Origin of the Plutonic Mafic Rocks of Southern Nova Scotia

Origin of the plutonic mafic rocks of southern Nova Scotia

CARLOS A. R. DE ALBUQUERQUE Department of Geology, Saint Mary's University, Halifax, Canada B3H 3C3

ABSTRACT with Taylor's (1969) classification of volcanic rocks of island arcs,

based mostly on the Si02 and K20 contents. Mafic rocks of the Appalachian orogenic belt of southern Nova The geochemistry of norite, phlogopite-plagioclase peridotite, Scotia were analyzed for major elements, Ba, Sr, Rb, Zr, Nb, Y, and hornblende-biotite tonalite, and high-K diorite is described here. rare-earth elements. These plutonic rocks comprise plagioclase These rocks form small outcrops spatially associated with the to- peridotite, norite, high-K diorrte, and tonalite. Comparisons with nalitic, trondhjemitic, and granitic rocks of the Acadian southern volcanic rocks and other plutonic calc-alkalic rocks indicate that plutons or with the metasedimentary rocks of the Appalachian belt mafic rocks of orogenic belts are very similar in composition to of Nova Scotia in the southern region of this province (Fig. 1). continental tholeiitic basalts, whereas the plagioclase peridotite is The gabbroic rocks of orogenic belts have been considered as chemically comparable to picrites. The high-K diorites have dis- mafic calc-alkalic rocks, and there are surprisingly few detailed tinctive geochemical characteristics and show only a few analogies studies of such rocks (for example, Nockolds and Mitchell, 1948; with volcanic rocks. Towell and others, 1965; Best and Mercy, 1967). In light of the re- It is suggested, on the basis of the modeling of the data, that the cent developments in our knowledge of the basaltic and andesitic noritic magma could have been originated by partial melting of rocks of various geologic environments, it appeared worthwhile to upper-mantle material with light rare-earth element abundances make a comparative study of these volcanic rocks and of the mafic five to ten times those of chondrites and heavy rare-earth element calc-alkalic rocks of the Appalachian belt of Nova Scotia. abundances two to five times the chondritic abundances. The plagioclase peridotite can be derived from a contaminated magma, ANALYTICAL METHODS although the contamination appears to be of mantle (or lower crust) origin. An origin by hybridization of mafic magma with a The major elements were determined by the same techniques granitic liquid for the high-K diorites is consistent with the data (wet methods, X-ray fluorescence, and flame photometry) as de- available. scribed earlier (de Albuquerque, 1977). The trace elements were determined by energy-dispersive X-ray INTRODUCTION fluorescence and spark source mass spectrometry. The precision and accuracy of the methods used were given in de Albuquerque In orogenic belts such as the North American Cordillera, igneous (1977). Rare-earth elements (REE) were determined by spark- rocks ranging in composition from gabbro to granodiorite or gra- source mass spectrometry and the light rare-earth elements (LREE) nite form distinct plutonic associations (Larsen, 1948; Best and also by X-ray fluorescence. The U.S. Geological Survey standard Mercy, 1967). However, in some Paleozoic orogenic belts mafic rocks G-l, G-2, W-l, and BCR-1 were used as standards or un- rocks are rare, while granodiorite and granite are the most common knowns for evaluation of the accuracy of the determinations. plutonic rocks (Kolbe and Taylor, 1966; Oen, 1970; de Albuquerque, 1971, 1973; Gulson, 1972). Small plutons of mafic GEOLOGIC SETTING or intermediate rocks have been identified in these orogenic belts where they occur in close spatial association with the granodiorites The geology of the southern region of the province of Nova and granites. Although the petrogenetic significance of those mafic Scotia has been described by Taylor (1967) and the geologic setting rocks appears to have been overlooked for some time after the of the plutons also by me (de Albuquerque, 1977; Fig. 1). The pla- pioneering studies of Nockolds (1934) and Deer (1935), recently gioclase peridotite is an intrusive rock into the metasedimentary some authors have pointed out the special nature of some of these rocks of the Meguma group of Cambrian-Ordovician age. The rocks (de Albuquerque, 1971; Gulson, 1972; Gulson and others, radiometric age of the peridotite, determined by the Rb/Sr method 1972). In addition to "normal" igneous rocks such as gabbro or using phlogopite, is 362 ± 22 m.y. (R. F. Cormier, 1975, personal norite, some of the mafic rocks are characterized by high contents commun.). of biotite and amphibole, the latter mineral being typically an ac- The norite forms a small outcrop on the mainland and several tinolitic hornblende (Gulson and others, 1972; de Albuquerque, small islands not far from the shore. It appears to be completely 1974; Kanisawa, 1974). Gulson and others (1972) recognized the surrounded by the Barrington biotite tonalite. The age of the biotite difficulties in classifying some of these rock types in terms of con- (Rb/Sr method) is 362 ±18 m.y. (R. F. Cormier, 1975, personal ventional petrography and proposed a classification by analogy commun.). Contact relations between the norite and the Barrington

Geological Society of America Bulletin, Pan 1, v. 90, p. 719-731, 11 figs., 3 tables, August 1979, Doc. no. 90806.

