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NEW EVIDENCE FOR SUPRA-SUBDUCTION ZONE OPHIOLITES IN THE VARDAR ZONE FROM THE VERMION MASSIF (NORTHERN GREECE): IMPLICATION FOR THE TECTONO-MAGMATIC EVOLUTION OF THE VARDAR OCEANIC BASIN
No full textThe Vardar zone represents a main Tethyan suture, which testifies for the existence of a Mesozoic oceanic basin between the Pelagonian and the Serbo-Macedonian continental realms. The Vardar zone in the Greek sector is subdivided into three sub-zones: the Almopias (including the western, central, and eastern Almopias ophiolites), the Paikon (continental volcanic arc), and the Guevgueli (ensialic backarc basin).
The Vermion massif consists of three tectonic units belonging to the Pelagonian domain, which are tectonically overlain by three tectonic units associated with the Almopias sub-zone. All these units include ophiolitic rocks (mantle harzburgites and ophiolitic mélanges) referred to the western and central Almopias ophiolites.
Mantle harzburgites have a very depleted nature and represent portion of the supra-subduction (SSZ) mantle developed in an intra-oceanic arc setting. The Pelagonian mélange formations display intense ductile and pervasive metamorphism and mainly include materials derived from the Pelagonian margin (calc-alkaline rocks and Triassic marbles) and materials derived from the Vardar oceanic basin (MORBs and alkaline within-plate rocks), as well as minor material derived from the intra-oceanic arc setting (very low-Ti boninites). The Almopias units mainly include intra-oceanic arc materials, such as low-Ti island arc tholeiitic and boninitic rocks associated with minor MORBs.
The new evidence from the Vermion massif compared with literature data on other magmatic rocks within the Vardar zone suggests a much more complex scenario than previously depicted for the tectono-magmatic evolution of the Vardar ocean and provides new constraints for its possible evolution as follows.
(1) In the Late Permian-Early Triassic the rifting of the continental lithosphere between the future Pelagonian and Serbo-Macedonian continental margins was associated with calc-alkaline and alkaline within-plate (WPB) magmatic events. (2) From the Mid Triassic, the continuous extension between the Pelagonian and Serbo-Macedonian continental margins led to the generation of the oceanic crust of the Vardar ocean in which E- and N-MORB, as well as alkaline WPB (seamounts) were erupted. (3) In the Early-Mid Jurassic, the Vardar oceanic basin was affected by convergence with development of an intra-oceanic arc characterized by tectonic extension trigged by slab-rollback. During this phase typical SSZ ophiolites, such as low-Ti IAT and very low-Ti boninitic rocks, as well as very depleted mantle harzburgites were produced. (4) The Mid-Late Jurassic closure of the Vardar ocean was characterized by the formation of the mélange successions of the Vermion ophiolites, which is associated with the emplacement of the ophiolitic sequences onto the Pelagonian continental margin. (5) Meanwhile, the extension tectonics moved eastward to the backarc in consequence of the continuing slab-rollback. This led to the Late Jurassic formation of a new spreading centre (Almopias ocean) with generation of MORB crust now represented by the Eastern Almopias ophiolites. Afterwards, the plate margin moved eastward in consequence of the continuing convergence between the Adria and Eurasia plates and a new subduction started in the eastern part of the Almopias ocean beneath the Serbo-Macedonian continent leading to the formation of the Late Jurassic (-Early Cretaceous?) Paikon volcanic arc and Guevgueli backarc basin. (6) The final emplacement of the Pelagonian, Western-,Central-, and Eastern- Almopias ophiolites, Paikon arc, and Guevgueli complex occurred from the Early Cretaceous to Tertiary
Petrogenesis and tectono-magmatic significance of volcanic and subvolcanic rocks in the Albanide-Hellenide ophiolitic mélanges
No full textOphiolitic mélanges associated with ophiolitic sequences are widespread in the Mirdita-Subpelagonian zone (Albanide-Hellenide orogenic belt) and consist of tectono-sedimentary “block in matrix-type” mélanges. Volcanic and subvolcanic basaltic rocks included in the main mélange units are studied in this paper with the aim of assessing their chemistry and petrogenesis, as well as their original tectonic setting of formation.
Basaltic rocks incorporated in these mélanges include: (1) Triassic transitional to alkaline within-plate basalts (WPBs); (2) Triassic normal (N-MORB) and enriched (E-MORB) mid-ocean ridge basalts; (3) Jurassic N-MORBs; (4) Jurassic basalts with geochemical characteristics intermediate between MORB and island arc tholeiites (MORB/IAT); (5) Jurassic boninitic rocks. These rocks record different igneous activities, which are related to the geodynamic and mantle evolution through time in the Mirdita-Subpelagonian sector of the Tethys.
