Mineralogical insights for very high temperature conditions during Cretaceous mantle exhumation at the northern Iberian passive margin: the sapphirine-bearing supradetachment deposits of the North Pyrenean Zone in the Lherz area
Résumé
In order to decipher the mode of exhumation of the Pyrenean peridotites and the mechanisms of extreme thinning
of the continental crust, consistent information from the direct geological environment of the mantle bodies has
to be collected. Therefore, we focused our field investigations on the metasedimentary formations that occur in
direct contact with the Lherz lherzolites in the inverted Aulus Basin of Albian-Cenomanian age, completed by a
microscopic observation and microprobe analysis of more than 50 thin sections. This mineralogical approach pro-
vides robust constraints on the lithology of the units progressively exhumed during the successive stages of mantle
unroofing. We show that various polymictic breccias, microbreccias and sandstones are exposed in the immedi-
ate vicinity of the mantle bodies and associated Upper Triassic meta-ophites. Among these detrital sediments, the
sapphirine-bearing sandstones (Monchoux, 1970; 1972) are located on the northern edge of the Lherz lherzolite.
At this site, we describe a progressive transition from the lherzolite body towards the surrounding massive lime-
stones. This transition consists of breccias and sandstones, whose lithology varies according to the distance to the
lherzolite. The mm-sized clasts are mainly composed of Al- and Mg-bearing minerals, such as sapphirine, phlo-
gopite (altered to vermiculite), gedrite, kornerupine, metamorphic Al-spinel and enstatite, in a carbonate cement.
Polymineralic clasts have been deposited along with the monomineralic debris, commonly defining a grain-size
sorting. In addition, minor isolated clasts originating from the disaggregation of various rock types are observed
in the sapphirine-bearing sandstones suite. These clasts include: lherzolite, micaschist, quartzite, alkaline gabbro,
meta-ophite, metaevaporite and marble fragments. Sandstones composed of alternating beds of mineral debris de-
riving from these rock-types are also exposed close to the mantle body. All the sources of these detrital elements
can be identified in the Aulus Basin, apart from the protolith of the sapphirine-bearing rocks which remains enig-
matic. Microprobe analyses allowed an estimate of the average composition of this protolith. We found that the
best candidate is a mix of evaporitic clays and dolomite, typical of the Keuper-Rhetian sediments, that evolved
under the HT-LP conditions of the Pyrenean metamorphism. The presence of inclusions with evaporitic affinity
(Cl-apatite and anhydrite) in the sapphirine and kornerupine crystals, revealed by electron microscopy and Raman
analyses, strongly supports this hypothesis. In addition, earlier observations of anhydrite enclosed in metamor-
phic enstatite strengthen this interpretation (Foucard, 1997). Accordingly, we propose that the sapphirine-bearing
rocks and associated sedimentary rocks originated from the transformation of sediments of Keuper-Rhetian age
through cataclasis and metasomatism during the Cretaceous metamorphic event coeval with mantle exhumation.
This complete transformation occurred along the extensional detachment fault which was progressively exposing
the lherzolites to the seafloor. The cataclastic debris were abandoned on the unroofed detachment surface and
rapidly reworked through sedimentary processes.