719

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tonalite are difficult to observe because of the lack of outcrop. It is modal percentage of plagioclase is higher than that normally ac- possible that near the contact the norite has been modified by con- cepted for peridotites, this name is preferred because the chemistry tact metamorphism owing to the intrusion of the tonalite. The of the Nova Scotia rocks is similar to that o f peridotites associated modified "norite" shows extensive development of an actinolitic with calc-alkalic rocks such as those of the Scottish Caledonian hornblende apparently at the expense of pyroxene. complexes. The olivine shows a little iron oxide along fractures. The hornblende-biotite tonalite is intrusive into the Halifax for- Clinopyroxene and orthopyroxene are frequent. The composition mation of the Meguma group. of the clinopyroxene is Wo^En^sFsu).;, while that of or- The high-K diorite forms a relatively large outcrop near Birch- thopyroxene is En78.5Fs18i5 (analyses of separated minerals by wet town (Shelburne County), and other small bodies occur in the methods and X-ray fluorescence). Brown hornblende with pleo- vicinity of the Birchtown outcrop. This diorite is surrounded by the chroism Z brown, Y light brown, and X pale yellow appears to be a Shelburne trondhjemite, and the contacts between the two rock primary mineral. The crystals are often rimmed by green types appear to be sharp. The areal extension of the outcrops of hornblende. Phlogopite (ZY yellow-brown, X pale yellow) forms those rocks cannot be exactly determined owing to lack of expo- large patches enclosing crystals of other minerals. The plagioclase is sure. However, they appear not to exceed 1 km. Contact relation- zoned and calcic (An84). However, the rims of the crystals reach a ships are also difficult to establish, for the same reason. No appar- composition of An54. ent changes occur in the high-K diorite near the contact with the The norite is a medium-grained, massive rock. Slightly zoned

trondhjemite. plagioclase (An60 to An5;i), clinopyroxene fWo4,En47Fsi2), and or-

thopyroxene (En69Fs27.o) are subhedral. Brown hornblende (ZY PETROGRAPHY brown, X pale yellow), a primary mineral, and biotite (ZY red brown, X yellow) apparently also primary, occur in small amounts. Modal compositions of the rocks are given in Table 1. Near the contacts with the biotite tonalite the norite becomes a The plagioclase peridotite is a medium-grained rock with large hornblende gabbro: quartz, plagioclase, brown hornblende, green flakes of phlogopite, as much as 1 cm in diameter. Although the hornblende, and biotite.

METAMORPHIC ROCKS GRANITIC ROCKS (Amphibolite facies) Muscovite-biotite [-•-" j Halifax formation granodiorite and granite

'}>» Biotite trondhjemite I G I Goldenville formation Figure 1. Generalized geological map of southern Nova Scotia, based on Nova Scotia Department of Mines 1965 map and on DOLE RITE (Triassic) Biotite tonalite Taylor's (1967) survey. Schists that possibly belong to Goldenville PLUTONIC MAFIC ROCKS formation are included in Halifax formation.

I [plagioclase peridotite • I Hornblende - biotite I tonalite I •• I Norite 01 High-K diorite

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The hornblende-biotite tonalite is a medium-grained rock with a and others (1972) have proposed a classification by analogy with

slight orientation of the biotite. The plagioclase is zoned (An51 to volcanic rocks. This classification is adopted here, although the An2g) and subhedral. Hornblende with pleochroism Z bluish-green, Nova Scotia rocks are more mafic than those from Australia. Y brownish green, X light yellow, and biotite (ZY brown, X light yellow) are the only ferromagnesian minerals. Quartz is interstitial. GEOCHEMISTRY The high-K diorite is a massive, medium- to fine-grained rock. It is dark green owing to the large amounts of amphibole and biotite. Major Elements

The plagioclase is subhedral and zoned An67 to An2S. Light-green amphibole (Z light bluish green, Y light brownish green, X pale yel- Chemical analyses of representative samples of plagioclase low) and biotite, with pleochroism ZY yellow brown and X pale peridotite, norite, tonalite, and high-K diorite are given in Table 1. yellow are common and appear to have crystallized during the The plagioclase peridotite has a chemical composition similar to same period, as deduced from their complex petrographic relation- those of peridotites and pyroxenites of the Caledonian igneous ships. Biotite may fringe crystals of amphibole, while the am- complexes (Nockolds, 1941), except for the relatively high K con- phibole forms rims around other crystals of biotite. Quartz is in- tents of the former rocks. That composition is also similar to those terstitial, and ilmenite and sphene are the accessory minerals. of picrites, which constitute the most mafic rock type of the tholeii- It is difficult to classify these rocks according to the established tic volcanic association (Walker and Poldervaart, 1949; Stanton standards based on mineralogy or chemical composition. Gulson and Bell, 1969; Warden, 1970). Although rocks with the composi-

TABLE 1. CHEMICAL ANALYSES, NORMS, AND MODES OF NOVA SCOTIA ROCKS

Peridotite Norite Tonalite High-K diorite 1 2 3 4 5 6 7 8 9 10 11

Si02 45.97 45.65 45.28 52.13 52.79 50.48 52.86 58.85 57.55 56.07 58.19 Ti02 0.52 0.67 0.50 0.71 0.59 0.97 1.11 1.11 0.82 0.90 0.78 AI2O3 9.61 11.33 8.88 18.37 18.18 16.90 16.85 17.64 13.35 12.60 11.85 Fe2Oa 1.99 0.90 1.29 1.15 2.02 1.57 FeO 10.29* 8.23 10.75* 4.70 5.46* 7.43* 6.44* 4.53 5.90 5.67 5.96 MnO 0.17 0.17 0.18 0.08 0.13 0.13 0.12 0.08 0.11 0.12 0.11 MgO 22.49 20.73 24.20 7.44 8.08 10.31 8.80 3.51 7.87 8.29 8.04 CaO 7.38 7.52 7.06 11.02 10.79 9.05 8.94 6.25 6.59 7.34 7.05 Na20 1.17 1.37 1.06 2.88 2.67 2.45 2.93 3.57 2.13 2.09 2.19 K2O 0.85 0.94 0.76 0.58 0.44 0.83 0.55 1.72 2.39 2.52 2.04 P2O5 0.12 0.16 0.11 0.18 0.06 0.16 0.14 0.34 0.34 0.53 0.37 + t + f + H2O 1.43 1.20 1.22 0.67 0.81 1.29 1.26* 0.90 1.43 1.47 1.63 H2O- 0.12 0.06 0.13 0.05 0.10 0.10 Total 100.00 100.08 100.00 99.72 100.00 100.00 100.00 99.90 99.68 99.72 99.88

