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The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 701
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
1Demonstrator, Department of Geology, Meiktila University
2Demonstrator, Department of Geology, Meiktila University 3Lecturer, Dr, Department of Geology, Mandalay University
# Corresponding author: [email protected]
Geochemistry and Geochronology of Igneous Rocks Exposed in Tawma-
Nabebin Area, Singaing Township, Mandalay Region
Wai Yan Phyo1#
, Thae Su Htwe2, and Tin Aung Myint
3
Abstract
Tawma - Nabebin area is situated near Singaing Township, Mandalay Region. It is located between East Longitude 96° 07′ and 96° 15′ and North Latitude 21° 40′ and 21° 45′ in one
inch topographic map no. 93 C/2. Granitoid rocks in Myanmar occur in three north–south
trending linear belts over a distance of more than 1500 km from Kachin State in the north
through Mogok, Mandalay, Taungoo and Mon State to the Tanintharyi Region in the south.
The study area is located within the central granitoid belt and tectonically, it lies within the N-
S trending Mogok Metamorphic Belt (MMB). The major element data, using the construction
of variation diagrams, display the data as bivariate or trivariate plots. Field and petrochemical
data suggest that hornblendite, hornblende diorite, biotite microgranite and other granites
intruded along the highly deformed metamorphic rocks of the study area. The igneous rocks
exposed in the area consist of hornblendite, hornblende diorite, tourmaline granite, biotite
granite, biotite microgranite, leucogranite, garnet-biotite granite and porphyritic biotite
granite. Metamorphic rocks are biotite muscovite schist, biotite schist, epidote quartzite, diopside calc-silicate rock, and white marble. Hornblendite is the oldest igneous and might
have been formed before the continental epiorogenic uplift. Generally, the granitoid rocks are
classified into I-type and S type. All granitic rocks of the study area fall in S-type except
hornblendite and some hornblende diorite which are typical I-type characters. The age of the
igneous activity in the study area might have been formed during Tertiary period.
Keywords: Geochemistry, I-type granite, S-type granite, geochronology of igneous rocks
Introduction
In this paper describes a suite of granitoid rocks exposed at the Tawma - Nabebin
area, covering about 48 km2 in the near Singaing Township, Mandalay Region, Central
Myanmar (Fig. 1), described previously. The area is located within latitudes 21° 40′ N and
21° 45′ N and longitudes 96° 07′ E and 96° 15′ E and lies about 25 km south of Mandalay.
Mogok Belt including the study area lies within the western part of the Sibumasu Block (Also
known as the Shan- Thai Block, Shan –Tennasserim Block, the Sinoburmania Block or the
Burmese Malayan Block), along the northwestern margin of the Shan Plateau and southwards
between the north- south trending Sagaing fault and Shan Scarp (Searle and Haq, 1964).
Therefore, MMB is the key tectonic position in the SE Asia tectonics.
The aim of this paper is to describe the petrology, geochemistry and geochronology
of these granitoid rocks in the Tawma - Nabebin area and to discuss their petrogenesis and
tectonic setting.
Regional Framework and Local Geology
The study area lies between the western margin of Shan-Tanintharyi Belt and the
eastern margin of Central Cenozoic Belt. It is also situated within the N-S trending Mogok
Metamorphic Belt of highly deformed metamorphic units (Searl and Haq, 1964). The rock
assemblages of Mogok Metamorphic Belt not only occur at Mogok and its environs but also
are widespread throughout Thabaikkyin, Madaya, Mandalay and Kyaukse areas. The Mogok
Metamorphic Belt including the study area extends over 1500km along the western margin of
702 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Shan-Thai Block. The metamorphic units of the study area were regarded as Precambrian age
(La Touche, 1913 and Chhibber, 1934). Recently, various workers such as Mitchell et al.,
(1999, 2007), Bertrand et al.(2007), Bertrand and Ranging (2003). Morley et al., (2002),
Barley et al., (2003), Searle et al., (2007), Khin Zaw et al., (1990 & 2014 a&b) studied the
Mogok Belt and expressed their opinions on the geology, metamorphism, magmatism and
tectonisim of the Mogok Metamorphic Belt. Myint Thein (1975) study that most granodiorite
and diorite rocks of the Belin area and Kyaukse area have originated metasomatically from
epidote-biotite schists and epidote calc-silicate rocks on the basis of the field and
petrographic evidences. The metamorphic units of Jurassic-Cretaceous age probably crop out
in the Thakhinma Taung and the Tawzu Area, north of Mandalay. In this area, the pelitic and
calcareous metasediments of Jurassic-Cretaceous age are exposed at the Belin and Sunye
area. Medium to high grade metamorphic rocks of the study area were regarded as
Precambrian to Paleozoic age (Bender, 1983). Than Than Nu (1990) described the petrology
of the Belin and Nwale Hill, Singaing Township, Mandalay Region. Lay Lay Khaing (2007)
stated that on the igneous processes governing the emplacement of the Belin intrusives in
Singaing Township, Mandalay Region. Thet Tun (2009) study "Petrology and petrogenesis of
the granitoid rocks exposed along the Shan boundary fault system between Belin-Kyaukse
and Yamethin area, Mandalay division". He stated that the granitoid magmatism of the Shan
scarp region formed in relation to the pre-dated collosion of India with Eurasia during Mid-
Jurassic to Cretaceous Time. Geochemistry and petrogenesis of igneous rocks exposed in
Tawma-Nabebin area, Singaing Township, Mandalay region described by Thae Su Htwe
(2016).
