Petrology and Geochemistry of the Metamorphic System of Santorini

The sediments into which the volcanic material penetrated suffered later a metamorphism, and therefore it is deduced that the age of the sediments is probably Triassic to Jurassic. Based on the critical minerals of antigorite, stilpnomelane, chlorite etc. the facies of metamorphism is defined as greenschist facies. The products of anatexis are described (granite - porphyries). Tataris found fossils of Paleocene in the upper strata of the metamorphic system and he accepted the age of this formation as probably up to Eocene. However, between the Triassic - Jurassic volcanism and this Eocene, metamorphic rocks of a thickness of 250 m interpose, though the complete stratigraphic series is absent. Therefore we have to accept either that these intermediate horizons were eroded, or that the Eocenic formations overlaying the Triassic - Jurassic sediments have been overthrust on them.

INTRODUCTION

Rocks of non-volcanic origin are found on the island of Thera only in the south-east part, which also constituted the more ancient island, before the manifestation of volcanism, whose beginning has been placed in the Upper Pleiocene.

These rocks appear in the following areas:

• In the area of the mountain Profitis Elias - Messa Vouno (Selada - Kamari)
• In the area of Platynomos, south of the village Emborion
• In Pyrgos
• In Monolithos
• In the precipitous walls of the caldera and in the coves of Athenios and Thermia

The nucleus of the mountain mass of Profitis Elias - Messa Vouno is composed of phyllites, which appear in the ridge between Profitis Elias - Messa Vouno (in the area of Perissa, over the ridge of Selada, at a height of 275 m. and from Selada to Kamari village), in some ravines in Emborion and Gonia as well as in the walls of the caldera, below the volcanic layers of Athenios and Thermia.

The extent of these non-volcanic rocks appears int he enclosed sketch (Fig. 1).

Our present knowledge concerning the metamorphic basement of Santorini, is based on the words of Fouqué, Reck, Neumann van Padang, Philippson, Papastamatiou, and Tataris.

Accoridng to Fouqué the phyllites include psammites, graywackes and conglomerates. In some areas they contain also crystallic limestones, especially in the outcrops of the caldera. Fouqué also observed chlorite and mica flakes.

Later on, Papastamatiou made a study of the crystallic limestones of the island. He microscopically identified feldspars and quartz in some samples of phyllites and hence he concluded that these phyllites belong probably to old volcanic tuffs of the phyllite series.

On the thin covering overlying crystallic limestone he found by himself sections of gigantic Megalodon of the triassic age. So relying on the fossils he identified the age of the crystallic limestones as Triassic, and probably of the Upper Triassic.

Concerning relation between these limestones and the underlying phyllites, he accepts that it is probably tectonic, after his investigations at Messa Vouno, where bands of schists in the limestone are observed.

Concerning the present day beliefs of investigators about the age of the phyllites of Santorini, A. Philippson, with some reservation, expresses the opinion that the Santorini phyllites are early Palaezoic, relying only on the lithologic similarities between these phyllites and the phyllites of Amorgos.

Renz considers that the Amorgos schists are equivalent to the schists of Athens and the Perani series of Salamis, whose age is also problematic. But he found the fossiliferous Triassix, overthrust, in his opinion, over the schists of Amorgos.

Newman van Padang accepts for the Santorini phyllites a probable Permocarboniferous age, and that the underlying crystallic limestones are slantingly overlayed on them.

Lastly Tataris, on the basis of the fossils he found in a small dolomitic limestone in the semimetamorphic conglomerate of the underlying Triassix limestone (Miliolidae, Laffiteina, Gastropodes etc.) determines the age at least of the upper parts of the semimetamorphic system as Postcretaceous (Montian).

In comparison to the semimetamorphic Eocene flysch of the island of Anaphi he believes that in this formation as well the age will be raised to the Eocene. Given that, as referred to above, the semimetamorphic system underlies the crystallic limestones it can be concluded that the limestones are overthrust on the semimetamorphic system.

GEOLOGY OF THE METAMORPHIC SYSTEM

The phyllites appear in many places tectonically disturbed and mylonitized, as for example in Athenios and Thermia. The direction and inclination of them vary.

