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

• Published in: Geochronology
• Main Authors: J. E. Pohlner, A. K. Schmitt, K. R. Chamberlain, J. H. F. L. Davies, A. Hildenbrand, G. Austermann
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2020
• Subjects: Geology; QE1-996.5; Stratigraphy; QE640-699; Newfoundland
• Online Access: https://doi.org/10.5194/gchron-2-187-2020; https://doaj.org/article/07caf393ee6747f9bccbc7d1268f18ac

Baddeleyite ( ZrO 2 ) 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. 207 Pb∕ 206 Pb 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 ∕ Pb relative sensitivity calibration affected 206 Pb∕ 238 U 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 206 Pb∕ 238 U dates, indicating that their discordance is dominated by recent Pb loss due to fast pathway diffusion or volume diffusion. Hence, 207 Pb∕ 206 Pb dates are more reliable than 206 Pb∕ 238 U 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 207 Pb∕ 206 Pb dates and the degree of discordance may indicate preferential 206 Pb loss, possibly due to 222 Rn mobilization. In such cases, the most reliable crystallization ages are concordia upper intercept dates or weighted means of the least discordant 207 Pb∕ 206 Pb ...