Origin of the Plutonic Mafic Rocks of Southern Nova Scotia
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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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/8/719/3418914/i0016-7606-90-8-719.pdf by guest on 01 October 2021 720 C.A.R. DE ALBUQUERQUE 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/8/719/3418914/i0016-7606-90-8-719.pdf by guest on 01 October 2021 PLUTONIC MAFIC ROCKS OF SOUTHERN NOVA SCOTIA 721 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.