Geology Reference
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isotopic chemistry as well as textural characteristics suggest a genetic relationship
between the phaneritic lithics and their host phonolitic pumices.The Stage 4 tephras
are not related to earlier phases of basaltic to trachytic magmatism (Stages 1
3).
They have distinct isotopic compositions and cannot be reliably modelled by
fractional crystallization processes.The differences between the explosive phonolitic
(Stage 4) and effusive trachytic (Stage 2
-
3) eruptions are mainly due to different
pre-eruptive pressures and temperatures, causing closed- versus open-system
degassing. Based on thermodynamic and thermobarometric modelling, the phon-
olites were derived from deeper (subcrustal) magma storage and rose quickly, with
volatiles trapped until eruption. By contrast, the trachytes were stored at shallower
crustal levels for longer periods, allowing open-system volatile exsolution and
degassing before eruption.
The volcanic history of the island, has been described by Tsuboi (1920), who
described the island as Utusryo or Ullungdo. According to volcanologists, the
eruption of lave started
-
first with the eruption of alkali basalt followed by a long
period of erosion, then there was out-pouring of trachytic lavas. This was followed
by collapse of the apical part of the trachytic body. In the
final stage there was
eruption of leucite-bearing lavas.
The alkali basaltic lavas are characterized by the presence of phenocrystal
olivine, titanaugite and plagioclase in a groundmass of plagioclase, titanaugite,
magnetite and glass. The second stage of evolution of the volcano was accompanied
by eruption of 47 distinct
flows of latite, trachyte, hornblende-bearing trachyte,
aegirine-augite-bearing phonolite, aegirine-augite trachyte, amphibole-bearing tra-
chyte and biotite trachyte. The last stage of evolution was associated with eruption
of leucite-bearing tephritic phonolites through a triangular depression near the top
of the volcano. The lavas were poured out at the beginning from the bottom part of
the caldera and afterward from its northwest corner forming a small cone at a place
called Arpong; where there was accumulation of lapilli and pumice around the vent
through which they were ejected. The last lava
flow was discharged from this vent
to
flow northward. This vent is 800 m long, 300 m wide and 20 m thick. Under thin-
section the lavas from Arpong shows the presence of phenocrystal pyroxene,
amphibole, sanidine and labradorite and microphenocrysts of anorthoclase, olivine
and apatite in glass. Pyroxene phenocrysts are of two types: titanaugite and
aegirine-augite. The aegirine-augite is weakly pleochroic, whereas titanaugite has
purplish tint with hour-glass structure. The rocks also show presence of tabular
biotite crystals, apatite needles and rare amount of olivine in the groundmass.
Minute laths of K-feldspar and plagioclase are associated with round crystals of
leucite, aegirine-augite and magnetite. Chemical analyses of a K-rich rock from
Ulleung Island are given in Table
4.20
(Fig.
4.35
).
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