Impact-generated pseudotachylitic breccia in drill core BH-5 Hättberg, Siljan impact structure, Sweden

Pseudotachylitic breccia (PTB) in the form of cm-wide melt breccia veinlets locally occurs on the exposed central uplift of the 380 Ma Siljan impact structure. The host rock to the PTBs is the so-called Järna granite of quartz monzonitic to syenodioritic composition. The nearly 603 m long BH-5 drill core from Hättberg, near the centre of the Siljan central uplift, contains numerous veins and pods of PTB. In particular, two major zones of 60 m combined width contain extensive PTB network breccias (30% actual melt breccia component), with individual melt breccia occurrences up to >1 m in length. Core logging and petrographic and geochemical analysis of the core have been performed, and the data are interpreted to suggest the following. (1) The impact event caused low to moderate (at essentially