Georgia Pe-Piper

The nature of Triassic extension-related magmatism in Greece: evidence from Nd and Pb isotope geochemistry

The widespread Triassic volcanic rocks of Greece, dismembered during the Hellenide orogeny, are used to interpret the nature of Triassic rifting. Four assemblages of volcanic rocks are distinguished on geochemical criteria: (1) a predominant subalkaline basalt–andesite–dacite series with a high proportion of pyroclastic rocks; (2) minor shoshonites; (3) alkali basalt and (4) MORB. The stratigraphic and palaeogeographic distribution of these rock types is synthesized. New Pb and Nd isotopic data are used to discriminate between hypotheses suggesting that either subduction or extension was responsible for the Triassic volcanism. In the subalkaline basalt assemblage, e Nd is negative with depleted mantle model ages >1.5 Ga. Pb isotopic compositions are mostly close to the very distinctive compositional field of Cenozoic extensional rocks of the Aegean area, with very high 207 Pb/ 204 Pb for relatively low 206 Pb/ 204 Pb ratios. These isotopic data confirm interpretations based on trace elements that subalkaline basalts were predominantly derived from melt-depleted peridotite in the sub-continental lithospheric mantle as a result of extension. Small areas of enriched hydrous mantle partially melted to yield shoshonitic magmas. Nd and Pb isotopic compositions of the alkali basalts are quite different from those in other rock types and suggest a HIMU mantle source component derived from a small plume, which also influenced MORB compositions. Distribution of these various rock types is used to constrain palaeogeographic reconstruction of Triassic micro-continental blocks.

Late Cenozoic, post-collisional Aegean igneous rocks: Nd, Pb and Sr isotopic constraints on petrogenetic and tectonic models

Nd isotopic composition has been determined for 16 igneous rocks, representing the wide geochemical, spatial and temporal range of post-collisional, late Cenozoic magmas in the Aegean area. Nd isotopes are used to further interpret previously published Pb and Sr isotope data. The overall pattern of late Cenozoic volcanism resulted from rapid extension, with thermal effects causing melting of hydrated, enriched, subcontinental lithosphere to produce widespread K-rich magmas. Slab break-off and intrusion of hot asthenosphere caused partial melting of rift-related continental margin basalts at the detachment point to generate adakitic magmas. Further outboard, mafic magma from enriched lithospheric mantle melted thickened lower crust to produce the granitoid plutons of the Cyclades. Nd isotopic variation in these varied rock types correlates with pre-Cenozoic palaeo-geography. Proterozoic subduction-related enrichment in Th and U, together with other large-ion lithophile elements, produced distinctive Pb isotope composition. This was later modified where Mesozoic subduction of terrigenous sediment was important, whereas subduction of oceanic carbonate sediments produced enrichment in radiogenic Sr and low Ce/Sr ratios. Late Cenozoic magmas sourced in eastern Pelagonian zone sub-continental lithospheric mantle have Nd model ages of about 1.0 Ga, and generally high 87 Sr/ 86 Sr and high 207 Pb/ 204 Pb (∼ 15.68) and 208 Pb/ 204 Pb (∼ 39.0) for low 206 Pb/ 204 Pb (∼ 18.6), but rocks to the west have more radiogenic Pb and higher Ce/Sr as a result of greater subduction of terrigenous sediment from the northern Pindos ocean. Magmas sourced from sub-continental lithosphere beneath the Apulian continental block were strongly influenced by subduction of oceanic crust and sediments north of the passive margin of north Africa. Subduction of Nile-derived terrigenous sediment in the east resulted in Nd model ages of 0.7 to 0.8 Ga and radiogenic Pb isotopes. Greater subduction of oceanic carbonate in the west resulted in magmas with higher 87 Sr/ 86 Sr and lower Ce/Sr. The strongly negative e Nd for adakites in the central Aegean rules out a source from subducted oceanic basalt, and the adakite magma was probably derived from melting of hydrated Triassic sub-alkaline basalt of continental origin. Where trachytic rocks are succeeded by nepheline-normative basalts (e.g. Samos), Nd isotope data imply that early partial melting of the enriched subcontinental lithospheric mantle involved hydrous amphibole and phlogopite, but once these minerals were consumed, younger magmas were produced by partial melting dominated by olivine and orthopyroxene.

