SIGNIFICANCE of RIEBECKITE and FERROHASTINGSITE in MICROPERTHITE GRANITES Peur, C

American Mineralooist Vol. 57, pp. 1404-1412 (1972\

SIGNIFICANCE OF RIEBECKITE AND FERROHASTINGSITE IN MICROPERTHITE GRANITES Peur, C. Lvous, Boston Uniuers'ity,College ol Basic Studies,Boston, Massachusetts02215

Arsrnecn

Riebeckite and ferrohastingsite occur separately in closely related hypersolvus granites of eastern Massachusetts. Mineralogical, chemical, and phase equilibria data suggest that, the Quincy riebeckite granite and the Peabody ferrohastingsite granite crystallized at similar temperatures, most, likely 6b0-750"C. The chemical composition of the parent magma and oxygen fugacity played important rolbs in determining the composition of the amphibole. Parent composition favored a calcium-iron-aluminum amphibole, ferrohastingsite, NaCazFer+rFd+AlzSiOzs(OH):, in the Peabody, and a soda-iron amphibole, riebeckite, NazFeF+sFd+zSi8O22(OH)2, in the Quincy. Higher oxygen fugacities favored the ferric-rich amphibole, in the Quincy; lower oxygen fugacities favored the ferric-poor arrphibole, ferrohastingsite, in the Peabody. Mineral relations in the Quincy suggest,an increase in oxygen fugacity with decrease in temperature. Two new chemical analyses, one of the Quincy granite riebeckite and one of the Peabody granite ferrohastingsite, are reported. INrnooucrrox Riebeckite (or arfvedsonite)and ferrohastingsiteare found in one- feldspar granites of Nigeria, New Hampshire, and eastern Massa- chusetts.The riebeckiteand ferrohastingsitegranites typically occur in separateplutons. In New Harnpshireand Massachusetts,a small amountof riebeckitegranite is assoeiatedwith ferrohastingsitegranite in the same pluton. The Quincy and Peabodygranites of easternMassachusetts will be usedto addressthe problem of the spatial independenceof riebeckite and ferrohastingsite.New chemicaldata are presentedfor the Quincy riebeckiteand the Peabodyferrohastingsite. This paper will attempt to explain the field relations of these amphibolesin light of their chemistry, mineralogiial and whole-rock chemical data, and phase equilibriastudies. The spatial relationsof the alkali graniteplutons of easternMassa- chusettsare shown in Figure 1. The Quincyl and RattlesnakeHill are riebeckitegranites, and the Peabodyand Cape Ann are ferrohastingsitegranites.

'Investigators should clearly state whether they are referring to the Quincy, Peabody, or Cape Ann granites, because using Quincy granite for a.ll three has causedconfusion in the literature (see Toulmin. 1964).

t404 BIEBECKITE IN GRANITES 1405

cRANtrEt MASS. = QUINCY GRANITE

--' -T-'-'-'-l RATTLESNA iirt"^D,uli..r enanrre

l'lrlrl ot02030 MILES 30 71 30 Frc. 1. Location of the granite plutons.

Mrxnner,ocrcAr,DATA Modal data for the Quincy and Peabodygranites are summarized in Table 1. Pyrochlorein the Peabodyand fluorite in the Quincy suggest that mineralizerswere presentduring the crystallizationof the magmas.The similar youngergranites of Nigeria (Borley, 1963) contain, in addition, cryolite and topaz, again indicating the presenceof mineralizers,particularly fluorine. The alkali amphibolesof Nigeria (Borley, 1963) and of Massachusettsand Rhode Island (Lyons, un- publisheddata) also contain considerableamounts of fluorine sub- stituting for OH. The absenceof aegirineand aenigmatitefrom the Peabodyand fayalite from the Quincy are noteworthy.Fayalite has not been reported from the Peabody,although it does occur in the closely-relatedCape Ann granite and is commonto the ferrohastingsite granitesof New Hampshireand Nigeria. The Peabodyand Quincy graniteshave counterpartsin New Hamp- shire and Nigeria; all three areashave ferrohastingsiteand riebeckite (or arfvedsonite)granites with strikingly similar mineralogicalcom- positions. Tbpically one granite is a riebeckite- or arfvedsonite- aegirinegranite and the other is a ferrohastingsite-fayalitegranite. 1406 PAUL C. LYONS

labL 1 ATD CMMICAL DAII

X@AL DATA CI{EMICAL DATI qriacyr !ruod8 lrcrthitr* 62.7 6t.z sig Qu{tt 29.o 25.o zr% .2o .2o (.2o) .2o .w

trbrrobrstlng!ltc o.0 6.lr ^\% Lt.57 l-t.l? 12.09 11.61 12.98 12 1A 13.08 Rirbcckitc 5.i tr rcr\ 2.25 t.?I z.9L 2.2g .81 .2lt <2 l'.Blrfu. 1.8 0.o .Fto .93 J,.26 1.55 1.25 2.85 2.81

