Phase diagram sections applied to amphibolites: a case study from the Ossa-Morena/Central Iberian Variscan suture (Southwestern Iberian Massif)

• Published in: Lithos Vol. 68; no. 1; pp. 1 - 21
• Main Authors: López Sánchez-Vizcaı́no, V., Gómez-Pugnaire, M.T., Azor, A., Fernández-Soler, J.M.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.05.2003
• Subjects: Eclogite facies; Fluid phase; Metabasites; Pseudosections; Variscan belt; Variscan belt; Eclogite facies; Metabasites; Fluid phase; Pseudosections
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/S0024-4937(03)00017-3

Amphibolites included in the Ossa-Morena/Central Iberian suture of SW Iberia offer the opportunity of investigating the subduction and collision events, followed by exhumation, which affected this major boundary during the Variscan orogeny. This suture contact is marked by a band of highly deformed and metamorphosed rocks, namely, the Central Unit. The main deformation affecting this unit is a ductile shearing that produced an intense planar–linear fabric. This shearing has two components: a dominant left lateral one and a subordinate dip-slip normal one, which can be taken to be responsible for the exhumation of this unit in Devonian–Carboniferous times. The rocks studied in the present paper are garnet-free amphibolites and garnet-bearing amphibolites intercalated between gneisses. They appear as lens-, dike- or elongated-shaped bodies, their thickness ranging between a metre and several hectometres. The investigation of their metamorphic evolution has been carried out by computing phase diagram sections (pseudosections) for specified bulk rock compositions. This method enables us to achieve a twofold goal: (i) to model the P– T path undergone by the rocks during the metamorphism in question, and (ii) to assess the influence of the fluid phase abundance on the formation and the preservation of the mineral assemblages as well as on the density decrease of the rocks during exhumation. The metabasites of the Central Unit underwent a metamorphic evolution, characterized by an initial eclogite facies event with peak pressures around 1.9 GPa and temperatures about 550 °C, during which grossular-rich garnet and jadeite-rich clinopyroxene coexisted with chlorite, glaucophane, and paragonite. The former two hydrous minerals disappeared during the subsequent stage, characterised by a slight decompression with strong heating. At the peak temperature (≈725 °C and 1.6 GPa), important amounts of water were released owing to paragonite breakdown. This water produced (i) the formation of large amounts of amphibole coexisting with jadeite-rich clinopyroxene and sodium-rich plagioclase, and (ii) a sharp density decrease of the rocks, which must have contributed to their exhumation. Fast uplift is consistent with the deduced sharp isothermal decompression (1.18 GPa). This was, in turn, followed by simultaneous strong cooling and decompression (≈455 °C and 0.3 GPa), which produced the calcium enrichment observed in plagioclase and the jadeite decrease and diopside enrichment in clinopyroxene. This final retrograde evolution occurred at fluid absent conditions, thus allowing the preservation of previously formed high-pressure assemblages.