zpravodajství životního prostředí již od roku 1999

Ofiolity - to je Kypr

Ofiolity - to je Kypr
Článek pouze v angličtině

In Cyprus one has a unique opportunity to visit a "relatively" undisturbed section of the Earth's interior.  As a result of Africa's movement northward the island has been pushed up from from the depths of the Mediterranean and subsequent erosion has exposed a complete cross-section of the ocean crust and portions of the mantle.  The cross section below (modified and simplified from the Cyprus Geological Survey) goes from the mantle dunites all the way up into the sediments that buried the ocean floor.


Starting on the left of the cross section there are exposures of rock called Dunite.  This rock is a good model for the estimated composition of the upper mantle.  An ultramafic rock composed of iron-magnesium silicate minerals, this weathered sample surely does not display the character of the rock since it is primarily dark minerals.  Hopefully there will be some cross-sectional views of it here in the future. (the labeling is courtesy of the Cyprus Tourism Organisation's Atalanti nature trail that begins on the western side of the Troodos Square)

In some areas these regions have been faulted and solutions of economic minerals have invaded them.  Below is a vein of dark Chromite.

Such economic materials give rise to many mines in this area. Here a chromite mine.

Above the Dunite in the Mt Olympus area is a rock very familiar to those who walk the landscape of Marin, Serpentinite.  Serpentinite is a hydrated version of the Dunite...that is, it has had significant amounts of water added to it.  It also contains significant deposits of asbestos minerals that were once mined on the margins of the Troodos Mountains. The fibrous lines running through this rock are composed of Chrysotile asbestos.

This abandoned asbestos mine lies on the eastern side of Mt. Olympus in the Troodos Mts. and is being remediated to lessen the potential problems from the asbestos and the unstable slopes created by the mining.

The image below shows a transitional zone between the Dunite (the finer, non crystalline layers shown in the rock below) and crystalline plutonic rocks that overlie the Dunite.  Here it is a Pyroxenite in other places it is the Harzburgite. Eventually the Dunite plays out and the other lighter minerals take their place in the stratified mantle, eventually giving way to the lighter portions of what we call the crust.

Above the transition zone one reaches the bottom of the crust.  The first rock encountered is gabbro, seen below as a coarsely grained or pegmatitic gabbro.  It was surprising to me that many of the Gabbros are quite light in color with significant feldspars AND in some places there are actually plagioclase rich granitic rocks on the gabbros.

Above the gabbro there are dikes of gabbroic composition and are envisioned as the feeder zones to the surface eruptions of the basaltic rocks characteristic of the ocean floor. At this location on the northern side of the Troodos Mountains, all of the landscape is composed of dikes.

As you move upwards into the pillow lavas you see fewer and fewer dikes here a few cut across the landscape trending from upper left to lower right.  This is in the Alambra-Ayia Varvara area of the Cyprus archeological expedition that I worked during the summer of 1999.  It is in this type of setting that the historic copper mining of Cyprus is concentrated.

It is in mineralized sediments that are associated with the pillow sediment boundary that there are some very interesting occurrences. I had the luck of meeting Michele Morrisseau, who found hotspring fossils in Cyprus back in 1990. He loaned me a couple of the rock samples with fossils and sedimentary features to photograph. The first photo is a sample that contains a couple of tube worms according to the staff of the British Museum.  Keep in mind that this sample is nearly pure pyrite (look what happens if it is photographed with a flash). There is an article in the journal Geology; November 1999; v.27; no.11; p. 1027-1030 on these late Cretaceous fossils.  The second and third samples show pronounced layering of the pyrite, believed to have formed around ancient black smokers that were supporting the biological community.

A webpage from the UK's national museum of natural history on another sulfide deposit with similar fossils from a Silurian-age (430 million years ago) fossil assemblage from the Yaman Kasy VMS deposit in the Urals of Russia.Here is Michele and his son were showing me an excellent outcrop of the pillows, a closeup of the pillows and a couple of pictures of the interesting zeolite mineralization that have been formed within the pillows by the movement of the heated seawater, (#1 is a vertical view of the latticing, #2 is a side view, and #3 is a closeup of a spherical crystallizing zeolite mineral)

Michele also directed me to a mine that I might be able to find some samples. The following day I went to the abandoned Kampia mine.  An overview of the mineThe shoreline of the highly acidic mine lake.   A closeup of the shoreline mineralization upon evaporation. An outcrop of weathered pyrite that had the general appearance of a black smoker (it could have been due simply to weathering!).  A closeup of the surface showed some tube like structures...could it be a black smoker with tube worms? 

Moving upwards the pillows dominate, shown here in the upper pillow lavas near the expedition site. The upper pillows are hydrothermally altered and there are black smoker like deposits of manganese found on top of the pillows, rather like those in the Marin Headlands.  Jaspers are also common in the pillow lavas.

The pillow lava sediment boundary is pretty obvious, just notice how the road and road cuts changes color as it reaches the base of this hill.  The white is an overlying limestone that predominates in Cyprus.  There is very little chert like we have in Marin over the pillow lavas.

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