The Secrets of Prof. Verschure's Rosetta Stones

[Andy Resnick]

This sample is from Honstjern:

Honstjern carbonatized damtjernite-like explosion breccia. This sample was obtained a few hundred meters away from the Tveitan carbonatized damtjernite-like explosion breccia. As before, this is a melanocratic aphanitic rock with nodular texture, many nodules have a core containing relic amphibole.

Carbonate vein and veinlets. Although the gross texture also resembles carbonatized damtjernite samples 74 Fen 50 N-6A, N-6B, and N-6C; there are significantly fewer opaques, biotite phenocrysts are absent, and chlorite is present. Many of the nodules have a complex interior structure:

Large feldspar xenocryst with amphibole inclusions, appears to have a dark carbonate reaction rim.

Groundmass is extremely fine-grained, consisting of carbonate, possibly biotite flakes, abundant apatite crystals, and innumerable minute rounded inclusions.

Edit: Ha! I forgot the mention the whole reason I selected this sample... Even though these two rocks (Honstjern and Tveitan carbonatized damtjernite-like explosion breccias) look identical and located so close to each other, K-Ar dating models place the two samples 200 million years apart. I'm not sure that puzzle was ever solved.

 

[Andy Resnick]

The Fen complex (about 1 square mile in size) has enormous diversity of rock types (I've counted between 15-17) and is the type locality for at least 6 of those. Here are 2 samples from type locality Holla, Telemark county, Norway:

Silicocarbonatite (Hollaite) from type locality Holla, Norway. Sometimes referred to as pyroxene søvite.

Euhedral prisms of pyroxene, anhedral grains of apatite, minor biotite set in a groundmass of coarse-grained anhedral interlocking crystals of calcite.

Euhedral prisms of pyroxene, anhedral grains of apatite, minor biotite set in a groundmass of coarse-grained anhedral interlocking crystals of calcite. The pyroxenes are pleochroic in shades of light yellow-green to apple-green, published analysis claims exist on the acmite-diopside-hedenbergite series. Unlike pyroxenes found in all the other rocks at Fen, the silicocarbonatite pyroxenes are only weakly zoned and show little grain-to-grain compositional variation.

There are a few euhedral prisms that I can't identify:

On the carbonate background, there are three distinct prisms: an elongated on emerging from the upper left corner and two stubby prisms along the lower third of the image. There are also some smaller fragments scattered. I think they are pyroxenes and the anomalous birefringence could indicate jadeite.

In one of the two samples, pyroxenes are enriched in Fe³⁺(dark green color) compared to the other.

 

[Andy Resnick]

This sample is (most likely) a Søvite.

Søvite is a carbonate rock of variable composition. The rock was called Søvite by Brogger (1921) after Søve farm and agricultural college. Ankerite and dolomite occur in variable amounts and in places can become quite dominant, in which case the rock is called rauhaugite. The mode of formation of the Søvite is one of the central problems of the Fen area and is important for the understanding of the genesis of the area as a whole and of peralkaline rocks in general. As best I can tell, the evidence that this rock is magmatic comes from how it occurs- in dykes and “cone sheets”- and the existence of an active carbonate volcano (Ol Doinyo Lengai, in Tanzania). The contrary evidence is that some rocks at Fen (the carbonated damtjernites, for example) indicate that Søvite could result from hydrothermal metasomatic replacement of silicates with carbonates. This particular sample has only zoned phlogopite (clear to orange pleochroism) and carbonate. Lack of apatite in noteworthy, and the phlogopite is more birefringent than "normal"- here, the birefringence extends to high third order, possibly due to the presence of titanium or iron.

I have to note that since I have no way to (optically) verify that the carbonate is calcite instead of dolomite/ankerite, this rock could in fact be rauhaugite.

 

[Andy Resnick]

I recently came across this website describing the history of the Fen complex (as a mining site) which told me the origin of all these samples:

"[From 1967-1971] Research group for rare earths (FSJ) carried out the first systematic survey of rare earths in the Fen Complex, mainly in Gruveåsen where the old iron mines were located. This was one of several periods of mapping and exploration activity in the Fen Complex during the Cold War. None of the campaigns ended with new mining, but contributed with research and increased knowledge of the field, which has become important later."

