The Impact Concept

The author has studied only a few of the drill-cores, and a preliminary attempt to re-interpret the geology from the concept of an impact event is presented. The interpretation, suggesting an impact stratigraphy (first proposed by F. E. Wickman in 1988) is only tentative, and more in-depth studies are needed before different formations can be more accurately interpreted. As evidenced from the shock metamorphism (described in the following paragraph), the general sequence of rock units and the appearance of the breccia formations an impact model would best explain the Duobblon Structure. The following description and discussion assumes that the structure formed by an impact, even though there is not yet final confirmation for such a proposal.

Shock metamorphic textures

Shock metamorphic features have been observed in three thin sections from three different drill sites. PDFs in 5 quartz grains have been measured with an universal stage with the following results:

2 grains with Miller index {1 0 1 3} with the symbol ©, 1 grain with (0 0 0 1) with the symbol c, 1 grain with {5 1 6 1} with the symbol x, and 1 grain with {1 0 1 3} and {1 12 1} with the symbols © and s, respectively.

Typically only one set of PDF is seen in quartz grains. Two sets of PDFs in quartz have been found in a thin section from Dh 78013 at 245.6 m (Fig 4). The PDFs in quartz are mainly non-decorated or very weakly decorated.

Fig. 4. Two sets of PDFs in quartz. Drill-hole 78 013, 245.6 m. Crossed nichols.

The crystallographic orientation of the PDFs against quartz optical axis has been obtained by U-stage studies, resulting in five forms of features parallel to: {1 0 I 3}, {11 2 1}, {5 1 6 }), (0 0 0 1), and {2 13 1}. The main orientation is {1 0 1 3}, which is typical for impact-affected rocks. PDFs in feldspar (usually in K-feldspar) have been found in thin sections from three different sites (Dh 85003 at 197.7 m, Dh 78013 at 245.6 m and Dh 7872). The obtained data are however not sufficient to deduce the formation pressure.

Partly circular to semicircular textures in quartz grains have been noted in two thin sections and is interpreted as ballen quartz (Grieve et al 1996).

Target Rocks

The target rocks are granite, meta-greywacke and graphitic schist. The granite has been given an age of 1.87 Ga (based on U/Pb analyses of zircons, Skiold 1988).

In the westernmost part of the Duobblon Structure, a sparse network of cm-wide breccia veins intersects the basement granite, which upward grades into a matrix- supported autochthonous breccia. Einarsson (1979)

interpreted the veins as "fractures filled with mud and weathered residue containing larger quartz grains and granite clasts in a dense greyish green matrix". To the east, the basement consists of greywacke overlain by autochthonous schist breccia derived from the same sediments. In drill-cores pesudotachylitic-like veinlets have been observed both in the granite basement and in brecciated schist. A more detailed investigation and description is, however, needed before a correct classification of these veins can be presented.

Autochthonous Breccia

A matrix-supported autochthonous breccia with granite clasts overlies the granitic basement, whereas the schist (the Paksjo Schist) is overlain by a monomict schist breccia. In the eastern part the Sorsele granite is in intrusive contact with rotated blocks of basement greywacke with bedding planes striking more or less perpendicular to the contact and with way-up directions to both the west and the east (Fig. 3). The thickness of the autochthonous breccia varies considerably and can reach 100 m.

Assumed Impact Melt Rock

The monomict breccia grades upward into what Einarsson (1979) describe as lithophysae-bearing acid volcanic rocks. In the following this unit is interpreted as an impact melt rock. The assumption is based on planar deformation features in quartz, the fine-grade cherty-like appearance, perlitic textures (indicating that the rock was formed as glass) and the inclusion of more or less melted granite and schist clasts.

The rock ranges in thickness, from 20 m in the western parts (Dh 78001) to 140 m in the central parts of the structure (Fig. 4). As a whole, the rock is heterogeneous on a smaller, local scale (due to varying number of clasts). In some drill-holes of the structure it is more homogeneous, where the rock unit normally has a chert-like appearance, due to recrystallisation and hydrothermal alteration. The rock is, however, not completely recrys-tallised and in place exhibits perlitic textures (e.g., Einarsson 1979, Fig 44, and observations in Dh 78301 at 68.8 m).

In the lower part and at different levels higher up, it contains more of less melted granite/and or schist clasts (Fig. 5a). The clasts occur at different levels and in varying stages of preservation (Fig. 5b). The more altered granite grades into clasts of chert-like appearance (Fig. 5c). It is, however, not possible to exclude or to prove that these are of volcanic origin and existed in the assumed target rocks. Earlier authors (Einarsson 1979; Lindroos and Smellie 1979) described the material as dominated by fine-to medium grained clasts of granite and volcanic rocks with very variable appearance and consisting of rock types, that are not represented within the Duobblon area.

Feldspar and quartz phenocrysts are common throughout the rock (Fig. 5e). The feldspars (0.3-5 mm in diameter) have mainly albite composition, and generally are sericitised. The quartz phenocrysts show resorption and embayment features. Occasionally the rock has a pseudo-layering with alternating green and red colours. Welded particles are commonly present, and the matrix consists of a devitrified felsitic mass with grain size <1 mm. Accessory minerals include titanomagnetite, ilmenite, magnetite, apatite, titanite, epidote, calcite, pyrite, and zircon (Smellie 1982).

