Using Δ O of Zircon to Determine the Magmatic Evolution and Degrees Of

Home , Monzogranite

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

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-

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 nt ic 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). 2 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.

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

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

with the closing of the Iapetus Ocean. However, the exact timing of closing remains

unresolved (i.e., Robinson et al., 1998; Murphy and Keppie, 2005).

Spanning an area of 7300 km2, the South Mountain batholith is the largest granitoid complex of the Acadian Orogen, and was emplaced syn- or post-kinematically

(MacDonald and Horne, 1988; Benn et al., 1999) into the Meguma terrane during the

Late Devonian (ca. 370 Ma; MacDonald and Clarke, 1991). A composite, differentiated peraluminous body, the SMB contains 13 discrete plutons divided into early Stage I, composed of granodiorite and monzogranite, and late Stage II, composed of monzogranite, leucomonzogranite, and leucogranite (MacDonald, 2001). The SMB is unconformably overlain by Early Carboniferous terrestrial clastic sedimentary rocks of the Horton Group (MacDonald and Horne, 1988). The Early Carboniferous age of this unconformity shows that the batholith was unroofed shortly after emplacement (ca. 10

Ma; MacDonald and Horne, 1988).

Whole rock and isotope studies of the batholith indicate a crustal protolith for the

SMB. While Longstaffe et al. (1980) conclude from whole rock 18O data that anatexis of the Meguma terrane is a possible source of the batholith, neodymium isotopic studies

(i.e. Clarke et al., 1988) reject the Meguma Group as a major SMB protolith. Seismic reflection data from Keppie and Dallmeyer (1987) suggest that the Meguma terrane was thrust over the Avalon terrane basement, and Eberz et al. (1991) conclude that this

Avalonian basement could be a source of the Devonian peraluminous granitoids

emplaced in the Meguma terrane. Additionally, in a recent study of controls on

peraluminosity in the SMB, Clarke et al. (2004) find no geochemical evidence to suggest

another SMB protolith beside the Avalonian basement. Summarizing these studies,

MacDonald (2001) concludes that underplating and partial melting of Avalonia beneath the Meguma terrane most likely generated the large volumes of granitic magma emplaced as the SMB and related granitic bodies of southwestern Nova Scotia.

Halifax Pluton

The Halifax pluton (HP) is located at the southeastern edge of the SMB (Fig. 1).

This 1060 km2 body (MacDonald, 2001) has relative discrete boundaries and crops out in

the Halifax metro area, as well as in coastal exposures along the Atlantic Ocean between

Halifax Harbor and St. Margaret’s Bay. These factors make the Halifax pluton one of the

most studied plutons in the batholith (see MacDonald and Horne, 1988; Shupe et al.,

1989; Kontak et al., 2002; Clarke, 2003). The HP is the largest Stage II pluton in the

SMB (MacDonald, 2001).

The mapped units of the pluton (Fig. 1) are differentiated by Canadian researchers

using mafic mineral and mica content, as well as the presence of megacrystic textures

(MacDonald and Horne, 1988; MacDonald, 2001). The Harrietsfield muscovite-biotite

monzogranite and the Halifax Peninsula coarse-grained leucomonzogranite are at the

pluton’s core, while the Sandy Lake and Peggy’s Cove biotite monzogranites form the

respective “rims” at the pluton’s northern and southern edges. A granodiorite unit is

mapped along the pluton’s northern contact with the Meguma terrane country rocks, while along the southern coastline a mafic porphyry crops out at the point of Cape

Sambro (MacDonald, 2001). MacDonald and Horne (1988) describe the HP as a

compositionally zoned body with a normally-zoned rim and reversely-zoned core. An

inferred contact with the Meguma group is located just off the Atlantic coast

(MacDonald, 2001).

The Peggy’s Cove monzogranite is a 1-3 km wide band along the Atlantic Coast,

and is probably the outermost southern unit of the HP (Fig. 3; MacDonald, 2001).

Variably megacrystic with occasional swarms of pegmatite and aplite dikes, the Peggy’s

Cove unit is rich in metasedimentary xenoliths and enclaves of diverse compositions and

sizes. The unit varies regionally in xenolith type and abundance, megacryst presence and

orientation, accessory mineral assemblage, prevalence of magmatic and mafic enclaves,

and additional magmatic and reaction textures (Fig. 4). Frequency and variations of

enclaves and xenoliths in the unit are detailed by Imtiaz (2007). Magmatic garnet is

common in some locations, and several sites have excellent exposures of reaction

textures and pegmatite and aplite dikes.

Cranberry Head

Cranberry Head is a small (<100 m2) point located northwest of Peggy’s Cove

(Fig. 3). The outcrop contains a spectacular exposure of swirling biotite schlieren and meter-scale “intermediate enclaves” rimmed with megacrystic feldspar and diverse communities of xenoliths (Fig. 5). There are no mapped pegmatitic or aplitic dikes.

Biotite schlieren are common through out the Peggy’s Cove monzogranite, but at

Cranberry Head they occur in meter-scale swirls spiraling out from megacrystic feldspar centers (Fig. 4C) or continuous centimeter-wide lines that extend for meters.

