Oxygen isotope perspective on Precambrian crustal growth and maturation William H. Peck* Elizabeth M. King John W. Valley
Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA
ABSTRACT In this study we contrast insights on Precambrian crustal growth and maturation from radiogenic and oxygen isotope systematics in the Superior (3.0–2.7 Ga) and Grenville (1.3–1.0 Ga) Provinces of the Canadian shield. Oxygen isotope ratios in zircon provide the best evidence of supracrustal input into ancient orogens. Archean Superior Province zircons have relatively low δ18O values and a limited range (5.7‰ ± 0.6‰), while Proterozoic Grenville Province zircons have elevated δ18O values and a wider range (8.2‰ ± 1.7‰). These data reflect fundamental differences in crustal evolution and maturation between the Superior and the Grenville Provinces. In the Grenville Province, radiogenically juvenile supracrustal material with high δ18O values was buried (or subducted) to the base of the crust within 150 m.y. of initial crust production, causing high magmatic δ18O values (δ18O [zircon] ≥ 8‰) in anorthosite suite and subsequent plutons. Information about large volumes and rapid recycling of Grenville crust is not accessible from radiogenic isotope data alone. The Grenville data contrast with the restricted δ18O values of Superior Province magmatism, where subtle (~1‰) elevation in δ18O occurs only in volumetrically minor, late to postorogenic (sanukitoid) plutons. Differences in sediment δ18O values between the Superior and Grenville Provinces are predominantly a function of the δ18O of source materials, rather than differences in chemical maturity or erosion styles. This study shows that zircon is a robust reference mineral to compare igneous processes in rocks that have undergone radically different histories. Keywords: oxygen isotopes, zircon, Precambrian, crustal growth, Superior Province, Grenville Province. INTRODUCTION Determining the growth and evolution of continental crust through time is a primary goal of isotope geochemistry. Oxygen isotope ratios provide a signature for low-temperature, near-surface weathering, diagenesis, and alteration. However, oxygen isotopes have been underutilized in studies of crustal evolution, in part because oxygen isotope ratios are often reset during postmagmatic alteration. Even in relatively retentive minerals, such as quartz, postmagmatic recrystallization can obscure magmatic signatures (e.g., Valley and Graham, 1996; King et al., 1997). In contrast, oxygen isotope ratios of nonmetamict igneous zircons generally preserve magmatic compositions because zircon is a refractory mineral, is resistant to recrystallization, and oxygen diffusion in zircon is slow (Valley et al., 1994; Watson and Cherniak, 1997; Peck and Valley, 1998; Peck et al., 1999). In addition, analysis of zircon dated by U-Pb allows the oxygen isotope ratio to be tied to a particular igneous or metamorphic event. The high precision of laser fluorination and the oxygen retentivity of zircon facilitates the recognition of previously unknown, subtle but important variations in δ18O between and among igneous suites (e.g., King et al., 1998; Valley et al., 1998b). We are able to detect voluminous, rapid recycling of supracrustal material in the Grenville Province, as well as subtle (~1‰) variation in *E-mail:
[email protected]. Geology; April 2000; v. 28; no. 4; p. 363–366; 3 figures.
δ18O between igneous suites in the Superior Province. Our approach shows good promise for identifying supracrustal recycling in metamorphosed plutonic and volcanic rocks, even if there is essentially no age contrast between igneous rocks and assimilants or source materials. Future detailed studies of oxygen isotope ratios in zircon could help distinguish (1) high- versus low-temperature hydrothermally altered ocean crust in magma source regions, (2) derivation from supracrustal rocks versus late assimilation of supracrustal material during intrusion and crystallization, or (3) comagmatic versus simply cotemporal magmatic suites. In this study we contrast oxygen isotope ratios of zircons from 176 rocks of the Precambrian Superior and Grenville Provinces (Fig. 1), and compare them with published radiogenic isotope results. OXYGEN ISOTOPE RATIOS OF ZIRCON Oxygen isotope analyses were made of ~2 mg splits of zircon by laser fluorination at the University of Wisconsin, where average precision is better than ±0.1‰ for samples and standards (Valley et al., 1995). Multiple analyses of one or more splits of nonmetamict, low-magnetism zircon from 106 Archean rocks (King, 1997; King et al., 1997, 1998; Valley et al., 1998a) from the western Superior Province and from 70 rocks of Proterozoic age (Valley et al., 1994, 1998; Peck, 1996; Peck et al., 1997, 1999) from the southern Grenville Province are shown in Figure 2.
