Pyroxene-Olivine-Quartz Assemblages in Rocks Associated with the Nain Anorthosite Massif, Labrador

Transcription

1 Pyroxene-Olivine-Quartz Assemblages in Rocks Associated with the Nain Anorthosite Massif, Labrador by DOUGLAS SMITH Department of Geological Sciences, The University of Texas at Austin, Austin, Texas {Received 9 April 1973; in revised form 22 June 1973) ABSTRACT Unusually iron-rich pyroxene and olivine occur in rocks associated with the Nain anorthosite massif, Labrador. Adamellite and granodiorite contain orthopyroxene (inverted from pigeonite) as iron-rich as Ca^FesjMgn; comparison with experimental data suggests a minimum pressure of crystallization of 5 kb. Some of these iron-rich pyroxene crystals have broken down, apparently upon decreasing pressure, to yield intergrowths of less iron-rich orthopyroxene (near Ca 7 Fe 7,Mg sl ), ferroaugite, fayalite (near Fo 9 ), and quartz. Other rocks, monzonites, contain pyroxenes with calcium-poor cores and ferroaugite rims, as well as crystals composed of broad lamellae of ferroaugite and orthopyroxene in sub-equal proportions. Analysis of one such crystal with unusually thin and closely spaced lamellae yielded a bulk composition of CawFeogMgu. Such pyroxenes probably crystallized near or above the crest of the augite-pigeonite two-phase region, probably above 925 C. This high temperature suggests that the monzonites crystallized from relatively dry magmas. If they represent a residual fraction derived from the same magma as the anorthosite, then that magma must have been nearly anhydrous. Pigeonite rather than orthopyroxene was the primary magmatic Ca-poor pyroxene in most of the Nain rocks studied here. Nucleation rates apparently were low under subsolidus conditions, and low-ca pigeonite (CaiFe7 8 Mg H) ) is present in grains where orthopyroxene did not nucleate as pigeonite cooled and exsolved ferroaugite. Iron-rich orthopyroxene (Ca,Fe7 8 Mg lt ) crystallized instead of pigeonite in a Greenland quartz syenite that contains more abundant hydrous phases. INTRODUCTION THE stability fields of pyroxene rich in the ferrosilite component (FeSiO 3 ) are sensitive to pressure and temperature. Occurrences of such pyroxenes together with fayalitic olivine and quartz are of particular interest, since Ca-poor pyroxene is stabilized by pressure relative to equivalent assemblages containing silica and olivine. Ferrosilite-rich pyroxenes typically occur in the monzonite-adamellite (or mangerite) rock suite associated with anorthosite massifs. Some rocks about the Nain massif, Labrador, contain not only pairs of unusually iron-rich pyroxenes but also textural evidence of the breakdown of pyroxene to assemblages with fayalitic olivine and quartz. Petrographic and electron probe studies of some of these Nain rocks are reported here, together with limited data on rocks from several other localities. Preliminary results on several rocks were discussed by Smith (1971a). This study was designed, first, to try to define conditions of crystallization of some of the {Journal of Fetrolou, Vol. IS, Part 1, pp ,1974]

2 ASSOCIATED WITH THE NAIN ANORTHOSITE MASSIF 59 rocks associated with the Nain anorthosite, and second, to test and supplement recent experimental data on the stability of Fe-rich pyroxenes. Relatively few studies of pairs of Fe-rich, igneous pyroxenes have been published; consequently, these pyroxene pairs are less well known than the more magnesian ones common in gabbroic rocks. GEOLOGIC SETTING A number of rock types, including granodiorite, monzonite, and adamellite, 1 are associated with the anorthosite massif of Nain near the north-east coast of Labrador. The massif is the most northerly known representative of the belt of anorthosites which extends from the coast of Labrador south-west at least to the Adirondacks. The Nain rocks are north of the Grenville front, and they do not appear to have been subjected to any of the major metamorphic events which affected many other massifs in the belt (Wheeler, 198). The mineral assemblages present may reflect conditions of emplacement and subsequent cooling histories of the intrusions. Wheeler (1955, 190, 198) has suggested that all the rock types belong to an anorthosite-adamellite complex derived from a single parent magma. De Waard & Wheeler (1971) assembled chemical and petrologic data for a selected group of rocks of the anorthosite-adamellite series, and they concluded that a continuous differentiation series was present. The differentiation sequence outlined by de Waard and Wheeler is characterized by a systematic increase in quartz content with increasing values of the ratio of alkali feldspar to plagioclase, and it includes granodiorite and adamellite. The monzonites studied here do not fit this scheme, since they have different quartz contents but similar feldspar ratios compared to the adamellites. Morse (1972) has suggested that the anorthosites and adamellites were derived from different magmas. Heath & Fairbairn (198) obtained Rb/Sr data from three samples of the Nain adamellite suite; they concluded that either the samples were not comagmatic or that there had been substantial contamination by country rock. The Nain rocks studied here are from three distinct masses of the 'adamellite series': they were mapped and collected by E. P. Wheeler II (198, and written communication), and his identification numbers have been retained. BREAKDOWN OF PYROXENE TO ASSEMBLAGES CONTAINING OLIVINE AND QUARTZ Wheeler (195, 198) described textures which document the breakdown of Ca-poor pyroxene to assemblages containing ferroaugite, olivine, and quartz in some rocks of the 'adamellite series'. Three rocks, two adamellites and one granodiorite, were selected for determination of the compositions of the minerals involved in this reaction. All three rocks have the dark color characteristic of 1 These rock types might be called opdalite, mangerite, and farsundite, respectively, using the terminology of de Waard (198), since the rocks contain orthopyroxene and olivine.

