Baddeleyite microtextures and U-Pb discordance: insights from the Spread Eagle Intrusive Complex and Cape St. Mary’s sills, Newfoundland, Canada

Baddeleyite (ZrO2) is widely used in U-Pb geochronology, but different patterns of discordance often hamper accurate age interpretations. This is also the case for baddeleyite from the Spread Eagle Intrusive Complex (SEIC) and Cape St. Mary’s sills (CSMS) from Newfoundland, which we investigated combining high precision and high spatial resolution methods. Literature data and our own observations suggest that at least seven different types of baddeleyite–zircon intergrowths can be distinguished in nature, among which we describe xenocrystic zircon inclusions in baddeleyite for the first 20 time. Baddeleyite 207Pb/206Pb dates from secondary ionization mass spectrometry (SIMS) and isotope dilution thermal ionization mass spectrometry (ID-TIMS) are in good agreement with each other and with stratigraphic data, but some SIMS sessions of grain mounts show reverse discordance. This suggests that matrix differences between references and unknowns biased the U-Pb relative sensitivity calibration, possibly due to crystal orientation effects, or due to alteration of the baddeleyite crystals, which is indicated by unusually high common Pb contents. ID-TIMS data for SEIC and CSMS single baddeleyite 25 crystals reveal normal discordance as linear arrays with decreasing 206Pb/238U dates, indicating that their discordance is dominated by recent Pb loss due to fast pathway or volume diffusion. Hence, 207Pb/206Pb dates are more reliable than 206Pb/238U dates even for Phanerozoic baddeleyite. Negative lower intercepts of baddeleyite discordias and direct correlations between ID-TIMS 207Pb/206Pb dates and degree of discordance indicate preferential 206Pb loss, possibly due to 222Rn mobilization. In such cases, the most reliable crystallization ages are concordia upper intercept dates or weighted means of the least discordant 30 207Pb/206Pb dates. We regard the best estimates of the intrusion ages to be 498.7 ± 4.5 Ma (2σ; ID-TIMS upper intercept date for one SEIC dike) and 439.4 ± 0.8 Ma (ID-TIMS weighted mean 207Pb/206Pb date for one sill of CSMS). Sample SL18 of the Freetown Layered https://doi.org/10.5194/gchron-2019-21 Preprint. Discussion started: 29 January 2020 c © Author(s) 2020. CC BY 4.0 License.


Introduction
Baddeleyite, a monoclinic ZrO2 polymorph, is currently one of the most commonly used minerals in U-Pb geochronology, especially for mafic rocks, which are traditionally difficult to date (e.g., . It grows most readily during the late stage of igneous crystallization from a silica-undersaturated residual melt, and can co-exist with zircon at conditions near silica saturation (Heaman and LeCheminant, 1993;. Where both minerals co-45 exist, geochronologists often prefer baddeleyite dates because (1) baddeleyite forms almost exclusively during the late stages of igneous crystallization, facilitating age interpretation, (2) it is rarely inherited from country rock, and (3) it is more resistant to Pb loss than zircon (e.g., Heaman and LeCheminant, 1993), as it remains crystalline even at high radiation doses (Lumpkin 1999). However, small degrees of U-Pb discordance are common in baddeleyite and cannot be eliminated by chemical abrasion techniques (Rioux et al., 2010). Additionally, crystals are often too small for mineral separation (Söderlund and Johansson, 50 2002), but can be analyzed in situ by secondary ionization mass spectrometry (SIMS; e.g., Schmitt et al., 2010;Chamberlain et al., 2010) or laser ablation ICP-MS (e.g., Renna et al., 2011).
Various mechanisms have been proposed to explain baddeleyite discordance, leading to contradicting approaches for interpreting ages from discordant analyses. During metamorphism or hydrothermal events, fluids with high SiO2 activity often cause partial reaction of baddeleyite to polycrystalline zircon. Contributions of such zircon rims can produce discordant 55 baddeleyite analyses due to isotopic mixing (e.g., Heaman and LeCheminant, 1993;Söderlund et al., 2013;Rioux et al., 2010).
Strategies for mitigating this problem are selective dissolution of baddeleyite and not zircon prior to ID-TIMS (isotope dilutionthermal ionization mass spectrometry) analysis (Rioux et al., 2010), or employing high spatial resolution methods (SIMS or LA-ICP-MS) to analyze baddeleyite domains free of metamorphic zircon. Other mechanisms for baddeleyite discordance have been proposed, including alpha recoil (Davis and Sutcliffe, 1985;Davis and Davis, 2018), Pb loss due to fast pathway diffusion 60 (Rioux et al., 2010;Söderlund et al., 2013;, and isotopic disequilibrium due to 231 Pa excess (Amelin and Zaitsev, 2002;Crowley and Schmitz, 2009) or 222 Rn loss (Heaman and LeCheminant, 2000). For accurate age interpretation of a given sample, the relevant discordance mechanism(s) must be identified, and therefore a more complete understanding of discordance in baddeleyite is needed.
In this study, we present geochronological and microtextural data on early Paleozoic dikes and sills of the Spread Eagle 65 Intrusive Complex (SEIC) and Cape St. Mary's sills (CSMS) of the Avalon Zone of Newfoundland. These rocks are variably affected by low-grade metamorphism, allowing us to study the effect of alteration on baddeleyite, which is important to consider for most rocks that are typically selected for baddeleyite dating. To identify baddeleyite discordance mechanisms, we applied U-Pb geochronology by SIMS and ID-TIMS on the same baddeleyite crystals, combined with detailed micropetrographic characterization of baddeleyite by scanning electron microscopy (SEM) and in situ SIMS dating. We extend the 70 comparison of SIMS and ID-TIMS dating to the essentially unaltered baddeleyite from the Duluth gabbro (sample FC-4b, Schmitt et al., 2010) and Freetown Layered Complex (sample SL18, Callegaro et al., 2017). Besides deciphering baddeleyite discordance, we present previously undocumented types of baddeleyite-zircon intergrowths and document potential pathways for common Pb incorporation in baddeleyite.

