HP–LT Variscan metamorphism in the Cubito-Moura schists (Ossa-Morena Zone, southern Iberia)

Abstract

Multi-equilibrium thermobarometry shows that low-grade metapelites (Cubito-Moura schists) from the Ossa-Morena Zone underwent HP-LT metamorphism from 340-370 • C at 1.0-0.9 GPa to 400-450 • C at 0.8-0.7 GPa. These HP-LT equilibriums were reached by parageneses including white K mica, chlorite and chloritoid, which define the earliest schistosity (S 1 ) in these rocks. The main foliation in the schists is a crenulation cleavage (S 2 ), which developed during decompression from 0.8-0.7 to 0.4-0.3 GPa at increasing temperatures from 400-450 • C to 440-465 • C. Fe 3+ in chlorite decreased greatly during prograde metamorphism from molar fractions of 0.4 determined in syn-S 1 chlorites down to 0.1 in syn-S 2 chlorites. These new data add to previous findings of eclogites in the Moura schists indicating that a pile of allochtonous rocks situated next to the Beja-Acebuches oceanic amphibolites underwent HP-LT metamorphism during the Variscan orogeny.

Comment on “Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites” by A. Azor et al.

“…These assertions are not justified for two reasons: first, as discussed in TC5013 PIN AND RODRÍGUEZ: COMMENTARY section 2, their data were most likely not obtained on oceanic rocks, but rather on postcollisional intrusives, and therefore are not directly relevant to the problem of the putative Rheic Ocean in that region; second, the interpretation of the broad OMZ/SPZ boundary zone in terms of an oceanic suture is well documented by the typical NMORBs occurring as tectonic slices south of Aracena [e.g., Castro et al, 1996;Pin et al, 2008], and as blocks in the Peramora Melange of the Pulo do Lobo unit [Eden, 1991;Eden and Andrews, 1990]. Further, the occurrence of HP/LT metamorphism in the southern part of the Ossa-Morena Zone in Portugal provides circumstantial evidence for an oceanic suture in this area [e.g., De Jong et al, 1991;Fonseca et al, 1999;Booth-Rea et al, 2006]. Azor et al [2008, paragraph 22] ''favor the interpretation of the BAA unit as a narrow and very ephemeral realm of oceanic-like crust'' generated in the 330 -340 Ma time span.…”

“…On the basis of these questionable age data, Azor et al [2008, paragraphs 22 and 23] claim that “the BAA unit cannot represent an oceanic realm related to the Rheic Ocean,” that “the Rheic suture is not represented at all in the BAA unit,” and that there are “some doubts on the tectonic meaning of the OMZ/SPZ boundary.” These assertions are not justified for two reasons: first, as discussed in section 2, their data were most likely not obtained on oceanic rocks, but rather on postcollisional intrusives, and therefore are not directly relevant to the problem of the putative Rheic Ocean in that region; second, the interpretation of the broad OMZ/SPZ boundary zone in terms of an oceanic suture is well documented by the typical NMORBs occurring as tectonic slices south of Aracena [e.g., Castro et al , 1996; Pin et al , 2008], and as blocks in the Peramora Melange of the Pulo do Lobo unit [ Eden , 1991; Eden and Andrews , 1990]. Further, the occurrence of HP/LT metamorphism in the southern part of the Ossa‐Morena Zone in Portugal provides circumstantial evidence for an oceanic suture in this area [e.g., De Jong et al , 1991; Fonseca et al , 1999; Booth‐Rea et al , 2006]. The Middle to Late Devonian age of the Peramora Melange [ Eden and Andrews , 1990] and the Late Devonian radiometric ages measured for the HP/LT metamorphism [ Moita et al , 2005] imply that the intervening oceanic lithosphere between the OMZ and SPZ was at least Mid‐Devonian in age.…”

The elusive nature of the Rheic Ocean suture in SW Iberia

“…Five geological units with different tectonometamorphic evolutions can be distinguished at the OMZ‐SPZ boundary. All of these units run parallel to this collisional boundary (Figure ) and are introduced below from north to south: An allochthonous unit with mid‐ocean ridge basalt (MORB)‐featured metamafic rocks, eclogites and high‐pressure metasediments, namely, the Cubito‐Moura unit, thrust onto the southern OMZ [ Fonseca et al ., ; Araujo et al ., ; Booth‐Rea et al ., ; Ponce et al ., ]. The southernmost edge of the para‐autochthonous OMZ: a 2–6 km wide belt of high‐temperature/low‐pressure metamorphic rocks (gneisses, marbles, migmatites, and amphibolites) and gabbros [ Bard , ; Crespo‐Blanc , ; Díaz Azpiroz et al ., ]. The Beja‐Acebuches (BA) unit: a conspicuous strip of metamafic rocks, from greenschists to metagabbros (locally ultramafic rocks), classically considered the suture boundary between the OMZ and the SPZ [e.g., Bard , ; Crespo‐Blanc , ; Quesada et al ., ; Díaz Azpiroz and Fernández , ]. This unit crops out continuously along the boundary, with a maximum outcrop‐width of 3.5 km. The Pulo do Lobo unit: a group of detrital low‐grade metasedimentary formations cropping out extensively south of the BA unit.…”

Rheic Ocean ophiolitic remnants in southern Iberia questioned by SHRIMP U‐Pb zircon ages on the Beja‐Acebuches amphibolites

“…Finally, north of the BAA unit, the Moura‐Cubito Schists (Figures 2b and 2c) intercalate slices of basic rocks, partly with oceanic affinity. These schists show high‐pressure/low‐temperature metamorphism [ Booth‐Rea et al , 2006] with eclogite facies relics in some of the amphibolite intercalations [ Fonseca et al , 1999]. Accordingly, the Moura‐Cubito Schists have been interpreted as an obducted accretionary complex rooted to the south in the suture zone [ Araújo et al , 2005].…”