Cranberry Head East Dover 06SMB20 06SMB12 8.15±0.03‰ 7.71±0.03‰ Fig. 5

West Dover Peggy’s Cove 06SMB19 06SMB23 8.14±0.03‰ 8.14±0.06‰

Indian Point 06SMB11 8.23±0.03‰ Lower Prospect Cape Sambro Key HX05HX 06MP01 8.24±0.05‰ 8.26±0.02‰ Harrietsfield Pennant Point muscovite-biotite monzogranite A 06SMB07 t l 8.23±0.03‰ Halifax Peninsula a n coarse-grained leucomonzogranite t i c O Tantallon c e fine/medium-grained leucomonzogranite a n Peggy’s Cove biotite monzogranite mafic prophyry Kilometers Meguma Group metasediments Goldenville Formation 0 3 6 9 12

Figure 3. Sampling sites along the Atlantic coast of the Halifax pluton. Sample numbers are in bold and the results of zircon oxygen isotope analysis are included in italics. The Cranberry Head locality is shown in detail in Figure 5. 9 10

A B

C D

E F

Figure 4. Field photos from sampling sites in the Halifax Pluton. (A) Aligned feldspar megacrysts at the Indian Point sampling site, as described by Clarke (2003). (B) Aggre- gation of mafic enclaves in the mafic porphyry at Cape Sambro. (C) One of many biotite schlieren swirls observed in the Peggy’s Cove monzogranite at Cranberry Head. (D) Pegmatitic intermediate enclave rim at Cranberry Head with abundant mafic enclaves and metasedimentary xenoliths, preferentially exposed due to differential weathering of the monzogranite. (E) Hornfels facies xenolith likely sourced from the Meguma country rock. (F) Meter-scale mafic enclave reacting with and being digested by the monzogranite. Located at the Indian Point sampling site. 11

Cr an be rr y A He ad 06SMB20 100 m 8.15±0.03‰

06SMB22 enclave 8.19±0.02‰

monzogranite sampling site 06SMB21 8.21±0.03‰

garnet bearing xenoliths N B pegmatitic rim

C 30 cm diverse enclaves

D E

Figure 5. Features of the Cranberry Head locality. (A) The distribution of meter-scale enclaves, two of which were sampled for δ18O(zrc) analysis. (B) Field photo and (C) rendered sketch of one sampled enclave (SMB21) showing associtation with smaller enclaves and pegmatitic “rim” of feldspars containing metasedimentary xenoliths that are often garnet bearing. (D) Enclave with a wider rim (delineated in photo) containing diverse community of xenoliths and small enclaves. (E) An individual sitting in the midst of a large enclave for scale. The rim arcs across the center of the picture, continuing over the edge of the cliff towards the ocean. 12

Thirteen distinct intermediate enclaves shown in Figure 5 are roughly elliptical, with a long axis ranging from 1 to 10 meters. Most have distinct centimeter- to decimeter-scale rims of white megacrystic feldspar with a pegmatitic texture.

Additionally, these rims tend to contain a congregation of diverse centimeter-scale xenoliths and enclaves (Fig. 5C), a community similar to that found scattered through out the monzogranite. Some enclave-associated xenoliths are clearly of metasedimentary

(Meguma?) origin, and are typically garnet-bearing (Fig. 5B). There is no clear relationship between these enclaves and the biotite schlieren.

Indian Point

Located outside the village of Prospect, this locality has no exposed felsic dikes, but instead contains knots or balls of megacrystic feldspars with a pegmatitic texture.

These “conglomerations” also contain diverse xenoliths and small mafic enclaves, resembling the pegmatitic rims of the Cranberry Head intermediate enclaves. Decimeter- scale, fine-grained mafic enclaves and lone garnet-bearing xenoliths are also present. At

Indian Point, abundant feldspar megacrysts align in flow patterns as described by Clarke

(2003; Fig. 4A).

Peggy’s Cove

The type locality for the Peggy’s Cove monzogranite is a prominent point at the southern mouth of St. Margaret’s Bay (Fig. 3). This exposure is cross-cut by multiple large (>1 m thick) aplitic and pegmatitic dikes, recently investigated in detail by Kontak et al. (2002). Lone garnets typically rimmed with biotite coronas and garnet bearing xenoliths are common at Peggy’s Cove.

ANALYTICAL METHODS

Nine different sites along the Atlantic Coast of the Halifax Pluton were sampled

during the summer of 2006, with 10-15 lb samples taken for zircon extraction. In

addition to six samples from the Peggy’s Cove monzogranite, two intermediate enclaves

at Cranberry Head were sampled, as well as the mafic porphyry that crops out at the tip of

Cape Sambro (Fig. 3 and 5). An additional monzogranite sample, collected by J.S.

Lackey in spring of 2005 at Lower Prospect and processed separately, was integrated into

this study, bringing the total number of samples prepared for 18O(zrc) analysis to ten.

