Harrow and Hillingdon Geological Society
Naxos: geological evolution of a gneiss dome
Naxos, north of Santorini, is the largest island in the Aegean Sea. The speaker has had 15 or 16 trips to the island with final year undergraduates at the University of Bristol because the geological history of the island can be determined in the field with nothing more than a hand lens.
Gneiss domes are abundant around the world in orogenic belts. They are structural domes cored by high-grade metamorphic rocks – migmatites, granitoids and gneisses – and range in age from Archaean to the present day. Naxos is a modern example, with uplift about 10 million years (MY) ago. Vertical and lateral flow of the crust to create the dome structure has redistributed heat and material within the crust. This is an important process in crustal evolution.
With the African plate to the south and the Eurasian plate to the north, there is a major plate boundary in the Mediterranean Sea. The area suffered compression throughout the Mesozoic and Tertiary, with the African plate subducting beneath the Eurasian plate. There are now only remnants of the Mesozoic ocean floor north of Egypt and Libya. The Aegean is undergoing extension. It is a very active seismic area with concentrations on the arc-like southern boundary (the plate margin) to the south of Crete and the North Anatolian Fault in northern Turkey and a scatter of earthquakes through the Aegean Sea. The sea floor has huge fractures/trenches marking the position of faults and Turkey is being pushed westwards by the Arabian plate. From 200MY to 100MY the African plate moved eastwards and then turned northwards, pivoting on Gibraltar. Naxos sediments were deposited in the Neo-Tethys Ocean.
Overview of geology
Naxos is now all metamorphic rocks. Extensive Mesozoic limestones have been converted to marble and they are interbedded with schists. The Naxos dome has a migmatite/gneiss core. The west of the island has a large granodiorite body. Overall Naxos has a relatively simple structural pattern, with the metasediments dipping in all directions from the central dome.
The Granodiorite has a very sharp contact with the gneisses and is full of large mafic enclaves, which are not xenoliths but fragments of early differentiated magma. The granodiorite is very coarse-grained, indicative of slow cooling because it was intruded into hot rocks (the gneisses). It represents the roots of a volcano, ie the magma chamber.
Economic deposits are principally marble and there are corundum deposits ((not gem-quality rubies), which have been mined for centuries.
The main rock types are Mesozoic limestones and detrital sediments, which show evidence of repeated exposure to the atmosphere with the formation of soils. The classic metamorphic sequence is seen of slate – phyllite – schist – gneiss – migmatite, with the diagnostic minerals chlorite – biotite (melts at 400oC) – garnet (melts at 500oC) – kyanite – sillimanite (melts at 700oC+). The sequence indicates partial melting to form migmatites at between 40 and 20km depth.
The outer parts of the island have fine-grained rocks with biotite and garnet in the metasediments, illustrating the compression/burial stage with intense isoclinal folding. The schistose rocks are coarser but still tightly folded with mica, garnet and the aluminium silicates kyanite and sillimanite. The gneissose rocks lack mica and have much coarser banding. The migmatite core is very heterogeneous and some parts have the original gneissose banding. Partial melting is shown by light-coloured patches of quartz and feldspar (leucosome), which have been melted on a very small scale and darker patches of biotite, which has not melted, as a melanosome residue. At 30km depth, quartz and feldspar melt at 650oC but biotite does not melt until 900oC.
The chemistry of the source material is important. Muscovite melts early and produces water, which aids melting of other minerals. If muscovite is lacking, rock which might otherwise melt at 700oC will not melt until 1,000oc+. Sitting in the middle of the island are lumps of marble, which does not melt until 1,400oC.
Aluminium silicates (Al2SiO5) comprise kyanite (named after its blue colour – kyanos being Greek for blue), white sillimanite and andalusite. Kyanite forms under high pressure, sillimanite at high temperatures and andalusite at lower pressures. The source material is kaolinite, the reaction being:
Al2Si2O5 → Al2SiO5 + SiO2 + 2H2O
The largest marble quarries are in the migmatite zone, where the marble has very coarse crystals because they reached the highest temperatures. The marble has been used in the Delos lions, at Dephi and in the Temple of Demeter on Naxos.
The corundum deposits are formed from bauxites, a type of tropical soil (Al(OH)3). SiO2 is leached from kaolinite to form bauxite then, as temperature and pressure increase, H2O is lost to form diaspore (Al(OH)), which converts to corundum (Al2O3, ruby red) as further water is lost. This provides evidence that the carbonates and muds were exposed to the atmosphere and repeatedly leached. There are emery boudinage deposits.
How did the rocks get back to the surface from 30-40km depth?
The 3 possible explanations are gravity, subduction roll-back/slab break-off and buoyancy.
There is evidence for roll-back in that the older volcanoes are in the north of the Aegean Sea and the younger ones in the south, which is what one would expect if the subduction angle increased. Break-off of the subducting slab would lead to a rebound effect.
Compression ceased and the rocks are now being stretched and there was buoyant uprise of the migmatite dome, leading to somewhat bizarre folding as the cover rocks fall of the dome. It is estimated that it took 5-7MY to come up from 30km depth.
There is evidence of the rocks passing through the brittle/ductile transition at 300oC with aligned kyanite along the shear zone and thin beds of limestone pulled into rods (tectonites).. Elsewhere there is lots of evidence of brittle deformation in normal faulting and apparent cleavage (fractures) in garnet. As temperature and pressure fall, kyanite converts to sillimanite and we can trace the pressure/temperature changes the rocks have undergone. As pressure and temperature rise, biotite converts to garnet then to kyanite and finally to sillimanite with reduced pressure.
There is evidence of repeated fault movement. Banded chert in the west of the island has a hydrothermal origin, with the fluids coming out of fractures as the dome rose. The granodiorite has pseudotachylite, a glass formed from frictional melting.
The rising dome had shed schists, marbles and gneiss but not the migmatite at the time more recent sediments were deposited.
Gneiss domes recycle deep crustal material to shallow level and all is visible in the field in Naxos with just a hand lens.
The last 8,000 years have seen human activities producing Apollon Kouros in the 6th century BC, the Flerio aqueduct at the end of the 6th century BC, the Panagia Drossiani chirch in the 7th century AD and Naxos town, which has been continually inhabited since 1207.