Sixth Hutton Symposium on the Origin of Granitic Rocks
Some characteristics and a possible genesis of
extremely high heat producing Mesoproterozoic granites
from Mount Painter Province, South Australia
K. Kromkhun 1, J. D. Foden2
1 Geology and Geophysics, University of Adelaide, kamonporn.kromkhun@student.adelaide.edu.au
2 Geology and Geophysics, University of Adelaide, john.foden@adelaide.edu.au
The Mount Painter (MP) and Mount Babbage
basement inliers (Figure 1) in the northern
Flinders Ranges, South Australia, expose
Mesoproterozoic granites and associated
felsic volcanics with very high U and Th
contents. These suites intrude late
Palaeoproterozoic
sediment-dominated
basement.
Figure 1: Location of Mesoproterozoic granite at
Mount Painter Province, South Australia.
The high heat production granites (HHPG)
have heat production values (HP average of
16 μWm-3) that are 3-4 times those of normal
granites (Figure 2). Associated volcanic
rocks and mafic dykes are also characterized
by high HP (average of 10 and 18 μWm-3,
respectively).
The Yerila granite (YG) in particular
contains extreme U and Th contents (average
U of 121 ppm and Th of 422 ppm) with HP
values up to 112 μWm-3 (average 61.8 μWm3
). The Yerila and Mt Neil granites are also
associated with mafic magmas (quartz
lamprophyre or diorite) occurring both as
dykes and as mingled schlieren or enclaves.
The irregular contact of the hosted mafic
enclaves and the common K-feldspar
University of Stellenbosch – July 2007
orientation in both YG and enclave suggests
magma mingling and mixing. An implied
petrogenetic relationship is reinforced by the
mafic suite’s very high U-Th. Other mafic
dykes in the area have “normal” low U-Th
contents and may be Neoproterozoic.
Figure 2: Heat production values (values at the
top bar), U and Th contents (values at the below
bar) of I- and S-type granites, A-type granites,
Mount Painter granites, Yerila granites and high
HP mafic dykes (Stewart and Foden, 2001;
unpublished data and this study).
The YG is typically medium- to coarsegrained, with tabular K-feldspar and quartz
phenocrysts within a quartz, microcline,
plagioclase, biotite, hornblende groundmass
and contains abundant accessory mineral
including zircon, titanite, allanite, fluorite
and apatite. Biotite generally shows damage
haloes around radioactive minerals. Large
euhedral allanite shows internal zoning. The
enclaves and mafic dykes associated with the
YG are quartz-bearing hornblende-biotiterich lamprophyres or diorites and are Kfeldspar-plagioclase-phyric with the same
accessory minerals as the YG.
The MP granites and volcanics are
metaluminous to weakly peraluminous. All
units
show
A-type
and
alkaline
Sixth Hutton Symposium on the Origin of Granitic Rocks
characteristics. The SiO2 content of MP
granites varies from ~65 to 78 wt% while the
YG is among the most mafic (SiO2 average
of 69 wt%). The YG has high K2O,
Fe/(Fe+Mg) and Rb/Sr. The high HP mafic
dykes have higher FeO, MgO, MnO, P2O and
TiO2 (~2.2 wt%TiO2) with lower SiO2 and
K2O contents than the YG. Incompatible
trace elements (including HFSE and REE) in
the YG and the lamprophyre/ diorite dykes
and enclaves are very enriched. The Th/U
ratio of both granites and mafic magmas has
a narrow range averaging ~ 3.24 suggesting
magmatic control rather than hydrothermal U
mobilisation.
A key question is what is the source of the
Yerila and related MP HHP granites? Based
on preliminary data these granites have initial
εNd values at 1565Ma that average -2.0
(McLaren et al, 2006, Stewart and Foden , 2001)
and are significantly higher than the local
crust (εNd = -6 to -4), implying a role of a
mantle-derived component. Very limited data
on the contemporary mafic samples suggests
a role for both fractional crystallisation of
highly enriched mafic parent magmas
coupled with mixing trends produced by
mingling between mafic melts and felsic
differentiates, presumably in upper crustal
magma chambers. SiO2 – TiO2 and SiO2-Ce
variation (Figure3) illustrates these separate
trends. Both Ti and Ce show initial
enrichment with silica resulting from
fractional crystallisation. This is followed by
rapid Ti and Ce decline with further silica
enrichment due to allanite and titanite
saturation. The YG suite mostly falls on this
part of the fractionation trend. By contrast
many of the other MP HHG series define
linear mixing trends due to back-mingling
with mafic parents.
Figure 3: Bivariant plots of TiO2 (wt%) and Ce
(ppm) versus SiO2 (wt%) showing fractionation
crystallisation of allanite and titanite
Our interim conclusion is that the HHP
granite suite is derived by fractional
crystallisation from a parental mafic magma
that is unusually incompatible element –
enriched. The source of this magma is
probably in the sub-continental lithospheric
mantle. Future work using Nd-Sm and U-Pb
isotopic data will be used to further define
this unusual mantle source and to determine
the age of enrichment.
The magma system probably evolved as a
series of stacked chambers in which
crystallisation occurred with periodic
replenishment by new mafic magma batches.
References
McLaren, S., Sandiford, M., Powell, R., Neumann, N., and Woodhead, J., 2006, Palaeozoic intraplate crustal
anatexis in the Mount Painter Province, South Australia: Timing, thermal budgets and the role of
crustal heat production: Journal of Petrology, v. 47, p. 2281-2302.
Stewart, K. & Foden, J., 2001. Mesozoic granites of South Australia. University of Adelaide; Primary Industries
and Resources SA.
University of Stellenbosch – July 2007