TIMING AND EMPLACEMENT OF INJECTION DIKES AT THE GARDNOS IMPACT STRUCTURE

Gardnos (Norway, 60°40’N, 9°00’E) is a 5-6 km impact structure in crystalline target rocks [1, 2]. Field work in 2010 identified the first known injection dike into the Gardnos Breccia of the crater floor. While most mineral grains in the Gardnos Breccia show no shock metamorphic effects, equivalent mineral grains in the dike show textures indicative of higher shock. Specifically, biotite in the dike shows severe kinking visible in both plane- and cross-polarized light, anomalous interference figures, and the development of planar microstructures which have an amphibole-like symmetry. Similar textures in biotite have been reported at the Reis structure [3].

A combination of field relationships and this difference in petrographic shock textures allows for a determination of the relative timing of events after impact: 1) shock deformation of the uppermost units, 2) brecciation (but not shock deformation) of the crater floor, and then 3) injection of the more strongly shocked material into the unshocked Gardnos Breccia.

[1] French et al., (1997), Geochim. Cosmochim. Acta, 61, 873-904 [2] Kalleson et al. (2010), Meteorit. Planet. Sci, 45, 798-827. [3] Schneider (1972), Contr. Mineral. and Petrol., 37, 75—85