Figure (1). Location map of the Tawma - Nabebin area, Singaing Township, Mandalay
Division.
Study area
The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 703
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Petrography of the Granitoid Suite
Methods of Study
Fieldwork included sampling of representative rock units, the measurement of
geological structures and geological mapping using tape and compass traverses, assisted by
GPS positioning. Unknown minerals were more than fifty representative rock samples
collected from the study area have been selected and analyzed for the petrologic studies.
Petrography was studied using 80 thin-sections, and modal analyses of the igneous rock units
were made using a mechanical points counter in conjunction with a petrological microscope.
For petrochemistry, chemical data are obtained by the XRF analysis at MURC (Mandalay
University Research Center) in Mandalay City. The major and trace element oxides from
XRF analysis are shown in tables (1) and (2). Granitoid rocks are classified and named on the
basis of modal analyses, plotted using total alkali versus silica, (TAS) diagram of Cox et al.,
(1979) adapted by Wilsom (1989) showing the field of plutonic rocks of the study area.
Their detailed mineralogy and textures are described below. For the present study mineralogy
and major element oxides are used to describe, name and classify rocks. In most cases, the
chemistry alone is the criterion for classification. The major element data, using the
construction of variation diagrams, display the data as bivariate or trivariate plots. This type
of diagram is used to show the interrelationship between elements in the data-set and it is
from these relationships that geochemical processes may be inferred. These diagrams were
drawn using Tridaw, Corel Draw 11, SPSS and Petrograph software.
General Statement
The igneous units exposed in the Tawma – Nabebin area are mainly various granite,
leucogranite, hornblende diorite, hornblendite and pegmatite. Based on the constituents'
minerals assemblages, the metamorphic rocks of the study area are grouped into metapelites,
metacarbonates and skarns. According to the previous works, field relationship, petrography
and stratigraphical evidence, the igneous rocks sequence of the study area can be subdivided
into eight major rock types as shown in figure (2).
Hornblendite
This unit is mainly exposed in the north-eastern part of the study areas near Mogaung
village. It is coarse grains texture, hard and compact nature and consists of hornblende only,
it is hornblendite. They are constituted of mafic minerals only and very dark or very dark
green in color and show a pitted surface in many places. This unit is intruded by pegmatite
and aplite dykes in some places (Fig. 3.a). Hornblendite displays hypidiomorphic granular
texture and is mainly composed of hormblende with minor amount of biotite, sphene, apatite,
plagioclase and tourmaline (Fig. 3.b).
Hornblende Diorite
This unit is best seen in the northern parts of Shwetawya monastery, Taungdanshe
Taung and Balin prison quarry. It consists of mainly plagioclase and hornblende, coarse
grained texture, hard, compact and dark greenish color and manifestation on pitted surface
(Fig. 3.c). Microscopically, the rock is mainly composed of hornblende, plagioclase and
biotite. The accessory minerals are epidote, apatite, magnetite, sphene and quartz.