Regional geology 75
The Avalon Peninsula consists of rocks that were formed as part of the microcontinent Avalonia during the Neoproterozoic and early Paleozoic (e.g., Williams, 1979;Murphy et al., 1999; Figure 1). The Cambrian Adeyton and Harcourt groups (Hutchinson, 1962;King, 1988; Figure 2), which unconformably overlie Precambrian rocks, consist of well-preserved marine sediments with intercalated pillow basalts and mafic tuffs. The feeder pipes or conduits of these volcanic rocks make up a mafic intrusive complex, called the "Spread Eagle Gabbro" (McCartney, 1967) or "Spread Eagle Gabbro and equivalents" 80 (King, 1988). To avoid confusion, we define this unit, consisting of at least eleven dikes, as the Spread Eagle Intrusive Complex (SEIC). Lower Ordovician sedimentary rocks are exposed only on Conception Bay islands northeast of the study area (King, 1988). They are roughly coeval with Avalonia's presumably early Ordovician separation from Gondwana (e.g., Cocks et al., 1997;Murphy et al., 2006;Linnemann et al., 2008Linnemann et al., , 2012Pollock et al., 2009Pollock et al., , 2012. The Cambrian pillow basalts and their feeder pipes are confined to the western Avalon Peninsula, which experienced deformation and pervasive low-grade 85 metamorphism during the Acadian Orogeny (McCartney, 1967), lasting from ca. 420-360 Ma (van Staal and Barr, 2012;Willner et al., 2014).

Spread Eagle Intrusive Complex (SEIC) and Cambrian volcanic rocks
Cambrian shales of the Chamberlain's Brook Formation and Manuels River Formation contain pillow basalt flows and/or basaltic metatuffs in five different localities (McCartney, 1967;Greenough, 1984;Greenough and Papezik, 1985b;Fletcher, 90 2006). Additionally, our field observations suggest their presence also within the overlying Elliot Cove formation (sensu stricto King, 1988). Several lines of indirect field evidence substantiate that these volcanic rocks were fed by the SEIC dikes or pipes which crop out in their vicinity (Greenough, 1984;Greenough and Papezik, 1985b). The SEIC intrusions are arranged in a N-S trending array (Figure 1), and are usually subcircular pipes with diameters of several hundreds of meters (McCartney, 1967).
assemblage, but better preserved than the volcanic rocks, which almost completely lack primary minerals (Greenough and Papezik, 1985a,b). Geochemical features of the SEIC rocks are similar to rift basalts, suggesting that Avalonia experienced extensional tectonics in the Cambrian (Greenough and Papezik, 1985b;Greenough and Papezik, 1986a; new whole-rock major and trace element data are given in the supplements). Early attempts at radiometric dating of the SEIC failed, as alteration 100 hampered Rb-Sr whole-rock methods and zircons were not found (Greenough, 1984). Samples of the current study were collected from several SEIC feeder pipes (Table 1).

Cape St. Mary's sills (CSMS)
In addition to the occurrence of the Cambrian igneous rocks described above, the Cambrian succession is intruded by the CSMS in the southwestern Avalon Peninsula, especially in the upper Cambrian Gull Cove Formation (Fletcher, 2006). The 105 sills are up to 60 m thick and consist mostly of gabbro, but the thickest sills also include up to 3 m thick granophyric dikes and pockets (Greenough and Papezik, 1986b;Fletcher, 2006). The gabbros and granophyres are both unusually rich in hydrous minerals, notably amphibole and biotite, and volatile complexing was probably an important differentiation process of the subalkaline parental magma (Greenough and Papezik, 1986b). Trace element patterns indicate that the magma was generated in a mantle source that had been metasomatically enriched by subduction zone-derived fluids (Greenough et al., 1993;110 supplementary data of this study). A CSMS granophyre was the first terrestrial rock for which baddeleyite dating was performed (Greenough, 1984 and pers. comm.). These ID-TIMS analyses of multiple-crystal aliquots yielded an early Silurian U-Pb discordia upper intercept date of 441 ± 2 Ma, but all analyses are discordant to 2.0-3.5% (Greenough et al., 1993).
Combining this date with a paleolatitude of 32° S ± 8°, the CSMS were interpreted as the result of an igneous event after Avalonia's separation from Gondwana, but before complete closure of the Iapetus (Hodych and Buchan, 1998). Samples of 115 the current study were collected from a 60 m thick sill at the southwestern coast of Lance Cove (Table 1).