Aliquots of each sample were sent to the Geoanalytical Laboratory at Washington

State University for X-ray fluorescence (XRF) analysis of 28 major and trace elements,

following the methods of Johnson (1999). Zircon was separated from the remaining

sample by standard crushing, density, and magnetic separation techniques (Valley et al.,

1994; Lackey et al., 2002; Lackey et al., 2005) at Macalester College. Leeching of the

zircon separates with nitric acid to remove non-magnetic phosphate and sulfide minerals

was followed by treatment with hydrofluoric acid, which dissolves regions in zircon crystals that have been subject to radiation damage and are susceptible to oxygen isotope diffusion after crystallization (Valley, 2003). Powdering of acid-treated separates with a boron-carbide mortar and pestle limited grain-size effects and maximized the efficiency of laser fluorination.

Oxygen isotope analyses were performed at the University of Wisconsin stable isotope laboratory by CO2 laser fluorination, following techniques described by Valley et

al. (1995). A gas source Finnigan MAT 251 mass spectrometer was used to measure

isotope ratios. The analyses were standardized against the University of Wisconsin Gore

Mountain garnet standard (UWG-2), which has an accepted 18O value of 5.80‰. On the

day of analysis, the UWG-2 was measured at 5.74‰ and the correction was 0.06‰.

PETROGRAPHY

There is little modal variation between the samples of Peggy’s Cove

monzogranite in thin section. Compositions for all the samples, including the mafic

porphyry and intermediate enclaves, fall within the monzogranite field of the IUGS

classification diagram (Streckeisen, 1976), with generally equal amounts of alkali

feldspar, plagioclase, and quartz (Fig. 6). Biotite is a significant constituent of the all

monzogranite samples, and has modal values ranging from 10-15%. Biotite makes up

approximately 20% of the mafic porphyry. Minor amounts (1%) of magmatic muscovite

occur in samples SMB07, SMB12, SMB19, SMB20, and SMB22 (intermediate enclave).

Trace amounts of apatite, zircon, and Fe-Ti oxides are common in all thin sections

examined (Fig. 7D).

Igneous textures

The mafic porphyry is distinctly porphyritic in thin section when compared to the

monzogranite. The intermediate enclave samples are relative equigranular (Fig. 7C),

lacking the megacrysts common in the monzogranite. The variably megacrystic nature of

the monzogranite samples made accurately estimating modal composition in thin section

difficult because centimeter-scale feldspar megacrysts, commonly the alkali feldspars, can take up the area of an entire thin section.

Quartz

90 90

60 60

granite granodiorite 20 20

5 5 Alkali Feldspar 10 35 65 90 Plagioclase

Peggy’s Cove monzogranite intermediate enclave mafic porphyry

Figure 6. Ternary quartz-alkali feldspar-plagioclase (QAP) classification diagram (after Streckeisen, 1976). Modal compositions for the nine Halifax pluton samples all fall within the granite field. This study follows Canadian researchers (MacDonald, 2001), refering to this field of the diagram as “monzogranite”. 16

A B Kfs Plag

Bt Plag

Qtz Qtz

C D Kfs

Zrc Plag Bt Qtz Qtz Kfs

Figure 7. Petrographic textures under cross E polarization unless otherwise noted. (A, B) Common textures of the Peggy’s Cove monzogranite. Plagioclase is characterized by Carlsbad and penetration twinning, as Bt well as concentric compositional zoning with cores altering to sericite. Orthoclase alters to sericite in veins and patches. (C) Chl Texture of an intermediate enclave (SMB21). The rock has a monzogranite modal composition, but is more finely crystalline and equigranular. (D) Biotite containing apatite and zircon with clear halos of radiation damage. Note the undulatory extinction of quartz in the lower left of the photomicrograph. (E) Biotite altering to chlorite under plain polarization. 17

There is abundant sericite in the feldspars of the Peggy’s Cove monzogranite. In

thin section, most of the cores of plagioclase crystals are partially or completely altered to

sericite (Fig. 7B), while many alkali feldspars are altered along cleavage planes.

Chloritization of biotite is common (Fig. 7E).

Cathodoluminescence

Qualitative analysis of zircon using cathodoluminescence reveals a wide variety of zircon sizes and morphologies. Inherited cores with magmatic overgrowths are common, and compositional zoning is visible (Fig. 8; Corfu et al., 2003).

RESULTS

Whole Rock Geochemistry

Whole rock geochemical data are presented in Table 1 and plotted with previously published data compiled by MacDonald (2001) in Figure 9. Weight percent silica in the monzogranite samples varies from 66-73 wt. %. The sample from Peggy’s Cove

(SMB23) has the lowest silica value and is an outlier, with a weight percent of 65.90 wt.

