Multimethod U–Pb baddeleyite dating: insights from the Spread Eagle Intrusive Complex and Cape St. Mary's sills, Newfoundland, Canada

<p>Baddeleyite (<span class="inline-formula">ZrO₂</span>) is widely used in U–Pb geochronology but analysis and age interpretation are often difficult, especially for samples which have experienced post-intrusive alteration and/or metamorphism. Here, we combine high spatial resolution (secondary ionization mass spectrometry, SIMS) and high-precision (isotope dilution thermal ionization mass spectrometry, ID-TIMS) analyses of baddeleyite from the Spread Eagle Intrusive Complex (SEIC) and Cape St. Mary's sills (CSMS) from Newfoundland. Literature data and our own detailed microtextural analysis suggest that at least seven different types of baddeleyite–zircon intergrowths can be distinguished in nature. These include secondary baddeleyite inclusions in altered zircon, previously unreported from low-grade rocks, and likely the first discovery of xenocrystic zircon inclusions mantled by baddeleyite. <span class="inline-formula">²⁰⁷Pb∕²⁰⁶Pb</span> baddeleyite dates from SIMS and ID-TIMS mostly overlap within uncertainties. However, some SIMS sessions of grain mounts show reverse discordance, suggesting that bias in the U&thinsp;<span class="inline-formula">∕</span>&thinsp;Pb relative sensitivity calibration affected <span class="inline-formula">²⁰⁶Pb∕²³⁸U</span> dates, possibly due to crystal orientation effects, and/or alteration of baddeleyite crystals, which is indicated by unusually high common-Pb contents. ID-TIMS data for SEIC and CSMS single baddeleyite crystals reveal normal discordance as linear arrays with decreasing <span class="inline-formula">²⁰⁶Pb∕²³⁸U</span> dates, indicating that their discordance is dominated by recent Pb loss due to fast pathway diffusion or volume diffusion. Hence, <span class="inline-formula">²⁰⁷Pb∕²⁰⁶Pb</span> dates are more reliable than <span class="inline-formula">²⁰⁶Pb∕²³⁸U</span> dates even for Phanerozoic baddeleyite. Negative lower intercepts of baddeleyite discordia trends for ID-TIMS dates for SEIC and CSMS and direct correlations between ID-TIMS <span class="inline-formula">²⁰⁷Pb∕²⁰⁶Pb</span> dates and the degree of discordance may indicate preferential <span class="inline-formula">²⁰⁶Pb</span> loss, possibly due to <span class="inline-formula">²²²Rn</span> mobilization. In such cases, the most reliable crystallization ages are concordia upper intercept dates or weighted means of the least discordant <span class="inline-formula">²⁰⁷Pb∕²⁰⁶Pb</span> dates.</p> <p>We regard the best estimates of the intrusion ages to be <span class="inline-formula">498.7±4.5</span>&thinsp;Ma (2<span class="inline-formula"><i>σ</i></span>; ID-TIMS upper intercept date for one SEIC dike) and <span class="inline-formula">439.4±0.8</span>&thinsp;Ma (ID-TIMS weighted mean <span class="inline-formula">²⁰⁷Pb∕²⁰⁶Pb</span> date for one sill of CSMS). This first radiometric age for the SEIC is consistent with stratigraphic constraints and indicates a magmatic episode prior to opening of the Rheic Ocean. Sample SL18 of the Freetown Layered Complex (FLC), Sierra Leone, was investigated as an additional reference. For SL18, we report a revised <span class="inline-formula">201.07±0.64</span>&thinsp;Ma intrusion age, based on a weighted mean <span class="inline-formula">²⁰⁷Pb∕²⁰⁶Pb</span> date of previous and new baddeleyite ID-TIMS data, agreeing well with corresponding SIMS data. Increasing discordance with decreasing crystal size in SL18 indicates that Pb loss affected baddeleyite rims more strongly than cores. Our SL18 results validate that the SIMS in situ<span id="page188"/> approach, previously used for Precambrian and Paleozoic samples, is also suitable for Mesozoic baddeleyite.</p>