Petrology and geochemistry of target rocks from the Bosumtwi impact structure, Ghana, and comparison with Ivory Coast tektites

The 10.5 km diameter Bosumtwi crater in Ghana, West Africa, is the most likely source crater for the Ivory Coast tektites, as the tektites and the crater have the same age (1.07 Ma), and there are close similarities between the isotopic and chemical compositions of the tektites and crater rocks. The crater is excavated in 2.1–2.2 Ga old metasediments and metavolcanics of the Birimian Supergroup. Here we present the first integrated petrographic and geochemical study of rocks from the Bosumtwi impact crater. A variety of target rocks from the Bosumtwi impact structure were selected to represent the major rock types that have been described before, resulting in four groups: shale, phyllite-graywacke, and two different types of granites (from dispersed dikes and from the so-called Pepiakese intrusion at the northeastern side of the crater). These rocks were analyzed for their major and trace element composition and their petrographic characteristics. In addition, representative samples were also analyzed for their O, Sr, and Nd isotopic compositions. The target rocks do not show any unambiguous evidence of shock metamorphism (i.e., planar deformation features, PDFs). Distinct impact-characteristic shock effects (PDFs) were identified only in clasts within suevitederived melt fragments. The compositional range of the target rocks is significantly wider than that of the Ivory Coast tektites, but overlap the tektite compositions. A best-fit line for the Bosumtwi crater rocks in a Rb-Sr isotope evolution diagram yields an “age” of 1.98 Ga, and an initial Sr/Sr ratio of 0.701, which is close to results previously obtained for granitoid intrusions in the Birimian of Ghana. Our Nd isotopic data yield depleted mantle model ages ranging from 2.16 to 2.64 Ga, and eNd values of 217.2 to 225.9‰. Harmonic least-squares (HMX) mixing calculations were able to reproduce the composition of Ivory Coast tektites from a mixture of Bosumtwi country rocks that include about 70% phyllite-graywacke, 16% granite dike, and 14% Pepiakese granite. The oxygen isotopic composition of the metasedimentary rocks and granite dikes (dO 5 11.3–13.6‰) and the tektites (dO 5 11.7–12.9‰, also this work) agree fairly well, while the Pepiakese granites have lower values (dO 5 8.6–9.0‰), indicating that only minor amounts of these rocks were incorporated in the formation of the Ivory Coast tektites. The large variation in Sr and Nd isotopic compositions of the target rocks do not allow the unambiguous identification of distinct endmember compositions, but in both a Sr/Sr vs. 1/Sr plot and an eSr vs. eNd diagram, the tektites plot within the area occupied by the metasedimentary and granitic Bosumtwi crater rocks. Thus, our data support the interpretation that the composition of the Ivory Coast tektites is similar to that of rocks exposed at the Bosumtwi impact structure, indicating formation during the same impact event. Copyright © 1998 Elsevier Science Ltd