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Generalour description of fenite is too restrictive

8th Jan 2018 02:29 GMTTony Peterson Expert

I was about to attempt to reclassify my photograph of a coarse wollastonite fenite from Ice River, but Mindat's definition is restricted to mimics of igneous syenites and ijolites. Wikipedia's definition is both incomplete and overbroad (not all skarns should be considered fenites!) but at least allows for a wider range of mineralogies. The final assemblage is a product of both the original wall rock composition (anything!!) and the origin of the fenitizing fluids.


The original definition from Fen, Norway, and followed up most importantly with studies of fenites in Africa by Michael LeBas, simply stated that fenites were igneous mimics derived by flooding wall rocks with (most importantly) alkalies, Ca-Ba etc. H2O, CO2, F, and other easily mobilized elements. Fenites are thus obviously common around carbonatites and alkaline silicate rocks, but are not restricted to that environment. The original description at Fen included a wide and important zone of melanite-bearing rocks; melanite is not even included in the mineral list. And neither is wollastonite, which is one of the most common minerals in fenites. I shall have to upload a photo of a spectacular comb-layered wollastonite fenite from the Archean Kaminak carbonatite, Nunavut, in my possession!


Unless I'm mistaken, I do have the ability to edit these rocks terms. But I don't see an edit button on the page. Could someone please remind me how this is done? And the entry has no references, either................!!@(^&$##*!!


Tony

8th Jan 2018 04:48 GMTPavel Kartashov Manager

Hi Tony,

it seems to me, that you yourself confusing fenites with skarns.

Fenites are alkaline metasomatites of magmatic stady. Earlier fenites are potassik (deposition of microcline), the later ones are sodic (aegirinization and albitization sometimes with feldspatoids deposition).

Fenites forms only over felsik rocks (shists, slates, quartzites, sandstones, gneisses, granites, syenites and similar rocks). Kontakts of alcaline igneous rocks with sedimentary/metamorphic carbonate rocks producing exactly skarns.

Wollastonite or melanite can't to be named typical minerals of fenites.


Some magnesial skarns able to contain some nepheline (for example Slyudyanka and Belaya Vyemka areas in Prebaikalie), what don't made them fenites sensu stricto.

8th Jan 2018 14:00 GMTTony Peterson Expert

Appreciated Pavel, but your statement about melanite is incorrect (I'm sick at home and need to get to the books in my office, so I'll get back to this later). My experience regarding wollastonite contradicts yours - perhaps things are different in Canada? (also a pretty large country).

8th Jan 2018 19:52 GMTPavel Kartashov Manager

04134390015651884758890.jpg
Country sizes don't influences on definition of scientific terms.


Wollastonite is typical mineral of calcic skarns (of low depth and moderate temperatures). In fenites you'll rather meet agrellite or pektolite instead of simple wollastonite.


Garnets of ugrandite row, especially in rock-forming quantities, also are typycally skarn minerals. Titanoan andradite don't contains alkali and isn't alkaline mineral (of fenite stage), but able to be of pre-fenite metamorfic assemblage member.


In southern Korea I observed fenitization of metaluminous shists stratum under influence of leucogranites intrusion. Biotite-plagioclase-quartz assemblage was changed by microcline-quartz-albite-magnetite-zircon fenites. Pokets of marbles within shists also were involved into this process and were replaced by andradite-melanite-calcite-wollastonite-hematite-fluorite skarns. It is interesting that epidote of shists was changed by aluminous ferriallanite in fenites (the main concentrator of Ca there) and Fe3+-rich ferriallanite in skarns.

10th Jan 2018 20:53 GMTRalph Bottrill Manager

Interesting, i’m no expert on these but like Pavel I was brought up to believe fenites were specifically alkali-enriched alteration zones formed by alkali metasomatise about certain intrusives. They can be associated with other types of alteration, eg. Iron and calcium rich zones but these are not strictly fenites, I think. Associated Ca rich rocks are usually called skarns, I’m not sure if there is a specific name for Ca-metasomatised rocks other than riding items or endoskarns, but that’s slightly different, where the intrusive gets Ca-metasomatised. I guess if in doubt we would usually describe such rocks by their constitution rather than implied genesis, eg. as an andradite quartz hornfels, if the origin and precursor is in doubt.


From the Glossary of geology we have: “A quartzo-feldspathic rock that has been altered by alkali metasomatism at the contact of a carbonatite intrusive complex. The process is called fenitization. Fenite is mostly alkalic feldspar, with some aegirine, subordinate alkali-hornblende, and accessory sphene and apatite.” I’m not sure “quartz’s-feldspathic” is always right, but I guess that’s a common assemblage.


Wikipedia says:”Fenite is a metasomatic alteration associated particularly with carbonatite intrusions and created, very rarely, by advanced carbon dioxide alteration (carbonation) of felsic and mafic rocks. Fenite alteration is known, but restricted in distribution, around high-temperature metamorphic talc carbonates, generally in the form of an aureole around ultramafic rocks. Such examples include biotite-rich zones, amphibolite-calcite-scapolite alteration and other unusual skarn assemblages. The process is called fenitization.” All this seems a little weird, with no mention of alkaline alteration or little on mineralogy. I have seen fenites (orthoclase-rich metasomatise mudstones) around syenites with no sign of CO2 alteration. I’m also confused again about the association with ultramafic rocks as again where are the alkalis? Plus they suggest things like biotite rich zones are Skarns!?


The BGS metamorphic classifications describe fenites as K and Na-metasomatised and desilicated crustal rocks associated with alkaline intrusives, fair enough but a little vague.


You should be able to edit rock names as you do minerals, but before we change anything we do need to chase some references to see more recent definitions, not to hand at present as I’m on holidays.

10th Jan 2018 22:58 GMTPavel Kartashov Manager

A source of fluid for fenitization not necessary should to be represented only by carbonatites. Fenitization zones are known around nephelin-syenite massifs (Khibiny, Lovozero, Cherry Mts), alkaline granites (Verkhnee Espe, Khan Bogdo) or sometimes along abissal fault zones (without any intrusive alkaline rocks) as it is in Western-Uralian fenite zone.


This is fenitized precambrian quartzites from Northern boundary of Lovozero massif - their association is quartz >> aegirine> narsarsukite>magnesioriebeckite.

https://www.mindat.org/photo-394556.html

https://www.mindat.org/photo-394557.html

https://www.mindat.org/photo-215835.html

This is very similar fenites from Turii Mys massive - we see here more aegirine and pecktolite appears.

https://www.mindat.org/photo-387089.html

This is fenites after shists around alkaline granite intrusion - https://www.mindat.org/photo-205866.html

This is product of fenitization of usual granites under influence of alkaline syenites - https://www.mindat.org/photo-205429.html

This is fenitized by leucogranite intrusion biotite shists about which I wrote above - https://www.mindat.org/photo-142966.html

This is fenitization zone around deep fault zone without visible alkaline rocks close to them -

https://www.mindat.org/photo-74868.html

https://www.mindat.org/photo-104931.html

https://www.mindat.org/photo-202424.html

11th Jan 2018 04:50 GMTGeoff Van Horn Expert

Hi Pavel, can you delete the periods after html on you 5th, 6th and 7th example?

11th Jan 2018 10:16 GMTUwe Kolitsch Manager

Geoff: you can also delete the full stop in the browser line and hit enter.

11th Jan 2018 12:19 GMTPavel Kartashov Manager

Fixed.

11th Jan 2018 13:20 GMTRalph Bottrill Manager

Interesting rocks thanks Pavel, could you add "fenite" to the rock list for some of these please?
 
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