The most spectacular textures are spherulitic textures and cavities, which have attracted the attention of earlier investigators. Spherulites are sporadic and can reach 4 mm in size, exhibit radial growth structures, and are nucleated around quartz fragments (Smellie 1982). The lithophysae are spherical in shape or sometimes elliptical. The diameter is usually less than 5 cm (good illustrations in Einarsson 1979).

The lithophysae are cryptocrystalline in the outer margin with the same mineralogical composition as the matrix. Some of the lithophysae have a concentric growth pattern. In the inner parts of the lithophysae appear flattened, rather similar to ignimbritic textures.

In the microscope the matrix of the lithophysic rock is sericitised and has often well preserved perlitic texture, but mostly a devitrified micro-felsitic groundmass is seen. In the matrix a few mineral clasts of potassium feldspar and quartz can be seen.

The zircons in the rock are commonly zoned and, in addition to their common association with opaque minerals, occur sporadically throughout the rock (Smellie 1982). Major element analyses indicate that the rock is calc-alkaline in composition with only minor variation along its vertical and its horizontal extension.

Fig. 5 A. Rounded granite clast and flattened melt clasts. Drill-hole 85 003. 185.5 m. B. Irregular formed clasts with reddish rim and mineral clasts in a fine-grained matrix. Drill-hole 78 013, 325.7 m. C. Irregular, densely packed clasts with a chert-like appearance. Drill-hole 78 013, 327.9 m. D. Fine-grained, partly banded rock, consisting of feldspar grains and a few quartz grains (one with 2 sets of PDFs, see Fig. 2). E. Rock with <1 cm size granite clasts and diffuse chert-like clasts. From this section, one quartz grain with one set of PDF has been recorded. Drill-hole 85 003, 194.1 m.

Fig. 5 A. Rounded granite clast and flattened melt clasts. Drill-hole 85 003. 185.5 m. B. Irregular formed clasts with reddish rim and mineral clasts in a fine-grained matrix. Drill-hole 78 013, 325.7 m. C. Irregular, densely packed clasts with a chert-like appearance. Drill-hole 78 013, 327.9 m. D. Fine-grained, partly banded rock, consisting of feldspar grains and a few quartz grains (one with 2 sets of PDFs, see Fig. 2). E. Rock with <1 cm size granite clasts and diffuse chert-like clasts. From this section, one quartz grain with one set of PDF has been recorded. Drill-hole 85 003, 194.1 m.

The lithophyse-bearing melt rock is up to 15 m thick in the field. At one outcrop lithophysae have been seen to occur in two layers separated by breccia (Einarsson 1979). At another locality, a 3-m-thick schist breccia grades upwards into 5 m of polymict breccia with mainly granitic clasts, and uppermost follows a 5-m-thick greyish green lithophysae-rich rock. This unit in between breccia formations, was earlier interpreted as an ignimbrite, and has given an U/Pb zircon age of 1803±15 Ma age (Skiold 1988).

Suevitic Ash?

The lithophysae-bearing volcanic rocks are overlain by what has been interpreted as a grey to red tuffitic sandstone (Fig. 5d). Larger fragments of quartz, microcline-perthite and sericitised plagioclase is found in a very fine-grained sericitised matrix. The finest fraction consists of small angular quartz and potash feldspar fragments in a dense matrix of sericite and calcite with varying amount of epidote. It is here interpreted as a finegrained suevitic, based on two sets of PDFs in quartz. The rock is sometimes difficult to separate from the lower fine-grained melt unit.

Polymict Breccia

An upper polymict breccia overlies the rock, above interpreted as a suevitic ash, without a distinct contact. In the central part of the rocks exposed in outcrop the thickness of the polymict breccia is approximately 160-200 m.

The lower part of the polymict breccia is interpreted as a coarse-grained suevite breccia with clasts of granite, schist and "porphyries" (which either are melted granite clasts or have a volcanic origin).

Part of the upper part of the polymict breccia has more rounded clasts and could represent a re-deposited breccia. A more in depth study might be able to separate different units.

The Gippervare Formation

The Gippervare Formation concordantly overlies the polymict breccia. According to Einarsson (1979) and Lindroos and Smellie (1979), the formation is about 1000 m thick and extends over an area of 30 km with a maximum width of 2 km. The rocks have an overall rhyolitic composition (Einarsson 1979). Two main types can be distinguished: in the lower a part grey dense rock rich in quartz phenocrysts and higher up a coarser grained rock with feldspar phenocrysts. Coarse, fragment-bearing rocks with clasts up to 30 cm in size occur in the central part of the sequence. Einarsson (1979) suggested that these rocks could largely be of ignimbritic origin, forming relatively thin units with large areal extent. The position on top of the polymict breccia indicates that the formation could have originated as a fine-grained suevite. No investigation of the presence of shock metamorphic minerals has yet been performed. For a more detailed description of the Gippervare Formation the reader is referred to the publication of Einarsson (1979).

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