Hornblende displays hypidiomorphic granular texture and size ranges from 0.01mm to
0.03mm diameter. (Fig. 3.d)
704 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Figure (2). Geological map of the Tawma - Nabebin area Singaing Township, Mandalay
Region (after. Than Than Nu 1990)
Granite Suite
Biotite granite is well exposed near Tawma Village, Kyaukkyi Taung, Taungdanshe
Taung, western and southern part of the Nwale Taung. This rock shows hard, compact and
light grey color on weather surface and light grey to white on fresh surface. It exhibits
medium grained, homogeneous in texture and some granite rocks are showed medium to
coarse grained nature. Some places, columnar structures have six sides and occur in granite
rocks. Exfoliation is the common character of this granite (Fig. 3.e). Xenolilths of country
rocks are found in the biotite granite (Fig. 3.g). This unit is intruded by pegmatite and aplite
dykes in some places, estimating that these pegmatite and aplite are younger than biotite
granite. This biotite granite almost everywhere intruded either regionally metamorphosed
rocks or older intrusions. Porphyritic granite is found as both a core zone in the central part of
the granitoid plutons and individual plutons. It displays the light grey, coarse grained,
porphyritic texture. Quartz and feldspar occur as porphyroblast. Garnet bearing biotite
granite, tourmaline granite and biotite microgranite are well exposed northeast of monastery
and Belin quarry in the study area. It shows white to brownish on fresh surface and light grey
color when weathered. Migmatites are well exposed in the central part of the study area
which is associated with biotite granite and biotite gneiss in many places (Fig. 3.h).
Ptygmatic folds are irregular and isolated fold structures that typically occur as tightly folded
veins or thin layers of strongly contrasting lithology. Leucogranite found as small stocks at
Belin prison quarry and either small or large bodies in some places at Shwemyintin Taung in
the lower part of the mountain ranges. It is coarse-grained and essentially composed of felsic
minerals such as feldspar and quartz. Pegmatites generally occur as small dykes and vein
61 62 63 64 65 66 67 68
61 62 63 64 65 66 67 68
34
33
32
31
30
29
28
27
Q
34
33
32
31
30
29
28
27
The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 705
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
which intruded all metamorphic and igneous units. It shows whitish color on fresh surface
and brownish yellow on weather surface, medium to very coarse-grained, leucocratic rocks.
In some places, small aplite dykes and quartzofeldspathic veins are fairly observed. Most
pegmatite dykes have NNW-SSE trend. In thin section, granite contains orthoclase, quartz,
biotite, plagioclase and accessory minerals (apatite and magnetite). The plagioclase
composition ranges from albite to oligoclase, and the simple twins are very common.
Perthitic orthoclase enclosed within the anhedral plagioclase indicates that the plagioclase is
early crystallized than the perthitic orthoclase. Biotite shows anhedral to subhedral
appearance and strong pleochroism. Some crystals occur as flaky aggregates along grain
boundaries (Fig. 3.f). It is partially altered to chlorite. Muscovite occurs as prismatic form
with one set of perfect cleavage. The size ranges from 0.5 to 2 mm in length. Garnets are
locally abundant in these rocks, and form as anhedral to subhedral grains. Tourmaline occurs
as fails small crystal. Myrmekitic texture is observed at the quartz-feldspar intergrowth. Most
orthoclase show untwined. They are altered to kaolinite and sericite.
Chemical Characteristics of Igneous Rocks
In general, granitic rocks have SiO2 (68-76%), Al2O3 (12-17%), CaO (0.2-2%), MgO
(0.1-0.7%), Fe2O3 (0.1-2%), TiO2 (0.02-0.2%) and Na2O+K2O (3-10%). Based on XRF
analytical data and Harker’s variation diagram (Fig. 4a), major element oxides (CaO, MgO,
Fe2O3, and TiO2) are negatively correlated with SiO2. Na2O, K2O and Al2O3 have a positive
correlation with SiO2.
Wilsom (1989) uses TAS diagram, adopted from Cox et al. (1979), to give a
preliminary classification of plutonic igneous rocks which is one of the most useful
classification schemes. For the present study chemical data, i.e., the sum of the Na2O+K2O
content (total alkalis, TA) and the SiO2 content (S) are taken directly from a rock analysis as
wt% oxides and plotted onto the classification diagram. This diagram, (Fig. 4.b), points out
the bulk composition of igneous rocks is granite. A further criterion is referenced to the
O’Connor (1965) normative Ab-Or-An diagram.
