New insights into the geological evolution of the west Musgrave Complex

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New insights into the geological evolution of the
west Musgrave Complex

• H. Smithies, H. Howard, P. Evins, C. Kirkland and
  M. Wingate

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• Basement
• What is it?
• Not dominated by metasedimentary rocks
• Nd-model age 1.9-2.0 Ga

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• Oldest exposed rocks
• Composite gneisses derived from
• volcanic, volcaniclastic, and clastic
• protoliths that were thought to be
• deposited c.1600 1540 Ma??
• BUT
• Our data show no indication of intrusion
• or extrusive of felsic magmas older than
• c. 1400 Ma in the west Musgrave
• Complex.

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Interpretive Bedrock Geology
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• Oldest exposed rocks
• Composite gneisses derived from
• volcanic, volcaniclastic, and clastic
• protoliths that were thought to be
• deposited c.1600 1540 Ma??
• BUT
• Our data show no indication of intrusion
• or extrusive of felsic magmas older than
• c. 1400 Ma in the west Musgrave
• Complex.

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Deposition of the protolith is constrained
between c. 1360 and 1293 Ma. Wirku
Metamorphics.
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The age spectra for detrital zircons in the Wirku
Metamorphics define at least four geographically
separate groups.
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• Oldest exposed rocks

• Most likely, is that the differences in detrital
  patterns but similarity in depositional age means
  that we are simply seeing slightly different
  stratigraphic levels within a single depositional
  basin that evolved across the entire west
  Musgrave Complex between c. 1360 and c. 1293 Ma.
• The various lithotectonic zones that we have
  identified contain no exotic component and
  might simply reflect varying levels of exposure

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• The 1336-1293 Mount West Orogeny
• Wankanki Supersuite

Hbl-biot-(cpx) metamorphosed monzogranites and
equivalent high-grade gneiss and migmatites first
identified by Gray (1971) and thought to be
restricted to a small region south of the Mann
and Hinckley Faults. Mainly constrained to a
narrow period between c. 1326 and 1312 Ma.
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• The 1336-1293 Mount West Orogeny
• Wankanki Supersuite

Now known to be far more extensive in outcrop
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• The 1336-1293 Mount West Orogeny
• Wankanki Supersuite

• Calc-alkaline, I-type magmas typical
  volcanic-arc geochemical signature (whatever that
  implies??)
• ? No gabbros
• ? Much older Nd model age (1.9-2.0 Ga)
• (continental arc?)
• (inherited signature?)

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• The 1219-1155 Musgravian Orogeny
• Pitjantjatjara Supersuite

Intense deformation and widespread high-grade
crustal reworking between c. 1200 and 1160 Ma
(e.g. Edgoose et al., 2004).
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• The 1219-1155 Musgravian Orogeny
• Pitjantjatjara Supersuite

Granites of the Pitjantjatjara Supersuite
dominate outcrop in the BATES region. New U-Pb
SIMS (SHRIMP) data can be divided into distinct
age groups.
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c. 1219 - 1200 Ma early Musgravian Orogeny c.
1190 - 1155 Ma late Musgravian Orogeny
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Compositionally homogeneous age-groups (suites)
HOLT
BATES
BELL ROCK
BLACKSTONE
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• The 1219-1155 Musgravian Orogeny
• Pitjantjatjara Supersuite

All zones show Pitjantjatjara magmatism
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• The 1219-1155 Musgravian Orogeny
• Pitjantjatjara Supersuite

Granites of the Pitjantjatjara Supersuite are
significantly less abundant in the southwest.
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• The 1219-1155 Musgravian Orogeny
• Pitjantjatjara Supersuite

There is a systematic variation in abundance of
Pitjantjatjara granites
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In the Tjuni Purlka Tectonic Zone the metamorphic
ages are continuous.
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• The 1219-1155 Musgravian Orogeny
• Pitjantjatjara Supersuite

The Tjuni Purlka Tectonic Zone was tectonically
active throughout the entire Musgravian
Orogeny Much of the deformation that actually
defines the zone is itself related to the
Musgravian Orogeny.
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• The 1219-1155 Musgravian Orogeny
• Pitjantjatjara Supersuite

Granites of the Pitjantjatjara Supersuite fall
into the within plate granite field on the
tectonic discrimination diagrams and the A-type
fields
High-T granites
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• The 1219-1155 Musgravian Orogeny
• Pitjantjatjara Supersuite

The granites within the Tjuni Purlka Tectonic
Zone show extremely well-developed A-type
characteristics. Melting of a dehydrated and
refractory source at even higher temperatures
than those required for the rest of the
Pitjantjatjara magmas.
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What does all this mean??
Basement melted during the Mount West Orogeny and
was too refractory to melt again during the
Musgravian Orogeny(s) Basement dehydrated
during Mount West Orogeny and melted at
high-temperature during the Musgravian
Orogeny(s) Basement melted during the Mount West
Orogeny and refractory residue remelted under
extremely high-T conditions during the Musgravian
Orogeny possibly aided by shear-related
rehydration No need for exotic tectonic zones
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• The 1219-1155 Musgravian Orogeny
• Pitjantjatjara Supersuite

Tectonic environment ??? within-plate
chemistry simply reflects high-T melting of a
dehydrated and/or refractory crustal source
does that necessarily exclude a subduction- or
near subduction zone- environment??? What we do
have is very thin crust and very high heat-flow
extension!! Could be within-plate, but there
are alternatives
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• Conclusions

• Geologically very complicated but the pieces
  are certainly falling into place
• Notion that the c. 550-570 Ma Petermann Orogeny
  has had the main control on present structural
  trends is probably not true
• Very systematic changes in the abundance of
  various granite age- and compositional- groups
  and these are very useful in deciphering the
  geological history
• All recognised lithotectonic zones share the same
  geological components just differ in how these
  components are manifested suggesting that there
  are no exotic tectonic components within the
  exposed west Musgrave Complex