U-Pb geochronology methods
The occurrence and intergrowth of baddeleyite with other phases in SEIC and CSMS was investigated by SEM on polished thin sections, relying on backscattered electron (BSE) imaging and energy dispersive X-ray spectrometry (EDS) with additional information from cathodoluminescence (CL) imaging. When baddeleyite and/or zircon crystals were too small for 120 mineral separation (<50 µm), they were analyzed in situ in thin section by SIMS for U-Pb dates (Table 1). For samples with larger crystals, polished epoxy grain mounts were prepared from handpicked separates, followed by SEM imaging and SIMS analysis. Every SIMS session was followed by re-imaging of the analysis craters by SEM to identify analyses with contributions from adjacent phases. Selected crystals were removed from grain mounts with a fine needle for single crystal ID-TIMS analyses. 125 SIMS analyses were performed using a CAMECA ims1280-HR ion probe at Heidelberg University. Oxygen flooding of the sample chamber was employed to mitigate crystal orientation effects and improve secondary ion yields Chamberlain et al., 2010;Li et al., 2010), using an oxygen pressure of 2.0×10 -5 to 3.0×10 -5 mbar. The primary ion beam was https://doi.org/10.5194/gchron-2019-21 Preprint. Discussion started: 29 January 2020 c Author(s) 2020. CC BY 4.0 License.
focused to a diameter of about 10-15 µm. In cases where an even higher spatial resolution was needed, the field aperture of the secondary beam was closed to a square of 5-8 µm. For baddeleyite and zircon analyses, analytical procedures were adapted 130 from Schmitt et al. (2010). The U/Pb relative sensitivity calibration (RSC) against the UO2/U ratio accounts for differences in Pb ionization caused by spot-to-spot differences in sputtering behavior. For this, Phalaborwa baddeleyite (Heaman, 2009) was always used as primary reference material. For grain mount sessions, FC-4b baddeleyite  was included as secondary reference material. Zircon analyses were calibrated using the reference materials AS3 (Schmitz et al., 2003) for U-Pb ages and 91500 (Wiedenbeck et al., 2004) for U concentrations. Zirconolite from sample FP7G was analyzed to 135 investigate the influence of its matrix on U-Pb baddeleyite dates when the primary ion beam overlaps onto both minerals.
For ID-TIMS analyses of samples FP6D and S2E, baddeleyite dissolution and chemistry were adapted from Rioux et al. (2010).
Baddeleyite crystals were plucked from the SIMS grain mount, spiked with a mixed 205 Pb/ 233 U/ 235 U tracer (ET535) and dissolved in HCl acid. Solutions were pipetted into beakers to separate them from undissolved zircon domains. Pb and UO2 from baddeleyite were loaded onto single rhenium filaments with silica gel without ion exchange cleanup. Isotopic 140 compositions were measured on a Micromass Sector 54 TIMS at the University of Wyoming. Common Pb corrections of SIMS and ID-TIMS analyses were made using the model of Stacey & Kramers (1975) at 400 Ma. The decay constants and 238 U/ 235 U ratio are from Steiger and Jäger (1977). Concordia coordinates and uncertainties were calculated using IsoplotR for SIMS (Vermeesch, 2018) and PBMacDAT for ID-TIMS (Ludwig, 1988).

Secondary reference baddeleyite from the Duluth gabbro and Freetown Layered Complex (FLC)
Reference baddeleyite FC4b is from the anorthositic series of the Duluth Complex, part of the Middle Proterozoic (ca. 1.1 Ga) North American Midcontinent Rift system (Paces and Miller, 1993). The sample was collected from the anorthositic series of the complex, and has been described as an olivine-phyric gabbroic anorthosite (Hoaglund, 2010). FC4-b baddeleyite has yielded dates of 1096.84 ± 0.45 Ma ( 206 Pb/ 238 U; all uncertainties stated are 2σ) and 1099.6 ± 1.5 Ma ( 207 Pb/ 206 Pb) by ID-TIMS 150 analysis . Our new SIMS data for FC-4b are from baddeleyite crystals with petrographic properties comparable to those of Schmitt et al. (2010).
Sample SL18 is an olivine gabbronorite from the Freetown Layered Complex (FLC) in Sierra Leone, which is part of the Central Atlantic Magmatic Province (CAMP). SL18 consists of plagioclase and augite with minor olivine and accessory baddeleyite and apatite. Large baddeleyite crystals (with U contents of 1-4 ng) produced a weighted mean 206 Pb-238 U date of 155 198.777 ± 0.047 Ma by ID-TIMS (Callegaro et al., 2017), but these data show some scatter and the mean date was generated by 7 analyses out of a total of 11 analyses.
Furthermore, this baddeleyite date is significantly younger than all zircon U-Pb ID-TIMS dates from CAMP samples (Blackburn et al. 2013;Davies et al. 2017)  to determine a robust U-Pb crystallization age. We present additional single crystal ID-TIMS data of SL18, obtained with exactly the same methodology as for Callegaro et al. (2017) at the University of Geneva, but with baddeleyite crystals from the same separate that are 10-30 times smaller. 165