DE(,rcrc Tr .2 lfgo .0] .0L .08 .O5 .02 .11 Blotlt. .1 .J lho tr .O5 t! .oZ .o8 .10 .09

lregrctit .2 .7 Cao .l+b .L9 .31 .Ll I.O)r f ln

HcDtltc tr o.0 Ns?o 1.21 lL03 lJ.6 lr.lo L.19 3.99 1.09 fhcnitc tr tr K2o \.62 L.58 U.53 b.5L t 6. 5.ar

Auiglatit 0.o 13taoplyUlt6 tr ? Eluorit o.o lFochhe

Ily31lt.

S*Lcite tr tr

12 anrlylcri Dind conrtltunts ao apprdinatc (Iftrren, F- "1 19ll; Iyoror upub.). constitucnts cc apqcinatc (Touluin, 1951r; /lt!s;:i ": 3 T'""*r_*nor ryons, upub.). of 3 airalyscr f|l&cn, l9U). 'rba of 2 enelyaaa (Tdlrjr, 1951r). *CoroLsts of deocl.ir ad rlbttci iacludcr mircr aEout8 sln of s3pcatc albitc. placct, prrtly katophorit .

The ferrohastingsitegranites of the NorthwestAdirondacks (Budding- ton, 1948)contain similar mineralsto the comparablegranites of New Hampshire (Billings, 1956) such as microperthite,minor plagioclase, quartz, biotite, and apatite. The New Hampshire ferrohastingsite graniteshave, in addition,traces of fayalite. NIEBECKITE IN GNANITES

.,.Reer!rrnMrsrnY The chemicalcompg6itions of the Quincy and Peabodygranites are given in Table 1. Tle Quincy is higher in silica and soda,but lower in alumina,potash, and lime than is the Peabody.There is no signifi- cant differencebetween the total iron contentsof the two granites; the major differenceis in the oxidation state of the iron: the Quincy is ferric-rich and the Peabodyis feric-poor. Obviously,the oxidation state of the iron partly conditionedthe nature of the amphibolewhich crystallized from the magmas.This will be discussedlater in the paper. The ferric-rich nature of the Quincy is consistentwith its mineralogy,riebeckite and aegirine.The three chemicalanalyses of the Quincy granite consistentlyshow ferric ion dominant over ferrous ion. The ferric/ferrous ion ratio of the riebeckiie in the Quincy pegmatite (War- ren and Palache,1911) is nearly identical to that of the riebeckiteof the Quincy granite proper,which suggeststhat similar oxygenfugacities prevailedduring their crystallization. The higher silica content of the Quincy is reflected by more qtartz in its mode. The higher alumina and potash contentsof the Peabodyare shown by a greater modal amount of perthite and a more aluminous amphibole. The amphibole compositions (Table 2) indicate the dif- ferencesin lime and sodalthe Peabodyhas a calcium amphiboleand the Quincy has a soda amphibole.The marked distinction in the ferric and ferrous iron contents is clearly revealedby the amphiboles;the Quincy has riebeckite,Na2Fe2*rFet*rSirOrr(OH)r; the Peabodyhas ferrohast- *nFea*AlrSi6Or, ingsite,NaCarFe' (OH)r. AMpHrsoLpCnniursrny A summaryof the chemicalcompositions of the two amphibolesis givenin Table 2. The chemicalcomposition of the Peabodyamphibole is very similar to the compositionsof the ferrohastingsitesof Northern Nigeria (Borley and Frost, 1963) and the hastingsitesof the Adiron- dack hornblendegranites (Buddingtonand Leonard, 1953). The atomic proportions of the various constituentsin the two amphiboles are shown in columns (7) and (8) of Table 2. The formula of the Peabody ferrohastingsiteis: Na6."sK6.raCa1.76,Fe'*r.urMgo.a7Mns.13 Lis.s3, Fe8*.."eTio.23A-10.06,Alr.5ssi6.41, (OH)o.elFo.ra. The data for the Quincy amphiboleindicates an arfvedsoniticriebeckite, or, if you prefer, a riebeckitic arfvedsonite.The formula is: Nar.eeKr.rrCao.or,Fe'*r.ru Mgo.orMno.ro, Fet*r.url,io.rnTio.o., Alo.4nTio.lESi".nr,(OI[)r.rrFo.ur. In contrastingthe chemicalcompositions of the amphibolesof the Quincy and Peabodygranites, note that the major differencesindicated 1408 PAUL C. LYONS

