GEOCHEMISTRY AND PETROGENESIS OF METABASITES FROM THE KTB ULTRADEEP BOREHOLE, GERMANY

The metabasites from the KTB pilot and main drill holes can be divided into two major associations. The first is the metagabbro-amphibolite suite which is composed principally of metagabbros, basalts, and basal tic andesites with tholeiitic P-MORB affinities. The second is the amp hibolite-gneiss suite that is characterized by alternating layers of m etabasites which intimately interfinger with metasedimentary rocks. Me tabasites from the amphibolite-gneiss suite are alkaline, and they inc lude metamorphosed basanites, basalts, basaltic andesites, and trachy- andesites. A distinctive subgroup of the amphibolite-gneiss suite are basanitic metabasites which are interlayered with marble. With regard to trace elements, the alkaline rocks are distinguished from the thole iites by steeper slopes of their spidergrams and REE patterns, and dis tinctively higher Nb concentrations (20-50 vs 3-20 ppm). Oxygen isotop e studies establish that the most primitive metabasites of both series have mantle like delta(18)O values between 5 and 7 parts per thousand . More differentiated samples have delta(18)O values reaching 8 parts per thousand, and some alkaline metabasites have values as high as 10. 5 parts per thousand, indicating extensive interaction with sedimentar y material. In general, epsilon Nd values decrease with differentiatio n. Gabbroic samples from the tholeiitic suite have highest epsilon Nd( O) values at approximately +5, whereas andesitic samples have values a s low as -5. At least three components are needed to explain the trace element and isotope trends found in the metabasites: (a) an enriched asthenospheric MORE-type mantle; (b) a within-plate plume source; and (c) continental detritus with the composition of paragneiss. Melts fro m the two mantle sources mixed extensively, and the andesitic rocks ap e those with the highest contribution from the sedimentary component. Most probably all ZEV rocks were deposited in an oceanic basin close t o an active continental margin where magmas were alternately derived f rom a tholeiitic mantle source as well as highly enriched within-plate magma source.