Low-P metamorphism following a Barrovian-type evolution. Complex tectonic controls for a common transition, as deduced in the Mondoñedo thrust sheet (NW Iberian Massif)
Introduction
Thermal models of orogenic regions characterized by crustal thickening have shown that their thermal evolution follows rather systematic patterns. For intermediate values of the more common parameters intervening in the thermal history, mesocrustal domains undergo a medium-pressure metamorphic evolution (England and Thompson, 1984). In fact, this Barrovian-type evolution has been considered an indicative for collision tectonics (Thompson and England, 1984). However, for most middle and lower crustal levels, the medium-P metamorphic history follows from a previous one of high-P roughly synchronous with the maximum burial. Nevertheless, because of the inferred clockwise shape of the P–T–t paths developed in the medium to higher P parts of the belt, evidence for the early maximum P segment of the P–T–t path is usually obliterated by the subsequent recrystallization at peak T, specially when the P–T path reach a high-T thermal peak.
Numerical experiments show that most of the P–T paths inside a thickened crust tend to approach the low-P metamorphic field after the thermal peak and during strong decompression. When exhumation is only due to erosion, England and Thompson (1984) have suggested that the low-P field is not reached by the common P–T paths. But, as a late event of low-P is the norm rather than the exception in collisional orogenic belts, one has to conclude that erosion is not the only process responsible for the low-P metamorphic gradients. Abundant magmatism may help the trajectories to enter the low-P domain of the P–T space, but late-orogenic extension has also been claimed to explain broad regions with low-P metamorphism England, 1987, Thompson and Ridley, 1987. Many regional contributions have described the quick exhumation of deep crustal sectors in the footwall of large extensional detachments Sandiford, 1989, Platt, 1993, Ruppel, 1995. These sectors appear nowadays as dome-shaped, low-P plutono-metamorphic complexes Dewey, 1988, Reinhardt and Kleeman, 1994, Escuder Viruete et al., 1994, Escuder Viruete et al., 2000. Both plutono-metamorphism and extension are the mechanisms most commonly used to explain the low-P metamorphism in collisional belts.
This contribution presents a study carried out in the Mondoñedo thrust sheet and its authochthon, in the NW of the Iberian Massif (Fig. 1), which forms part of the Variscan orogenic belt. Our results indicate that regional low-P metamorphism develop in sectors of collisional orogenic wedges that register a rich and complex structural evolution. The low-P event occurred there both in the hangingwall and the footwall of a huge contractional structure. In the thrust sheet itself, the low-P metamorphism took place after a medium-P event clearly related to crustal thickening, but in the footwall, the low-P metamorphism is the only significant event of recrystallization. Moreover, the complex structural and metamorphic relationships established between the upper and lower blocks of the Mondoñedo thrust fault give insights into the evolution of the Variscan orogenic wedge with time. Histories like the one described here may be relatively common in orogenic belts, though could they have been remained unnoticed in many cases.
Section snippets
Geological setting
A first deformation episode of recumbent folding with E vergence characterizes the Variscan orogeny in the internal parts of the NW Iberian Massif. This was followed by ductile and brittle thrusting toward the E, and subsequent open steep folding (Fig. 1).
The Mondoñedo nappe is a large crystalline thrust sheet, formed essentially by low to high-grade metasediments ranging in age from Upper Proterozoic to Lower Devonian. Metapelites are the most common lithologies, followed by quartzites and,
Synmetamorphic shear zones
The first structures developed in the study area were large recumbent folds. These folds reflect an episode of crustal shortening and thickening, with the deformation widely distributed and developing a regional cleavage affecting all the metasediments. The associated regional metamorphism was of intermediate pressure (Miyashiro, 1961) with kyanite–sillimanite, so common in mesocrustal levels of many orogenic belts (Thompson and England, 1984). The metamorphic zonation, of Barrovian type,
Metamorphic zones and isograds
The regional distribution of metamorphic zones is shown in Fig. 4, Fig. 5. They have been traced through the study of mineral parageneses in significant geological sections. A fundamental conclusion is the difference in the metamorphic zonation between the thrust sheet and its relative autochthon. In the former, the evolution is one of medium-P, essentially Barrovian, but with a continuation towards temperatures somewhat higher than those registered in the type area of Scotland (Yardley, 1989).
P–T paths
One of the main tools to analyse the metamorphic evolution of a complex area are the P–T paths followed by the most significant lithologies. Their geometry has a marked dependence of the dynamic history of each orogenic sector England and Thompson, 1984, Thompson and England, 1984, Davy and Gillet, 1986, England, 1987, Thompson and Ridley, 1987. After a careful petrographic analysis, two techniques can be used to establish the P–T paths. One is the use of geothermometers and geobarometers
Interpretation of the P–T paths
A synthesis of the P–T evolution of the study area is shown in Fig. 9. Paths 1, 2 and 3 correspond to the upper, middle and lower parts of the Mondoñedo thrust sheet and paths 4 and 5 to its relative autochthon, above and below the lower extensional detachment respectively. Fig. 3 depicts the structural location of the P–T paths shown in Fig. 9.
Metamorphism and the structural evolution
The relationships of metamorphism with the structural evolution are sketched in Fig. 10. The successive structural stages are deduced from overprinting criteria among macrostructures, fabrics and mineral assemblages. The tectonic event that formed the large recumbent folds provoked the most important crustal thickening undergone by the Mondoñedo nappe and induced the medium-P metamorphism. The relative delay between the increase of pressure, which is instantaneous, and the increase of
Discussion
The metamorphic history of orogenic domains depends of the dynamic evolution of each particular region. However, an extended generalist interpretation tends to ascribe every type of metamorphism to a general kind of dynamic evolution. So, early collisional events are commonly linked to high- and, specially, medium-P metamorphic histories. The late stages of orogenic activity, often marked by crustal extension and abundant magmatism, are related to wide recrystallization in the low-P domain. In
Acknowledgments
This work forms part of IGCP Project No. 453, Uniformitarism revisited: a comparison between modern and ancient orogens. Toby Rivers and Cecilio Quesada are thanked for constructive and insightful reviews.
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