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Unraveling the exhumation history of high-pressure ophiolites using magnetite (U-Th-Sm)/He thermochronometry

Abstract : Magnetite is a ubiquitous oxide in ultramafic and mafic rocks, which is present in a large range of geological and tectonic settings. In the case of high-pressure ultramafic rocks, exhumation timing is commonly constrained by geochronometers in nearby lithologies. The development of the magnetite (U-Th-Sm)/He method (MgHe) has opened new perspectives to refine the exhumation history of such rocks. However no thermal history has been deduced using MgHe data so far. Here we applied MgHe dating to magnetite from an HP-LT alpine ophiolite body (Rocher Blanc) from the Schistes lustres units (Western Alps, France) where part the P-T-t history is constrained to further refine the thermal history and demonstrate validity of the method. Textural and geochemical characterizations of the magnetite grains reveal two crystallization environments leading to distinct grain morphologies, euhedral and pseudo-euhedral, with different amounts of mineral inclusions (titanite, chlorite and augite-aegirine). Both magnetite types crystallized during the same retrograde path at T > 250 degrees C, under hydrothermal conditions shown by Si and V oscillatory zoning and low Ti content. MgHe ages derived from euhedral magnetite crystals range from 14.8 +/- 2.1 to 20.9 +/- 3.0 Ma, between zircon and apatite fission track (ZFT) and (AFT) ages deduced from the literature. Pseudo-euhedral grains display older ages interpreted as resulting from the implantation of He from U-rich neighboring minerals such as titanite. For the first time, MgHe data are inverted along with AFT and ZFT data to refine the thermal history of an ophiolite related to their exhumation. The MgHe data suggest that the Rocher Blanc sample has undergone an exhumation in ductile conditions related to subduction dynamics and underplating processes, followed by an exhumation reflecting denudation by erosion under brittle conditions. This exhumation regime change is found to take place at similar to 20 Ma, as opposed to a less constrained similar to 26 Ma cooling as would be inferred from the ZFT and AFT ages alone. Considering published thermochronometric data along the Western Alps, we propose that the exhumation is controlled by the upward indentation of the Apulian mantle producing regional tilting and associated erosion. With the present Schistes lustres ophiolite example, we demonstrate that the MgHe method can be used as a low-temperature thermochronometer (closure temperature in the 200-250 degrees C range) that aids in unraveling the cooling history associated with exhumation of high-pressure ultramafic and mafic rocks at the brittle to ductile transition. (C) 2020 Elsevier B.V. All rights reserved.
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Stéphane Schwartz, Cécile Gautheron, Richard Ketcham, Fabrice Brunet, Marianna Corre, et al.. Unraveling the exhumation history of high-pressure ophiolites using magnetite (U-Th-Sm)/He thermochronometry. Earth and Planetary Science Letters, Elsevier, 2020, 543, pp.116359 (IF 5,273). ⟨10.1016/j.epsl.2020.116359⟩. ⟨hal-02911613⟩

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