Prof. Verschure's thin section collection from the Fen and Hor localities

Join the discussion
Registration is free. Ask a follow-up in this thread, or start your own.
Sign up Log in
184 replies · 69K views · last reply
Thin section under cross-polarised light, labelled 14 FEN 283: upper part shows coarse interlocking blue and white crystals, lower part a fine-grained mass of multicoloured birefringent grains.


Melteigite (Ijolite) intrusion adjacent to fenitized country rock (telemark gneiss), Fen complex. The intrusion consists predominately of a dense mat of anhedral augite prisms, some zoned, with abundant euhedral titanite (sphene) and fine-grained mixtures of muscovite and (likely) aegirine, likely altered nepheline.

Thin section photomicrograph in plane-polarized light (left) and crossed polars (right), with a 1 mm scale bar at lower right; the crossed-polar view shows bright interference colours among grains in a dark matrix.


Side-by-side thin section micrographs: plane-polarized light on the left shows a pale tan feldspar grain beside a green altered mineral mass; crossed polars on the right show colourful green and yellow birefringent minerals against grey and black grains. Scale bar 1 mm.


Side-by-side thin section micrographs in plane-polarised light (left) and crossed polars (right), scale bar 100 µm; dense green mineral mat with brownish euhedral prisms and pale grains.


Side-by-side thin section micrographs at 1 mm scale, plane-polarised light on the left and crossed polars on the right, showing dense green pyroxene prisms with bright patches and small colourful birefringent grains.


Thin section photomicrograph in plane-polarised light showing a black opaque grain with irregular edges, surrounded by pale grey and green mineral grains with fractures, and a 100 µm scale bar at lower right.


Thin section photomicrograph under crossed polarised light with a 100 µm scale bar, showing large pinkish-brown and grey mineral grains with bright interference colours and a black opaque patch.


Side-by-side thin section micrographs in plane-polarised light (left) and crossed polars (right), scale bar 100 µm, showing a fine-grained mineral mat with green and colourless grains and bright interference colours.


Two side-by-side thin section micrographs, plane-polarised light on the left and cross-polarised light on the right, with a 100 µm scale bar at lower right; the right panel shows colourful interference patterns in fine-grained mineral aggregates.

The Telemark gneiss has been fenitized, evidenced by the perthic K-feldspar with weakly albitized borders. Some pyrite. A feature that appears to be a vein consists of carbonate (likely calcite) and chessboard albite.

Side-by-side thin section micrographs in plane-polarized light (left) and crossed polars (right), scale bar 1 mm; pale carbonate and albite patches with green and brown mineral grains and striped feldspar crystals.
 
Earth sciences news on Phys.org
Thin section photomicrograph under cross-polarised light of strongly fenitized gneiss, showing a mottled mosaic of bright blue, white and dark grains with a vertical discontinuity running down the centre. Paper label reads FEN 282 II.


Strongly fenitized gneiss, Fen Complex. Sample primarily consists of dusty K-feldspar, chessboard albite, and chlorite. Chlorite is olive green in PP and either low order or anomalous “Berlin blue” in XP.

Side-by-side thin section micrographs of fenitized gneiss in plane-polarised light (left) and crossed polars (right), with a 1 mm scale bar; grains show dark opaque minerals and olive-green chlorite.


Side-by-side thin section micrographs at the same scale, plane-polarised light on the left and crossed polars on the right, with a 1 mm scale bar. Olive-green chlorite and dusty brownish feldspar, with dark opaque grains and white-to-grey patches.


Side-by-side photomicrographs of a thin section, plane-polarised light on the left and cross-polarised light on the right, with a 1 mm scale bar; green chlorite and granular, dusty feldspar fill the field, with opaque grains scattered throughout.


Side-by-side thin section micrographs under plane-polarised light (left) and crossed polars (right), with a 1 mm scale bar; pale feldspar grains with dark opaque minerals and olive-green chlorite in the fenitized rock.