Spread Eagle Intrusive Complex (SEIC)
The SEIC rocks have well-preserved igneous textures, but major and accessory minerals are frequently replaced by parageneses indicative of alteration and low-grade metamorphism. Grain size and colour index can vary considerably, ranging from finegrained melanogabbros to coarser-grained monzonites and monzosyenites, which are almost pegmatoidal in rare cases. 170 Plagioclase is always completely altered to albite. Many samples also contain large amounts of K-feldspar. In the less potassic samples, K-feldspar is concentrated in interstitial areas or leucocratic pockets, often together with accessory minerals. Minor quartz is often present in baddeleyite-bearing and baddeleyite-free rocks, but usually in secondary pockets and veins.
Clinopyroxene is replaced by chlorite to a variable extent, often forming pseudomorphs. Some samples have essentially unaltered clinopyroxene, but also contain chlorite pseudomorphs. Ilmenite is largely replaced by titanite ± rutile ± magnetite. 175 Other common secondary minerals are calcite, epidote, prehnite and/or pumpellyite, and accessory sulphides (pyrite, chalcopyrite, galena, sphalerite). Apatite is ubiquitous.
All samples contain Zr-bearing accessory phases. Baddeleyite occurs in many samples, usually as <20 µm long, lath-shaped euhedral crystals. Crystals 20-80 µm in length occasionally occur in some samples, and FP6D is the only sample with large textures. The most common case is that baddeleyite crystals contain zircon domains mostly at their rim, but commonly penetrating into the core. This largely pseudomorphic replacement texture is accompanied by feather-like zircon coronas around the crystal (e.g., Figure 4d). Baddeleyite preservation tends to be better if the rock is less altered, but also if crystals are large, as the presence of rather modest zircon overgrowths in the strongly altered monzonite FP6D indicates. Baddeleyite inclusions in K-feldspar usually lack zircon intergrowths. In samples FP1F and FP1I, clusters of baddeleyite needles are 185 enclosed by zircon crystals (Figure 3a). The enclosing zircon is sometimes almost euhedral, but often with feather-like zircon overgrowths. A special feature we identified in FP12A is that some baddeleyite crystals have zircon inclusions up to 3 µm wide and at most 12 µm long (Figure 4f-k). Secondary zircon overgrowth on the enclosing baddeleyite was rarely observed.
Besides that, FP12A contains prismatic euhedral baddeleyite crystals essentially free of zircon, but with a notch on one prism plane that penetrates into the crystal's core (Figure 4e, l). Zircon crystals without baddeleyite intergrowth are sometimes present 190 also in baddeleyite-bearing rocks. Some of these crystals have an amoeboid surface and a scarred interior. Baddeleyite-free rocks are often either melanocratic or rich in quartz. They contain zircon as the only Zr-bearing mineral, forming euhedral (≤ 20 µm) or anhedral (≤ 50 µm) crystals. Only one sample (FP7G) contains zirconolite CaZrTi2O7 (all mineral formulae given as stochiometric end-members from Anthony et al., 2001) and rare pyrochlore (Ca,Na)2Nb2O6(OH,F) in addition to baddeleyite https://doi.org/10.5194/gchron-2019-21 Preprint. Discussion started: 29 January 2020 c Author(s) 2020. CC BY 4.0 License. and zircon. Zirconolite occurs mainly in the vicinity of baddeleyite crystals of similar size and form ( Figure 3f). It sometimes 195 has an altered appearance and/or significant Si contents. In rare cases it is partly replaced by titanite + zircon (Figure 3g).
In contrast to the gabbros, granophyres are strongly leucocratic (> 70 vol.-% albite). Albite crystals are typically 1 × 0.5 cm large in the interior of granophyre pockets, whereas the outermost ca. 5 cm of the pockets are somewhat less leucocratic with smaller albite crystals. Nonetheless, a strong contrast of grain size and mineralogy characterizes the sharp contact between 205 gabbros and granophyre pockets. Granophyre samples S2C (center of a pocket) and S2E (transition of a pocket center to the gabbro contact) are largely identical in major phases (albite, clinopyroxene, ilmenite, chlorite, biotite ± Ti-rich hornblende).
However, the accessory mineral assemblages are highly different. In S2C, zircon is the only Zr-bearing phase with contact to

SIMS data
The SEIC SIMS data are presented in Figure 6 (summary in Table 2; detailed data in supplementary Tables S1-S6). In situ 225 baddeleyite analyses of sample FP7G yielded weighted mean dates of 529.9 ± 21.4 Ma ( 206 Pb/ 238 U; all uncertainties specified in the text are 2σ) and 497.8 ± 73.2 Ma ( 207 Pb/ 206 Pb). For FP12A baddeleyite, the weighted mean dates are 508.2 ± 11.2 Ma ( 206 Pb/ 238 U) and 546.6 ± 83.6 Ma ( 207 Pb/ 206 Pb). Many baddeleyite analyses show surprisingly high contents of common Pb. As a general practice, analyses with high common Pb (<90% radiogenic 206 Pb) were excluded from weighted mean calculations.
In the grain mount sessions, FC-4b baddeleyite was analyzed as a secondary reference in addition to the Phalaborwa  Table 2; Table S6). The 206 Pb/ 238 U dates from these sessions are in good agreement with the CAMP age of ~201.5 Ma based on worldwide samples using zircon (Blackburn et al., 2013;Davies et al., 2017).

ID-TIMS data
ID-TIMS analyses of baddeleyite from SEIC (sample FP6D) and CSMS (sample S2E) yielded normally discordant data that 260 form linear arrays (Figure 8; Table 3 Additional ID-TIMS data for very small baddeleyite crystals of sample SL18 (Figure 9c; Table 3) yielded 206 Pb/ 238 U dates that are clearly younger than those of the larger crystals of SL18 published in Callegaro et al. (2017). Common Pb contents of SL18 baddeleyite are lower than those of SEIC and CSMS baddeleyite.

