Geological and Geochemical Evolution of the Santorini Volcano: a Review

INTRODUCTION

The continuous interest the international scientific community has maintained in the volcanic centre of Santorini has resulted in the gathering, over the past twenty years, of large volumes of data pertaining to all aspects of its geological evolution. Numerous published studies ranging from the seismic structure of the area and the geology of the pre-volcanic basement to the volcanic stratigraphy, explosive volcanology and caldera development, geochemistry of lavas and the origin of the currently active ore-forming hydrothermal systems, have substantially improved our understanding of the processes involved in the genesis and evolution of this volcanic centre. Therefore, it was considered appropriate at this stage to briefly review selected aspects of the existing information on the tectonic and volcanic evolution of Santorini and highlight their implications for the origin of the erupted magmas.

TECTONIC SETTINGS

The Santorini volcanic centre is part of the Aegean island arc which is an arcuate chain of a dozen inactive or dormant volcanoes, approximately 500 km long and 20-40 km wide and extending from the middle of the eastern coast of mainland Greece, through the central Aegean to the western coast of Turkey (Fytikas et al. 1976; Ninkovich and Hays 1971).

The central Aegean has a crustal thickness of 30-35 km, high heat flow, approximately three times higher than the surrounding eastern Mediterranean, and relatively low P-wave velocities for the underlying mantle (7.5-7.7 km sec^-1). It exhibits positive Bouguer and magnetic anomalies and is dominated by extensional tectonics with north-east and south-east trending faults which played an important role in the structural control of the volcanic and hydrothermal activity in the area of Santorini (Angelier et al. 1982; Boström et al. 1990; Erickson 1970; Huijsmans 1985; Jongsma 1974; 1975; Le Pichon and Angelier 1979; Makris 1978a, 1978b; Makris and Stobbe 1978; Makris and Vees 1977; McKenzie 1978).

Volcanic activity in the area started ~3 my ago in response to the subduction of the African plate beneath the Aegean microplate in a north-east direction at an angle ~30° and a relative rate of convergence of 3.5 cm/year. The sinking slab is postulated to have reached depths in the order of 220 km with the volcanic arc corresponding to a depth of 130-150 km. The subduction processes along the Hellenic trench are regarded as having initiated 13 my ago (Angelier et al. 1982; Ferrara et al. 1980; Fytikas et al. 1976; Hays 1972; Papazachos and Comninakis 1978). More recent work on the geodynamic evolution of the Aegean, however, proposed penetration depths of the sinking slab in the region of 600 km and an age for the initiation of subduction exceeding 26 my.

The Aegean arc remains exceptional in that the continental crust composes the upper parts of both overriding and subducting plates (Makris 1978) and that extensional tectonics dominate the arc-fore-arc area down to the Hellenic trench (Le Pichon and Angelier 1979). Further to the south of the trench the area is dominated by compression which is reflected by the presence of the Mediterranean ridge, an accretionary ridge formed by the piling up of the upper 3-4 km of sediments which failed to subduct (Le Pichon et al. 1982).

TEMPORAL EVOLUTION OF THE SANTORINI VOLCANIC CENTRE

Santorini is a group of five islands, Thera, Therasia, Aspronisi, Palaea Kameni and Nea Kameni (Fig. 1) with an areal extent of 75 km². The first three islands are arranged in a semicircle and form a fracture caldera 8 x 5 km across. The flooded caldera reaches a depth of 370 m with Palaea Kameni and Nea Kameni islands rising at the centre of it. In the past 3 my of intermittent volcanic activity on Santorini several volcanoes were active, operating successively or concurrently. At present volcanic activity is restricted to the Kameni islands with all the older volcanoes being extinct.

