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

He, Qiang; Zhang, Shao-Bing; Zheng, Y-F; Xia, Qiong-Xia; Rubatto, Daniela (2019). Geochemical Evidence for Hydration and Dehydration of Crustal Rocks During Continental Rifting. Journal of geophysical research, 124(12), pp. 12593-12619. American Geophysical Union 10.1029/2019JB018508

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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.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences

UniBE Contributor:

Rubatto, Daniela

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0148-0227

Publisher:

American Geophysical Union

Language:

English

Submitter:

Daniela Rubatto

Date Deposited:

20 Jan 2020 10:24

Last Modified:

06 Jun 2020 02:30

Publisher DOI:

10.1029/2019JB018508

BORIS DOI:

10.7892/boris.138618

URI:

https://boris.unibe.ch/id/eprint/138618

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