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| Titel |
Enhanced anatexis as a consequence of mantle-derived magma intrusion in the middle crust: a case study from the Eastern French Massif Central |
| VerfasserIn |
Simon Couzinié, Jean-François Moyen, Arnaud Villaros, Jean-Louis Paquette, Jane H. Scarrow, Christian Marignac |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089898
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| Publikation (Nr.) |
 EGU/EGU2014-4111.pdf |
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| Zusammenfassung |
| The post-collisional stage of orogens corresponds to a dramatic change in mountain belts
dynamics. During this period, large volumes of granitic melts are generated in the crust thus
impacting its rheology and overall behavior. Evolving from compression/transpression to
extension/transtension enhances exhumation of high-grade metamorphic rocks and
subsequent decompression crustal melting. However, other processes can trigger anatexis
such as heat or fluid fluxes from the mantle and the crust.
The Early Carboniferous nappe stack of the Eastern French Massif Central (EFMC)
underwent two successive low-pressure melting events at the end of its evolution, during the
Late Carboniferous. They are particularily evident in the southern edge of the Velay
Complex, a 100 km-diameter migmatite-granite dome. The M3 “pre–Velay” event
corresponds to water-saturated melting in the amphibolite facies at T < 750 °C,
P ≥ 5 kbar. It is recorded by cordierite-free, stromatic migmatites and was quite
long-lasting since available U–Th–Pb monazite ages span from 335 to 310 Ma. At that
time, crustal melts mainly remained trapped in the source and few granite plutons
were associated with this event. Contrarily, the M4 “Velay” anatexis occurred under
granulite-facies conditions at 760 < T < 850 °C and 2 < P < 5 kbar. The M4
cordierite-bearing migmatites are nebulitic to diatexitic as a consequence of biotite
breakdown which led to disruption of the solid framework of melanosomes and enhanced
melt extraction. This widespread melting event is synchronous with emplacement of the
cordierite-bearing restite-rich S-type Velay granite at ca. 305 Ma. Then, the EFMC
records an evolution in melting conditions with a clear heat input at the M3–M4
transition.
The EFMC anatectic crust is intruded by widespread, Mg–K–rich biotite–rich
diorites locally called “vaugnerites”. These mantle-derived melts emplaced in a
partially molten setting, as evidenced by mingling features between vaugnerites and
anatectic melts, as well as the presence of hybrid granitoids including a “vaugnerite”
component. In situ (LA–ICP–MS) U–Pb zircon and monazite dating of vaugnerites or
coeval granites in the Southern Velay area yielded ages mostly indistinguishable
within analytical uncertainties, spanning from 307.4 ± 1.8 to 303.7 ± 3.1 Ma. Thus,
mantle-derived magmas emplaced at ca. 305 Ma which is the very transition from M3 to
M4.
This striking synchronism between enhanced crustal melting and mantle-derived
magmatism suggests that vaugnerites could be the cause of the M3–M4 transition. Depending
on the volume involved, the emplacement of hot (ca. 1000 °C) melts in mid crustal
levels would have supplied significant amounts of heat. Vaugnerites could also
be the manifestation of a (yet unconstrained) process enhancing the conductive
mantle heat flux to the crust. For instance, delamination of a lithospheric mantle root
or slab break-off would result in generation of mantle-derived melts as well as
increase the heat conduction into the crust. Therefore, the relevant system that must
be considered to study late-orogenic periods is not only the crust but the whole
lithosphere, taking into account mass/heat transfer from the mantle to the overlying crust. |
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