By THAN HTUN (GEOSCIENCE
MYANMAR)
Mount Popa Region
The Taungni, Gwegon and
Sebauk Tuffs
This article is a continuation of Episode 50 on the Mount Popa Region, extracted from Chapter XXX of The Geology of Burma by HL Chhibber (1934).
Taungni Hill: The rocks of Taungni
Hill consist of silicified and unsilicified tuffs, the former displaying
various colours, the latter a cream-to-white tint. In the silicified tuffs’
cavities, well-developed colourless quartz crystals are sometimes seen. The
hard, silicified, fine-grained rocks occasionally show coloured rhythmic
banding of iron oxide owing to the diffusion of the iron salts through these
cryptocrystalline siliceous, another case of the Liesegang phenomena. The older
andesites are also seen on the southern slopes of this hill, where they occupy
a lower position; being extrusive rocks, they must be older than the overlying
white tuffs. This fact was further confirmed by finding specimens of rhyolites
(and allied tuffs) containing xenoliths of andesite. Further, weathered
fragments of andesite are enclosed in the white tuffs.
The second main occurrence of these
white tuffs is to the east and south of Gwegon village. They do not call for
any special mention. Specimens of red and variegated jaspers are associated
with them.
The third notable occurrence of the
tuffs is to the north and northeast of the village of Sebauk. The tuffs, as
usual, are white, friable, and present a chalky appearance. The rock is well
bedded, and north of Sebauk, it shows south-westerly dips, where it caps the
Irrawaddy Series and exhibits potholes in the stream channels. About half a
mile north of Sebauk it presents a steep scarp. The Irrawaddy sandstone forms
the lower portion overlain by tuff; the steepness of the scrap is due to the
more resistant covering of tuff preserving the underlying soft sand-rock
removed from the neighbourhood. Another occurrence is in the valley about one
mile west of Taunggala, where chalky white tuff is interbedded with Irrawaddian
sands, etc.
Biotite-Hornblende-Gneiss: The rock
from the hill east of Okshitkon village appears to be a
biotite-hornblende-gneiss, and was found associated with andesite and other
tuffs. The specimens of these gneisses and mica-schists associated with white
tuffs, described below, show that the latter have undergone metamorphism,
largely of a thermal character; this was brought about by subsequent volcanic
discharges, heated gases, and vapours, aided by regional metamorphism due to
earth-movements which occurred at the close of the Tertiary period.
Mica-schist: A specimen of mica
schist from two miles due south of Taunggala, associated with white tuff near
Sebauk, is light-grey in colour and consists of muscovite and quartz. Under the
microscope, a thin section shows a granular mosaic of quartz and colourless
flakes of muscovite. Most probably this schist was formed by earth movements
which folded the white partially silicified tuffs. Similar metamorphism of fine
tuffs into silky schists has also been recorded by Geikie.
Age: These rocks appear to be older
than the lavas of Mount Popa. They are interbedded with Tertiary sands west of
Konde village. A thin covering of the Irrawaddian is seen capping them; they
seem contemporaneous with the closing of the Irrawaddian epoch (Pliocene
period). Further, these white tuffs are older than the interbedded black tuffs,
as they occupy a lower position in the Irrawaddian sands and have undoubtedly
taken part in the folding movements. In contrast, the black tuffs are almost
horizontal. But the white tuff is younger than the older flow of andesite and
the Kyaukpadaung lavas, as fragments derived by denudation from the former
rocks form the white tuff.
The Interbedded Black
Tuffs and Ashes.
The Black Tuff and Ashes are exposed all
around the slopes, especially on the west, of the plateau on which Popa stands,
interbedded with the Irrawaddian rocks on which the plateau is built. The rock
is generally a coarse-grained ash that often passes into coarse tuffs where the
pieces of andesitic rocks have been cemented by finer ash. With these, similar
black basal tuffs underlying the lavas of Mount Popa may also be included as
they are not very different petrographically. The ash is usually a stratified
deposit, and its best sections are seen near Sinzin village and at the pagoda
point about two miles to the west. Here, the almost horizontal bedding is so
fine and regular that steps leading to the pagoda have been cut out of these
rocks. Similar beds, over which the younger andesite has flowed, crop out
opposite the pagoda.
These rocks, like the older white
tuffs, are interesting in that they furnish evidence regarding the date of
commencement of the volcanic activity around Mount Popa itself.
The rocks are interbedded with the
upper Irrawaddian, suggesting thereby that the Popa volcano was first active
towards the close of the Pliocene period. Sir Edwin Pascoe notes that “activity
commenced in Irrawaddian times, but the later and larger flows of augite and
hornblende andesite and the clastic material derived therefrom where in all
probability post-Irrawaddian”.
These interbedded rocks consist
largely of soft ash grading into black basaltic or andesitic tuff which
consists of pieces of lava cemented by the softer ashes.
Andesitic Tuff: The rock outcropping
opposite the pagoda, about two miles west of Sinzin village, is grey, and dark
green pyroxene and clear feldspar can be recognized with the pocket lens. Under
the microscope, a thin section consists of laths of feldspar and pyroxenes.
