Troodos Ophiolite

The Troodos Mountain range is a fragment of an ancient oceanic crust, known worldwide as Troodos Ophiolite Complex. Its formation, as well as its subsequent uplift to its present position was implemented under complex geological processes, due to the convergence between the African and Eurasian tectonic plates. In the early 19th century, the term ophiolite, which is derived from the Greek words <<ophis and lithos>> that mean <<snake and rock >> respectively, was initially introduced to describe serpentinite rocks, whose general appearance resembles that of a green snake. Later, and especially since the 1960s with the acceptance of the plate tectonics theory, the term ophiolite is used to describe a series of mafic and ultramafic igneous rocks and chemical sediments. These rocks beginning from the upper towards the lower stratigraphic units are the following:

    a) Umbers interbedded with radiolarian cherts and mudstones.

    b) Volcanic rocks, mainly of Pillow Lava flows.

    c) Sheeted dykes, mainly of doleritic composition.

    d) Cumulate or plutonic rocks.

    e) Mantle sequence rocks.

The Troodos Ophiolite Complex has been formed approximately 92 million years ago (Upper Cretaceous time), along a small segment of seafloor spreading axis above a subduction zone of the African plate below the Eurasian plate. It is considered as the most stratigraphically complete, best exposed and best-preserved ophiolite sequence in the world (along with the Oman ophiolite). Since the 1960’s, it has attracted great interest from geoscientists from around the world. These ophiolites (Troodos and Oman) are members of an ophiolite sequence that runs along the northern boundary of the Arabian and African tectonic plates and marks the closure of the ancient Neotethys Ocean that existed in the area. The Mediterranean Sea is a remnant of this ancient ocean.

The Troodos Ophiolite has a characteristic elongated domal structure. The stratigraphy of the ophiolite shows a topographic inversion, with the rocks of the lower units outcropping around the highest peak of the mountain range, while the rocks of the upper units appear progressively towards the flanks of the mountain, creating successive ring-shaped exposures. This apparent inversion is related to the diapiric rising of its core and to the differential weathering and erosion of its rocks. A characteristic valley with an East-West direction that is exposed at the southern part of the mountain range, the Arakapas valley, is considered as a part of a fossilized transform fault.

The Pera Pedi Formation consists of umbers, radiolarites, and radiolarian mudstones, which constitute the first sediments deposited on top of the ophiolitic rocks and more specifically by infilling depressions on top of the pillow lava flows. The umbers are dark brown chemical sediments, that occur locally in small bodies of limited horizontal extend with a thickness of a few meters. The typical section near the Pera Pedi village, exhibits a thickness of 20 meters. In other areas of Cyprus, such as the Drapeia area, these deposits can reach a thickness of 35 meters. Umbers are fine-grained, layered, or massive sediments, which occasionally contain volcanoclastic material and radiolarite layers. In general, the umbers are formed on the ocean’s seafloor along mid-ocean ridges, where significant hydrothermal activity takes place. As a result, black smokers are formed that extrude hydrothermal fluids, rich in iron and manganese, at temperatures up to 350oC.

The volcanic rocks of Troodos consist of pillow lavas and lava flows. Based on the mineralogical composition, the color, and the ratio of dykes, the volcanic rocks are generally divided into the Upper Pillow Lava and the Lower Pillow Lava horizons. The Upper Pillow Lavas consist of 80-90% of Pillow Lavas and 10-20% of dykes. The composition of the lavas is basaltic with varieties of olivine basalts. Generally, the Upper Pillow Lavas exhibit a red surficial color due to the presence of iron oxides in the rocks. The Lower Pillow Lavas mainly consist of basalts and andesites, with a ratio of 1:1 of pillow lavas and dykes. In this horizon, the main sulphide ore bodies are identified. The maximum thickness of these two sequences is approximately 1.5 km, while their contact is not always clearly delimited. More detailed research indicated, that the volcanic rocks can be divided into more horizons based on their geochemistry. The pillow lavas have a characteristic spherical to ellipsoidal shape, with a more common diameter between 30 to 170 cm as a result of submarine volcanic activity. Their surface is glassy due to rapid cooling, while their interior is vesicular due to the gasses within the molten lava. The dykes represent the feeder channels of the overlying younger lavas. These dykes can be vertical, inclined or horizontal, with their number increasing towards the lower part of the volcanic rocks near the boundary with the Basal Group. The Basal Group is a transitional zone, marked by a large number of dykes that are intruding lavas at the lower part of the volcanic sequence, near the boundary with the Sheeted Dyke Complex. Its thickness, ranges from a few hundred meters up to 2.3 kilometers and is comprised by dykes (95-100%) and pillow lava flows (up to 5%).

The sheeted dyke complex (diabase) was formed by the solidification of the magma in the channels, through which it intruded from the magma chambers at the bottom of the oceanic crust, feeding at the same time the submarine extrusion of lava on the seafloor. This continuous process resulted in the creation of a series of parallel dykes made up of up to 100% dykes, which in-fill the empty space created by the divergence of the tectonic plates. The sheeted dyke complex outcrops nearly everywhere across the Troodos Mountain range, forming an elliptical ring, which surrounds the plutonic rocks of Mount Olympus and is surrounded by the extrusive rocks. The intrusive rocks are fine- to medium- crystalline, have a basaltic to doleritic composition with northwest to southeast direction, and are almost vertical, apart from regions affected by later tectonism phases.

