I visited a prominent dark-colored knoll in Cyprus which is mostly composed of olivine. It is probably made of mantle material, right? Ancually no, it is not. It seems to be part of a lava flow.
The knoll is located near village Margi (15 km S of Nicosia). Olivine makes up about 60% of the rock in places but it grades out relatively quickly to near zero upwards which indicates that olivine phenocrysts probably formed by gravitational settling and accumulation in a topographic depression which acted as a trap to the more dense olivine crystals. It would be very difficult to find another explanation because lavas that rich in olivine are not known at present. Ultramafic lavas did exist in the geological past when the Earth’s heatflow was much higher. Such ultramafic lavas are known as komatiites but they mostly formed in the Archean. Since that time, Earth has cooled considerably and lavas with ultramafic composition can not form (there is not enough heat to melt large enough part of mantle rocks).
That may be all fine and understandable but one thing is unclear for me. How should I name such rocks? They are not basaltic rocks according to the TAS diagram. Is this diagram applicable in this case or should I just say that it is olivine cumulate lava without attempting to name it more precisely? Anyway, this seems to be yet another example that defiantly demonstrates how many pitfalls our common classification schemes contain.
Example of a relief inversion? This knoll parhaps once filled a topographic depression.
Closer looks reveales that the rocks are dark green because of abundant olivine phenocrysts about a millimeter in size.
Obviously, I am not the first geologist here. Holes left by coring are easily spotted and in my opinion represent a problem which deserves a longer discussion. I do not think that there is something wrong with taking samples for scientific study but I definitely think that it should be used only if absolutely necessary. And maybe some efforts should be made to hide these holes. Otherwise, geologists in the long run risk the anger of public for their reckless “littering” and distructing of natural sights which will remain visible for a long time. I’ve seen such holes in many places.
Here is another example (five coring holes). Can someone help to explain what kind of structure it is and how to interpret it?
I am not sure it is correct interpretation but I think that these layers might be individual lava flows?
What makes you think it is lava? The layering is consistent with being part of a cumulate intrusion. That would explain the variation in mineralogy as well.
The structures could be related to serpentinisation
I didn’t accidentally find this knoll. It was mentioned in a field geology guidebook as an ultramafic lava. It is also far away from deep intrusions. These intrusions are in the Troodos mountains but this knoll is not.
Very interesting.
I didn’t doubt you BTW, just interested to know more. Looking at the photos on my phone they did remind me of olivine gabbros I’ve known.
I think the petrological term you’re looking for is “picrite” or “picritic basalt”: http://en.wikipedia.org/wiki/Picrite_basalt
Simon, it is somewhat awkward looking for a typical lava indeed and there are still unanswered questions for me. As usually, most of these questions tend to emerge later when I can’t anymore seek answers in situ.
Yes, ‘picrite’ is probably acceptable, although this term also has its restrictions. In this particular case the lava may be even more ultramafic than is allowed for picritic basalt.
During my undergrad trip around Cyprus, our group stopped at this exact hill! I can’t find my field notes at the moment, but all I know from a labeled photo is that we did call it a “picritic basalt hill”.
It is a picrite hill and is very much part of the ophiolite sequence that makes up the Troodos mountains.