Geochemical evolution of granites in
northern NEO and its tectonic
implications
AIG NEO Seminar
5 June 2015, Theodore Club, 333 Adelaide Street Brisbane
Joseph Tang
Geological Survey of Queensland, Department of Natural Resources and Mines
Talk outline
• Significance of granite geochemistry in
understanding the NEO
• Highlight the challenges of using archival
granite geochemistry esp. major elements
data
• Demonstrate REE and isotopic data to
constrain source models
• Tectonic implication and mineralisation
potential
Granite in this presentation refers to granitic or plutonic rocks. Co-magmatic
volcanic geochemistry is used throughout the presentation for comparison.
• Assessment of over 9500 archival
data
• Attributing and filtering data
• Many, many hundreds of plots
Significance of Granite Geochemistry
• Granites act as probes to image subterranean
source and to provide information about the
upper mantle and/or lower crust (Chappell &
White, 1992)
• Granite chemistry characterises the magma
source (igneous or sedimentary)
• The geochemical evolution identifies magmageneration and differentiation processes, as well
as the conditions of emplacement
• The evolution of magma chemistry relates to
compositional variations in the source, and
provides vital clues regarding tectonic setting
Nature of Granite Geochemistry
• Granite represent
unroofed rocks from
>1 km depth
• Geochemistry is
modified between
source and
emplacement site
• Chemistry within a
pluton evolves with
time and between
locality within a
pluton
• Granite cross cuts
stratigraphy and
has no time
reference, which
needs to be dated
Northern NEO Granites
Granites in this presentation are limited to the Wandilla & Yarrol, Gympie Provinces and the Triassic basins
New England Orogen
• Wandilla Province Carboniferous
accretionary wedge
• Yarrol Province Late Devonian–
Carborniferous forearc basin
• Gympie Province Mainly Triassic marine
• Triassic basins e.g. Esk Trough
• Calliope Province late Silurian– Middle
Devonian old Oceanic Island Arc
• Connors–Auburn Province Volcanic Arc
• Marlborough Province Nappe
Granite Age Distribution
Age distribution is based on the availability of radiometric dating, and may not be a
true representation of magmatic age of all granites.
Granite distribution in Northern NEO
Granites of southern NNEO
Major Element Geochemistry
Peacock 1931 Alkali Diagram
• Middle-Late
Devonian: Bimodal
gabbroic and
intermediate rock
• Carboniferous-early
Permian: Bimodal
gabbroic and felsicintermediate rocks
• Late PermianMiddle Triassic:
Continuous spectrum
of gabbro to granite.
Variations in total
alkali content with
two trends
• Late Triassic: Two
populations of
gabbro-diorite and
granodiorite-granite.
Almost a continuous
trend between the
two.
• Post Triassic: Two
populations? or
continuous trend from
diorite to granodiorite
Miyashiro 1974 FeO*/MgO vs SiO2
• Middle-Late
Devonian: Mainly
tholeiite rocks
• Carboniferous-early
Permian: Mainly calcalkaline rocks but
gabbro-diorite has
transitional tholeiite
characteristics.
• Late Permian to
Middle Triassic:
Mixture of calcalkaline and tholeiite
rocks with two parallel
trends of calc-alkaline
and tholeiite evolution.
• Late Triassic: Two
populations of
gabbroic tholeiite
rocks and a calcalkaline felsic rocks.
• Post Triassic:
Predominantly tholeiite
rocks but has
transitional calcalkaline intermediate
rocks.
Maniar & Piccoli 1989 Alumina saturation
• Middle-Late
Devonian:
Metaluminous I-type
• Carboniferous-early
Permian: Bimodal
population of both Itype and S-type
granites
• Late Permian-Middle
Triassic: Mainly I-type
granites with a small
population of S-type
like plutons.
• Late Triassic: I-type
granites with minor
transitional S-type
• Post-Triassic: I-type
granites.
• Spurious data for
anomalous Mg# due to
extremely low Na2O
and K2O values are
not peraluminous
• No peralkaline granites
Trace Element Tectonic Discrimination Diagrams
Tectonic Discrimination Diagram
• Middle-Late
Devonian: Volcanic
arc granite
• Carboniferous-early
Permian: Volcanic arc
granite with transitional
within plate granites
• Late Permian-Middle
Triassic: Mainly
volcanic arc granites
with transitional I-A
types within plate Atype granites
• Late Triassic: Mainly
volcanic arc granites
with transitional I-A
types to true A-types
within plate granites
• Post Triassic:
Volcanic arc granites
Tectonic Discrimination Diagram
• The NEO granites fit
well within the VAG field
and extends onto the
WPG field.
• Compared with well
researched plutons, the
scatter of data could
result from poor data, or
represent partial melts
from the same parental
depleted mantle or
primitive lower crustal
source
• I-type granites are
interpreted as
disequilibrium partial
melts with refractory,
anhydrous, lower crustal
residues of charnokitic
composition.