The plutonic or cumulate rocks are the products of fractional crystallization and the subsequent concentration of the crystals at the bottom of the magma chamber beneath the zones of seafloor spreading. The main cumulate rocks include dunite, wehrlite, pyroxenite, gabbro, and plagiogranite, which are observed in small discontinuous occurrences.

Convection currents in the Earth’s mantle transfer heat and mass to the surface, due to vertical differences in temperature and density of the material. Temperature variations create unstable gravity conditions and vertical transportation of hot and cold material. Within the asthenosphere, in areas where the convection currents move upwards, provoking the partial melting of the mantle at a depth of approximately 60 kilometers, which in turn ascends forming the magma chambers at depths of about 4-6 kilometers below the ocean floor. The majority of these magma chambers are a dynamic open system, where magma is continuously entering from below and subsequently moves upwards through channels that simultaneously feed the submarine lava extrusions.

When the magma remained for long periods within the magma chambers, heat was lost due to its transfer to the surrounding rocks and by the influx of seawater. With the gradual drop in temperature, fractional crystallization of minerals began. Initially, the minerals olivine ((MgFe)2SiO4) and chromite (FeCr2O4) were crystallized and cumulate at the bottom of the magma chamber, forming the rock dunite as well as concentrations of chromite. With further cooling of the magma, crystallization, and settling of clinopyroxene (CaMgSi2O6) crystals together with olivine and chromite, formed the rock wehrlite. At higher levels of the magma chamber and with the continuous temperature drop the mineral plagioclase ((Ca,Na)(Al,Si)AlSi2O8) was crystalized, which together with olivine and clinopyroxene formed the different types of gabbroic rocks. The remaining magma within the magma chamber was now rich in silicon dioxide (SiO2) and with further crystallization formed the rock plagiogranite, which usually occurs in small bodies on the higher levels of gabbros and are often cross-cut by diabase dykes. Moreover, the fractional crystallization process was interrupted repeatedly due to the influx of new magma within the magma chambers, which resulted in the creation of alternating white and dark thin gabbroic layers.

The cumulate rocks occur in two different areas of the Troodos Mountain range, one around the highest peak of Olympus Mountain and the second at the Limassol Forest. Dunites are exposed around the Olympus Mountain, extending northward towards the village of Agios Nikolaos Kakopetrias, where the biggest chromite ore bodies were identified. Its thickness in this area ranges from 150 to 200 meters. It mainly consists of olivine minerals, however other minerals in smaller proportions could be present, such as chromite and clinopyroxene. Upwards, the dunite is succeeded by wehrlite with an intermediate zone of clinopyroxene-bearing dunite. Wehrlite consists of olivine (40-90%) and clinopyroxene (10-60%), with a small proportion of chromite (0-2%). Occasionally, the plagioclase mineral is identified within wehrlites with a proportion of up to 10%. Upwards, the wehrlite is succeeded by pyroxenite and finally by gabbro. Gabbro rocks include olivine-gabbro and pyroxene-gabbro depending on their mineralogical composition. At the higher stratigraphic levels of gabbros, small discontinuous bodies of plagiogranite occur, followed by the Sheeted Dyke Complex.

The mantle sequence is considered as the residual material, which remained after the partial melting of the upper mantle and created a rising magma of basaltic composition, from which the other units of the ophiolitic sequence were formed. It consists of approximately 90% of harzburgite, 10% of dunite, and up to 2% of chromite. The circulation of seawater, at temperatures lower than 500oC through harzburgite and dunite, caused the serpentinization of the original minerals (mainly olivine) up to 50-80%. As a result, these minerals transformed into serpentine minerals, such as antigorite, lizardite, and chrysotile, with a chemical composition of Mg3[Si2O5](OH)4. Asbestos mineralization appears in veins with fibers growing perpendicular to the direction of the veins.

The best exposures of harzburgite with dunite bodies are observed around the highest peak of Olympus Mountain. In the broader area south of the Amiantos Mine, the harzburgite rocks have been completely altered into serpentinite. Occasionally, the harzburgite has a “foliated” texture, due to the parallel arrangement of the orthopyroxene and chromite crystals. It can also exhibit a banded texture, characterized by alternating rich and poor bands of orthopyroxene minerals. These bands have a thickness of up to 10 cm and are of a few meters in length with distinct boundaries. This texture resulted from the deformation and recrystallization (in plastic conditions) of the upper mantle’s residual material, after the creation of the basaltic magma (at temperatures between 1,000-1,200oC). Such conditions prevail below the divergent boundaries of tectonic plates.

Sporadically, dunite bodies of various shapes and sizes are observed within the harzburgite. Within some of these dunite bodies economically exploitable chromite concentrations are identified. The majority of larger dunite bodies have an elongate shape with their longest dimension parallel, with respect to the foliation of the harzburgite. The foliation in dunite, is defined by chromite grains and is usually parallel with that of the adjacent harzburgite, indicating that the dunite was formed, then deformed and recrystallized together with the harzburgite.

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