Petrological modifications in granitic rocks from the siljan impact structure: evidence from cathodoluminescence

Cathodoluminescence (CL) and microprobe analyses were performed on quartz, plagioclase and K-feldspar from granitic rocks in the Siljan impact area, central Sweden. In the upper 2200 m and in the vicinity of the Gravberg-1 well, secondary reddish luminescing quartz and brownish luminescing albite (Ab > 98%) and K-feldspar (Or > 95%) are the dominant minerals. Primary magmatic quartz, plagioclases (oligoclase-andesine) and K-feldspar show blue, yellowish and bluish luminescence colours, respectively. The reddish luminescence colour of quartz is interpreted to have been induced by the shock heating episode of the impact and subsequent elevated temperature in the granites. The impact shock pressure effects, as shown by shatter cones and planar features, were retained within granite dominated by blue luminescing quartz. Most alteration of the feldspar took place during the cooling episode that followed the impact heating. Post-impact fracturing facilitated the infiltration of meteoric water, which has further enhanced alteration of feldspars.