1. Geology
RARE EARTH ELEMENT DISTRIBUTION IN ROCKS OF THE UGATKIN
PALEO CALDERA (CHUKCHI DISTRICT, RUSSIA)
PhD Ildar Kalko
Scien. Dmitry Bychkov
PhD Irina Nikolaeva
Prof. Vsevolod Prokofiev
PhD stud. Yulia Sidorina
Lomonosov Moscow State University, Russia
ABSTRACT
The Ugatkin paleo caldera is situated in the external zone of the Okhotsk-Chukchi
volcanic belt. There are three volcanic units representing contrast volcanic rocks. The
oldest unit is composed of andesitic tuff and lavas. Middle aged rocks are rhyolitic lavas
and ignimbrites. Acid obsidian unit is the youngest. Subvolcanic rocks are represented
by rhyolites, andesites and basalts.
There are several zones with silver LS and polymetallic mineralization in the caldera.
Samples were analyzed by ICP-MS and XRF.
The composition of rocks is typical of calc-alkaline series formed in island arc setting.
The rocks are enriched in light rare earth elements ((La/Lu)n = 2.4-11.3) that indicates
homogeneity of the magmatic source. The rocks are characterized by the depletion of
Eu ((Eu/Eu*)n = 0.6 – 1) which is due to incomplete melting of the source plagioclase.
Keywords: Rare earth elements, volcanic belt, Chukchi Peninsula
INTRODUCTION
The Ugatkin paleo caldera is situated in the external zone of the Okhotsk-Chukchi
volcanic belt (OChVB). The location of the caldera is shown in figure 1.
We examined chemical and mineralogical composition of 12 rock samples.
Analysis on major constituent content was conducted in the Institute of Geology of Ore
Deposits, Petrography, Mineralogy and Geochemistry, Russian Academy of Sciences
(Moscow).
The REE content was determined by the ICP-MS technique using an Element-2 mass
spectrometer at the Laboratory of experimental geochemistry of the Faculty of Geology,
Moscow State University [2]. Samples were digested using a standard procedure in a
hot mixture of concentrated acids (HF:HNO3:HCl in the proportion 5:1:1) by heating in
a microwave oven. About 0.1 g rock sample was loaded into a 7cm3 SavillexTM vials
with 3.5 ml of the acid mixture. After microwave heating, the sample with HCl was
evaporated three times under a lamp with vapor withdrawal using a water-jet pump. The
residue was quantitatively transferred into 50 ml of 3% HNO3 solution. Solution was
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diluted by a factor of 100, and indium was introduced as an internal standard. The
accuracy of analysis was checked by measuring the BCR-2 standard and blank prepared
using the same procedure. The deviation from the certified values was no higher than
10%.
For petrographic study thin sections were prepared. Rock samples contain 5-10% of
chlorite which might have caused slight redistribution of rare earth elements.
Data from the report on geological mapping of 1:50,000 scale [1] were used for
reference and comparison.
Figure 1. Location of the Ugatkin paleo caldera
GEOLOGICAL SETTING
Volcanic and volcanoclastic rocks of Upper Cretaceous age are identified as subalkaline
and alkaline felsic ones (fig. 2). Composition of suites ranges from andesites to
rhyolites, mainly represented by felsic rocks. Subvolcanic bodies are represented by
porphyry andesites, diorites and rhyolites (fig.3). Ore bearing rocks have porphyry
texture with phenocrysts of plagioclase [3]. Geophysical study predicted shallow (1.5
km deep) felsic batholiths in the area. The age of rocks is about 87 Ma [4]. There are
zones of quartz veins with IS silver-gold mineralization in the area. Wallrocks are
altered to varying degrees to potassic, propylitic and argillic assemblages. For research
1. Geology
we selected less-altered samples, although small amount of chlorite, epidote, mica and
quartz was present. Locations of samples are shown in figure 2.
Figure 2. Geological map of the Ugatkin paleo caldera
Most of the rock samples contain phenocrysts in crystallized glass. Rock samples #1
and #2 are the most altered: plagioclase in sample #1 is partially replaced by carbonate
and chlorite replaced biotite both in samples #1 and #2. Quartz veinlets 30 m thick
were diagnosed in sample #10.
Half of the samples were analyzed for macro-component composition by XRF. The
results were compared with the data of previous studies [1].
On TAS diagram (fig.4) the circles of rock composition are grouped in narrow area
within VIII, IX, XIII, XIV and XV sections or at their borderlines. Subvolcanic rocks
are more felsic or more mafic than volcanic rocks. Probably there were several
chambers intensified at different stages of volcanic activity.
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Figure 3. Thin sections photos (samples from 1 to 12)
1) propylitic alteration of rhyodacite; 2) andesite-dacite; 3) dacite; 4) dacitic ignimbrite;
5) andesite-dacite tuff; 6) dacite; 7) rhyolite; 8) rhyodacite; 9) trachyte or trachidacite;
10) dacite; 11) trachyte; 12) rhyolite (Bi – biotite; Pl – plagioclase; Q –quartz; Kfs – Kfeldspar; Ep – epidote; Chl –chlorite)
AFM classification (fig. 5) of rocks shows calc-alkaline trend which is typical of rocks
formed in island arc conditions [5].
1. Geology
Figure 4. TAS and AFM classification diagrams for rocks (data from previous study [1]
are included). 1) Upper Cretaceous intrusive rocks. 2) Upper-Lower Cretaceous
subvolcanic rocks. 3) Upper Cretaceous volcanic rocks.
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The REE content shown in diagrams was normalized to the CI chondrite composition
[6]. The results of REE analysis of the rocks revealed that alteration did not obliterate
the REE distribution features of the wall rocks (fig.4).
The REE patterns of the rocks show a pronounced negative Eu anomaly (Eu/Eu* = 0.61). Their total REE content ranges from 56 to 182 ppm. Redistribution of elements does
not occur in weakly altered rocks but there are a number of samples that demonstrate
significant depletion of light rare earth elements probably due to hydrothermal
alteration.
Figure 4. REE distribution patterns of the Ugatkin paleo caldera volcanic rocks
CONCLUSIONS
The Ugatkin caldera was formed in Upper Cretaceous epoch in island arc conditions.
Low variation of element content is the evidence of magmatic source homogeneity.
REE distribution indicates incomplete melting of the source plagioclase.
Redistribution of elements does not occur in weakly altered rocks but there are a
number of samples that demonstrate significant depletion of light rare earth elements
probably due to hydrothermal alteration.
ACKNOWLEDGEMENTS
This study has been supported by the Russian Science Fund, project no. 14-13-01386.
1. Geology
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