Another example of søvite:

Søvite is a carbonate rock of variable composition, occupying a large area in the eastern part of the Fen area. Mica minerals (typically phlogopite and tetraferriphlogopite), magnetite, pyrochlore and apatite are found in the Søvite in variable quantities. This particular sample has zoned tetraferriphlogopite (identification from the gold-yellow and orange-pink pleochroism) and minor amounts of apatite on a background of subhedral carbonate, likely calcite.

The presence of tetraferriphlogopite places the source location at either one of two quarries separated by just 450m: the Cappelen quarry (Søve Mines) or the Hydro quarry at Fen.

At some crystal orientations, high magnification views of the phlogopite booklets are striking- intricately patterned second-order interference colors.

Could this be an artifact of the grinding/polishing preparation? As another example, the birefringence pattern in those four adjacent large phlogopite crystals (shown above) is continuous even though the crystal orientations are vary discretely.

 

[Andy Resnick]

Another søvite:

Based on the distinct pleochroism patterns (gold-yellow and orange-pink for tetraferriphlogopite cores and gold-dark brown for the mantles), this sample seems to have tetraferriphlogopite mantled by some other variety of phlogopite.

My first guess, titanium-bearing phlogopite, is incorrect because that is found in Damtjern, and the phlogopite in those samples looks different, so it’s probably iron-bearing phlogopite.

The sample most likely has niobium-bearing pyrochlore (koppite) mostly converted to columbite.

While reading, I discovered that in addition to niobium-bearing pyrochlore (koppite), there are also rare earth element (REE)-bearing perovskites (knopite)…. pyrochlore-koppite/perovskite-knopite sounds like a strange double naming coincidence… in addition to those two, there is also a niobium-bearing perovskite (dysanalyte) and all three are likely present within the Fen complex. Plentiful rounded apatite crystals, and where there are small aggregations of fibrous amphibole (possibly stilpnomelane?) the carbonate is dominated by a mist of minute inclusions.

Opaques include illmenite, magnetite, and pyrite.

 

[Andy Resnick]

Moving on to another type-locality rock type, this is a meltegeite:

Melteigite, likely from type locality Melteig Farm, Nome, Telemark, Norway. Melteigite is part of the Ijolite series (urtite - ijolite - melteigite - vipetoite/Jacupirangite) which mainly consists of aegirine-augite and up to 30% nepheline. In fact, most of the rock types in the Fen complex are defined in terms of the mix of nepheline, pyroxene, and/or carbonatite: other than the disturbed crustal gneiss, there is rarely quartz or feldspar present in the rocks. Here, the nepheline has been completely altered. Nowadays, this rock type is classified as a melanocratic nephelinolite.

This sample has anhedral titanaugite (some with complex zoning) and subhedral nepheline (mostly pseudomorphed to chlorite , muscovite and epidote), minor amounts of carbonate and elongated, rounded, apatite crystals.

The presence of epidote implies a hydrothermal alteration process. In the following epi-darkfield image are common alteration products of nepheline: blue is sodalite, yellow (cancritine), and near the cancritine is a grain of apatite, kind of green-grey (to my eyes).

The chlorite shows anomalous birefringence: brown is typical of optically postive crystals, blue is typical of optically negative crystals)

 

[Andy Resnick]

Moving to the end of the ijolite series, this sample is a vipetoite:

Vipetoite, likely from type locality Vipeto Farm, Nome, Telemark, Norway. A local name for a variety of pyroxenite containing abundant titanian augite and hornblende with biotite, primary calcite, and occasional albite and nepheline. As pointed out by Sæther (1957) the name was actually misspelt by Brögger in the belief that the locality was named Vibeto instead of Vipeto. The correct spelling should have been vipetoite but vibetoite is in common usage. Synonymous with Jacupirangite, a variety of alkali pyroxenite consisting essentially of titanian augite with minor amounts of titanomagnetite, nepheline, apatite, perovskite and melanite garnet.