The rock, from about 1 ½ miles SSW of
Popaywa village, taken from the outer edge of the plateau, is soft and
grey-coloured with pieces of andesite enclosed in it. The thin section is
remarkable for its large crystals of brown hornblende showing reaction rims,
with augite and clear feldspar set in a glassy matrix.
The Volcanic Ash: It is light grey in
colour and soft and fine-grained. Under the microscope, it appears to consist
of very minute laths of feldspar and fine pyroxene granules cemented by dark
glass. The specimens may represent consolidated volcanic dust.
The Younger Andesites and
Basalts, with Associated Tuffs and Conglomerates.
The andesitic lavas of the Mount Popa
area north of Lat 20° 53’and extending up to the village of Legyi (near
Seiktein, which marks the northernmost limit of lava), appear younger than the
andesites described above. In the field, the younger andesites are generally
distinguished by well-developed squarish phenocrysts of dark-green augite and
sometimes of brownish-black hornblende. However, in places, these phenocrysts
are not so well marked. The rock is of grey, greenish-grey, or greyish-blue
colour. It is remarkable that besides undisturbed flows, piles of disintegrated
boulders of lava are strewn all around Mount Popa north of the latitude given
above. Wherever lava has flowed down steep slopes it has generally split up
into blocks, and this is best seen on the slopes of the plateau surrounding
Mount Popa. Sometimes these blocks are resting on the Tertiary sands. These
small accumulations of broken boulders represent disintegrated flows, and the
upper surface of the boulders often shows signs of having come from the surface
of a flow, exhibiting a ropy and slaggy appearance. In the centre, there is the
enormous mass of Mount Popa itself, mostly a tuff and lava cone, rising at
least 3,000 feet above the plateau on which it stands. Sir Edwin Pascoe notes
that what is left of the crater wall is formed of breccia or agglomerate.
In the crater, we find good exposures
of agglomerate which we may give the name of “crater agglomerate.” This deposit
is best seen in the streams near the villages of Taungbaw, Gyaingywa,
Kanzatkon, etc. The top of the main mountain is covered with layers of volcanic
ashes, tuffs, cinders, bombs, etc., of varying thickness, approaching 100 feet
or more in places.
Volcanic conglomerates and tuffs are
sometimes associated with these andesites, and in places extend far into the
slopes of Mount Popa. The cuttings made by streams expose small and large
fragments of andesite cemented by ash or other volcanic detritus.
The figure shows some of the more
important flows around the crater, and the following table indicates their
petrographic characters: -
Several bedded flows are seen in the
crater walls; this is particularly the case in the eastern wall. During a
subsequent visit, the author observed several distinctly bedded flows below ∆ marked NNE of 4981. The
average thickness of the flows is roughly ten feet. The walls are so
precipitous that it is difficult to map some of them separately on the
available one-inch maps.
The Taunggala Type.
Hornblende-Augite-Andesite.
The hill known as Taunggala
(Taungkalat), “2,417,” forms a remarkable feature on the southwestern slopes of
Mount Popa; in the section, it is roughly circular and rises precipitously on
all sides. It consists of a plug of igneous rock intruded into Irrawaddian
sands, which, in general, dip away from the igneous mass. Taunggala is a
typical subsidiary neck on the flanks of the main volcano. The rock is
dull-grey in colour, often with a purplish tinge due to weathering, and is
characterised by elongated crystals of hornblende. It has a coarser and more
even-grained texture than most igneous rocks in this area. Its specific gravity
is 2.74.
The groundmass is holocrystalline and
consists of feldspar laths with abundant granules of augite and magnetite. It
appears certain that the magma was supersaturated with hornblende, evidenced by
many long pseudomorphs. Consequently, hornblende was the first mineral to form;
it was then reacted upon by the residual magma, resulting in the corrosion of
the hornblende crystals and their consequent filling with augite crystals or
groundmass.
In some cases, the corroded crystals
serve as nuclei around which augite crystals develop; hence, the
supersaturation of magma with hornblende is proved. Some are seen partly with
feldspar, augite, and groundmass. In other sections, feldspar and augite show
ophitic or sub-ophitic relations, and there is evidence to prove that the
crystallization of augite and feldspar proceeded simultaneously in some cases.
In some sections, the feldspar has
been altered to a pale yellow or pale pink mineral which is isotropic and may
be analcite. Islands of unaltered feldspar are often to be seen. The alteration
of feldspar to analcite, both in the phenocrysts and groundmass and the
development of the greenish tinge in the hornblende may have been brought about
by the escaping gases and vapours which are generally emitted during the waning
phases of an eruption. Associated with magnetite are some primary haematite,
which is orange-red.
It is also seen as inclusions in
hornblende and other minerals. Crystals of specular iron ore are found
associated with these rocks and are locally called Popa “sein”, or Popa
“diamonds”. (To be continued)
References:
Chhibber, HL, 1934: The Geology of
Burma, Macmillan, and Co Limited, St Martin’s Street, London.
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