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Some characteristics and a possible genesis of extremelyhigh heat producing Mesoproterozoic granites fromMount Painter Province, South Australia

K. Kromkhun 1, J. D. Foden2

1 Geology and Geophysics, University of Adelaide, kamonporn.kromkhun@student.adelaide.edu.au2 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 latePalaeoproterozoic sediment-dominated

basement.

Figure 1: Location of Mesoproterozoic granite atMount Painter Province, South Australia.

The high heat production granites (HHPG)

have heat production values (HP average of

16 Wm-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 Wm-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 Wm-3 (average 61.8 Wm-

3). 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

orientation in both YG and enclave suggests

magma mingling and mixing. An implied

petrogenetic relationship is reinforced by the

mafic suites 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 thetop bar), U and Th contents (values at the belowbar) of I- and S-type granites, A-type granites,Mount Painter granites, Yerila granites and highHP mafic dykes (Stewart and Foden, 2001;unpublished data and this study).

The YG is typically medium- to coarse-

grained, with tabular K-feldspar and quartz

phenocrysts within a quartz, microcline,

plagioclase, biotite, hornblende groundmass

and contains abundant accessory mineral

including zircon, titanite, allanite, fluoriteand 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-biotite-

rich lamprophyres or diorites and are K-

feldspar-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

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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 (SiO 2 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? Basedon preliminary data these granites have initial

Nd values at 1565Ma that average -2.0

(McLaren et al, 2006, Stewart and Foden , 2001)

and are significantly higher than the local

crust (Nd = -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 crustalmagma 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 fractionationcrystallisation 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 crustalanatexis in the Mount Painter Province, South Australia: Timing, thermal budgets and the role ofcrustal heat production: Journal of Petrology, v. 47, p. 2281-2302.

Stewart, K. & Foden, J., 2001. Mesozoic granites of South Australia. University of Adelaide; Primary Industriesand Resources SA.

University of Stellenbosch July 2007