Abundant pyrite, Ilmenite and titanite/leucoxene. The titanium oxides monerals appear both as pseudomorphed euhedral prisms and as fine granulated dispersions.

Photomicrograph in plane-polarised light of a thin section of fenitized rock, showing clusters of pale grey angular grains with dark pits and orange-yellow patches; scale bar reads 1 mm.


Reflected-light photomicrograph with a 100 µm scale bar showing bright metallic grains, likely pyrite, with a golden-yellow grain and darker grey granular material between them.


Not sure how to interpret what appears as a discontinuity these images, down the midline:

Thin section photomicrograph in plane-polarised light of altered gneiss with pale green and yellowish mineral grains, with dark brown granular aggregates scattered across it; scale bar reads 100 µm.


Cross-polarised thin section micrograph at 100 µm scale showing fine interlocking feldspar grains with bright white and grey birefringence, scattered brownish granular aggregates, and a dark opaque mass at upper right.

Possibly a relic twin plane that is preserved by the orientation of chlorite booklets? The granular aggregates are titanium oxide, not sure which variety.
 
Thin section photomicrograph under crossed polars of a fenitized gneiss, showing a mosaic of bright blue-white and dark blue interference-coloured mineral grains with a dark crack running down the centre; a paper label reads Fen 282.


Strongly fenitized gneiss, Fen Complex. On the left side: large grains of chessboard albite.

Side-by-side thin section micrographs: plane-polarised light on the left showing pale grains with dark opaque patches and greenish alteration; crossed polars on the right showing bright and dark interference colours of laths and grains. Scale bar 1 mm.


Side-by-side thin section micrographs: left in plane-polarised light, pale grey-lilac with dark specks; right in cross-polarised light, showing blue-grey, grey and white interference colours and elongated, lath-like crystals. Scale bar: 1 mm.


Chlorite is light green in PP and either low order or anomalous brown in XP (in contrast to Fen 282 ii).

Side-by-side thin section micrographs in plane-polarised light (left) and cross-polarised light (right), scale bar 100 µm; green chloritic matrix with large pale grains, one showing twinning striations.


Side-by-side thin section micrographs in plane-polarised light (left) and crossed polars (right) at 100 µm scale, showing a green altered crystal aggregate with radiating, fan-like internal textures surrounded by pale grains.


In places, anhedral grains of cancrinite (clear in PP, low relief, and 3rd order interference colors) are intermingled with anhedral grains of K-feldpar, chessboard albite, and chlorite.

Two side-by-side thin section micrographs at the same scale: left in plane-polarised light showing pale grains with olive-green patches and small black opaque specks; right in cross-polarised light showing grey, blue and pink interference colours; scale bar 1 mm.


In places, thin fingerlike agglomerations of granular green pyroxene project into a clear, largely inclusion-free mineral, presumably quartz. The quartz appears as clear low relief in PP with few to no inclusions and a feathery low order birefringence pattern in XP- extreme undulose extinction? It's unclear what's happening:

Side-by-side thin section micrographs: left in plane-polarised light shows pale grey mineral with dark green irregular grains; right in cross-polarised light shows blue and grey birefringent streaked grains with dark green patches. Scale bar 100 µm.


Photomicrograph of a thin section in plane-polarised light showing dark and light blue-white elongated bands running diagonally from lower left to upper right, with a 30 µm scale bar in the lower right.


Side-by-side thin section micrographs at 100 µm scale: left in plane-polarised light showing pale grains with dark green veinlike bands, right in crossed polars showing the same veins against a bright granular birefringent matrix.


Side-by-side thin section micrographs at 10 µm scale: left plane-polarised light shows green chlorite grains along a boundary with pale colourless mineral; right crossed polars shows dark blue-green and bright blue-white birefringent grains.
 
Cross-polarised thin section photomicrograph of a breccia: fine angular clasts in a dark matrix on the left, with coarser blue-grey interference-coloured crystals on the right; label reads 15 FEN 266.


