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Multimethod U–Pb Baddeleyite Dating: Insights from the Spread Eagle Intrusive Complex and Cape St

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Multimethod U–Pb Baddeleyite Dating: Insights from the Spread Eagle Intrusive Complex and Cape St Geochronology, 2, 187–208, 2020 https://doi.org/10.5194/gchron-2-187-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Multimethod U–Pb baddeleyite dating: insights from the Spread Eagle Intrusive Complex and Cape St. Mary’s sills, Newfoundland, Canada Johannes E. Pohlner1,2, Axel K. Schmitt1, Kevin R. Chamberlain3,4, Joshua H. F. L. Davies5,6, Anne Hildenbrand1, and Gregor Austermann1 1Institut für Geowissenschaften, Universität Heidelberg, Im Neuenheimer Feld 234–236, 69120 Heidelberg, Germany 2Unit of Earth Sciences, Department of Geosciences, University of Fribourg, Chemin du Musée 6, 1700 Fribourg, Switzerland 3Department of Geology and Geophysics, University of Wyoming, 1000 E. University Ave., Laramie, WY 82071-2000, USA 4Faculty of Geology and Geography, Tomsk State University, Tomsk 634050, Russia 5Department of Earth and Atmospheric Sciences, Université du Québec à Montréal, 201, Avenue du Président Kennedy, H2X 3Y7, Montréal, QC, Canada 6Department of Earth Sciences, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland Correspondence: Johannes E. Pohlner ([email protected]) Received: 16 December 2019 – Discussion started: 29 January 2020 Revised: 27 May 2020 – Accepted: 17 June 2020 – Published: 8 July 2020 Abstract. Baddeleyite (ZrO2) is widely used in U–Pb is dominated by recent Pb loss due to fast pathway diffu- geochronology but analysis and age interpretation are of- sion or volume diffusion. Hence, 207Pb=206Pb dates are more ten difficult, especially for samples which have experienced reliable than 206Pb=238U dates even for Phanerozoic bad- post-intrusive alteration and/or metamorphism. Here, we deleyite. Negative lower intercepts of baddeleyite discordia combine high spatial resolution (secondary ionization mass trends for ID-TIMS dates for SEIC and CSMS and direct spectrometry, SIMS) and high-precision (isotope dilution correlations between ID-TIMS 207Pb=206Pb dates and the de- thermal ionization mass spectrometry, ID-TIMS) analyses of gree of discordance may indicate preferential 206Pb loss, pos- baddeleyite from the Spread Eagle Intrusive Complex (SEIC) sibly due to 222Rn mobilization. In such cases, the most reli- and Cape St. Mary’s sills (CSMS) from Newfoundland. Lit- able crystallization ages are concordia upper intercept dates erature data and our own detailed microtextural analysis sug- or weighted means of the least discordant 207Pb=206Pb dates. gest that at least seven different types of baddeleyite–zircon We regard the best estimates of the intrusion ages to be intergrowths can be distinguished in nature. These include 498:7 ± 4:5 Ma (2σ ; ID-TIMS upper intercept date for one secondary baddeleyite inclusions in altered zircon, previ- SEIC dike) and 439:4 ± 0:8 Ma (ID-TIMS weighted mean ously unreported from low-grade rocks, and likely the first 207Pb=206Pb date for one sill of CSMS). This first radio- discovery of xenocrystic zircon inclusions mantled by bad- metric age for the SEIC is consistent with stratigraphic con- deleyite. 207Pb=206Pb baddeleyite dates from SIMS and ID- straints and indicates a magmatic episode prior to open- TIMS mostly overlap within uncertainties. However, some ing of the Rheic Ocean. Sample SL18 of the Freetown SIMS sessions of grain mounts show reverse discordance, Layered Complex (FLC), Sierra Leone, was investigated suggesting that bias in the U = Pb relative sensitivity cal- as an additional reference. For SL18, we report a revised ibration affected 206Pb=238U dates, possibly due to crys- 201:07 ± 0:64 Ma intrusion age, based on a weighted mean tal orientation effects, and/or alteration of baddeleyite crys- 207Pb=206Pb date of previous and new baddeleyite ID-TIMS tals, which is indicated by unusually high common-Pb con- data, agreeing well with corresponding SIMS data. Increas- tents. ID-TIMS data for SEIC and CSMS single baddeleyite ing discordance with decreasing crystal size in SL18 indi- crystals reveal normal discordance as linear arrays with de- cates that Pb loss affected baddeleyite rims more strongly creasing 206Pb=238U dates, indicating that their discordance than cores. Our SL18 results validate that the SIMS in situ Published by Copernicus Publications on behalf of the European Geosciences Union. 