THE GARDNOS IMPACT STRUCTURE, NORWAY - PETROLOGY AND GEOCHEMISTRY OF TARGET ROCKS AND IMPACTITES

The Gardnos structure, Norway is an approximately circular area of ano malously fractured and brecciated rock, about 4.5 km in diameter, empl aced in a metamorphic terrane composed chiefly of granitic gneisses wi th minor amphibolite and quartzite. The original recognition of Gardno s as a deeply eroded impact structure between 900 and 406 Ma old has b een followed up by detailed petrographic and chemical studies of appro ximately thirty samples of target rocks and various types of shocked r ocks (impactites). Deep erosion of the structure has erased the origin al rim, removed much of the crater-fill deposits, and exposed large ar eas at or near the original crater floor. However, a wide variety of d istinctive impactites -fractured and blackened quartzites in the sub-c rater basement rocks, lithic breccias, and melt-bearing breccias-are s till preserved. These impactites show petrographic and chemical charac teristics that confirm an impact origin: distinctive Planar Deformatio n Features (PDFs) in quartz and feldspar, incipient melting of feldspa r clasts in the melt-bearing breccias, close matches between the chemi cal composition of the breccias and mixtures of the target lithologies , and the detection of an extraterrestrial component. A minor extrater restrial component (less than or equal to 0.15%) was detected in the m elt-bearing breccias, based on significantly elevated Ir and Os conten ts and lower Os-187/Os-188 ratios compared to those in the target rock s. The Gardnos impactites are significantly enriched in C (5-10X) over the exposed target rocks. This may reflect the presence of a C-rich s hale overlying the metamorphic basement at the time of impact; this id ea is supported by delta(13)C values of -28.1 to -31.5 parts per thous and measured in the impactites. Mixing calculations show that the chem ical compositions of the impactites can be reproduced by mixtures of t arget 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 co mponent. The deeply eroded state of the structure and the preliminary state of detailed geologic mapping make crater reconstruction difficul t. One possible scenario involves the impact of a 300 m diameter stony meteorite that released 10(19) J of energy and formed a transient cav ity 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 containi ng 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 Ga rdnos 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 impa ctites. Copyright (C) 1997 Elsevier Science Ltd.