Geology Reference
In-Depth Information
5.8 Trace Element Geochemistry for Ringgit-Beser
Complex (Indonesia)
The arc lavas from Ringgit-Beser complex have been classi
ed into calc-alkaline,
high-K calc-alkaline, shoshonitic and leucititic rock types by Edwards et al. (1994).
The classi
cation of these four rock types is shown in a K
2
O versus SiO
2
diagram
(Fig.
5.23
). The incompatible trace element data for the ma
c and evolved lavas
belonging to excess K (EK), high K (HK) and high K-calc-alkaline (HK calc) series
are summarized in a chondrite-normalized trace and minor element plots
(Fig.
5.24
a
c; Edwards et al. 1994). These data are similar to typical island arc
element patterns in having LILE and LREE enrichments related to HFSE and
HREE.
The EK series lavas show little change with MgO variation in the incompatible
trace element content. The Rb content in these lavas is higher relative to Ba and Th.
The chondrite-normalized lavas have similar or slightly lower LILE contents than
those of the in-plate lavas of Cameroon ocean island basalts. The K-series lavas of
the complex have low Sr/Nd ratios similar to those of EK series. Edwards et al.
observed that all lava series have negative Ti and Zr anomalies in addition to Nb
anomalies i.e. typical of arc lavas (Fig.
5.24
a
-
c). They further observed that the
negative HFSE anamolies are present in the high magnesian lavas and are not
related to magnetite and/or hornblende fractionation. They observed that the tran-
sition from calc-alkalic to high K-lava composition is related to LILE, LREE and
HFSE contents, and a reduction of negative Nb anomaly and increase in the LREE/
HREE ratios.
The
87
Sr/
86
Sr ratios of the lava series range from 0.70419 to 0.70549 and the
143
Nd/
144
Nd ratios from 0.512674 to 0.512821 (Fig.
5.25
). The highly potassic
-
Fig. 5.23 A plot of K
2
O
versus SiO
2
for the calc, HK
calc, K. and EK series rocks
of Ringgit-Beser (after
Edwards et al. 1994)
7
6
5
4
3
2
1
Calc-alkaline
42
44
46
48
50
52
54
56
58
SiO wt. %
2
EK series
K series
HK calc series
Calc series
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