Kinematics of the central Hellenides

Structural analysis along 24 cross sections crosscutting several windows in the central Hellenides provides the sense of nappe movements as well as the location of destroyed oceans lying between the Apulian and Eurasian continents from the Mesozoic. Orogeny took place in two phases: The first phase, “the Eo-Hellenic” phase, was initiated by convergence of the Apulian and Pelagonian plates with west directed subduction and closure of the Pindos Ocean. Late Jurassic obduction of oceanic lithosphere over the western margin of the Pelagonian plate was followed by footwall imbrication, mylonites and sheath folds. During the late Cretaceous, uplift was associated with ductile normal faulting at depth and tectonic unroofing at shallow crustal levels. The second phase, “the Meso-Hellenic” phase, comprised the closure of the Ambelakia Ocean at the eastern margin of the Pelagonian plate and continental subduction along the eastern margin of the Apulian plate. West directed subduction of the Ambelakia Ocean was associated with eastward directed ductile thrusting, folding and blueschist metamorphism. Blueschist formed within a simple duplex structure at depth and was subsequently overthrusted in the late Eocene onto the Olympos microcontinent, which acted as a major obstacle to the eastward directed nappe movements. Up to 150-m-thick cataclasites, kink folds and a spaced cleavage were formed during the late stage of the continental collision. “A subduction” along the eastern margin of the Apulian plate caused kink folding and reimbrication of the western parts of the Pelagonian basement. Since the Oligocene, the overthickened crust collapsed by means of low-angle normal faults.

The Miocene igneous rocks in the Basal Unit of Lavrion (SE Attica, Greece): petrology and geodynamic implications

The Miocene igneous rocks in the Basal Unit of the Lavrion area form part of the granitoid province of the central Aegean. Undeformed, subvertical dykes of quartz-syenite to granodiorite and granite porphyries, and a little deformed but variably altered granodiorite stock intrude metamorphic rocks of the Basal Unit. A 9.4 ± 0.3 Ma K–Ar age on feldspar for a dyke rock provides a minimum age for the igneous activity in the Basal Unit. East–west orientation of porphyry dykes is indicative of a regional extensional stress field with roughly north–south direction. Substantial extension in the Basal Unit after granodiorite emplacement is evident from widespread quartz veining associated with hydrothermal alteration of the granodiorite and the occurrence of mineralized tension gashes cutting the hydrothermally altered hornfelses. Final emplacement of the Blueschist Unit over the Basal Unit by extensional detachment post-dates contact metamorphism of the rocks surrounding the granodiorite. Geochemical diagrams show a continuous range of compositions from the dykes to the granodiorite. Radiogenic isotope compositions are compatible with a common magmatic source for the two lithologies. Elemental variations, as well as the considerable geochemical similarity of the dyke rocks to the Hercynian paragneiss of the central Cyclades, indicate that crustal melts were significant components during the evolution of the igneous rocks with fractional crystallization as an important process during later stages of evolution. The granodiorite displays geochemical signatures indicative of a significant mafic mantle-derived magma component.

Spatial and temporal variation in Late Cenozoic back-arc volcanic rocks, Aegean Sea region

Abstract The Aegean Sea and adjacent areas comprise thinned continental crust forming an extensional back-arc basin behind the South Aegean Arc. In this back-arc region are many small volcanic centres, with a wide range of volcanic products. This Late Miocene to Quaternary volcanism is volumetrically minor, in contrast to the voluminous igneous activity of the South Aegean Arc. The age and major and trace element geochemistry of these back-arc volcanic rocks are reviewed from the literature: where necessary, new geochemical analysis have been made. Five magma types are recognised: continental alkaline basalts; sodic basalts with incompatible element enrichment and Nb depletion; trachytes; shoshonites; and calc-alkaline andesites. All but the continental alkaline basalts appear geochemically related to subduction processes. There is a regular distribution of rock types in time and space: the subduction-related rocks occur in a broad arc between the modern South Aegean Arc and the Early to Middle Miocene volcanicity of the central Aegean region. Sodic basalts occur in the most inboard regions, and are later than the trachytes. Shoshonites and calc-alkaline magmas occur in the most outboard regions. This back-arc volcanism occurs only in areas of abundant shallow seismicity. However, areas of extension and seismicity in the northeast Aegean, distant from the subducting slab, lack volcanoes. With the exception of the continental alkaline basalts, the magmas are derived only from mantle influenced by the aseismic part of the Hellenic subduction slab between 200 and 400 km depth. Back-arc extension, associated with rotation of the Aegean microplate and strike-slip motion on the North Anatolian Fault system, has created a fault system allowing these magmas to reach the surface. Analogous processes may be common in the final stages of closure of ancient orogens.

