Geology Reference
In-Depth Information
calc-alkaline and high K-calc-alkaline lavas are interspersed. The volcanic eruption
of Ringgit complex took place 1.2 and 1.0 Ma ago, whereas eruption of Beser
complex took place 0.8 and 0.6 Ma ago. The calc-alkalic basalts and high K-calc-
alkalic trachybasalts and basaltic trachyandesites are characterized by a lot of
plagioclase phenocrysts (An
89
-
94
) and augite; olivine may be present in more ma
c
varieties.
The high K-series lava is constituted of basanite, tephrite, tephritic phonolite,
trachybasalt, basaltic trachyandesite and trachyandesite. The lavas of Ringgit-Beser
complex are not aphyric and contain up to 25 vol% phenocrysts. Clinopyroxenes
are twinned and zoned and are salites or diopsides. Pyroxenes are thus, similar in
chemistry to those from K-rich lavas of Muriah and Batu Tara. More ma
c varieties
contain olivine phenocrysts (Fo
88
-
90
) and more potassic lavas contain leucite. The
trachytic lavas are devoid of leucite but are constituted of plagioclase and biotite
phenocrysts. Edwards et al. (1994) found that most evolved potassic lavas contain
sanidine
hornblende. Titano-magnetite and apatite are common accessory phases,
and in case of highly ma
±
c lavas magnesio-chromite is present. The basanites and
trephrites are characterized by the presence of clinopyroxene
±
leucite
±
nephe-
line
magnetite in the groundmass. Trachytic lavas are constituted of plagio-
clase + clinopyroxene + magnetite
±
±
±
nepheline in the glassy matrix.
The EK volcanic series are characterized by basanite, tephrite and phono-tephrite.
Zoned and twinned Al
-
Ti-rich salite and diopside along with leucite, are usually
present as phenocrysts. Phonotephrites often contain plagioclase (An
91
) and/or
phlogopite and more ma
leucite
c variety contains olivine (Fo
91
-
92
). The accessory phases
include titanomagnetite
±
apatite
±
sphene. The groundmass is constituted of
clinopyroxene
magnetite.
Edwards et al. (1994) noted that the K and EK series rocks show variable K
2
O
content and have almost similar chemistry and cannot be separated on the basis of
K
±
leucite
±
olivine
±
nepheline
±
plagioclase
±
Si relationship. In fact the most magnesian EK series lavas have greater K
2
O
content than potassic lavas with similar MgO content. These two series of lavas can
be distinguished from each other on the basis of their Ba and Nb content. The K
2
O
content of both series of rocks ranges between 0.98 and 6.35 wt%, and is in fact
higher than that of most island arc lavas (usually <2.0 wt%) and except for calc-
alkalic rocks the K
2
O/Na
2
O ratio is >1.
The Ringgit-Beser lavas have a wide MgO content ranging between 1.6 and
18.1 wt%, though the average is slightly greater than 4 wt%. The SiO
2
content of
the lavas ranges between 42.9 and 54.2 wt%. The calc-alkalic lavas have bulk
chemistry plotting near the critical plane of silica-undersaturation (Yoder and Tilley
1962). The alumina content ranges between 9.3 and 9.9 wt%. The majority of EK
series lavas have Al
2
O
3
content >13 wt%, a value, which overlaps with those of in-
plate basalts. The Fe
2
O
3
content ranges from 6.6 to 12.3 wt% and the lavas of EK
series tend to be lower in Fe content. The CaO content varies between 7 and 14.8 wt
%, but the majority of the samples have CaO content >10 wt%. The maximum lime
content is found in lavas with 45 vol% clinopyroxene phenocrysts. The TiO
2
content is <1.12 wt%.
-
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