Geology Reference
In-Depth Information
Ultrapotassic rocks are characterized by phenocryst of phlogopite, embedded in
a groundmass containing leucite, diopside, olivine and Fe-Ti
Ti oxides, and are
reported from Kajan River, Kalimantan. The phlogopites are reported to show
reverse pleochroism, which are usually observed in phlogopites from lamproites.
The K
2
O/Al
2
O
3
and K
2
O/Na
2
O ratios of the rocks from Kalimatan are 0.4 and 2.0,
respectively. The rocks from this locality are syenitic leucitites. In the nearby
locality around Karamu River of central Kalimatan, minettes have been described
by Bergman (1987). Analyses of these rocks are summarized in Table
4.3
.
Iddings and Morley (1915) and Rittmann (1951) studied the volcanic rocks of
Indonesia. Leucite-bearing lavas were erupted in particular by Mt. Mouriah volcano
(now extinct), which occurs northeast of Semarang, Java (Nelson and Whiteford
1983). Associated with this volcano are two other small volcanoes: Paliaian and
T
-
Tülering. K-rich phonolitic lavas containing modal leucite were erupted from a
small volcano called Lorous. Leucite-bearing rocks in association with phonolites
also occur in the Bawean Island between Java and Borneo. Mt. Mouriah is largely
made up of tuffs and breccia with small massive lava
ü
flows of mainly leucite
tephrites and leucitites with a few olivine-bearing varieties. These rocks are of late
Tertiary age. Analyses of some of the rocks given in Table
4.3
indicate that the SiO
2
content varies from 44 to about 53 %; the Al
2
O
3
and CaO contents are rather high,
whereas the TiO
2
contents are very low. Mineralogically these rocks are charac-
terized by leucite, augite, K-feldspar, and calcic plagioclase in various proportions.
Nepheline is usually absent in the rocks of Mt. Mouriah, whereas olivine and biotite
are occasionally present. The interplay of subduction zone and interpolate processes
pertaining to the genesis of K-rich silica-undersaturated rocks have been discussed
by Edwards et al. (1991).
In Celebes, olivine leucitite, leucite trachyte and leucitite tuff occurs at various
localities along the coast from Cape Mandar to Cape William and near the Bay of
Mamudju. Leucite basanite is found in Oldeidu Kiki in the Matinang Mountains,
southeast of Bowool in Northern Celebes. These lavas are associated with trachytes
and trachyandesites. Potassium-rich rocks have been reported in Pic de Maros
Mountain, located between Maros and Tjambo, north of Makassar in Celebes. These
rocks occur as intrusive bodies of shonkinites, consisting of augite, K-feldspar, biotite,
and rare plagioclase. In some localities such as Gentungen, the shonkinites contain
pseudoleucite and are very similar to the shonkinitic rocks of Montana, U.S.A. These
shonkinites grade into nepheline syenite, syenite porphyry, and trachyte.
4.1.5.3 Other Localities in South East Asia
Bergman (1987) described a group of rocks called, cocites near the border of Laos
and North Vietnam in Indochina. They occur at Coc pia as dykes, where they cut
across Mesozoic alkaline syenites and granites and are
fine-graind, comprising
olivine, diopside, phlogopite, magnetite, sanidine and leucite. At Sincao, north east
of Laidchau, cocite group of rocks have been emplaced in Triassic sediments, and
the dykes may be much younger in age. Some potassic rocks known as cocites are
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