Regional implications of geochemistry and style of emplacement of Miocene I-type diorite and granite, Delos, Cyclades, Greece

Abstract The Miocene plutons of the Cyclades were emplaced in a subduction setting during regional back-arc extension of continental crust, that led to flat-lying mid-crustal detachment faulting. Mapping of the island of Delos shows that quartz diorite and tonalite were emplaced as dykes in country rock of schist and marble within shear zones parallel to the extension direction. Mafic magmas were followed by numerous small batches of felsic magma, with magmatic and ductile deformation synchronous with magma emplacement. Late granite dykes occupy brittle fractures in the more deformed rocks. Mafic and intermediate rocks show a bimodal distribution of incompatible trace elements, with one group of broadly tholeiitic character and the other with substantial enrichment in Sr, Nb, and HFSE, but low Th and Ba. These differences appear to be inherited from two distinct mafic sources that are different from the mafic source for the plutons of the eastern Cyclades. Voluminous granodiorite results from these mafic magmas fractionating and/or mixing with felsic crustal material, some of which was derived by anatexis of a sedimentary protolith, indicated by high B and Mn. Some late granites appear derived from partial melting of Hercynian paragneiss. Regionally, the shear zones appear to be feeders to more extensive granitic plutons located at space produced at ramps in detachment fault zones. The shear zones parallel the Mid-Cycladic Lineament, a broad zone of displacement between two crustal blocks rotating in opposing directions as rollback took place at the Hellenic subduction zone. Distinctive geochemical features in Miocene igneous rocks suggests that these two blocks had quite different geological histories. The localisation of plutonism and core complexes near the Mid Cycladic Lineament suggests that this crustal-scale shear played a role in bringing subduction-derived magmas to mid-crustal levels. The heat supplied by the mafic magmas promoted ductile deformation high in the crust, where extension was concentrated, leading to the formation of core complexes. The regional extension resulted in progressive shallowing of the position of the granite solidus within the crust, leading to welding of the Mid-Cycladic Lineament, which is no longer seismically active.

Postorogenic shoshonitic rocks and their origin by melting underplated basalts: The Miocene of Limnos, Greece

Potassium-rich volcanic rocks of the shoshonite suite are common features of postorogenic extensional settings inboard from subduction zones. Various petrogenetic processes and tectonic settings have been proposed for their origin. Early Miocene volcanic rocks of Limnos, part of the northeast Aegean shoshonite belt, show distinctive geochemical features that allow their petrogenesis to be well constrained. The rocks are principally trachyandesites and dacites. Very strong fractionation of light and middle rare earth elements (REEs), similar to that found in adakites, is inconsistent with a mantle source, but it can be modeled by melting of meta-basalt enriched in incompatible elements. A comparison with experimental melting of metabasaltic amphibolite requires small degrees of dehydration melting of amphibole, plagioclase, clinopyroxene, and minor garnet at a temperature >950 °C. Melting was triggered by mantle-derived magma, evidenced by repetitive zoning in clinopyroxene with Cr-rich cores. Nd and Sm isotopes suggest that some of this magma was similar to lamproite found elsewhere in this shoshonite belt and some was of asthenospheric origin. The amphibolite source is inferred to be subduction-enriched metabasalt that underplated the crust during pre-Mesozoic subduction. The regional trigger for dehydration melting was upwelling of asthenosphere as a result of slab detachment. The geochemistry and radiogenic isotopes of other shoshonitic rocks in the northeastern Aegean suggest a similar origin, but with higher degrees of partial melting of base-of-crust metabasaltic amphibolite. Similar processes appear likely for shoshonitic magmatism in some postcollisional settings elsewhere.

Evolution of boninites and island-arc tholeiites in the Pindos Ophiolite, Greece

Whole-rock geochemical and Sm/Nd isotope data are presented for a representative suite of crustal rocks from the Pindos Ophiolite in order to resolve the origin of the geochemical signature of the boninites. Comparison is made with Triassic MORB from the Avdella melange and with other ophiolitic rocks of northwestern Greece. Hydrothermal alteration results in large scatter in Sr and K and some variability in Ba, Th and U. The Pindos boninites contain high Zr and Hf with respect to REE, characteristic of many boninites. Pb, La/Sm and Nb decrease with decreasing TiO 2 from MORB to IAT, but then increase in the boninites. Nd isotopic values expressed as e Nd decrease systematically with decreasing TiO 2 , from 7–8 in IAT to 0.6–3.0 in boninites. As mantle wedge harzburgite became increasingly depleted, another magma source contributed significant amounts of Pb, REE and probably Nb. The Pb and other large-ion lithophile elements may have been transported in aqueous solutions from the subducting slab, but the REE and Nb imply an ocean-island basalt (OIB)-type source within the mantle wedge. This OIB source is a consequence of mantle plume activity during late Triassic rifting.