Teblc 2 1IIPIIIBOIEDATA _* (1) si02 L8.7lr L{3Jer,{3ls ',J};. s[2e J.5J' s,. {li" J:h [ao3 2.3L 2.36 7.29 9.08 9.7h 8.51 rlrE J$9 i..t9 Tioz 1.L8 t.53 .67 2.88 3.10 1.89 .llE .ooo .05 rcloj 13.22 t2.9L 5.75 6.59 7.06 6.11 Tr' .t5? .oo

rbo .19.95 N.92 27.78 23.09 2\.8? 26.30 Ti .028 .23 l[nO .7b .76 .97 .77 .83 .90 rb(rrr) 1.532 .79 Itgo .29 .30 1.90 1.83 a.96 a,93 tu(rr) 2,7t5 3.62 r4eo .35 .38 u.d. .0lr .olr .olr E .o?o .ll? cao 3,6I 3.76 9.5t+ 9.oJ 9.73 9.61+ !r| .101 .13 Fzo 6.Q1 S.ge 2.86 1.99 2,L5 2,5L Ii .2w .03 (zo Lu l.2z .99 1,2o 1.29 1.11r ce .63b 7,70

F f lt 1.16 .o5 .tp .4> .4> Na 1.820 .8o B2O* a.96 1.61r t.ot .56 .6 .83 r. .2W .zl+ HaO- .27 .28 .88 .# .e5 oE 1.728 .91 Total 100.81 10O.2lr100.00 9.38 99.38 100.1r r .578 .21+ -0:s .W ,IL9 .oz .18 .19 .t9 iff,T-Te-$' ffi 9-i'F WE

(l) Rietrcldte, QuiacEr grettc, rcr ana\rsis; Tailashi A8arl, analyst. til LT::Fl,"i !*fry_s.r*b, sorrectedfor lrpur$ies (:.ol quartr, z.Blt qejfltrc). fErrohaltlngsitc, leabody granltc, Toulntn (1!6[); conEcted for jtmftlcJ. granitc, *a"lp:9+"S"11c, tcaboff. w analyslsi Tadashi A!ai, Enalyst . -rcrrohstllgritcr_leabody Eranite, cotrected for irFrities @.4 qr,z, .31 bLobf:@). ltee of (3) and (5). El.etogkil?, Qdncjr_grantie (2)i_I{o:_gf ioB on basis of ZIr (O,OHrF) rbltohastingsltc. lcaboqr granitc (6); ilo. ot lons onlasis'oi-2-t-to,OBrF).

t,hc_pnaclslon of tlr ttata for thc uaJor cmstittrcnts of thc tro n6r mlyscs pcfcsnt. is l.-2 Ths ir€cislo! b tlrc flZO+ concentratl,onr ic pocr snd nay be ix ^high as 509 dcvlatioa fron. thc EaD.- The lE:aclsLon for thi rluiaru is XoF dsvlatloD fron thc rcuo in the whole-rockchemical compositions are reflectedin the amphibole compositions;thatis, the Quincy riebeckiteis higher in Si and Na but lower in Al and Ca than the Peabodyamphibole. It appearsthat the compositionof the amphibolewas partly controlledby the chemical compositionof the parentgranite magma.