%. Additionally, this sample has the highest Al2O3 weight percent of any sample

analyzed (16.22 wt. %). The intermediate enclaves are geochemically distinct from the

monzogranite samples and similar to the mafic porphyry, with silica values from 67.40-

67.80 wt. %. Major elements inversely co-vary with silica content, with the exception of

P2O5, and analyses from this study generally plot within the data range of previous

studies (Fig. 9). All samples are weakly peraluminous (A(mol)/CNK(mol)>1.1; Clarke,

G B E I

K A

C F D J H 100 µm

Figure 8. Cathodoluminesence images of zircons from the Peggy’s Cove monzogranite, sample SMB23. This imaging technique highlights compositional differences within the crystals, and reveals different kinds of compostional zoning and that reflect magmatic growth (A, C, D, G, K) and overgrowth of zircon on rounded inherited cores (B, D, E, F, H, I, J, K; Corfu et al., 2003). TABLE 1. WHOLE ROCK MAJOR (WT. %) AND TRACE ELEMENT (PPM) DATA. monzogranite intermediate enclaves maﬁc porphyry samples SMB07 SMB11 SMB12 SMB19 SMB20 05HX05 SMB23 SMB21 SMB22 MP01

SiO2 72.13 70.69 71.95 71.10 69.53 72.97 65.90 67.80 67.40 67.73

TiO2 0.32 0.38 0.34 0.36 0.44 0.26 0.59 0.55 0.66 0.48

Al2O3 13.90 14.22 13.67 14.27 14.71 13.98 16.22 15.01 15.36 15.43 FeO* 2.18 2.37 2.39 2.34 2.91 2.21 3.74 3.52 4.27 2.97 MnO 0.06 0.05 0.06 0.06 0.07 0.05 0.08 0.08 0.10 0.07 MgO 0.70 0.74 0.67 0.76 0.80 0.5 1.16 1.06 1.23 1.51 CaO 1.16 1.53 1.26 1.57 1.43 0.87 2.06 1.62 2.12 2.49

Na2O 3.31 3.23 3.30 3.44 3.32 3.04 3.58 3.25 3.60 3.62

K2O 3.99 4.44 3.85 3.91 4.59 4.8 4.26 4.31 3.22 3.60

P2O5 0.17 0.14 0.14 0.14 0.14 0.2 0.20 0.16 0.18 0.19 Total 97.92 97.79 97.64 97.95 97.93 98.88 97.79 97.36 98.13 98.09

Ni 0 4 2 3 4 4 6 4 7 10 Cr 13 15 12 14 16 - 20 20 22 35 Sc 6 6 7 6 8 - 11 10 11 9 V 24 30 25 26 31 - 49 40 48 42 Ba 321 569 322 532 511 - 660 558 467 430 Rb 202 163 187 155 186 - 184 177 183 184 Sr 108 147 105 158 134 - 191 175 161 162 Zr 115 142 133 139 159 - 204 206 248 137 Y 20 27 27 22 30 - 26 30 36 19 Nb 8 8 8 7 9 - 10 11 12 8 Ga 20 18 19 18 20 - 20 20 23 20 Cu 1 3 3 3 3 22 4 4 3 5 Zn 42 47 50 46 54 45 79 72 83 56 Pb 18 23 19 20 24 4 31 30 19 16 La 24 21 23 23 26 - 31 31 37 21 Ce 35 44 39 40 50 - 61 60 77 44 Th 10 11 11 10 12 - 12 12 14 11

Nd 19 25 24 21 25 - 34 31 36 24 19 20

0.800 17.00 0.700 16.50 0.600 16.00 0.500 2 3 2 15.50 2 0.400 O TiO 2 Al15.00 0.300 Al TiO 0.200 14.50 0.100 14.00 0.000 13.50

5.50 0.110 5.00 0.100 4.50 0.090 4.00 0.080

FeO*3.50 MnO MnO0.070 FeO* 3.00 0.060 2.50 0.050 2.00 0.040

2.50 3.00

2.00 2.50 2.00 1.50 1.50 MgO CaO

MgO 1.00

CaO 1.00 0.50 0.50 0.00 0.00

0.250 1.45 peraluminous 1.35 0.200

1.25 5

O 0.150

1.15 2

A/CN P

A/CNK 1.05 0.100 0.95 0.050 0.85 metaluminous O 0.75 2 0.000 P 66.00 68.00 70.00 72.00 74.00 76.00 66.00 68.00 70.00 72.00 74.00 76.00

SiO2 SiO2

Peggy’s Cove monzogranite intermediate enclave mafic porphyry Peggy’s Cove monzogranite, previous studies mafic porphyry, previous studies

Figure 9. Major elements plotted against silica content (normalized wt. %), including additional data compiled by MacDonald (2001). Peraluminous rocks have a A/CNK > 1.1, delineated on plot. 21

1992), with A/CNK values ranging from 1.07 in the mafic porphyry to 1.19 in the sample from Lower Prospect (Fig. 9; Table 2).

Trace element values (ppm) are plotted with previously published data corresponding to the major element data (Fig. 10; MacDonald, 2001). Samples from this study appear to be uniformly enriched in Zr/Nb relative to other data from the Halifax pluton, probably as a result of instrumentation differences. Otherwise, monzogranite data from this study generally agree with previous studies. Spider diagrams (Fig. 11) normalized to CI chondrite (McDonough and Sun, 1993) also show that the data are consistent with previous geochemical studies of the Halifax pluton.

