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Using Δ O of Zircon to Determine the Magmatic Evolution and Degrees Of Using 18O of zircon to determine the magmatic evolution and degrees of contamination in Peggy’s Cove monzogranite, Halifax pluton, Nova Scotia Kendra Murray Senior Integrative Exercise March 9, 2007 Submitted in partial fulfillment of the requirements for a Bachelor of Arts degree from Carleton College, Northfield, Minnesota Table of Contents Abstract Introduction……………………………………………………………………...... 1 Geologic Setting………………………………………………………………....... 5 Halifax Pluton Analytical Methods……………………………………………………………….. 13 Petrography……………………………………………………………………….. 14 Igneous Textures Cathodoluminescence Results……………………………………………………………………………... 17 Whole Rock Geochemistry 18O Zircon Discussion…………………………………………………………………………. 26 Oxygen Isotopes Implications for Post-magmatic Isotope Exchange Conclusions………………………………………………………………………... 34 Acknowledgements……………………………………………………………...... 35 References Cited………………………………………………………………….. 36 Using 18O of zircon to determine the magmatic evolution and degrees of contamination in Peggy’s Cove monzogranite, Halifax pluton, Nova Scotia Kendra Murray Carleton College Senior Integrative Exercise March 9, 2007 Advisors: Cameron Davidson, Carleton College Jade Star Lackey, The College of Wooster Abstract The Halifax pluton is the largest discrete granitoid body of the Late Devonian peraluminous South Mountain batholith complex associated with the Acadian Orogeny. We report the first 18O values in zircon from the Peggy’s Cove monzogranite, a unit on the outer edge of the Halifax pluton, which vary from 7.71-8.26‰. Small, but systematic E-W regional variation in 18O values suggests heterogeneous magmatic contamination, and field observations of meter-scale enclaves agree with a model of magma mingling and heterogeneous mixing. These data agree with previous whole rock and isotope studies that indicate a dominantly sedimentary source rock for the South Mountain batholith. The data also show that the monzogranite is not in isotopic equilibrium with zircon, perhaps due to late-stage isotopic exchange with a high 18O reservoir. Zircon has proved to be a useful tool for parsing out the magmatic history of these granitic rocks with a whole rock composition that is an amalgamation of the source magma(s), uppercrustal contamination, and post-crystallization alteration. Keywords: South Mountain Batholith, zircon, isotope geochemistry, S-type granites, magma contamination, peraluminous composition 1 INTRODUCTION The Canadian province of Nova Scotia contains a complex record of the Paleozoic orogens associated with the docking of Laurentia and Gondwanaland and the closing of the Iapetus Ocean (Keppie and Dallmeyer, 1987; Keppie, 1993; Robinson et al., 1998; Murphy and Keppie, 2005). Plutonic rocks that form in convergent margins contain information critical to understanding the formation and recycling of Earth’s crust, and the South Mountain batholith (SMB, Fig. 1) is one of the major North American granitic bodies characteristic of such tectonic settings (Halliday et al., 1981; Keppie and Dallmeyer, 1987; MacDonald and Horne, 1988; Horne et al., 1992; Clarke et al., 2004). Aluminum-enriched (peraluminous) granite complexes such as the SMB form by partial melting of sedimentary rocks, fractional crystallization of metaluminous magmas, or subsoludius processes (Clarke, 1981; Halliday et al., 1981; Zen, 1988). The SMB is classically described as having supracrustal sedimentary source rocks (Smith and Turek, 1976; Smith, 1979; Longstaffe et al., 1980; Clarke, 1981; Halliday et al., 1981). Such “S- type” granites form at relatively low temperatures and are compositionally controlled by sedimentary fractionation via surficial processes (Chappell and White, 2001) and mingling with lower crustal melts or wall rocks during emplacement and crystallization. Oxygen isotope ratios are useful for understanding the complex history of magmatic, hydrologic, and thermal alteration in the Earth’s crust (Taylor and Sheppard, 1986). Different “reservoirs” of material in the Earth have distinct isotopic signatures, measured relative to Standard Mean Ocean Water (SMOW, Fig. 2). Primitive mantle material typically has a 18O value of 5.7±0.3‰ (Rollinson, 1993). When rocks interact with meteoric water, enrichment in 18O occurs because either (1) the 18O re- 2 Kilometers 0 35 70 140 Avalon Terrane y a B s ’ H Cobequid-Chedabucto t e a Fault System r li a fa g r x a H M a . r Meguma Terrane t b S o r South Mountain Batholith A tla nti c 70˚W 65˚W 60˚W 55˚W O cean Quebec 50˚N 50˚N Kilometers Halifax Pluton 0 3 6 9 12 Island of Early Carboniferous Sediments Sandy Lake Newfoundland biotite monzogranite Gulf of St. Lawrence Windsor Group megacrystic biotite granodiorite New Late Devonian Igneous Rocks Brunswick Harrietsfield mafic prophyry muscovite-biotite monzogranite an Maine ce 45˚N O Halifax Peninsula Cambro-Ordovician 45˚N tia (U.S.A.) y o d Sc tic coarse-grained leucomonzogranite un n Meguma Group F a tla Kilometers of ov A y N Sable Tantallon Halifax Formation Ba 0 200 Island fine/medium-grained leucomonzogranite slate, minor greywacke 65˚W 60˚W 55˚W Peggy’s Cove Goldenville Formation biotite monzogranite greywacke, minor slate Figure 1. Location map for the South Mountain batholith and Halifax pluton (MacDonald, 2001). 