96 °
80°
64°
56°
56 °
Superior Province 48°
G 500 km
nv
ill
e
ov
inc
e
48°
AMB
N 96°
re
Pr
80 °
64°
Figure 1. Superior and Grenville Provinces of Canadian shield; dots show sampling localities. AMB is Allochthonous monocyclic belt of Grenville Province.
Pretectonic to syntectonic (mainly tonalitetrondhjemite-granodiorite [TTG]) plutons, from granite-greenstone and metasedimentary subprovinces of the Superior Province, have remarkably restricted δ18O (zircon) values of 5.6‰ ± 0.5‰ (1 standard deviation [sd], n = 42, 3.00–2.67 Ga), and are consistent with δ18O (zircon) in equilibrium with mantle-derived rocks (King et al., 1998). These values correspond to whole-rock δ18O values of ~7.1‰ at magmatic conditions (for SiO2 contents of 60–70 wt%). These data are similar to zircon data from volcanic rocks, which have an average δ18O of 5.4‰ ± 0.8‰ (1 sd, n = 46, 2.74–2.69 Ga), and are not distinguishable from mantle-derived zircons (δ18O = 5.3‰ ± 0.2‰) that would be in equilibrium with olivine from mid-ocean ridge basalt, ocean-island basalt, and mantle peridotites (Valley et al., 1998b). The TTG data underrepresent plutons from metasedimentary subprovinces; three plutons from the paragneiss-rich English River subprovince have an average δ18O (zircon) of 6.6‰ ± 0.2‰ (1 sd, ~2.70 Ga). Late tectonic to posttectonic, Mgand light rare earth element–enriched plutons with sanukitoid affinities also have slightly elevated δ18O (zircon) values of 6.5‰ ± 0.4‰ (1 sd, n = 17, 2.70–2.68 Ga). The sanukitoid data suggest that a small but significant high δ18O component from sediments and/or altered ocean crust was important in crustal growth, especially during the last stages of crust production in the Superior Province (King et al., 1998). Grenville Province zircons (1.34–1.05 Ga) have an average δ18O of 8.2‰ ± 1.7‰ (1 sd, n = 70; Fig. 2). This average is ~2.5‰ higher than the aver363
# samples
40
Superior Province 3.0-2.7 Ga
30 Volcanic (2.74-2.69 Ga) Plutonic TTG (2.74-2.67 Ga) Plutonic Sanukitoid (2.70-2.68 Ga)
20 10
3
# samples
20
4
5
6
7
8
9
10 11 12 13 14
Grenville Province 1.3-1.0 Ga Pre-AMCG (1.34-1.18 Ga) AMCG (1.18-1.13 Ga) Post-AMCG (1.09-1.05 Ga)
10
3
4
5
6
7
8
9
10 11 12 13 14
Figure 2. Oxygen isotope ratios of zircons from Superior and Grenville Provinces. Superior Province samples show tight range of relatively low and primitive δ18O values (5.7‰ ± 0.6‰). Grenville Province samples are more evolved and have higher average δ18O (indicating substantial supracrustal input) as well as larger range in oxygen isotope ratios (8.2‰ ± 1.7‰). TTG is tonalite-trondhjemite-granodiorite; AMCG is anorthosite-mangerite-charnockite-granite suite; VSMOW is Vienna standard mean ocean water.