Rock texture is allotriomorphic granular, consisting of anhedral grains of pyroxenes, amphibole, carbonates, and abundant apatite.

Some grains have a poikilitic texture, pyroxene oikocrysts enclosing a variety of apatite, biotite, and carbonate chadacrysts.

Opaques likely pyrite and ilmenite (often associated with leucoxene).

 

[Andy Resnick]

Posting a day early today, this sample is an urtite/ijolite:

Urtite/ijolite adjacent to fenitized gneiss. The cutoff between the two is a nepheline concentration of 70%: more than that is an urtite, less than that is ijolite (and still less is melteigite, then vipetoite). Country rock component is fenitized gneiss, primarily perthite and chessboard albite in highly lobulated grains. Within the urtite component, there is a prominent carbonate veinlet. Nepheline largely altered to muscovite/sericite, pyroxene often altered to amphibole.

Large pyroxene grain partially altered to secondary amphibole (hydration reaction).

A relatively high concentration of sphene (possibly more than 10%)and as the boundary is approached, sphene is oxidized into opaques (illmenite and leucoxene).

Nepheline has been completely altered, mostly into muscovite. Near the carbonate veinlet are some photogenic features; here there is carbonate (calcite?) that transects altered nepheline:

 

[Andy Resnick]

This sample is (IMO) one of the highlights of the collection:

Rødberg. An altered carbonatite composed of hematite-stained granular calcite, dolomite and sometimes ankerite. Rødberg is only found in one location over the entire earth ( https://www.mindat.org/min-48029.html ). Rødberg is a source for Thorium and results from hydrothermal alteration of carbonatite. Perhaps surprisingly, it took me a long time to identify this sample because I am color blind- both PP and XP images, to my eyes, are very nearly black and white (or greyscale):

It wasn’t until I imaged using epi-darkfield that the vivid red color and distinctive texture became apparent:

Also, while the geological context of the word ‘pneumolysis’ is archaic (referring to petrological alterations due to gas emitted by magma), the medical use of the word is not, where it means “the surgical procedure of separating adhesions that bind the lung or lung tissue to the chest wall or other structures.” Indeed, the texture of hematite in this sample does remind me of alveoli, small structures within a lung.

 

[Andy Resnick]

For this sample (and the next few), I made use of the text “Igneous Rocks: A Classification and Glossary of Terms. (Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks), R.W. Le Maitre (Ed.)”, to match samples with lists of type-locality Fen rocks provided in a few publications. This one is Kamperite:

Kamperite. Type locality: Kamperhoug, Fen Complex, Telemark, Norway. Defined in “Igneous Rocks: A Classification and Glossary of Terms” as a local name for a medium- to fine-grained highly potassic dyke rock composed of almost equal amounts of orthoclase and biotite with minor oligoclase. It would now be defined modally as a K-feldspar syenite on the QAPF diagram.

The biotite is a late crystallization product. Some biotite grains have small inclusions of apatite, zircon, or epidote (possibly piemontite).

Minor opaques, possibly illmenite or magnetite.

 

[Andy Resnick]

This sample is Ringite:

Ringite. Type locality: Ringsevja, Fen Complex, Telemark, Norway. A local name for a coarse-grained carbonatite containing aegirine and alkali feldspar, considered to be a mixture of carbonatite and syenitic fenite.

Fenite (named for Fen, the type locality) is an alkali syenite found in the peripheral parts of the Fen area primarily consisting of alkali feldspar (here, likely albite and anorthoclase) and aegirine (which has replaced the original biotite during fenitization). Fenite can have a highly variable composition reflecting the variety both mineral chemistries and geologic processes that have occurred, and I will work through the many different fenite samples in due time. Meanwhile.... ringite. One of the interesting (to me) aspects of this sample is the ease with which the alkali feldspars can be distinguished, using either PP or epi-darkfield imaging.

The albite is mostly clear while the anorthoclase is milky/cloudy. This sample has several metal oxides, including (I think) lucoxene:

in addition to iron oxides:

The red hematite has an appearance similar to rodberg, here it is confined to the interstices of carbonate grains, again mimicking alveoli.

