American Mineralogist, Volume 76, pages 942-955, 1991

Petrology of orthoamphibole-cordieritegneisses from the Orijiirvi aire , southwestFinland

Jnr, S. ScnNnronnulN Departmentof Geology,Pomona College, Claremont, California 91711, U.S.A. Ronnnr J. Tnq,cv Department of Geological Sciences,Virginia Polytechnic Instirute and State University, Blacksburg,Viryinia 24061, U.S.A.

Ansrru.cr In this paper we report comprehensivedata on mineral assemblagesand mineral chem- istry for 34 orthoamphibolite gneissesfrom Orijiirvi, Triiskbdle, and Perniii in south- western Finland, classic areas first reported on by Eskola 75 years ago. In addition we present an analysis of phaserelationships in these samples.Most of the protoliths for our samples are apparent altered mafic volcanics of Archean age. By far the most common assemblageis quartz + plagioclase * cordierite + + + ilmenite, although we observed five samples containing coexisting anthophyllite and gedrite and severalcontaining almandine-rich garnet. Temperaturesestimated from garnet-biotite are 550-600 oCfor the samples,and pressureestimated by other authors is about 3 kbar. The metamorphism of theserocks was thereforedifferent from two other well-studied orthoam- phibolite localities: about 50 'C higher I and similar P to the Post Pond Volcanics of Vermont, and 50 "C lower Z and several kbar lower P than the Ammonoosuc Volcanics in southwesternNew Hampshire. Analysesof coexistingcordierite and orthoamphibolesindicate that the sameassemblage occurs for a wide range in composition of these minerals, particularly Mg/@g + Fe). Although we cannot rule out systematicvariations in such variables as f, and pHrO, we believe that the assemblagesare of high variance and therefore the mineral compositions are controlled by bulk composition. The few sampleswe found that had coexisting with cordierite and anthophyllite or gedrite provided clues to the behavior of this mineral. Cummingtonite and cordierite have reactedin several samplesto form thin rims of gedrite in a texturally late, and probably prograde, reaction whose progressmust be dependent upon pNaAlOr. An unusual texture found in several sampleswas the development of armored alumi- nous enclavesin which the highly aluminous minerals corundum, spinel, and hdgbomite (along with magnetite) are separatedfrom host gedrite by a moat of cordierite. These evenly spaced enclaves may represent compositional heterogeneitiesinherited from the protolith, which have evolved texturally during metamorphism. What is particularly in- teresting about these enclavesis an abrupt compositional change in adjacent gedrite in which Al content rises dramatically at enclave margins, whereasMg and Si contents drop concurrently. The compositional change reflects coupled lattice difftrsion of Mg and Si toward the enclave (to grow cordierite rims) and Al out of the enclave on a scaleof up to 200 pm. Fe does not appear to have participated in this mass transfer.

INtnooucrroN 1950;Tilley, 1937;Floyd, 1965;Vallance, 1967;Grant, I 968; Froese,I 969; de Rosen-Spence,19691- Chinner and Cordierite-anthophylliteand cordierite-gedriterocks are Fox, 19741'James et al., 1978; Schermerhorn, 19781Ka- chemically unusual rock types characterizedby enrich- mineni, 1979;Franklin et al., 1981;Robinson et al., 1982; ment in Mg, Al, and Ti and by moderate to extreme HudsonandHarte, 1985;Reinhardt, lgST;Schumacher, depletion in alkalis and Ca. Unlike many other meta- 1988). Hypotheses forthe origin of cordierite-orthoam- morphic rocks, they cannot be linked simply to one kind phibole rocks include (l) contemporaneous metamor- of protolith, for example as pelitic schists to shalesand phism and metasomatism (Eskola, l9l4; Tllley, 1937; amphibolites to mafic volcanics. The origin of protoliths Floyd, 1965);(2) a residuum from partial melting (Grant, forcordierite-orthoamphibolerocksisaproblemthathas 1968; Hoffer and Grant, 1980);and (3) chemical alter- long been recognizedby metamorphic petrologistsand is ation of the protolith, followed by metamorphism (Tilley the basisfor numerouspapers (e.g., Eskola, 1914, 1915, and Flett, 1929;Tuominen and Mikkola, 1950;Vallance, 0003-004x/9l /0506-0942$02.00 942 SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE.CORDIERITE 943

1967;Froese,19691, Chinner and Fox, 1974;Schermer- horn, 1978;Schumacher, 1988). Studies of cordierite-orthoamphibole rocks have been inspired, at least in part, by their common association with massive Fe-Cu-Zn-Pb sulfide deposits,as well as by the enigma of production of their protoliths. Occurrences of cordierite-orthoamphibole have a wide geographicdis- tribution, for example,in Canada(de Rosen-Spence,1969; Froese,19691- Lal and Moorhouse, I 969; Kamineni, I 975, 1979;James et a1.,1978), in New England(Robinson and Jaffe,1969a;- Robinson et al., l97l1, Schumacher,1983; Spear, 1980), Cornwall (Tilley and Flett, 1929 Tllley, 1937;Chinner and Fox, 1974),Australia (Vallance, 1967), Norway (Stout, 1972), and India (Sharma and McRae, 1981).The classiclocality for the study ofcordierite-or- thoamphibole rocks, however, is in southwesternFinland in the vicinity of Orijiirvi. Fig. l. Generalizedgeologic and locationmap of the Trisk- The rocks of Orijiirvi were made famous in the early bijle-Orijiirvi area,southwestern Finland, from Eskola(1914), gneiss 1900s by Eskola's studies of them in the formulation of showingthe relationshipof intrusivegranite to the and which orthoamphibolitesamples the metamorphic facies concept and the study of meta- the amphibolitebelt from the werecollected. somatism(Eskola, 1914, 1915, 1950). Tuominen (1951) criticized the idea that metasomaticprocesses could have produced the unusual rock compositions observedat Ori- jiirvi, and Schermerhorn(1978) reexaminedthe role of in these rocks and show how our data confirm or refute metasomatismin the origin of theserocks. Other research them. In addition, we will discussin detail the formation in the Orijiirvi region has concentrated on the regional of aluminous enclavescontaining corundum, spinel, and structure(Tuominen, 1957; Bleekerand Westra, 1987), htigbomite in a gedrite gneissfrom the Pernid area.These the synorogenicintrusive granitesincluding thosein whose enclavesformed in a diffusion processthat produced sig- contact aureolethe cordierite-orthoamphibolegneisses are nificant chemical zoning in gedrite and that may have developed (Mikkola, 1955; Tuominen, 1961, 1966a, important implications for the diffusional behavior of Al 1966b), and the massive sulfide ore deposits (Varma, in metamorphic rocks. 1954; I-,atvalahti, 1979). This paper presents the first modern systematicpetrologic investigation of the cordi- Gnor-ocrc sETTTNG erite-orthoamphibole rocks, in particular a microprobe The Orijiirvi area(Fig. l) is located in the Svecofennian study of mineral compositions to test the topological hy- supracrustal belt, which is approximately 1800 m.y. in pothesesmade by Eskola regarding the stability of vari- age (Simonen, 1980). Trending roughly east-westacross ous -bearing assemblages. southern Finland, the belt consistsof varying proportions In this report, we present data on the petrography and of metamorphosed sedimentary and volcanic rocks. In mineral chemistry of orthoamphibole and clinoamphi- the Orijarvi vicinity, the lower Svecofenniangroup reaches bole, as well as other minerals, from a large sampling of a maximum thicknessof 3000 m and is composedmostly orthoamphibolites from Orijiirvi, Perni6, and Triiskbdle. of felsic volcanics, primarily pyroclasticsand lavas, with The study was initially undertaken to test the hypothesis some agglomeratesand tuffs. Limestones, forma- of Froese(1969) that there might be systematicreduction tions, and arenaceoussediments occur as thin interbeds. in Fe/(Fe + Mg) of ferromagnesianminerals, including The middle Svecofenniangroup is 500-1000 m thick and amphibole, in the vicinity of metamorphosed massive consistsof intermediate and mafic pyroclasticsand lavas sulfide deposits causedby sulfidation reactions (seealso with minor carbonate layers. The upper Svecofennian Nesbitt and Essene,1983). We did not find such system- group, less than 3000 m thickness, is composed of are- atic variations, but concluded that a reexamination of naceous and aryillaceous sediments that are now phyl- Eskola's classic locality using modern techniques was lites, mica schists,and gneisses.A detailed description of worth reporting. the stratigraphy can be found in Mikkola (1955). We will therefore (l) describe the bulk compositions Supracrustalrocks of the Orijiirvi region were folded, that can produce the assemblagecordierite * orthoam- regionally metamorphosed,and intruded by granodiorite phibole + biotite so common in southwesternFinland; during the Svecokarelianorogeny (1800-1900 Ma; Si- (2) discussthe substitution mechanismsthat have oper- monen, 1980).The first phaseof folding, F', produced ated in the orthoamphibole; (3) report the element par- isoclinal folds with axesplunging gently to the east and a titioning between coexisting cordierite, orthoamphibole, prominent foliation in all rock types. The secondgener- biotite, and garnet; and (4) review Eskola's observations ation of folds, Fr, is also isoclinal but with subvertical and predictions of the chemography of the assemblages axesplunging steeplyto the southwest(Tuominen, 1957). 944 SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE

