Deciphering the metamorphic evolution of the Pulo do Lobo metasedimentary domain (SW Iberian Variscides)
<p>The Pulo do Lobo domain is one of the units exposed within the orogenic suture zone between the Ossa-Morena and the South Portuguese zones in the SW Iberian Variscides. This metasedimentary unit has been classically interpreted as a Rheic subduction-related accretionary prism formed during...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-04-01
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| Series: | Solid Earth |
| Online Access: | https://www.solid-earth.net/11/469/2020/se-11-469-2020.pdf |
| Summary: | <p>The Pulo do Lobo domain is one of the units exposed within the orogenic
suture zone between the Ossa-Morena and the South Portuguese zones in the SW
Iberian Variscides. This metasedimentary unit has been classically
interpreted as a Rheic subduction-related accretionary prism formed during
pre-Carboniferous convergence and eventual collision between the South
Portuguese Zone (part of Avalonia) and the Ossa-Morena Zone (peri-Gondwanan
terrane). Discrete mafic intrusions also occur within the dominant Pulo do
Lobo metapelites, related to an intra-orogenic Mississippian transtensional
and magmatic event that had a significant thermal input. Three different
approaches have been applied to the Devonian–Carboniferous phyllites and
slates of the Pulo do Lobo domain in order to study their poorly known
low-grade metamorphic evolution. X-ray diffraction (XRD) was used to
identify the mineralogy and measure crystallographic parameters (illite
“crystallinity” and K-white mica <span class="inline-formula"><i>b</i></span>-cell dimension). Compositional maps of
selected samples were obtained from electron probe microanalysis, which
allowed for processing with XMapTools software, and chlorite semiempirical and
thermodynamic geothermometry was performed. Thermometry based on Raman
spectroscopy of carbonaceous material (RSCM) was used to obtain peak
temperatures.</p>
<p>The microstructural study shows the existence of two phyllosilicate growth
events in the chlorite zone, the main one (<span class="inline-formula"><i>M</i><sub>1</sub></span>) related to the
development of a Devonian foliation <span class="inline-formula"><i>S</i><sub>1</sub></span> and a minor one (<span class="inline-formula"><i>M</i><sub>2</sub></span>)
associated with a crenulation cleavage (<span class="inline-formula"><i>S</i><sub>2</sub></span>) developed in middle–upper
Carboniferous times. <span class="inline-formula"><i>M</i><sub>1</sub></span> entered well into epizone (greenschist facies)
conditions. <span class="inline-formula"><i>M</i><sub>2</sub></span> conditions were at lower temperature, reaching the
anchizone–epizone boundary. These data accord well with the angular
unconformity that separates the Devonian and Carboniferous formations of the
Pulo do Lobo domain. The varied results obtained by the different approaches
followed, combined with microstructural analysis, provide different
snapshots of the metamorphic history. Thus, RSCM temperatures are higher in
comparison with the other methods applied, which is interpreted to reflect a
faster re-equilibration during the short-lived thermal Mississippian event.
Regarding the metamorphic pressure, the data are very homogeneous: very low
celadonite content (0 %–10 %) in muscovite (and low values of K-white mica
<span class="inline-formula"><i>b</i></span>-cell dimension; 8.995 Å mean value), indicating a low
pressure–temperature gradient, which is unexpected in a subduction-related
accretionary prism.
<br/><br/>
<strong>Highlights</strong>
</p><ul><li>
<p id="d1e286">A multidisciplinary approach has been applied to study the metamorphism of
the Pulo do Lobo metapelites.</p></li><li>
<p id="d1e290">Devonian metamorphism entered epizone conditions.</p></li><li>
<p id="d1e294">Carboniferous metamorphism reached the anchizone–epizone boundary.</p></li><li>
<p id="d1e298">The inferred low-pressure gradient is incompatible with a subduction-related
accretionary prism.</p></li></ul> |
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| ISSN: | 1869-9510 1869-9529 |