 

[Andy Resnick]

Continuing to identify a "legion of obscure rock types named after equally obscure European villages", this is Sannaite:

Sannaite. Type locality: Sannavand, Fen Complex, Telemark, Norway. A variety of lamprophyre composed of combinations of olivine, titanian augite, kaersutite (kaersutite is a dark brown to black double-chain calcic titanium-bearing amphibole) and Ti-rich biotite phenocrysts with alkali feldspar dominating over plagioclase in the groundmass which also contains nepheline. Defined within the lamprophyre classification, this set of samples appear to be sequential thin-section preparations from the same rock:

This is evidenced by, for example, the prominent (pseudomorphed) nepheline phenocryst in 52 ii and 52 iii and rounded amphibole phenocryst in the lower edge of 52 i and middle of 52 ii. These samples are strongly porphyritic with large phenocrysts of both amphibole and (pseudomorphed) nepheline, there are also, based on the hexagonal habit, smaller phenocrysts of (pseudomorphed) biotite. Any biotite that was present has been pseudomorphed to chlorite with inclusions of (relatively) coarse-grained leucoxene.

This rock does not appear to contain olivine or augite, so my assignment of sannaite could be incorrect. Even so, alkali feldspar >> plagioclase in the groundmass and possibly nepheline (altered to paragonite) in the groundmass as well.

The nepheline phenocryst has been pseudomorphed into fine sheaves of paragonite (muscovite).

Pac-Man is amphibole (kaersutite?) while his meal and eye is amphibole and/or biotite that has been altered to chlorite and leucoxene.

Possibly two small grains of anatase?

 

[Andy Resnick]

Switching over to a better-studied rock type: Fenites.

Fenite. Type locality: Fen Complex, Telemark, Norway. Fenite is an alkali syenite and is found in the peripheral parts of the Fen area with highly variable composition. From an economic perspective, fenites are enriched in rare earth elements and so are an attractive object to study. Metasomatism (chemical changes due to fluid interaction/transport) of the country rocks by the Fen Complex was named by Brøgger “fenitisierung” (fenitization) and the metasomatized gneiss “Fenit” (fenite), and this process has been observed at other localities on the earth. It has been shown that multiple (at least 3) fenitization events have occurred at Fen, with each instance corresponding to a “pulse” of magmatic-derived fluids that transport materials through brecciated rock, changing the chemistry by both leaching and deposition. Because metasomatism is highly dependent on both the original rock composition as well as the fenitizing fluids, fenites display a wide range of appearances and compositions (similar to the variability of lamprophyres at Fen).

The first fenitizing event (fenitization-1) produced aergirine and sodic amphibole at the expense of hornblende and biotite in country rock, the second event formed stilpnomelane and other oxides at the expense of the fenitization-1 minerals, and the third, caused by interaction between rocks and groundwater-derived hydrothermal fluids infiltrating the eastern part of the complex, led to oxidation of ferrocarbonatite into Rødberg.

This particular sample has been classified in a paper as exhibiting both weak fenitization-1 and weak fentization-2. Green pyroxenes (aegirine) and blue amphiboles (riebeckite, and arfvedsonite) are considered the most prominent fenitization minerals.

Fenitization-1 is characterized by the breakdown of primary hornblende and biotite. Both minerals show incipient formation of fine-grained reaction coronas, with an outer zone of aergirine and an inner zone of albite and/or microcline. The coronas occur only where hornblende and biotite crystals were in contact with quartz.

Primary feldspars (perthite and plagioclase), remain largely unaltered, except for the development of turbidity by very fine-grained mineral matter. Fenitization-1 minerals can range from less than 5% (weak) to greater than 10% (strong) and exceed 90% of the total rock volume.

Fenitization-2 is a later process resulting in the replacement of Fenitization-1 minerals, primary gneiss minerals, and also minerals of the Fen Complex (Ijolites and carbonatites). Fenitization-2 produced low-temperature crystallization of hydrous minerals (Na-amphiboles, new greenish biotite, new chlorite, sericite and stilpnomelane), carbonates, quartz and opaque minerals. Fenitization-2 minerals are fine grained and seldom exceed 10% of the total rock volume.