Sresrr,nv Rnr,euoNs The separateoccuffence of riebeckite (or arfvedsonite)and ferro- hastingsitein granitesof Nigeria,New Hampshire,and easternMassa- chusettssuggests either that riebeckite(or arfvedsonite)and ferrohast- RIEBECKITE IN GRANITES 1409 ingsite do not occur togetherin equilibrium or that they have a very limited field where both coexistin equilibrium. Toulmin (1964).has found traces of riebeckite(?) associatedwith the ferrohastingsiteof the Peabody and Cape Ann granites.These traces are closely asso- ciated with pyroxene and, probably, are secondarycrystallizations. Huang (1958) reportedriebeckite from a peculiar riebeckite-aegirine melagranitefrom the Wichita Mountains, Oklahoma.The riebeckite occurswith "hornblende" which has a B too low to be consistentwith ferrohastingsite.Ernst (1968) suggeststhat these two amphiboles representan equilibriumassociation. Ernst (1962)has investigatedthe phaserelations of syntheticsoda- iron amphiboles.Riebeckite is stable to about 500'C under oxidizing conditionsdefined by the hematite-magnetitebuffer. The solid solution riebeckite-arfvedsoniteis stableto temperaturesof about 700'C, with oxygen fugacities defined by the wiistite-iron buffer. Below about 900 bars fluid pressurewith oxygen fugacities defined by the wiistite-iron buffer, the assemblagequartz-aenigmatite-aegirine is stable between 650-750'C,except for fluid pressuresbelow about 200 bars wherethe temperature siability range is 600-800'C. At temperaturesbelow 650"C to 675oC,this assemblageis unstable relative to quartz * riebeckite-arfvedsonite.At fluid pressuresof 700-900bars, there is a field for fayalite * quartz * aegirineseparating these two fields.The fayalite-bearing field also separatesthe same two fields at higher oxygenfugacities (magnetite-wiistitebuffer). The absenceof fayalite in the Quincy granite thereforesuggests either fluid pressuresbelow 700 bars or oxygen fugacitieslower than those definedby the magnetite-wiistite buffer, or, most likely, both. In summary, the occurrenceof quartz, aegirine, and aenigmatite, and absenceof fayalite in the Quincy granite suggeststemperatures of 650-750'C, fluid pressuresbelow 700 bars, and oxygen fugacities lower than those defined by the magnetite-wiistite buffer. The associationof riebeckiteand magnetitein the Quincy (Warren, 1913) suggestsa changeto higher oxygen fugacitiesas crystallizationcon- tinued at lowertemperatures. Ernst (1968) suggestedthat the temperatureof crystallization of riebeckitein the pegmatitephase of the Quincy granite was 500 i 50'C. In the Quincy granite proper,the associationof riebeckitewith alkali feldspars,which probably crystallizedabove the solvus (660"C) in the systemAb-Or-HzO (Tuitle and Bowen,1958), indicates a temperature of riebeckitecrystallization perhaps as high as 700oC.The pegmatite riebeckite (Warren and Palache, 1911) has significantly lower concentrationsof Ca, Al, and F when comparedwith the Quincy 1410 PAAL C. LYONS riebeckiteof the main body. Ernst (personalcommunication, 1971) suggeststhat the high concentrationsof these constituentsin the Quincy riebeckitewould probably stabilize this amphiboleto near- solidusma gmatic temperatures. Phase relations of hastingsite minerals have not been determined (Ernst, 1968).However, Ernst (1968)suggests that the P-?relations of the ferrohastingsitesare probably similar to ferropargasite,but with a more extensivef 6,-T stability ra,nge.The ferrohastingsiteof the Beverly syenite, which is associatedwith the Peabodygranite, has inclusionsof euhedralmicroperthite crystals (Toulmin, 1964),suggesting crystallization of the amphibole above 660oC. Tuttle and Bowen (1958) have shown that the Peabody ferrohastingsite (presumably their Quincy granite riebeckite) is unstable in the presenceof excesswater at a temperature of 625oC. This amphibole may be stable to higher temperatures at lower water fugacities, as suggestedby the above mineral relations for the Beverly syenite. The soda/potash ratios of the microcline microperthitesof the Quincy and Peabodygranites are nearly identical (Lyons and Wolfe, 1971).These ratios and their similar feldspar Ca content (chemical data of Toulmin, 1964;optical work of Warren, 1913)indicate a close temperaturecorrespondence between the two granites.The feldspars indicatecrystalization above 660'C. Dnvnr,oplrpNror OxrprzrNcCoNnrrroxs The factorsthat led to a developmentof more oxidizingconditions in the Quincy magma,as comparedto the Peabodymagma, are a mat- ter of speculation.However, a clue is given in the iron contentsof the granite porphyries associatedwith the Quincy granite in the Blue Hills area. Warren (1913)showed that the normal graniteporphyry has a dif- ferent Fe3./Fe2*ratio as comparedto the porphyries near rhyolite contacts.This is shownbelow: NormaI Granite Contact Contact Porphyry Porphyry Porphyry

wt. percent Fe3* T.I7 1.53 1.87 wt. percent Fe'* 1.63 1.33 1.18

Total Fes*, Fe2* 2.80 2.86 3.05 Note that the total iron contents in the three rocks are similar. However, in the normal granite porphyry Fe'* is dominant, whbreasin RIEBECKITE IN GRANITES l4Ll

