Geochronological and thermometric evidence of unusually hot fluids in an Alpine fissure of Lauzière granite (Belledonne, Western Alps)

• Main Authors: E. Janots, A. Grand'Homme, M. Bernet, D. Guillaume, E. Gnos, M.-C. Boiron, M. Rossi, A.-M. Seydoux-Guillaume, R. De Ascenção Guedes
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-01-01
• Series: Solid Earth
• Online Access: https://www.solid-earth.net/10/211/2019/se-10-211-2019.pdf

<p>A multi-method investigation into Lauzière granite, located in the external Belledonne massif of the French Alps, reveals unusually hot hydrothermal conditions in vertical open fractures (Alpine-type clefts). The host-rock granite shows sub-vertical mylonitic microstructures and partial retrogression at temperatures of &lt;&thinsp;400&thinsp;<span class="inline-formula">^∘</span>C during Alpine tectonometamorphism. Novel zircon fission-track (ZFT) data in the granite give ages at 16.3&thinsp;<span class="inline-formula">±</span>&thinsp;1.9 and 14.3&thinsp;<span class="inline-formula">±</span>&thinsp;1.6&thinsp;Ma, confirming that Alpine metamorphism was high enough to reset the pre-Alpine cooling ages and that the Lauzière granite had already cooled below 240–280&thinsp;<span class="inline-formula">^∘</span>C and was exhumed to &lt;&thinsp;10&thinsp;km at that time. Novel microthermometric data and chemical compositions of fluid inclusions obtained on millimetric monazite and on quartz crystals from the same cleft indicate early precipitation of monazite from a hot fluid at <span class="inline-formula"><i>T</i></span>&thinsp;&gt;&thinsp;410&thinsp;<span class="inline-formula">^∘</span>C, followed by a main stage of quartz growth at 300–320&thinsp;<span class="inline-formula">^∘</span>C and 1.5–2.2&thinsp;kbar. Previous Th-Pb dating of cleft monazite at 12.4&thinsp;<span class="inline-formula">±</span>&thinsp;0.1&thinsp;Ma clearly indicates that this hot fluid infiltration took place significantly later than the peak of the Alpine metamorphism. Advective heating due to the hot fluid flow caused resetting of fission tracks in zircon in the cleft hanging wall, with a ZFT age at 10.3&thinsp;<span class="inline-formula">±</span>&thinsp;1.0&thinsp;Ma. The results attest to the highly dynamic fluid pathways, allowing the circulation of deep mid-crustal fluids, 150–250&thinsp;<span class="inline-formula">^∘</span>C hotter than the host rock, which affect the thermal regime only at the wall rock of the Alpine-type cleft. Such advective heating may impact the ZFT data and represent a pitfall for exhumation rate reconstructions in areas affected by hydrothermal fluid flow.</p>