18O Zircon

18O(zrc) values vary from 7.71-8.26‰ (Table 2). Monzogranite values cluster between 8.14±0.06‰ and 8.15±0.03‰ in the West and 8.23±0.03‰ and 8.26±0.02‰ in the East, excluding the sample from East Dover (SMB12, 18O = 7.71±0.03‰). Two trends appear in 18O values plotted against the major elements that also reflect this east to west variation within the Peggy’s Cove unit (Fig. 12), with the eastern samples and intermediate enclaves (located in the west) enriched in 18O relative to the western samples with similar weight percent silica. The intermediate enclave samples SMB21 and SMB22 from Cranberry Head fall within a standard deviation of each other, with respective values of 8.21±0.02‰ and 8.19±0.03‰. The mafic porphyry (MP01) has the highest 18O(zrc) value, 8.26±0.02‰.

TABLE 2. VARIOUS CALCULATED VALUES FROM THIS STUDY'S WHOLE ROCK AND 18O DATA

Standard Calculated Calculated Calculated Calculated Calculated Calculated sample A/CNK 18O(zrc) Dev (±) 18O(WR) 18O(qtz) 800 C 18O(qtz) 750 C 18O(qtz) 700 C 18O(qtz) 650 C 18O(qtz) 600 C SMB07 1.17 8.23 0.03 10.24 10.53 10.76 11.02 11.33 11.70 SMB11 1.10 8.23 0.03 10.16 10.53 10.76 11.02 11.33 11.70 SMB12 1.15 7.71 0.03 9.72 10.01 10.24 10.50 10.81 11.18 SMB19 1.12 8.14 0.03 10.09 10.44 10.67 10.93 11.24 11.61 SMB20 1.13 8.15 0.03 10.00 10.45 10.68 10.94 11.25 11.62 05HX05 1.16 8.24 0.05 10.26 10.53 10.76 11.03 11.34 11.70 SMB23 1.16 8.14 0.06 9.76 10.43 10.66 10.93 11.24 11.60 SMB21 1.14 8.21 0.03 9.98 10.51 10.74 11.00 11.31 11.68 SMB22 1.07 8.19 0.02 9.90 10.49 10.72 10.98 11.29 11.66 MP01 1.19 8.26 0.02 9.99 10.56 10.79 11.05 11.36 11.73 average 10.01 10.44 10.67 10.94 11.25 11.62

Note: Peraluminosity index, δ18O(zrc) and calculated values for the expected δ18O of whole rock and quartz assuming equilibrium during crystallization. 22 23

60 800 50 700 600 40 500 V30 400 Ba V 20 300 200 10 100 0 0

300 250

250 200 200 150 Rb Rb150 Sr 100 100 50 50 0 0

40 40 35 35 30 30 25 25 Y Y20 La La20 15 15 10 10 5 5 0 0

4.00 300 3.50 250 3.00 2.50 200 O/Rb

2.00 2 150 Rb/Sr

Rb/Sr K 1.50 K2O/Rb100 1.00 0.50 50 0.00 0

25 10.00

20 8.00

15 6.00

Sr/YSr/Y Zr/NbZr/Nb 10 4.00

5 2.00

0 0.00 66.00 68.00 70.00 72.00 74.00 66.00 68.00 70.00 72.00 74.00 SiO SiO2 2

Peggy’s Cove monzogranite intermediate enclave mafic porphyry Peggy’s Cove monzogranite, previous studies mafic porphyry, previous studies

Figure 10. Trace element concentrations (ppm) plotted against silica content (normalized wt. %) with additional data compiled by MacDonald (2001). 24

1000 A

100 sample/standard sample/standard10

1 Cs Rb Ba Th Nb La Ce Pb Sr Nd Zr Y

1000 B

100

sample/standard 10

1 Cs Rb Ba Th Nb La Ce Pb Sr Nd Zr Y

Peggy’s Cove monzogranite intermediate enclave mafic porphyry Peggy’s Cove monzogranite, previous studies mafic porphyry, previous studies

Figure 11. (A) Trace element data from this study normalized to CI chondrite (McDonough and Sun, 1995). (B) Our data plotted with previously published data from the Halifax pluton, compiled by MacDonald (2001). 25

75.00 17.00 74.00 16.50 73.00

16.00 2

72.00 3 O

71.00 2 15.50 SiO

Al Al2O3 70.00 15.00 69.00 68.00 14.50 67.00 14.00 O(zrc) (‰) O(zrc)

18 1.80 3.00 δ 1.60 2.50 1.40

1.20 2.00 1.00 1.50 0.80 CaO CaO MgO 0.60 1.00 0.40 0.50 0.20 0.00 0.00

12 1.20 10 1.18 8 1.16 1.14

Ni 6 1.12 A/CNK A/CNK 4 1.10 2 1.08 0 1.06 8.10 8.15 8.20 8.25 8.30 8.10 8.15 8.20 8.25 8.30 18 δ18O(zrc) (‰) δ O(zrc) (‰) Monzogranite Enclave Mafic Porphyry

8.28 8.26 8.24 8.22 Eastern Sites 8.20

O(zrc) (‰) O(zrc) 8.18 18 δ 8.16 8.14 Western Sites 8.12 8.10 55.00 60.00 65.00 70.00 75.00