in the porphyroblasts.Orthoamphibole, most commonly anthophyllite and more rarely gedrite, occurs intergrown with cummingtonite and is observed on the weathered surfaceas long prismatic bladesor as fanlike bunches.At Perni6, the texture ofthese rocks is developed spectacu- larly into arrays of amphibole florettes up to 9 cm in diameter embeddedin dark purple cordierite. Mineral assemblagesobserved in the orthoamphibole gneissesare given in Table l. The most common assem- blageis quafiz + plagioclase* anthophyllite + cordierite + biotite * ilmenite. About one-quarter of the samples have gedrite in addition to, or in place of, anthophyllite and l0o/o have garnet in addition to cordierite. Cum- mingtonite occurs as an accessorymineral in some sam- ples. Six samples contained coexisting orthoamphiboles Fig. 2. Detailedlithologic map of the areabetween Lakes (OR-26A, OR-27D, OR-40B, OR-40C, PN-3C, and Orijiirvi and Miiiirijarvi in the vicinity of the Orijiirvi massive 2l 8-B), and four samplescontained coexistingcumming- sulfidemine, showingthe locationsof the OR-prefixsamples tonite and anthophyllite (OR-26A, OR-26B, OR-40C, and (Table l) which comefrom the mine area(shown by crossed 215). In only two samplesdid gedrite coexist with cum- pickssymbol). Base map courtesy of Heiki Puustajiirvi(personal mingtonite (OR-40C and OR-26A). Magrretite,rutile, and communication). pyrrhotite are relatively common minor constituents, whereas corundum, spinel, and h6gbomite have a very The rocks were regionally metamorphosed to the am- restricted occurrence in aluminous enclaves in gedrite phibolite facies at P-Z conditions estimated to be 3 kbar gneissesfrom Perniti and Triiskbdle. and 650 (Latvalahti, 1979). "C Synorogenicintrusion of In thin section,orthoamphiboles most commonly have granodiorite causedcontact metamorphism at the same a highly elongated,bladed appearance,and may occur in or slightly lower P- Z conditions (Eskota, I 9 I 5). The cor- radiating sprays or even in florettes as described above dierite-orthoamphibole rocks formed in por- the lower (Fig. 3). Anthophyllite shows a grcat variation in color, tion of the lower Svecofenniangroup in the contact au- with the more Fe-rich grains being a considerablydarker reole of the large synorogenic intrusion. gray-brown than the Mg-rich ones, which are essentially The Orijiirvi area lies in southwestern Finland near colorless.Most gedrite also has pronounced color and is 60"13'N and 23"32'E. Orijiirvi and Pernitj sampleswere pleochroic from gray-blue to gray-brown. It may be dis- collected from the Orijiirvi mining region in Finland dur- tinguished from coexisting anthophyllite by its darker ing the summer of 1980 by J.S.S.She collectedsamples color. Coexistinganthophyllite + gedrite showsextensive from an area approximately 3 km by 2 km; locations are but uneven distribution of fine microscopic exsolution shown in the large-scalegeologic map of Figure 2. In lamellae, as has been reported for other occurrences addition, Triiskb6le were samples collected by P. M. Or- (Spear, 1980). Schumacherand Czank (1987) have re- ville in 1967 and were obtained from his collections at ported lamellae severalthousand angstromswide of ches- Yale University. terite and jimthompsonite in anthophyllites from Orijiir- vi, PnrnocnapHy AND MTNERALcHEMrsrRy based on HRTEM studies. Cummingtonite may be very difficult to distinguish from anthophyllite if the ori- Mineral assemblages entation of a grain is not appropriate to show the non- The dominant metamorphic rocks of the supracrustal parallel extinction. Small blades of cummingtonite are belt are fine-grainedgneisses (leptites) and amphibolites. commonly intergrown with sprays of anthophyllite in two- The gneissesare composed of quartz and feldsparswith amphibole rocks or may even occur as thin rims on an- subordinate amounts of muscovite, biotite, cordierite, thophyllite. Where cummingtonite is present, it cannot hornblende, and Fe-rich garnet.The amphibolites consist be demonstratedto be in contact with cordierite. In fact, mostly of hornblende, cummingtonite, and plagioclase in one sample (OR-26A), amphibole crystalswithin large (about Anro),with lesserbiotite and ilmenite. Of greatest poikiloblastic cordierite have cummingtonite cores and interest in this study are the orthoamphibolite gneisses gedrite rims, and only the gedrite is in contact with cor- that occur with skarns and sulfide ores in the contact dierite (Fig. a). This clear reaction texture suggeststhat aureoleof the granodiorite. the lack of coexistenceof cummingtonite * cordierite Recognition of the cordierite-orthoamphibole rocks in may be a late metamorphic or even retrogradeeffect, and the field is facilitated by their characteristic appearance. that coexistenceof cummingtonite + cordierite may be Elongated anhedral porphyroblasts of blue-gray cordier- possible under some metamorphic conditions. This con- ite up to 5 by 8 cm are unevenly distributed on the rock trastswith Eskola's(1914) assertionthat theseminerals surface and protrude prominently to form a knotty tex- never coexist and constitutesimportant evidenceregard- ture. Quartz, orthoamphibole, and biotite are intergrown ing phaserelations, to be discussedbelow. SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE 945