The greatest appearance of them is in Athenios where they extend along the caldera wall for a length of 1700 m. approximately and a height of 220 m., and after a small interruption from Athenios, they appear again at Thermia for a length of about 250 m. and a height of 55 m. above sea level.

Frequently they enclose lenses of crystallic limestone of fluctuating thickness up to 3 meters. These crystallic limestones are sometimes fine and sometimes coarse grained. Sometimes they contain sericite, muscovite or chlorite. Their color is light grey, greenish to reddish or pale yellow. In some places they appear strongly alterated with material of disintegration such as talc, argilaceous material and iron oxides. Frequently they are traversed by quartz veins, sometimes parallel to the schist layers and sometimes irregularly. Veinlets of calcite are observed likewise.

In the places of Athenios and Thermia and near the sea level, heavier metamorphic types appear (see petrographic section), of a black grey to black green color. In Thermia an outcrop of actinolite - hornblende - epidote schist of small extent is met, with pyroxene. On the boundary zone of this outcrop layers are observed of hematite and limonite of some thickness (30 - 40 cm), as well as chrysocolla of emery green color, and cuprite. The surrounding rocks were enriched locally by epidote.

South of Athenios harbour were found stratified in the old sediments pyroclastic rocks of porphyritic tecture with phenocrysts of pyroxenes; these pyroclastic rocks, in spite of the metamorphism that took place, kept their structure and their initial mineralogical composition, so that they can easily be recognised today within the sericitic calcitic schists. Metamorphic tuffaceous sediments (hyaloclastites) were identified as well.

In many places the metamorphic system is traversed by veins of granitic porphyry rocks. These veins traverse this outcrop, irregularly almost perpendicularly, in places they follow the schistosity of the surrounding rocks. Probably they follow the tectonic irregularities if it.

In many of the Athenios pyllites a sulfide mineralisation is observed. This must be probably the hydrothermal solutions that penetrated in from the faults and discontinuities of the rock formation. They appear frequently in veins, or in small nets, or as disseminated ore. Of this, the most valuable outcrops have been exploited in the past. The greastest thickness of the veins amounts to 70 cm. The nearby rock formation is frequently silicified, even though with restricted enrichment of the ore.

The identified minerals under the metallographic microscope are:

1. gralena
2. ironpyrite
3. sphalerite
4. marcasite
5. some magnetite
6. arsenopyrite
7. coveline
8. goethite

In the area of Profitis Elias - Messa Vouno a metamorphic conglomerate is also observed, apart from the phyllites and schists. This is composed of phyllitic and limestone pebbles of a size up to a fist.

Near Perissa the phyllite is frequently of a conglomerate structure with mostly prolonged limestone and phyllitic pebbles, up to 4 - 5 m long and 0.5 m thick.

PETROGRAPHY

In the metamorphic system we distinguish the following petrographic sequences:

A:    Metamorphic rocks of sedimentary origin

B:    Metamorphosed mafic and semimafic igneous rocks

C:    Low-grade metamorphosed granite porphyries

A:   Metamorphic rocks of sedimentary origin:

In this sequence were identified the following petrographic types:

1. Muscovite-biotite schists (Athenios)
2. Muscovite-calcite-quartz schists to quartzites (Athenios, Emborion)
3. Muscovite (or sericite) stilpnomelane-quartz schists (Athenios, Emborion)
4. Chlorite-mica schists (Athenios, Emborion, Perissa)
5. Quartzites (Athenios, Emborion, Perissa, Selada, Kamari) and stilpnomelane quartzites (Athenios)
6. Phyllites and calc phyllites (Athenios, Emborion, Pyrgos, Perissa, Selada, Kamari)
7. Calcite-dolomite-antigorite-stilpnomelane schists (Athenios)
8. Calcite marbles and dolomite marbles, in layers into the schists and phyllites (Athenios, Selada near by Ancient Thera)
9. Metamorphic sandstones (probably arkoses and graywackes) (Athenios, Perissa, Selada, Kamari)
10. Metamorphic conglomerates (conglomerates schists) and metamorphic breccias (Perissa, Ancient Thera, Selada, Kamari)

B:   Metamorphosed mafic and semimafic igneous rocks

1. Actinolite-hornblende-epidote-pyroxene-albite-chlorite schists and actinoilite-epidote-pyroxene-sericite-albite schists (Thermia)
2. Hyaloclastites with or without pyroxene (Thermia, Athenios - hyaloclastites without pyroxene probably and near Ancient Thera)
3. Metadolerite (hornblende dolerite with glaucophane). outcrop of a lens-shape and small in extent, approximately in the middle of the road to Athenios
4. Glaucophane-epidote-chlorite schists (pebble in the sericitic schists, on the road to Athenios and at a height of 120 m. approx.)

