Geochemistry and petrogenesis of metabasites from the KTB ultradeep borehole, Germany

Abstract

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 basaltic andesites with tholeiitic P-MORB affinities. The second is the amphibolite-gneiss suite that is characterized by alternating layers of metabasites which intimately interfinger with metasedimentary rocks. Metabasites from the amphibolite-gneiss suite are alkaline, and they include 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 tholeiites by steeper slopes of their spidergrams and REE patterns, and distinctively higher Nb concentrations (20–50 vs 3–20 ppm). Oxygen isotope studies establish that the most primitive metabasites of both series have mantle like δ18O values between 5 and 7‰. More differentiated samples have δ18O values reaching 8‰, and some alkaline metabasites have values as high as 10.5‰ indicating extensive interaction with sedimentary material. In general, ɛNd values decrease with differentiation. Gabbroic samples from the tholeiitic suite have highest ɛNd(0) values at approximately +5, whereas andesitic samples have values as 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 MORB-type mantle; (b) a within-plate plume source; and (c) continental detritus with the composition of paragneiss. Melts from the two mantle sources mixed extensively, and the andesitic rocks are those with the highest contribution from the sedimentary component. Most probably all ZEV rocks were deposited in an oceanic basin close to an active continental margin where magmas were alternately derived from a tholeiitic mantle source as well as highly enriched within-plate magma source.