ISSN 1028334X, Doklady Earth Sciences, 2010, Vol. 432, Part 1, pp. 587–592. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © G.A. Shatkov, N.G. Berezhnaya, Ye.N. Lepekhina, N.V. Rodionov, I.P. Paderin, S.A. Sergeyev, 2010, published in Doklady Akademii Nauk, 2010,
Vol. 432, No. 3, pp. 360–364.
587
The Streltsov ore cluster lies within the Argunsk
median mass (microcontinent) of the Central Asian
mobile belt and has unique reserves of highquality
ores (about 300 000 tons of uranium) [1, 5]. It is situ
ated in the Tulukuev volcanotectonic structure (VTS)
(220 km
2
) that consists of a collapse caldera (180 km
2
)
and three centers of acid volcanism at the western edge
of the caldera (Fig. 1). The VTS structure consists of
the lower and upper layers and the bottom. The strati
graphic sequence of volcanogenic–sedimentary
deposits and their relations with subvolcanic rocks are
reliably established, but in the structure there are no
paleontological remains and the available values of
rock ages calculated using the K–Ar and Rb–Sr meth
ods [1, 3, 7] vary significantly for the particular hori
zons of rocks and dykes. Precise results are obtained
only for uranium ores, 135
±
1 Ma, by uranite [4].
The question on the genesis of the Streltsov ore
cluster comprising 19 deposits remains point of fierce
disputes [1–3, 5, 7]. Some authors provide arguments
that rhyolites might be the source of uranium in
deposits [2, 3, 6]. To solve the geochronological part of
this problem, we performed dating of zircons from the
rocks of the lower and upper layer in the Tulukuev
caldera using the local SIMS SHRIMPII method
(Fig. 2). To select the dating places, the internal struc
ture of the zircons was studied by the method of cath
ode fluorescence (CF). Table 1 presents the morpho
logical and geochemical properties of magmatic zir
cons and their respective concordant U–Pb age. The
massspectrometry measurements and calculations
were made according to the standard procedure
described in [8, 9]. Figure 2 shows the position of
imaging points on the concordance graphs.
The lower layer (
J
2–3
) of rocks is composed of three
covers of andesite–basalts that are interlaid by two
covers of lavas and tuffs of trachydacites. The section
bottom consists of horizons of sedimentary rocks. The
total thickness of the lower layer reaches 800 m. The zir
cons from trachydacites in the upper (sample 1087/70)
and lower (sample 191a/67) covers have similar mor
phology and internal structure by the KL (Table 1).
The age of zircons from the lower and upper covers of
trachydacites is 162
±
2 (sample 191a/67) and 156
±
2
(sample 1087/70) Ma, respectively (Fig. 2). These age
values are interpreted as the time of magmatic crystal
lization of the rocks.
U–Pb (SIMS SHRIMP–II) Age of Volcanic Rocks
from the Tulukuev Caldera (Streltsov UraniumOre Cluster,
Eastern Transbaikalia)
G. A. Shatkov, N. G. Berezhnaya, Ye. N. Lepekhina, N. V. Rodionov,
I. P. Paderin, and S. A. Sergeyev
Presented by Academician D.V. Rundkvist December 24, 2009
Received January 12, 2010
Abstract
—The precision dating (U–Pb local by zircons, SHRIMP–II) of volcanic rocks in the unique ura
niumbearing structure of Transbaikalia is performed for the first time. The basic conclusions are as follows.
The volcanic activity in the Tulukuev caldera covers the period of not less than 30–35 mln years, within the
period from (not later than) 162 to 128 mln years. Two stages of caldera evolution are established: the early
(trachydacite–basalt) stage up to 154 mln years and the late (trachybasalt–rhyolite) stage from 142 to 128 mln
years, with a 10 mln year break, which caused the deep erosion of the lower layer. Three phases of rhyolite
magmatism are substantiated. The first one, 142 mln years, is the ejection of ignimbrites (microfelsitic rhyo
lites); the second one, 137–135 mln years, is the outflow of lavas of sanidine–morion rhyolites and subvol
canic and ring dyke intrusions. The third phase, 128 mln years, is connected with the occurrence of cesium
bearing perlites in the southwestern part of the caldera. The age of the granite–porphyries of the Krasnoka
mensk stock almost coincides with the precision data of the age of the uranium ores [4]. It is found that zir
cons from the granite–porphyries within the ore field of the Argunsk deposit have an anomalously high con
tent of uranium. This fact can additionally testify to the timeandspatial closeness of magmatism and pro
cesses of ore formation.
DOI:
10.1134/S1028334X10050089
Karpinskii AllRussia Research Institute of Geology,
Saint Petersburg, Russia
email: vsegei@vsegei.ru
GEOLOGY