In this diagram, (Fig. 4.c), the majority fall in the granite field and a few in the
Trondhjemite field. In addition, the fractionation trend is well fitted to the Bowen’s reaction
series, i.e., calcic to sodic composition chemical designation in most common use is based on
silica percentage. For the study area SiO2 content ranges from 68 to 74 % in leucogranite,
biotite microgranite, biotite microgranite, tourmaline granite, garnet biotite granite,
porphyritic biotite granite and up to 75% in pegmatite. Based on the above criterion, the
magma responsible for the igneous rocks of the study area is acidic in composition.
In the Al2O3-CaO-(Na2O+K2O) diagram (Fig. 4.d), the igneous rocks of the study area
fall in the field of peraluminous. It is substantiated by NK/A versus A/CNK diagram. In this
diagram, (Fig. 4.e), all of the igneous rocks fall within the peraluminous field except
hornblendite and some hornblende diorite fall in metaluminous. Concerning the mineral
chemistry, the igneous rocks of the study area have the mole percent alumina is greater than
the sum of lime, soda and potash (Al2O3 > CaO+Na2O+K2O) and the norm contains
corundum. Shand (1949) classified four groups of rocks in terms of alumina saturation.
Following the Shand’classification, these rocks belong to peraluminous suite.
Based on the silica and alkali content, igneous rocks have been classified into two
major series: alkaline and subalkaline. In SiO2 versus total alkali diagram (Fig. 4.f) all of the
igneous rocks of the study area fall in the field of subalkaline. The dividing line was
following the work of Irvine and Baragar (1971).
706 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Figure (3). (a). Photograph showing pegmatite vein (dyke) intruded in massive nature of
hornblendite: (3.b). Photomicrographs showing biotite (Bt), feldspar (Fel) and
hornblende (Hbl) found in hornblendite, under XN: (3.c). Photograph showing
close up view of hornblende diorite at Taungdanshe Taung: (3.d)
Photomicrographs showing biotite (Bt), feldspar (Fel) and hornblende (Hbl)
found in hornblende diorite, under XN: (3.e) Exfoliation of biotite microgranite
exposed at Taungkantlant area:(3.f) Photomicrographs showing biotite (Bt),
quartz (Qtz) and feldspar (Fel) found in biotite granite, under XN: (3.g)
Xenolilths of different country rocks found in the biotite granite:(3.h) Ptygmatic
folds in the Belin quarry (hammer for scale). Note the wavelength variation as a
function of layer thickness.
Hbl
Bt
Fel
(b)
(c)
(e) (f)
Bt
Qtz
Fel
(d)
Hbl
Bt Fel
(h)
Ptygmatic folds are irregular and
isolated fold structures
Granite
(g)
(a)
The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 707
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
The subalkaline series was further subdivided, (Tilley, 1950), into tholeiitic and calc-
alkaline by using AFM diagram. Figure (4.g) indicates almost all of the igneous rocks of the
study area fall in the calc-alkaline field and single fractionation trend. It is substantiated by
Si2O versus alkalinity index diagram. According to this diagram, leucogranite, biotite
microgranite and pegmatite belong to the calc-alkaline series (Fig. 4.h). Based on the above
criterion, the igneous rocks of the study area are genetically related to the subduction related
plate tectonic processes.
The plots of Na2O+K2O-Fe2O3-MgO diagram (Fig.4.g) show that alkali is enriched
during the late stage of crystallization. In addition, Na2O-K2O-CaO diagram (Fig. 4.i) shows
the later stage of magmatic evolution trend showing an increase in K2O and Na2O with
depletion of CaO, i.e., fractionation trend follows the Bowen’s reaction series. Therefore, it
can be concluded from figures (4.c), (4.g), (4.h) and (4.i) that igneous rocks of the study area
were derived from single parental magma.
The above-mentioned points indicate the granitic rocks of the study area belong to
acid clan, peraluminous nature and calc-alkaline (subalkaline) series but hornblendite fall
within alkaline series (Fig. 4.f). For the type of granitoids, various plots are used to
distinguish I- and S-type. The figure (4.j), ACF diagram, indicates that the majority of
igneous rocks from the study area belong to S-type. In addition, some distinctive chemical
properties such as the molar ratio of Al2O3/ Na2O+CaO+K2O is greater than 1% and CIPW
normative corundum content is mostly >1% in igneous of the study area point out S-type
nature. According to the CIPW Norm calculation, hornblendite and hornblende diorite fall in
the field of I- type but some hornblende diorite can be seen at the boundary of I-type and S-
type because ilmenite and magnetite have equal amount of percent.. It is also found that all
granite fall in the field of S- type because ilmenite is typical present in norm calculation (Fig.