In the above images, transformation of the (green) pyroxene into (blue) amphibole can be seen, as can the growth of a few brownish stilpnomelane flakes.

Stilpnomelane often forms "bushy" borders with quartz:

Apatite, allanite, sphene and zircon occur as accessory minerals:

For the above images, starting at the top left-of-center is a grain of apatite. Moving around clockwise, we see the palimpsestic (a new favorite word!) replacement of (likely) hornblende into aegirine and albite (Fenitization-1) as well as opaque oxides (fenitization -2). Continuing around is a large grain of (ilkely) Allanite-(Nb), in the lower left corner is a square crystal of (likely) zircon, and more towards the center, an anhedral grain of (I think) sphene. The center of this image is dominated by a grain of perthite (and a crack in the sample). A small flake of biotite is in the upper left, and the lack of a reaction corona provides supporting evidence for the identifications of both apatite and feldspar.

I have at least a dozen thin sections that are uncovered (bare rock surface, no Canada balsam or coverglass) and so cannot be used for optical minerology. I’m not sure what to do with them- my priority is to not damage the samples- but I have been thinking about applying a thin film of immersion oil to the rock surface in order to hold in place a coverslip. The index of refraction of immersion oil very closely matches that of canada balsam.

In addition to the many samples of fenitized gneiss showing various degrees of fenitization, I also have samples of fenitized carbonatites and fenitized ijolites. Stay tuned!

 

[Andy Resnick]

Another fenite:

Fenite. Type locality: Fen Complex, Telemark, Norway. This sample was been classified as having moderate fenitization-1 and weak fenitization-2, so it preferentially shows the effects of fenitization-1 metasomatism. Moderate Fenitization-1 is characterized by the nearly complete breakdown of both hornblende and biotite by the formation of wide Na-pyroxene + albite- microcline reaction coronas.

Na-pyroxenes (aegirine) also form irregular patches and vein-like aggregates.

Primary plagioclase is partly replaced by fine-grained, clear aggregates of chessboard albite, this feature in particular is also diagnostic for fenitization:

Sæther (1957) called the formation of clear chessboard albite and microcline aggregates ‘matrix formation’. Primary quartz and microcline perthite are still present. Adjacent K-feldspar grains show albitization following along the boundary between grains (intergranular albite). Interestingly, each half of the albite seam has the same crystallographic orientation as the *opposite* K-feldspar, as can be visualized by inserting a quarter-waveplate when imaging XP:

The mechanism for this process (“dissolution–precipitation”) is the subject of a considerable body of literature, most of which I don't fully understand (yet).

 

[Andy Resnick]

Continuing with examples showing increasing levels of fenitization:

Fenite. Type locality: Fen Complex, Telemark, Norway. This sample was been classified as having strong fenitization-1 and weak fenitization-2, so it should preferentially show the effects of fenitization-1 metasomatism. While this sample’s fenitization-1 minerals comprise 24% of the total rock volume, future samples will show fenitization-1 minerals comprising up to 95% of the total rock volume. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase. This sample still has primary gneiss minerals (74% of the total rock volume, according to the paper). However, biotite and hornblende have been completely broken down. Coronas of Na-pyroxene surrounding chessboard albite may be present, but Na-pyroxene generally occurs as veins and irregular aggregates. The original gneissose texture is still recognizable in places. The strongly fenitized-1 rocks also contain clear, euhedral alkali feldspar crystals forming a matrix of chessboard albite and microcline without preferred orientation.

Fenitization-2 minerals are also present, comprising 2% of the rock volume. One example (below) is the breakdown of aegirine (Na-pyroxene) into arfvedsonite (sodium amphibole).

I used the large language model "NotebookLM" for optical criteria to distinguish riebeckite from arfvedsonite- both are sodium amphiboles, differing only in that riebeckite has 2 Na ions per unit cell while arfvedsonite has 3. Based on the training materials I uploaded, NotebookLM provided a few differences, primarily in terms of pleochroism. Based on the program output (and the original paper :) ), the blue amphibole in this sample is arfvedsonite.

