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Petrology of Orthoamphibole-Cordierite Gneisses

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Petrology of Orthoamphibole-Cordierite Gneisses 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 + anthophyllite + biotite + 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 cummingtonite 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 amphibole-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, iron 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
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