Volcanic activity on Santorini started 3 my ago at the Akrotiri peninsula (Fig. 1). This volcano operated discontinuously for 1.5 my and produced both lavas and pyroclastics (Ferrara et al. 1976; Pichler and Kussmaul 1980). The Thera volcanic centre operated from 1 my to 37,000 years ago and produced mainly pyroclastic deposits with subordinate amounts of lava (Friedrich et al. 1977; Friedrich 1978; Heiken and McCoy 1984; Huijsmans 1985; Seward et al. 1980). The north-eastern volcanic centres comprise Megalo Vouno, Mikros Profitis Ilias and Skaros volcanoes. Megalo Vouno is a shield volcano more than 100,000 years old which produced mainly lava flows and subordinate amounts of pyroclastics. Mikros Profitis Ilias also produced lava flows with minor amounts of pyroclastics and operated almost concurrently with the former volcano. Skaros is also a shield volcano which ceased ~13,000 years ago and produced lava flows and ignimbrites (Druitt and Sparks 1982; Huijsmans 1985; Pichler and Friedrich 1976; Pichler and Kussmaul 1980). On the island of Therasia a number of distinct volcanic centres have been recognized, having produced mainly lava flows and some pyroclastics (Huijsmans 1985; Pichler and Kussmaul 1980).

     Caldera development and post-caldera volcanic activity: Approximately 100,000 years ago a paroxysmal eruption of the Thera volcanic centre resulted in the production of a plinian pumice deposit and the development of the southern sector of the present caldera (Heiken and McCoy 1984). In 1390 BC, on the other hand, a tremendous eruption covered the whole Santorini area with a conspicuous white layer of ash. It is estimated that ~19 km³ of pyroclastic material was produced during this event which was responsible for the extension of the caldera to the north. Subsequent caldera collapse created the present topography (Friedrich 1978; Heiken and McCoy 1984). In historical times two new volcanic islands formed within the flooded caldera. In 197 BC and AD 726 Palaea Kameni island was formed, whereas intermittent eruptions in 1570, 1707, 1866, 1925, 1940-41 and 1950 gave rise to Nea Kameni. The black and glassy post-caldera lavas formed domes and block flows (Huijsmans 1985). Significantly, the most recent eruptions were preceded by strong intermediate depth (< 160 km) earthquakes which could correspond to magma generation processes above the subducting slab.

The crust beneath the Kameni islands is characterized by pronounced negative gravity anomalies (Yokoyama and Bonasia 1978). Moreover, these islands are the loci of discharge of metal-rich warm hydrothermal solutions and fumarolic activity (Butuzova 1978; Boström and Widenfalk 1983; Smith and Cronan 1984). These phenomena are taken as evidence for the presence of a degassing magma chamber which has introduced hydrothermal convection within and below the volcanic carapace of the Kameni islands.

PETROLOGICAL EVOLUTION OF THE SANTORINI LAVAS

Detailed accounts of the petrography and petrology of the Santorini lavas are given by Huijsmans (1985) and Nicholls (1971). According to the classification scheme of Peccerillo and Taylor (1976) the Santorini lavas range from basalts and basaltic andesites to dacites, rhyodacites and rhyolites. In general they are porphyritic to glomeroporphyritic with phenocrysts set in a fine-grained intergranular to intersertal groundmass. In the andesites and the more silicic composition the groundmass becomes hyalo-ophitic to hyalopilitic and quite often trachytic. Phenocryst phases are plagioclase, olivine, clinopyroxene and magnetite. Orthopyroxene becomes a major phenocryst phase in the more silicic compositions replacing olivine. In such compositions apatite and ilmenite commonly occur as microphenocryst phases. The earliest phenocrysts to crystallize are plagioclase and olivine followed by clinopyroxene and magnetite. Phenocrysts may be euhedral, subhedral or anhedral and often exhibit disequilibrium textures such as complex zoning, reaction, resorption, roundness etc. which in certain cases render difficult the distinction between phenocryst proper and xenocrysts. Xenolith occurrence is virtually restricted to the older Akrotiri basalts and the younger post-caldera lavas. Basalts and basaltic andesites occur as lava flows in the northern shield volcanoes, Akrotiri and Therasia, and also as intercalations within the Thera pyroclastic deposits.

