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 mos t likely source crater for the Ivory Coast tektites as the tektites an d the crater have the same age (1.07 Ma), and there are close similari ties between the isotopic and chemical compositions of the tektites an d crater rocks. The crater is excavated in 2.1-2.2 Ga old metasediment s and metavolcanics of the Birimian Supergroup. Here we present the fi rst integrated petrographic and geochemical study of rocks from the Bo sumtwi impact crater. A variety of target rocks from the Bosumtwi impa ct structure were selected to represent the major rock types that have been described before, resulting in four groups: shale, phyllite-gray wacke, and two different types of granites (from dispersed dikes and f rom the so-called Pepiakese intrusion at the northeastern side of the crater). These rocks were analyzed for their major and trace element c omposition and their petrographic characteristics. In addition, repres entative samples were also analyzed for their O, Sr, and Nd isotopic c ompositions. 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 cl asts within suevite-derived 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 th e Bosumtwi crater rocks in a Rb-Sr isotope evolution diagram yields an ''age'' of 1.98 Ga, and an initial Sr-87/Sr-86 ratio of 0.701, which is close to results previously obtained for granitoid intrusions in th e Birimian of Ghana. Our Nd isotopic data yield depleted mantle model ages ranging from 2.16 to 2.64 Ga, and epsilon(Nd) values of -17.2 to -25.9 parts per thousand. Harmonic least-squares (HMX) mixing calculat ions were able to reproduce the composition of Ivory Coast tektites fr om a mixture of Bosumtwi country rocks that include about 70% phyllite -graywacke, 16% granite dike, and 14% Pepiakese granite. The oxygen is otopic composition of the metasedimentary rocks and granite dikes (del ta(18)O = 11.3-13.6 parts per thousand) and the tektites (delta(18)O = 11.7-12.9 parts per thousand, also this work) agree fairly well, whil e the Pepiakese granites have lower values (delta(18)O = 8.6-9.0 parts per thousand), 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 no t allow the unambiguous identification of distinct endmember compositi ons, but in both a Sr-87/Sr-86 vs. 1/Sr plot and an epsilon(Sr) vs. ep silon(Nd) diagram, the tektites plot within the area occupied by the m etasedimentary and granitic Bosumtwi crater rocks. Thus, our data supp ort the interpretation that the composition of the Ivory Coast tektite s is similar to that of rocks exposed at the Bosumtwi impact structure , indicating formation during the same impact event. Copyright (C) 199 8 Elsevier Science Ltd.