3 0 DOUGLAS SMITHPYROXENE-OLIVINE-QUARTZ IN ROCKS charnockites, and all contain nearly anhydrous assemblages: one rock contains less than a per cent amphibole plus biotite of probable primary origin, one rock has only a trace of amphibole, and the third rock apparently lacks primary hydrous minerals. Textural relationships are different in each of the selected rocks, though all contain orthopyroxene, olivine, quartz, and ferroaugite. Textures are particularly clear in one adamellite (2-1299). Orthopyroxene, the most abundant ferromagnesian mineral, occurs in grains with two sets of pyroxene exsolution lamellae; one set is fine (less than 1 micron thick), closely spaced, and approximately parallel to (100), while the other set is thicker (most to 10 microns thick), sparsely distributed, approximately parallel to the b crystallographic axis of the orthopyroxene host, and at a high angle to c. Poldervaart & Hess (1951) noted that, in the inversion of pigeonite to orthopyroxene, the orthopyroxene commonly 'will develop in such an orientation that it retains the b and c crystallographic axes of the parent pigeonite'. The thicker lamellae in the orthopyroxene are interpreted as '(001)' lamellae in original pigeonite grains. The two sets of lamellae are interpreted as evidence that the orthopyroxene represents inverted pigeonite, with inversion taking place as described by Poldervaart and Hess. Most of these grains show no sign of later reaction to olivine. Some of the orthopyroxene, however, is intergrown with olivine, quartz, and ferroaugite in what appear to be reaction products of another mineral, probably pigeonite (Fig. 1). Olivine and ferroaugite occur only in such intergrowths with quartz, and their abundance appears compatible with an origin by isochemical breakdown of pigeonite. In contrast, olivine and ferroaugite occur in discrete crystals in other rocks, such as granodiorite Textural evidence suggests that orthopyroxene, pigeonite, ferroaugite, and olivine were primary minerals in this rock, though it is conceivable that the orthopyroxene was produced by subsolidus, intergranular recrystallization. Conclusive textural evidence for pyroxene breakdown to yield olivine and quartz is also present; the evidence is based on textures like those described by Wheeler (195, 198). Some rocks appear to have metamorphic textures; an example of this type, adamellite 2-158, contains augen-like clusters of feldspar grains in a relatively fine-grained matrix. Orthopyroxene, free from exsolution lamellae visible with an optical microscope, occurs together with olivine, ferroaugite, and quartz. Many olivine grains contain numerous inclusions of quartz, and some orthopyroxene grains have irregular inclusions of ferroaugite. Conclusive evidence of pyroxene reaction is not present, but the intricate nature of some of the intergrowths of olivine, quartz, orthopyroxene, and ferroaugite suggests that they formed by recrystallization of grains of pigeonite. Electron probe microanalyses were made on all ferromagnesian silicates in each rock (Table 1). A large number of grains were photographed in polished thin section and analyzed at many points for Ca, Fe, and Mg (pyroxenes and some olivines) or Fe, Mg, and Mn (olivines). Selected points were then reoccu-

4 ASSOCIATED WITH THE NAIN ANORTHOSITE MASSIF 1 pied and analyzed for three or more additional elements out of the group Al, Ti, Si, Mn, and Na.1 Particular attention was paid to zoning and to analysis of grains in different textures. Most olivine grains are homogeneous within 3 per cent fayalite. Ferroaugite grains commonly have fine exsolution lamellae, but most grains are relatively homogeneous for analyses made with a beam IS to 30 microns in diameter. Many orthopyroxene grains show variable Fe-Ca in microanalyses: these grains are interpreted as inverted pigeonite (Figs. 1, 3D), as described above. The analyses include the fine (100) lamellae. Variations of the proportions of these lamellae from one analyzed area to another may account for much of the observed Fe-Ca variation. The analyses of orthopyroxene inverted from pigeonite are averages of areas analyzed with a broad beam; though the analyses include thefine(100) lamellae, they do not include the sparse, broader set. Therefore, they do not represent original pigeonite compositions. Some of these analyses of orthopyroxene plus (100) lamellae indicate -7 mole per cent wollastonite (1, 3, Table 1), unusually 1 Synthetic compounds were used for standards. All data for Ca-Fe-Mg and Fe-Mg-Mn partial analyses were processed with several computer programs written for this purpose, while and 9 element analyses were processed by the procedures and programs of Hadidiacos et al. (1971) (data obtained at Geophysical Laboratory) and Bence & Albee (198) (data at U. Texas). Fro. 1. Portion of a grain of inverted pigeonite which has broken down to less iron-rich orthopyroxene, ferroaugite, olivine, and quartz. The orthopyroxene has moderate relief and is transected by a set of ferroaugite lamellae roughly parallel to the length of the figure. Intergrown olivine (high relief), quartz, and ferroaugite are distributed in irregular aggregates through the crystal. Compositions of these grains are among those plotted as solid symbols in Fig. 2. A grain in the same rock which did not break down to yield olivine is shown in Fig. 3D. Length of field, 1-3 mm. Adamellite

5 SiO, TiO, AI.O. Fe as FeO MnO MgO CaO Na.O TOTAL o Si Ti Al Fc Mn Mg Ca Na Ca:Mg:Fe SiO, TiO, Al.O, Fe a» FeO MnO MgO CaO Na,O TOTAL o Si Ti Al Fo Mn Mg Ca Na Ca:Mg:Fe :13: OO :15:41 TABLE 1. Representative electron-probe microanalyses of iron-rich pyroxenes and olivines :11: :7: :20:73 n :18: :17: O :1: n.d O02 :9: :18: :18: n.d :10:90 Not analyied :23: nm O :7: n.d :7: :2: :1: :21:41 nm. Not detected :19: ^ :21: n.d :10: :17: :15: :18: :14: : :9: : MOO o od o r IH H I O X w % m I d> H N 5 O O

6 ASSOCIATED WITH THE NAIN ANORTHOSITE MASSIF 3 calcic values for orthopyroxene. As pigeonite broke down to orthopyroxene plus augite, some of the released augite may have crystallized as fine (100) lamellae in the orthopyroxene. If so, then the analyses of orthopyroxene plus (100) lamellae may not reconstruct true orthopyroxene compositions. Some of KEY TO TABLE 1 Adamellite Near the south-cast corner of the large adamellite body mapped by Wheeler (198, fig. 1), at about 5 50' N., 2 40' W., Labrador. 1. Orthopyroxene with unresolved (100) lamellae; no reaction to olivine, quartz. 2. Ferroaugite lamella in above crystal. 3, 4, and 5 are from an intergrowth resulting from the breakdown of a pigeonite crystal: 3. Orthopyroxene with unresolved (100) lamellae. 4. Ferroaugite. 5. Olivine. Adamellite 2-158, Rock described by Wheeler (198, Table 1); analyses are of three intergrown grains.. Ferroaugite. 7. Orthopyroxene. 8. Olivine. Granodiorite Near the south-east corner of the large adamellite body mapped by Wheeler (198, Fig. 1), at about 5 53' N., 2 45' W., Labrador. 9. Orthopyroxene; no reaction to olivine, quartz. 10. Ferroaugite intergrown with olivine and quartz. 11. Olivine intergrown with ferroaugite and quartz. Monzonite Rock described by Wheeler (198, Table 1). 12. Calcium-poor core of clinopyroxene grain. 13. Rim of same grain. 14. Primary olivine, not intergrown with quartz. 15. Ferroaugite intergrown with olivine and quartz. 1. Olivine intergrown with quartz, probably product of pyroxene reaction. Monzonite Small body with gneissic structure in the marginal zone between anorthosite and paragranulite, at 5 31' N., ' W., Labrador. 17. Primary pyroxene. 18. Broad ferroaugite lamella in composite crystal. 19. Orthopyroxene lamella coexisting with lamella of Analysis Olivine in equant crystal, no intergrowths. 21. Olivine intergrown with quartz, probable product of pyroxene reaction. Adamellite (mangerite) Phqm. Same outcrop as sample P-4-45 of A. R. Philpotts (19), Grenville Township, Quebec. 22. Orthopyroxene. 23. Large ferroaugite crystal (less calcic than small crystals). 24. Small ferroaugite crystal (more calcic than large crystals). Quartz syenite Graahs Fjelde body, Greenland. Sample and number are from the Geological Survey of Greenland. 25. Ferroaugite (representativeca variable). 2. Orthopyroxene (representativeca variable). Quartz syenite Patussoq Fjord, Greenland. Sample and number are from the Geological Survey of Greenland. 27. Hedenbergite (uniform). 28. Fayalite (uniform).