SiO 2 Figure 12. δ18O(zrc) (‰SMOW) plotted against selected major (wt. %) and trace (ppm) elements. The sample from East Dover (δ18O(zrc) = 7.71‰) is excluded from this analysis. Lines indicate the two suggested regional trends in the data, where the samples in the east and the intermediate enclaves have higher δ18O(zrc) values than the western sites. 18 The trend is supported by the plot of δ O(zrc) vs. SiO2. However, the peraluminosity index (A/CNK) does not appear to follow this trend. The mafic porphyry, the eastern most sampling site, has the highest δ18O(zrc) value but does not fit into either trend. 26

Given these values of 18O(zrc), we calculate equilibrium 18O values for whole

rock and phases such as garnet and quartz (Valley et al., 2003). Valley et al. (2003)

report the following experimental relationship between different phases A and B:

18 18 6 2 OA – OB = AA-B10 /T (T in Kelvin) (1)

By choosing magmatic temperatures, we use published coefficients to predict what the

18O value should be for one phase based on data from another. At 800˚C quartz in

equilibrium with this study’s zircons would have a 18O value from 10.24-10.56‰, while

at 600˚C the values would be from 11.18-11.73‰ (Table 2; Fig. 13). The authors report

600˚C as the lowest temperature for which this model is accurate. Similarly, Lackey et

al. (2007) use the following linear relationship between silica content and 18O(WR):

18 18 Calculated O(WR) = (wt. % SiO2)(0.0612) – 2.4982 + O(zrc) (2)

Given values for 18O(zrc) and weight percent silica for a sample, we can calculate what

the 18O(WR) should be if the zircons are in isotopic equilibrium with the rock. These values vary between 9.71-10.26‰ (Table 2; Fig.13).

DISCUSSION

Whole rock geochemistry confirms that the Peggy’s Cove monzogranite is peraluminous (Fig. 9), and suggests that it is dominantly sourced from a sedimentary protolith. These data are supported by field observations of Al-rich phases such as magmatic garnet and coexisting biotite and muscovite. The abundant presence of metasedimentary xenoliths and common polygenetic zircon crystals (Figs. 4 and 8) also

12 A range of δ18O (WR) reported by Longstaffe et al. (1980) 11

9 O (‰)

18 d18O

δ 8

5 67 68 69 70 71 72 73 74 75

SiO2 Peggy’s Cove monzogranite mafic porphyry intermediate enclave calculated δ18O (WR) (‰)

B 12

10 range of δ18O (qtz) reported by Longstaffe et al. (1980) 9 O (‰) 18

δ 8

5 67 68 69 70 71 72 73 74 75

SiO2

Peggy’s Cove monzogranite mafic porphyry 800˚C 700˚C 600˚C intermediate enclave 750˚C 650˚C

Figure 13. Plots of the equilibrium δ18O (‰) of (A) whole rock and (B) quartz calculated from our δ18O (zrc) values in the Peggy’s Cove monzogranite and mafic porphyry. When those calculated values are compared to published oxygen isotope data for the Halifax pluton (the shaded regions; Longstaffe et al., 1980), it is clear that the rock is not in isotopic equilibrium with zircon. 28

indicate a sedimentary source for the Halifax pluton. This result agrees with other

geochemical and isotopic studies on the greater pluton and batholith scale (i.e. Longstaffe

et al., 1980; Clarke et al., 1988; Eberz et al., 1991; MacDonald, 2001; Clarke et al.,

2004), and gives us a geochemical context in which to place our 18O(zrc) data.

Oxygen Isotopes

Variations in 18O(zrc) data from the Peggy’s Cove monzogranite are small but

systematic, excluding the outlier 7.71‰ (SMB12, discussed below), and range from 8.14-

8.26‰. These high 18O(zrc) values support the whole rock geochemical data suggesting

a sedimentary protolith for the Halifax Pluton. S-type granites are characterized by

18O(WR) > 9‰ (Clarke, 1992), which translates into an equilibrium 18O(zrc) value of

approximately 7‰ for rocks such as the monzogranite with approximately 70% SiO2.

Values of 18O for the mafic porphyry and Peggy’s Cove monzogranite exhibit a geographic trend. The western sites have lower 18O(zrc) values (8.14-8.15‰) than

those in the east (8.19-8.26‰) for equivalent weight percent silica values (Fig. 12).

Other major elements plotted against 18O(zrc) show similar trends (Fig. 12), and it is at

least qualitatively clear from this small sampling that the intermediate enclaves, located

in the West, are more chemically similar to the monzogranite and mafic porphyry in the

East than their host monzogranite at Cranberry Head.

One interpretation of this geographical variation is that the Peggy’s Cove

monzogranite heterogeneously incorporated metasedimentary country rock during

crystallization. In the east, where 18O(zrc) values indicate enrichment in 18O,

contaminating material was perhaps more uniformly incorporated into the melt prior to or

during crystallization. In contrast, the data from western sampling sites indicate less complete mixing of 18O-enriched material. Field observations of xenoliths and enclaves

caught in the midst of digestion (Fig. 4F) indicate that this heterogeneous magmatic

contamination was in process even as the melt was in the last stages of crystallization.