Fig. 3. Photomicrographshowing spray of coarseantho- Fig. 4. Photomicrographof sampleOR-26A showingreac- phylliteblades in fine-grainedmatrix ofbiotite, qtartz,and pla- tion of cummingtoniteagainst enclosing cordierite to form re- gioclase.Horizontal field of view is 2.4 mm. actionrim of gedrite.Note the very sharpboundary between cummingtoniteand gedrite.Horizontal field of view is 1.2mm. Cordierite is readily identified in thin section by its typical spongy, inclusion-filled appearanceand by the ples,however (e.g., 218, 336),cordierite rinds of uniform common pleochroic haloes around tiny included mona- thickness armor aluminous enclaves containing corun- zites. Polysynthetic twinning at 60' anglesoccurs but is dum, spinel, and htigbomite and separate these highly not common. Cordierite is ubiquitous in orthoamphibo- aluminous phases from surrounding orthoamphibole. lites, but generallydoes not show any particular textural These enclaves form a texture of special interest in the relationship to orthoamphibole blades. In several sam- orthoamphibolites and will be describedin detail below.

TABLE1, Mineralassemblages and selected compositional data

GED CUM CRD' BT- GRT- PL* orz MAG PO SPL OR-78 X 0.278 0.406 30 X X OR-84 X 0.265 0.375 31 X X OR-gA X 0188 0.243 X X X OR-9E X 0.190 0.278 20 X X X oR-124 X 0.173 0.220 X X X oR-128 X 0.368 0.492 0.838 X X oR-15 X 0.279 0.365 85 X X oR-18B X 0.200 0.254 45 x x oR-254 X 0.348 0.468 X oR-26A X XX 0.340 0.518 21 X X oR-268 X X 0.365 28 X x oR-27D X X 0.306 0.461 15 X oR-288 X 0.318 0.457 X X oR-31C X 0.327 0.430 X X oR-358 X 0.237 0.320 90 X oR-37 X 0.210 0.275 X X oR-40B X X 0.365 0.518 50 X x oR-40c X XX 0.370 0.515 48 X X oR-41 X 0.248 0.377 X X X oR-42C X 0.269 0.403 X X PN.1A X 0.228 0.29s X X x PN-2B X 0.333 0.850 X X PN-3A X 0.307 0.420 0.807 X X X PN-3B X 0.300 0.392 0.795 X X X PN-3C X 0.240 0.325 X 213 X 0.210 0.282 x X 215 x 0.225 0.341 X X X 216 x 0.240 0.328 X X x 216-8 x 0.256 0.336 36 X X x 218-A X 0.180 0.212 X XX XX 218-B X X 0.186 0.209 3 X XX XX 336 X 0.145 X XX 343 X on X 343-A X 0.266 0.777 XX X 'Fe/(Fe + Mg). -. Mol% An. 946 SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE

3.0 beam current, and l-2 pm spot size. Simple oxides and silicates were used as standards,and data were reduced using standard matrix corrections. Selectedsamples were chosen at random for WDS analysis on an ARL SEMQ 2.0 microprobe at Virginia Tech to confirm the earlier anal- 343 o \ 218_A yses,and excellent agreementwas found. The composi- tions reported in the tables are not averages,but rather are single analysestaken from groups of data for each 1.0 sample. Each reported composition is representativeof

I that mineral in that sample.Structural formulas were cal- +L. culated on an anhydrous basis assuming 23 O atoms for tt I amphibole, ll O atoms for biotite, and l8 O atoms for 0.0 cordierite. 0.3 0.4 0.5 0.6 0.7 Representativeanalyses of orthoamphiboles and cum- Mg/(Mg+Fe+Mn) mingtonite are presentedin Table 2. The names antho- phyllite and gedrite have been used to describevarieties Fig. 5. Plot of Al,o, content vs. Mg/(Mg + Fe + Mn) for of orthoamphiboles with A-site plus ratAlof less than or anthophyllite (squares)and gedrite (triangles). Coexisting or- greaterthan 1.0, respectively,following the suggestedno- thoamphibole pairs are indicated by tie lines. Note the trend menclatureof Leake (1978). Ideal end-membermagne- toward decreasedA1.", with increased Mg/@g + Fe + Mn). sioanthophyllite is taken to be Mg,SirO.,(OH),, and ideal Three amphibole samples(two gedrite, one anthophyllite) that gedrite to be Naor(Mg,Fe)rrAl,5Si6AlrOrr(OH), (Robin- fall within a broadly gap (see defined orthoamphibole miscibility sonet al., l97l). text) are indicated. In assemblagesof anthophyllite + cordierite from Ori- jiirvi and Triiskbtile, the anthophyllite contains 0-4 wto/o Microprobe analyses AlrO. and its ratio Mg/(Mg + Fe) varies from 0.45 to Polished thin sections of all samples listed in Table I 0.70. Anthophyllite from the Pernid locality contains 2.5- were analyzed using the electron microprobe. Analyses 7 wto/oAlrO, and has Mg/(Mg * Fe) varying from 0.52 were made on a Cameca MS-64 equipped with Tracor to 0.62. There is an apparent subtle inverse relationship Northern TN-2000 EDS system at the Department of between the AlrO. content and Mg/(Mg + Fe) for both Geology and Geophysics, Yale University. Operating orthoamphiboles which can be seenin Figure 5. In par- conditions included 15 kV accelerating potential, 5 nA ticular, anthophyllite showsa tendencytoward increased

Tlaue 2. Representativemicroprobe analyses of amphibole

oR-8A OR-l2A OR-128 OR-26A oR-26A oR-26A OR-268 OR-268 OR-27D OR-27D OR-408 A-AAA u ACAGA sio, 52.10 55.54 49.79 52.74 42.94 52.31 53.24 54.61 51.76 42.61 51.06 Tio, 0.16 0 0.12 0 0.46 0 00 0 0.47 0 Alro3 2.83 0.31 3.24 0.90 14.64 0.71 1.88 0.67 3.00 15.50 2.28 FeO 24.13 18.54 29.66 28.94 28.82 29.31 22.86 22.40 26.54 25.87 29.25 MnO 0,50 0.35 0.43 0.26 0.41 0.16 0.19 0.67 0.19 0.54 0.49 Mgo 17.52 22.85 13.59 15.43 10.54 1s.64 19.01 19.34 15.99 11.64 15.24 CaO 0.16 0.19 0 0.13 0.20 0 0.22 0.30 0.24 0.14 0.28 Naro 0.44 0.43 0.71 0.10 1.26 0 0.57 0.32 0.53 1.71 0.40 KrO 000 000 00 0 0 0.'t2 Total 97.85 98.21 97.53 98.50 99.27 98.13 97.97 98.31 98.25 98.48 99.11 23 O atomtormulas Si 7.643 7.880 7.555 7.844 6.424 7.82',1 7.740 7.892 7.642 6.340 7.601 AI 0.357 0.051 0.445 0.156 1.576 0.125 0.260 0.108 0.358 1.660 0.399 Total 8.000 7.931 8.000 7.950 8.000 7.946 8.000 8.000 8.000 8.000 8.000 AI 0.129 0 0 134 0.003 1.006 0 0.063 0.006 0.164 1.058 0.001 Ti 0.018 0 0.14 000 00 0 0.052 0 Fe 2.960 2.200 3.762 3.599 3.606 3.664 2.779 2.707 3.276 3.219 3.641 Mn 0.062 0.042 0.056 0.032 0.054 0.020 0.023 0.082 o.o24 0.069 0.062 Mg 3.835 4.835 3.073 3.421 2.35t 3.485 4.120 4.165 3.521 2.583 3.382 Ca 0.024 0.029 0 0.020 0.033 0 0.034 0.047 0.037 0.022 o.044 Total 7.028 7.106 7.039 7.O75 7.050 7.169 7.019 7.007 7.022 7.003 7.130 Na 0.126 0.115 0.208 0.028 0.367 0 0.159 0.088 0.150 0.493 0.116 K 000 000 00 0 0 0.024 Total 0 126 0.115 0.208 0.028 0.367 0 0.159 0.088 0.150 0.493 0.140 Fe(Fe + Mg) 0.436 0.313 0.550 0.513 0.60s 0.512 0.403 0.394 0.482 0.555 0.518 - A : anthophyllite;G : gedrite; C : cummingtonite. SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE 947

