Geochemical relationship between REE and PGE in mafic and ultramafic rocks and their petrogenetic significance - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 249

the diagram in the coordinates (total REE - Pd/Pt) we can assume the existence of
two separate trends (Figure 10.13, 2-4). The position of figurative points in the
diagrams in the coordinates (Yb - Pd/Ir) and (Yb - Pt/Ir) assumes discontinuity
of geochemical characteristics of the ultramafites and gabbro. In these rocks with
increasing contents of less refractory Pd and Pt against more refractory Ir contents,
the contents of Yb are increasing (Figure 10.14). In general, these data do not con-
tradict the assumption that during the formation of ultramafic restites and gabbros
composing this massif, a multidirectional fractionation of REE and PGE occurred.
10.3 KOKPEKTINSKY MASSIF (SOUTH URALS,
KAZAKHSTAN)
Kokpektinsky massif is located in the basin of the Kokpekty River (South Urals)
and structurally confined to the area of Main Urals deep-seated fault [Petrology, …,
1991; Fershtater & Bea, 1996; Fershtater, 2004]. It is exposed over an area of
70 sq · km. and has a form that is elongated to north-eastern direction. This massif
is considerably gabbro and is directly connected with the large Kempirsaysky ultra-
mafic protrusion located to the east of it; they both comprise the Kempirsay ophio-
lite association. The structure of Kokpektinsky massif includes troctolites, olivine
gabbro and gabbro, and also the located among them isolations of serpentinized
dunites and wehrlites of various size. The rocks of the massif are cut by dikes of
pegmatite gabbro and anorthosites. Among the gabbros of Kokpektinsky massif
close to the contact with Kempirsaysky ultramafic protrusion there were xenoliths
of ultramafites forming this protrusion found. The xenoliths size varies from a few
centimeters to hundreds of meters, the shape - from angular to rounded and the
contacts with others gabbros - from sharp to gradual. According to our observa-
tions, the Kokpektinsky massif has intruded after the formation of Kempirsaysky
ultramafic protrusion [Petrology, …, 1991], which gives grounds to consider it as
a polygenic ultramafic-mafic body. It is assumed that whole variety of rock types
that compose the massif was formed as a result of crystallization of primary mantle,
as well as contaminated mafic melts that appeared under the active collaboration
of primary melts with ultramafic restite substance. According to the results of dat-
ing by different isotopic methods (Rb-Sr, Sm-Nd, U-Pb, Ar-Ar) it was ascertained
that Kempirsay ophiolite association was formed in the range from 420 to 396 Ma
[Edwards & Wasserburg, 1985; Sharma & Wasserburg, 1996; Sharma et al ., 1995;
Melcher et al ., 1999].
Judging by the data obtained by ICP-MS method, the rocks of Kokpektinsky mas-
sif are largely depleted by REE, the chondrite-normalized contents of which are gener-
ally lower than in N-MORB basalts (Table 10.8, Figure 10.15). As we can see by the
level of REE accumulation, the wehrlites and anorthosite are inferior to non-olivine
and olivine gabbro, while the chondrite-normalized Yb contents in them are slightly
higher than La contents. Note that in a sample of anorthosite there is no significant
excess of chondrite-normalized LREE contents over HREE observed, as typical of
many samples enriched with plagioclase. In addition, in the rocks of this massif, espe-
cially in the olivine gabbro and troctolites, there is a slight Eu excess, and the lowest
values of (Eu/Eu*) n parameter were identified in wehrlites, while the highest - in the