INTRODUCTION
The Trà Bồng shear zone (TBSZ) is exposed on a length of
100 km, extending in east-west direction from the Khâm Đức Townlet through Trà
Bồng and plunges under the Quaternary cover when reaching the
Numerous shear zones accompanied by activities of
deformation and metamorphism characterize the geologo-tectonic structure of the
Vietnamese territory. They usually have the NW-SE trend that changes gradually
to E-W, such as the Sông Mã, Sông Cả, Đại Lộc - Khe Sanh, etc. zones. The rocks
of different primary lithological composition exposed along these zones are
mylonitized, even ultramylonitized. Strike-slip activities accompanied by
deformation-metamorphism took place in main relation to the Indosinian movement
in about 245 Ma [3, 5] and renewed in younger stages [6]. On the structural
pl.an the TBSZ has the parallel structural direction with the Đại Lộc -
On the structural plan the TBSZ extends nearly 100 km
from the Khâm Đức area, where it connects with the Pô Kô River fault zone, then
extends in E-W direction to the Trà Bồng Townlet and plunges under Quaternary
formations when reaching the
Figure
1. Structuro-geological schema of the TBSZ
(simplification after geological maps at 1:200,000 scale)
Note: 1) Micaschist with C/S structure of the Khâm Đức Group; 2) Gneissic schist of the Ngọc Linh Group; 3) Schist of the Sa Thầy Formation; 4) Mylonite of the TBSZ; 5) Granite; 6) Deformed granite; 7) Granodioritic and dioritic orthogneiss; 8) Mafic and ultramafic rocks; 9) Quaternary sediments; 10) Faults; 11) S1 structural direction of laminated deformation; 12) Right shear, 13) Trà Bồng Fault; 14) Hưng Nhượng Fault; 15) Pô Kô Fault; 16) Sampling locality.
The TBSZ is
formed by crystalline schist, amphibolite and intrusives of granodioritic -
dioritic composition. It is bounded by two great faults: the Trà Bồng Fault in
the north and the Hưng Nhượng one in the south. The width of the zone
approximates 8 km that gradually decreases westward, when the zone changes the
direction and joins the Pô Kô shear zone (Fig. 1). Almost all the rocks in the
shear zone are very strongly deformed forming mylonites, even ultramylonites.
Granite of the Hải Vân Complex penetrates the above deformed-metamorphic
formations.
The study on the
petro-structural characteristics has been carried out along sections
perpendicular to the structural direction of the zone at Trà Bồng, Trà My and
Khâm Đức. The results have been showing that almost all quartzite,
micaschist, quartz schist, amphibolite and granite, granodiorite express very
strong level of deformation and change into gneissic schist and mylonite.
Mylonites have all the characteristic tectonic-L texture, almost impossible to
distinguish the bedding or banded structure of primary rocks. The foliation
feature is rather homogeneous along the zone, among that the structure of
crystalline foliation is characterized by the alternation between mica-rich and
quartz-feldspar-rich beds. The foliation direction changes in the interval of
90-110° and the foliation plane always dips southward with the dip changing
from 70 to 80°, locally nearly vertical (Fig. 1b). The surface of these schists
usually contains sillimanite, quartz and amphibole that are elongated forming
the lineations and almost parallelly directed to the horizontal surface with
the inclining angle of 0-10° (Fig. 1b). This structural feature shows that
these schists were formed under the impact of the strike-slip process. The
petro-dynamic indices (S/C, d, s) studied at different scale have been showing that the strike-slip
activities correspond to the right shear direction (Fig. 1a, b).
The petrologic
studies have been showing that mylonites have different petrographic
composition, consisting mainly of quartzite, quartz-mica schist, dioritic
orthogneiss, granodioritic orthogneiss and amphibolite. The analysis of
paragenetic mineral associations of the pelitic rocks has been showing the
major part of them is characterized by the association of
quartz-muscovite-biotite- fibrous sillimanite ± andalusite. Under the microscope
these minerals are all directed. As for the rocks of dioritic and granodioritic
composition, they were formed by such minerals as: K-feldspar with
inverse-perthitic texture, zonated yellow-brown hornblende, residual
clinopyroxene and titanium-rich biotite. The presence of muscovite-biotite -
fibrous sillimanite existing in equilibrium in metapelite and the existence of
high-temperature minerals in metamorphic rocks of intrusive origin show that
these mylonites were formed in high-temperature condition. Besides, the
presence of andalusite proves the low mean pressure condition. The existence of
these mineral associations shows that the studied rocks were metamorphosed and
deformed in the conditions of rather high temperature and low mean pressure, and
belong to the lower part of amphibolite facies.