Norms Q 10.97 8.93 7.20 10.58 Or 4.85 5.40 4.33 3.40 2.58 4.88 3.25 10.27 14.38 15.18 12.30 Ab 10.15 11.95 9.17 25.77 23.80 21.90 26.25 32.45 19.47 19.12 20.10 An 17.93 21.42 16.60 35.36 36.07 32.53 31.13 27.37 20.16 17.91 16.84 Di 13.46 11.10 13.16 14.35 13.35 8.90 9.76 1.42 8.68 12.58 13.14 Hy 5.13 6.09 3.03 18.14 23.25 17.92 27.31 13.86 25.28 23.46 23.47 Ol 47.54 40.80 52.80 0.68 12.20 0.47 Mt 2.02 0.94 1.37 1.22 2.15 1.67 11 0.70 0.91 0.68 0.99 0.82 1.34 1.54 1.57 1.16 1.28 1.11 Ap 0.24 0.32 0.23 0.37 0.13 0.33 0.29 0.72 0.72 1.12 0.79

Modes Olivine 19.6 23.7 26.9 Plagioclase 18.7 20.5 15.0 59.0 63.2 60.7 47.3 57.6 31.4 33.3 33.5 Orthopyroxene 22.2 21.2 28.2 14.5 12.7 12.5 Clinopyroxene 14.6 11.4 10.5 9.6 12.2 14.7 3.5 Hornblende 12.9 18.5 11.1 13.4 9.4 9.7 41.5 14.0 29.2 29.8 31.0 Biotite 10.8 4.3 7.6 3.4 1.7 1.7 6.7 17.2 23.9 23.4 21.6 Quartz 0.5 0.3 10.1 14.5 11.4 12.5 K-feldspar 0.4 0.6 Accessories 1.2 0.4 0.7 0.5 0.5 0.7 1.Ò 1.1 0.6 1.5 1.4

Samples: 1, 2, 3: Phlogopite-bearing plagioclase peridotite (S-164, S-165, S-166); Liverpool, lat 44°00'51"N, long 64°40'16"W. 4, 5, 6, 7: Norite (S-102, S-143, S-149, S-156); Murray Cove, Barrington Passage, lat 43°30'44"N, long 65°39'06"W. Samples S-143 and S-149 were collected on islands in Murray Cove. 8: Hornblende-biotite tonalite (S-103); Cockerwit Passage, Barrington Passage, lat 43°33'06"N, long 65°46'04"W. 9, 10: High-K diorite (S-l, S-5); Birchtown Quarries, Shelburne, lat 43°44'32"N, long 65°24'00"W. 11: High-K diorite (S-25); Small outcrop southwest of the Birchtown Quar- ries, lat 43°44'21"N, long 65°24'13"W. * Total iron as FeO. Includes H20~.

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tion of picrite have been identified among the plutonic rocks as- hornblende-biotite tonalite is similar to those of diorites and tona- sociated with alkali basalts, they are not common in the calc-alkalic lites of calc-alkalic series such as those of the Southern California plutonic association. batholith (Larsen, 1948). The norite is chemically similar to mafic rocks of the Caledonian The main difference between the Nova Scotia rocks and those of complexes of Scotland (Nockolds, 1941; Nockolds and Mitchell, other complexes of plutonic or volcanic rocks is the Mg contents 1948) or to the San Marcos gabbro of the Southern California and Mg/Fe ratios, which are relatively high in the Nova Scotia batholith (Larsen, 1948) or, among volcanic rocks, to continental rocks. The plot of the plagioclase peridotite, norite, and tonalite tholeiites — namely, the chilled diabase of the Palisades sill, New and also of the spatially related tonalitic to granitic rocks of south- Jersey (Walker, 1969), or the Skaergaard marginal gabbro (Wager ern Nova Scotia (de Albuquerque, 1977) in the AFM diagram (Fig.

and Brown, 1967). Although the high A1203 content of the norite is 2) defines a. trend of very little iron enrichment ; this is not similar to more similar to those of high-Al203 basalts, the affinities with this the calc-alkalic trend of differentiation. The apparent similarity of rock type are not confirmed by the comparison of the contents of the trend defined by the Nova Scotia tonalite, granodiorite, and other elements or of element ratios. The composition of the granite with that defined for comparable rock types of the Southern

rt,Oj+F«o

Figure 2. AFM diagram of rocks of Nova Scotia southern plutons. Solid line shows, for comparison, trend of rocks of Southern California batholith. Nova Scotia rocks: P = peridotite; N = norite; D = high-K diorites; HT := hornblende-biotite tonalite; BT = biotite tonalite; T = trondhjemite; G = granite; GD = granodiorite.

