In the volcanic-sedimentary sequence (teschenite association, Outer Western Carpathians), layers of very fine-grained sediment with an extremely high microcline content (up to 80%, the rest is calcite) were found in shales in close contact with the effusive volcanic (highly altered mygdaloidal trachybasalt?). Feldspar is up to 5 μm in size, hypidiomorphic to idiomorphic. The rock lies clearly outside the volcanic body, but is also finely amygdaloidal.
Is it possible that it is a sediment from hydrothermal activity or what is it about ??? Is there any literature on similar rocks?
Dear Dr. Matysek,
It is always a bit difficult to hand down a remote-controlled expertise.
1. I was first pondering over the so-called keratophyre issue which also forms fine-grained ash tuff ?
It is in my opinion not a normal petrogenic but hydrothermal alteration process which brought about 80 % K feldspar together with calcite
2. is the XX shape in the last SEM image pointing to adularia XX? High K-feldspar contents may occur in the low sulfidation-type alteration (?)
3. I know abnormally high sanidine contents from hotspot volcanism taking place in the Mongolian plateau during the Tertiary and Quaternary. This vast eruptive igneous activity brought about mainly alkaline basanites and olivine basalts. There , sanidine formed instead of leucite. The rocks also are accountable for placers enriched in sanidine, olivine (“peridot”), garnet (enriched in pyrope) and last-but-least sparked the discovery of 50 micro-diamonds. The K2O/Na2O ratio is below 5 and did not qualify the area as highly prospective for diamond-bearing "lamproites". What is yours?
4. A reworking has to be excluded due to the fine grain-size (?). Are there agglomerates or breccias?
These are some thoughts which are still rather un-oriented in one or the other direction.
At the moment I would go along with points 3 and 4.
HGD
SUNKEL, G. (1990): Devonischer submariner Vulkanismus im Ostsauerlarid (Rheinisches Schiefergebirge): Vulkanautbau, Magmenzusammensetzung und Alteration.-Bochumer geol. u. geotechn. Arb., 34: 250 S., 96 Abb., 17 Tab., 11 Taf.; Bochum.
It could be, there are some informations in this publication wich can help you.
Thomas
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