Download Poster NGC 2013 Transitional I-S type characteristics in the Main

Poster NGC 2013
Transitional I-S type characteristics in the Main Range Granite of Peninsular
Malaysia
Azman A. Ghani1, Michael Searle2, Laurence Robb2 & Sun-Lin Chung3
Department of Geology, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia
2
Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK
3
Department of Geosciences, National Taiwan University, Taipei, Taiwan
1
The dominantly Triassic Main Range Granite of Peninsular Malaysia that occurs west of the
Bentong‒Raub suture zone was previously regarded exclusively as S–type granite. Among the S-type
characteristics of the granite are, (a) high initial 87Sr/86Sr isotope ratio > 0.710, (b) low Na2O content,
< 3.2% Na2O in rocks with ~ 5% K2O, (c) narrow range of felsic rock (SiO2: 65.95 to 77.4%), (d) high
K2O/Na2O ratio, 1.4-2.8, (e) usually ilmenite-bearing (f) contains pelitic or quartzose metasedimentary xenoliths and (g) narrow range of highly evolved rocks with SiO2. However, the present
review shows that the granites also have many features that are more characteristic of I-type granites.
They are (a) Al-rich minerals such as sillimanite and cordierite are generally absent, (b) occurrence of
primary titanite and pale green amphibole especially in the northern part of the batholith, (c)
occurrence of mafic, hornblende bearing enclaves, (d) increasing peraluminosity towards the most
differentiated rocks (cf. S-type granite: increasing peraluminosity towards the most mafic varieties)
and (e) similar trends in P2O5 vs. Rb and P2O5 vs SiO2 and (f) A-B plots of Debon and LeFort, (1983)
and Villaseca et al., (1988).
Aluminium saturation index of the Main Range granite are mildly metaluminous to
moderately peraluminous with ACNK value ranging from 0.92 to 1.18 (Figure 1). Thus, only some
20% of the Main Range samples plot in the S–type field according to Chappell criteria (ACNK =1.1).
More than 50% of amphibole bearing granite samples plot below ACNK < 1 compared to the
amphibole free granite which has only about 4% of the samples plotted in the same area. The trend
increases with increasing SiO2 which is different to the trend observed in S-type granites from the
Lachlan Fold Belt and consistent with the I-type features. Thus it is likely that the source rocks of the
Main Range Granite were either mildly peraluminous or mildly metaluminous in character. Partial
melting of these source rocks will give rise to a mildly peraluminous magma. It is however a part
from the partial melting, peraluminousity can be brought about by mechanisms such as (1) reaction
with host rocks, (2) fractional crystallization of metaluminous magmas, (3) interaction with
hydrothermal fluids during late- or post-magmatic stages
In P2O5 vs. SiO2 plot, the Main Range Granite produce a decreasing trend with increasing
SiO2 content which is comparable to the general trend produced by the I‒type granites. Plots of the
Main Range Granites on the P2O5 vs. Rb diagram also produce a trend similar to the I-type granite
pattern. The variability in the abundances of P between the strongly fractionated I- and S-type granites
is related to a higher apatite solubility in peraluminous melts, P becoming progressively more
abundant in the felsic S-type melts during fractionation whereas in I-type melt P decrease as a result
of fractional crystallization of apatite.
Clemens (2003) suggested that terranes dominated by sediments with a few igneous rocks or
terranes dominated by andesitic to basaltic igneous successions with minor intercalated sediments
may produce a magma with transitional I- and S-type characteristics. The presence of andalusite,
although in minor amounts, suggests that at least part of the Main Range Granite magma was Alsaturated. The presence of titanite and the higher Fe2O3 contents in the ilmenite in the amphibolebearing granites indicate that the magma was derived from relatively oxidized magma. The
moderately peraluminous nature of the bulk Main Range Granite, without containing cordierite,
Fe‒Mg garnet or sillimanite, is most consistent with derivation from metasedimentary source rocks
undersaturated with respect to Al2SiO5. The high K2O content and high K2O/Na2O ratio of the Main
Range granite compared to the Lachlan Fold Belt reflects differences in source characteristics. It is
likely that the sedimentary source material of the Main Range Granite has a greater proportion of high
K/Na clay like illite and hydromuscovite.
Poster NGC 2013
REFERENCES
Clemens, J.D., 2003. S-type granitic magmas - petrogenetic issues, models and evidence. Earth
Science Reviews 61, 1–18.
Debon, F., LeFort P., 1983. A chemical - mineralogical classification of common plutonic rocks and
association. Transactions of the Royal Society Edinburgh: Earth Sciences 73,135–149.
Villaseca,C., Barbero, L., Herreros, V., 1998. A re-examination of the typology of peraluminous
granite types in intracontinental orogenic belts. Transactions of the Royal Society Edinburgh: Earth
Sciences 89. 113–119.
1.2
Biotite granite
Amphibole bearing granite
Microgranite/mesogranite
S Type
1.1
ACNK
I Type
Peraluminous
1
Metaluminous
0.9
65
70
75
80
SiO2 (wt%)
Figure 1: ACNK vs SiO2 plot for the Main Range Granite of Peninsular Malaysia