Occurrence, textures and interrelations of accessory minerals 270
Zirconium-bearing accessory minerals in mafic magmas form during late stages of crystallization in more differentiated interstitial melt (Heaman and LeCheminant, 1993;. In our study, abundance and crystal size of accessory minerals lack a strong correlation with whole rock Zr content (see supplements), but the more coarse-grained samples tend to contain larger baddeleyite and zircon crystals. Regardless of crystal sizes, baddeleyite and zircon in SEIC and CSMS rocks form different types of intergrowths. Baddeleyite in mafic rocks is typically of igneous origin, but metamorphic 275 processes can cause it to react to polycrystalline zircon (Heaman and LeCheminant, 1993). Metamorphic zircon is therefore expected to be the most common type of zircon intergrown with baddeleyite in SEIC and CSMS rocks, which all show petrological evidence for low-grade metamorphism. Although probably less common, magmatic zircon overgrowths on preexisting baddeleyite can also form during late-stage igneous crystallization due to an increased SiO2 activity in the melt (e.g., Renna et al., 2011). Such igneous zircon overgrowths on baddeleyite have rather euhedral crystal faces and straight interfaces 280 with baddeleyite (Renna et al., 2011). By contrast, metamorphic zircon shares a more irregular crystal interface with baddeleyite and has an anhedral exterior, described as "raspberry texture" (Heaman and LeCheminant, 1993) or "frosty appearance" (Söderlund et al., 2013). For SEIC and CSMS, the typical textural features of igneous zircon overgrowth on baddeleyite are rarely displayed (Figure 5l), whereas features of metamorphic replacement zircon are frequently observed.
which we interpret as a result of volume enlargement by the addition of silica during metamorphism. The presence of such coronas can therefore help to distinguish zircon with a baddeleyite precursor from primary zircon in altered igneous rocks.
The extent of baddeleyite replacement by zircon in this study often depends on the host minerals. Baddeleyite surrounded by chlorite shows replacement by zircon more commonly than baddeleyite in albite or epidote group minerals, and baddeleyite inclusions in K-feldspar mostly lack zircon. We attribute this to local variations in the SiO2 activity during metamorphism: the 290 chloritization of pyroxenes liberates large amounts of Si, whereas alteration of alkali feldspars has a neutral Si balance. Si release sometimes also causes replacement of zirconolite by titanite + zircon (Figure 3g However, Cape St. Mary's sills have experienced only subgreenschist facies (Greenough and Papezik, 1986b), or at most lower greenschist facies conditions. Hence, secondary baddeleyite inclusions in zircon can also form at low temperatures, and lowtemperature reactions of zircon to baddeleyite and vice versa can occur within the same dike.

Zircon inclusions in baddeleyite 305
A peculiar texture in sample FP12A is baddeleyite with zircon inclusions (Figure 4e-k). Most of these baddeleyite crystals lack zircon overgrowth, and the baddeleyite mantle is coherent even if it is as thin as 1 µm. Thus the zircon crystals clearly represent inclusions and are not parts of a metamorphic rim locally penetrating into the crystal interior. In SIMS analysis of these zircon inclusions, the primary beam overlapped onto baddeleyite and zircon, but at least one zircon crystal gave a 206 Pb/ 238 U date considerably above the Cambrian intrusion age using either a baddeleyite-based or a zircon-based RSC ( Figure  310 10). As baddeleyite is reasonably expected to record the age of dike intrusion, the older age indicates that the zircon inclusions predate this magmatism, and must therefore be of xenocrystic origin. We explain this by assimilation of zircon-bearing country rock by a hot, low SiO2 activity magma, where zircon is undersaturated and dissolves (see e.g., Boehnke et al., 2013). Slow Zr diffusion in the melt limits the zircon dissolution rate, so that the melt adjacent to the zircon will develop an exponentially decreasing Zr concentration gradient (e.g., Harrison and Watson, 1983). Hence, partially dissolved xenocrystic zircon will be 315 surrounded by a halo of elevated Zr concentration in the zircon-undersaturated magma. Such a halo of elevated Zr concentrations is a preferential location for baddeleyite nucleation, even if the bulk of the magma remains undersaturated with regard to baddeleyite. Once a nucleus is formed, baddeleyite will grow preferentially where Zr concentration is highest, this is https://doi.org/10.5194/gchron-2019-21 Preprint. Discussion started: 29 January 2020 c Author(s) 2020. CC BY 4.0 License.
at the dissolution interface. If a coherent baddeleyite mantle is formed, the zircon xenocryst becomes shielded from further dissolution. 320 In contrast to the apparent rarity of this texture suggested by the lack of previous reports, we document  xenocrysts that are up to ~250 Ma older than baddeleyite, and of others that are only slightly older. Besides zircon cores, FP12A and all other samples of our study lack any other recognizable xenocrysts or xenoliths. It appears that the remaining minerals of the assimilated country rock became readily resorbed.
It is possible that this texture has been overlooked in the past, as detailed micropetrographic investigation is necessary to detect it. This is especially true for large baddeleyite crystals typically targeted for ID-TIMS analysis. Alternatively, this texture may 330 be in fact rare, as its formation depends on numerous factors: 1) A melt with low SiO2 activity is needed, being zircon-undersaturated, but close to baddeleyite saturation.
2) The country rock must have zircon, but should not be too siliceous, because otherwise zircon would be stabilized and baddeleyite destabilized. If the country rock is only weakly consolidated, zircon liberation is facilitated.
3) High temperatures and low crystal fraction of the magma are necessary to assimilate country rock effectively. However, 335 this speeds up Zr diffusion and zircon dissolution as well, compromising baddeleyite nucleation and growth. 4) If the zircon crystal is small or the relative crystal orientations of zircon and baddeleyite are unfavorable, baddeleyite may fail to enclose zircon before the latter dissolves completely. This may leave a notch on the baddeleyite crystal, such as in Because of the drastic consequences of zircon intergrowths for geochronology, it is important to carry out careful 340 micropetrographic investigation of baddeleyite crystals, especially for samples with complex metamorphic histories. At least seven different types of baddeleyite-zircon intergrowths have to be considered (compiled in Table 4), including three types not present in our samples: granular baddeleyite droplets can rim zircon that decomposed to baddeleyite + SiO2 as a result of impact shockwave heating (e.g., El Goresy, 1965;Wittmann et al., 2006). The inversion of this reaction was found in a shergottite sample, where primary baddeleyite is often partially rimmed by polycrystalline zircon in the vicinity of impact melt 345 (Moser et al., 2013;Darling et al., 2016). This is the only baddeleyite-zircon intergrowth type where baddeleyite is affected by shockwave metamorphism (Moser et al., 2013;Darling et al., 2016). Besides that, feather-like polycrystalline baddeleyite reaction rims on mantle-derived zircons in kimberlites were found to be the result of desilicification reactions (Kresten, 1973).
These seven intergrowth types can occur in combination, complicating textural interpretation. Dating by SIMS or LA-ICP-MS provides the high spatial resolution that is required to unravel the age relationships of baddeleyite-zircon intergrowths. 350 Alternatively, dissolution in hydrochloric acid alone may avoid including zircon domains in ID-TIMS baddeleyite analyses (e.g., Rioux et al., 2010). https://doi.org/10.5194/gchron-2019-21 Preprint. Discussion started: 29 January 2020 c Author(s) 2020. CC BY 4.0 License.