7 4.' DOUGLAS SMITHPYROXENE-OLIVINE-QUARTZ IN ROCKS the apparent variation in calcium content of orthopyroxene inverted from pigeonite may reflect the extent to which augite was retained as (100) lamellae. The sparse, broad lamellae of ferroaugite in orthopyroxene typically contain mole per cent woliastonite component. Consideration of the proportions and compositions of the orthopyroxene and lamellae indicates that primary pigeonite crystals contained about 10 mole per cent woliastonite component. 80 Mole percent Fio. 2. Compositions of pyroxene and olivine in adamellite and granodiorite. Open circles represent compositions of orthopyroxene grains which did not break down to yield olivine in adamellite and granodiorite Other grains in the same two rocks and in adamellite reacted to an assemblage of less iron-rich orthopyroxene and ferroaugite (filled circles), olivine (solid bar), and quartz. Each circle represents an average analysis of one grain; analyses of orthopyroxene inverted from pigeonite, however, include fine (100) lamellae but do not include the sparse basal set formed before inversion. Some (100) lamellae in orthopyroxene probably contain augite formed during inversion of pigeonite; the presence of such lamellae presumably is responsible for the unusually calcic orthopyroxene analyses. The dashed line outlines the area in which pyroxene is not stable at 925 C and 1 atmosphere (Smith, 1972). Compositions of ferromagnesian minerals in the three rocks are plotted in Fig. 2. Each point represents the average composition of material analyzed at a number of spots in one grain. Compositions of pyroxene and olivine intergrown with quartz in textures which appear to have resulted from pigeonite breakdown are plotted as solid symbols. Though these solid symbols represent data from three rocks with different bulk compositions and different proportions of phases, they plot in well-defined groups. The ranges of the ratio Fe/(Fe+Mg) for the intergrown minerals are only for ferroaugite, for

8 ASSOCIATED WITH THE NA1N ANORTHOSITE MASSIF 5 orthopyroxene, and for olivine, so the intergrown assemblages in the three rocks appear to have equilibrated under similar conditions. Compositions of pyroxene in grains which did not react to yield olivine are plotted as open circles; two rocks ( and ) are represented. These open circles are at distinctly more iron-rich compositions than the solid circles which represent orthopyroxene intergrown with olivine, ferroaugite, and quartz in the same two thin sections. In theory, the breakdown of pyroxene to yield such intergrowths might occur in two ways. First, pyroxene might react with melt during crystallization, as illustrated by Yoder, Tilley, & Schairer (193, Fig. 18, Point D) in their discussion of quaternary reaction points in the pyroxene quadrilateral. This possibility is unlikely for these rocks, since pigeonite or orthopyroxene would disappear by reaction with liquid rather than equilibrate to less Fe-rich, Capoor compositions like those found in the breakdown intergrowths. A subsolidus reaction seems more likely. Wheeler (195) has cited additional evidence for a subsolidus origin based on textures in an adamellite from the same area. He noted that ferroaugite occurs in plates within intergrowths of olivine and quartz, and that the optic orientation of ferroaugite within the plates is like that of augite in basal exsolution lamellae formed in pigeonite. He deduced that the breakdown to olivine and quartz occurred after exsolution had begun in pigeonite in the subsolidus region. It is not clear whether olivine and quartz formed before or after pigeonite inverted to orthopyroxene. Pyroxene breakdown to yield olivine may be attributed to the fact that all analyses of orthopyroxene plot within the area of the 'forbidden zone' of the pyroxene quadrilateral outlined by Smith (1972) for synthetic pyroxenes at one atmosphere and 925 C (Fig. 2). The 'forbidden zone' is the area of the pyroxene quadrilateral in which Fe-rich pyroxenes are not stable relative to assemblages with olivine and silica (Lindsley & Munoz, 199). The size of this area is a function of pressure and temperature. The actual boundary within the quadrilateral cannot be a smooth curve, since it is determined by equilibria of both orthopyroxene and clinopyroxene with olivine and silica. With increasing pressure, more Fe-rich pyroxenes become stable, and the boundary of the zone contracts towards the composition of pure ferrosilite. Fe-rich, Ca-free orthopyroxenes are stabilized relative to olivine plus quartz by decreasing temperature as well as by increasing pressure. The effects of temperature changes on the stability of pigeonite with respect to assemblages containing olivine and quartz are probably minor compared to those of pressure changes, but relevant data are not yet available (Smith, 1972). The breakdown of pigeonite grains to isochemical assemblages containing olivine and quartz in these rocks is interpreted to have occurred in response to a decrease in pressure as the rocks were uplifted, perhaps long after melt crystallization. Other pyroxene grains in the same thin sections did not break down, apparently because the assemblage of olivine-ferroaugite-quartz could F