C:   Low-grade metamorphosed granite porphyries

1. Biotite-granite porphyries with stilpnomelane (Athenios)

Chemical analyses of the above petrological types are included in Tables 1 and 2.

The above rocks appear almost metamorphosed. This metamorphism is greater generally in the deeper horizons, even though it is observed sometimes strongly in various places, selectively and independently of the rock depth.

The metamorphosed semimafic igneous rocks appear on the way to Athenios, south of Athenios harbour and at Thermia. They are composed mostly of heterogeneous material, that presupposes an initial tuffic material. The mineralogical composition and especially the diopsidic pyroxene, speaks itself for the initial andesitic material.

These metamorphosed volcanic rocks are found as stratified lenses or as whole layers in the old sedimentary series, which series today appears, due to the metamorphism, as sericitic limestone schists, quartzites etc.

MINERALOGICAL COMPONENTS (Sequences A and B)

Pyroxene appears in great crystals, as we say porphyroblasts. They show frequently severe fracturing and deformation, as well as alteration to chlorite. Apparently big porphyroblasts were formed after a post-tectonic recrystallization (south of Athenios).

Based on its optical properties (colorless to pale green, (+) 2V = 60^o , c/nγ = 38^o) we consider that it probably belongs to diopside.

Hornblende appears in the actinolitic schists and hyaloclastites, with the following optical properties:

Pleochroism: nα = light yellow - light red to orange

                   nβ = emerald green

                   nγ = blue green

c/nγ = 22^o, (-) 2 V = 72^o

The chemical composition of horneblende, determined by microprobe analysis is ⁽¹⁾:

Fe₂O₃: 17.31

MgO:     8.60

SiO₂:   46.38

Al₂O₃: 12.60

CaO:   11.15

Na₂O:   1.57

K₂O:     0.14

Number of cations of a basis of 24 oxygens:

Fe''':      2.00

Mg:       1.90

Si:        7.00

Al:        2.24

Ca:       1.80

Na:       0.52

K:         0.00

Based on the above chemical composition and optical properties, it is considered probable, that this hornblende corresponds to a tschermakitic hornblende.

Common hornblende occurs also in the metadolerite.

• Actinolite. Colorless to pale green, with oblong radially placed crystals, c/nγ = 12^o , (-) 2V = 76^o (Ca₂Fe₅ = 85%, Ca₂Mg₅ = 15%)

• Epidote. It was determined only in the actinolitic schist of Thermia, with optic properties:

(-) 2V = 70 - 71^o for the red light

(-) 2V = 63 - 67^o for the violet light   r>υ, nγ/a = 30^o

Therefore it is defined as pistacite.

• Antigorite. Colorless to pale green in thin section. All grains are plate-like parallel to (001), and many have a rectangular outline due to (010) and (100) cleavages. Optically negative with positive elongation and very low refringence and birefringence. The optic angle is between 40^o and 51^o.

• Stilpnomelane. Lamellar in brittle flakes. It occurs in thin plates often showing a radiate or sheaf-like arrangement. The perfect basal cleavage is not as regular nor as continuous as in mica and it has also an imperfect cleavage perpendicular to (001). Pleochroic as follows:

nα = golden yellow

nβ = nγ = reddish brown

• Biotite. Only in few petrological types biotite was determined.

• Muscovite. It occurs in the chloritic schists and phyllites. In the hyaloclastites it comes often by the metasomatism of pyroxene with clear chlorite pseudomorphs after pyroxene. Anomalous interference colours are observed in some chlorites (penninite?)

• Calcite
• Dolomite
• Albite

• Glaucophane. It occurs in the glaucophane - epidote - chlorite schist of Athenios but in such very small crystals that the determination of its optical properties was impossible.

