First
approximation to the petrography and mineral chemistry of the Maladeta
and Andorra-Mont-Louis plutons (SIMPROP Project).
Introduction: what is SIMPROP?
Simprop is a coordinated project with the main goal of studying the relationship between
the plutonic and volcanic processes of siliceous magmas as part of the same
magmatic system, with special emphasis on their temporal, structural,
petrologic and geodynamic relationships, based on the multidisciplinary study
of the Permo-Carboniferous magmatism of the Catalan Pyrenees. SIMPROP is
divided in two sub-projects: one
will be carried out by CSIC researchers that will
deal with the study of the magmatic products. The other will address the
characterization of structure from a geophysical perspective and will be
carried out by IGME.
Geological setting
The
Maladeta and the Andorra-Montlouis plutons constitute two of the main variscan
granitic bodies outcropping in the Pyrenean Axial Zone (Fig.1). These bodies
were intruded mainly during the last stage of the variscan orogeny (Late
Carboniferous-Early Permian) between 310 Ma and 290 Ma (between
310 Ma and 290 Ma (Lopez-Sanchez et al., 2018; Esteban et al., 2015; Denèle et
al., 2014; Druguet et
al.,
2014; Maurel et
al.,
2004; Evans et al., 1998; Romer and Soler, 1995) and uplifted
afterwards during the Alpine orogeny.
Figure
1. Geological
map of the area. a) situation of the different variscan
bodies outcropping in the pyrenees.
Modified from french Geoportail
(https://www. geoportail. gouv.fr/), Institut
Cartogràfic
i
Geològic
de Catalunya
(ICGC) and Porquet
et al., 2017. b) detailed map of the two studied bodies with location of the samples studied until date.
Modified from Pereira et al., 2014.
Petrography
The first
remarkable difference between the volcanic sample studied
(S-1) and the granitic ones (S-2,
S-21, S-40, S-49, S-50, S-55, S-58) is the absence/presence of hornblende (Fig.
2). Whereas in volcanic rocks hornblende forms phenocrysts up to 1 mm, this mineral has not been observed until date in plutonic rocks. Some references of this mineral in the granites are found in the bibliography (Maurel et al., 2004), however in low concentrations.
Figure
2:
comparison between a volcanic (a, b) and a plutonic sample (c, d) from two
different thin sections of the Andorra – Mont-Louis pluton in transmitted
microscopy in parallel light (a,c) and polarized light (b,d).
Plag:
plagioclase, Kf: K-feldspar, Qtz:
quartz, Hbl:
hornblende, Bt: biotite.
Plutonic
samples present an equigranular
texture with quartz, plagioclase, K-feldspar and biotite as main minerals (Fig.
3a-g). Muscovite
has
only been found until date in some samples from the Maladeta
pluton (Fig.
3h).
Figure 3:
different features of the granitic samples under transmitted microscopy in
polarized light. a-d) plagioclase with polysynthetic twining, zoned and often
altered to sericite. K-feldspar presents perthites
(e,f) and can be observed interstitially filling fractures along quartz crystals (g) . h) muscovite in
the Maladeta
sample i)
prehnite with some reaction and dissolved borders appears cutting biotite.
Biotite often presents zircon and apatite inclusions (Fig. 4). Its remarkable to observe the abundance of ilmenite inclusions in the inner part of the Andorra- Mont-Louis pluton (Fig. 4b,c) whereas its presence is rare and small in the external part of the pluton and in the Maladeta massif (Fig. 4a).
Figure 4:
different inclusions in biotite grains under EDS. It is remarkable the
difference of ilmenite inclusions between the center and the rim of the Andorra
– Mont-Louis massif. Ap: apatite, ilm:
ilmenite, Zr:
zircon.
Mineral geochemistry
Regarding
the mineral chemistry, plagioclase ranges from albite to labradorite
being oligoclase and andesite the
most frequent composition (Fig. 5). No relevant difference is
observed between the different granitic facies and between the two different plutons.
Figure
5:
composition of different feldspar grains from three different samples analyzed
using microprobe.