9 DOUGLAS SMITHPYROXENE-OLIVINE-QUARTZ IN ROCKS not nucleate within them. Data discussed later support the concept of nucleation barriers to the growth of new phases in pyroxene grains at subsolidus temperatures in these rocks. The orthopyroxenes (e.g. analysis 1, Table 1) contain little more than one total weight per cent of components such as MnO and A1 2 O 3 which are not represented in the pyroxene quadrilateral. Since fayalite occurs in these rocks and the pyroxenes contain little sodium, the pyroxenes are unlikely to have significantly more ferric iron than synthetic equivalents in experiments buffered by assemblages of fayalite-magnetite-quartz. Their compositions are thus generally comparable to those of synthetic 'pure' pyroxenes. The more Fe-rich, unreacted crystals plot close to that join in the pyroxene quadrilateral with a Fe/(Fe+Mg) ratio of Smith (1972) found that synthetic pyroxenes on this join with less than 15 per cent wollastonite are not stable below 3-5 kb at 925 C. For pigeonites with only 10 per cent wollastonite, minimum pressures must be nearer 5 kb at 925 C. Effects of temperature upon this minimum pressure are unlikely to be large (Smith, 1972, p. 1423), but experimental data are lacking. It is likely, however, that the 925 C temperature falls within or near the crystallization range of the magmas which formed the nearly anhydrous adamellites. Present data, therefore, suggest a minimum pressure of crystallization of about 5 kbar for the more iron-rich pyroxenes preserved in these Nain rocks. PYROXENE-OLIVINE ASSEMBLAGES IN MONZONITES Two monzonites were selected for study because they contained pyroxenes with calcium contents intermediate between those of normal augite (Wo^^) and pigeonite (Wo^), as well as intergrowths of olivine and quartz. Both samples were collected by Wheeler from masses at contacts between anorthosite and country rock. The two rocks are similar in that they consist primarily of plagioclase and alkali feldspar with modal per cent ohvine plus pyroxene in about equal proportions. Less than 1 per cent biotite plus amphibole is present. They contain at most several per cent modal quartz, primarily intergrown with olivine. Feldspars were characterized by Ca, K, and Na determinations with the electron probe. Plagioclase and perthite in one of the monzonites (2-178) are in the ranges An34_ 37 and Or^.^, respectively. These feldspars are in anhedral, equant grains, and the texture may reflect feldspar recrystallization below the solidus. The other monzonite (2-1599) contains An^^ plagioclase as well as mesoperthite and unmixed alkali feldspar which is too complex texturally to characterize in bulk by microanalysis. Many pyroxene crystals in one monzonite (2-1599) are zoned, with cores and rims that contrast in birefringence and extinction angle (Fig. 3c). Other grains resemble rim pyroxene and lack cores; these grains are ferroaugite. Ferroaugite in these grains and in rims of zoned pyroxene typically has one thin set of exsolution lamellae visible in thin section; it appears homogeneous when analyzed

10 ASSOCIATED WITH THE NAIN ANORTHOSITE MASSIF 7 FIG. 3. (A) Lamellae of ferroaugite (Ca 44 Fe 4 img I8 ) and orthopyroxene (Ca,Fe, 8 Mgi,) formed from a crystal of intermediate wollastonite content. Lamellae are about 0-03 mm wide and were considered too coarse to average by microanalysis. Thin (001) lamellae are visible within the dark lamellae of ferroaugite. Length of field, 0-2 mm, crossed nicols. Monzonite (B) Rare crystal in rock with lamellae sufficiently fine to average by microanalysis. Average composition Caj 4 Fe 8 Mg 18. Length of field, 0-2 mm, crossed nicols. (c) Typical zoned crystal in monzonite Average compositions of core and rim are Ca 18 Fej7Mg 15 and Ca«Fe <4 Mg u, respectively. Length of field, 1 mm, crossed nicols. (D) Orthopyroxene (lower left) separated by a lamella of ferroaugite from low-ca pigeonite (upper right). The orthopyroxene appears dark, the low-ca pigeonite light. Electron probe traverse shown in Fig. 5 was made along the short white line. Length of field, 0-5 mm, crossed nicols. Adamellite with an electron beam 20 microns in diameter, near Ca^Fe^Mg^ (Fig. 4B). Rim-core contacts are sharp in some crystals and gradational over a narrow zone in others. Most cores contain a set of visible lamellae, and some are composed of well defined, alternating lamellae of nearly equal width. Cores are less calcic than rims; analyses made with a broad electron beam on six cores plot

11 8 DOUGLAS SMITHPYROXENE-OLIVINE-QUARTZ IN ROCKS in the range 13 to 31 mole per cent wollastonite, an unusual range for pyroxenes in plutonic rocks (filled circles, Fig. 4B). The hypothesis of Smith (1971a) that the core and rim compositions bracket a two-phase region must be discarded, since additional microanalyses represent intermediate compositions. The intermediate compositions are averages of lamellae plus host, and exsolution and diffusion may have modified or obscured primary compositional variations. ACo Mg Fe FIG. 4. Compositions of pyroxene in monzonites (circles) and (squares). (A) Analyses of pairs of coexisting exsolution lamellae. (B) Analyses which appear to represent primary pyroxene. Each plotted point represents the average of a number of analyzed spots. Grain cores in rock are shown as filled circles, while grain rims and small, uniform grains are plotted as open circles. Most grains in rock are composed of lamellae too coarse to average by microanalysis; the lamellae are nearly equal in abundance, and the grains have bulk compositions with intermediate wollastonite contents. The analyses may reconstruct real pyroxene compositions formed from a magma, however, since some of the cores are relatively uniform in composition when analyzed with a beam 30 microns in diameter. For instance, 1 point analyses on one core yielded an average composition of Ca 2 Fe 59 Mg 15, with a standard deviation of less than ±3 per cent wollastonite. Crystals composed of alternating, broad lamellae of ferroaugite and orthopyroxene in approximately equal proportions and up to 008 mm wide are common in the other monzonite (2-178). The lamellae are remarkable for their width and straight boundaries, and they resemble twin lamellae at first glance (Fig. 3A). Ferroaugite compositions cluster near Ca^Fe^Mg!,, orthopyroxene compositions near CagFeTgMgu (Fig. 4A, Table 1). Since the lamellae are present in nearly equal proportions, they appear to have formed from pyroxenes of intermediate calcium content in the range Wo^^. Apparently clinopyroxene crystals in this compositional range unmixed to broad lamellae

