The Delamerian orogenic cycle: inferences from the provenance record of the Koonenberry Belt
Glen Phillips1,2 & Emma Johnson2,3 1 Regional Mapping & Exploration Geosciences, Geological Survey NSW 2 Discipline of Earth Sciences, The University of Newcastle 3 Research School Earth Sciences, The Australian National University
The Delamerian Orogenic cycle is the oldest of the Tasmanides.
It defines the onset of over 500 million years of subduc:on-‐related tectonism along the eastern margin of Australia.
It was responsible for the genera:on of major known ore deposits (Cadia,
Western Victoria).
The Tasmanides & the Delamerian
Given that our explora:on tenement is
the whole state of NSW; we need mineralisa:on models at that scale.
In part, regional-‐scale models such as these are underpinned by geodynamic
reconstruc:ons.
The Tasmanides & the Delamerian
A. To some degree. But a major gap in our understanding is the :me and the mechansism(s) responsible for the onset of the Delamerian orogen.
Antarc7ca
Passive margin in eastern Australia
Q. Do we understand the Delamerian?
Arrowsmith Volcanics Unconformity: passive to ac:ve margin sediments
Mount Wright Volcanics Turbidites – Teltawongee Gp.
No detrital mica (Kara Fm.)
Lots of detrital mica (Teltawongee Gp.)
Samples
Samples are from the Kara Forma7on Detritus shedding into passive margin basins. (or incipient ac:ve margin?)
Samples are from the Teltawongee and Ponto Groups Detritus shedding into the ac:ve margin basins. Should be arc related?
Samples are from the Kayrunnera and Mutawintji Gp. Detritus shedding into foreland basins.
Detrital Zircon – Igneous and Metamorphic records
Huh?
Passive margin (Kara Forma7on) Detritus sourced from con:nental
interior
Ac7ve margin (Teltawongee & Ponto Gp.) Detritus sourced from the margin
Foreland Basin (Kayrunnera & Mutawintji Gp.)
Detritus sourced from the orogen
n=198
n=397
n=299
n=198
Detrital Ru7le – Metamorphic Record
Passive margin (Kara Forma7on) Detritus sourced from con:nental
interior
Ac7ve margin (Teltawongee & Ponto Gp.) Detritus sourced from the margin
Foreland Basin (Kayrunnera & Mutawintji Gp.)
Detritus sourced from the orogen
Huh?
Implica7ons 1 Geodynamic SeWng 1. The provenance of sediments deposited in the ac:ve margin basins during the
Cambrian (i.e., Teltawongee and Ponto groups) is not consistent with the postulated forearc seYng.
2. The provenance of these rocks is more consistent with deposi:on in a back-‐arc seYng.
Re-‐assess the reconstruc7on
SubducFon zone further to the east.
What are the Mount Wright Volcanics?
Inconsistencies with the local source hypothesis 1. No significant igneous / metamorphic complex of this age in
eastern Australia.
2. Sediment and geochemical record is consistent with a passive margin seYng along eastern Australia between 600-‐550 Ma – not a renowned seIng for zircon producFon
Two possibili:es : 2. A distant source from the interior
of Gondwana
Implica7ons 2 Source of the 550-‐600 Ma detrital zircons
Two possibili:es
If the sediments are derived from a local, now concealed source: 1. U-‐Pb in zircon defines the longevity of the tectonothermal
event -‐ posing the ques7on: is the Delamerian orogen older than 520 Ma?
2. Hf in zircon informs us on the crustal evolu:on of the
Neoproterozoic source – what were the magma:c and geodynamic condi:ons prior to the Delamerian?
700-‐620 Ma No change through :me and minimal zircon produced.
620-‐580 Ma Rapid departure toward –ve values; mel:ng evolved crust. 570-‐540 Ma Returning towards +ve values; increased juvenile input.
Zircon produc7on through 7me
520-‐490 Ma Delamerian Orogen Strong juvenile input – development of an external orogen.
Zircon produc7on through 7me
What could this mean (assuming a proximal source for detritus)
1. Crustal mel:ng in a ria environment
2. Introduc:on of more isotopically depleted rocks during riaing
3. Magma:c break – crustal thickening?
4. Development of an external orogen
1 2
3 4