Challenges in baddeleyite geochronology by SIMS
Despite many examples of good agreement between SIMS and ID-TIMS data (e.g., the SIMS 207 Pb/ 206 Pb date agrees with the ID-TIMS 207 Pb/ 206 Pb and upper intercept dates of FP6D, the SIMS in situ dates agree with both ID-TIMS dates of S2E, and 355 all dates of SL18 and FC-4b agree), some SIMS sessions yielded dates that deviate significantly, notably in case of grain mount sessions and/or 206 Pb/ 238 U dates. Although SIMS dates of baddeleyite are often not less accurate than ID-TIMS dates (see Sect. 6.3 and 6.4), baddeleyite poses analytical challenges that are largely specific for in situ methods. These are beam overlap on adjacent phases, possible bias in the U-Pb relative sensitivity calibration (RSC) and unusually high common Pb contents (the latter also applies to ID-TIMS analyses). 360 Small crystal sizes of baddeleyite (< 10 µm) often result in primary beam overlap on adjacent phases during SIMS sessions.
This does not necessarily affect the accuracy of baddeleyite dates if the adjacent minerals are U-and Pb-free (e.g., chlorite), but otherwise, especially in case of intergrowths with zircon and zirconolite, accuracy of baddeleyite dates can be severely affected. But even so, estimations of the baddeleyite crystallization ages are possible if the extents of potential resulting inaccuracies can be assessed. This requires knowledge about (1) the approximate U-Pb crystallization ages of baddeleyite and 365 the contaminating phase -we suppose that the zircon rims of SEIC samples formed at ca. 400 Ma during the Acadian Orogeny, being ca. 20% younger than SEIC baddeleyite, and that some of the zircon rims experienced Pb loss; (2) possible differences in U content of the involved minerals, where baddeleyite tends to have the same (this study) or higher (Heaman and LeCheminant, 1993) U contents than zircon, and lower U contents than zirconolite ; and (3) matrix effects that lead to different U-Pb relative sensitivities. As observed from the RSC, baddeleyite 206 Pb/ 238 U dates shift to 370 younger ages when computed against a zircon RSC (-54 to +3% total, -23% average), and zircon dates become older vice versa (+1 to +121%, +43% average). Similar bias results from baddeleyite-zirconolite beam overlap, although presently this bias cannot be quantified due to lack of well-characterized zirconolite reference materials. Notably, when SIMS baddeleyite analyses contain metamorphic zircon contributions, their younger age is partly compensated by the RSC bias, so their influence on data quality is less severe than in cases where zircon is coeval or older than baddeleyite. 375 Precision and accuracy of SIMS 206 Pb/ 238 U dates strongly depend on the quality of the RSC. Variable degrees of reverse discordance in different SIMS grain mount sessions of the same sample most likely reflect difficulties in quantification of matrix effects. RSC accuracy depends on numerous factors, but crystal orientation effects have been long recognized as particularly important for SIMS U-Pb dating of baddeleyite (Wingate and Compston, 2000). Although oxygen flooding of the sample chamber proved to be effective for reducing crystal orientation effects, residual bias remains Li 380 et al., 2010). In case of grain mounts, tabular crystals will be preferentially oriented with their c axis parallel to the sample surface. This may be a cause of matrix mismatch in grain mount analyses, whereas in situ mounts, which lack reverse discordance in this study, are expected to have more random distributions of crystal orientations. Analysis of a sufficiently large number of crystals (> ca. 25), and randomizing crystal orientations of grain mounts, are advisable strategies for obtaining https://doi.org/10.5194/gchron-2019-21 Preprint. Discussion started: 29 January 2020 c Author(s) 2020. CC BY 4.0 License. pathway diffusion (e.g., along twin-planes) affects both systems similarly, making 207 Pb/ 206 Pb ages most accurate (Davis and Davis, 2018).
Metamorphic zircon overgrowth was absent in baddeleyite used for ID-TIMS analysis of S2E and SL18. Even in FP6D, where it is petrographically evident, the ID-TIMS data appear to be free of a significant isotopic component of metamorphic zircon. 420 This confirms that baddeleyite and zircon can be separated successfully with the method of Rioux et al. (2010), using only hydrochloric acid for dissolution. Consequently, discordance should be attributed to baddeleyite itself instead of zircon intergrowths. Our ID-TIMS analyses show linear arrays that are typical for varying degrees of recent Pb loss. The portion of radiogenic Pb lost by alpha recoil can be predicted based on crystal shapes (Davis and Davis, 2018) and is generally <0.3% for the crystals of this study. However, many of the ID-TIMS data here indicate that Pb loss exceeds this extent by more than 425 one order of magnitude (Table 3). Hence alpha recoil plays only a subordinate role, depending on the U zonation of the crystals.
Fast pathway and/or volume diffusion is therefore likely to dominate baddeleyite discordance in FP6D, S2E and SL18.
Intriguingly, the discordia trends for samples FP6D and S2E have negative lower intercepts and show a positive correlation of the 207 Pb/ 206 Pb date with the percentage of discordance (Figure 8). We interpret preferential loss of 206 Pb, possibly due to 222 Rn mobility, to be responsible for this pattern. Our data suggest that this excess 206 Pb loss increases with overall Pb loss, meaning 430 that the least discordant analyses are least affected by this bias. The mechanisms for Pb loss from baddeleyite remain unclear.
The radiation dose does not seem to be crucial: except for the in situ session of S2E, negative correlations between U content and 206 Pb/ 238 U dates appear to be absent in SIMS and ID-TIMS data of our samples (cf. Söderlund et al. 2013).
Baddeleyite rims appear to be more strongly affected by Pb loss than cores. This is shown by the ID-TIMS data of SL18 (Figure 9c), which show younger 206 Pb/ 238 U dates for the smallest crystals, which have a larger surface to volume ratio than 435 the larger crystals. This effect is not as obvious in the SIMS data, owing to much larger uncertainties. Furthermore, ID-TIMS data tend to be more discordant than SIMS data from the same grains (Table 3 vs. Tables S1, S6). The SIMS spots were typically placed in the centers of the grains, but dissolution of the plucked grains for ID-TIMS analysis included the rims as well. Similarly, SIMS 207 Pb/ 206 Pb dates tend to be younger than ID-TIMS 207 Pb/ 206 Pb dates, possibly due to increased 206 Pb loss of the rims. Fast pathway diffusion and/or volume diffusion are both possible explanations of intensified Pb loss from the 440 crystal rims, but to differentiate between these processes, both the U zonation within the crystals and the post emplacement thermal history of the sample are not sufficiently known.