 

[Andy Resnick]

Fenite. Type locality: Fen Complex, Telemark, Norway. Fenitization-1 minerals comprise 51% of the total rock volume of this sample, having been classified as having strong fenitization-1. In addition, it is classified as weak fenitization-2, so it should preferentially show the effects of fenitization-1 metasomatism. The macroscopic texture has been described as a “mortar texture”, with large phenocrysts of microcline perthite within a fine-grained groundmass of chessboard albite.

Strongly fenitized-1 rocks contain clear, euhedral alkali feldspar crystals forming a matrix of chessboard albite and microcline without preferred orientation. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase. Of the primary gneiss minerals, only microcline perthite remains (46% rock volume). What began as thin intergranular albite seams has now become thick bands of chessboard albite. The original gneissose texture is barely recognizable.

Na-pyroxene now occurs entirely as veins and irregular aggregates:

Fenitization-2 minerals comprise 1% of the rock volume. Three of those minerals: opaque oxides, calcite, and stilpnomelane (the thin seam), are shown here:

Lastly, a poorly-exposed photo of chessboard albite with a quarter wave plate (mica plate) inserted:

 

[Andy Resnick]

Continuing with strongly fenitized-1 gneisses:

Fenite (fenitized gneiss). Type locality: Fen Complex, Telemark, Norway. This sample’s fenitization-1 minerals comprise 83% of the total rock volume. This sample was been classified as having strong fenitization-1 and weak fenitization-2, so it should preferentially show the effects of fenitization-1 metasomatism. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase. Of the primary gneiss minerals, only microcline perthite remains (15% rock volume).

Fenitization-2 minerals comprise only 1% of the sample. Although the paper mentions ‘Fenitized-2 apatite’ is present as an accessory mineral, I think, based upon the optical properties in both transmitted and reflected light, is fenitized titanite, not fenitized apatite. The primary evidence is by comparing epi-darkfield images of this sample and of apatite, found in sample 67 Fen 23 ii- there are clear differences in appearance. The images below predominately show (I think) fenitized titanite (and pleochroic pyroxene).

This sample has a relatively high concentration of titanite- the paper claims 1%- and also claims they are xenocrysts. I'm not sure that's correct, but I'm not an authority on the subject...

Fenitization (defined as the leaching of SiO2 and enrichment of Na2O) is quite complex; I've been working through a more comprehensive paper "Carbonatite-related contact metasomatism in the Fen complex, Norway: effects and petrogenetic implications" (https://www.cambridge.org/core/jour...implications/00AA1631F359FD10A73327CB7BEEB65F). which is, to be honest, somewhat beyond my expertise. I'm working on it! :)

 

[Andy Resnick]

This sample is almost completely altered:

Fenite. Type locality: Fen Complex, Telemark, Norway. This sample was been classified as having strong fenitization-1 and weak fenitization-2, so it should preferentially show the effects of fenitization-1 metasomatism. This sample’s fenitization-1 minerals comprise 95% of the total rock volume. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase.

According to the relevant paper, the only remnant from the primary gneiss is accessory titanite (4% rock volume). That said, I believe the lightly dusted grains along the right side are primary gneiss (K-feldspar):

Fenitization-2 minerals comprise only 1% of the sample, and although the paper mentions ‘Fenitized-2 apatite’ [as accessory mineral] I think, based on the optical properties, is actually fenitized titanite.

Comparison of epi-darkfied images of apatite (67 Fen 23 ii) and these show clear differences. Worth mentioning here (for comparison later) is the habit of these titanite grains- anhedral with relatively rough edges.

Biotite present is classified as “Fenitized-2 Biotite”; optical appearance (clear to very light brown pleochroism, characteristic “birds-eye’ extinction pattern mostly absent) is markedly different from biotite found in either unaltered country rock or the phlogopite found within lamprophyres found within the Fen intrusion.