δ18 O (Zircon) ‰VSMOW
age δ18O (zircon) from the Superior Province, and the range of values is more than two times as large. Igneous zircons were analyzed from >85% of Grenville samples; the remainder contain metamorphic zircon or igneous zircon where U-Pb systematics have been disturbed by metamorphism (Chiarenzelli and McLelland, 1993). These disturbed zircons also have oxygen isotope ratios that reflect igneous values (Valley et al., 1994). Early Grenville crust building, represented by ca. 1.3–1.2 Ga calc-alkaline orthogneiss (McLelland et al., 1996), has δ18O (zircon) values of 6.9‰ ± 1.5‰ (n = 12). Anorthosite suite (anorthositemangerite-charnockite-granite; AMCG) plutons (ca. 1.18–1.13 Ga; McLelland et al., 1996) have δ18O (zircon) values that range from 6.0‰ to 13.5‰ (average = 8.8‰ ± 1.8‰, n = 37). PostAMCG magmatism has an average δ18O (zircon) of 7.8‰ ± 0.9‰ (n = 21). Thus, early (preAMCG; 1.3–1.2 Ga) magmatic rocks have δ18O values only slightly higher on average than those from the Superior Province. After ca. 1.18 Ga, plutons have oxygen isotope ratios on average ~2‰ higher than the oxygen isotope ratios of initial crust-building magmas, indicating substantial supracrustal input in the source regions of AMCG and post-AMCG magmas. COMPARISON OF OXYGEN AND RADIOGENIC ISOTOPE SYSTEMATICS Superior Province Published lead and neodymium isotope ratios suggest that Superior Province TTGs are derived from Late Archean depleted mantle and contaminated by 1%–10% sediments (Henry et al., 1998), within the constraints allowed by zircon oxygen isotope systematics (King et al., 1998). The Mg and light rare earth element (REE) enrichments in sanukitoid plutons are generally attributed to their being melts of metasomatized mantle (Shirey and Hanson, 1983; Stern et al., 1989). Nd and Pb isotope data have been interpreted as indicating significant crustal contamination in sanukitoid plutons (Henry et al., 1998). Slightly elevated δ18O values (i.e., ~6.5‰) for zircon from sanukitoid plutons are consistent with 364
derivation from mantle that has been metasomatized by subducted slab (or sediment) derived fluids (King et al., 1998). The subtle elevation of δ18O in sanukitoids is less likely to be the result of crustal contamination during ascent and emplacement (King et al., 1998), because average (bulk) Superior Province crust has relatively low δ18O values (average whole rock = 7.8‰; Shieh and Schwarcz, 1978; zircon ≈ 5.8). Grenville Province Proterozoic samples in this study are from the Allochthonous monocyclic belt (~3 × 104 km2) in the southwestern Grenville Province (the Central metasedimentary belt,Adirondack Highlands, and Morin terrane; Rivers et al., 1989), ~100–200 km southeast of the Proterozoic-Archean boundary at the Grenville Front (see Fig. 1). Neodymium and lead isotope data from whole-rock samples (high 208Pb/ 204Pb, 207Pb/ 204Pb, and 206Pb/ 204Pb, high ε Nd values, and 1.3–1.4 Ga Nd model ages) from these intrusive suites suggest that this part of the Grenville Province does not contain a significant percentage of Archean crust or sedimentary rocks (Marcantonio et al., 1990; Daly and McLelland, 1991; McLelland et al., 1993; DeWolf and Mezger, 1994). It has been proposed, on the basis of Nd isotope and trace element data, that AMCG suite granitoids are the products of partial melting of ca. 1.3 Ga calc-alkaline rocks (Daly and McLelland, 1991; McLelland et al., 1993, 1996). Oxygen isotope data indicate an additional component; the ~2‰ elevation in δ18O between suites could indicate addition of a voluminous (>~30%), high δ18O contaminant (such as Grenville Supergroup paragneiss, δ18O [whole rock] ≈ 14‰; Shieh and Schwarcz, 1978) to ca. 1.3 Ga calc-alkaline rocks during genesis of AMCG granitic rocks. An alternative explanation is that AMCG granitoids are derived for the most part from supracrustal rocks with Middle Proterozoic model ages (McLelland et al., 1996). Southern Grenville paragneisses have juvenile Nd model ages that range from 1.5 to 2.1 Ga (Marcantonio et al., 1990; Daly and McLelland, 1991; McLelland et al., 1996), but a majority of