It occurs also in the metadolerite of Athenios, where it is formed, replacing the hornblende. Its optical properties are:

(-) 2V = 28^o for the red light

(-) 2V = 40^o for the violet light   r<υ and optic axial plane (010)

The small optic axial angle and the position of the optic axial plane calls for crossite.

DEFINITION OF THE PETROGRAPHIC FACIES OF METAMORPHISM

In the above described petrographic types, the following index-mineral-assemblages were determined:

.   Calcite - dolomite - antigorite - stilpnomelane

.   Epidote - actinolite - hornblende - chlorite - pyroxene - albite

.   Glaucophane - albite - epidote - chlorite

.   Albite - crossite

.   Stilpnomelane - quartz

The critical minerals formed are: muscovite, chlorite, stilpnomelane, antigorite, albite, horneblende, and locally crossite, glaucophane and epidote.

The mineral composition of the above assemblages and especially the presence of antigorite and stilpnomelane, defines the metamorphic facies belonging to the greenschist facies.

The existence of stilpnomelane in the later penetrated granite porphyries shows also their low-grade metamorphism.

C: GRANITE PORPHYRIES

In the many places in the metamorphic system of Athenios, ring-dikes of acid granitic rocks are observed.

They are rocks of light gray to green color and concrete texture. Phenocrysts of feldspars and quartz are seen macroscopically. These vein outcrops cross irregularly the metamorphic system, with a thickness of 3 to 8 m.

Their greatest occurrence is located near the old stairs, approximately 60m north of it. Reck mentions it incorrectly as a hypersthene andesite.

Under the microscope they show a holocrystalline porphyritic structure and the same mineralogical composition and therefore all of them belong to the same petrological type of the granite porphyry.

Mineralogical composition of the granite porphyries

• Plagioclase. The plagioclase crystals show often a zonal growth. Their percentage in anorthite changes from An₂₇ to An₃₅. In some less acidic veins it approaches An₄₅ in the nucleus. It appears often completely alterated to sericite.

• Alkali feldspar. The alkali feldspar is represented by sanidine. It occurs in big phenocrysts and twins (carlsbad twinning) as well as in smaller crystals in the groundmass.

(-) 2V = 20 - 25^o.

Inclusion of plagioclase and biotite are common.

• Quartz. It appears in big phenocrysts frequently idiomorphic or in rounded grains. Liquid inclusions of both water and CO₂ are especially common. Gaseous bubbles often accompany the liquid inclusions.

The relatively large magnitude of the crystals of quartz, the magmatic corrosion and reaction rims composed of feldspar, which often surrounds them, strengthen the belief that the quartz crystals are xenocrysts.

• Biotite. The main femic material of the rock appears in phenocrysts as well as in smaller crystals, frequently alterated in chlorite. It contains inclusions of feldspars, magnetite and needles of rutile.

• Calcite. It appears sometimes in a very restricted proportion.

• Orthite. Observed with a characteristic strong pleochroism.

• Stilpnomelane. Found in poryphyries shows the low-grade metamorphism of the rock.

PETROCHEMICAL OBSERVATIONS

In Table 2 are presented 6 chemical analyses of granite prophyries from various vein intrusions, and in Table 3 the Niggli-values, the "basis", the magmatic types, as well as the respective norms. The graphic representation of the values of Q, L, M, π, and γ, as well as the k - mg velues, are illustrated on the triangle Q, L, M and the k - mg diagram (fig. 2 and 3).

From the petrochemical calculations and graphic representations it is clear that these rocks belong to the weak Mediterranean type, the "Yellowstone Park" type, according to Burri.

According to Streckeisen the granite porphyries are classified among the granitic rocks (Fig. 4).

Even so, the very high value of Si (276 - 316) and K (.63 - .93) in combination with the microscopic investigation, show that the phenomena of assimilation and anatexis played a very important role during the formation of these rocks. One could admit that the genesis of these acid rocks is connected with the phenomena of anatexis of the upper mantle, as well as with contamination by material of nonvolcanic phyllitic origin, probably as a result of the movement of the tectonic plates in the Aegean area.

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