riagO+KtO

TABLE 2. TRACE-ELEMENT CONTENTS OF ROCKS

Peridotite Norite Tonalite High-K diorite BCR-1 1 2 3 4 5 6 8 9 10 11

Sr 287 324 273 597 592 472 365 202 270 260 307 Ba 462 494 411 188 167 222 324 565 796 586 629 Rb 29 30.5 25 19 16 29 55 89 107 81 46 Zr 92 105 88 106 102 127 153 131 173 154 161 Nb 7.8 8.8 8.0 9.5 9.0 11 9.5 7.8 13 8.7 13.5 Y 13.5 11 11 18 11 18 24 20 24 21 32 La 16 18 13.5 11 8.0 14.5 14 19 25 21.5 27 Ce 33.5 43 30 25.5 21 34.5 32.5 42 53.5 48 56 Pr 3.5 5.2 3.1 3.3 2.1 4.5 5.8 7.5 5.8 7.2 Nd 13.8 22.5 11.3 15.5 8.7 18 20.5 23.5 28.5 25.5 27.5 Sm 2.3 3.6 1.85 3.3 1.7 3.4 4.3 5.1 4.9 4.6 6.5 Eu 0.70 1.0 0.60 1.1 0.63 1.15 0.80 1.1 1.1 1.0 2.1 Gd 2.5 4.0 2.35 3.2 1.9 4.2 3.6 3.6 5.0 4.3 7.2 Tb 0.38 0.60 0.35 0.50 0.30 0.65 0.60 0.65 0.90 0.70 1.1 Dy 2.15 2.75 1.72 3.2 2.0 3.7 3.2 3.6 5.0 4.2 6.1 Ho 0.45 0.55 0.35 0.60 0.37 0.75 0.60 0.80 1.0 0.80 1.2 Er 1.3 1.6 1.05 1.8 1.0 2.1 1.6 2.4 2.5 2.6 3.5 Yb 1.2 1.4 1.1 1.6 1.2 2.2 1.6 2.4 2.5 1.8 Lu 0.27 0.27 0.15 0.25 0.35 0.25 0.5

Note: See Table 1 for sample locations and descriptions.

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California batholith (Larsen, 1948) is probably coincidental, as the K/Na ratios and K and Na contents of the high-K diorites are simi-

Nova Scotia granodiorite plots closer to the (Na20 + K20) apex lar to those of some shoshonitic volcanic rocks (compare Jakes and than the granite. Similarly, the high-K diorite has higher K contents White, 1969). than the hornblende-biotite tonalite, and the latter rock plots nearer the alkali apex than the high-K diorite. These observations Trace Elements are at variance with the trends expected and observed in series of differentiation. Therefore, a more complex model is considered The trace-element contents of representative rocks of the Nova here for the origin of the plutonic mafic and granitic rocks of south- Scotia southern plutons are given in Table 2. ern Nova Scotia. A detailed comparison of various trace elements of the Nova The high-K diorites have a distinctive chemical composition, Scotia mafic rocks with those of volcanic rocks is made below. which is different from those of "normal" igneous rocks, as noted Meaningful comparisons with the mafic rocks of the plutonic earlier by Gulson and others (1972). In particular, the Mg contents calc-alkalic association cannot be made at this time owing to the and Mg/Fe ratios of those rocks are high and similar to those of lack of data on these rocks for most trace elements. gabbroic rocks, whereas the K contents are higher and the Na contents are lower than those of most gabbroic and dioritic rocks. The COMPARISON WITH VOLCANIC ROCKS

Recent extensive geochemical studies of volcanic rocks permit the detailed comparison of the geochemistry of the mafic plutonic Figure 3. K/Rb dia- rocks of Nova Scotia with the various magma types of volcanic gram. Diamonds = rocks. peridotite; squares = norite; circles = high-K diorite. Field bounded by K/Rb Diagram crosses = intermediate basalts; field bounded by The K/Rb ratios of volcanic rocks are plotted in Figure 3. The dotted line = Karroo oceanic tholeiites, with low contents of these elements and high dolerites; field bounded ratios, plot in a distinctive field. The volcanic rocks of island arcs by dash-dot line = K/Rb also have high K/Rb, except for the less mafic rock types, which variation of individual series of tholeiites, An- tarctica and New Zea- land. References (Figs. 3 through 8): 1, de Albu- querque and others (1972); 2, Baker (1969); 3, Baxter (1975); 4, Cann (1970); 5, Comp- ston and others (1968); 6, Cox and others (1967); 7, DeLong K(%) (1974); 8, Erlank and Hofmeyr (1966); 9, Ewart and Bryan (1973); 10, Ewart and others (1973); 11, Ewart and Stipp (1968); 12, Gass and others (1973); 13, Gill (1970); 14, Gill (1974); 15, Gulson (1972); 16, Gunn (1965); 17, Gunn (1966); 18, Hedge (1966); 19, Jakes and Gill (1970); 20, Kay and others (1970); 21, Kes- son (1973); 22, Lowder and Carmichael (1970); 23, Ridley (1970); 24, i i i i Taylor (1969); 25, 20 30 Taylor and others (1969); 26, Weigand and Rb Ragland (1970). (ppm)