Approaches to obtain the most accurate baddeleyite crystallization ages
With Pb loss as a dominant discordance mechanism, 206 (Li et al., 2009) Table 3, Figure 8). If the baddeleyite cores are less discordant than the rims, the cores should be preferentially targeted for analysis. In the case of ID-TIMS, mechanical abrasion 455 is potentially helpful in this respect, and there are documented cases where discordance was reduced by mechanical abrasion (e.g., Corfu and Lightfoot, 1996;cf. e.g., Greenough et al., 1993). A possible alternative to mechanical abrasion is to cut out baddeleyite cores with a focused ion beam before ID-TIMS analysis (White et al., in review). With SIMS, it is easier to preferentially target the cores. Hence, SIMS 207 Pb/ 206 Pb dates, which are not precise enough to reveal discordance patterns but potentially more accurate than ID-TIMS, are important as cross-checks, highlighting the power of the combined usage of high 460 spatial resolution (SIMS) and high precision (ID-TIMS) dating methods.

Intrusion ages of the SEIC, CSMS and FLC
Statements on the intrusion age of SEIC need to consider stratigraphic constraints in addition to the geochronological data. As the SEIC consists of the feeder pipes of the Cambrian volcanic rocks on the western Avalon Peninsula (Sect. 2.1), the SEIC is required to be coeval to this volcano-sedimentary succession. Biostratigraphic constraints suggest that the age span of 465 deposition of the Manuels River Formation roughly equals that of the Drumian stage (Hildenbrand, 2016;Austermann, 2016).
The Drumian stage is currently thought to span from 504.5-500.5 Ma with age uncertainties from these bounds of ca. 2 Ma (Peng et al., 2012). The occurrence of pillow lavas in the Chamberlain's Brook Formation and Elliot Cove formation expands the age range of Cambrian volcanism on the Avalon Peninsula to both pre-and post-Drumian ( Figure 2). Our SIMS and ID-TIMS data are thus in good agreement with these stratigraphic limits. We regard the ID-TIMS concordia upper intercept date 470 of FP6D (498.7 ± 4.5 Ma) as the best available estimate of the intrusion age of the corresponding feeder pipe. As indicated by the stratigraphic distribution of the volcanic rocks, the other feeder pipes may significantly pre-and/or postdate this age, but they lack sufficiently precise geochronologic data to establish a firm age range of magmatism. Nevertheless, the improved perception of the age of SEIC indicates that SEIC magmatism clearly predates the opening of the Rheic Ocean (see Sect. 2).
For the CSMS, zircon from the granophyre sample S2C is considerably altered and the degree of secondary Pb loss is often 475 very large, limiting its use for geochronology. Baddeleyite from sample S2E, although potentially also altered, better preserves the igneous age. Our ID-TIMS 207 Pb/ 206 Pb baddeleyite date (444.1 ± 4.4 Ma) agrees within error with that of Greenough et al. (1993). Their sample was derived from the same granophyric dike as ours (Greenough, pers. comm.). Greenough et al. (1993) analyzed bulk separates of baddeleyite, thus the large number of crystals per aliquot in their analyses may have obscured the discordance patterns that we observed in our single crystal analyses. Combining the data of both studies, we regard a weighted 480 mean ID-TIMS 207 Pb/ 206 Pb age of 439.4 ± 0.8 Ma (95% conf.; MSWD = 0.94; Figure 12) as the best available estimate of the intrusion age of the Lance Cove sill, using only the analyses with <3% discordance to minimize bias due to preferential 206 Pb https://doi.org/10.5194/gchron-2019-21 Preprint. Discussion started: 29 January 2020 c Author(s) 2020. CC BY 4.0 License.
loss. If this bias is significant even for the least discordant analyses, this 207 Pb/ 206 Pb date may be an overestimate, but the ID-TIMS upper intercept date of 437 ± 8 Ma, which would be more accurate in this case, does not lead to a better age estimate due to inferior precision. The date that we report does not rule out the possibility that other sills of Cape St. Mary's are 485 significantly older or younger.