188 J. E. Pohlner et al.: Multimethod U–Pb baddeleyite dating approach, previously used for Precambrian and Paleozoic 2010; Söderlund et al., 2013; Schaltegger and Davies, 2017), samples, is also suitable for Mesozoic baddeleyite. and isotopic disequilibrium due to 231Pa excess (Amelin and Zaitsev, 2002; Crowley and Schmitz, 2009) or 222Rn loss (Heaman and LeCheminant, 2000) have been proposed to 1 Introduction explain baddeleyite discordance, but none of these are uni- versally accepted as the dominant baddeleyite discordance Baddeleyite, a monoclinic ZrO2 polymorph, is one of the mechanism. most commonly used minerals in U–Pb geochronology, espe- It is necessary to investigate to which degree reliable bad- cially for mafic rocks, which are traditionally difficult to date deleyite dating is possible when several of the abovemen- (e.g., Schaltegger and Davies, 2017). It forms most readily tioned challenges come together. Here, we present a case during the late stage of igneous crystallization from a silica- study of dating the Spread Eagle Intrusive Complex (SEIC) undersaturated residual melt and can coexist with zircon at and Cape St. Mary’s sills (CSMS) of the Avalon Zone of conditions near silica saturation (Heaman and LeCheminant, Newfoundland. These early Paleozoic mafic dikes and sills 1993; Schaltegger and Davies, 2017). Where both minerals were affected by low-grade metamorphism and contain abun- coexist, geochronologists have tended to prefer baddeleyite dant texturally complex baddeleyite, as commonly found in dates because it is (1) a primary igneous mineral, facilitating the geologic record. Moreover, the approximate intrusion age age interpretation; (2) rarely inherited from country rock; and of the SEIC is constrained by its stratigraphic context. We (3) more resistant to Pb loss compared to zircon (e.g., Hea- applied U–Pb geochronology by SIMS and ID-TIMS on the man and LeCheminant, 1993), as it remains crystalline even same baddeleyite crystals, combined with detailed micropet- at high radiation doses (Lumpkin, 1999). U–Pb baddeleyite rographic characterization of baddeleyite by scanning elec- dating has proved powerful in solving numerous problems in tron microscopy (SEM) before and after SIMS analysis. Our earth and planetary sciences (e.g., Olsson et al., 2011; Moser comparison of SIMS and ID-TIMS dating also includes es- et al., 2013; Wall and Scoates, 2016; Davies et al., 2017; sentially unaltered baddeleyite from the Duluth gabbro (sam- White et al., 2020). ple FC-4b; Schmitt et al., 2010) and Freetown Layered Com- However, many of these studies were able to work with plex (sample SL18; Callegaro et al., 2017), leading to a criti- rather large, texturally simple, unaltered and concordant bad- cal evaluation of possibilities and limitations in dating small, deleyite crystals. Such favorable conditions are the exception texturally complex and/or altered baddeleyite crystals. Based rather than the rule for large parts of the geologic record. on our micropetrographic and geochronologic data, we dis- In fact, crystals are often too small for mineral separation cuss various types of baddeleyite–zircon intergrowths, possi- (Söderlund and Johansson, 2002), prohibiting high-precision ble mechanisms of baddeleyite discordance and the reliabil- ID-TIMS (isotope dilution-thermal ionization mass spec- ity of different types of baddeleyite dates (e.g., 206Pb=238U, trometry) analysis. Instead, they can be analyzed in situ by 207Pb=206Pb, concordia upper intercept). Many of these im- secondary ionization mass spectrometry (SIMS; e.g., Schmitt plications are also significant for unaltered baddeleyite. et al., 2010; Chamberlain et al., 2010) or laser ablation induc- tively coupled plasma mass spectrometry (LA-ICP-MS; e.g., Renna et al., 2011). In situ methods require relative sensitiv- 2 Regional geology ity corrections, which may be complicated by crystal orien- tation effects (Wingate and Compston, 2000), although these The Avalon Peninsula consists of rocks that were formed effects for SIMS can be reduced by oxygen flooding (Schmitt as part of the microcontinent Avalonia during the Neopro- et al., 2010; Li et al., 2010). Furthermore, baddeleyite can terozoic and early Paleozoic (e.g., Williams, 1979; Mur- be intergrown with other Zr-bearing minerals, especially in phy et al., 1999; Fig. 1). The Cambrian Adeyton and Har- rocks with a metamorphic or hydrothermal overprint, where court groups (Hutchinson, 1962; King, 1988; Fig. 2), which fluids with high SiO2 activity often cause partial reaction of unconformably overlie Precambrian rocks, consist of well- baddeleyite to polycrystalline zircon (Heaman and LeChem- preserved marine sediments with intercalated pillow basalts inant, 1993; Söderlund et al., 2013). In situ dating after care- and mafic tuffs. The feeder pipes or dike-like conduits of ful micropetrographic investigation can resolve such inter- these volcanic rocks make up a mafic intrusive complex, growths, and for ID-TIMS, baddeleyite can be selectively called the “Spread Eagle Gabbro” (McCartney, 1967) or dissolved in hydrochloric acid, leaving zircon rims essen- “Spread Eagle Gabbro and equivalents“ (King, 1988). To tially undissolved (Rioux et al., 2010). However, even badde-
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