4.j).
Table (1 & 2 ) Major- and minor- element analyses ( in Wt.% ) and norms of the
plutonic rocks from the study area.
Table (1). Major and minor element analyses (in Wt.%) and norms of the study area.
.
708 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Table (2). Major and minor element analyses (in Wt.%) and norms of the study area.
Figure (4). (a). Variation diagram of selected major oxides vs
SiO2 in granitic rocks of the
study area. Arrows indicate the fractionation trend with increasing silica content.
Figure (4). (b) Chemical classification of plutonic
rocks using total alkali versus silica,
(TAS) diagram of Cox et al. (1979)
adapted by Wilsom (1989) showing the
field of plutonic rocks of the study
area.
Figure (4). (c) Normative albite-orthoclase-
anorthite diagram for the granitic
rocks of the study area, with
dividing lines of O’connor (1965).
The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 709
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Figure (4). (d) Al2O3-CaO-(Na2O+K2O)
diagram for the granitic rocks of
the study area.
Figure (4). (e) Aluminous saturation indices
(Shand, 1949) of the granitic
rocks from the study area.
(Source: Winter, 2001)
Figure (4). (f) Diagram showing the
subalkaline nature of the igneous
rocks of the study area. (Source:
Irvine and Barager, 1971)
Figure (4). (g) Fe2O3(T) -(Na2O+K2O) –MgO,
AFM diagram in terms of alkalis,
total Fe and Mg for granitic rocks
of the study area. The dividing
line is based on the work of
Irvine and Baragar (1970).
Figure (4). (h) Alkalinity index of the
igneous rocks of the study area.
(field names are after Wright, and
the dashed line separates alkaline
from calc-alkaline, after Khin
Zaw 1986)
Figure (4). (i) Na2O-K2O-CaO diagram for
granitic rocks of the study area
710 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Figure (4). (j) ACF diagram for the granitic
rocks of the study area. Molar
ratios: A- Al2O3 +Na2O+K2O, C-
CaO, F- Fe2O3+MgO (after
Hyndman, 1985)
Condition of the Crystallization of the Granites
Almost all of the analysed rocks of the study area possess >40% normative
Ab+Or+Qtz and >40% normative Or+An+Qtz. These ratios are plotted in ternary diagrams
(Fig. 5 & 6).
In figure (5), the quartz-feldspar boundaries at water pressure of 2kbar and 10kbar are
shown projected onto the anhydrous base of the tetrahedron, after Tuttle and Bowen (1958).
The blue line represents the Ab/An piercing points projected onto the plane Ab-Or-Qtz at 5kb
water pressure (Wiebe, 1974). With reference to figure (6), almost all the points of biotite
microgranite and leucogranite lie within the ternary minimum of 2kb and 10kb, and one point
of biotite microgranite lies at the 5kb curve. Assuming PH2O = Ptotal , the minimum PH2O
allowed by the scattered points are 2kb and 5kb respectively.
In addition in the normative Or-An-Qtz diagram (Fig. 6), almost all of the biotite
microgranite and some leucogranite lie between the ternary minimum of 1kb and 5kb. If the
igneous rocks were assumed as crystallization at minimum pressure of 2kb, their liquidus
temperatures could have been cited from the diagram that shows the relationship between
differentiation index and temperature at 2kb water pressure (Fig. 7). According to this
diagram, the liquidus temperatures for biotite microgranite and leucogranite are 690°C and
650°C respectively.
Assuming that they were emplaced at PH2O of 10kb, the temperature of crystallization
of the rocks would become considerably lower. The depth of emplacement of these granitic
rocks is considered to be epizonal to mesozonal. Points 1 suggest the epizonal intrusion
(<1Km), Points 2-5 show the mesozonal intrusion (5-20 Km) and point 6 indicates the
catazonal intrusion. According to the depth-temperature relation diagram of (Marmo, 1956),
leucogranite will crystallize at 23.5 km, tourmaline granite, biotite microgranite, garnet
biotitegranite and porphyritic granite would be at 24-26 km, biotite granite might probably
be at 24.5 km, hornblende diorite would be at 29 km and hornblendite might be probably be
at 33 km. Based on these facts, the granitic rocks of the area might intrude at the deeper
crustal level according to the depth-temperature relation diagram and then, they evolved and
intruded into the shallow crustal level because of the presence of skarn minerals, xenoliths
Tourmaline granite
Leucogranite
Garnet biotite granite
Porphyric biotite granite
Biotite microgranite
Biotite granite
Hornblende diorite
Hornblendite
The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 711
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
and destruction of the country rocks.The depth of crystallization of the igneous rocks for the
study area can be interpreted from the schematic depth-temperature relation diagram (Fig. 8).