A1,",with decreasingM/(Mg * Fe), although a similar, 0.5 though less clearly defined, tendency also occurs in the gedrite. In the assemblagesof gedrite + cordierite from 0.4 all three sample areas,gedrite samplesvary from 9 to 17 wto/oAlrOr, with the bulk of the gedrite samplesbetween 13 and 16 wo/o and their ratios of Mg/(Mg * Fe) range o 0.3 = from 0.38 to 0.70.Stout (1972,p. ll0) noted a similar a pattem in anthophyllite from Telemark, Norway, in which I 0.2 anthophyllite with less than 2.0 Fe atoms per formula unit had no 16rAl,whereas those with greater Fe showed a strong positive correlation between Fe and t6tAl. 0.1 Other minerals analyzed as part of the study include biotite, cordierite, gamet, and plagioclase.Representative 0.0 biotite analysesare given in Table 3, and summary chem- 0.0 0.5 1.0 1.5 2.0 ical data for all four ferromagnesianminerals and plagio- 14IAI clase are given in Table l. Data for minerals in the alu- minous enclavesare in a separatesection below. Fig. 6. Plot illustratingthe relationofA-site occupancyand IorAlfor anthophyllite (squares)and gedrite (triangles)'Antho- Element distributions phyllite-gedritepairs are shown by tie lines,and three amphibole samplesthat fall within the miscibilitygap are indicated. Note important Figures 5 and 6 summarize compositional that the anthophyllitesamples scatter around a line ofslope0.5, featuresof the orthoamphiboles using plots suggestedby suggestinga relatively regular ratio of edeniteto tschermakite previous studies (Robinson et al., l97l; Spear, 1982; substitutionsamong the gloup. Robinson et al., 1982).Figure 5 illustrates the character- istic enrichment of Fe in gedrite compared to coexisting anthophyllite. The Fe-Mg distribution is quite similar to stabilization of cordierite in the bulk compositions with that found by Spear(1980, 1982).Note also,as pointed lower Mg/(Mg + Fe). Lower modal cordierite would re- out above, the trends ofincreasing Al", with decreasing sult in more Al being available for incorporation into the Mg/(Mg + Fe) in both anthophyllite and gedrite. Spear clihoamphiboles. (1982) did not observe this effect for orthoamphiboles The data in Figure 6 for orthoamphibole pairs also with a wide range of composition from Vermont. The nicely illustrate the gedrite-anthophyllite miscibility gap chemical trends are likely causedby the progressivede- (Spear,1980;Robinson et al., 1982).Despite the limited

Taete2-Continued

oR-408 OR-40C oR-40c oR-40c PN-28 PN-3C PN-3C 215 215 218-8 218-8 336 343-A GA GCA A GAC AG GG 40.92 50.99 44.36 51.97 50.89 50.51 42.58 52.31 51.55 53.57 46.10 46.99 42.40 o 24 0.52 00 000.41 0 00 0.28 0 0 16.55 3.06 12.54 1.42 4.26 5.59 16.33 3.70 2.79 3.21 13.41 13.39 15.80 27.74 27.86 28.32 29_15 26.19 21.96 22.72 22.22 22.42 19.71 18.30 17.03 24.12 0.68 0.43 0.26 0.41 0.32 0.40 0.48 0.78 0.50 0 0 0.23 0.33 10.04 14.54 10.93 15.01 15.66 18.65 13.75 19.21 19.41 21.59 18.49 19.99 13.62 0.48 0.024 0.50 0.3s o.32 0.41 0.44 0.18 0.35 0.14 o.21 0.19 0.30 1.50 0.031 1.00 0 0.87 1.01 1.70 0.55 0 0.61 1.61 0.90 1.56 00 00 0 0.12 0 0 00 000 98.15 97.95 97.91 98.31 98.51 98.65 98.41 98.96 97.02 98.88 98.40 98.71 98.13 23 O atomformulas 6.186 7.612 6.687 7.764 7.503 7.311 6.252 7.531 7.576 7.594 6.584 6.632 6.277 1.814 0.288 1.313 0.236 0.497 0.689 1.748 0.469 0.424 0.406 1.416 1.368 1.723 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 8.000 1.135 0.151 0.915 0.013 0.244 0.265 1.077 0.159 0.059 0.129 0.843 0.859 1.033 0.026 0.059 00 0 0 0.045 0 00 0.029 0 0 3.506 3.478 3.570 3.642 3.228 2.658 2.788 2.675 2.755 2.336 2.185 2.010 2.986 0.086 0.054 0.033 0.051 0.039 0.049 0.060 0.095 0.062 0 0 0,027 0.041 2.263 3.238 2.457 3 343 3.441 4.025 3.010 4.122 4.252 4.564 3.938 4.206 3.008 0.078 0.038 0.080 0.055 0.050 0.063 0.069 0.28 0.056 0.022 0.033 0.029 0.047 7.094 7.018 7.055 7.104 7.002 7.060 7.049 7.079 7.1U 7.051 7.028 7.131 7.115 0.439 0.087 0.291 0 0.248 0.282 0.482 0.153 0 0.169 0.446 0.247 0.447 00 00 0 o.022 00 00 000 0.439 0.087 0.291 0 0.248 0.304 0.482 0.153 0 0.169 0.446 0.247 0.447 0.608 0.518 0.592 0.521 0.484 0.398 0.481 0.402 0.393 0.339 0.357 0.323 0.498 948 SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE

0.5 A A 0.4 (E z 0.3 A E CL o U.C .= E o A o I 0.2 l1 o . r I ..lt q) 0.1 ll IL o) = o.o 0.0 0.0 0.2 0.4 0.6 0.8 1.0 E X Albite (PIag)

