St. David's area, Pembrokeshire / Dyfed, southwest Wales, U.K.

metatuff [305 kb]

metatuff [97 kb]

"Rock of the Month # 53, posted for November 2005" ---

Sample 112.16 has been described as an halleflinta. It was collected in southwest Wales in 1973, by a roadside south of Pont Clegyr, northwest Pembrokeshire (now Dyfed), some 1.5 km east of the small town of St. David's, on the prominent peninsula along the north side of St. Brides Bay. In hand specimen it is an extremely fine-grained massive rock, of a delicate pale apple-green colour, a silicified or metamorphosed acid tuff. Not appreciably magnetic, no reaction with dilute acid. There is a very fine wavy pattern, somewhat suggestive of a flow texture, on the pale orangey sub-mm oxidized outer layer, and minor dendrites of secondary oxides of iron and manganese.

The digital photomicrograph, 2005, depicts this very fine-grained rock, Nominal magnification 100X, crossed-polarized transmitted light, long-axis field of view 0.8 mm. The hand specimen, including the sawn offcut from which the thin section was prepared, shows the essentially massive, featureless nature of the rock, and the evident lack of both primary and tectonic fabrics. Coin diameter = 23 mm.

Halleflinta is a Scandinavian term for fine-grained tuffaceous rocks (bedded volcanic ash and derived volcanogenic clastic sediments), applied especially in Sweden and Norway. Some rocks interpreted in this fashion may actually be fine-grained shear zones, of crushed and even partially-melted rock, and as such may be potential host rocks for epigenetic mineral deposits (Ojakangas and Marmo, 1991, 1992).

This is a rather attractive pale green rock, a pale orangey thin film of iron oxide on surface, massive, extremely fine-grained and homogeneous within, of a porcellaneous aspect. Outer surface has sub-mm (?) flow texture of minute runnels. There is rather little to see in a conventional microscope: a granular groundmass (94% of mode) which is very fine-grained, individual equant grains rather ill-defined, grain size circa 10-20 µm. Low relief and low 1st-order interference colours. The rock may be largely feldspars, but no twinning or other diagnostic features are visible. There are Fe-Ti oxides (rutile and leucoxene), granular, often brown and translucent, individual grains mostly 10-40 µm. Strong white internal reflections in reflected light. Hairline fractures are infilled by quartz veinlets (1%). These cracks are straight and continuous on a cm scale, but just 30-40 µm wide.

In summary, a granular, massive rock, all grains <50 µm in diameter, composed of ill-defined, commonly equant feldspar, oxide and silica crystals. No evidence for carbonate in the matrix, No obvious phenocrysts, nor lithic clasts, nor porphyroblasts. An example of halleflinta, a metamorphosed acid tuff. This metatuff appears massive, very fine-grained, homogeneous, with an abundance of tiny Fe-Ti oxide or alteration products. The original interpretation of halleflinta appears plausible in this case, although elsewhere they have been reinterpreted as alteration zones and/or sheared lithologies, as noted above.


OJAKANGAS,RW and MARMO,JS (1991) Early Proterozoic leptite and halleflinta (tuff and tuffite) sequences of southern Finland reinterpreted as shear zones. GSA Abs.w.Progs. 23 no.5, 486pp., 97, San Diego.

OJAKANGAS,RW and MARMO,JS (1992) Early Proterozoic leptite and halleflinta (tuff & tuffite) sequences of southern Finland reinterpreted as shear zones: significance to Lake Superior geology. Abs. 38th Annual Meeting, Institute on Lake Superior Geology, vol.38, part 1, 97pp., 80-81, Hurley, WI (May 1992).

Graham Wilson, 07 January 2006, extended 09 January 2006

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