the plutonic rocks in this study intrude country rock with model ages at the younger end of this range (ca. 1.5 Ga; McLelland et al., 1996). Some post-AMCG granites show inheritance of zircon with ca. 1.15 Ga ages (McLelland et al., 1997), indicating at least partial derivation from AMCG suite materials. This is consistent with the similarity of average oxygen isotope ratios between AMCG and post-AMCG plutonic rocks (Fig. 2). Figure 3 shows oxygen isotope ratio of zircon plotted against εNd values of whole rock (Morrison et al., 1985; Shirey and Hanson, 1986; Marcantonio et al., 1990; Tilton and Kwon, 1990; Beakhouse and McNutt, 1991; Daly and McLelland, 1991; Stern and Hanson, 1991; McLelland et al., 1993; Henry et al., 1998). Initial neodymium ratios of Grenville rocks in this study correlate with age: Early plutonism has the highest ε Nd values observed (>3), consistent with extraction from a Middle Proterozoic depleted mantle (Daly and McLelland, 1991; McLelland et al., 1993). AMCG and post-AMCG samples have εNd values between 0 and 3 (Marcantonio et al., 1990; Daly and McLelland, 1991; McLelland et al., 1993), indicating at most a minimal role for older crust in their genesis. Oxygen isotope ratios in Grenville zircons correlate with ε Nd (Fig. 3), forming an array away from 1.3 Ga depleted mantle. This array is suggestive of mixing with Proterozoic metasedimentary rocks. Note that these metasedimentary end members are proxies for supracrustal material at depth; mixing calculations suggest that the isotopic compositions of some plutons cannot be simple binary mixtures between mantle and exposed country rock (e.g., Marcantonio et al., 1990). Archean sedimentary rocks would have extremely low ε Nd values in the Middle Proterozoic. The δ18O of Superior Province samples have a more limited range than Grenville samples, while the average ε Nd for Superior Province samples is the same as 2.7 Ga depleted mantle (Fig. 3). INSIGHTS FROM OXYGEN ISOTOPES Supracrustal Recycling Oxygen isotope compositions are sensitive to interaction at low temperatures where fractionations are largest, and thus surface waters are typically low in δ18O, while products of weathering and low-temperature alteration are high in δ18O (>10‰; Muehlenbachs, 1986; Longstaffe, 1987). The high magmatic δ18O values in Grenville Province igneous rocks are consistent with derivation from source rocks that have interacted, either directly or indirectly, with surface waters at low temperatures. This signature is not dependent on an age contrast between Grenville igneous and metasedimentary rocks. If one were to calculate crustal growth rates in the southern Grenville Province, radiogenic isotope data from AMCG rocks (>30% of the ~3 × 104 km2 study area) indicate that these rocks are juvenile Middle Proterozoic additions to only GEOLOGY, April 2000
εNd
Figure 3. Oxygen isotope ra6 tios of zircon versus wholeSuperior Province rock εNd (calculated for time Plutonic TTG 1.3 Sanukitoid of crystallization). In general Grenville Province there is little variability in Su4 Pre-AMCG perior Province, although AMCG sanukitoids have lower ε Nd Post-AMCG 18 and higher zircon δ O val2.7 ues. In Grenville Province 2 samples, δ18O (zircon) correlates well with εNd . FurtherAdirondack more, anorthosite-mangeritemetasedimentary rocks charnockite-granite (AMCG) 0 and pre-AMCG suites plot in distinct fields. AMCG and post AMCG-suite Grenville samples fall on mixing lines Depleted Mantle (age in Ga) -2 Frontenac between depleted mantle and metasedimentary rocks average Grenville metasedimentary rocks from Adiron4 6 8 10 12 14 16 dacks (McLelland et al., 1996) and Frontenac terrane (Marcantonio et al., 1990). Metaδ18O (Zircon) VSMOW 18 sedimentary rock δ O is plotted as δ18O (zircon) in high-temperature equilibrium with average Grenville paragneiss (Shieh and Schwarcz, 1978; whole-rock δ18O ≈ 14‰). TTG is tonalite-trondhjemite-granodiorite; VSMOW is Vienna standard mean ocean water.
slightly older (
