphibole, biotite or garnet recently in an elegant fashion by Thompson et al. (1982) in rocks of higherFeO/MgO. In most greenschists,these and by Laird in Robinsonet al. (1982).In theseworks, minerals appearas the result of divariant CFM reactions. the changesfrom actinolite to hornblendeand from albite The usual order of appearancewith increasing grade of to oligoclasewere modelled as continuous reactions. metamorphism is actinolite, biotite, garnet (Winkler, Abbott (1982)presented a petrogeneticgrid for amphib- 1979;Turner, 1968;Dobretsov et al., 1972).However, olite and granulite facies metabasites, based on phase garnet may appear at a lower grade than the biotite if the relationshipsin the CFM projection (Q : CaO+Na2O K2O contentof the rock is very low, in which casebiotite +K2O-Al2Or, F: FeO-Fe2O3,M = MgO). The CFM may not appear at all. Normally the K2O content is high projection is useful for illustrating mineral assemblagesin enoughto sustainbiotite (or someother K-bearingminer- metabasitescontaining qvar1"z,magnetite and one or two al, Laird, 1980).For the purposesofthis note, thereis no feldspars. The grid consists of 26 P-T regions, each need to consider CFM topologies without amphibole, characterizedby a distinct CFM topology. Boundaries biotite and garnet. between the P-T regions are mineral reactions involving The most likely CFM topology for the greenschist combinations of four of the CFM minerals hornblende, facies is shown in Figure la. The topology is derived from biotite, garnet, epidote, clinopyroxene, orthopyroxene Abbott's (1982)CFM topology l, 5 or 6 by the additionof and fayalite. Quartz, feldspar and magnetiteare involved chlorite. All of the three-phase CFM assemblagesin as neededfor balancing the reactions. Figure la are naturally occurring (Abbott, 1982;for the Some of the lowest grade CFM topologies(e.g., P-T chlorite-bearingassemblages, Winkler, 1979; Turner, regions1,5, and 6 in Figure 3 of Abbott, 1982)would be 1968;Dobretsov et al.,1972;Spear,1982).The partition- consistentwith the greenschistfacies if (l) the amphibole ing of FeO and MgO, Xr"o : FeO/(MgO+FeO),between were actinolite instead of hornblende, (2) the plagioclase chlorite and other minerals in metabasitesis Xp"6 (Chl) > were albite and (3) if the CFM topologies were modified Xr"o (Act), XF"o (Chl) = Xn.o (Hnb), XF"o (ChD l Xr.o 0003-004xE4/03M-0250$02.00 250 ABBOTT: GREENSC HI ST-AM PHIB OLITE TRANSITIO N 25r

Epi is Chl+Epi, Chl+Epi+Amp, Chl+Amp, Chl+Amp+ Bio, Bio+Amp, Bio*Amp+Epi. The chlorite-bearing * Quortz assemblagesare typical of the greenschistfacies where +Plogioclose the amphibole is actinolite and the plagioclaseis albite. +M ognetite The assemblageswithout chlorite are typical of the am- t K-teldspor phibolite facies where the amphibole is hornblende and the plagioclaseis oligoclase.Very commonly metabasites (metabasalts)are in the Amp+Epi+Chl field when the greenschist-amphibolitetransition takes place (Thomp- A sonet al., 1982;Laird rn Robinsone t al., 1982).For bulk / / compositions higher in FeO/MgO, the greenschist-am- Bio \ \ phibolite transition may occur in one of the chlorite- absent assemblages.For compositions lower in Gor Chl Gor Chl FeO/MgO, the facies transition may occur in the amphibole-absentassemblage Chl+ Epi. Topology 1,5or 6 I suggestthe greenschist-amphibolitetransition takes Low T Low K2O place in Abbott's (1982)lowest grade CFM topology. If the K2O content of the rock is very low, biotite may not