Figure (5). Normative Qtz-Ab-Or-H2O
diagram for the granitic rocks
of the study area.
Figure (6). Weight percent of normative quartz-
orthoclase-anorthite variation for
granitic rocks of the study area. The
boundary curves and minima at 0.5 to
5kb. (after Tuttle and Bowen, 1958)
Figure (7). Temperature- differentiation
index diagram for the
igneous rocks of the study
area, at PH2O = 2kb. ( after
Piwinskii and Wyllie, 1970)
Figure (8). Schematic depth-temperature relation
diagram showing the position of the
igneous rocks of the study area. (After
Marmo, 1958)
Emplacement and Depth of Intrusions
Field and petrochemical data suggest that hornblende diorite, porphyritic biotite
granite, leucogranite, biotite microgranite and other granites intruded along the highly
deformed metamorphic rocks of the study area. The emplacements of these igneous bodies
were accomplished by forceful intrusion. Pegmatite and aplite dykes are later intruded into
the biotite microgranite bodies. Hornblendite is the oldest igneous and might have been
formed before the continental epiorogenic uplift. The relationships of the igneous and
712 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
metamorphic rocks of the area imply that the latest metamorphism immediately preceded the
emplacement of the microgranite.
The field evidences and petrochemical criteria point out the depth of emplacement of
igneous intrusions from the study area. The following facts are used in estimating the depth
of emplacement.
(1) Temperature of country rock is greater than 450oC. The country rock belongs to
amphibolite facies.
(2) Migmatites are observed in the country rock in Belin Quarry.
(3) Mostly discordant structural relations to the country rocks.
(4) No graditional contact between country rock and intrusions.
(5) Abundance of associated pegmatite and aplite containing tourmaline, beryl, rubellite,
garnet and others.
(6) Contact metamorphic zones are few and small.
(7) The chilled border zones are present.
(8) Xenoliths of the country rocks are found.
These points collectively suggest that the depth of igneous emplacement of the study
area may be estimated at catazone through mesozone to epizone.
Geochronology
Based on the K/Ar dating method, Geological Survey and Exploration Project (1978)
stated that the bluish microgranite near Yesin yields 24 ± 2.7Ma (Upper Oligocene). (in Ma
Oo, 1995).
Numerous analyses by the Ar39
/ Ar40
method (Bertrand et al., 2001) indicate that the
undeformed Kabaing Granite (Mogok area) that intrudes the Mogok Series yields 39
Ar/40
Ar
age of 15.8 1.1 Ma that postdates the ductile stretching along the belt.
Barley et al., (2003) stated that the sensitive high-resolution ion microprobe U-Pb in
zircon geochronology for the Mogok Metamorphic Belt shows that strongly deformed
granitic orthogneisses near Mandalay contain Jurassic (~170Ma ) zircons that have partly
recrystallized during (~43Ma) high-grade metamorphism. Metamorphic overgrowths to
zircon in the orthogneisses near Mandalay date a period of Eocene (~43Ma) high-grade
metamorphism possibly during crustal thickening related to the initial collision between India
and Eurasia (at 65 to 55 Ma). This was followed by emplacement of syntectonic hornblende
syenites and leucogranites between 35 and 23 Ma. In addition, they point out the zircon cores
of augen gneiss from Kyanigan hills have a mean age of 170.1 ± 1.1Ma, suggesting the
magmatic age of the orthogneiss protolith.
Using U-Th-Pb dating method, Searle et al., (2007) point out two Tertiary
metamorphic events affected the Mogok Metamorphic Belt. The first was the Paleocene
event that ended with intrusion of crosscutting postkinematic biotite granite dykes at
(~ 59Ma). A second metamorphic event spanned late Eocene to Oligocene (at least from 37,
possibly 47, to 29Ma). This resulted in synmetamorphic melting producing garnet and
tourmaline bearing leucogranites at 45.5 ± 0.6 Ma and 24.5 ± 0.7 Ma. The later metamorphic
event is older than 24.5 ± 0.7 Ma - the age of leucogranites that crosscut all earlier fabrics.
