Gardner, RL, Piazolo, S orcid.org/0000-0001-7723-8170, Daczko, NR et al. (1 more author) (2020) Microstructures reveal multistage melt present strain localisation in mid-ocean gabbros. Lithos, 366-367. 105572. ISSN 0024-4937
Abstract
In this work, we examine the tectono-metamorphic evolution of gabbroic rocks of the Atlantis Bank oceanic core complex, South West Indian Ridge, focusing on multistage strain localisation associated with exhumation. We study a sample from the core complex footwall below the depth of seawater hydrothermal fluid-rock interaction. We identify a succession of increasing strain localisation where early solid-state deformation of an olivine gabbro is succeeded by two episodes of melt present deformation resulting in increasing localisation of both strain and melt flux into a narrowing zone.
The early melt-absent solid-state deformation occurs dominantly in the dislocation creep regime, and localises strain at the tens of metre scale. This evolved into melt present deformation characterised by (i) grain size reduction by replacement reactions, and (ii) dislocation creep and incipient grain boundary sliding in coarse- and fine-grained parts, respectively. This is based on (1) microstructures indicative of the former presence of melt, (2) a shift to lower calcium plagioclase, and (3) reaction textures involving partial replacement of olivine and diopside by fine grained enstatite and hornblende exhibiting only minor internal deformation features.
A narrow (2–3 mm) high strain zone which cuts the earlier shear zone foliation at ~30°, exhibits strong and near complete grain size reduction by melt-rock reaction and tightly spaced foliation within a millimetre-wide high strain zone. Here, strain localisation is contemporaneous with a second melt influx shown by (1) complete olivine replacement by fine grained hornblende and enstatite exhibiting no crystallographic preferred orientation, (2) microstructures indicative of the former presence of melt, and (3) changes in the mineral chemistry of diopside, plagioclase, enstatite and hornblende. We suggest deformation was dominated by melt assisted grain boundary sliding allowing higher volumes of melt flux, a high degree of weakening and deformation focused in the narrow shear zone than during the porous melt flow in the wider, early shear zone.
This combined microstructural and microchemical study highlights that the recognition of changes in deformation regime, their time relationships and potential melt related deformation are critical for understanding, and therefore modelling, progressive strain localisation, melt flux and the rheological evolution of oceanic detachment faults.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2020 Elsevier B.V. This is an author produced version of a paper published in Lithos. Uploaded in accordance with the publisher's self-archiving policy. |
Keywords: | Scientific ocean drilling; Oceanic core complex; Strain localisation; Melt microstructures; Melt-present deformation |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Inst of Geophysics and Tectonics (IGT) (Leeds) |
Depositing User: | Symplectic Publications |
Date Deposited: | 02 Jun 2020 13:03 |
Last Modified: | 17 May 2021 00:38 |
Status: | Published |
Publisher: | Elsevier BV |
Identification Number: | 10.1016/j.lithos.2020.105572 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:161340 |
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