Mélange units formed mainly through sedimentary processes are characterised by the prevalence of materials derived from the SSZ environments, whereas in mélange units where tectonic processes prevail, the oceanic materials predominate. By contrast, no compositional distinction between structurally similar mélange units is observed, suggesting that they may be regarded as a unique mélange belt extending from the Hellenides to the Albanides, whose formation was largely dominated by the mechanisms of incorporation of the different materials.
Most of the basaltic rocks surfacing in the MOR and SSZ Albanide-Hellenide ophiolites are incorporated in mélanges. However, basalts with island arc tholeiitic affinity, although they are volumetrically the most abundant ophiolitic rock types, have not been found in mélanges so far. This implies that the rocks forming the main part of the intra-oceanic arc do not seem to have contributed to the mélange formation, whereas rocks presumably formed in the forearc region are largely represented in sedimentary-dominated mélanges. In addition, Triassic E-MORBs, N-MORBs, and WPBs included in many mélanges are not presently found in the ophiolitic sequences. Nonetheless, they testify the existence throughout the Albanide-Hellenide belt of an oceanic basin since the Middle Triassic
Tectono-magmatic significance of volcanic rocks from the Albanide-Hellenide ophiolitic mélanges
No full textOphiolites of the Albanide-Hellenide Subpelagonian Zone are widely associated with mélange units, which consist of polygenetic thrust sheets formed by various combination of tectonic and sedimentary processes operating at subduction zones during tectonic emplacement of ophiolites. Subpelagonian mélanges incorporate tectonically eroded and/or fragmented blocks of the subducting oceanic plate, as well as materials derived from the forearc region in the upper plate plate. Composition and petrology of volcanics included in these mélanges record the magmatic activities developed from the early stages of generation of oceanic lithosphere (including seamounts) up to its consumption in a converging margin setting and also provide information on the mélange formation and evolution. The Rubik Complex (Albanides) and the Avdella Mélange (Pindos, Hellenides) are typical tectono-sedimentary mélanges, which include Triassic-Jurassic basalts with both mid-ocean ridge (MORB) and ocean island (OIB) affinities. This suggests that they formed by incorporation of oceanic materials from the subducting plate. The tectono-sedimentary Koziakas Mélange (Hellenides) consists of a variety of different volcanic rocks: (1) transitional to alkaline basalts, trachyandesites, and trachytes; (2) both normal (N-) and transitional (T-) MORBs; (3) boninitic volcanics. No datings are available for these litho-types, although blocks of associated radiolarian chert indicate both Triassic and Jurassic ages. The Agoriani Mélange (Othrys, Hellenides), originated mainly through sedimentary processes, includes N-MORBs, alkaline OIBs, boninitic clasts and blocks, and MORB/IAT intermediate basalts. The mélange from the Argolis Peninsula (Hellenides) can be subdivided into four different mappable units. In the Lower and Upper Units sedimentary processes prevail with volcanics largely predominated by boninitic rocks, whereas N- and T-MORBs and alkaline OIBs are rare. By contrast, tectonic processes mainly characterize the Middle and Wedged Units. Volcanics in the Middle Unit are alkaline OIBs and Triassic and Jurassic N- and T-MORBs, whereas the Wedged Unit is exclusively composed of OIBs, thus representing a seamount fragment. In the Rubik and Koziakas Mélanges some boninitic dykes crosscut different tectonic slices, referred to both MORB and OIB sequences. This implies that tectonic incorporation of MORB-type and OIB-type materials occurred prior to the development of boninitic magmatism
Mid-ocean ridge and supra-subduction affinities in the Pindos Massif ophiolites (Greece): Implications for magma genesis in a proto-forearc setting
No full textThe Pindos ophiolitic massif is considered an important key area within the Albanide-Hellenide ophiolitic belt and is represented by two tectonically distinct ophiolitic units: 1) a lower unit, including an intrusive and a volcanic section; and 2) an upper ophiolitic unit, mainly including mantle harzburgites. Both units share similar metamorphic soles and tectono-sedimentary mélanges at their bases.
The intrusive section of the lower unit is composed by an alternation of troctolites with various ultramafic rock-types, including dunites, lherzolites, olivine-websterites, olivine-gabbros, anorthositic gabbros, gabbros and rare gabbronorites.
