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THE TAS-KYSTABYT MAGMATIC
BELT (NORTHEAST ASIA):
FIRST U-Pb (SHRIMP) AND
Sm-Nd DATA
*Diamond
and Precious Metal Geology Institute SB RAS, Yakutsk, Russia,
prokopiev@diamond.ysn.ru
**West
Virginia University, Morgantown, USA, jtoro@wvu.edu
***Stanford
University, Stanford, USA, miller@pangea.stanford.edu
The
Tas-Kystabyt magmatic belt extends north-northwestward for 300 km in
the southeastern part of the Verkhoyansk-Kolyma region. It comprises
a discontinuous chain of granitoid plutons, the large Taryn
subvolcano,
and volcanogenic-sedimentary rock fields. The belt is located in the
area of a knee-like bending of the Adycha-Taryn fault zone, at the
boundary between the Verkhoyansk fold-and-thrust belt and the
Kular-Nera terrane. From geological and geochronological data,
different-aged magmatic rocks occur within the belt limits (Rudich,
1959, Popov, Kuznetsov, 1987; Shkodzinsky et al., 1992; Mishin,
1994; Bakharev et al., 1997; Layer et al., 2001).
We
studied the largest magmatic formations of the belt such as the
rhyolite complex, the Taryn subvolcano, and granitoids of the
Nel’kan pluton. U-Pb dating of zircon grains was performed
using SHRIMP RG (Stanford, USA). For each sample we analyzed 10 to
11 spots on zircon grains including cores and rims. The obtained
isotopic ages are generally concordant and have very low or no
common Pb. Below we report 207
Pb-corrected
206
Pb/238Pb
mean ages for the magmatites.
The
rhyolite complex comprises volcanic rocks of a
sedimentary-volcanogenic sequence (700-850 m) delineated in the
southwestern margin of the belt. The sequence is composed of
felsorhyolites of uniform mineral composition as well as agglomerate
and vitric crystalloclastic rhyolite tuffs. In the lower horizons of
the sequence, mudstones, siltstones, and tuffaceous sandstones are
present. From the early studies of flora in the tuffaceous
sandstones (Popov, Kuznetsov, 1987) it was established that the
rhyolite complex has a Neocomian (Berriasian-early Valanginian) age,
and that it overlies, with a stratigraphic unconformity, the
Callovian-Kimmeridgian volcanogenic-sedimentary rocks. We
estimated the age of crystallization of the rhyolite complex at
152.2±1.4
Ma (MSWD=1.34) (Late Jurassic, Kimmeridgian),
which makes the flora-based dates questionable.
The
dacite complex composes the Taryn subvolcano (about 1500 km2
in area) in the axial part of the belt. The subvolcano extends
north-northwest for nearly 100 km. The sedimentary host rocks of
Triassic and, locally, Lower and Middle Jurassic age, plunge under
the subvolcano, and are complicated by normal faults. The dacites
intrude the rhyolite complex, and have 40Ar-39Ar
ages of 134–138 Ma. The initial Sr R0
isotope composition is 0.7100-0.7110 (Tectonics…, 2001). The
obtained U-Pb dates estimate the age of crystallization of the
dacites at 149.9±1.2
Ma (MSWD=0.58) (Late Jurassic, Kimmeridgian-Volgian boundary time).
The
large (1200 km2)
Nel’kan pluton occurs in the northern part of the belt. It is
made of biotite adamellites grading, in the endocontact zone, into
granodiorites. The granites form bodies of irregular shape in the
central part of the pluton. It was earlier interpreted, from Rb-Sr
dates and model calculations, that adamellites and granites were
emplaced at 157–152 Ma (R0=0.704)
and 136–134 Ma (R0=0.7070),
respectively (Shkodzinsky et al., 1992). The 40Ar-39Ar
ages available range from 144.8 to 139.8 Ma (Layer et al., 2001;
Tectonics…, 2001). The
U-Pb dating yielded the crystallization age of 147.8±1.1
Ma for the granitoids of the Nel’kan pluton (MSWD=0.88).
Sm-Nd
isotope studies of the Nel’kan pluton granitoids and the Taryn
subvolcano dacites were carried out. The
rocks have a Mesoproterozoic Nd model age – TNd
(DM-2st) = 1527 and 1458 Ma, and negative values of ºNd
(T) = –6.9 and –6, respectively, which is indicative of
the dominant role of continental crust material in the source.
Inherited
cores of two zircon grains from the granitoids of the Trud pluton
were earlier dated at 1.6 and 1.9 Ga (Prokopiev et al., 2007). All
the dates obtained form two groups of concordant values on a
discordia with a lower intercept of 153±3
Ma (age of crystallization of granitoids) and the upper intercept of
1880±26
Ma (age of crust protolith). At the same time, the granitoids have a
Mesoproterozoic Nd model age – TNd
(DM-2st)=1457 Ma and a negative value of ºNd
(T)= –6, which is also suggestive of the most important
contribution of the continental crust component.
Thus,
the data obtained indicate that crystallization of the largest
magmatic formations in the belt was practically synchronous during
the Late Jurassic (Kimmeridgian-Volgian). The rhyolites and the
Taryn subvolcano dacites were formed much earlier than in the Early
Cretaceous. The earlier obtained Early Cretaceous 40Ar-39Ar
ages (Bakharev et al., 1997; Tectonics…, 2001) are likely to
reflect the cooling time of the plutons and the fact that isotope
systems were disrupted during the tectonomagmatic activation in the
region. The origin of the magmatic belt is unclear. On the Rb –
(Y+Nb) diagram, the data points of the rocks plot at the boundary
between the island-arc, within-plate and collisional granitoids
(Tectonics…, 2001).Their synchronous emplacement with the
volcanites of the Uyandina-Yasachnaya magmatic arc and granitoids of
the Main belt in the axial part of the Upper Chersky orogen
(Prokopiev et al., 2007) suggest its relation to
subduction-accretion processes occurring on the eastern margin of
the North Asian craton in the Late Jurassic.
This work was supported by the
Stanford University and grants 07-05-00743, 06-05-96070, 06-05-96069
from the Russian Foundation for Basic Research and ONZ - 7.10.2.
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