Magma generation and crustal accretion as evidenced by supra‐subduction ophiolites of the Albanide–Hellenide Subpelagonian zone

Abstract

Ophiolites of the Mirdita–Subpelagonian zone form a nearly continuous belt in the Albanide–Hellenide orogen, including mid‐ocean ridge basalt (MORB) associations in the western Mirdita sector and supra‐subduction zone (SSZ) complexes, with prevalent island arc tholeiitic (IAT) and minor boninitic affinities in the eastern part of the belt (i.e. eastern Mirdita, Pindos, Vourinos). In addition, basalts with geochemical features intermediate between MORB and IAT (MORB/IAT) are found in the central Mirdita and in the Aspropotamos sequence (Pindos). These basalts alternate with pure MORB and are cut by boninitic dykes. The distinctive compositional characteristics of the mafic magmas parental to the different ophiolitic suites can be accounted for by partial melting of mantle sources progressively depleted by melt extractions. Partial melting processes (10–20%) of lherzolitic sources generated pure MORB, leaving clinopyroxene‐poor lherzolite as a residuum. Approximately 10% water‐assisted partial melting of this latter source, in an SSZ setting, may in turn generate basalts with MORB/IAT intermediate characteristics, whereas IAT basalts and boninites may have been derived from 10–20% and 30% partial melting, respectively, of the same source variably enriched by subduction‐derived fluids. In addition, boninites may also have been derived by comparatively lower degrees of hydrated partial melting of more refractory harzburgitic sources. A generalized petrologic model based on mass balance calculations between bulk rock and mineral compositions, indicate that most of the intrusives (from ultramafic cumulates to gabbronorites and plagiogranites), as well as sheeted dykes and volcanics (from basalts to rhyodacites) forming the bulk crustal section of the SSZ ophiolites, may be accounted for by shallow fractional crystallization from low‐Ti picritic parental magmas very similar in composition to IAT picrites from Pacific intraoceanic arcs. The most appropriate tectono‐magmatic model for the generation of the SSZ Tethyan ophiolites implies low velocity plate‐convergence of the intraoceanic subduction and generation of a nascent arc with IAT affinity and progressive slab roll‐back, mantle diapirism and extension from the arc axis to the forearc region, with generation of MORB/IAT intermediate basalts and boninitic magmas.

Origin of mantle peridotites from the Vourinos Ophiolite Complex, Greece, as deduced from Cr-spinel morphological and chemical variations

“…However, this interpretation does not seem to explain sufficiently the formation of the harzburgites in the Voidolakkos district. In particular, the low MREE and HREE contents of the Vourinos harzburgites compared to undepleted mantle and Alpine lherzolites, confirm that they represent mantle residues (e.g., Godard et al 2000;Takazawa et al 2003;Zhou et al 2005) after more than one melting episodes (e.g., Beccaluva et al 2005;Saccani et al 2008; Fig. 5 MREE-HREE depletion during a single melting event is geologically unattainable (Arai 1994a).…”

“…On the basis of petrological data the Vourinos peridotites have traditionally been viewed as refractory residues produced by multi-stage partial melting of an initially undepleted mantle source (e.g., Beccaluva et al 2005;Saccani et al 2008). However, this interpretation does not seem to explain sufficiently the formation of the harzburgites in the Voidolakkos district.…”

massif du Koniambo, Nouvelle-Calédonie :

“…Des études récentes (Robertson et Shallo, 2000 ;Robertson, 2002 ;Beccaluva et al, 2004) (Pearce, 1980 ;Jenner et aï., 1991 ;Robertson, A., 2002 ;Beccaluva et ai, 2004).…”

“…Bien que plusieurs auteurs supportent la théorie voulant que la genèse des complexes ophiolitiques soit reliée exclusivement aux rides océaniques (Nicolas, 1989 ;Nicolas et al, 1994 ;Boudier et Nicolas, 1995 ;Boudier et al, 1996), d'autres favorisent un modèle de genèse plus complexe dans lequel l'environnement supra-subductif est générateur d'une grande proportion de faciès géologiques observés au sein des complexes ophiolitiques (Lippard et al, 1986 ;Searle et Cox, 1999 ;Robertson, 2002;Beccaluva et al, 2004 (Boudier et Nicolas, 1985 ;Godard et al, 2003 ;Einaudi et al, 2003 ;Python et al, 2003 ;Takazawa et al, 2003). La zone MTZ se caractérise par fa présence de dunite, de troctoîite, de gabbro et de wehrlite (Boudier et Nicolas, 1995 ;Nicolas et Prinzhofer, 1983 Corse et dans l'Apennin septentrional (Nicolas et Jackson, 1972 ;Juteau et Maury, 1999 (Juteau et Maury, 1999 ;Griselin, M, 2001).…”

“…La zone MTZ se caractérise par fa présence de dunite, de troctoîite, de gabbro et de wehrlite (Boudier et Nicolas, 1995 ;Nicolas et Prinzhofer, 1983 Corse et dans l'Apennin septentrional (Nicolas et Jackson, 1972 ;Juteau et Maury, 1999 (Juteau et Maury, 1999 ;Griselin, M, 2001). Cette suture marque l'emplacement d'une paléozone de subduction à vergence nord, qui a absorbé plus de 2 000 km de fonds océaniques téthysiens, avant la collision complète de l'Inde avec FEurasie à la fin de î'Éocène (Juteau et Maury, 1999 Plusieurs complexes ophiolitiques de la région de la Méditerranée présentent des composants indubitablement reliés à un environnement supra-subductif (Robertson et Shallo, 2000 ;Robertson, 2002 ;Beccaluva et al, 2004 …”