Zircons - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 152

Kha-7a

Kha-7

Kha-7

Kha-7a

Figure 4.6 Chondrite-normalized REE (1) and multielemental (2) patterns for zircons from horn-
blende gabbros of Khayalygsky massif (Tuva, Russia) (data Table 4.5).

While isotopic dating of hornblende gabbros of Chernoistochninsky and Reftinsky
massifs (Urals, Russia) by accessory zircons in individual samples of the latter their,
REE composition was determined [Krasnobaev et al ., 2004]. In these zircons the chon-
drite-normalized HREE contents predominate over the contents of LREE, as indicated
by the steep positive slope of the REE patterns. The zircons from the rocks of Cher-
noistochninsky massif have higher overall level of REE accumulation (La - 40 t.ch.,
Lu - 3000 t.ch.) compared with the mineral from rocks of Reftinsky massif (La - 0.6 t.ch.,
Lu - 1300 t.ch.). Nevertheless, a positive Ce anomaly on patterns of zircon from Cher-
noistochninsky massif has a much lower intensity than this anomaly in the mineral
from rocks of Reftinsky massif. In the spectrum of the latter the positive Eu anomaly
is of minimal importance, while it is very clearly shown in the spectrum of zircon from
Chernoistochninsky massif.
Ronkin & Nesbit [1997] studied the REE composition of zircons from certain
kinds of rocks that form the Berdyaushsky massif (Urals, Russia), including gabbro.
It was ascertained that in the zircons from this gabbro the chondrite-normalized La
contents are around 0.015-03 t.ch., which is about the same level as the Pr content.
The determined Ce content in these zircons is about 1-8 t.ch., which causes the pres-
ence of Ce positive anomalies in their patterns. At the patterns intervals between Pr
and Lu they have a form of almost straight positively inclined lines complicated by
weak negative Eu anomalies.
Interesting data was obtained during studies on REE composition of zircons
from gabbros that underwent brittle-plastic deformation and are dredged in Markov
depression (Mid-Atlantic Ridge) [Zinger et al ., 2010] (Table 4.1, Figure 4.7). Accord-
ing to the U-Pb isotope dating of zircon grains from the gabbros, it was determined
that their age varies from 2.3 to 0.7 million years. The authors of this paper concluded
that under the plastic deformation, accompanied by infiltration of the intergranu-
lar fluids of unknown nature, the studied zircon crystals have undergone an uneven
enrichment with several trace elements, including REE, U, Th, Hf, P and Y. On the
example of zircon crystals from particular sample of gabbro it was found that the 238
U/ 206 Pb parameter varies significantly even within individual grains of the mineral.
At the same time, the values may differ in order, to some extent correlated with the