12 , ASSOCIATED WITH THE NAIN ANORTHOSITE MASSIF 9 of augite and pigeonite; the unmixed clinopyroxenes were intergrown on planes approximately parallel to (001). Subsequently, much of the pigeonite inverted to orthopyroxene; optical data suggest that in a few crystals pigeonite was preserved. The relatively large size of the lamellae in comparison with average grain sizes makes it difficult to estimate the compositions of the original pyroxene grains with a higher degree of accuracy. A bulk composition of C^FeiaMgig was obtained for one grain with exceptionally thin, close spaced lamellae (Fig. 3B, Table 1). Most of the pyroxene grains apparently had similar wollastonite contents, though a range of primary compositions may have been present. The high proportion of pyroxene grains intermediate in wollastonite content is obscured by the distribution of points in Fig. 4B, since almost all of these crystals broke down to broad lamellae. The descriptions above represent the majority of grains in each monzonite, but they do not portray the complexity of compositions and textures of pyroxenes present. Monzonite contains a few crystals with broad, alternating lamellae of ferroaugite and orthopyroxene, like those in rock The latter rock also contains a few smaller crystals with sparse, thin lamellae. Some of these are ferroaugite (bulk compositions near Ca 40 Fe 42 Mg 18 ), while others (bulk compositions near CaujFe^Mgu,) are low-ca pigeonite with lamellae of ferroaugite. Compositions of such crystals are plotted as primary phases in Fig. 4B, so that they may be contrasted with compositions of pairs of exsolution lamellae in Fig. 4A. Olivine is present in two distinctly different textures in the monzonites. Most olivine occurs in relatively large crystals generally free from intergrowths and inclusions. Some olivine occurs in smaller grains which are intricately intergrown with quartz and ferroaugite. Compositions of the two textural varieties of olivine are distinct. In one monzonite (2-1599) olivine in large, inclusion-free grains contains 9-7 mole per cent forsterite, while olivine intergrown with quartz contains 7-3 per cent, with very little overlap; respective compositions in the other monzonite are 11-8 and 8- mole per cent forsterite. No Ca-poor pyroxene is preserved in the intergrowths of olivine-quartzferroaugite, but the textures suggest that reaction or breakdown of orthopyroxene or pigeonite led to the formation of many of them. Reaction of pyroxene with residual melt seems more likely than simple breakdown, both because olivine in intergrowths is more iron-rich than that in the more abundant, intergrowth-free grains and because no orthopyroxene or pigeonite is preserved in the intergrowths. If this interpretation is correct, more calcic pyroxene should crystallize during and after reaction of less calcic pyroxene with the melt (Yoder et al., 193). The occurrence of pyroxene grains with Ca-poor cores and ferroaugite rims in one monzonite (2-1599) is consistent with the inferred pyroxenemelt reaction. The intermediate compositions analyzed could form if crystallization took place at temperatures near or above the crest of the augite-pigeonite

13 70 DOUGLAS SMITHPYROXENE-OLIVINE-QUARTZ IN ROCKS miscibility gap. Most crystals studied do not appear to be continuously zoned, but evidence of such zoning might have been removed as exsolution lamellae were formed. Lamellae are so large in most grains of intermediate pyroxene in the other monzonite (2-178) that it is not clear whether a range of compositions was present or whether all crystals had the same composition (Wo^ as the analyzed crystal. The less abundant ferroaugite and pigeonite in this rock may have formed under subsolidus conditions, as discussed later. NUCLEATION, GROWTH, AND EQUILIBRATION OF PYROXENES Three lines of evidence indicate that it was difficult for new phases to nucleate within pyroxene in the Nain rocks, but that diffusion processes within crystals were adequate to ensure growth once micleation occurred. First, some iron-rich orthopyroxene crystals persisted metastably without reaction, while in the same thin sections other crystals broke down to yield less Fe-rich orthopyroxene, olivine, ferroaugite, and quartz. Second, exsolution lamellae of ferroaugite and orthopyroxene are exceptionally wide in many crystals in the two monzonites, perhaps because few nuclei were present for lamellae growth. Finally, low-ca pigeonite was recognized in the Nain rocks by a combination of optical study and microanalysis. Evidently it is present because orthopyroxene failed to nucleate in some grains with decreasing temperature. Two equant grains about 0-2 mm in diameter consisting mainly of low-ca pigeonite were analyzed in monzonite The clinopyroxene has small positive optic angles (2Fin range 0-30 ) and a higher birefringence than orthopyroxene. Both analyzed grains consist of low-ca pigeonite (CajFe^Mgao) with thin lamellae of ferroaugite (Ca 45 Fe3gMg 17 ); approximate bulk compositions of the two grains derived from analyses of a number of points with a broad beam are Ca u Fe 70 Mg 19 and Ca 7 Fe 74 Mg 19. Low-Ca pigeonite also was recognized in a composite grain with orthopyroxene in an adamellite (2-1299). Apparently orthopyroxene nucleated with declining temperature at one center in the grain (Fig. 3D), but the structural conversion from pigeonite stopped at a barrier formed by a (001) lamella of ferroaugite. Results of a traverse of an electron beam across the lamella (Fig. 5) appear to indicate that orthopyroxene contains considerably more calcium than coexisting low-ca pigeonite. Actually, the two phases may be similar in composition. It is likely that much of the calcium in the analyzed 'orthopyroxene' resides in unresolved (100) lamellae which formed during or after inversion, while in clinopyroxene, calcium continued to separate into previously formed basal lamellae. Wheeler (198) made detailed universal-stage measurements to establish the presence of 'pigeonite' with 'a negligible calcium content' in the core of a zoned pyroxene crystal in another of the monzonites (2-1599) discussed here. Relatively iron-rich low-ca pigeonite has been described in other igneous rocks only as thin exsolution lamellae in host crystals of ferroaugite (e.g. Binns et al., 193); such lamellae have been termed 'clinohypersthene', to indicate that

14 ASSOCIATED WITH THE NAIN ANORTHOSITE MASSIF 71 Opx Aug Clino px Aug 80-4 o O 8 t ^ o Microns FIG. 5. Reduced data from an electron probe traverse with 2-micron steps along the white line shown in Fig. 3D. Orthopyroxene (left) with unresolved (100) lamellae is separated from low-ca pigeonite (right) by a lamella of ferroaugite parallel or nearly parallel to clinopyroxene (001); the low-ca pigeonite (clinopx) includes another lamella of ferroaugite. Adamellite they contain less calcium than normal pigeonite. The term 'low-ca pigeonite' is used here to emphasize that the mineral formed from normal pigeonite as ferroaugite exsolved during cooling. Though such low-ca pigeonites are rare, indirect evidence from other rocks indicates that it is difficult for orthopyroxene to form nuclei in the pigeonite crystal structure. Bonnichsen (199) described textures that indicated groups of pigeonite crystals inverted to single crystals of orthopyroxene in a metamorphosed iron formation, and he inferred that orthopyroxene nucleated with difficulty in pigeonite but grew readily once a nucleus was established. Von Gruenewaldt (1970) and Morse (199) described similar relations in the Bushveld and Kiglapait complexes: Von Gruenewaldt commented on the 'sluggishness' of the inversion of pigeonite. Co