For the FLC, our new ID-TIMS and SIMS data suggest that the intrusive age based on a weighted mean 206 Pb/ 238 U date reported in Callegaro et al. (2017) is likely too young and reflects some degree of Pb loss. We showed here that smaller baddeleyite crystals from SL18 yield younger ID-TIMS 206 Pb/ 238 U dates due to more intense Pb loss likely due to fast pathway or volume diffusion. Therefore, we regard the 207 Pb/ 206 Pb ID-TIMS date (201.07 ± 0.64 Ma) as the best estimate for the baddeleyite 490 crystallization age of SL18. This age is in agreement with the SIMS dates and all other age constraints from both the FLC and the CAMP Callegaro et al., 2017). This new age does not change the overall interpretations of Callegaro et al. (2017).

Conclusions 495
A case study of mafic dikes and sills from western Avalon Peninsula, Newfoundland, Canada shows complex textures and age relations for baddeleyite and zircon in mafic rocks that underwent sub-to lower-greenschist metamorphism. Based on new and published microtextural observations, at least seven different types of baddeleyite-zircon intergrowths have to be considered when using baddeleyite as a geochronometer. A previously undocumented type that we discovered in SEIC dikes is xenocrystic cores of zircon mantled by igneous baddeleyite overgrowths. This study also shows that a combination of high precision and 500 high spatial resolution methods is required to extract reliable age information from baddeleyite. The accuracy of SIMS in situ analyses is affected by calibration bias, possibly due to crystal orientation effects. Unusually high common Pb contents in SEIC and CSMS baddeleyite are probably a consequence of alteration. Baddeleyite discordance detected in ID-TIMS singlecrystal analyses is primarily caused by secondary Pb loss, which comprises a component along a zero age intercept discordia, but also preferential loss of 206 Pb, possibly due to 222 Rn mobility. Any kind of Pb loss makes 207 Pb/ 206 Pb ages more reliable 505 than 206 Pb/ 238 U ages, even for Phanerozoic baddeleyite. Preferential 206 Pb loss is detectable by direct correlations between 207 Pb/ 206 Pb dates and degree of discordance. In this case, the most accurate age is either the concordia upper intercept date or the 207 Pb/ 206 Pb dates of the least discordant analyses. Potential remedies for Pb-loss are to preferentially target the less discordant cores of baddeleyite, either by SIMS, or possibly by applying mechanical abrasion prior to ID-TIMS analysis.      https://doi.org/10.5194/gchron-2019-21 Preprint. Discussion started: 29 January 2020 c Author(s) 2020. CC BY 4.0 License. Figure 10: Baddeleyite with and without a zircon inclusion, before and after SIMS analysis, with corresponding 206 Pb/ 238 U dates (Ma; 2σ uncertainties) calculated using Phalaborwa baddeleyite calibration (black) and AS3 zircon calibration (gray). The fact that the date of the mixed baddeleyite-zircon analysis is older than the other baddeleyite dates for both calibrations indicates a real age difference rather than bias caused by applying a baddeleyite-based calibration for the zircon component of the analysis.    ins = in situ, gm = grain mount *2σ uncertainty multiplied with the sqareroot of the MSWD for samples with MSWD > 1 **number in parentheses is without analyses that have high common Pb (<90% radiogenic 206 Pb) or contain contributions from other U-bearing minerals. These analyses are excluded from weighted mean calculations. For zircon, a range of dates is given instead of weighted means, as the crystals may belong to several generations and/or have undergone strongly variable Pb loss ***not including zircon inclusions in baddeleyite. Analyses were acquired during sessions FP12A baddeleyite1-3.