Fig. 7. Plot showingNa in the A-site vs. albite contentof plagioclase.Symbols are the sameas in Figures5 and 6. Note -0.5 that theNa contentof A-sitesin gedriteseems to correlatemore 0.5 1.0 1.5 2.0 stronglywith plagioclase compositionthan that of A-siteoccu- ln (Mg/Fe)- Cord pancyin anthophyllite,which seemsto be independentof the Na contentof coexistingplagioclase. Fig. 8. Plot showingthe distributionof Fe and Mg between orthoamphibolesand cordierite.Symbols are the sameas in the previousfigures. The numbers on thediagram indicate the slopes number of pairs, some inconsistencyin gap width can be of the least-squaresbest-fit lines through each set of data,and seenand probably reflects the range in temperature over thereforethe meanIl (Mg-Fe)(see text) for anthophyllite-cor- which these samplesequilibrated. In fact, three samples dierite(0.48) and gedrite-cordierite (0.36). from the higher-Z western end of the field area in Triisk- biile and Perni6, 343,218-A, and PN-3A, all contain one amphibole which falls well within the gap. We presume The garnets are sufficiently close to almandine-pyrope that these samples must have equilibrated at tempera- solid solutions to allow use of the garnet-biotite ther- tures above the crest of the gedrite-anthophyllite solvus mometer of Ferry and Spear(1978), with almandine and which Spear(1980) suggests is at about 600.C (+25"C). pyrope activities calculated using the model of Hodges We have used data on coexistinggarnet and biotite from and Crowley (1985).Calculated temperatures are 575 "C threesamples, OR-128, PN-3A, and PN-38. to calculate for PN-3A, 580'C for PN-3B, and 600'C for OR-12B. metamorphic temperaturesfor the Orijiirvi-Pernid area. These temperatures are reasonably consistent with data

Tmm 3. Representativemicroprobe analyses of biotite

oR-l28 oR-27D 218-B sio, 38.25 36.10 35.64 38.98 35.70 40.19 Tio, 1.54 1.34 1.68 1.05 1.91 0.72 Al2o3 18.54 18.06 18.28 16.40 17.43 15.35 FeO 15.46 19.83 19.27 14.18 21.48 10.02 MnO 0 0 o.25 0 0.26 0 Mgo 14.38 11.49 12.77 16.89 11.09 20.66 Na.O 0.65 0.54 0.72 0.80 0.50 0.95 KrO 8.02 8.23 7.99 7.77 7.94 7.82 Total 96.84 95.59 96.60 96.25 96.31 95.71 11 O atomformulas si 2.773 2.725 2.662 2.831 2.702 2.874 AI 1.227 1.275 1.338 1.169 1.298 1.126 Total 4.000 4.000 4.000 4.000 4.000 4.000 AI 0.357 0.332 0.271 0.235 o.429 0.168 Ti 0.084 0.076 0.095 0.057 0.109 0.038 Fe 0.937 1.252 1.203 0.861 1.359 0.600 Mn 0 0 0.015 0 0.016 0 Mg 1.554 1.294 1.442 1.828 1.25',1 2.202 Total 2.932 2.954 3.006 2.981 3.164 3.008 Na 0.091 0.080 0.105 0.112 o.o74 0.132 K 0.742 0.792 0.761 0.720 0.766 0.713 Total 0.833 o.872 0.866 0.832 0.840 0.845 Fe/(Fe + Mg) 0.376 o.492 0.458 0.320 0.521 o.2'14 SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE 949

1.0 -c 3OO CL I E l .9 o o (E Ka =o'tz,. O1 0.5 t, o I ;\ ! o lr aa Ko=o'o+ o Z\K. =0.61 lr I ao o) ct) 0.0 I =0 = I E c A -0.5 -1 -1 012 0.4 0.6 0.8 1.0 1.2 1.4 1.6 - In (Mg/Fe)- Biot ln (Mg/Fe) Cord

Fig. 9. Plot showingthe distributionof Fe and Mg between Fig. 10. Plot showingdistribution of Fe and Mg between orthoamphiboleand biotite. Symbolsare the sameas in the cordieriteand biotite. MeanIQ (Mg-Fe)is 0.64.The relatively previousdiagrams. Numbers on thediagram show the meanKo low scatteris an indication of the small temperaturerange of (Mg-Fe)for anthophyllite-biotite(0.77) and gedrite-biotite (0.6 1). equilibrationfor the samPles. Note that the scatteris slightly greaterin this diagramthan in Figure8, possiblybecause ofgreater degtee ofpostequilibration readjustmentin biotitecompositions. Fe-Mg distribution between ferromagnesianphases in the orthoamphibolites is displayed in Figures 8, 9, and 10, which are similar to plots usedby Spear(1982). The reported by Schreurs (1985) and with the observed or- straight lines have slopes of 1.0 and are drawn by eye thoamphibole compositions and are about 15-65 "C through each set of data. The vertical position of each higher than the Post Pond orthoamphibolites (Spear, line gives the mean rKu(Mg-Fe) for that mineral pair. The 1980) and about 50-75 oC lower than Amphibole Hill distribution of data in Figure 8 (orthoamphibole vs. cor- (Robinsonet al., l97l). dierite) is considerably more regular than that in Figure The relation of A-site occupancy to t4lAl is shown in 9 (orthoamphibole vs. biotite). This effectis undoubtedly Figure 6. For most samplesthere is a clear correlation of causedby a relative insensitivity of orthoamphibole-cor- A-site and latAl which reflects the operation of the eden- dierite K" (Mg-Fe) to changesin Icompared to orthoam- ite substitution superimposedon Tschermak's substitu- phibole-biotite Il (Mg-Fe). The mean K. (Me-Fe) values tion. The three higher-Z samples noted above not only for the Orijiirvi samplesare 0.36 (range: 0.34-0.39) for fall within the solvus limits, but also seem to be anom- gedrite-cordierite,0. 4 8 (0.4 3-0. 54) for anthophyllite-cor- alously low in A-site occupancy for the tatAl content. dierite, 0.61 (0.48-0.72) for gedrite-biotite, and 0.77 This indicates a predominanceof the Tschermak over the (0.61-0.98) for anthophyllite-biotite. Mean Ko (Mg-Fe) edenite substitution in these samples,which may be ei- for biotite-cordierite is 0.64 (see Fig. l0). These values ther causedby higher temperature or by low Na concen- are about l5-25o/o higher than similar ratios from the trations in the bulk rock. The general scatter ofthe data Post Pond volcanics, Vermont (Spear, 1982) and un- in Figure 6 is very likely causedby random differencesin doubtedly reflect higher metamorphic temperatures in Na content from rock to rock. Na in the A-site reflects Finland (about 550-600 "C) than in Vermont (535 "C). the extent of the edenite substitution, which should be controlled by the activity of NaAlO, in the rock and PrHsn RELATToNS therefore by the activity ofalbite in the plagioclasein a The assemblagesreported in this study are of consid- quartz-bearingrock. Figure 7 is a plot of Na in the A-site erable interest becauseof the wide range of bulk com- of anthophyllite and gedrite vs. the mole fraction of albite positions represented by the orthoamphibole-bearing in plagioclase.Despite the paucity of data, there appears rocks. Figure I I illustrates the chemographic relation- to be a positive correlation for gedrite, but virtually no ships of mineral compositions in Al-Fe-Mg subspaceof correlation, or perhaps a slight negative one, for antho- KNFMASH for the ferromagnesianphases anthophyllite, phyllite. There is a similarly poor correlation for or- gedrite, cummingtonite, cordierite, and garnet,but exclu- thoamphibole-plagioclasepairs reported by Spear( 1980), sive of biotite. It is clear that this diagram does not func- but the rangein plagioclasecomposition he observed(ap- tion well as a phase diagram because of significant proximately An,o-Anoo)is considerably less than that of amounts of unrepresentedcomponents in several of the the Finnish samples(An,o-Anro). phases,notably Ca and Mn in garnet, and Na in gedrite. 950 SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE

