Zircons - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 156

The isotope-geochronological and geochemical studies were recently carried out of
the large collections of zircon samples from the Archean and Paleoproterozoic hyper-
sthenic gneisses forming Irkutny granulitic-gneiss block of Sharyzhalgaisky ledge in
south-western part of the Siberian craton (Russia) [Turkina et al ., 2011]. Based on the
identified distinguishing features of zircons, including the morphology of their crys-
tals, isotopic age and REE composition, the authors singled out three generations of
this mineral. The first, represented in the cores of the crystals with relict growth zon-
ing, is defined as a magmatic generation. The zircons relating there to (age 3.16 bil-
lion years) are characterized by relatively high values of (Lu/Gd) n parameter (11-36)
and very intense positive Ce anomalies. Zircons of the second generation, which is
defined as metamorphogenetic (age 3.04 billion years), are present in the form of

multiplanar crystals, shells and nonzonal cores of crystals. These zircons are depleted
by REE and are characterized by much lower values of the (Lu/Gd) n parameter, than
the zircons of the first generation. The third generation of zircon is the latest and also
identified as metamorphogenetic one. Zircons of this generation (age

∼

1.85 billion
years) are represented by narrow rims surrounding the crystals of zircon of the earlier
generations. They are characterized by medium values of (Lu/Gd) n parameter.
Fedotova et al . [2008] studied the REE composition of zircons from subalkaline
magmatic rocks, syenites and tonalites of the Karelian region and metamorphic rocks
of granulite facies of the Ukrainian shield. Zircons from tonalites and syenites, which
crystallized in equilibrium with the melt, have positive slopes of distribution pat-
terns of REE chondrite-normalized contents. They are complicated by the maxima
of Ce and minima of Eu. Zircons from metamorphic rocks, which crystallized under
granulite facies, have the REE patterns dramatically different from the minerals of the
syenite and tonalites lower concentrations of HREE, which is due to the competing
influence of coexisting garnets.
Origin and REE compositions of zircon in jadeitite from the Nishisonogi meta-
morphic rocks (Kyushu, Japan) were examined by Mori et al . [2011]. The cores of
zircon grains are richer in Y, Th, Ti and REE, but the rims are richer in Hf and U.
Chondrite-normalized REE patterns of the cores indicate higher (Sm/La) n ratios, lower
(Yb/Gd) n ratios and larger positive Ce anomalies compared with those of the rims.
Granitoids and acid effusive rocks. The REE composition of zircons, presented
in high-silicon plutonic and volcanic rocks that differ in composition and conditions
of formation, have been studied with various degree of detail (Table 4.6). In addition,
the zircons from granite and dacite, that are common in some parts of Japan, were
investigated regarding the REE distribution among the first [Nagasawa, 1970]. In this
study it was shown that the zircons from Osumi and Takakuma granitoidal massifs
have chondrite-normalized contents of the elements increasing in the row from Ce
(200-400 t.ch.) to Yb (2000-4000 t.ch.) (Figure 4.9, 2, 3). Moreover, in the zircons
from dacites that are forming Torihama and Ito massifs, the level of Ce accumulation
turned out to be even higher (Figure 4.9, 4).
Later on, the studies were concentrated on the zircons from tonalities of Quottoon
complex (British Columbia, Canada), the content of REE in which, as well as their total
amounts, show considerable variation (Table 4.6). Chondrite-normalized contents of
the elements in them are dramatically increasing in the row from La (1-20 t.ch.) to Yb
(600-6000 t.ch.), while the configuration of REE patterns and intensity of positive Ce
anomalies are only marginally affected (Figure 4.9, 1).

∼