 

[Andy Resnick]

This is one of the more interesting samples in the collection:

Chilled margin of Ijolite intrusion. One side is fenitized gneiss, the other is fenitized melteigite.

The geniss was been classified as having strong fenitization-1 and weak fenitization-2, while the melteigite has been characterized as having strong fenitization-2. Strong Fenitization–1 is characterized by a major reduction in the amount, or nearly complete disappearance, of primary quartz and primary microcline perthite and the complete disappearance of primary plagioclase. Fenitization of the country rock (telemark gneiss), the first of several metasomatic events, was driven by the release of volatiles and fluids from the ijolite intrusion (on the other side of the sample) resulting in loss of SiO2 and Al2O3 and influx of Na/K/Ca-rich fluids (an alkaline reaction).

The relevant papers indicate that the melteigite was fenitized later, undergoing a low temperature hydrothermal metasomatic event. Evidence for this is the presence of stilpnomelane, a fenitization-2 mineral (not shown here). The melteigite is pierced by a mixture of albite, prehnite, and epidote- filled veinlets.

Nepheline (with pyroxene grains) has been completely altered to (most likely) a zeolite.

Here, the opaque shows evidence of metasomatism; it appears to be pyrite enclosed within a shell of magnetite:

In this sample, titanite (with epidote inclusions) has a significantly different appearance, as compared to the previous samples- note how the crystallites are much smoother and also potentially a slightly lower relief:

 

[Andy Resnick]

This is a very specific type of fenite:

Pulaskitic fenite. ‘Pulaskitic’ means this rock is classified as a nepheline-bearing syenite (field 7' on the QAPF diagram) composed of alkali feldspar (perthite) and nepheline (altered to sericite). From the relevant paper, “Close to the Fen Complex, a maximum 50 m-wide zone only rarely contains quartz, plagioclase or orthoclase/microcline. These rocks – fenites in the sense of Brøgger (1921) and probably similar to Kresten’s (1988) contact fenites – are mainly made up by aggregates of Na-pyroxene in a matrix of sub-to euhedral mesoperthite (in places with Carlsbad or Baveno twinning) with discontinuous intergranular rims of exsolved albite. The texture of these rocks is magmatic.”

 

[Andy Resnick]

The next fenitized gneiss example:

Fenitized gneiss. I’m not sure what feature was marked in blue pen- I didn’t observe anything unusual in that region. This sample is weak F1 and moderate F2, so alterations preferentially show effects of F2. This second metasomatic event is much less well defined (both in terms of originating fluid(s) and succession of mineral growth) as compared to F1. For example, whereas the source material for F1 is known (fluids and volatiles driven by the ijolite intrusion), the source material(s) driving F2 is unknown and may actually consist of successive pulses of fluids, each with differing metasomatic chemistries.

A later paper (https://www.cambridge.org/core/jour...implications/00AA1631F359FD10A73327CB7BEEB65F) subdivides the Fenitization-2 metasomatic event into discrete fenitization events. “Verschure and Maijer (1984) have distinguished between two metasomatic events in the fenite aureole, the first (here called A1) producing acmite or sodic amphibole, the second (A2) forming stilpnomelane at the expense of these minerals. A third, later metasomatic event of aureole extent (A3), was caused by interaction between rocks and groundwater-derived hydrothermal fluids infiltrating the eastern part of the complex, leading to oxidation of ferrocarbonatite to hematite carbonatite, locally known as 'Rødberg', i.e. 'red-rock’.”

Fenitization-2 (meaning both A2 and A3) is a hydrothermal process resulting in the replacement of Fenitization-1 minerals, primary gneiss minerals, and also minerals of the Fen Complex (Ijolites and carbonatites). Fenitization-2 produced low-temperature crystallization of hydrous minerals (Na-amphiboles, new greenish biotite, new chlorite, sericite and stilpnomelane), carbonates, quartz and opaque minerals. Fenitization-2 minerals are fine grained and seldom exceed 10% of the total rock volume.