15 72 DOUGLAS SMITHPYROXENE-OLIVINE-QUARTZ IN ROCKS The diversity of compositions of separate pyroxene grains in one of the monzonites (2-178) may reflect limited intergranular recrystallization of pyroxene under subsolidus conditions rather than magmatic processes. Most grains in this rock have broken down to broad lamellae, and these grains have bulk compositions near or in the range Wo^a,,. Smaller grains are present, however, that have bulk pigeonite (near Wo 10 ) and ferroaugite compositions (near Wo 40 ). These smaller grains are less extreme in composition than the broad exsolution lamellae of obvious post-magmatic origin, but all pyroxene compositions in the rock plot very close to the tie-line connecting compositions of exsolution lamellae (Fig. 4 A, B). This systematic relationship would not be expected if the compositional diversity were a magmatic feature. Perhaps the smaller grains formed in the monzonite as magmatic pyroxenes with intermediate wollastonite contents broke down at subsolidus temperatures, and nuclei formed outside grains of igneous pyroxene in some parts of a thin section and not in others. Philpotts (19) has described both igneous and metamorphic pyroxenes in single samples of norite related to anorthosite; some pyroxene in these norites also must have formed by local nucleation and recrystallization. CRYSTALLIZATION TEMPERATURES OF MONZONITES Pyroxenes with calcium contents intermediate between those of typical pigeonite (Wo 10 ) and augite (Woj^s) are rare in plutonic rocks, as a miscibility gap typically separates these compositions at temperatures of igneous processes in the crust. Most described occurrences of such intermediate pyroxenes are in volcanic rocks and lunar rocks, where their presence has been ascribed to metastable crystallization resulting from rapid cooling (Muir & Tilley, 194; Smith & Lindsley, 1971). Occurrences in slowly crystallized, intrusive rocks are restricted to kimberlites, which originated at high temperatures of the mantle (e.g. Boyd, 199), and to a few rocks with iron-rich silicates. Philpotts (19) described a subcalcic ferroaugite (Wo M ) from a Quebec mangerite. Binns (197) analyzed Fe-rich, calcic pigeonite and subcalcic augite in several meteorites. Such pyroxenes may be stable crystallization products, since the critical temperature of the augite-pigeonite miscibility gap decreases with iron-enrichment in the pyroxene quadrilateral (e.g. Smith, 1972; Ross et al., 1973). Compositions of Nain pyroxene grains with intermediate wollastonite contents and with lamellae sufficiently fine to average by microanalysis are plotted in Fig. together with phase boundaries at 15 kb for synthetic pyroxenes with Fe:Mg of 85:15. The analyses of the Nain pyroxenes plot in the quadrilateral between joins of constant Fe:Mg of 75:25 and 85:15. The critical temperature for the miscibility gap on the 85:15 join for synthetic pyroxenes lies between 915 and 955 C at 15 kb (Smith, 1972). On the 75:25 join it is at 980±10 C at 15 kb (Grover et al., 1972). The miscibility gap outlined at 15 kb may be useful in interpreting phase relations of natural, impure pyroxenes at pressures near 5 kb, the minimum value suggested for a Nain adamellite. Grover et al.

16 ASSOCIATED WITH THE NAIN ANORTHOSITE MASSIF 73 (1972) have calculated that critical temperatures of the two-phase gap on ironrich joins may decrease with declining pressure at a rate of 2-5 C per kilobar. They recognized, however, that the excess volume of mixing used to calculate this correction may not be appropriate (e.g. Smith, 1972), and that the derived 50 0 Mole percent wollastonite En 15 Fio.. Intermediate wollastonite contents of apparently primary pyroxene in monzonites and 2-178, shown together with phase relations on the 85:15 join determined by Smith (1972). The only compositions shown are those which could be successfully averaged by microanalysis. Most grains in monzonite are composed of broad lamellae of orthopyroxene and ferroaugite in nearly equal proportions (Fig. 3A); these lamellae are too coarse to be averaged. correction may be too large. A minimum value of 85 C at 5 kb for critical temperature on the 85:15 join can be calculated, applying the maximum pressure correction of Grover et al. to the low temperature bracket of Smith (1972) at 15 kb. This temperature is too low in the light of a comparison between experiments at 1 atmosphere and 15 kb. Ross et al. (1973) made homogenization experiments on lunar augites with compositions near the 85:15 join at 1 atmosphere. They did not outline the crest of the two-phase region, but their interpretation indicates that a miscibility gap about 10 mole per cent wollastonite in width is present at 90 C and 1 atmosphere, while Smith (1972) bracketed this 10 per cent gap between 915 C and 950 C at 15 kb. This comparison suggests that the critical temperature of the two-phase region does not decrease with pressure, and that it lies near 950 C at 5 kb on the join Fe: Mg:: 85:15. Compositions of Nain pyroxenes may not match the critical composition of the