The volcanic and subvolcanic sequence of the lower unit can geochemically be subdivided into three groups of rocks: 1) basalts and basaltic andesites of the lower pillow section showing a clear high-Ti affinity; 2) basaltic andesites of the upper pillow section with high-Ti affinity, but showing many geochemical differences with respect to the first group; 3) very low-Ti (boninitic) basaltic and basaltic andesitic lava flows separating the lower and upper pillow sections, and dykes widespread throughout the Pindos ophiolites.
These different magmatic groups originated from fractional crystallization from different primary magmas, which were generated, in turn, from partial melting of mantle sources progressively depleted by previous melt extractions. Group 1 volcanics may have derived from partial melting (ca. 20%) of an undepleted lherzolitic source, while group 2 basaltic rocks may have derived from partial melting (ca. 10%) of a mantle that had previously experienced MORB extraction. Finally, the Group 3 boninites may have derived from partial melting (ca 12-17%) of a mantle peridotite previously depleted by primary melt extraction of Group 1 and 2 primary melts.
In order to explain the coexistence of these geochemically different magma groups, two petrogenetic models formerly proposed for the Albanian ophiolites are discussed
GEOCHEMISTRY AND TECTONO-MAGMATIC SIGNIFICANCE OF HP/LT METAOPHIOLITES OF THE ATTIC-CYCLADIC ZONE IN THE LAVRION AREA (ATTICA, GREECE)
No full textThe Lavrion area correspond to the northwestern end of the Attic-Cycladic Complex and mainly consists of metamorphic rocks formed during the Eocene high-pressure/low-temperature (HP/LT) and the Late Oligocene-Early Miocene medium-pressure metamorphic events. These metamorphic rocks are found in two superimposed tectonic units that are: the Kamariza unit, which includes metavolcanic rocks and the overlying Lavrion blueschist unit, which is largely represented by metaophiolites.
Protoliths of metavolcanic rocks in the Kamariza unit are calc-alkaline basalts displaying marked enrichment in Th, U, and LREE and depletion in Ta, Nb, Hf and Ti, which account for a genesis from a depleted mantle source further enriched by subduction components. The Lavrion blueschist unit mainly includes metavolcanic rocks with tholeiitic composition, as well as subordinate metagranites and metavolcanic rocks with calc-alkaline affinity. The tholeiitic metavolcanic rocks are mainly represented by enriched-type (E-) mid-ocean ridge basalts (MORB) and subordinately by normal-type (N-) MORB. E-MORB chemistry implies a genesis from a depleted asthenospheric source modified by an OIB component, whereas N-MORB has chemical features, which are typical for rocks generated in a mid-ocean ridge setting from a primitive asthenospheric source.
Previous works suggested that the magmatic protoliths of similar HP/LT metamorphic rocks from elsewhere in the Cyclades reflect the occurrence of an arc to back-arc tectonic setting, which developed during the Cretaceous closure of the Pindos oceanic basin. However, recent geological studies have shown that the Lavrion metamorphic units, unlike similar units from the Cycladic zone, represent Triassic Pelagonian sequences metamorphosed under the HP/LT conditions typical of the Cycladic zone. The geochemical and petrological characteristics of the Lavrion metamorphic rocks support this conclusion. In particular, calc-alkaline protoliths display many similarities with the Triassic calc-alkaline rocks associated with the rift of the Gondwana, whereas MORB-type protoliths are similar to the Triassic MORB found in the Subpelagonian ophiolitic mélanges. The magmatic protoliths of the Lavrion HP/LT metamorphic rocks are thus compatible with a paleotectonic evolution, which encompasses the Triassic continental rift followed by the early oceanization stage of the Pindos ocean and were emplaced on the border between the Pelagonian continental margin and the Pindos basin. These rocks were probably incorporated into mélanges during the Jurassic closure of the Pindos basin, and finally involved in the Eocene and Late Oligocene-Early Miocene metamorphic events that affected the Cycladic zone
Tectono-magmatic significance of Triassic MORBs from the Argolis Peninsula (Greece): Implication for the origin of the Pindo ocean
No full textThe Triassic age for the beginning of spreading of the Neo-Tethyan-Pindos Ocean, although proposed by some authors, is poorly constrained because it is generally based on limited data on Triassic radiolarites, which are tentatively associated with volcanic rocks represented by basaltic sequences mainly showing within-plate (alkaline) affinity or, subordinately, ranging from transitional WPB to transitional MORB compositions. The Middle Unit of the central-northern Argolis (eastern Peloponnesus, Greece) consists of a composite tectonic association of various types of thrust sheets, some of which include coherent sequences of basalts topped by radiolarian cherts previously attributed to the Middle-Late Jurassic (Baumgartner, 1985). However, recent biostratigraphic data (Bortolotti et al., 2001) indicate that several thrust sheets are Middle-Late Triassic in age. The Jurassic volcanics are represented by MOR basalts. By contrast, the nature of the Triassic basalts is still unknown, and ma..