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show crustal ratios (of less than 300). In both associations the K/Rb characterized by low Rb/Sr ratios, with a relatively small variation ratios remain constant or decrease slightly with an increase in the K of the Sr contents. Island-arc suites such as those from New Zea- contents. The continental tholeiitic basalts and dolerites show only land (Ewart and Stipp, 1968) and Tonga (Ewart and others, 1973) very little fractionation of Rb relative to K. This is Shaw's (1968) show a distinctive trend of Rb enrichment with a small variation of "main trend." Relatively high K/Rb ratios (~ 300) are, however, the Sr contents. However, the tholeiitic rocks of Tonga (Ewart and characteristic of the Karroo dolerites. The intermediate-type basalts others, 1973) have variable Sr contents for a small variation of Rb. of the Red Sea (Gass and others, 1973) and Mauritius (Baxter, The Rb/Sr ratios of these rocks are similar to those of oceanic 1975) have K/Rb ratios with values overlapping those of some tholeiites. A trend similar to that of island-arc suites is also ob- island-arc rocks and continental tholeiitic rocks. Alkali basalts served in tholeiites such as those from Antarctica (Compston and from both oceanic and continental environments have variable others, 1968) and eastern North America (Weigand and Ragland, ratios (300 to 700) for relatively high K contents (0.80%). This 1970). However, considerable variation of the Sr contents is seen in ratio in the plagioclase peridotite, norite, and tonalite is similar to continental tholeiites. It is noteworthy that tholeiites of continental those of continental tholeiites. The K/Rb ratio of the high-K diorite margins, such as those from the western United States and Japan is similar to those of island-arc rocks and continental tholeiites for (Hedge, 1966), have low FLb/Sr ratios (0.02 to 0.05), more similar K contents of about 2%. to those of oceanic tholeiites. Intermediate-type basalts show higher ratios (0.03 to 0.10) and relatively high Sr contents. The K/Ba Diagram Karroo basalts (Cox and others, 1967), among continental tholeiites, have ratios and Sr contents similar to those of The patterns of K/Ba are similar to those defined for the K/Rb intermediate-type basalts. Alkali basalts of both continental and ratio, except for some overlap of the fields of continental tholeiites, oceanic environments have Rb/Sr in the range 0.02 to 0.05, intermediate-type basalts, and alkali basalts. The plagioclase although the Sr contents are systematically above 500 ppm, higher peridotites and the norites plot in the field of continental tholeiites than those of most other basaltic rocks. (Fig. 4). It is apparent from Figure 5 that there is considerable scatter for continental tholeiites, although rocks from one region do not show Rb/Sr Diagram much scatter when plotted in the diagram. The plagioclase peridotites have higher Rb/Sr ratios than the norites, while those of the The Rb and Sr contents of rocks occurring in various geologic latter rocks are comparable to those of some alkali basalts, Karroo environments are plotted in Figure 5. The oceanic tholeiites are basalts (tholeiitic), and some island-arc rocks. The Rb/Sr ratios and

Figure 4. K/Ba diagram. Symbols as in Fig- K/Ba =10 ure 3. Field bounded by crosses = intermediate basalts.

CONTINENTAL THOLEIITES

J I L I I I I

B a (pprM

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Rb contents of the high-K diorites are comparable only to those of Nova Scotia plot in the field of tholeiites, and the high-K diorites some island-arc rocks such as andesites and dacites from New Zea- plot in the area where there is an overlap of the fields of tholeiites land (Ewart and Stipp, 1968). and island-arc rocks.

Ti02-Zr/P205 Diagram Ti02-Y/Nb Diagram

Some of the diagrams of Floyd and Winchester (1975) have been The plot of the volcanic rocks of island arcs in this diagram modified to include the basaltic and andesitic rocks of island arcs defines a new field, approximately paralleling that of the oceanic and the intermediate-type basaltic rocks (after Gass and others, tholeiites. These fields can be characterized by large variations of

1973; Baxter, 1975; Figs. 6 through 8). the Y/Nb ratios with relatively small variation of the Ti02 contents. Most mafic volcanic rocks of island arcs plot outside the fields The Ti02 contents of the island-arc rocks are low compared to defined by Floyd and Winchester (1975; Fig. 6). Although there is those of tholeiites (Fig. 7). The intermediate-type basaltic rocks plot some overlap with the fields of continental and oceanic tholeiites, it in the area of overlap of the fields of alkali basalts and continental appears possible to define a field of island-arc rocks characterized tholeiites. The mafic rocks of Nova Scotia plot in the area where

by relatively lowTi02 and/or low Zr/P205 ratios. Intermediate-type there is an overlap of the fields of tholeiites and island-arc rocks for basalts plot mostly in the field of alkali basalts. The mafic rocks of low Ti02 contents and Y/Nb ratios.

Rb (ppm)

Figure 5. Rb/Sr diagram. Symbols as in Fig- ure 3. 0 = tholeiites of continental margins.

Sr (ppm)

