"Rock of the Month # 52, posted for October 2005" ---

Sample 321.11, from the Kragero area, southern Norway, Scandinavia, Europe (Cappelen, 1965). This is a massive granular rock, of dappled appearance. It is a metagabbro with a well-reserved igneous texture of interlocking bluish-grey plagioclase feldspar and brownish-black hornblende, typical grain size 4-8 mm, not appreciably magnetic, no effervescence in dilute HCl. This digital image, 2005, depicts a pale, coarse amphibole with dark rutile crystals in albite matrix. Nominal magnification 50X, plane-polarized transmitted light, long-axis field of view 1.6 mm.

In thin section, this rock is dominated by albitic plagioclase feldspar in equant grains, grain size typically 0.1-0.4 mm (an estimated 74 volume percent of the rock). A pale amphibole, probably actinolitic hornblende, appears to be recrystallized, and like the feldspar, occurs in mm-scale aggregates of sub-mm domains, thought to reflect preservation of precursor igneous texture (24%). The TiO₂ polymorph rutile is an abundant accessory phase, often hosted by the amphibole, in crystals up to 1.2 mm long (2%). Small prisms of apatite are also present.

The Bamble sector of southern Norway is noted for its diverse geology, particularly amphibolite to granulite facies metamorphic rocks. This includes a range of gabbroic to ultramafic intrusions (Alirezaei and Cameron, 2002). The region contains assemblages of rock types reminiscent of parts of the Grenville province in eastern North America (Starmer, 1976; Dahlgren et al., 1993). The middle- Proterozoic age of the Sveconorwegian rocks is similar to much of the Grenville. Hydrothermal dolomites occur here as lenses, breccias and veins / dykes cutting amphibolites and metagabbros - they can be distinguished from normal metasedimentary marbles by their chemistry (enrichments in rare-earth elements, Ni, Co, Cr and Sc; depletion in Ba and Sr relative to the nearby metasedimentary marbles - Dahlgren et al., 1993).

The rock is a mosaic of equant, polygonal grains of feldspar and amphibole, with the appearance of a high degree of recrystallization (e.g., numerous perfect 120° triple points on albite grain boundaries). The rutile is often nestled within the amphibole, and may itself display 120° grain boundary terminations. Thus, we have a recrystallized metagabbro of a peculiar form, composed in the main of albite and amphibole, estimated colour index 26 percent. More strictly, a rutile-bearing albitite, which in Norway acquired the locality-inspired varietal name krageroite.

References

ALIREZAEI,S and CAMERON,EM (2002) Mass balance during gabbro-amphibolite transition, Bamble sector, Norway: implications for petrogenesis and tectonic setting of the gabbros. Lithos 60, 21-45.

CAPPELEN (1965) Cappelens Bil-og Turistkart, dobbeltblad 1-2. J.W. Cappelens forlag, Oslo, tourist map, sheets 1-2 (1:325,000 scale).

DAHLGREN,S, BOGOCH,R, MAGARITZ,M and MICHARD,A (1993) Hydrothermal dolomite marbles associated with charnockitic magmatism in the Proterozoic Bamble shear belt, south Norway. Contrib.Mineral.Petrol. 113, 394-409.

STARMER,IC (1976) The early major structure and petrology of rocks in the Bamble series, Sondeled- Sandnesfjord, Aust-Agder. Norsk Geologisk Tidsskrift 327, 77-97 [journal of the geological society, the Norsk Geologisk Forening].