The Gallejaur structure, Northern Sweden

Abstract

Rocks, interpreted as impact generated lithologies, occur in a large area surrounding the Gallejaur magnetic structure at latitude 65°10′/longitude 19°30′ in northernmost Västerbotten County in northern Sweden. These rocks comprise a variety of different types of breccias: authigenic/autochthonous monomict breccias from the underlying rock units (monomictly brecciated basement), parautochthonous, monomict breccias, and polymict melt breccias. No convincing planar deformation features (PDFs) in shock metamorphic minerals have yet been identified.

The Gallejaur structure is located in the central part of the Skellefte mining district in the Precambrian Baltic Shield. The ca. 1.9 Ga old Skellefte district is an extensively mineralized, mainly felsic, submarine volcanic belt. The predominant lithologies are acid to mafic volcanic and volcanoclastic rocks, interbedded with, and overlain by, graphitic schists and greywackes. The rocks, which in the present article are described as impact-generated, have been called the Vargfors Group and overlie the Skellefte volcanics and sediments with an angular unconformity. In general, the older rocks are deformed and folded, which results in a more or less vertical position, whereas the younger rock sequence is flat-lying and undeformed outside of the regional shear zones. The youngest rocks in the area, intruding the Skellefte district supracrustals and probably the impact-generated rock, are A/I-type granitoids belonging to the Revsund-Adak granite suite, which have been dated at ca. 1.80 to 1.78 Ga.

The highly magnetic ring around the centre of the structure is interpreted as an impact melt body with varying amounts of more or less absorbed clasts of various basement lithologies. Density measurements indicate a mafic composition, implying that the original rocks were andesitic to basaltic. The gravity anomaly of the central rise region, above +15 mGal (150 gu), is high. This gravity high is surrounded by an encircling gravity low of about −20 mGal amplitude. The central uplift area is between 10 and 12 km in diameter according to the gravity and magnetic anomaly maps, which corresponds to a final crater diameter of 50–60 kilometers, and a transient cavity of about half that size.

In the center of the Gallejaur ring structure a fine-grained crystalline rock of monzonitic composition is found, which is loaded with rounded clasts of volcanic origin. The origin is not yet established, but the lack of deformation and location within the inferred central uplift indicate that the monzonitic rock could have an origin as crystallized impact melt. The Gallejaur monzonite has been dated at 1873 ± 10 Ma, which is taken as the age of the impact event. The monzonite and an underlying porphyritic rock seem to be differentiates of one melt body.

In a northwest-southeast zone across the structure occur water-deposited immature sediments (pelitic, arkosic, and conglomeratic), which are interpreted as fill.

The original impact structure has been affected by later deformation events, which are mainly represented by shearing, faulting and erosion.