3 mantle value = 5.7± 0.3 metamorphic water magmatic water meteoric water sea water limestone argillic sediments detrital sediments metamorphic rocks granitoids andesites and rhyolites MORB bulk Earth chondritic meteorites -40 -30 -20 -10 0 5.7 10 20 30 40 δ18O (‰) Figure 2. The distribution of oxygen isotope reservoirs on Earth. Standard Mean Ocean Water (SMOW) has a δ18O value of 0.0‰ and the mantle has a value of 5.7 ± 0.3‰ (modified from Rollinson, 1993). 4 equilibrates under the influence of the water, as the lighter 16O isotope is preferentially mobilized in hydrothermal alteration (Taylor and Sheppard, 1986) or (2) surficial weathering processes mobilize 16O and the remaining detrital material is enriched in 18O and incorporated into sedimentary rocks. Oxygen isotope values vary in the Earth by about 100‰ (Rollinson, 1993), with meteoric water depleted in 18O and sedimentary rocks enriched in 18O. Rocks that have interacted with the Earth’s surface at some stage in their evolution have higher 18O values (Taylor, 1968). Thus, 18O analysis offers a powerful tool for deciphering various sources of melts and contamination during magmatic evolution. Recent investigations of oxygen isotope ratios in zircon (cf. Valley et al., 1994; King et al., 2000; Lackey et al., 2005; Valley et al., 2005; Lackey et al., 2006) reveal the particular usefulness of this phase in obtaining unaltered magmatic oxygen isotope data. Zircon is one of several refractory igneous minerals characterized by slow intra- crystalline oxygen diffusion (Peck et al., 2003; Valley, 2003; Page et al., 2006). Additionally, zircon has been rigorously characterized because of its substantial use in geochronology (Davis et al., 2003). In phases that exchange oxygen readily, the isotopic signatures of magmatic evolution can be overprinted by post-crystallization hydrothermal alternation, subsolidus recrystallization, and diffusion. In contrast, Valley et al. (1994) find that zircon crystals contain a preserved record of magmatic conditions and variations, such as contamination from sources with distinct 18O signatures. Analysis of oxygen isotopes of whole zircons is a useful tool for differentiating magmatic chemical evolution from post-crystallization processes, and also offers a “snapshot” of conditions in an evolving magmatic system. 5 Previous oxygen isotope studies of the SMB report high whole rock (WR) 18O values. Longstaffe et al. (1980) report that 18O(WR) across the SMB varies from 10.1- 12.0‰, with samples from the Halifax pluton ranging from 10.7-11.7‰. Additionally, the authors find enrichment of coexisting minerals in the expected order for magmatic isotope partitioning in coexisting phases (quartz > feldspar > muscovite > biotite; Taylor, 1968) and conclude that the relatively high 18O values are representative of the magmatic oxygen isotope values for the SMB. Longstaffe et al. (1980) also analyze 18O(WR) of the Meguma Group clastic metasedimentary rocks, and report values between 10.0-12.9‰. Other more recent isotope studies (Chatterjee et al., 1985; Kontak et al., 1991; Clarke et al., 1993) confirm trends observed by Longstaffe et al. (1980). In this study, we use oxygen isotope ratios of zircon paired with whole rock geochemistry to investigate outcrop- and regional-scale magma mingling and mixing in the Halifax pluton of the SMB. This zircon analysis offers a new tool for deciphering the magmatic history of dirty peraluminous granites such as the SMB. Adding to knowledge of crustal growth and recycling is critical to understanding the chemical differentiation of the lithosphere. GEOLOGIC SETTING The province of Nova Scotia is composed of the Avalon terrane to the north and Meguma terrane to the south, separated by the Cobequid-Chedabucto fault system (Fig. 1; MacDonald, 2001). Traditional interpretations of the tectonic setting describe the Cambro-Ordovician metasedimentary rocks of the Meguma terrane docking with the Laurentian coast and previously accreted Avalonia terrane during the Devonian Acadian 6 Orogeny (cf. Keppie, 1993; Benn et al., 1999). However, Murphy and Keppie (2005) suggest that recent paleogeographic reconstructions provide evidence for accretion of both the Avalon and Meguma terranes onto Laurentia by the Early Silurian, coincident
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