American Mineralogist, Volume 73, pages261-273, 1988 Pyroxene exsolution:An indicator of high-pressureigneous crystallization of pyroxene-bearingqv rtz syenite gneissfrom the High Peaks region of the Adirondack Mountains Plur- W. Or,r,rr..q. Department of Geology and Geography,Vassar College, Poughkeepsie, New York 12601, U.S.A. How.q.nr W. Jlrrn, Er.rzlnnrs B. J.q.rrn Department of Geology and Geography,University of Massachusetts,Amherst, Massachusetts01003, U.S-A- Ansrn-q.cr Relict, coarse,high-temperature inverted pigeonite lamellae in host augite and augite lamellae in host inverted pigeonite are locally preservedin metamorphosedigneous rocks from the Adirondack Mountains of New York State. The exsolution history of these pyroxenesconsists of (l) crystallization of high-temperatureaugite or pigeonite, (2) exso- lution of coarse "00l" lamellae of augite in host pigeonite and pigeonite in host augite, (3) fautting of lamellae and then of host along (100) as lattice parameterschange during cooling, (4) inversion of pigeonite to orthopyroxene,and (5) decomposition of the lamellae to produce intergrowths ofaugite and orthopyroxene along (100) ofhost grain. Thesetextures have been observedin pyroxenesthat spanthe full rangeofcompositions found in Adirondack anorthositic and syenitic rocks. An inverted pigeonite from a pyrox- ene-bearingquartz syenite gneisscollected in the Mount Marcy quadrangleyields a rein- tegratedcomposition of Wo,r,EnorFsro, indicating that igneouscrystallization of this rock took place at pressuresgreater than or equal to approximately 9 kbar. gioclase,pyroxene, hornblende, and garnet, occurs at the INrnorucrroN margins of the massif and in localized shearzones within This paperdescribes pyroxene exsolution textures found the core of the massif. Syenite gneissestypically overlie in metamorphosed igneous rocks from the Adirondack the gabbroic anorthosite gneissfound at the margins of Mountains of New York State.These exsolution textures the anorthosite massif. are petrologically significantin that they allow one to "see Valley and O'Neil (1982)and Valley (1985)presented through" the effects of regional metamorphism and to evidencethat anorthosite in the Adirondacks was intrud- study the igneoushistory of the rocks that contain them. ed at relatively shallow levels (<10 km) and then later Pyroxenesdescribed in this report were found in rocks underwent granulite-facies metamorphism. This was a that range in composition from gabbroic anorthosite to significant finding in that it was the first actual evidence pyroxene-bearingquartz syenitegneiss and were collected that Adirondack anorthosite was not intruded under during our geologic mapping of the Mount Marcy and pressure-temperatureconditions similar to those that oc- Santanoni quadranglesin the High Peaks region of the curred during regional metamorphism. This finding gave Adirondacks. Sample locations are shown on Figure 1, suppolt to the suggestion,made on the basis of analogy and sample descriptionsare given in Appendix 1. with Labrador anorthositemassifs, that anorthositein the The Marcy anorthosite massif crops out in a roughly Adirondacks may have intruded under anorogenicrather heart-shapedpattern and coversapproximately 3000 km'z than synorogenicconditions (Berg, 1977;Emslie, 1978). (Fig. l). It has generallybeen charucterizedas consisting Evidence presentedin this paper suggests,however, that of a core of anorthosite surrounded by gabbroic anor- pyroxene-bearingquartz syenitesclosely associatedwith thosite(Buddington, 1939,1969; Davis, 1971).Mapping and commonly thought to be coeval with the Adirondack by Ollila (1984),however, has shown that there are sig- anorthosite were intruded at depths greater than l0 km nificant amounts of gabbroic anorthosite within the core and perhapsas great as 30 km. The most important evi- regions of the massif. Core rocks are characterizedby denceleading to this conclusionis igneousexsolution tex- igneous textures. Subophitic gabbroic anorthosites and tures in Fe-rich pyroxenesfrom quartz syenites.Low-Ca gabbros in which garnet coronas around mafic minerals pyroxenesthat exhibit these textures are sufrciently Fe- are the only evidenceof regional metamorphism are rel- rich that they only could have crystallized at higft pres- atively common. Gabbroic anorthosite gneiss,character- sures.In order to develop this argument, it is first nec- ized by plagioclaseaugen in a granoblasticmatrix of pla- essaryto briefly describeexsolution in pyroxenes. 0003-004x/88/0304-026I $02.00 26r 262 OLLILA ET AL.: PYROXENEEXSOLUTION 74030' 44ogo' ARANACLAKE LAKEPLACID AU SAELE ,'/ | Bloomingdole /\. | /- t ,, z. /' r -' ll ),1' "H,l )\'):. ' 't/'/ -',t\ /\ \.zt. \ | l:.