So, the
petro-structural study has been allowing to draw the following conclusions: the
TBSZ passed through the metamorphic process characterizing the low mean
pressure - rather high temperature conditions in the process of plastic
deformation. The kinematic peculiarity of the shear zone is characterized by
right strike-slip. For determining the age of this tectonic activity the
authors have been taking the mylonite samples VN.284, VN.286 for age analysis by
the 40Ar-39Ar method.
II. PETROGRAPHIC FEATURES OF ANALYZED SAMPLES
Aiming to date
the activities of the TBSZ, the authors have been collecting samples, the
formation of which was directly related to the moving process. Therefore, the
two mylonite samples of diorite and granodiorite composition (VN.284 and
VN.286) were directly collected in the TBSZ, among them the sample VN.284 was
taken from the margin of the zone coinciding with the position of the Trà Bồng
Fault. As for the sample VN.286, it is collected in the centre of the zone.
These two different localities will allow to study on the evolution in space of
the shear zone. The sampling localities are presented in the Figure 1a.
a) The mylonite
sample VN.284 (15°15’08” N; 108°34’34” E) has granodioritic composition bearing
quartz, K-feldspar, albite, andesine, hornblende, biotite, epidote, sphene and
zircon. Almost all these rock-forming minerals are stretched and directed,
proving the crystallization during the metamorphic process accompanying the ductile
deformation.
b) The mylonite
sample VN.286 (15°14’14”N; 108°37’59”E) has dioritic composition,
characterizing the destruction in high temperature proved by the banding of
quartz, albite, Ti-rich biotite, apatite, zircon and Fe oxides.
For analyzing the
forming age of these mylonites, the syn-moving biotite from the above samples
was taken for analyzing by the 40Ar-39Ar method.
III. RESULTS OF 40Ar-39Ar DATING
The
syn-metamorphic-deformed biotite separated from mylonite samples was sent to
IV. DISCUSSION AND
CONCLUSIONS
The advantage of the Ar-Ar isotopic dating method is
that, it allows to recognize from low to high thermo-tectonic events in the
same analyzed sample. Usually, Ar occurs from the radiation process from K
existing in K-bearing minerals, keeping K in their lattice. When the minerals
are impacted by a thermo-tectonic event, according to the degree of impact a
part or all occurring argon is liberated from their lattice. If the mineral is
impacted by a low temperature event, only a part of argon is lost in the
position of low-energy levels (in the margin or in defects of the lattice
structure). As for the argon stored in high-energy levels in the centre of the
crystal lattice, it is lost only under the impact of metamorphic process at the
temperature high enough for surpassing the threshold of its closing temperature.
Therefore, with the technics of partial gas push the 40Ar-39Ar
method allows to reflect different events that other methods cannot have.
The results of 40Ar-39Ar age
analysis have been showing that the biotite sample VN.286 gives the plateau age
of 223 Ma (Fig. 2b). This value corresponds to 85% 39Ar liberated in
high temperature. Besides, the age spectrum still gives a low-temperature value
corresponding to over 10% of liberated 39Ar. The results calculated
for these low-temperature levels are 100 Ma (Fig. 2b). The biotite sample
VN.284 gives an older age. The calculated average age corresponding to 95% of
liberated 39Ar is 243 Ma (Fig. 2a). Similar to the sample VN.286 the
age spectrum of this sample gives also information on the low- temperature age
clearly expressed through the corresponding plateau age in the first 3 levels.
These first 3 levels corresponding to 5% liberated Ar give the average age
value of 70 Ma (Fig. 2a). Due to the rather great errors of the levels, the
authors have been checking the results by establishing the ischrone line on the
inverse isochrone diagram for these first 3 levels. The obtained results give
also the age values of about 68.7 ± 6 Ma (Fig. 2c), corresponding to the value
of 36Ar/40Ar ratio of the primary atmospheric argon of
295.5 Ma. So, the two ways of demonstration have been giving the approximate
values, proving that the samples have been recognizing a low-temperature phase.