Miocene magnesian andesites and dacites, Evia, Greece:adakites associated with subducting slab detachment and extension

Abstract Mid-Miocene volcanic rocks are rare in the Aegean region, although early Miocene and late Miocene-Quaternary volcanism is widespread. At Oxylithos (island of Evia), 14 Ma dacites form a dome or sub-volcanic complex. Phreatomagmatic eruptions formed nearby rhyolitic pyroclastic surge deposits. The calc-alkaline dacites contain bronzite, Mg-rich clinopyroxene, phlogopite and plagioclase phenocrysts. The magma results from mixing of a Mg-rich andesitic magma, similar to that in the nearby island of Skyros, with more felsic magmas represented by the rhyolitic pyroclastics. The dacites are geochemically similar to adakites, which are derived by partial melting of eclogitic subducted oceanic crust and have low Y and Yb and high Sr Y ratio. 87 Sr 86 Sr ≈0.7095 is found in both dacite and rhyolite. Lead isotopic composition from the high-Mg andesite from Skyros, with 207 Pb 204 Pb = 15.70 and 208 Pb 204 Pb = 38.90 , forms a linear trend with Evia dacite and rhyolite with 207 Pb 204 Pb = 15.71 and 208 Pb 204 Pb = 39.05 . The high temperatures required to produce such magma resulted from decompression due to extension of the Aegean basin at the same time as the initial intrusion of the detached subducted slab in the western Aegean that has been imaged by seismic tomography. The Oxylithos rocks extend the known occurrences of adakite series rocks: this series is not restricted to sites with subduction of young oceanic crust.

The South Aegean active volcanic arc: relationships between magmatism and tectonics

Abstract Two principal volcanic associations, together with a third minor association, occur in the South Aegean active volcanic arc, differing in magma type, age, spatial distribution, relationship to faulting, and petrogenesis, even though geophysical data indicate a continuous subducted slab at 130 – 150 km beneath the volcanic centres. Variation in magmatism is related to changes in tectonics during the evolution of the arc, as a result of collision of African continental crust with the Aegean-Anatolian microplate, that set up changing patterns of strike-slip faulting in the arc. A synthesis is presented of the age and the variation in major and trace elements and radiogenic isotopes of the rocks of the arc. The western part of the arc (including Aegina, Methana, and the older rocks of Milos and Santorini) has typical arc-related andesite – dacite volcanism, predominantly of Pliocene age, associated with E-W listric faulting with slow slip rates. Nd and Sr isotopes and trace elements show that magmas resulting from volatile-induced melting in the asthenospheric mantle wedge subsequently either underwent assimilation with fractional crystallisation (AFC) or mixed with local partial melts within the lithospheric mantle. Viscous felsic magmas were likely trapped in the lower crust. In contrast, the mid to late Quaternary of the central and eastern part of the arc (Milos, Santorini, Nisyros) consists of lavas and voluminous pyroclastics, with lava compositions including both tholeiitic and calc-alkaline minor basalt, andesite, dacite and minor rhyolite. These younger magmas resulted from melting both of hydrated mantle (calc-alkaline magmas) and depleted asthenospheric mantle (tholeiitic magmas), influenced by regional extension, which is greatest in the central part of the arc. This young volcanism began at the same time as the ENE-trending strike-slip faulting that resulted from collision with an indentor of thinned continental crust of the African plate. Stepover faulting and extension on early Quaternary NE-trending strike slip faults as a result of this mid- to late Quaternary ENE-trending sinistral strike slip motion provided pathways for magmas (including mantle-derived felsic magmas) to rise and fractionate and also pathways for water to enter shallow magma chambers. The third minor group of rocks comprises felsic lavas at Crommyonia and Kos, principally of Pliocene age, derived from mid-crustal anatexis. The ultimate cause of magmatism in the South Aegean active volcanic arc is subduction-related release of hydrous volatiles, but there are important differences between the petrogenesis of the older western and younger eastern parts of the arc.