Garnets - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 47

L-12 (crystal A)
L-12-2-c

L-13 (crystal A)

L-12 (crystal B)

L-12-4-c

L-13-10-r

L-12-3-r

L-12-1-r

L-13-9-r

L-13 (crystal B)

L-13-1 1-r

Figure 1.8 Chondrite-normalized REE patterns for garnets from eclogites from Atbashi metamorphic
complex (Kyrgyzstan) (data Table 1.5).

On the basis of results of analyses of the chemical composition of garnets from
eclogites of the Atbashi complex as well as coexisting omphacites, we estimated the
PT -parameters of metamorphism of their protoliths [Lesnov et al ., 2004b]. Judging
by them, during the formation of the peripheral zones of garnet crystals and coex-
isting omphacites of sp. L-12 the pressure reached 23 kbar and the temperature

(according to different geothermometers) varied in the range

∼

720-760°C. The for-
mation of internal and middle zones of garnet crystals occurred at a higher pressure
(

∼

570-620°C). Therefore, during the forma-
tion of the eclogite represented in the sp. L-12 the growth of garnet crystals from the
inner to the peripheral areas was carried out under the conditions of increasing tem-
perature and decreasing pressure, i.e. on progressive stage of metamorphism. Similar
calculations were made for sp. L-13 and they showed that the marginal zones of the
garnet grains were formed at a pressure of 22.7 kbar and temperature (according to
different geothermometers) of 840-880°C. The calculations carried out by using the
data on the composition of omphacite grains that are present in the inner zones of
garnet crystals in the form of micro-inclusions, indicate a higher pressure (

∼ 23.8 kbar) and a lower temperature ( ∼

∼ 23.7 kbar)

and a lower temperature (from 690 to 740°C).
Thus, the garnets from eclogites of the samples from Atbashi complex have a
progressive chemical zoning, which becomes apparent in the enrichment with Mg
and depletion by Ca of the peripheral zones of their crystals. They were formed in
the progressive stage of metamorphism of protolith under conditions of increasing
temperature. Towards the periphery of garnet crystals the heavy REE content is con-
sistently decreasing, whereas the contents of light elements remain approximately at
the same level as in the internal zones of the crystals. This kind of zonal distribution