Though the 18O(zrc) value for one intermediate enclave is not statistically

different from the values in the western part of the Peggy’s Cove monzogranite, the

intermediate enclave data tend to indicate that they are enriched in 18O/16O relative to the

monzogranite that hosts them. This suggests that the enclaves are geochemically distinct

from the monzogranite at Cranberry Head. Textures observed in the field, including the smaller grain size within the enclaves, the intermediate enclave’s distinct ellipctical shape, and feldspar rims, support this assertion. Additionally, the occurrence of thirteen discrete intermediate enclaves in one location is likely indicative of some magmatic mingling process.

MacDonald (2001) briefly describes a coastal outcrop in the Halifax Peninsula leucomonzogranite on Pennant Point (Fig. 3) where meter-scale angular granodiorite

“xenoliths” are bordered with centimeter-scale aplite dikes and containing metasedimentary xenoliths. The field descriptions and photos of these features closely resemble our “intermediate enclaves” at Cranberry Head. However, an important difference is the sharp angularity of the bodies at Pennant Point. According to

MacDonald (2001), this angularity suggests an emplacement sequence where the

“xenolith” material is emplaced by stoping, followed by crystallization and fracturing.

Then aplite dikes intrude along the fractures and the leucomonzogranite exploits those fractures, “stoping” the xenoliths during its subsequent emplacement. It is possible that

the enclaves at Cranberry Head are related to these features in the Harrietfield

leucomonzogranite, though the intermediate enclaves are characterized by coarser pegmatitic rims and lack the sharp angularity of the xenoliths at Pennant Point.

Additionally, at Cranberry Head the pegmatite borders themselves contain a diverse community of centimeter-scale enclaves and xenoliths, an association that does not have clear geochemical explanation, though in the field this relationship is clear. These factors continue to raise questions about the formation of the pegmatitic texture and its chemical relationship to the xenoliths, enclaves, and magmas.

Surprisingly, the mafic porphyry has the highest 18O(zrc) value at 8.26±0.02‰;

this 18O/16O enrichment relative to the monzogranite might be caused by the porphyry’s proximity to the Meguma country rocks. 18O(WR) typically increases with weight

percent silica because more felsic magmas have a greater percentage of high-18O

minerals such as quartz and feldspar (Valley, 2003). The mafic porphyry is not

significantly more mafic that the monzogranite; indeed, the lowest silica content we

report is not from the porphyry but from the type locality of the Peggy’s Cove

monzogranite (65.90%). However, the porphyry does have a higher modal percentage of

biotite, a low 18O mineral (Valley, 2003). The porphyry is mapped at the outer most rim

of the pluton (MacDonald, 2001) and is likely close to the contact with Meguma

metasediments. These country rocks (18O(WR) = 10.0-12.9‰; Longstaffe et al., 1980)

could be a source of post-emplacement contamination for the porphyry and the

monzogranite. Though the capacity of a felsic magma in the upper crust to assimilate a

significant amount of wall rock is limited (Pitcher, 1993), more mafic magmas have

higher temperatures and thus a greater capacity to assimilate cold metasedimentary

material. MacDonald and Horne (1988) observe that the early, more mafic Halifax

Pluton units have abundant xenoliths, while the later leucogranites are xenolith-poor,

even when located close to the pluton boundary. Thus, the abundance of

metasedimentary xenoliths may correlate to these data and help explain regional variation

in 18O(zrc) not only in the Peggy’s Cove monzogranite, but also on the scale of the

larger pluton.

The outlying 18O value of 7.71±0.03‰ at East Dover (SMB12) is generally

excluded from this discussion, though it is representative of other data collected during

our field season that may reveal a larger 18O(zrc) trend in the Halifax Pluton. The only sample we collected from the mapped Harrietsfield monzogranite unit, analyzed for a related study, has a similar low 18O(zrc) value of 7.62±0.03‰ (Nowak and Lackey,

unpublished data). The Harrietsfield unit is the core of the pluton, which MacDonald and

Horne (1988) interpret as the Halifax Pluton’s most evolved unit. Emplacement

mechanisms for the pluton proposed by Canadian workers suggest that the outer units,

including the biotite monzogranite, granodiorite, and mafic porphyry, intruded first,

followed by the more felsic leucogranites at the core of the pluton (MacDonald and

Horne, 1988). In this model the early melts, already enriched in 18O from the sedimentary source rock, would likely mix with the high 18O Meguma metasedimentary

rocks during emplacement, driving up 18O in zircon and the magma as a whole. This

melt would clear the way for later magmas to intrude without significant contact with

country rock. It is possible that geologic mapping in the East Dover area is wrong, and

our sample SMB12 is not actually Peggy’s Cove monzogranite, but one of the units

mapped in the pluton’s core. If additional sampling of the various HP units for 18O(zrc)

analysis results in values that continue this trend of low 18O (zrc) in more “evolved”

units, we will be better able to quantify and constrain the relationship between emplacement and magmatic evolution of the Halifax pluton and contamination by 18O enriched sources.