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Nb/Y-Zr/P205 Diagram The REE patterns of the high-K diorites show a moderate fractionation of HREE relative to LREE and no fractionation of HREE. Intermediate-type basalts define an area overlapping the fields of (Fig. 9, bottom). All rocks have a small negative Eu anomaly. LREE alkali basalts and continental basalts (Fig. 8). The data for island- abundances are higher than those of the norite, but the HREE are arc rocks are scanty, and therefore no attempt was made to include only slightly higher than in the norite and peridotite. High-K dior- these rocks in the diagram. The mafic rocks of Nova Scotia plot in ites from Australia have higher REE abundances with more frac- the field of continental tholeiites. tionated patterns than those of the Nova Scotia rocks (Fig. 10, b). The patterns of the latter rocks are more similar to those of the Rare-earth Elements low-Si diorites, except for the absence of a negative Eu anomaly in these rocks. The high-K diorites have higher REE abundances and a The REE pattern of the norite is similar to those of continental much smaller negative Eu anomaly than the hornblende-biotite to- tholeiites and characterized by a moderate fractionation of the nalite. heavy rare-earth elements (HREE) relative to the light (LREE), very little fractionation of the HREE, and lack of an Eu -anomaly (Fig. DISCUSSION 9, center). Figure 10, a, shows that while the REE patterns of the continental tholeiites are moderately fractionated, the REE abun- The major-element composition of the plagioclase peridotite is dances are different in basalts from various regions. The San Mar- similar to those of picrite basalts associated with- tholeiites. The cos gabbro of the Southern California batholith also has a similarity in chemical composition of the norites and of continental moderately fractionated pattern, although it shows a positive Eu tholeiitic rocks is noteworthy. These extend TO trace-element abun- anomaly. REE patterns and abundances of some island-arc ande- dances, as is evident from inspection of Figures 3 through 8 and 10. sites (Taylor, 1969; Gill, 1974) are very similar to those of the no- Although the data are scarce, the plagioclase periodotite and norite rite. The REE pattern of the plagioclase peridotite differs from that appear to be similar in composition to the peridotitic and gabbroic of the norite in that it shows a larger enrichment of LREE and a rocks of other orogenic belts, such as those of the Southern small negative Eu anomaly (Fig. 9, top). The hornblende-biotite to- California batholith (Larsen, 1948) and Scottish Caledonian com- nalite has a REE pattern similar to that of the peridotite, except for plexes (Nockolds, 1941; Nockolds and Mitchell, 1948). the existence of a large negative Eu anomaly in the former rock, Turner and Verhoogen (1960) had already considered the while REE abundances are slightly lower in the tonalite (Fig. 9, bot- tholeiitic magma type as the parental magma of calc-alkalic mafic tom). rocks. In light of the evidence presented above, it appears possible to identify the mafic magmas of the continental, plutonic calc- alkalic suites with the continental tholeiites magma type. Differ- entiation of continental tholeiitic magma in the plutonic conditions prevailing in orogenic belts may originate the calc-alkalic series, possibly extending in composition to tonalite or granodiorite. A 4.0 mechanism of differentiation in which fth plays a major role and leads to the crystallization of calc-alkalic rocks was proposed by INTERMEDIATE Osborne (1962) and later applied and elaborated on by Best and "BASALTS Mercy (1967) for the Guadalupe complex of the Sierra Nevada

3.0 - CONTINENTAL THOLEIITES

O F= 2.0 - OCEANIC THOLEIITES

~ ISLAND ARC ROCKS

0.05 0.10 0.15 Zr/PzOs Y/Nb

Figure 6. Ti02-Zr/P205 diagram (after Floyd and Winchester, Figure 7. Ti02-Y/Nb diagram (after Floyd and Winchester, 1975). Symbols as in Figure 3. 1975). Symbols as in Figure 3.

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batholith. Alternatively, it is possible that the model of partial melt- mantle material with composition olivine 50% (Ol0.5o), clinopyrox-

ing suggested by Green and Ringwood (1968) is valid for most ene 25% (Cpxo.25), and orthopyroxene 25% (Opx0.2s). The rocks of the series. This hypothesis is not in conflict with the obser- hypothetical residuum has the composition of lherzolite vations made above on the analogy between continental tholeiitic (Olo.62Cpxo.15Opxo.23), assuming that the magma resulted from the magmas and calc-alkalic magma. melting of clinopyroxene and orthopyroxene in the proportion 2:1 and ignoring possible variations in the composition of the residuum Origin of the Noritic Magma due to the incongruent melting of these mineral phases. The partition coefficients used are those for basaltic rocks (Table 3). The hypothesis of an origin of the noritic magma by partial melt- If the norite crystallized from a residual magma, it is likely that ing of oceanic tholeiite above a Benioff zone is not supported by the the parental magma had the composition of olivine tholeiite. Frac- geochemical data. In particular, the high Mg contents and Mg/Fe tionation of an olivine tholeiitic magma with olivine in the liquidus ratios of the norite cannot be explained unless the parental material would lead to an increase of the concentrations of elements such as had even higher contents of Mg and Mg/Fe ratios. Such high values the REE in the liquid, because the partition coefficients {sll) are less are not normal for oceanic tholeiites. The possibility of derivation than unity for olivine. This could explain the high REE concen- of the noritic magma from garnet-peridotite or eclogite is not com- trations in the norite relative to oceanic tholeiites and island-arc patible with evidence such as the REE patterns. These patterns of basalts. However, this mechanism does not explain the fractiona- the norite show little fractionation, which indicates that garnet was tion of LREE relative to HREE in the norite pattern. Crystallization not an important constituent of the residuum. However, the of 30% olivine from the olivine tholeiitic magma would leave the geochemical data (major-element composition, REE, and other liquid enriched in REE with no change in the pattern (Fig. 11). If trace-element abundances and patterns) are consistent with a this model is considered, and after similar calculations as for the model involving generation of the mafic magma in the upper man- model above, the agreement is good with the norite abundances for tle in equilibrium with a peridotite residuum. In particular, the REE concentrations of LREE and HREE in the parental mantle material pattern of the norite is similar to those of liquids in equilibrium of five and two and one-half times, respectively, those of chondrites with a residuum in which the REE patterns are dominated by cli- (Fig. 11). Constraints on the extent of fractionation may be placed, nopyroxene and orthopyroxene, although the pattern of the paren- however, in this model, as the Mg contents of the norite are rela- tal material is not chondritic (Fig. 11). tively high. If orthopyroxene were the liquidus phase, a model of If it is assumed that the norite crystallized from a primary magma, the high Mg contents (and high Mg/Fe ratios) indicate relatively high degrees of partial melting. However, the REE pattern and abundances of the norite fit this model only if the magma was derived from either undepleted or, perhaps more probable, enriched mantle material, with LREE abundances about ten times those of chondrites and HREE abundances five times those of chrondrites (compare Fig. 11, top). These calculated values were obtained for a degree of partial melting of the order of 20% of

\ 3.0

ALKALI Nb/Y BASALTS

2.0

^INTERMEDIATE BASALTS • on High-K Diorite 0 Tonalité 1.0

_l I 1 1 1 1 1 1 I 1 1 I 1 1 i_ 0.05 0.10 0.15 La Ce Pr Nd (Pm) Sm Eu Gd Tb Dy Ho Er (Tm) Yb Lu Zr/PaOs Figure 9. REE abundance patterns, normalized to chondrite

Figure 8. Nb/Y-Zr/P205 diagram (after Floyd and Winchester, abundances, of Nova Scotia mafic rocks: plagioclase peridotite, 1975). Symbols as in Figure 3. norite, high-K diorite, and hornblende-biotite tonalité.