• Partial melting of these
charnokitic residues
yielding co-magmatic Atype granites
REE and isotopic geochemistry and magmatic
source modelling
REE Pattern
• Middle-Late
Devonian: Flat,
enriched MORB or
lower crustal signature
• Carboniferous-early
Permian: Crustal and
MORB signatures
• Late Permian-Middle
Triassic: Lower to
upper crustal
signatures
• Late Triassic:
Enriched MORB, lower
crustal and crustal
signatures, possible
ocean ridge granite?
• Post Triassic:
Granulite, lower
crustal and crustal
signatures
Mantle
Isotopic modelling
Significance of specialised
granite research- Lower to
Late Triassic granites
Radiogenic isotopes
•Stable and radiogenic
isotopes provide better
indication of source
characterisation
•Station Creek Igneous
Complex has very primitive
signature despite being calcalkaline with crustal
signatures
• Marianas and Aleutian islandarcs sourced from depleted
asthenospheric mantle source
similar to the MORB, with
oceanic sediments
• Higher 87Sr/86Sr ratios in
Mexico, Ecuador, Cascade and
South Sandwich Islands arcs
were attributed to crustal
contamination or the addition
of components derived from
the subduction slab
• Mixing two isotopically
distinct sources and/or
assimilating upper-crustal
rocks by depleted mantle
derived magma
• two-components mixing
model uses
(Sr/Nd)MORB/(Sr/Nd)CR
ratios of 1.5 to 4
• Neara Volcanics has a
different source region
from partial melting of
isotopically primitive
source involving
terrigenous sediments on
seawater altered oceanic
crusts.
• Station Creek rocks formed
by mixing depleted mantle
or primitive lower crustal
sources with upper crustal
rocks ranging from 4-14%
upper-crustal component
(CR)
Summary
TIME INTERVAL
Geochemical Observations
Implications
Middle-Late
Devonian
Bimodal, tholeiite, metaluminous, I-type gabbro and
intermediate rocks with MORB or lower crustal
Lower crustal or upper mantle sourced magma in volcanic arc
setting. Possible in extensional tectonism.
MINERALISATION
Carboniferous-early
Permian
Bimodal, transitional tholeiite gabbroic and calcalkaline felsic-intermediate rocks. Bimodal
population of both I-type and S-type granites.
Volcanic arc granite with transitional within plate
granite with crustal and MORB signatures.
Upper mantle MORB or lower crustal magma in extensional
setting. Partial melting of sediments to form S-types granites.
MINERALISATION associated with I-Types
Late Permian-Middle
Triassic
Continuous spectrum of gabbro to granite with
tholeiite and calc-alkaline characters. Two mixing
trends. Mainly I-type granites with possible minor Stypes. Volcanic arc granites with transitional to
within plate granites. Rocks have lower to upper
crustal signatures.
Late Triassic
Two populations of tholeiitic gabbroic rocks and a
calc-alkaline felsic rocks with a continuous mixing
trend. I-type granites with minor transitional S-type.
Mainly volcanic arc granites transitioning to within
plate granites. Granites have enriched MORB, lower
crustal and crustal signatures, possible ocean ridge
granite?
I-type granites are interpreted as disequilibrium partial melting
of depleted mantle or primitive lower crustal with refractory,
anhydrous, lower crustal residues of charnokitic composition.
Volcanic arc setting. Little evidence of MORB sourced magma.
Primarily a compression tectonics.
DRY
Mixed lower, crustal and mantle MORB geochemical signatures
indicate a mantle plume causing partial melting of crust. A-type
granites are partial melting of charnokitic residues yielding comagmatic I- and A-type granites. Mixture of subduction and
extensional tectonism.
LOW POTENTIAL MINERALISATION
charnokitic melts are dry.
Post Triassic
Continuous trend from tholeiitic diorite to
transitional calc-alkaline granodiorite. I-type volcanic
arc granites. Granites have granulite, lower crustal
and crustal signatures
signatures. Volcanic arc granite.
Evidence shows partial melting of crustal in a volcanic arc
setting. No indication of extension.
Summary
• Granite geochemistry is used to interpret rock source
and tectonic setting
• Major elements data have limited application and are
useful for identifying rock types and classification
• Trace element chemistry are tools for identifying
processes and possible source regions
• REE and isotopic data are used to model source
characterisation
• Tectonic constraints requires age dates
• Combination of all the above will provide
mineralisation potential
References
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CHAPPELL B. W. & WHITE A. J. R. 1992. I- and S-type granites in the
Lachlan Fold Belt. Transaction of the Royal Society of Edinburgh: Earth
Sciences 83, 1-26.
LANDENBERGER B. 1996. Petrogenesis and tectono-magmatic
evolution of S-type and A-type granites in the New England Batholith.
PhD. Thesis, University of New England, unpublished.
STEPHENS C. J. 1991. The Mungore Cauldron and Gayndah Centre
Late-Triassic large-scale silicic volcanism in the New England Fold Belt
near Gayndah, southeast Queensland. University of Queensland, PhD.
Thesis, unpublished.
TANG J. E. H. 2004. The Petrogenesis of the Station Creek Igneous
Complex and Associated Volcanics, Northern New England Orogen.
Queensland University of Technology, PhD. Thesis, unpublished.