U‐Pb dating of interspersed gabbroic magmatism and hydrothermal metamorphism during lower crustal accretion, Vema lithospheric section, Mid‐Atlantic Ridge

Abstract

New U/Pb analyses of zircon and xenotime constrain the timing of magmatism, magmatic assimilation, and hydrothermal metamorphism during formation of the lower crust at the Mid-Atlantic Ridge. The studied sample is an altered gabbro from the Vema lithospheric section (11°N). Primary gabbroic minerals have been almost completely replaced by multiple hydrothermal overprints: cummingtonitic amphibole and albite formed during high-temperature hydration reactions and are overgrown first by kerolite and then prehnite and chlorite. In a previous study, clear inclusion-free zircons from the sample yielded Th-corrected 206 Pb/ 238 U dates of 13.528 ± 0.101 to 13.353 ± 0.057 Ma. Ti concentrations, reported here, zoning patterns and calculated Th/U of the dated grains are consistent with these zircons having grown during igneous crystallization. To determine the timing of hydrothermal metamorphism, we dated a second population of zircons, with ubiquitous <1-20 μm chlorite inclusions, and xenotimes that postdate formation of metamorphic albite. The textures and inclusions of the inclusion-rich zircons suggest that they formed by coupled dissolution-reprecipitation of metastable igneous zircon during or following hydrothermal metamorphism. Th-corrected 206 Pb/ 238 U dates for the inclusion-rich zircons range from 13.598 ± 0.012 to 13.503 ± 0.018 Ma and predate crystallization of all but one of the inclusion-free zircons, suggesting that the inclusion-rich zircons were assimilated from older hydrothermally altered wall rocks. The xenotime dates are sensitive to the Th correction applied, but even using a maximum correction, 206 Pb/ 238 U dates range from 13.341 ± 0.162 to 12.993 ± 0.055 Ma and postdate crystallization of both the inclusion-rich zircons and inclusion-free igneous zircons, reflecting a second hydrothermal event. The data provide evidence for alternating magmatism and hydrothermal metamorphism at or near the ridge axis during accretion of the lower crust at a ridge-transform intersection and suggest that hydrothermally altered crust was assimilated into younger gabbroic magmas. The results of this study show that high-precision U-Pb dating is a powerful method for studying the timing of magmatic and hydrothermal processes at mid-ocean ridges.

The temporal and spatial distribution of magmatism during lower crustal accretion at an ultraslow-spreading ridge: High-precision U–Pb zircon dating of ODP Holes 735B and 1105A, Atlantis Bank, Southwest Indian Ridge

“…The presence of inherited zircons is consistent with previous evidence for anomalously old gabbro and inherited zircons at Atlantis Bank (Schwartz et al, 2005) and on other ridges (Lissenberg et al, 2009;Rioux et al, 2012Rioux et al, , 2015b. Schwartz et al (2005) found that zircon 206 Pb/ 238 U dates from four surface gabbros were older than both the 206 Pb/ 238 U dates of nearby samples and the predicted magnetic age.…”

Zircon petrochronology in large igneous provinces reveals upper crustal contamination processes: new U–Pb ages, Hf and O isotopes, and trace elements from the Central Atlantic magmatic province (CAMP)

“…The high-temperature NMB crystals appear to correlate with lower δ 18 O values (Supplementary data). The low temperatures for most of the CAMP samples are consistent with those found in zircon from mid-ocean ridge gabbros (Rioux et al 2015b) and gabbro intrusions (Fu et al 2008). They are also consistent but slightly higher than the Ti temperatures from quartz crystals (calculated assuming the same Ti activity of 0.6) found in the same evolved granophyric pockets as the zircon crystals (Fig.…”

Geochronological and petrological constraints from the evolution in the Saxon Granulite Massif, Germany, on the Variscan continental collision orogeny

“…Considering the progressive mylonitization and recrystallization of the protoliths with release of U, Th and REE from decomposing igneous minerals on the one hand, and the rise in the U-Th-Pb and REE concentrations between the metamorphic populations as well as the fluid-assisted dissolution-reprecipitation process that created the predominant population of metamorphic zircon on the other hand, we interpret the metamorphic zircon populations as products of near-peak and retrograde metamorphism. The elevated Th/U ratio and the less negative Eu anomaly in the retrograde metamorphic zircon population imply higher redox conditions with increasing oxidation of U 4+ to less zircon-compatible U 6+ (see Rioux et al, 2015), and of Eu 2+ to Eu 3+ that partitions preferentially into zircon. An unzoned zircon core with a U-Pb age of 380 ± 9 Ma represents the near-peak metamorphic population but has U, Th and REE concentrations rather similar to those of the protolith zircon (Table 1, Appendix S9) from which the trace element composition seems to be inherited (see Goodge, Fanning, & Bennett, 2001;Kryza & Fanning, 2007).…”