Impact-Induced replacement of plagioclase by K-feldspar in granitoids and amphibolites at the Kärdla crater, Estonia

The 4-km-wide and 0.5-km-deep Kärdla Crater, presently buried under a thin sequence of Upper Ordovician limestone, was formed in a complex target: the crystalline basement was covered by about 170 m of poorly consolidated Lower to Middle Ordovician and Cambrian sediments. The basement-derived granitoids and amphibolites that were subjected to low shock pressure (less than 8 GPa) and are at present either in allochthonous or autochthonous position, have undergone significant impact related chemical and mineralogical alteration. Altered rocks and minerals are fractured and brecciated at different scales. Optical microscopy has shown that plagioclase is altered (sericitization or saussuritization) and amphibole chloritized. Bulk chemical compositions indicate that sodium and calcium were removed and potassium was brought in. Powder XRD analysis suggests the disappearance of plagioclase and formation of cryptocrystalline orthoclase, in addition to medium- to coarse-grained microcline present in primary rocks. SEM, EDS and EPMA studies reveal that the main secondary minerals are orthoclase, replacing plagioclase in granites and amphibolites, and chlorite, replacing hornblende in amphibolites. The extent of chemical and mineralogical alteration is closely related to microfractures. However, alteration has selectively changed the minerals. Plagioclase, hornblende and biotite have changed dramatically, whereas zircon, quartz and also primary microcline remained almost unaffected. Geological and mineralogical data suggest that this alteration was related to fireball — target rock interaction during the impact process.