The Gardnos impact structure, Norway: Petrology and geochemistry of target rocks and impactites

Abstract

The Gardnos structure, Norway is an approximately circular area of anomalously fractured and brecciated rock, about 4.5 km in diameter, emplaced in a metamorphic terrane composed chiefly of granitic gneisses with minor amphibolite and quartzite. The original recognition of Gardnos as a deeply eroded impact structure between 900 and 400 Ma old has been followed up by detailed petrographic and chemical studies of approximately thirty samples of target rocks and various types of shocked rocks (impactites). Deep erosion of the structure has erased the original rim, removed much of the crater-fill deposits, and exposed large areas at or near the original crater floor. However, a wide variety of distinctive impactites —fractured and blackened quartzites in the sub-crater basement rocks, lithic breccias, and melt-bearing breccias—are still preserved. These impactites show petrographic and chemical characteristics that confirm an impact origin: distinctive Planar Deformation Features (PDFs) in quartz and feldspar, incipient melting of feldspar clasts in the melt-bearing breccias, close matches between the chemical composition of the breccias and mixtures of the target lithologies, and the detection of an extraterrestrial component. A minor extraterrestrial component (≤0.15%) was detected in the melt-bearing breccias, based on significantly elevated Ir and Os contents and lower 187Os/ 188Os ratios compared to those in the target rocks. The Gardnos impactites are significantly enriched in C (5–10X) over the exposed target rocks. This may reflect the presence of a C-rich shale overlying the metamorphic basement at the time of impact; this idea is supported by δ 13C values of −28.1 to −31.5%. measured in the impactites. Mixing calculations show that the chemical compositions of the impactites can be reproduced by mixtures of target rocks ranging from approximately 60–90 wt% granite gneiss, 0–30 wt% amphibolite, 0–12 wt% quartzite, and 3–19 wt% of a C-rich shale component. The deeply eroded state of the structure and the preliminary state of detailed geologic mapping make crater reconstruction difficult. One possible scenario involves the impact of a 300 m diameter stony meteorite that released 10 19 J of energy and formed a transient cavity 3 km in diameter that evolved to a complex crater 5 km in diameter with a central uplift of about 350 m. The original crater was filled with at least 0.3 km 3 of allochthonous melt-matrix breccias containing about 0.06 km 3 of impact melt. The structure underwent low-grade (greenschist?) metamorphism in Caledonian time (about 400 Ma ago) and was subsequently eroded to its present appearance. The value of the Gardnos structure for further cratering studies lies in its easy access to large areas of the original crater floor zone, in the preservation of a possibly complete sequence of crater-fill breccias beneath a cap of elastic sediments, and in the unusual carbon enrichment of its impactites.