Mg Fe

Fig. 11. TernaryFe-Mg-Al diagram illustrating rhe chem- ographyofferromagnesian phases and assemblages (exclusive of biotite)in the Orijiirvi orthoamphibolites,based on microprobe analyses.Anthophyllite is indicatedby filled circles,gedrite by Cummingtonito triangles,cummingtonite by diamonds,and garnet by squares. Mgo

In addition, there may well be a range in temperature as large as 50 "C within the group of samplesplotted, but it is nonethelessinteresting to note the rather wide spread of mineral compositions within the assemblagescordi- erite + gedrite * anthophylliteand cordierite + antho- phyllite * garnet (+ gedrite). The similarly large varia- tion within cordierite + anthophyllite assemblagesis to be expectedfor a two-phaseassemblage even without avai- ation in intensive parameters.The spread in three-phase triangles may be great enough, however, to be taken as evidence for local variation in bulk composition (most assemblagesare not particularly low variance)or in pHrO from rock to rock. Ifthe assemblagesportrayed in Figure I I are related by reaction, the cordierite-gedrite-antho- phyllite triangles would be predicted to shift toward the Mg side of Figure I I with progressof a multivariant re- action such as anthophyllite * cordierite + ab compo- nent plag : gedrite. of The involvement of albite com- MgO FeO ponent is required by the substantial Na content of the gedrite. Fig. 12. (a) Chemographicrelarionships (projected from SiO, Schematicphase relations in the generalizedcomposi- and H,O) in orthoamphibolites from Orijarvi (Triiskbiile) based tion spaceFeO-MgO-AlrO, have been shown by Robin- on observationsofEskola ( I 9 I 4) as interpretedby Robinson and son and Jafe (1969a)and by Robinson et al. (1982) for Jaffe (1969a). (b) Generalizedchemographic relationships from a wide variety of metamorphic grades.Robinson and Jaffe the observations of this study, shown in the AlrOr-MgO-FeO- (1969a)suggested that the observationsofEskola (1914) NaAlO, tetrahedron projected from SiO, and HrO. Note es- pecially just required a chemography such as that shown in Figure the composition subspace behind the front face of the tetrahedron in which multivariant three-phase l2a. Of particular interest to them was the issue of the assemblages, cordierite + anthophyllite + gedrite define planes. relationship of anthophyllite, cordierite, and cumming- tonite. They suggestedthat becausecummingtonite was slightly more Mg rich than anthophyllite, cummingtonite although the paucity of cummingtonite-anthophyllite pairs + cordierite assemblageswould be limited to Mg-rich in our data set precludes a decisive statement regarding compositions and would be reactedaway by progressive cummingtonite relations. Our best choice of a composi- leftward movement, with increasinggrade, of the three- tion space for illustrating the phase relations is the tet- phasetriangle defined by cummingtonite + cordierite + rahedron AlrOr-FeO-MgO-NaAlOr, shown in Figure l2b. anthophyllite in Figure l2a. The relative abundance of anthophyllite-gedrite pairs re- Our observations suggesta slightly more complicated quires that both phases be shown explicitly. Most or- chemography than that of Robinson and Jaffe (1969a), thoamphibolite bulk compositions fall within the space SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE 951 outlined by the planes defined by cordierite * antho- , lF .:.. ."4 \ phyllite + gedrite, although the few garnet-bearingsam- T' .-\ \ ples within the four-phasevolume at the Fe-rich end : -Or- fall Hbm.r -7J\ - \ of this three-phasespace. -'Crn+Mag One observation of this study, which was noted above ? *+"" and which bears on the role of cummingtonite in the phaserelations, is illustrated in Figure 4. In severalsam- ples, cummingtonite grains within large, spongy cordier- '\r ite grains have reacted with cordierite to form rims of orthoamphibole, but in this caseit is gedrite, not antho- phyllite as cited for this generalreaction texture by Rob- inson et al. (1982). The thin gedrite reaction rim shown i; in Figure 5 has a very sharp boundary againstboth cum- mingtonite and cordierite. Cummingtonite in this texture t Qrd typically has less than 1 wto/oAlrO., whereasgedrite has enclavefrorn between 15 and 17 wto/o.Although the rimming textures Fig. 13. Photomicrographof a typicalaluminous field of view is 2.2 mm. Gedrite(Ged) suggesta simple reaction betweenhost and inclusions and sample336. Horizontal host enclosesoxide aggregate consisting of magnetite(opaque). the Al can be accountedfor in reaction betweencordierite aluminousspinel (Spl), hiigbomite (Hbm)' and magnetite-corun- and cummingtonite, the gedrite also contains substantial dum symplectite,all separatedfrom gedriteby a rim of cordi- Na (> 1.5 wto/o),which is not available in the local envi- erite (Crd). The bar in the lower right cornerof the photogaph ronment. The sourceof Na is presumably plagioclase,the indicatesthe positionofthe microprobetraverse in gedriteshown only major Na reservoir in the rock. Therefore the reac- in Figure14. tion of cummingtonite plus cordierite to gedrite, which is obviously a multivariant net-transferreaction in the sim- ple FMASH composition space, must actually be de- phase, in their explanation. They also observed a rich scribed in NFMASH. Although our samplesdo not allow variety of aluminous assemblagescontaining the above us to demonstratethe effect,we can predict that the pres- minerals as well as spinel, staurolite, calcic plagioclase' enceor absenceofgedrite reaction rims probably depends and rutile. Schumacher and Robinson (1987) reported upon the plagioclase composition in the rock, i.e., on sapphirine, in addition, and performed a detailed petro- pNaAlOr. Therefore, gedrite would be found in rocks with logic analysis of the enclave-forming process. sodic plagioclase,whereas, in the presenceof calcic pla- Eskola did not report observing corundum in the en- gioclase, cummingtonite plus cordierite would remain claves, but in our samples it is an essentialconstituent' stable. The limiting plagioclasecomposition presumably In fact, we observedno sillimanite or quartz in enclaves, shifts toward higher anorthite content with increasing only various mixtures of corundum, spinel, magnetite' metamorphic grade. chlorite, and hiigbomite. This latter aluminate mineral is commonly ascribed to being secondaryafter spinel, but gives every appearanceof being primary in our samples. Ar,uurNous ENcLAvES A photomicrograph of a typical enclave is shown in Fig- Petrography and mineral chemistry ure 13. In this particular sample (PMO 336), point count- Armored enclavescontaining moderately to very alu- ing reveals that the total rock comprises 72o/obyvolume minous minerals have been reported from relatively few of gedrite-richhost rock and 280/oenclaves, of which 15.80/o gedrite-cordierite gneisses,including Triiskbdle (Eskola, are oxide cores and 12.2o/oare chlorite-cordierite rims. 1914,p. 179-l8l), Ontario (Jameset a1.,1978), India The host rock is 98.2o/ogedite, l.5olocordierite, and 0.30/o (Lal et al., 1984),and the Italian Alps (Droop and Buch- magnetite. The oxide cores to the enclaves ate 34.60/o er-Nurminen, 1984),although the most detailed report is magnetite-corundum symplectite, 25.60/odiscrete mag- from southern New Hampshire (Robinson and Jaffe, netite,37 .7o/oaluminum spinel, and 2.lolohtigbomite. En- 1969b Schumacherand Robinson, 1987). The report by clave rims are 75.5o/o cordierite and 24.5o/ochlorite. Eskola is of single crystals or polycrystalline massesof Electron microprobe analysesindicate that the mag- sillimanite separatedfrom the enclosinganthophyllite (or netite and corundum are essentially stoichiometric, ex- gedrite) gneissby thin rims of cordierite. Robinson and cept that the magnetite has detectable but minor VrO' phases Jatre (1969b, p. al0-418) observedthis texture in the (<0.5 wto/o).Compositions of the other in the en- New Hampshire rocks, although with corundum as well clave are as follows: cordierite-Mgr ,,FeorrAl, onSi,o,O,rl - as aluminum silicate, and describedit as a step in a series spinel- M g r ruF e lluTiooi F e 3.5eA 1,, ., O.r, h