Olivine in the Santorini basalts and basaltic andesites shows normal compositional zoning with Fo contents ranging from Fo85 to Fo53, with the highest Fo values usually found within the largest often embayed olivine crystals. Olivine in basaltic andesites has similar Fo contents with the exception of the basaltic andesites of Skaros where Fo reaches 91 mole %. Such high Fo values are not consistent with the chemical composition of these rocks. Experimentally determined olivine-liquid KDs for Mg-Fe require Mg values for the corresponding liquids in the order of 0.75 (Gerlach and Grove 1982; Ivine 1977) which is not to be found even in the most basic of the Santorini rocks sofar analysed, a high alumina basalt from C. Balos at Akrotiri (Nicholls 1978). The presence of such xenocrysts can be explained by mixing of parental magmas containing high pressure fractionation products with more evolved magmas of low pressure cotectic character (Nicholls 1978).

Further evidence for magma mixing is provided by the cyclic compositional variations observed within individual lava sequences, corresponding to different eruptive cycles. They are also reflected by abrupt changes in the chemistry of phenocryst phases when appropriate contrasts in the bulk composition occur between cycles. In general, the compositional variation of equilibrium minerals within individual flows is very restricted and the trend from the more basic towards the more silicic compositions of the host lavas is that of Ca depletion in clinopyroxene, Ab enrichment in plagioclase, Fa enrichment in olivine and Mg ratio decrease in orthopyroxene (Huijsmans 1985).

Quantitative assessment of intercrystalline and mineral-melt equilibria indicated that the Santorini basalts crystallized at 1150°-1200° C under P H₂O < P_lith = 1-2 Kbars, basaltic andesites at 1050°-1150° C under P_lith = 1-3 Kbars, and the andesites at 950°-1050° C under similar lithostatic pressures and P H₂O = 1-2 Kbars. The dacites and rhyodacites on the other hand crystallize at somewhat lower temperatures under P H₂O = P_lith = 1-2 Kbars. Calculated oxygen fugacities plot just above the Ni-NiO buffer, a rather typical feature of calc-alkaline suites (Huijsmans 1985).

CHEMICAL EVOLUTION OF THE SANTORINI LAVAS 

Biaxial plots of major and trace element data from the Santorini lavas (Huijsmans 1985; Mann 1983) display coherent variation trends and indicate that these rocks are cogenetic in the sense that they have evolved within discrete magma chambers, periodically replenished with parental magmas ultimately derived from a common source. Evidence outlined above, suggests that magma mixing processes were instrumental in the evolution of these magmas in magma chambers located at shallow levels in the crust (3-8 km depths).

On the other hand, plots of trace and minor elements which remained incompatible throughout the compositional range of the Santorini lavas (e.g. Th, Rb, La, K, Ta, Zr, etc; Mann 1983) display good linear correlations intersecting the origins of the axes, thus suggesting that fractional crystallization was the other major differentiation process responsible for the evolution of their parental magmas during ascent and/or within shallow magma chambers. In fact, trace element Rayleigh fractionation modelling, major element addition-subtraction diagrams and least-square mixing calculations (Mann 1983; Nicholls 1968; 1971) suggested that the basaltic andesites, andesites and dacites can be produced from an initial basaltic magma by up to 54%, 67% and 82% fractional crystallization, respectively.

