Geochemistry of impactites and basement lithologies from ICDP borehole LB-07A, Bosumtwi impact structure, Ghana

Abstract

In 2004, a drilling project by the International Continental Scientific Drilling Program (ICDP) at the Bosumtwi impact crater, Ghana (1.07 Myr old and 10.5 km in diameter), obtained drill core LB-07A, which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure. The LB-07A core consists of three sequences: 82.29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54.88 m of so-called lower impactite of monomict impact breccia with several suevite intercalations, and 74.53 m of meta-graywacke and altered shale of the basement, also containing a number of suevite intercalations. Major- and trace-element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein. Compositions of polymict impact breccias of the crater fill revealed by core LB-07A are compared with the compositions of the Ivory Coast tektites and the fallout suevites. The impactites of the LB-07A borehole appear well homogenized with respect to the silicate component, and little change in the ranges of many major- and trace-element differences is seen along the length of the borehole (except for Fe2O3, MgO, and CaO contents). Much scatter is observed for a number of elements, and in many cases this increases with depth. It is proposed that any variability in composition is likely the function of clast population differences (i.e., also of relatively small sample sizes). No systematic compositional difference between polymict lithic and suevitic impact breccias is evident. An indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies. The impactites of the borehole generally show intermediate compositions to previously defined target rocks. The fallout suevites have comparable major element abundances, except for relatively lower MgO contents. The Ivory Coast tektites are generally similar in composition to the LB-07A suevites, but broader ranges in MgO and CaO contents are observed for the LB-07A suevites.