Report by J. Schauroth
Introduction: The Troodos Ophiolite
The Mediterranean Sea is a remainder of a once much larger basin of the Tethys Ocean, which was closed off by plate tectonic movements over the last 100 million years. These movements continue today and will eventually lead to a complete closure in the next few million years, resulting in a mountain range similar to the Himalaya.
The Troodos mountain range is also the result of such movements. Formed in a complex seafloor spreading scenario where two pieces of oceanic lithospheres drifted apart, creating a gap for magma to rise up from the deep liquid mantle.
The rise of magma during spreading and its consequent solidification results in the formation of ophiolites, a sequence of geological layering. The word ophiolite originates from the Ancient Greek words ὄφις (snake) and λῐ́θος (stone) and is hinting towards the typical shimmering green and blue rock colours found in ophiolites.
The uppermost layers are part of the oceanic crust and reach depths of up to 6 km, but the underlying layers come from much deeper, belonging to the mantle. These mantle rocks are formed under high pressure and temperature at the bottom of the spreading centre, where melts from the mantle solidify. As these layers rise to the surface, their sequence is reversed, so that the deepest layers are found at the peak of the ophiolite instead.
The Troodos ophiolite is unique in the world for its exposure of the entire layered ophiolite sequence at the Earth’s surface, attracting geologists internationally.
Lithologies of Four Selected Vineyards
Tsiakkas winery and the three vineyards mentioned in this report are all located in the Troodos ophiolite. Two of the locations are within the sheeted dyke complex, one in the gabbro complex and one at the transition between both. Before going into the details of the four locations, ensuing is a detailed description of the lithologies of gabbro and sheeted dyke.
Gabbro is a plutonic rock, describing a magmatic rock that is formed from a melt that solidifies under the surface in a large magma reservoir (a pluton) and never extruded to the surface. Early ideas of magma reservoirs described a single large body within the crust. Today, it is understood that magma reservoirs in spreading centres are complex and constitute several bodies in different layers, which evolve into each other (fig.3).
Once the magma settles in the reservoir, temperature starts to decrease and crystals form. Due to their higher density, these crystals settle towards the bottom of the reservoir where gabbro forms. The mineralogy of the crystals that are growing is governed by the chemistry of the melt. In the case of the Troodos ophiolite, the magma reservoirs were fed by melts originating from the mantle. These mantle melts are relatively poor in Silicone and rich in Calcium, Natrium, Magnesium, Iron and Aluminum. At the other end of the range are granites, which are also plutonic rocks, but resulting from the melting of continental crust, enriched in Silicone and depleted of heavy elements.
Due to very slow cooling rates inside magma reservoirs, crystals have time to grow and can reach sizes up to a few centimetres. Gabbros are completely crystalline, which means their whole network is built by crystals. The mineralogy is plagioclase (40 – 75%), clinopyroxene (25 – 60%), orthopyroxene (<25%) and olivine (<25%).
The sample (fig.4) taken from the Kerami vineyard shows the crystal network in gabbro. The white crystals are feldspars, specifically plagioclase, and the black crystals are pyroxenes. Due to this network that formed under high pressures at depth, the crystals are tightly packed and gabbros tend to be impermeable with very low porosities. Partly weathered gabbros become crumbly and may retain water, and therefore can provide fertile grounds for cultivation.
The sheeted dyke complex is by far the most abundant exposed sequence of the Troodos ophiolite. In the ophiolite sequence it lies above the gabbro, with the dykes being the pathways of magma extruding out the reservoir. These dykes are formed when fractures are formed during the seafloor spreading process. Magma rises through the fractures towards the surface, where it erupts on the seafloor in the form of pillow lavas. The vast majority of magma however does not reach the surface and cools down within the dykes.
Dyke diameters can vary from a few centimetres up to a few metres (Fig. 5). The contact to the plutonic sequence of the gabbro is complex, in most exposures plutonic rocks also intrude sheeted dykes, indicating that construction of the sheeted dyke was almost complete before the intrusion of the pluton ceased. The sheeted dyke complex was formed by repeated injection of younger dykes into older ones, in most locations to the complete exclusion of any other country rock (previously present rock).
The sheeted dyke complex is composed of diabase (from the Greek word διάβαση – the crossing), which, like gabbro, is a mafic rock. In fact, the chemistry and mineralogy of diabase are identical with gabbro, low in silica, with the primary crystals being feldspars and pyroxenes.
The difference between gabbro and diabase are the crystal sizes. As the cooling rates are much faster within the small scale dykes compared to the large magma reservoirs, the size of the crystals are much smaller in diabase than in gabbro (Fig. 6). In some cases crystals are too small to be seen by eye and a hand lens is necessary. Very often the outer surface of the dykes cooled down so fast, it formed a ‘chilled margin’, that is a zone of up to a few millimetres where the cooling rates were too fast for any crystals to grow and the whole area is made up of glass (Fig. 6).
The orientation of the sheeted dykes is roughly N-S throughout the whole ophiolite complex, which is a result of the 90° rotation of the Troodos microplate, as the spreading axis was oriented E-W beforehand. Their inclination varies from nearly vertical in most cases to very shallow in some cases. Unlike gabbros, sheeted dykes therefore form very steep hills, which can result in poor soil cover. Diabase itself is massive with low porosity and therefore relatively impermeable. However, the fractures and dyke contacts provide pathways for permeable flow and water drainage, therefore the sheeted dyke complex generally promotes poor water storage. At the contact of sheeted dykes to gabbro, water is drained off and many springs can be found (for example in the village of Kakopetria).
