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EG vol 2


									2: 70    Extended abstracts of Conference Proceedings: What drives metamorphism and metamorphic reactions: heat production,
         heat transfer, deformation and kinematics? Convenors Treloar, P.J. and O’Brien, P. Kingston University. 1996

40Ar/39Ar and U/Pb dating in the Shaw Batholith and Coongan Belt (Pilbara
Craton, WA), implications for timing of metamorphism and structures.
                     1                       2                                                 1
Tanja E. Zegers , Jan R. Wijbrans , David R. Nelson3 & Stan.H. White
1Faculty of Earth Sciences, Budapestlaan 4, 3508 TA Utrecht, The Netherlands,
2Institute of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081HV Amsterdam, The
3Geological Survey of Western Australia, Mineral House, 100 Plain Street, E.Perth, WA6004,

The Pilbara craton in NW Australia consists of a mid Archean granitoid-greenstone terrain (2.8-3.6 Ga), characterised
by bulbous granitoids surrounded by greenstone belts. The granitoid-greenstone terrain is unconformably covered by
relatively undeformed Hamersley Group volcano-sediments. It provides excellent opportunities to study the structural/
kinematic evolution in both greenstones and granitoids, and their relation to magmatic and metamorphic events.
   As part of a structural/kinematic study of the Eastern Pilbara a number of 40Ar/39Ar step heating experiments were
performed on amphiboles from samples from the Shaw Batholith and Coongan Belt. In addition, five granitoid samples
were dated by U/Pb on zircons using the Curtin University SHRIMP.
   The aims of this dating study were: 1) to place temporal constraints on large scale structures in this area (Central
Coongan, Split Rock and Mulgandinnah shear zones). 2) to link two previous 40Ar/39Ar studies in the Pilbara (Wijbrans
& Mc Dougall, 1987 and Davids 1995) with the new data and interpret the results in terms of the thermal history
of various tectonic domains.
   Most argon spectra show well defined plateaus ages varying from 3530 to 2850 Ma. In the west and central Shaw
batholith plateau ages are between 3040 and 2850 Ma. These ages coincide with ages of late- to post-tectonic adamellites
at c. 3000 Ma, 2950 Ma and 2850 Ma (Bickle 1989) in the Shaw Batholith. The eastern part of the Shaw Batholith
and adjacent Coongan Belt show a number of >3000 Ma plateau ages. Sample T94/87, a amphibolitic metabasalt
adjacent to the North Shaw, gave the oldest plateau ages recorded in this study: 3530±7 Ma and 3468±6 Ma. A number
of well defined plateau ages in this area are between 3200 and 3300 Ma. These cannot be related to thermal pulses of
granitoid intrusions in this area. Actinolites from sample T93/182b gave a plateau age of 3197±22 Ma. These actino-
lites postdate the main foliation in the Central Coongan Shear Zone. Sample T94/203 constrains the cooling of the
metamorphic aureole of the North Corunna Downs granitoid to 3340 Ma.
   New SHRIMP U/Pb zircon ages confirm the uniform old age of early suite of granitoids of the Shaw Batholith. The
Mulgandinnah shear zone along the western margin of the Shaw Batholith is one of the youngest features, its age is
2.90 -2.94 Ga dated both by zircons from a synkinematic granite and argon ages of hornblende from mafic xenolith
included in the shear zone.
   We conclude that:
1- All plateau ages are within the age brackets defined by other dating methods (U-Pb and Pb-Pb) for this part of the
Pilbara Craton, indicating that with the 40Ar/39Ar method meaningful ages in this area can be obtained.
2- Cooling ages in the Shaw batholith are reset by mainly 3000 Ma and 2950 Ma granitoids, and to a lesser degree the
2850 Ma granitoids.
3- The Mulgandinnah Shear Zone was active prior to 2950 Ma, as this is the oldest plateau age of samples from this
amphibolite grade shear zone. This age is confirmed by the youngest zircon ages from a synkynematic granitoid.
4- The 3530±7 and 3468±6 Ma plateau ages of sample T94/87 are thought to be related to intrusion of the North Shaw
Suite (U-Pb zircon: 3467±6 Ma, McNaughton 1988). They indicate that there have been no major thermal events
in this part of the greenstone belts since intrusion of the North Shaw Suite.
5- In the Coongan Belt and at the contact with the Shaw Batholith ages between 3300 and 3100 Ma record a thermal
evolution that can not be directly related to intrusion of granitoids. It may be related to crustal thickening as indicated
by thrusts s.a. the Central Coongan Shear Zone, which must have been active prior to 3200 Ma.
6- The cooling ages of granites of the Corunna Downs Batholith, as indicated by plateau ages of contact metamorphic
hornblendes is between 3340 an 3400 Ma (Davids 1995).
   The observed range in ages of hornblendes from various lithologies in the Shaw Batholith and it surrounding green-
stones is similar to the range in magmatic emplacement ages for granitoids as obtained by U/Pb zircon and Pb/Pb
whole rock techniques. These similarities lead us to conclude that the thermal effects of granitoid emplacement have
remained local, and that the thermal evolution of the Shaw Batholith area can be understood in terms of a sequence of
discrete thermal pulses.
                                                        2: 71

Bickle 1989, Contrib. Min. Petrol .101, 361-376
Davids 1995, in print in Precam Res.,
McNaughton 1988, AGC abstr. vol. 272-273,
Wijbrans & Mc Dougall, 1987, E.P.S.L. 84. 226-242.

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