Geochemical Evidence for Hydration and Dehydration of Crustal Rocks During Continental Rifting

To understand the temporal sequence of geological processes such as magmatism, water-rock interaction, and metamorphism in fossil continental rifts, a combined study of petrography, mineral geochemistry, in situ garnet O isotopes, in situ zircon U-Pb ages and O isotopes, and pseudosection calculations was conducted for metagranites from a Neoproterozoic continental rift generated during the Rodinia breakup. The results provide insights into the operation of hydration and dehydration during continental rifting. In the metagranites from the northern margin of South China, three types of garnet (Garnet-I to -III) are distinguished. They were sequentially produced by hydrothermal alteration, metamorphic dehydration, and fluid metasomatism. All of these garnets show negative δ¹⁸O values of −19.3‰ to −14.5‰, in contrast to mantle-like δ¹⁸O values for magmatic zircon. The extremely negative δ¹⁸O values of hydrothermal Garnet-I require infiltration of the continental deglacial meltwater during the continental rifting, and before that zircon crystallized from normal δ¹⁸O magmas. Once the rocks were hydrothermally altered, the extreme ¹⁸O depletion was retained in all later products such as metamorphic Garnet-II and metasomatic Garnet-III. Pseudosection calculations indicate that the metamorphic dehydration occurred at 1.0–3.0 kbar and 630–690 °C during a reheating stage, corresponding to high thermal gradients of >60 °C/km. The high-temperature/low-pressure metamorphic rocks produced by such high thermal gradients are indicative of the continental rift setting. The mineral geochemistry records not only the temporal sequence of rift magmatism, water-rock interaction, and rift metamorphism but also the evolution of temperature and water action in the crust during the continental rifting.