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The age of metamorphism along the MMB has been the source of numerous
discussions: Bertrand et al., (1999) and Barley et al., (2003) have considered it to be
Oligocene to middle Miocene age according to Ar40
/ Ar39
method. Besides almost all of the
igneous rocks of the study area intruded and crosscut the metamorphic units indicating that
the age of metamorphism was earlier than the emplacement of igneous rocks.
Thus, it is reasonable to conclude that the age of the igneous activity in the study area
might have been formed during Tertiary period. But the hornblendite in the study area could
be Mesozoic time.
Granitic Type and Tectonic Environment
Generally, the granitoid rocks are classificated into four main types, namely: I-type
and S - type (Chappell and White, 1974), A - type (Colling et al., 1982 in Winter, 2010) and
M - type (White, 1979). I - type granites are generated in cordilleran subduction and post-
orogenic uplift regimes while S - type granites are the products of continental collision
(Beckinsale, 1979 in Pitcher, 1987). A-type and M-type granites are generated in anorogenic
and oceanic environments, respectively (White, 1979; Collision et al., 1982) (in El -sayed
M.M. et al.,2002).
On the basis of chemical analyses, i.e, major, minor and trace element concentration
by X - ray fluorescence spectrometry (EDXRF) method, the granitic rocks were classified as
various characters by various authors. In terms of major elements, all the granitic samples in
the present study area plot as sub-alkaline and calc-alkaline fields by Irvine and Baragar
(1971), high-K calc-alkaline series by Le Maitre et al., (1989), peraluminous nature by Shand
(1972) (in Winter, 2010) and calcic series by El . Sayed et al., (2002) respectively.
The distribution of the granitic rocks as discordant character, their calc-alkaline
affinity and peraluminous character, relatively high K2O content, felsic type having CIPW
corundum content > 1% strongly suggest that the granitic rocks in the present study area are
S-type which may be derived by partial melting of already peraluminous sedimentary source
rocks ( Winter, 2010).
Several schemes have been proposed for the classification of granites on the basis of
tectonic setting (Pitcher 1983, Pearce et al. 1984, Brown et al., 1984, Maniar and Piccoli
1989, Pearce 1996 in El. Sayed et al., 2002). Among these, the tectonic setting of granitic
rocks from the study area was studied by using major elements and trace elements with
reference to Mariar and Piccoli, 1989 and Pearce et al., 1984).
Maniar and Piccoli, 1989 classified the granitic rocks on the basis of tectonic setting
as follows:
(1) Island arc granitoids (IAG)
(2) Continental arc granitoids (CAG)
(3) Continental collision granitoids (CCG)
(4) Post orogenic granitoids (POG)
(5) Rift related granitoids (RRG)
(6) Continental epiorogenic uplift granitoids (CEUG)
(7) Oceanic Plagiogranites (OP)
714 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
According to Maniarand Piccoli (1989), tectonic discrimination diagrams based on
major oxides for the studied granitic rocks are shown in figures (9. a-d).
In (Fig. 9.a), K2O versus SiO2 diagram shows the distinction between oceanic
plagiogranite (OP) and other granitic rocks types. (IAG + CAG + CEUG + RRG) and all the
studied igneous rocks fall within the field of IAG + CAG + CCG + CEUG + RRG. In Al2O3
versus SiO2 and MgO and SiO2 diagrams (Fig. 9.b), the igneous rocks can be subdivided into
three groups (IAG + CAG + CEUG; POG and RRG + CEUG) and the studied igneous rocks
plot within the field of IAG + CAG + CCG (Fig. 9.c) but hornblende diorite and hornblendite
fall in CEUG. Shand’s diagram shows the distinction between CCG, CAG and IAG. In the
diagram (Fig. 9.d) all granitic rocks from the studied area occupy the field of CCG.
Figure (9). (a) K2O versus SiO2 diagram
showing the distinction between
OP and IAG + CAG+ CCG +
CEUG + RRG.
Figure (9). (b) Al2O3 versus SiO2 diagram
showing the distinction between
IAG + CAG + CCG, POG and
RRG + CEUG.