Magmatic-intrusive-breccia/explosion breccia intruding into country rock, Fen complex. The breccia is matrix supported. Most of the clasts are individual fragments of either quartz or feldspar and likely originate from splintering of adjacent country rock (Telemark gneiss):

Side-by-side thin section photomicrographs in plane-polarised light (left) and crossed polars (right) of a fine-grained rock with slender, angular, mostly colourless crystals in a dark brown matrix; scale bar 1 mm.


Clasts are angular and slender with a vaguely trachytic texture. Clast aspect ratio varies from about 1:3 to 1:6, with long sides ranging from about 0.1 mm to 2 mm in length, which I believe classifies all of the clasts as “sand”, so the breccia is well sorted. The ratio of clast to matrix is small, close to 50-50 and there are no open voids.

Matrix appears to be igneous, composed of a mixture of pyroxene and chlorite, perhaps reflecting incomplete chloritization? Etched alteration features are apparent.

Two side-by-side thin-section micrographs at 100 µm scale: left in plane-polarised light showing a pale grain surrounded by olive-green matrix; right under crossed polars showing bright interference-coloured grains in a dark matrix.


The matrix intrudes along microfractures within the country rock (Telemark gneiss).

Plane-polarised light photomicrograph of a thin section showing angular quartz and feldspar clasts in a dark chlorite-rich matrix along a diagonal band, with fine fractures across a pale mineral background. Scale bar reads 1 mm.


Cross-polarised thin section photomicrograph of a breccia: angular, slender quartz and feldspar clasts in a dark, iron-stained matrix, with a 1 mm scale bar at lower right.


The gneiss appears weakly altered; there is little to no perthic texture, no albitization, some plagioclase remains present. The gneiss is transected by a set of roughly parallel microfractures; the grains are anhedral. Identifying the minerals has been difficult, often I cannot tell if a grain is quartz or feldspar, and when I can identify a grain as feldspar (a ‘dusty’ appearance in PP), I cannot tell if it is a plagioclase or K-feldspar. Case in point:

Two side-by-side thin-section photomicrographs of a feldspar grain showing fine parallel twinning lines, with an inset close-up of the twinning; a 1 mm scale bar is in the lower right.


At first glance, this seems to be microcline. However, the twinning pattern is fine, indicating anorthoclase. But then, close examination shows the twinning pattern was caused by deformation rather than growth (the inset shows ‘flame twinning’which is diagnostic for deformation twinning). This matters because deformation twinning is associated with plagioclase, not K-feldspar. But there is a cross-hatched pattern which is not associated with plagioclase. Around and around I go…

But wait, there’s more! Some grains show a coarse cross-hatched twinning pattern, but the pattern exhibits 60/120 degree junctions rather than microcline’s 90 degree junctions. In PP, this mineral is clear with low relief, in XP the birefringence is low first order: grey-to-white.

Side-by-side thin section micrographs: left in plane-polarised light showing a pale matrix with dark patches and fractures; right in cross-polarised light showing bright and dark mineral grains with twinning, scale bar 1 mm.

(above): Prominent grain upper center has 60/120 junctions in XP but in PP is indistinguishable from the neighboring quartz grains along the right edge. Lower center, is a more disorganized region that includes a small grain of myrmekite. At bottom, a grain of feldspar with a deformation twinning pattern. The horizontal 'line' cutting across the middle of the image is an artifact.

Cross-polarised and plane-polarised thin section photomicrographs side by side of a breccia: angular feldspar and quartz clasts in a fine dark matrix, with a 1 mm scale bar.

(above): Matrix and clasts, most clasts are either quartz (clear) or feldspar (dusty appearance), some of the feldspar grains have atypical twinning: 60/120 junctions (bottom) and deformation twinning (top).

Thin section photomicrograph in plane-polarised light of a pale grey-yellow mineral grain with thin dark fractures, bordered by brownish altered material, with pinkish patches; scale bar 1 mm.


Cross-polarised thin section micrograph of an interlocking mineral grain mosaic with vivid magenta, violet, yellow and cyan interference colours; a scale bar reads 1 mm.