The possible nature of the primary magmas at Santorini was discussed by Nicholls (1978) who concluded that the high alumina basalts of Santorini originated from mantle derived primary magmas which during lithospheric ascent underwent high pressure fractional crystallization of ol-Cr-sp ± cpx (also Osborn 1976). Trace element and radiogenic isotope systematics of the Santorini lavas can in fact be instrumental in testing such a hypothesis. More HYG element ratios such as Zr/Hf, Zr/Nb and Nb/Ta are not affected by magmatic processes and reflect the characteristics of the source (Paritsis 1985; Saunders et al. 1980; Tarney et al. 1980). The Santorini basalts have Zr/Hf ratios in the order of 25 to 33, Zr/Nb 20 to 32 and Nb/Ta 13 to 14 (Mann 1983), very similar to those postulated for the primordial mantle (31.4, 18 and 15 respectively; Jagoutz et al. 1979), thus confirming the involvement of a primordial mantle derived component in their genesis. Moreover, their Ti/Zr and Zr/Y ratios plot within a mantle array (Fig. 2), defined by basaltic suites ultimately derived from the primordial mantle (Paritsis 1985). Their higher Zr/Y and Ti/Zr ratios relative to MORB can be attributed to petrogenetic processes characteristic of the genesis and evolution of such magmas (e.g. higher degrees of partial melting, removal of cpx and hb etc.; Paritsis 1985). Their LFS to HFS element ratios on the other hand (K/Zr, Th/Nb, La/Ta, La/Nb, Ba/Nb data in Mann 1983) are much higher than the corresponding values for the primordial mantle and N-type MORB (Fig. 3).

This is considered to represent a fundamental feature of island arc magmatic suites ranging from the primitive arc tholeiites to the most evolved shoshonites (Saunders et al. 1980; 1981). This is also depicted by the primordial mantle normalized plot of Fig. 4 which clearly illustrates the similarity between the HYG element characteristics of the Santorini basalts and other arc magmatic suites as well as their contrasting character relative to basalts from the ocean floor. The processes regarded as responsible for the decoupling of LFS from the HFS elements during the genesis of arc magmas include contamination of the mantle wedge with fluids from the dehydrating subducted sediment and mantle-crust interaction (Mitropoulos et al. 1987; Saunders et al. 1980).

The multi-component character of the source of the Santorini lavas, implicit in their HYG element characteristics, is also depicted by their ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios (data from Briqueu et al. 1986) which are shown in Fig. 5 together with the fields for MORB, island arcs and continental margin calc-alkaline suites, ocean islands and Pacific seamounts. Evidently the Santorini lavas plot within the mantle array albeit well outside the field defined by MORB. The lower Nd and higher Sr ratios could, therefore, be attributed to a two-component source derived by mixing of depleted MORB type mantle and a relatively enriched component which may represent subducted crust (igneous basement plus sediment) and/or its dehydration products or continental crust material or combinations of all such components. However, as discussed by Mitropoulos et al. (1987) the Santorini basalts possess Th/Ba and Th/Rb ratios which are too high to justify the involvement, at least to any significant degree, of an upper crustal component or of abyssal ocean sediments in their genesis.

By implication, the source of the Santorini lavas can be considered a mixture of depleted MORB type mantle with partial melting products of subducted ancient, hydrothermally altered, oceanic crust. Such a model could account for the trace element and radiogenic isotope characteristics of the Santorini lavas (details of the model in Saunders et al. 1980; 1981) by ensuring the primordial mantle signature of their HFS element ratios, the high LFS/HFS ratios and the evolution of certain more to less HYG element and radiogenic isotope ratios along mantle arrays.

Considering the intracontinental setting of the Santorini volcanic centre it is difficult to envisage how the processes involved in the genesis and evolution of the magmas so far erupted were seemingly unaffected by crustal or subducted sediment contamination. As was pointed out by Mitropoulos et al. (1987) this may be due to the fact that Santorini occupies the centre of the arc where crustal stretching is greatest thereby allowing the underlying asthenosphere to be directly involved in magma genesis. The same hypothesis, however, cannot be held true for the neighbouring island of Milos.

The extensional regime cannot be significantly different there. The erupted lavas, however, have a markedly more contaminated character in relation to Santorini (Briqueu et al. 1986; Mitropoulos et al. 1987). It is therefore apparent that there is no explanation forthcoming for the intra-oceanic arc character of the Santorini lavas, thus adding to the rather exceptional nature of this volcanic centre.

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