Figure (9). (c) MgO versus SiO2 diagram
showing the distinction between
IAG + CAG + CCG, POG and
RRG + CEUG.
Figure (9). (d) Shand’s index diagram
showing the distinction between
CCG, CAG and IAG.
Conclusion Remarks
The study area is located between the eastern Shan Highland in the east and central
Myanmar lowland in the west. It is also situated within the N-S trending Mogok
Metamorphic Belt of highly deformed metamorphic units. The Mogok Metamorphic Belt
including the study area extends over 1500km along the western margin of Shan-Thai Block.
The highest peak in the area is 440 m and all the ranges in the mountainous part have a
general N-S trend.
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Petrogenesis of these rock units are described on the bases of lithology, mineral
chemistry, field relationships and correlation to those of other areas. Biotite granite suite
exposed in the central and western part of the terrane is the only felsic plutonic suite. It is
dominated by the biotite bearing granite rocks: (1) biotite granite, (2) porphyritic biotite
granite, and (3) medium to fine-grained granites such as garnet biotite granite, biotite
microgranite, tourmaline granite, leucogranite and pegmatite. There is slightly compositional
and structural variation in all of these rocks. Two former units form in continuum in overall
composition, and hence, intermixed on varied scale. The biotite granites, often, grade to the
K-feldspar megacrystic and pegmatitic. They contain slightly foliation along the country rock
interface, suggesting a timing of emplacement during the warming stages of prograde
metamorphic kinematic effects. The following field and petrographic evidences lead to a
conclusion that the granite rocks in the present area are believed to be mainly formed by the
partial melting (anatexis) of the metasedimentary rocks rather than magmatic intrusions. But
hornbledite could come directly from the magma before orogeny. The mineralogical,
petrological and geochemical characteristic features of the granitic rocks in the area suggest
that they possess the S-type granites. The peraluminous S-type granites (mainly ilmenite
series) were formed by the partial melting (anatexis) of the metasedimentary rocks.
According to the CIPW Norm calculation, hornblendite and hornblende diorite fall in the
field of I- type but some hornblende diorite can be seen at the boundary of I-type and S-type
because ilmenite and magnetite have equal amount of percent.. It is also found that all granite
fall in the field of S- type. The depth of emplacement of these granitic rocks is considered to
be epizonal to mesozonal. Points 1 suggest the epizonal intrusion (<1Km), Based on these
facts, the granitic rocks of the area might intrude at the deeper crustal level according to the
depth-temperature relation diagram and then, they evolved and intruded into the shallow
crustal level because of the presence of skarn minerals, xenoliths and destruction of the
country rocks.
The age of igneous activity in this area is mainly based on field evidence and a few
radiometric age dating. East of the quarry the metamorphic rocks are intruded by
hornblendites and later granitic dykes and by the MEC granite. The MEC granite and Belin A
are similar. Mitchell et al., 2012; also obtained a zircon U–Pb age of 44.6 ± 0.5 Ma on the
planar diorite dyke. Searle et al. 2007; reported a U–Th–Pb zircon age of 59.5 ± 0.9 Ma on
the flow-banded granite dyke which cuts banded pegmatitic granite dykes and which from
field relationships is the youngest intrusion at Belin. Thus, it is reasonable to conclude that
the age of the igneous activity in the study area might have been formed during Tertiary
period. But the hornblendite in the study area could be Mesozoic time.
Generally, the granitoid rocks are classified into I-type and S type. All granitic rocks
of the study area fall in S-type except hornblendite and some hornblende diorite which are
typical I-type characters. On the basis of tectonic setting, granitic rocks of the study area are
in continental collision granitoids (CCG). But on the other hand, hornblendite and hornblende
diorite fall in continental epiorogenic uplift granitoids (CEUG) field.
Acknowledgements
We would like to express our gratitude to Dr. Ba Han, Rector of the University of Meiktila University,
Dr.Kay Thi Thin, Pro-Rector of the University of Meiktila University for their encouragement. We acknowledge
Professor Dr Zaw Min Thein, Head of Geology Department, Meiktila University for their encouragement. We
am deeply indebted to Professor Dr Than Than Nu, Head of Geology Department and University of Mandalay,
for her kind permission and advice to carry out this research work.
This research work could not be completed without getting helpful hands and advices from all teachers
in Geology Department, University of Meiktila.
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November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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