PETROLOGY, GEOCHEMISTRY AND ORIGIN OF PLIOCENE - PLEISTOCENE POTASSIUM-RICH VOLCANIC ROCKS FROM GREECE
A thesis submitted to the University of Manchester
for the degree of Doctor of Philosophy (Ph.D.)
in the Faculty of Science
Department of Geology
From middle Pliocene up to recently (5 - 0.5 Ma) K-rich intermediate to acidic volcanic rocks were erupted in various parts of Greece. The volcanic centres which are studied here are located in North Western Greece near the border with Yugoslavia (Voras), in the northwestern margin of the currently active South Aegean (Hellenic) volcanic arc between the city of Volos and North Evia (Volos - N. Evia centres) and about 50 km north of the central part of the South Aegean volcanic arc (Antiparos).
A detailed petrographic, mineralogical (involving chemical compositions of all the primary phenocrysts) and geochemical (comprising major and trace element analyses) study of representative samples was carried out to assess the nature and origin of the primary magmas, their subsequent evolution and the potential linkage of the magmatism with the regional tectonics of each area.
The Voras lavas are intermediate in terms of their silica content and belong to the high-K calc-alkaline and shoshonitic series of Peccerillo and Taylor (1976). They are variably enriched in their LILE, LREE and other incompatible elements like Zr, U and Th, and can be divided into three groups, using geographical and petrographic criteria, which display different degrees of enrichment of incompatible elements. From the major and trace element variation diagrams, REE chondrite normalised patterns and chondrite or MORB normalised spiderdiagrams it is concluded that several distinct, but similar, primary magmas originated from small degrees of partial melting, under hydrous conditions, of an inhomogeneous upper mantle lherzolite, enriched in LILE and LREE elements. This enrichment occurred as a result of either earlier or contemporaneous subduction processes. It is suggested that fractional crystallisation or crustal contamination processes did not significantly modify the primary magma compositions. Additionally, on the basis of the presence of mixed populations of phenocrysts of the same phase within the same samples, it is proposed that magma mixing processes played an important role in the genesis of some of the rocks.
The volcanic rocks of the five small Volos - N. Evia centres (i.e. Porphyrion, Microthebes, Achillio, Likhades Islands and Agios Ioannis) are also intermediate in composition, but have lower silica contents than the Voras rocks and belong to the high-K calc-alkaline series of Peccerillo and Taylor (1976). They are also enriched in LILE, LREE and other incompatible elements (i.e. Zr, U and Th) but not as much as some of the Voras rocks. By using the same criteria as for the Voras rocks, and some additional isotopic data, the following conclusions were reached. Each volcanic centre represents a geochemically distinct magma, although two of them (i.e. Likhades and Agios Ioannis) probably originated from the same source. The process that generated the primary magmas is, similarly to Voras, considered to be small degrees of partial melting, under hydrous conditions, of an inhomogeneous and enriched (with respect to its LILE and LREE contents) upper mantle. Again the rocks are thought to represent near primary compositions with fractional crystallisation and crustal contamination processes regarded as either insignificant on not involved at all during the evolution of the primary magmas. The melting originated as a result of the subduction of the South Aegean oceanic slab beneath the continental crust underlying the volcanic rocks. Some of the parental magmas of the Likhades centre have evolved through magma mixing processes as indicated by the occurrence of phenocrysts of the same phase with different compositions within the same samples.
The Antiparos lavas are high-silica rhyolites which have relatively high potassium and sodium contents. They are significantly enriched in some incompatible elements (i.e. Rb, Cs, U, Th and Nb) and extremely depleted in Ba and Sr. The rhyolites are metaluminous or slightly peraluminous. Their chondrite normalised patterns have significant negative Eu anomalies and middle REE depletions. On the basis of their major, trace element and normative compositions, their REE patterns, their initial87Sr/86Sr ratios, their K/Rb, Rb/U, Zr/Hf, Zr/Nb and Th/Ta ratios and their comparison with upper and lower crust compositions, with calc-alkaline high-silica rhyolites from the currently active South Aegean arc and with similar topaz-bearing rhyolites from the Western U. S. A., it is suggested that the primary melts of the Antiparos rhyolites were generated by small degrees of partial melting, under either dry or water-undersaturated conditions, of a felsic granulite source located in the middle to lower crust. Subsequent to their generation the primary magmas rose to a shallow magma chamber where they evolved by significant fractional crystallisation processes involving dominant alkali-feldspar and subordinate Ab-rich plagioclase, quartz and sphene. The heat for melting of the crust was provided by the rise of hot basic magma, generated by the subduction of the Aegean oceanic slab, near the base of the continental crust underneath Antiparos.