o Fe g o \ cDz x o (t) (\l o E1 \ o \ AI (Mg + Si)-10

0 20 40 60 80 100 120 Distance(pm) (Fe,Mg)O spiner Al2O3 Fig. 14. Gedritezoning profile for (Mg Al, + Si) and Fe Fig. 15. Ternary plot projectedfrom adjacentto the aluminousenclave from @e,Mg)O-Al,O.-SiO, sample336 shownin HrO showingthe phaserelations in aluminousorthoamphib- Figure 13. Note that (Mg + Si) has beenscaled so as to be olitesbefore and afterthe developmentof armoredaluminous consistentin magnitudewith Fe and Al. enclaves.The filled triangle indicates the approximate bulk com- positionof silica-undersaturatedgneisses containing gedrite + cordierite,such as sample336, that containcorundum before enclave margin, shown in Figure 14, demonstrates that enclaveformation. The reaction Ged + Crn : Crd + Spl + HrO the shift in Al is balanced by Mg and Si, in an almost causedthe formationof the enclaves(see text) and causedthe perfect Mg-Tschermak's substitution, with virtually no breakingof the tie line (dashed)from gedriteto corundumin changein Fe content along the traverse. favor ofthe new tie lines(solid), especially those from high-Al gedriteto cordierite,and from cordieriteto spinel,the latterof Genesisof Orijiirvi enclaves which isolatesgedrite from corundum.The line connectingthe The scenariowe envision for formation of the enclaves two gedritecompositions reflects the zoning shown in Figure14. starts with a coarsegedrite-rich gneisscontaining roughly evenly spacedintergrowths of magnetite and corundum, However, the above reaction does not fully explain the perhaps similar to the bits of symplectite left in the en- peculiar texture of the enclavesin which cordierite forms clave shown in Figure 13. In some gedrite gneisssamples, a complete rim and none of the oxides are in contact with we have seensuch magnetite-corundumintergrowths, but gedrite. The postreactionchemography of Figure I 5 only without any enclavedevelopment. We have no definitive precludesthe coexistenceof gedrite and corundum. explanation for the origin of these intergrowths, but be- In order to explain both the gedrite zoning (Fig. la) lieve they may represent original heterogeneitiesin the and the nucleation and growth of the cordierite rims, a altered volcanic protolith. Accepting the arguments of second processis required. This secondprocess is diffu- Robinson and Jaffe (1969b) that the enclave-forming re- sion, and, to model it, we defined an exchangecomponent actions occurredduring decompression,we arguethat un- MgSiAl_, that describesimperfectly but adequately the der conditions at, or just following, the peak of meta- shift in gedrite composition adjacent to enclaves(there is morphism, the coarsegedrite began to react with the oxide also a minor edenite exchangevector, NaAID_,Si , that intergrofihs. The assemblagecan be shown on a diagram we will ignore in this treatment). There is some crystal such as Figure 15, assumingthat magnetite was passive chemical justification for definition of a MgSiAl 2 com- and did not participate in the reaction. An approximate ponent in orthoamphiboles(see Stout, 1972). bulk composition for the rock is shown in the figure, as The initial net transfer reaction given above, in which is a representativegedrite composition (the gedrite point the volume of gedrite decreases(that is, in which the edges farther from the Al corner). The initial net transfer re- of the gedrite grains were partially resorbed),must have action would have been gedrite + corundum : cordierite enriched the remaining proximal gedrite in edenite com- * spinel + HrO. Note that the composition subspacein ponent becausethere was no local sink for Na. Edenite which reaction occurs is corundum saturatedrather than may therefore be consideredto have increasedin the re- quartz saturated, but that after the reaction the new tie sorbed gedrite rim as an inert marker, or Kirkendall, ef- line from spinel to cordierite isolatesthe bulk composition fect. Enrichment of tschermakite component in rim ged- from corundum. Figure 15 also shows the chemog- rite may also have been amplified in this manner by raphy following reaction, including the variable range of preferential consumption of Mg and Si and, to a much post reaction gedrite, shown as the bar, limited by the lesserextent, Fe, in production ofcordierite. The key to most Al-rich gedrite adjacentto the enclaves(see Fig. l4). this process is the inhibition of further nucleation or SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE 953 growth of spinel with continued resorption of gedrite in the readerto Schumacherand Robinson (1987) for a very reaction with corundum. Production of only cordierite at detailed generaltreatment of the processof enclave for- the enclave margins would have resulted in a significant mation. increaseof tschermakite component in the resorbedged- Finally, the substantial volume of chlorite found with rite rims. The resorption reaction therefore may have es- cordierite in the rim is almost certainly a secondaryhy- tablished a strong concentration gradient, and therefore dration of the cordierite, perhaps nearly contemporane- a negativechemical potential gradient, for the component ous with the formation of the cordierite itself. Note that MgSiAl , in the gedrite, resulting in net outward diffu- the Fel(Fe + Mg) of the chlorite and cordierite, as listed sional transfer of Al. above, are virtually identical. Localization of spinel in the interiors of enclavesaway from gedrite rims indicates that the spinel-producingnet SurvrlranY transfer reaction ceasedbefore the cordierite-producing We have examined the petrology of a number of cor- reaction. The relatively Fe-rich spinel produced by initial dierite-bearing orthoamphibolites from Eskola's (l9la) gedrite reaction with corundum must have nucleated on classiclocality of the Orijiirvi mine and adjacent areasin the preexistingaluminous minerals in the enclave,where- southwestern Finland. These rocks are similar to or- as the cordierite rim first nucleated on the aluminous thoamphibolites describedfrom other localities, notably minerals but then grew and thickened outward toward the well-describedlocalities from New England. A num- the gedrite, the source of Mg and Si. It is likely, though ber of samples contain relatively complex assemblages impossible to prove, that spinel nucleation difficulties, consistingof multiple ,cordierite, biotite, and coupledwith physical isolation ofpreexisting spinel grains garnet; we found several three-amphibole assemblages from the gedrite rims, kinetically retarded further spinel (anthophyllite, gedrite, and cummingtonite), as well as a production. Schumacherand Robinson (1987)proposed substantial number of anthophyllite + gedrite assem- a similar mechanism to explain the separationof high-Al blages.We have documented metamorphic conditions of minerals from gedrite by cordierite rims in the enclaves about 550-600'C for the Orijiirvi area that are in accord from southwestNew Hampshire. It is also likely that this with earlier studies and indicate that conditions are ap- coupled difusion process operated virtually contempo- proximately those of the crestof the orthoamphibole two- raneously with the net transfer reaction, rather than as a phaseregion (Robinson et al., 1982).Several of our sam- separate,Iater event. If the zoning in gedrite (Fig. la) ples from the higher temperaturepart of the area contain developed during peak metamorphism as a result of vol- single orthoamphiboles with compositions straddling the ume diffusion in a chemical potential gradient, then vir- gapsfound in the lower temperaturetwo-amphibole rocks. tually no Fe was transferred as compared to Mg, Si, and Phase relations in the cordierite-orthoamphibole Al. There are two possible reasonsfor this behavior: low gneisses,as shown in various projected subspaceswithin diffusional transfer velocity for Fe caused by inefficient the NFMASH composition space,are fairly regular and volume diffusion of this ion, and much lower driving indicate that most assemblagesare high variance to the force, i.e., chemical potential gradient, for Fe than for the extent that bulk compositions typically control mineral other components. There is no a priori reason to expect chemistry.The most interestingquestion concerning phase diffusion of Fe to be so much slower than for the other relations of the Orijiirvi gneissesis the involvement of species,and therefore the latter explanation seemsmore cummingtonite,dealt with by Eskola(1914) and Robin- likely. A reduced gradient in pFe relative to pMg could son and Jaffe (1969a). Our observationsand data suggest be due to the fact that the principal product mineral in that cummingtonite + cordierite was a stableassociation the rim-forming processwas an unzoned Mg-rich cordi- up to peak metamorphic conditions, but that either at or ente. near peak conditions these two minerals reactedto form The proposed diffusion scenario is analogousto that gedrite in common bulk compositions. The gedrite-form- proposedby Thompson (1959) for the developmentof ing reaction must have involved some sodic phase, pre- monomineralic zones in a binary chemical system. It is, sumably plagioclase,and may have been retarded in rocks however, complicated by the extra component and the with more calcic plagioclase,and therefore lower albite fact that one ofthe sourcesofa diffusing component is a activity. phasethat has the diffusing component as one of its solid The most petrographically unusual feature of the high- solution exchangevectors. Also in contrast to the er-gradesamples is the occurrenceof armored aluminous Thompson model, the diffusion process we describe is enclaves containing corundum, magnetite, spinel, h6g- strongly self-limiting in that there is a finite, and rather bomite, and cordierite, in which the cordierite separates small, amount of corundum at the Al-rich end of the all other phasesfrom contact with gedrite. Eskola (1914) diffusion pair. This may well be the limiting factor that reported much simpler enclaves of sillimanite and cor- restricts the scale of zoning in gedrite to about 100 pm, dierite from his samples.There are relatively few reports although volume diffusion rates, particularly for Al, may of similar texturally and mineralogically complex en- also be limiting. In either case,it is an important point claves, suggestingthat the phenomenon of enclave for- that volume diffirsion of Al on the scale of at least 100 mation is not common. The production of the minerals pm is possible under metamorphic conditions. We refer and texturesofthe enclavesis a result oftwo processes. 954 SCHNEIDERMAN AND TRACY: ORTHOAMPHIBOLE-CORDIERITE

