Geochronological and thermometric evidence of unusually hot fluids in an Alpine fissure of Lauzière granite (Belledonne, Western Alps)
<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 ret...
| Main Authors: | , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-01-01
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| Series: | Solid Earth |
| Online Access: | https://www.solid-earth.net/10/211/2019/se-10-211-2019.pdf |
| Summary: | <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 < 400 <span class="inline-formula"><sup>∘</sup></span>C during Alpine tectonometamorphism.
Novel zircon fission-track (ZFT) data in the granite give ages at
16.3 <span class="inline-formula">±</span> 1.9 and 14.3 <span class="inline-formula">±</span> 1.6 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 <span class="inline-formula"><sup>∘</sup></span>C and
was exhumed to < 10 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> > 410 <span class="inline-formula"><sup>∘</sup></span>C, followed by a main stage of quartz growth
at 300–320 <span class="inline-formula"><sup>∘</sup></span>C and 1.5–2.2 kbar. Previous Th-Pb dating of cleft
monazite at 12.4 <span class="inline-formula">±</span> 0.1 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 <span class="inline-formula">±</span> 1.0 Ma. The results attest to the highly dynamic fluid
pathways, allowing the circulation of deep mid-crustal fluids,
150–250 <span class="inline-formula"><sup>∘</sup></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> |
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| ISSN: | 1869-9510 1869-9529 |