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.