The Guarda Structure, NE-Portugal: A meteorite impact crater or not?
Zalinge, M.E. van
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The Guarda Structure is a circular structure located in the Iberian Massif, just northeast of the city Guarda, Portugal. The structure has a diameter of ± 35km and is formed in Variscan granitoids. Monteiro (1991) suggested that the Guarda Structure is a deeply eroded complex multi-ring impact crater. He mentioned that the circular topography and the hydrographic pattern in combination with decorated planar features in quartz crystals are evidence for an impact crater centered at 40˚ 37’N and 7˚ 6’W. However, convincing evidence is missing and it remains unclear whether the Guarda Structure is an impact crater. During this study, field data and rock samples were collected in the Guarda Structure and detailed light microscopy and SEM-analyses were done on potential shock microstructures in quartz, zircon and monazite, to prove whether the Guarda Structure is an impact crater or not. Typical impact rocks and impact geology were not found in the Guarda Structure. Compared to minerals from confirmed craters, quartz, zircon and monazite from the Guarda Structure lack all forms of shock microstructures. The circular topography of the structure and the associated circular drainage pattern could be formed due to lithology differences within the Guarda Structure or due to a dome-like structure. Light microscope analyses are good methods to study planar shock microstructures (PMs) in zircon and monazite. PMs in zircon probably develop over a wide shock pressure range >10-35 GPa, but show also some restrictions, since PMs mainly form in zircons that are >100 μm. Shocked monazites contain three types of PMs and granular texture that probably start to develop at shock pressures between >10-15 GPa and >30 GPa respectively.