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fractional crystallization indicates that the REE patterns of the re- A possible cumulative origin for this rock is not suggested by the sidual liquid are more fractionated than in the model involving geologic or petrographic studies, and this is confirmed by geochem- crystallization of olivine (Fig. 11). The parental material would ical evidence such as high K and REE contents. It is possible that an have a fractionated REE pattern relative to the chondritic pattern accumulate can have some "trapped" liquid, which normally even given the assumption that orthopyroxene is the liquidus would be enriched in elements such as Rb or REE. The high K/Rb phase. ratio of this rock, in spite of the fact that it is phlogopite-bearing, It may be added that the considerations made above for the does not lend support to this hypothesis. model of partial melting involving clinopyroxene are also valid for The K contents of the peridotite are relatively high, and this melting in a. hydrous mantle if amphibole were a mineral phase in- suggests that the rock has been contaminated by crustal continental volved in the process of partial melting. This is due to the similarity material. It has been observed that picrites may have relatively high of the values of the partition coefficients for clinopyroxene and K contents, which cannot be attributed to contamination by conti- amphibole of basaltic rocks. nental material, because the rocks occur in an oceanic environment. It is possible that contamination of a picritic magma, originated by Origin of the Plagioclase Peridotite fractionation or as a primary magma, by a relatively K-rich magma led to the formation of phlogopite. This would explain the high Ba The origin of the plagioclase peridotite is not unequivocally ex- contents of the plagioclase peridotite, while the relatively high K/Rb plained by the available data. The geochemical similarities with the ratio of this rock would preclude the hypothesis of the K-rich melt norite would lend support to the hypothesis of fractionation of a being a granitic magma with low K/Rb ratios. single magma type. However, the K contents and the REE abun- Although the conclusions drawn for the origin of the plagioclase dances of the plagioclase peridotite are higher than those of the no- peridotite can be only tentative, it appears that the most likely rite, an otherwise less mafic rock. Therefore the parental magmas hypotheses are those involving fractional crystallization of a of these rock types might be similar, but not identical. tholeiitic magma with subsequent contamination, possibly in the

Figure 10. Comparison diagram of REE abundance patterns (normalized to those of chondrites), a: Nova Scotia norite; San Marcos gabbro, after Towell and others (1965); Columbia river basalts, after Schmitt and others (1964); Steen Mountain basalts, after Helmke and Haskin (1973); Karroo basalts, after Philpotts and Schnetz- ler (1968). b: Nova Scotia high-K diorites; Australia high-K diorites, after Gulson and others (1972); average andesite, after Taylor (1969); island-arc andesite, after Jakes and Gill (1970); Namosi andesite, after Gill (1974).

La Ce Pr Nd (Pm)Sm Eu Gd Tb Dy Ho Er fTm) Yb Lu

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upper mantle, before emplacement, or that this rock crystallized 1974) and those of northern Portugal and Nova Scotia are mean- from a tholeiitic magma with relatively high contents of elements ingful and reflect different origins for those rocks from various such as K and Ba and the LREE. localities.

Origin of the High-K Diorites Relationships with the Granitic Rocks

Several geochemical characteristics of the high-K diorites are The ages of the ultramafic and mafic rocks of southern Nova similar to those of the more differentiated andesitic to dacitic rocks Scotia suggest that these rocks and the tonalitic to granitic rocks of of island arcs (Figs. 3 through 8). The analogies between high-K the same area were intruded during the same tectonic episode. The diorites of Australia and island-arc rocks led Gulson (1972) to pos- hypothesis of differentiation to explain the origin of the various tulate that the former rocks are the plutonic equivalents of island- rocks is difficult to accept in view of the observations made on arc andesites. However, a marked "mafic" geochemistry in rocks trends of variation. However, this does not preclude the possibility comparable to high-K diorites from northern Portugal has been of a causal relationship between the mafic and granitic rocks. In noted (de Albuquerque, 1971). In particular, the Mg/Fe ratios of fact, the occurrence of these two rock types in a spatial and, appar- these rocks and of the high-K diorites of Nova Scotia are high and ently, temporal association is probably the result of magmatic ac- more similar to those of gabbroic rocks. In fact, their geochemistry tivity in the crust originated by the ascent of mantle-derived basal- is unusual, with characteristics of both mafic rocks (high Mg/Fe tic magmas. These show geochemical features of tholeiitic or calc-

ratios) and shoshonites (Na20/K20 ratios of about 1). However, the similarities of the high-K diorites with shoshonites do not ex- TABLE 3. PARTITION COEFFICIENTS FOR tend to several trace elements, such as Nb and Zr. The inspection of BASALTIC ROCKS the K/Rb and K/Ba diagrams (Figs. 3, 4) shows that there is a con- vergence of these ratios for different rock types at K contents above Olivine Orthopyroxene Clinopyroxene 1%. This explains the apparent similarities of high-K diorites with (1 specimen) (2 specimens) (2 specimens) rocks otherwise very different in composition, such as alkali Ce 0.009 0.035 0.14 basalts. Nd 0.010 0.046 0.30 An origin such as that proposed by Gulson and others (1972) for Sm 0.011 0.074 0.47 high-K diorites is not precluded by the data available for the Nova Eu 0.010 0.070 0.45 0.012 0.12 0.60 Scotia rocks. However, these data are also consistent with a model Gd Dy 0.014 0.21 0.67 of hybridization of mafic tholeiitic magma of the orogenic belt by Er 0.017 0.31 0.63 granitic magma before emplacement (de Albuquerque, 1971.) It is Yb 0.023 0.46 0.60 possible that differences in composition between the high-K diorites of Australia (Gulston and others, 1972) and Japan (Kanisawa, Note: After Schnetzler and Philpotts (1970).

