On The Role Of Shock Melting And Anatexis In Breccia Formation: Southern Sudbury Impact Structure, Canada

The ca. 1.85 Ga Sudbury Igneous Complex (SIC) in Ontario, Canada, represents the partially eroded remnant of an impact melt sheet that overlies brecciated target rocks. At Crean Hill, along its southwest margin, it is underlain by contact metamorphosed basement rocks cut by breccias of various compositions. The breccias formed as melts with contact-parallel flow textures, which are defined by elongate wispy clasts that wrap around basalt clasts. The presence of melts in the footwall of large impact melt sheets may be due to shock melting during propagation of the shock wave, frictional melting along superfaults during modification of the crater, injections from the impact melt sheet, or anatexis during cooling of the impact melt sheet. The absence of nearby superfaults at Crean Hill precludes the formation of the breccia by frictional melting. Modeling of partial melt compositions during contact metamorphism produces chemical melts that are generally enriched in Zr and more felsic than the breccia matrices, negating the possibility of their formation by anatexis during contact metamorphism. The breccia matrices have significantly lower SiO2 and higher TiO2 than the SIC original melt composition, suggesting that they could not have formed by the injection of SIC melt into the fractured target rocks. Because the composition of the breccias mirrors that of their host rocks, the breccias are interpreted as pockets and dikes of locally-derived shock melt that formed during shock compression and were trapped in the basement rocks during cooling of the SIC. Thus, the breccias formed by shock melting during impact and were subsequently modified by contact metamorphism (T >= 750 degrees C) during cooling of the SIC, and by later syntectonic regional metamorphism at upper greenschist to amphibolite conditions.