These images show “Biotite replaced during Fenitization-2 by radiating needles of bluish Na-amphibole [A2], subsequently partially replaced by fine- grained magnetite and hematite. [A3] The replacement occurred where biotite was in contact with quartz. Other biotites within the same sample appear unaffected or completely replaced. The sample was taken 1750 m south of the Fen Complex, far outside the zone of Fenitization–1. Location of the sample: HSP post M00059, Økonomisk Kart Foreløpig Utgave 1971 (BW 020-5-1) coordinates: 138965-51753. “

Because fenitization is so variable, there's a lot of samples to work through. While the next several samples are examples of F2 (meaning both A2 and A3), there is another whole set of fenitization examples formed at the contact between intrusion and existing wall-rock (so-called "contact fenitization" and denoted C1 through C4), with the majority of contact fenitization occurring in carbonatites.

Lots to learn! #thinsectionthursday

 

[Andy Resnick]

Here's another example of a 'pulaskitic fenite', one that exemplifies the type:

Strongly Fenitized-2 Pulaskitic fenite sample from the Håtveittjørn Section, located 10m from contact between country rock and the Fen Complex. Classified as a strongly Fenitized-2 (low temperature hydration-carbonation, F2) fenite; acc Ap; F2 Op veins; 2% stilpnomelane in veins; 88% mesoperthite, 2% F2 Sphene, 2% sericite. Perthite unmixing texture in orthoclase grains with discrete albite rims (co-oriented albitization of K-feldspar). 2% F2 biotite, 2% F2 carbonate.

Below is F2 titanite (high relief, high backscatter), stilpnomelane (brown fibrous appearance) and feldspar:

Here is stilpnomelane, magnetite, and feldspar

The albitized K-feldspar is quite photogenic: this is within the region outlined in blue marker:

‘Pulaskitic’ means this rock is classified as a nepheline-bearing syenite composed of alkali feldspar (perthite) and nepheline (altered to sericite). Interestingly, unlike the previous examples of F1 gneisses shown before, in MA 68 the replacement of K-feldspar with albite is co-oriented rather than hetero-oriented. Hetero-oriented replacement occurs at grain boundaries between two minerals, while co-oriented replacement takes place inside the replaced mineral, a difference attributed to “intense sodium metasomatism”. F2 minerals include stilpnomelane, sericite, F2 carbonate and F2 titanite.

Perthite inclusions in sericite (altered nepheline?) do not show albitization, biotite inclusions altered to chlorite?

Veinlet of chloritoid (high relief, grey-green color, low birefringence)?

 

[Andy Resnick]

Here are 2 thin sections (presumably) from the same rock:

Fenitized Gneiss. Prof. Verschure classified this sample as weakly fenitized-1 (1% by volume) and moderately fenitized-2 (10% by volume). According to Andersen, this sample seems to exhibit A3 metasomatism rather than A2 metasomatism, evidenced both by the metasomatic replacements Albite + microcline + quartz + biotite + hornblende to Quartz + calcite + albite + chlorite + hematite and the lack of Na-amphibole.

Much of the primary gneiss remains intact- anhedral grains of quartz and plagioclase comprise about 50% of the rock volume, with primary perthite and additional 30%. What may have been vein-like aggregates of Na-pyroxene have been replaced by palimpsestic magnetite, calcite (possibly ferrocarbonatite), and quartz.

This sample presents one of the difficulties challenging easy understanding of the Fen complex geology: while veinlets of aegirine (Na-pyroxene) aggregates are characteristic of strong F1 metasomatism, there is significant primary gneiss present and almost a complete lack of K-feldspar albitization, a metasomatic alteration which is definitively characteristic of even weak F1 metasomatism.

Biotite has been completely replaced by chlorite and magnetite, and where primary biotite was in contact with quartz, an inner corona of (I think) diagenic quartz and outer halo of chlorite/opaques now exists.

Vershure reports that the biotite -> chlorite pseudomorph occurred pre-fenitization while the chlorite halos are F2 chlorite. That paper also claims that some opaques are associated with the primary gneiss but have been completely altered via F2 metasomatism, in addition to 3% post-F2 opaques. I am not sure if these claims are consistent with Andersen’s paper or not.