\ w&/ r /'r// // ^ ),--XV/r l t ,,F i-:)'-Y/ t >>, -\ -- >W:,'); ,"'iii2)'!)"":i l;L'/,lr'( r-r,r-4{{{ \ ,. I \ E/ /^\ /g-17 /'l\ | ..a/ - t /t I /\ / /ra - / -- .. / \'rr. t t ,'- ),)\ \ tll/l \ //l PARADOXL ,, r. zl i2Tilta/> l\' \ /'ltf \\ /,r, t \ \/ ./n-{ / 4tP.t5' z4030' t---l l-vT-Tl l++l ANORTHOSITE CALC - SILICATE ROCKS GRAN|TtC,SYENtTtC, 8 MIXED GNEISSES Fig. 1. Generalizedgeologic map of the Mount Marcy anorthosite massif. Locations for samples9-17 , AL-4, BA-5, GB-2, and PO-17are indicatedon the map. are functions of pressure, temperature, and chemical PynoxnNn ExsoLUTroN composition, the orientation of the best-fit planes is also Optimal phaseboundary theory was used by Robinson a function ofthese variables.The rapid expansionofpi- etal.(1971, 1977), Robinson(1980), andJaffeeral. (1975) geonite as it goes through the P2,/c = C2/c reversible, to explain why pigeonite lamellae in an augite host and nonquenchabletransition is of particular importance in arrgitelamellae in a pigeonitehost are oriented along non- interpreting exsolution temperatures. At temperatures rational planes.The theory statesthat monoclinic pyrox- above the stability of P2,/ c pigeonite,a and c ofpigeonite eneswith identical D dimensions and differing a, c, and are greater than a and c of augite.At temperatureswhere B will have two planesof exactdimensional fit. One plane, P2,/c pigeonite is stable, the reverse is true. Becauseof "001," is closeto (001), its orientationbeing controlled theserelationships, high-temperature lamellae of pigeon- by the diference between the c dimensions of the host ite in host augite or augite lamellae in host pigeonite ori- and lamellaepyroxenes. Another plane,"100," lies close ent in obtuseangle p [c rr "001" < c A (001)];whereas to (100) and is controlled by the differencebetween the c low-temperaturelamellae orient in acute angle0 Fig.2). dimensionsof the pyroxenes.Since the lattice parameters High-temperature lamellae are much coarser (approxi- OLLILA ET AL.: PYROXENE EXSOLUTION 263 mately 10-100 pm thick) and spacedfarther apart than lamellae formed at lower temperatures, which are less than I pm thick. Examples of high-temperature exsolu- tion in low-Ca pyroxenes (inverted pigeonite) are rela- tively common in plutonic igneousrocks and have been described from many localities. This type of exsolution was originally describedby Hess (1941, 1960) and has been described in Adirondack rocks by Davis (1971), Ashwal (1982), and Jaffe et al. (1983). Robinson et al. ( I 977) describedhigh-temperature ( - 1000'C) exsolution in host augitesfrom the Bushveld complex in South Af- rica and the Nain complex in Labrador, and more re- cently Livi (1987) has describedsimilar exsolutionfea- tures in pyroxenesfrom the Laramie anorthosite massif. Beforethe work of Ollila et al. (1983, 1984),this type of exsolution had not been recognizedin the Adirondacks. In contrast to the exsolution textures visible in mono- clinic pyroxenes, exsolution between orthorhombic and monoclinic pyroxenesis much simpler. The only plane that is at all similar between orthopyroxene and clino- pyroxeneis (100), and lamellaeof clinopyroxenein or- thopyroxenehost or orthopyroxenein clinopyroxenehost orient along this plane. As pointed out by Robinson Fig.2. L .oa.r basedon ol."*u,roo, o, orro*Ln"textures (1980),this type of exsolutionis common in metamor- for the exsolutionhistory ofigneous augite in the Adirondacks. (l) of homogeneousaugite; (2) phic pyroxenesand in magnesianigneous pyroxenes that The sequenceis crystallization exsolutionof coarse"001" pigeonitelamellae, which orient in crystallizedat temperaturesbelow the stability of pigeon- obtuseangle of the host augite;(3) faultingof pigeonitela- during the late B ite. This type of exsolution also occurs mellaeduring cooling (Robinson etal., 1977);(4) faultingof the stagesof exsolution of igneouspyroxenes. host augitealong (100), inversion ofpigeonite and decomposi- Exsolution textures in Adirondack rocks are compli- tion of"001" invertedpigeonite lamellae along (100) ofthe host catedby the fact that igneousrocks have been affectedby augite;(5) formation of pyroxene"mesoperthite"-all the ortho- a regional metamorphism. This metamorphism has re- pyroxenethat originallywas in "001" lamellaenow forms(100) sulted in complex exsolution textures that have been lamellaein hostaugite; (6) orthopyroxenelamellae coarsen, and poorly understood. In a few locations, however, igneous fine pigeonitelamellae, which orient in acuteangle B of host (100) rocks were only slightly deformed during regional meta- augite,exsolve from hostaugite. Note that low-temperature as to protrudebeyond morphism, and pyroxenesin rocks from these localities orthopyroxenelamellae may exsolveso the confinesofthe augitehost. retain recognizablehigh-temperature exsolution textures