CoNcr,usroms The chemistry of their alkali feldsparsand the amphibolephase cquilibria data suggestthat the euincy and peabodymagmas crystallized at similar temperatures,probably between 65G-7b0"c. Both amphiboles,riebeckite and ferrohastingsite,apparently crystallized at magmaticor near-magmatictemperatures. The peabody chemicalcompositions of the and euincy amphibores were controlledby at least two variables: compositionof the parent magma and oxygen fugacity. Composition favored a calcium_iron_

oxygenfugacity with decreasein temperature.Buma and others (lgzl) have explainedthe Eu depletionin the euincy feldspar,as compared to the Peabodyfeldspar, as the result of higher oxidizing conditions in the Quincy, which preventedthe acceptanceof Eus* into the feldspar structure. Their conclusionabout oxygen fugacity agreeswith that of the writer, which is based on amphiboleand rock chemical compositions.

Acrxowr,nn

Rnrnnuwcrs Brr,r,rNcs, (1956) M. P. The geologyof New Hamsphire,part II, Bedrock geology. N.H. SLaIePIann. Deuelop. Comm.,203 n. Bonr,nr, G. D. (196g) Amphibores from the younger granites of Nigeria, part l, chemical classification. Mineral Mag. 33, 85g_826. eNr M. T. Fnosr (lg63) Some observations on igneous ferrohastingsites. Mi,neral. Mag. 33, €/:6-fl62. l4L2 PAAL C. LYONS

BvnorNcrolr, A. F. (1948) Origin of granitic rocks of the Northwest Adirondacks. Geol. Soc.Amer. Mem,28,2l-43. -, aro B. F. Lnorvenr (1953) Chemical petrology and mineralogy of horn- blendes in northwest Adirondack granitic rocks. Amer. Mitueral. 33, 891-902. Buivre; G., F. A. Fnsv, eNr D. R. WoNrs (1971) New England granites: Trace element evidence regarding their origin and differentiation. Contri'b. Mineral. Petrologg 31,300-320. Enxsr, W. G' (1962) Synthesis, stability relations, and occurrence of riebeckite and riebeckite-arfvedsonite solid solutions. J. GeoI.70' 689-736. - (196s), Amphi.boles, Springer-Verlag, New York, 125 p. Eucsrnn, I[. P., aNn D. R. WoNns (1962) Stability relations of the ferruginous biotite, annite. ./. P etro\ogg, 3, 82-125. Ifuewc, w. T. (1958) Riebeckite granite in the wichita Mountains, oklahoma. GeoI. 9oc.Arner. Bull.69, 1191-1192. LyoNs, P. C., lNo C. Wnoo Womn (1971) Correlation of granites by soda/potash ' ratios. GeoI. Soc.Amer. BuII.82,2n23-2026. Tour.rvrnr, P. 3d (1964) Bedrock geology of the Salem Quadrangle and vicinity, Massachusetts.U.S. Geol, Stnu. BuIl. 1163-4' 79 p. Turrr,n, O. F., eNo N. L. Bownw (1958) Origin of granite in light o{ experimental studies in the system NaAISLOTKAISiTO-SiO-H,O, GeoI. Soc. Amer' Mem' 74,LB p. WannnN, C. H. (1913) Petrology of the alkali-granites and porphyries of Quincy and the Blue Hills, Massachusetts, U.S.A. Amer- Acad- Arts Sci., Proc, 49, 203-331. WannnN, C. I[., aNr C' Par,ecun (1911) The pegmatites of the riebeckite-aegerite granite of Quincy, Mass., U.S.A.: Their structure, minerals, and origin- Amer' Acad,. Arts Sci., Proc. 47,l2Fl68.

Momucri,pt receiued,,Jo:rtuarg I/1, 19/2; accepted lor publicati'on, March gl, 1912,