The Kerami vineyard is located between Fylagra and the village of Amiantos, on a southwest facing hill. At its base is a stream, which originates at a landslide about 2 kilometres away from the vineyard (Fig. 8). Just north of the landslide is the inactive mine of Amiantos, where asbestos was mined until 1988.
On the geological map the vineyard lies within the gabbro. In situ rocks surrounding this vineyard were found at the southern and western edges, as well as in the riverbed. The dominating lithology is gabbro with varying crystal sizes, primarily feldspar (white – clear) and pyroxenes (black). Some crystals are up to a few centimetres big, indicating very slow cooling rates. In the dry stone walls, some rocks with very fine grained, black veins were found.
Although the rocks from the walls are not in situ, it was assumed they are from within close vicinity. The hills surrounding the vineyard are smooth with gentle slopes and no steep cliffs, indicating that it is indeed gabbro that constitutes the close surroundings.
In the riverbed, some external lithologies were found, such as a sample with asbestos veins and a chalcopyrite bearing rock. Another sample shows signs of high grade metamorphism (deformation) with a slight blue colour, the minerals are suspected to be muscovite (golden brown mica) and cordierite (blue mineral present in high grade metamorphic rocks). These rocks must have originated upstream at the landslide scarp and then transported down to the vineyard’s location. Even if those high pressure rocks are not in situ at the vineyard, due to the presence of a variety of lithologies, bringing with it a variety in minerals and therefore elements, it is suggested that the soil of this area should have increased fertility.
Agia Kiriaki & Petralona
The vineyard of Agia Kiriaki is located east of the village of Agros. On the geological map it is located within the sheeted dyke complex, close to an inferred fault. A fault describes a discontinuity within the rock sequence. An inferred fault means that no actual exposure of the fault was identified, however, during mapping the geologists found evidence supporting its existence.
When approaching the vineyard, the larger, more prominent dykes are visible from a distance (Fig. 10). In the area many in situ rock exposures were located, all of them confirmed that this area is indeed located in the sheeted dyke complex. The dykes vary in diameter from a few millimetres to a few metres. Most are fine grained with crystal sizes of less than 1 millimetre.
Many exposures show evidence of multiple injections, with dykes cross cutting each other. Glassy rims, bearing witness of rapid cooling are frequently observed. Generally, the observation was made that the smaller the grain size, the more massive, tough and weather resistant the material is.
Higher up, at the northern edge of the vineyard, many of the diabase surfaces are covered in big, white, flaky crystals; these are secondary plagioclase crystals, suspected to have grown in veins with circulating hydrothermal fluids. Veins are common in areas with faults, as faulting creates fractures and thereby creates new pathways for hydrothermal fluid flow, resulting in mineral deposition.
The Petralona vineyard is located 1 km to the north of Agia Kiriaki. On the geological map it is located within the sheeted dyke complex. The lithologies at Petralona are very similar to Agia Kiriaki. The dykes are of varying diameters, from centimetres to metres, with the predominant lithology being fine grained. Chilled, glassy margins are common.
Tsiakkas Winery Vineyard
Out of the four locations, the geology surrounding the winery seems to be the most complex. On the geological map, the winery is located on the border between the sheeted dyke complex and gabbro. The vineyards rise about 150m from the base of the hill up to the winery. At all levels, the rocks are altered and heavily cross-cut by veins, indicating that at this location there might have been some tectonic activity after the emplacement of the ophiolite.
At the lower parts of the vineyard, the dykes are not straight like in the other vineyards. They are bent and deformed, often in a plastic-like manner. (fig.13)
The grain sizes vary from glassy, to fine-grained, to coarse-grained. The coarse-grained lithology resembles the gabbro at the Kerami vineyard. This could be a contact of gabbro and sheeted dykes (Fig. 14). Due to heavy erosion, especially of the coarse grained rocks, it is not possible to say if the gabbro intruded the dykes or vice versa. Going higher up in elevation, an outcrop of heavily altered dykes is observed, which is full with white veins. Those veins are a secondary formation and hint towards tectonic activity enabling circulation of hydrothermal fluids. Some dykes are bent, some are offset, some are crosscut, generally all of them are heavily weathered, crumbly and prone to erosion (Fig. 15, 16).
Generally, the fine grained lithologies seem to be more resistant to erosion. Some of the exposed rocks are covered with white secondary crystals, which stem from hydrothermal fluids circulating in permeable pathways. These are zeolite crystals, which are aluminosilicates and commonly found in veins in mafic rocks.
Conclusions and key facts
All four vineyards are located in the ophiolite, which is unique in the world for both its completeness of the horizontal rock sequences, as well as the complete exposure at today’s surface. This means that the vines are growing on rocks that are normally found kilometres deep under the Earth’s surface.
The gabbro does not exhibit any layering and its crystal network is randomly distributed. In addition, the heavy tectonic deformation led to the rock being crumbly and brittle, and as a consequence roots can grow deeply with ease.
This tectonic deformation also affected the sheeted dyke complex. While the layers of the dykes are still visible, white mineral veins spread through most rock exposures. In many places it can be observed that the roots follow these veins, which presumably provide easy pathways that are also more nutritious.
The white mineral veins mostly observed around the vineyards of the Tsiakkas winery are composed of zeolites. Zeolites provide open cavities in the form of channels and cages, and therefore are able to easily exchange cations. The internal surface area of these channels and cages are reported to reach as much as several hundred square metres per gram of zeolite, making zeolites extremely effective ion exchangers. Consequently, they have multiple beneficial properties for agriculture. They are able to regulate acidic and alkaline soils to neutral, they provide decontamination of cations, they increase crop yield and promote efficient nutrient use. Other possible uses being investigated include applications as a carrier of slow-release fertilisers, insecticides, fungicides, and herbicides.
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