The first is a late-stagereaction, possibly causedby de- Froese, E. (1969) Metamorphic rocks from the Coronation Mine and compression,in which coarsehost gedrite and small alu- surrounding area. Geological Survey ofCanada, Paper 68-5, 55-77. Grant, J.A. (1968) Partial melting of common rocks as a possiblesource minous intergrowths reactedto form cordierite and other of cordierite-anthophyllite bearing assemblages.American Journal of aluminous products. In Orijiirvi, at least some of the alu- Science266,908-931. minous intergrowths were corundum rich, whereasin New Hodges,K.V., and Crowley, P.C. (1985) Error estimation and empirical Hampshire they all appear to have contained kyanite or geothermobarometryfor pelitic systems.American Mineralogist, 70, sillimanite. The second,which accompaniedand ampli- 702-709. Hoffer, E., and Grant, (19E0) fied the J.A. Experimental investigation ofthe for- development of the unusual armored texture of mation of cordierite-orthopyroxeneparageneses in pelitic rocks. Con- cordierite rims, was caused by enrichment of resorbed tributions to Mineralogy and Petrology, 73, 15-22. gedrite maryins in tschermakite component. Strong ged- Hudson, N.F.C., and Harte, Ben (1985) K,O-poor, aluminous assem- rite zoning was produced by diffusion of Mg and Si blages from the Buchan Dalradian, and the variety of assemblagesin aluminous through the gedrite toward the enclaves (to grow bulk compositions in the amphibolite facies. American addi- Journaf of Science.285. 224-266. tional cordierite) and the consequentcounterdiffusion of James,R.A., Grieve, R.A.F., and Paul, L. (1978) The petrology of cor- Al away from the enclaves.The processcan therefore be dierite-anthophyllite gneissesand associatedmafic and p€litic gneisses viewed in sum as a complex volume interdiffusion of Mg, at Manitouwadge,Ontario. American Journal ofScience,278, 4l-63. Si, and Al, with very little involvement of Fe. The growth Kamineni, D.C. (1975) Chemical mineralogy of some cordierite-bearing rocks of cordierite near Yellowknife, Northwest territories, Canada. Contributions rims is therefore analogousto the develop- to Mineralogy and Petrology, 53,293-310. ment of monomineralic zonesthrough diffusion first dis- -(1979) Metasedimentary cordierite-gedriterocks of Archean age cussedby Thompson (1959).Although this processoc- near Yellowknife, Canada.Precambrian Research, 9, 289-301. curs only on a 100 tr"rmscale, it does clearly indicate Lal, R.K., and Moorhouse, W.W. (1969) Cordierite-gedrite rocks and gneisses mobility of Al in volume diffusion at metamorDhic associated ofFishtail Lake, Harcourt Township, Ontario. Ca- con- nadianJournal ofEanh Sciences.6. 145-165. ditions. Lal, R.K., Ackermand, D., Raith, M., Raase,P., and Seifert, F. (1984) Sapphirine-bearingassemblages from Kiranur, southernIndia: A study AcrNowlBocMENTS of chemographicrelationships in the NarO-FeO-MgO-Al,Or-SiOr-HrO system.Neues Jahrbuch {iir Mineralogie, 150, l2l-152. Field work by J.S.S. was supported by a Bates Fellowship, Jonathan Latvalahti, U. (1979)Cu-Za-Pb oresin the Aijala-Orijiirvi area,southwest Edwards puustajairvi College,Yale University. Ulla Latvalahti and Heiki Finland EconomicGeology, 74, 1035-1059. ofOutokumpu Finland, graciouslyprovided Oy, invaluable expertiseand kake, B.E. (1978) Nomenclature of amphiboles.Canadian Mineralogist, assistancein the field, as well as geologicmaps. The authorsalso gratefully 16,501-520. acknowledgethe help of Brian Skinner, SusanSwapp, and especiallythe Mikkola, T. (1955) Origin of ultrabasicsin the Orijiirvi region. 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