The
Maladeta and the Andorra-Montlouis plutons constitute two of the main variscan
granitic bodies outcropping in the Pyrenean Axial Zone (Fig.1). These bodies
were intruded mainly during the last stage of the variscan orogeny (Late
Carboniferous-Early Permian) between 310 Ma and 290 Ma (between
310 Ma and 290 Ma (Lopez-Sanchez et al., 2018; Esteban et al., 2015; Denèle et
al., 2014; Druguet et
al.,
2014; Maurel et
al.,
2004; Evans et al., 1998; Romer and Soler, 1995) and uplifted
afterwards during the Alpine orogeny.
Figure
1. Geological
map of the area. a) situation of the different variscan
bodies outcropping in the pyrenees.
Modified from french Geoportail
(https://www. geoportail. gouv.fr/), Institut
Cartogràfic
i
Geològic
de Catalunya
(ICGC) and Porquet
et al., 2017. b) detailed map of the two studied bodies with location of the samples studied until date.
Modified from Pereira et al., 2014.
Petrography
The first
remarkable difference between the volcanic sample studied
(S-1) and the granitic ones (S-2,
S-21, S-40, S-49, S-50, S-55, S-58) is the absence/presence of hornblende (Fig.
2). Whereas in volcanic rocks hornblende forms phenocrysts up to 1 mm, this mineral has not been observed until date in plutonic rocks. Some references of this mineral in the granites are found in the bibliography (Maurel et al., 2004), however in low concentrations.
Figure
2:
comparison between a volcanic (a, b) and a plutonic sample (c, d) from two
different thin sections of the Andorra – Mont-Louis pluton in transmitted
microscopy in parallel light (a,c) and polarized light (b,d).
Plag:
plagioclase, Kf: K-feldspar, Qtz:
quartz, Hbl:
hornblende, Bt: biotite.
Plutonic
samples present an equigranular
texture with quartz, plagioclase, K-feldspar and biotite as main minerals (Fig.
3a-g). Muscovite
has
only been found until date in some samples from the Maladeta
pluton (Fig.
3h).
Figure 3:
different features of the granitic samples under transmitted microscopy in
polarized light. a-d) plagioclase with polysynthetic twining, zoned and often
altered to sericite. K-feldspar presents perthites
(e,f) and can be observed interstitially filling fractures along quartz crystals (g) . h) muscovite in
the Maladeta
sample i)
prehnite with some reaction and dissolved borders appears cutting biotite.
Biotite often presents zircon and apatite inclusions (Fig. 4). Its remarkable to observe the abundance of ilmenite inclusions in the inner part of the Andorra- Mont-Louis pluton (Fig. 4b,c) whereas its presence is rare and small in the external part of the pluton and in the Maladeta massif (Fig. 4a).
Figure 4:
different inclusions in biotite grains under EDS. It is remarkable the
difference of ilmenite inclusions between the center and the rim of the Andorra
– Mont-Louis massif. Ap: apatite, ilm:
ilmenite, Zr:
zircon.
Mineral geochemistry
Regarding
the mineral chemistry, plagioclase ranges from albite to labradorite
being oligoclase and andesite the
most frequent composition (Fig. 5). No relevant difference is
observed between the different granitic facies and between the two different plutons.
Figure
5:
composition of different feldspar grains from three different samples analyzed
using microprobe.
Future work
• Determine
the connection
between the volcanic and the plutonic products (petrography, mineral and major
geochemistry).
• Crustal
or mantelic
source?
Determine the genesis of magmas:
mineralogy (cordierite and muscovite vs. hornblende and biotite), mineral
geochemistry, Nd/Sr
isotopes.
• Emplacement
mechanisms of
the magma: one magmatic pulse vs. different pulses? (diffusive modelling,
geophysical methods: magnetotellurics,
magnetic susceptibility, AMS,
density
and gravimetric studies .
• Dating the
different events and gaps à Schamuells, S.
• Crystallization
sequence of the granites and dyke emplacement.
When did they intrude?
References
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Aknowledgments
This research is supported financiall by SIMPROP- CGL2017-84901-C2-1-P. Thanks to Joan Marti (principal ID and PhD supervisor