(Above) most likely a quartz grain with recrystallization boundaries made vibrantly visible with a quarter-wave (also: glimmer or mica) plate. I can discern faint blue and yellow tints in PP, these correlate with the grain boundaries but it is unclear what is causing the tint.
 
Thin section photomicrograph under crossed polars of a breccia clast in dark blue background, with pale blue and grey angular grains of quartz and feldspar in a fine-grained matrix; a paper label reads Fen 264 along the left edge.


Magmatic-intrusive-breccia/explosion breccia intruding into country rock, Fen complex. Similar to Fen 266, the breccia is matrix supported. Most of the clasts are individual fragments of either quartz or feldspar and likely originate from splintering of adjacent country rock (Telemark gneiss).

Side-by-side thin section micrographs under plane-polarised light (left) and crossed polars (right), with a 1 mm scale bar; elongated pale and dark grains lie in a dark fine-grained matrix.


Clasts are angular and slender with a vaguely trachytic texture. Clast sizes and aspect ratios are different than Fen 266, here the breccia is poorly sorted and clast aspect ratio varies between (about) 1:4 and 1:10, so the clasts are more ‘slender’ than in Fen 266. Matrix appears to be igneous, composed of mostly granular pyroxene and little to no chlorite is present. The ratio of clast to matrix is larger than Fen 266, closer to 60-40 or even 70-30 and there are no open voids.

As in Fen 266, the matrix intrudes along microfractures within the country rock (Telemark gneiss).

Side-by-side thin section photomicrographs, plane-polarised light on left and crossed polars on right, of a poorly sorted breccia with angular elongated clasts in a dark matrix. Scale bar 1 mm.


Plane-polarised light photomicrograph of a thin section showing an angular, matrix-supported breccia of quartz and feldspar clasts with dark fine-grained matrix, scale bar 1 mm.

Cross-polarised thin section micrograph of a poorly sorted, matrix-supported breccia with angular, slender grey and white mineral clasts in a dark, fine-grained matrix; scale bar reads 1 mm.


The gneiss appears altered; some grains of feldspar (dusty in PP) show incipient sericitization and quartz (clear in PP) often shows undulose extinction. As with Fen 266, identifying most of the grains has been difficult:
Photomicrograph in plane-polarised light of breccia: angular pale grains separated by a dark, fine-grained matrix veined along a fracture, with a 100 µm scale bar at lower right.

Cross-polarised thin section micrograph showing angular quartz and feldspar grains in a fine, colourful matrix, with a 100 µm scale bar in the lower right.


if the large upper grain is quartz, then the lower grain is as well but the lower grain shows either deformation twinning or anorthoclase twinning- the pattern is not believable as undulose patterns in quartz. If the grains are feldspar then the grains should be dusty in PP, tho... right?

Side-by-side thin section micrographs in plane-polarised light (left) and cross-polarised light (right), scale bar 1 mm; cross-hatched twinning in feldspar grains, a green mineral vein, and a white grain.


Side-by-side thin section photomicrographs at 1 mm scale: plane-polarised light on the left, cross-polarised light on the right, showing mineral grains with cross-hatched twinning lamellae.

Several grains show a coarse cross-hatched twinning pattern, but the pattern exhibits 60/120 degree junctions rather than microcline’s 90 degree junctions. In PP, this mineral is clear with low relief, in XP the birefringence is low first order: grey-to-white. There is some correlation in PP and XP in that the density of inclusions approximately duplicates the 60/120 twinning pattern. In the upper image pair, the rectangular outline of what was likely biotite (in center) now consists of chlorite and quartz.

Side-by-side thin section photomicrographs, plane-polarised light on the left and crossed polars on the right, scale bar 1 mm; elongated angular clasts of quartz and feldspar in a dark brown matrix.

Large vertical grain is plagioclase, but twinning pattern is curved due to deformation. Below, a small grain of quartz with pronounced undulose extinction- also diagnostic of stress/strain deformation.

(and yes, I am counting down)