Gabbro, plagiogranite and associated dykes in the supra-subduction zone Evros Ophiolites, NE Greece

Abstract

-The incomplete Evros ophiolites in NE Greece form a NE−SW-oriented discontinuous belt in the Alpine orogen of the north Aegean. Field data, petrology and geochemistry are presented here for the intrusive section and associated mafic dykes of these ophiolites. Bodies of high-level isotropic gabbro and plagiogranite in the ophiolite suite are cross-cut by NE-SW-trending boninitic and tholeiitic-boninitic affinity dykes, respectively. The dykes fill tensile fractures or faults, which implies dyke emplacement in an extensional tectonic regime. The tholeiitic-transitional boninitic gabbro is REE-and HFS-depleted relative to N-MORB, indicating derivation from melting of a refractory mantle peridotite source. Associated boninitic dykes are slightly LREE-enriched, showing mineral and wholerock geochemistry similar to the gabbro. The plagiogranite is a strongly REE-enriched high-silica trondhjemite, with textures and composition typical for an oceanic crust differentiate. Plagiogranitehosted tholeiitic and transitional boninitic dykes are variably REE-enriched. Geochemical modelling indicates origin of the plagiogranite by up to 75 % fractional crystallization of basaltic magma similar to that producing the associated tholeiitic dykes. All mafic rocks have high LILE/HFSE ratios and negative Ta-Nb-Ti and Ce anomalies, typical for subduction zone-related settings. The mafic rocks show a similar trace-element character to the mafic lavas of an extrusive section in Bulgaria, suggesting they both form genetically related intrusive and extrusive suites of the Evros ophiolites. The field occurrence, the structural context, the petrology and geochemical signature of the studied magmatic assemblage provide evidence for its origin in a proto-arc (fore-arc) tectonic setting, thus tracing the early stages of the tectono-magmatic evolution of Jurassic arc-marginal basin system that has generated the supra-subduction type Evros ophiolites.

A Geochemical Study of Crustal Plutonic Rocks from the Southern Mariana Trench Forearc: Relationship to Volcanic Rocks Erupted during Subduction Initiation

“…The isotropic gabbros are older than the tholeiitic and boninite dikes, which appear to have formed in faults and fractures, suggesting the dikes formed in an extensional environment. Interestingly, the micrographs of the plagiogranites published by Bonev and Stampfli [2009] are remarkably similar to the tonalites of RD63 in the southern Mariana forearc, in particular the quartz-feldspar ratio and the granophyric texture. This similarity indicates that the plutonic rocks possibly underwent similar processes of differentiation at some point during their formation, regardless of their source and parental magma composition.…”

“…It is also inferred that the compositions become more IAT-and boninite-like trending toward the ancient trench [Bonev and Stampfli, 2009;Dilek and Furnes, 2009]. The Kizildag (Turkey), Troodos (Cyprus) and the Semail ophiolite (Oman) are slightly younger (early to late Cretaceous) but exhibit the same stratigraphic trend in lava compositions: MORB-IAT and boninite lavas.…”

“…The consensus is that these ophiolites formed in the Jurassic to Late Cretaceous and were later obducted and underwent various degrees of metamorphism. Gabbros show boninite affinity, such as REE and HFSE depletions relative to N-MORB suggesting their parental magma formed by melting of a refractory peridotite mantle source overlying the sinking slab [Bonev and Stampfli, 2009]. The plagiogranites are more REE enriched (Figure 6.6), and REE modeling done by Bonev and Stampfli [2009], using Rayleigh fractional crystallization and batch melting, suggested these high-silica rocks formed by at least 75% fractional crystallization of a basaltic source that is similar to tholeiitic dikes.…”

“…Gabbros show boninite affinity, such as REE and HFSE depletions relative to N-MORB suggesting their parental magma formed by melting of a refractory peridotite mantle source overlying the sinking slab [Bonev and Stampfli, 2009]. The plagiogranites are more REE enriched (Figure 6.6), and REE modeling done by Bonev and Stampfli [2009], using Rayleigh fractional crystallization and batch melting, suggested these high-silica rocks formed by at least 75% fractional crystallization of a basaltic source that is similar to tholeiitic dikes. The isotropic gabbros are older than the tholeiitic and boninite dikes, which appear to have formed in faults and fractures, suggesting the dikes formed in an extensional environment.…”

“…Sediment compositions for the Pacific Plate in the vicinity of the IBM arc are represented by average volcaniclastic sediments (V) and clay (CL) from Pearce et al [1999] and Elliott et al [1997]. NHRL in b) and d) is the Northern Hemisphere Reference Line from Hart [1984], and the Indian-Pacific boundary in c) is from Pearce et al [1999] [Bonev and Stampfli, 2009], Troodos [Dilek and Furnes, 2009;Flower and Levine, 1987], Pindos [Saccani and Photiades, 2004], Semail (Oman) [Alabaster et al, 1982;Ishikawa et al, 2002;Rollinson, 2009], Kizildag [Dilek and Thy, 2009]), Karmoy ophiolite (Norway) [Pedersen and Hertogen, 1990] and the Papuan ultramafic belt (PUB) [Jaques and Chappell, 1980]. RD63 plutonics are from southern Mariana forearc (this study [Dilek and Thy, 2009;Dilek and Furnes, 2009].…”