Evidence for Post-magmatic Isotope Exchange

Calculated oxygen isotope values in quartz from this study’s 18O(zrc) are not

consistent with the values reported by Longstaffe et al. (1980), indicating late-stage or

progressive oxygen isotope exchange with an 18O-enriched reservoir has occurred in the

Peggy’s Cove monzogranite. Longstaffe et al. (1980) find a 18O(qtz) average value of

11.6±0.2‰ (n=3) from the Halifax Pluton. To calculate quartz 18O values this high with

our zircon data, is it necessary to have a magmatic temperature of 600˚C (Table 2). This

temperature is too low for zircon crystallization in the Halifax pluton. More realistic

crystallization temperatures for peraluminous magmas, such as 750˚C (Huang et al.,

1981), predict an average 18O(qtz) value of 10.7 ± 0.16‰. Thus, it appears that zircon

and quartz from the Peggy’s Cove monzogranite are no longer in isotopic equilibrium, as

they presumably were during crystallization (Valley, 2003).

Similarly, the whole rock oxygen isotope calculations with our zircon data predict

a lower 18O value than reported by Longstaffe et al. (1980), refuting the authors’

conclusion that their whole rock data accurately represent the magmatic 18O/16O during

emplacement and crystallization. 18O(zrc) values from this study predict whole rock

oxygen isotope values ranging from 9.72-10.26‰ (Table 2), significantly lower than the

average 11.24±0.31‰ observed by Longstaffe et al. (1980; Fig. 13). Again, this

indicates that the whole rock 18O values reflects the complex isotopic history of the

Halifax pluton, including progressive contamination of the melt after zircon

crystallization and the effects of subsolidus recrystallization and alteration. Only stable

phases such as zircon can provide magmatic 18O values, especially in granitic rocks with

clear evidence of continuous mingling and wall rock assimilation during crystallization.

This implication of late-stage oxygen isotope exchange is supported by observed petrographic textures and other studies of SMB evolution. Feldspar alteration to sericite and the chloritization of biotite (Fig. 7) are products of hydrothermal alteration, and are common features of in rocks we sampled. This agrees with Clarke et al. (2004) who identify melt-fluid interaction as a key component of SMB evolution. Additionally, in a study of the decimeter- to meter-scale pegmatite-aplite sheets at Peggy’s Cove, Kontak et al. (2002) conclude that these dike are not sourced from late-stage fractionation of the monzogranite, but are geochemically and isotopically distinct. The authors suggest that pegmatite-forming fluids are sourced from the late-stage devolatilization of the abundant metasedimentary xenoliths. Thus, there are already models for late-stage mobile fluids in the SMB, and our study strengthens the hypothesis that this hydrothermal activity significantly altered whole rock geochemistry of the SMB. Using zircon as a petrogenetic tracer, it is possible to recognize magmatic and post-magmatic chemical processes in granitic rocks, and develop a more complete story of their convoluted magmatic evolution.

CONCLUSIONS

Whole rock geochemistry and 18O(zrc) values ranging from 7.71-8.26‰ indicate that the Peggy’s Cove monzogranite is a peraluminous granitoid with a dominantly sedimentary protolith, concurring with previous Halifax pluton studies. East-

West variation in 18O(zrc) values suggests that the melt heterogeneously mixed with

18O-enriched material during pluton emplacement, crystallization, or both. Field

observations, whole rock geochemistry, and 18O(zrc) values of meter-scale intermediate

enclaves at the Cranberry Head locality provide additional evidence for mingling and

mixing in the magma chamber. Additionally, 18O(zrc) values are in disequilibrium with

published whole rock and quartz oxygen isotope values from the pluton, indicating that

late stage isotopic exchange enriched the monzogranite in 18O.

Ongoing 18O data collection from other units in the Halifax pluton will build a

geochemical context for the two samples in this study that lie outside the small, systematic variations of the Peggy’s Cove monzogranite. Also, further cathodoluminescence of zircon can quantitatively characterize the zircon population being analyzed and give more insight into this dataset. It is clear from this study that

18O of zircon provides a particularly useful tool for deciphering the magmatic history of the Halifax pluton, for it can differentiate between magmatic and subsolidus geochemical evolution. For granitoid rocks, this distinction is critical for understanding crustal formation and recycling processes.

ACKNOWLEDGEMENTS

I would like to extend my deepest gratitude to my advisors: Cameron Davidson

(Carleton College) for his upbeat support, suggestions, and help with the endless laboratory work this research entailed, and Jade Star Lackey (The College of Wooster) for a fantastic field season, good-humored guidance through every stage of this process, and an introduction to the wonders of zircon. Thanks also to Hilary Saunders and Jade

Star Lackey for organizing the Keck Nova Scotia project and to my fellow Keck students for great adventures, especially the members of the igneous petrology subgroup, Sarah

Hale, Jessica Hark, Hina Imtiaz, and Brian Mumaw. Special thanks to Karl Wirth at

Macalester College for use of his laboratory and his patient demonstrations of zircon separation techniques, as well as John Valley and the stable isotope laboratory at the

University of Wisconsin at Madison. And finally, many thanks to my friends, loved ones, and particularly the students of Carleton geology for their delightful camaraderie and support.

Funding for this research was provided by the Keck Geology Consortium, the

Duncan Stewart Fellowship, and the Carleton College Geology Department.

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