17 74 DOUGLAS SMITHPYROXENE-OLIVINE-QUARTZ IN ROCKS two-phase region exactly, but the crest of the region (Smith, 1972; Grover et al., 1972) appears sufficiently broad that it is unlikely that compositions in the region Wo2o_3 0 could form more than 25 C below the critical temperature. Both the exact configuration of the two-phase region at 5 kb and the effects of minor elements on pyroxene miscibility are still unknown, but it seems unlikely on the basis of data outlined above that the Nain pyroxenes with intermediate wollastonite contents could have formed stably below 925 C. Melting experiments on syenites with iron-rich pyroxenes and olivines at 1 kb water-pressure have been reported by McDowell & Wyllie (1971). They found pyroxene and olivine to be near-liquidus phases and to persist to temperatures greater than 875 C, the limit of most of their experiments. The minerals persisted to at least 100 C over the solidus in two rocks. Data of Tuttle & Bowen (1958) and Merrill et al. (1970) indicate that solidus temperatures for alkali feldspar mixtures exceed 900 C at water-pressures less than 0-5 kb, and that these temperatures drop rapidly with increasing water content. The presence of pyroxenes of intermediate calcium contents, therefore, is consistent both with experiments on clinopyroxene solid solutions and with known melting relations of relatively dry magmas rich in alkali feldspar. Concurrently, the occurrence of such high-temperature pyroxenes supports the hypothesis that these monzonites formed from magmas undersaturated with water. The hypothesis that the magmas contained little water is consistent with the low abundance of primary hydrous silicates in these rocks. If the monzonites are a residual fraction of the magma from which the anorthosites were derived, then the parent magma of the anorthosites must have contained very little water. OTHER OCCURRENCES OF IRON-RICH PYROXENE IN PLUTONIC ROCKS Philpotts (19) described an adamellite (quartz mangerite sample P 4 45) from the Grenville township of Quebec with two iron-rich pyroxenes, one a subcalcic ferroaugite (Ca2 Mg 22 Fe 2 ) and one an inverted pigeonite (Ca^g^Fe,^. He obtained the ferroaugite composition by bulk chemical analysis of a mineral separate and the pigeonite analysis by optical measurements. A sample from the same outcrop was selected for electron-probe microanalysis to obtain a composition for the Ca-poor pyroxene and to compare results with those from Labrador. Pyroxene textures in the sample studied are different from those described by Philpotts, in that textural evidence of orthopyroxene inversion from pigeonite is lacking. The rock contains orthopyroxene (CaaFe^Mg^), relatively large ferroaugite grains (CaggFe^Mgaj), and relatively small ferroaugite grains (Ca^FesjMgjo) (Fig. 7, Table 1). Though the pyroxenes have similar Fe-Mg ratios to the ferroaugite analyzed by Philpotts, no subcalcic clinopyroxenes were found. The silicates are less iron-rich than those from Nain. The rocks in the Grenville township have been metamorphosed, and the differences between the results obtained here and those of Philpotts may reflect

18 ASSOCIATED WITH THE NAIN ANORTHOSITE MASSIF 75 different degrees of metamorphic equilibration in two rocks from the same outcrop. The fact that small ferroaugite grains are systematically more calcic than large grains in the same thin section may reflect differences in degree of equilibration on a very small scale, a situation similar to that which may have occurred in the Nain monzonites. If the pair of iron-rich pyroxene compositions 80 Mole percent FIG. 7. Average grain compositions in three rocks selected for comparison with Nain samples. Open squares, adamellite from Grenville Township, Quebec; open circles and closed circles, two quartz syenites from Greenland. All points represent pyroxene except the point near Fe 1O o, which represents nearly pure fayalite. The letter 'N' and accompanying lines indicate the area of points of Nain pyroxenes which equilibrated with ferroaugite, olivine, and quartz. described by Philpotts (19) did coexist in equilibrium, a two-phase region between Wo 9 and Wo 2 would be implied under magmatic conditions. Since this miscibility gap is centered about a composition of Wo , it is not compatible with the experiments of Smith (1972) or Grover et al. (1972) which indicate a gap centered about W A range of compositions of subcalcic ferroaugite is present in the Nain monzonites; the bulk analysis of Philpotts (19) might represent the average of such a compositional range, rather than defining a point on a solvus surface. It is also possible, however, that the presence of elements such as Al and Ti in natural pyroxenes may cause the configuration of the miscibility gap to differ from that defined for synthetic pyroxenes.

19 7 DOUGLAS SMITHPYROXENE-OLIVINE-QUARTZ IN ROCKS Some of the most iron-rich pyroxenes described in the literature are from the Precambrian terrains of Greenland (Ramberg & devore, 1951) and Norway (Griffin & Heier, 199). Pyroxenes and olivine in two quartz syenites from the Ketilidian intrusives of Greenland (Allaart et ai, 199) were analyzed (Fig. 7, Table 1). One rock contains hedenbergite (Ca 44 Fe5 B Mg 1 ), fayalite (Fe^MnaMgo.!), and quartz. The other contains primary orthopyroxene (Ca 2 Fe 79 Mg 19 ), ferroaugite (near Ca^Fe^Mg^) and altered olivine. Both contain amphibole. The Greenland rock with orthopyroxene contains more hornblende than the Nain rocks studied here which contain pigeonite, and the presence of primary orthopyroxene presumably reflects the lower crystallization temperatures of a more hydrous melt. The temperature interval for inversion of Fe-rich pigeonite to orthopyroxene at moderate pressures must lie above the water-saturated solidus temperatures of these quartz syenites but below the liquidus temperatures for dry adamellites, as would be expected from the experiments of Smith (1972). Compositions of Ca-poor pyroxenes intergrown with olivine, ferroaugite, and quartz in the Nain rocks plot in a linear group at the letter N in Fig. 7. The lines at this letter may define part of the boundary of the 'forbidden zone' at the temperature and pressure at which equilibration between the intergrown minerals ceased. Since the compositions of the Greenland orthopyroxene are more Fe-rich, the orthopyroxene could not have been stable at 1 atmosphere pressure, unless it crystallized at temperatures considerably lower than those of subsolidus equilibration of the Nain pyroxenes. The orthopyroxene contains about 77 mole percent ferrosilite (Fe/(Fe+Mn+Mg+Ca) = 0-77). If it crystallized near 750 C, then the pressure must have been near 2 kb, to be consistent with the data of Smith (1971) for synthetic orthopyroxenes. The more Fe-rich orthopyroxenes of Ramberg & devore (1951) (CagFe^MgjJ and Griffin & Heier (199) (Cs^Fe^Mg^) are from granulite terrains. Pressures at which these pyroxenes crystallized may be calculated when estimates of their temperatures of formation are available. CONCLUDING REMARKS Pyroxene-olivine assemblages in the granodiorite and two adamellites can be interpreted in the light of experiments on the pressure dependence of the stability of Fe-rich, synthetic pyroxenes. Compositions of the most Fe-rich crystals preserved suggest a minimum pressure of crystallization of 5 kb. Some pyroxene broke down below the solidus to form olivine and quartz: compositions of such reacted pyroxene plot in a narrow range of Fe: Mg, suggesting that the reacted assemblages equilibrated under similar conditions. The equilibration is consistent with the hypothesis that rocks in the Nain complex crystallized at depth and cooled slowly. Metastable crystallization seems unlikely in such an environment. If the pyroxene with intermediate wollastonite contents in the monzonites was stable at formation, then it must have crystallized near or above the crest of the

