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trachybasalt-trachyandesite association. Varne (1985) described nepheline-norma-
tive olivine leucitite and other related rocks, which erupted from the Quaternary
volcanoes of Soromundi and Sangenges on Sumbawa and Batu Tara volcano,
located northeast of Flores.
According to Foden and Varne (1980), the K-rich rocks of Tambora and Sangeang
Api (Fig.
4.5
b) occur in that sector of the arc, which is a relatively young arc
developed during Miocene. The Benioff zone lies at an intermediate depth below
these volcanoes. The rocks of Tambora and Sangeang Api are in general similar to
shoshonite suite but are more silica-undersaturated. According to Foden and Varne
(1980) the best known analogue of Tambora-Sangeang Api is the Roman Province of
Italy, where both high and low K-series rocks occur. The Sumbawan lavas are very
similar to the low-K
2
O series rocks and have relatively low Ti, Zr and P concentra-
tion. The leucite series of Sumbawa is considered to be similar to the rocks of African
rift region. It may be observed that concentration of such elements as K, Rb and Sr,
are very similar in both cases, but the rocks from the African rift region are charac-
terized by higher P
2
O
5
. The content of TiO
2
and Zr is 3
4 times more, and the Nb
content of the African rocks is more than ten times than those of Indonesian leucitites.
The K
2
O content correlates well with the Sr and Nd isotopic ratios.
Varne (1985) established that progressive increase in K
2
O/SiO
2
ratio in the lavas
with SiO
2
contents less than 53 wt%, is accompanied by
87
Sr/
86
Sr ratio ranging
from 0.704 to 0.707 and decreasing Nd content from +5.3 to
−
3.7 (Whiteford et al.
1979).
-
4.1.5.2 Ringgit Beser Complex
The volcanic complex is located in the Madura basin of East Java. The Complex is
200 km above the Benioff zone (Hamilton 1979) and lies to the north of the
volcanic axis. The crustal segment in East Java is oceanic and only 18 km thick.
Van Bemmelen (1949) thought that the volcanism at Ringgit-Baser complex started
during Plio-Pleiostocene. Edwards et al. (1994) thought that the volcano erupted for
a short period and became extinct during Late Pleistocene. The southern foot of the
volcano subsequently got folded and the main cone also collapsed. Shape of the
volcanic ridge is somewhat maintained in the north and the vertical southern wall is
also intact. The Beser ridge to the south displays anticlinal structure. The Ringgit
complex towards north is divided into two parts by WSW
ENE-trending graben.
There have been eruptions of three different types of volcanic rocks:
-
(1) Calc-alkalic,
(2) High K-calc-alkalic, and
(3) High K-alkaline series.
Edwards et al. further subdivided the high K-alkaline lavas into two subgroups
based on their Nb and Ba content: the enriched potassic series (EK) and the potassic
series (K). They observed that northern part of the complex comprises predomi-
nantly EK series lavas but in the southern part along with high K-alkaline lavas,
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