o. b. be possible. Under these circumstances,the greenschist- amphibolite transition probably takes place in the context Fig. 1. CFM projectionsfor greenschist grade and low of the CFM topology shown in Figure lb. amphibolitemetabasites. C = CaO+Na2O+K2O-AI2O3,F = = FeO-Fe2O3,M MgO. In eachprojection X marksa bulk Acknowledgments compositionexpressed by the mineralassemblage Chl+Epi. Amp= s-06;6ole,Bio : biotite,Chl = chlorite,Epi = epidote, I wish to thank Jo Laird, Harry Y. McSween,Loren A. 641 : garnet.a. Portionof the CFM projectionfor Abbott's Raymond,J. G. Liou, DavidA. Hewitt andJohn P. Hoganfor (1982)P-T reeionsl, 5 or 6 modifiedby the additionof chlorite. their helpfulcomments. b. HypotheticalCFM projectionfor low K2O. References Abbott, R. N., Jr. (1982)A petrogeneticgrid for high grade (Gar) and XF"o (Chl) ( Xr"o (Bio) (Laird, 1980). The metabasites.American Mineralogist, 67, 865-876. epidotein Figure I is Ca2Fe3+AlzSi:Orz(OH);more alu- Dobretsov,N. L., Khlestov,V. V. andSobolev, V. S. (1972) minousepidotes plot closerto C. TheFacies of RegionalMetamorphism at ModeratePressures. As the temperatureincreases, Xp"6 decreasesfor Gar, (Transl.D. A. Brown, AustralianNational University, Can- Bio and Chl in the various three-phaseCFM regionsin berra.1973). (1980) Figure la. Where the amphibole is actinolite, Xr"o (Act) Laird, J. Phaseequilibria in maficschist from Vermont. Joumalof Petrology,2l, l-37. for the three-phaseCFM region Act+Chl+Epi may in- Robinson,P., Spear,F. S., Schumaker,J. C., Laird,J., Klein, creaseslightly with increasingtemperature (Laird, 1980; C., Evans,B. W. andDoolan, B. L. (1982)Phase relations of Laird in Robinson et al., 1982).The three-phaseregion metamorphicamphiboles: Natural occurrence and theory.In expands with increasing temperature, such that the D. R. Veblenand P. H. Ribbe,Eds., Amphiboles: Petrology Act+Epi join rotates slightly to higher Xp"e and the and ExperimentalPhase Relations, p. l-227. Mineralogical Chl+Epi join rotatesto lowerXp"e. Wherethe amphibole Societyof America,Reviews in Mineralogy,Volume 98. in Figure la is hornblende, Xp"q (Hnb) decreases or Spear,F. S.(1981) An experimentalstudy ofhornblende stability remainsconstant with increasinggrade (Spear, l98l), and and compositionalvariability in amphibolite.American Jour- all three-phaseregions move to the right (to lower Xp"e) nalof Science.281.697J34. (19E2) with increasing temperature. With regard to the model Spear,F. S. Phaseequilibria ofamphibolites from thePost PondVolcanics, Mt. Cube Vermont.Journal of assemblage,Amp+Chl+Epi+Pla, of Thompson er a/. Quadrangle, Petrology,23,383426. (1982),this changein the projection CFM is accommodat- Thompson,J. 8., Jr., Laird, J. and Thompson,A. B. (19E2) ed principally by the reaction: Reactionsin amphibolite,greenschist and blueschist, Journal Epi + Chl: Amp ofPetrology,23,1-27. Turner,F. J. (1968)Metamorphic Petrology. McGraw-Hill, New Quartz, feldspar, Fe3+ oxide and H2O are involved as York. neededfor balancing the reaction. A metabasite,initially Winkler,H. G. F. (1979)Petrogenesis of MetamorphicRocks. in the Chl+Epi field, but wirh C(C+F+M) slightly less 5th Edition.Springer Verlag, New York. than Amp (X in Figure la and b) will be situated in different fields as the temperature increases. In order from low to high temperature,for Abbott's (1982)CFM Manuscriptreceived, December 21, 1982; topologiesI , 5 or 6 (Fig. la), the sequenceof assemblages acceptedforpublication, August 24, 1963.