Figure 11. Hypothetical REE patterns of "basaltic" magma calculated for models involving partial melting or fractional crystallization. Partial melting (top) = 20% 10 X CH. melting of "mantle" material of composition 01o.5oCpxo.250pxo.25, assuming that cli- F30.20 nopyroxene and orthopyroxene melt in the proportion 2:1. Composition of residual 20 - 5x CH. material = Olo.63Cpxo.14Opxo.23- REE abun- 5xCH. dances of parental material ten and live times those of chondrites for La and HREE, 10 b NO R ITE ' respectively (dash-dot-dot line), and five and two and one-half times the chondritic LÜ 2.5XCH.. abundances for same elements (dashed g 5h line). Equations of Shaw (1970) were used o for model involving equilibrium with con- H—I—I—I—f- H—I—I—I—I—I—h tinuous separation of liquid fractions. O X 50 - FRACTIONAL CRYSTALLIZATION Fractional crystallization (bottom) = model o involving magma derived from parental \ is: material with REE abundances five and two and one-half times those of chondrites for §20 F = 0.30 OL-^"".:«^ = 0.20 OPX La and HREE, respectively, and subsequent fractional crystallization with separation of 30% olivine (dash-dot-dot line), or of 20% 10 orthopyroxene (dashed line). Equations of Helmke and Haskin (1973) were used in these calculations. Partition coefficients are 5 -j J L i I I i I J L J L given in Table 3. La Ce Pr Nd (Pm) Sm Eu Gd Tb Dy Ho Er (Tro) Yb Lu

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alkalic rocks. However, the mixing of mafic and granitic magmas plain by a model of partial melting in which these rocks would be or of partly solid gabbroic material with granitic magma leads to the plutonic equivalents of high-K andesites, this cannot be ruled the crystallization of high-K diorites with geochemical characteris- out, on the basis of the evidence available. tics of both mafic and felsic calc-alkalic rocks. These hybrid rocks Island-arc basalts are chemically similar to oceanic tholeiites, and are, however, different from the intermediate rock types of the apparently there is a continuous geochemical gradation through calc-alkalic series. These differences relate to the higher Fe and Mg andesite to dacite of the island-arc series. This may indicate that contents (and Mg/Fe ratio) and higher K contents and K/Na ratios these various magma types result from different degrees of partial in the high-K diorites relative to those intermediate rocks. melting, as suggested by Green and Ringwood (1968) on the basis It is possible that the lack of large outcrops of noritic or gabbroic of experimental data. The very small variation in the Sr contents of rocks at such crustal levels may be due to the extensive melting of rocks of some island-arc series lends support to this theory, because metasedimentary rocks with the formation of granitic magmas that the presence of plagioclase in the residual materials "buffers" Sr in would prevent the ascent to higher crustal levels of the "basaltic" a model of partial melting, but not as effectively in a model of frac- magma (compare Fyfe, 1973). tional crystallization. Alternatively, it is possible that fractional crystallization of mafic magma plays the major role in the forma- Rb/Sr Trends tion of the calc-alkalic series. It may be noted that if the model of fractional crystallization is valid for both island-arc and orogenic One of the observations that can be made from the diagrams of belts, the evidence presented here points to the possible develop- Figure 5 is the trend of Rb enrichment with little variation of Sr ment of calc-alkalic series from both oceanic tholeiite magma or noted for island-arc rock series in particular. This probably indi- similar basaltic magma in island arcs and continental tholeiite cates equilibrium between the magmas and residual material con- magma in orogenic belts. taining a mineral phase (or phases) that effectively "buffers" Sr The analogy between continental tholeiites and mafic calc-alkalic (compare de Albuquerque, 1977, Fig. 9). Plagioclase would satisfy rocks lends support to a model of magmatic evolution in orogenic that requirement because its partition coefficients// > 1. However, belts involving the ascent of tholeiitic magma to the lower(?) crust the lack of a negative Eu anomaly in those rocks precludes the pos- leading to the underplating postulated by Fyfe (1973) and envis- sibility of large amounts of plagioclase being present in the re- aged earlier by Piwinskii and Wyllie (1968). The existence of these siduum. The quantitative modeling shows that a liquid with 265 mafic magmas at crustal depths would raise the geothermal gra- ppm Sr can be derived from a parental material with 75 ppm Sr by dient, and this could originate low-pressure metamorphism and 20% partial melting. The residuum would contain 10% plagioclase also trigger partial melting of the metasedimentary rocks of the and 20% clinopyroxene, the remaining 70% being olivine and/or orogenic belt with the formation of granitic magmas. orthopyroxene and/or othe® minerals with low values of KD(sll) for Sr. The REE pattern of such a melt would show a very small nega- ACKNOWLEDGMENTS tive Eu anomaly. If this hypothesis is valid, it implies that such island-arc rocks or other rocks with the same characteristics crys- This research was supported by the National Research Council tallized from melts that were in equilibrium with a plagioclase- of Canada. bearing solid, and therefore at depths of less than 35 km.

CONCLUSIONS REFERENCES CITED

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