Maybe a metamorphosed evaporite. Difficult.

Thanks Brian.

Why metamorphosed evaporite? Not clear the connection among all these silicates and the typical mineral paragenesis to be found in an evaporite.

Michele

The composition is not consistent with an igneous rock. An evaporite with detrital input might give you carbonate plus the calcium in the andesine. Alternatively a carbonate-altered diorite that has been metamorphosed.

I might expect some hornblende in either of my above explanations. So it remains a mystery.

Hi Michele,

you meant 5% calcite, right?

Could this simply be a metamorphic rock that is not cut in the right way (i.e. parallel to lineation, perpendicular to foliation) or simply some granofels? I have seen stuff like that also in the didactic collections of Pisa. If you metamorphose an igneous rock with intermediate composition in the presence of water, plagioclase may survive, but K-feldspar may react with water and ferromagnesian minerals to form biotite. I would also expect some epidote around (maybe)

Another guess is some sort of cumulite with biotite and plagioclase. I do not know if such a rock is possible.

Can you get a bulk-rock analysis, even a qualitative one, e.g. a bulk extracted from an X-ray map of the thin section

The mineral orientation at the edge of the thin section not very pronouncing and due to the microfotographic technique applied during petrographic microscopy. The major constituents are plagioclase, biotite, and titanite found in a microporphyritic texture with a granular matrix. Is there any quartz and how much? It seems a magmatic rock which underwent some alteration, e.g., endoskarn owing to the abormal modal composition. The plagioclase composition also plays a role (?). As it stands it has to be named plagioclasite or anorthosite with all the mafic constituents transformed into biotite.

HGD

You may laugh, Michele, but what about "plagioclase-biotite rock"?

Lamprophyre: mica malchite ?

Brian P. J. Stevens : thanks for the comment. Anyway, it remains to understand why there is no trace of quartz and/or alkali feldspar, assuming your hypothesis of a mixed clastic/evaporitic protolith. Calcite is low (5%, not 50% as wrongly reported. Thanks to Samuele Papeschi Samuele for having pointed out my mistake) and there is aboslutely no sign of any relict of weathered mineral. All the minerals are 100% fresh. There is no amphibole at all. All the greenish stuff is fresh (not chloritized) biotite (I say this, because the interference colours are high as in pristine biotite). Striking is the relatively abundant presence (mostly

Thank you again Samuele Papeschi . What you propose sounds interesting. I have to investigate this possibility. What is surprising is that there is no trace of any relict of any K-feldspar. It is possible to have it being replaced by biotite, but I would more commonly expect the opposite (i.e., biotite transformed into K-feldspar with increasing metamorphic grade. The presence of andesine (determined optically only) implies relatively high temperatures. No traces of epidote have been found.

It is definitively not a granofels because the hand speciment shows some anisotropy.

I would exclude also a cumulitic origin, because of the distribution of biotite and mostly completely anhedral plagioclase crystals.

Above all it is to understand where do all such titanite and sparry calcite (unrelated with weathering) come from.

Michele Lustrino if andesite has been determined optically this opens up a lot of possibilities (maybe that is metamorphic oligoclase). When you see metamorphosed psammites, you commonly find metamorphic plagioclase, but K (that in the sedimentary protolith is stored in detrital feldspars and clay minerals) normally ends up in minerals such as white mica and biotite at higher grade, with K-feldspar relics being extremely rare. In my opinion this can happen also in a metamorphosed igneous rock but I would feel more confident if we had the bulk rock chemistry and some mineral chemistry, otherwise what I am saying is highly speculative!

Thank you Harald G. Dill .

There is no trace o quartz. Plagioclase is mostly anhedral occurring defining a sort of blastoporphyritic texture (i.e., resembling the presence of large subhedral relicts), together with more abundant anhedral, occasionally showing triple junctions finer-grained granoblasts. However triple junctions are never well developed, and plagioclase edges are commonly interdigitate.

I thought to a sort of skarns too, at least to explain some calcite and, partially, also titanite. However I am wondering where all this TiO2 can come from.

I have difficulty to name such a rock as plagioclasite for three reasons: 1) this is not a reccomended IUGS name; 2) no cpx has been found and 3) plagioclase is no more than 30-40% of the modal minerals. Similarly, anorthosites should have >90% anorthitic (not andesinic) plagioclase, which is very different from what I found.

Thanks Gavril Săbău ,

It is not a matter of laugh, of course. The task is to give the name of such a rock. First of all I have to define if the rock is igneous or metamorphic and, if it is a metamorphic rock, what was the composition of the protolith and what were the causes that led to this mineral paragenesis. Plagioclase-biotite rock adds little information.

Cheers,

michele

Thanks again Samuele Papeschi .

No muscovite nor quartz is present. For this reason I believe that a psammitic/pelitic origin for this rock is improblable/unrealistic.

michele

Thank you Vladimir Shpikerman

What do you mean with "mica malchite"?

Lamprophyre of diorite composition: andesine, biotite (instead of amphibole), accessory titanite

Lamprophyre of diorite composition: andesine, biotite (instead of amphibole), accessory titanite

Lamrophyre of diorite composition: andesine, biotite (instead of amphibole), accessory titanite

Dear Michele,

apparently it's all what we've got at this stage. The purpose of naming a rock is to be able to recognise it when we see it, and thereby know what we are talking about. Adding information, and how much of it, depends on whether we have it or not.

On the other hand, many people discourage using genetic terms in favour of descriptive ones; this also seems to me the soundest possible approach.

From one sample found in a drawer we cannot speculate about its formation, past history a.s.o., unless maybe if we've met with something similar before. Just to make a point, it's a long way from malchite or metaevaporite to endoskarn.

Good luck with it,

Gavril

Agree, in this case a descriptive name is the best path

Michele Lustrino mine was just an example, based on my experience. I was not saying that the origin of this rock is metapelitic/metapsammitic.

Vladimir Shpikerman : malchite is not an IUGS reccomended name. Anyway, this rock should have hornblende, labradorite and quartz, which are not present in the rocks I have reported.

OK! Mica(!) malchite: Pl An30-59, green biotite (instead of hornblende), rarely quartz, accessory minerals apatite, magnetite, titanite. Common malchite (not mica) andesine and hornblende. I'm not insisting, I'm just guessing. Malchite is a fine-grained rock; it is difficult to estimate the grain size in the attached photographs. Thank you, Michele Lustrino, and let's finish.

Dear all: I agree with Prof. Dill, this is a metasomatic igneous rock, which initially probably was hornblende diorite. The amphibole probably was altered to biotite with a sort of K-alkaline metasomatism. The pictures don't help, surely. Regards. Sebastian.