Petrogenesis and tectonic significance of IAT magma-types in the Hellenide ophiolites as deduced from the Rhodiani ophiolites (Pelagonian zone, Greece)
No full textThe Rhodiani ophiolites are represented by two tectonically superimposed ophiolitic units: the “lower” Ultramafic unit and the “upper”
Volcanic unit, both bearing calcareous sedimentary covers. The Ultramafic unit consists of mantle harzburgites with dunite pods and chromitite
ores, and represents the typical mantle section of supra-subduction zone (SSZ) settings. The Volcanic unit is represented by a sheeted dyke
complex overlain by a pillow and massive lava sequence, both including basalts, basaltic andesites, andesites, and dacites. Chemically, the
Volcanic unit displays low-Ti affinity typical of island arc tholeiite (IAT) ophiolitic series from SSZ settings, having, as most distinctive chemical
features, low Ti/V ratios (b20) and depletion in high field strength elements and light rare earth elements.
The rare earth element and incompatible element composition of the more primitive basaltic andesites from the Rhodiani ophiolites can be
successfully reproduced with about 15% non-modal fractional melting of depleted lherzolites, which are very common in the Hellenide ophiolites.
The calculated residua correspond to the depleted harzburgites found in the Rhodiani and Othrys ophiolites. Both field and chemical evidence
suggest that the whole sequence of the Rhodiani Volcanic unit (from basalt to dacite) originated by low-pressure fractional crystallization under
partially open-system conditions. The modelling of mantle source, melt generation, and mantle residua carried out in this paper provides new
constraints for the tectono-magmatic evolution of the Mirdita–Pindos oceanic basin
Triassic mid-ocean ridge basalts from the Argolis Peninsula (Greece) tectono-magmatic implications for the origin of the Pindos ocean,
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Petrogenesis and tectonic setting of volcanic rocks from the Subpelagonian ophiolitic mélange in the Agoriani area (Othrys, Greece)
No full textIn the northwestern end of the Othrys Massif, the lowermost unit of the Othrys ophiolitic complex is represented by the Agoriani Mélange, which is a typical tectono-sedimentary mélange developed at a convergent margin, and comprises ophiolite-derived turbidites and debris flow deposits including, in turn, various oceanic-crust lithotypes. The volcanic rocks recorded in the Agoriani Mélange are represented by chemically distinct rock groups.
Group 1 is characterized by MOR-type basalt and basaltic andesite, showing high-Ti geochemical affinity, and flat HFSE patterns, as well as mild LREE depletion (LaN/SmN = 0.48-0.69) coupled with an overall enrichment for HREE. The chemistry of these rocks is compatible with about 20% partial melting of an undepleted MORB-type mantle source.
Group 2 is represented by basalts whose geochemical characteristics are intermediate between typical low-Ti island arc tholeiites and pure high-Ti MORBs. Nonetheless, the strong depletion of Th, Nb, and LREE (LaN/SmN = 0.02-0.20) and the mild depletion of HFSE are consistent with the compositions of magmas generated in supra-subduction zone settings from partial melting of refractory mantle sources. In particular, these rocks are compatible with about 10% partial melting from a mantle source that had experienced about 20% previous MORB melt extraction.
Group 3 includes basaltic andesites and andesites showing chemical features typical of very low-Ti (boninitic) rocks: that is, strong depletion of HFSE and depleted, U-shaped REE patterns. The chemical features displayed by Group 3 rocks are compatible with 10 - 20% partial melting of mantle sources (enriched in LILE and LREE by subduction-derived fluids), representing the residua after Group 2 primary melt extraction.
One sample is represented by alkaline basalt, as testified by the incompatible elemental ratios, as well as the marked LILE and LREE enrichments. The overall chemical features are comparable to those of typical ocean island basalts (OIBs), and are consistent with ca. 5% partial melting of a theoretical plume source.
According to the regional reconstruction of the Neo-Tethys, the volcanic lithologies included in the Agoriani Mélange are consistent with the magmatic activities that occurred in the Pindos oceanic basin from the Permian-Triassic rifting stage and Triassic-Jurassic oceanization (including seamounts) to the Middle-Late Jurassic intra-oceanic subduction.
Group 2 basalts, in particular, correspond to basalts with peculiar chemistry sporadically found in both north and south Albania. The genesis and tectono magmatic significance of these basalts is discussed in this paper on the bases of three possible models
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