20 ASSOCIATED WITH THE NAIN ANORTHOSITE MASSIF 77 augite-pigeonite miscibility gap, probably above 925 C. The complexity of pyroxene assemblages in these rocks may reflect local intergranular recrystallization at subsolidus conditions. The pressure and temperature estimates may be revised as phase relations for natural, impure pyroxenes are studied at intermediate pressure (5-10 kb). It is clear, however, that detailed studies of compositions of Fe-rich silicates in the 'mangerite suite' may aid in understanding conditions of crystallization. The importance of water in the genesis of anorthosites has been debated: some investigators have argued for crystallization from H 2 O-rich magmas (e.g. Buddington, 198), while others have proposed a 'dry' origin (e.g. Philpotts, 19; de Waard, 198). Data discussed here support the latter position, since if the monzonites represent a residual fraction of the anorthosite magma, then the parent magma of the anorthosites must have been nearly anhydrous. ACKNOWLEDGEMENTS E. P. Wheeler, n, generously contributed samples, petrographic data, and discussions of results. D. Bridgwater and the Geological Survey of Greenland and A. R. Philpotts (through D. H. Lindsley) also contributed rocks. Versions of this manuscript were constructively criticized by D. S. Barker, D. H. Lindsley, S. A. Morse, M. Ross, and E. P. Wheeler, II. The project was begun while I held a postdoctoral fellowship at the Geophysical Laboratory. It was finished at the University of Texas at Austin, with support from National Science Foundation Grant GA 3103 and the University Research Institute. The electron probe at the University of Texas at Austin was obtained with support of the Department of Geological Sciences Geology Foundation. REFERENCES ALLAART, H. H., BRIDGWATER, D., & HENRIKSEN, N., 199. Pre-quaternary geology of Southwest Greenland and its bearing on North Atlantic correlation problems. Am. Ass. Petr. Geol. Mem. 12, BENCE, A. E., & ALBEE, A. L., 198. Empirical correction factors for the electron microanalysis of silicates and oxides. J. Geol. 7, BINNS, R. A., 197. Stony meteorites bearing maskelynite. Nature, 213, LONG, J. V. P., & REED, S. J. B., 193. Some naturally occurring members of theclinoenstatiteclinoferrosilite mineral series. Ibid. 198, BONNICHSEN, B., 199. Metamorphic pyroxenes and amphiboles in the Biwabik iron formation, Dunka River area, Minnesota. Spec. Pap. miner. Soc. Am. 2, BOYD, F. R., 199. Electron-probe study of diopside inclusions from kimberlite. Am. J. Sci. 27A, & BROWN, G. M., 199. Electron-probe study of pyroxene exsolution. Spec. Pap. miner. Soc. Am. 2, BUDDINGTON, A. F., 198. Adirondack anorthositic series. N.Y. Stale Mus. and Sci. Service Mem. 18, DE WAARD, D., 198. The anorthosite problem: the problem of the anorthosite-charnockite suite of rocks. Ibid & WHEELER, E. P., II, Chemical and petrologic trends in anorthositic and associated rocks of the Nain massif, Labrador. Lithos, 4, GRJFFIN, W. L., & HEIER, K. S., 199. Parageneses of garnet in granulite-facies rocks, Lofoten- Vesterallen, Norway. Contr. Mineral. Petrol. 23,

21 78 DOUGLAS SMITHPYROXENE-OLIVINE-QUARTZ IN ROCKS GROVER, J. E., LINDSLEY, D. H., & TURNOCK, A. C, Ca-Mg-Fe pyroxenes: subsolidus phase relations in iron-rich portions of the pyroxene quadrilateral. Abstr. geol. Soc. Am. 4, GRUENEWALDT, G. VON, On the phase-change orthopyroxene-pigeonite and the resulting textures in the main and upper zones of the Bushveld complex in the eastern Transvaal. Spec. Pub. geol. Soc. S. Africa, 1, HADIDIACOS, C. G., FINGER, L. W., & BOYD, F. R., Computer reduction of electron probe data. Yb. Carnegie Instn Wash. 9, 294. HEATH, S. A., & FAIRBAIRN, H. W., 198. Sr^/Sr" ratios in anorthosites and some associated rocks. N.Y. Stale Mus. andsci. Service Mem. 18, LINDSLEY, D. H., & MUNOZ, J. L., 199. Subsolidus relations along the join hedenbergite-ferrosilite. Am. J. Sci. 27A, MCDOWELL, S. D., & WYLLIE, P. J., Experimental studies of igneous rock series: the Kungnat syenite complex of southwest Greenland. /. Ceol. 79, MERRILL, R. B., ROBERTSON, J. K., & WYLLIE, P. J., Melting reactions in the system NaAlSi 3 O 8 -HiO to 20 kilobars compared with results for other feldspar-quartz-h,o and rock- H,0 systems. /. Geol. 78, MORSE, S. A., 199. The Kiglapait layered intrusion, Labrador. Mem. geol. Soc. Am. 112, An alternative model for the anorthositic and associated rocks of the Nain massif, Labrador. Lithos, 5, MUIR, I. D., & TILLEY, C. E., 194. Iron enrichment and pyroxene fractionation in tholeiites. Ceol. J. 4, PHILPOTTS, A. R., 19. Origin of the anorthosite-mangerite rocks in southern Quebec. J. Petrology, 7, 1-4. POLDERVAART, A., & HESS, H. H., Pyroxenes in the crystallization of basaltic magma. /. Geol. 59, RAMBERG, H., & DEVORE, G., The distribution of Fe" 1 " 1 " and Mg++ in coexisting divines and pyroxenes. Ibid Ross, M., HLTEBNER, J. S., & DOWTY, E., Delineation of the one atmosphere augite-pigeonite miscibility gap for pyroxene from lunar basalt Am. Miner., in press. SMITH, DOUGLAS, Stability of the assemblage iron-rich orthopyroxene-olivine-quartz. Am. J. Sci. Ill, a. Iron-rich pyroxenes. Yb. Carnegie Instn Wash. 9, & LINDSLEY, D. H., Stable and metastable augite crystallization trends in a single basalt flow. Am. Miner. 5, Stability of iron-rich pyroxene in the system CaSiO,-FeSiO,-MgSiO,. Ibid. 57, TUTTLE, O. F., & BOWEN, N. L., Origin of granite in the light of experimental studies in the system NaAlSijO 8 -KAlSi,O r -SiO 1 -H,O. Mem. geol. Soc. Am. 74, 153. WHEELER, E. P., II, Adamellite intrusive north of Davis Inlet, Labrador. Bull. geol. Soc. Am., Anorthosite-adamellite complex of Nain, Labrador. Ibid. 71, Minor intrusives associated with the Nain anorthosite. JVT. State Mus. and Sci. Service Mem. 18, YODER, H. S., TILLEY, C. E., & SCHAIRER, J. F., 193. Pyroxene quadrilateral. Yb. Carnegie Instn Wash. 2,