International Geologiical Congress - Oslo 2008

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MPI-05 Large Igneous Provinces: Initiation, evolution and origin

 

Giant layered mafic intrusions and mafic magmatic shear zones of the c. 1075 Ma Giles Event

 

Paul M. Evins, Geological Survey of Western Australia (Australia)
Hugh Smithies, Geological Survey of Western Australia (Australia)
Heather Howard, Geological Survey of Western Australia (Australia)
 

 

The Giles Event in central Australia is a complex sequence of at least 5 mafic and 5 felsic magmatic pulses coupled with intense deformation that took place over a short time span during the Mesoproterozoic. Ongoing detailed mapping of the western Musgrave Complex has outlined a far greater volume of mafic magma than previously thought, as well as extensive interactions between these and locally voluminous felsic magmas. Textural evidence and field relationships (including contacts with coeval extrusive units) indicate that all magmatism from this event occurred at relatively shallow crustal levels.

Magmatism began with deposition of the Mummawarrawarra Basalt. This was followed by emplacement of the large (up to 2500 km2 and 10 km thick) layered mafic and ultramafic Giles intrusions that collectively form one of the largest layered mafic suites in the world. These were likely emplaced in the upper crust, at least locally, along an unconformity above the Mummawarrawarra Basalt. The sills were then tilted to varying degrees before intrusions of similarly voluminous unlayered gabbro were emplaced into an active, up to 5 km wide, strike-slip shear zone that cuts the layered suite. Formation of these upper crustal, syn-mafic-magmatic shear zones was accompanied by felsic magmatism. Felsic magma is preserved as stringers and xenolithic blebs mingled with mafic magmas. During the waning stages of deformation, felsic magmas were again introduced as rapakivi granite at deeper levels, acicular pyroxene-hornblende alkali-granite at higher levels and the trachydacite flows of the Smoke Hill Volcanics and Hogarth Formation. Concurrent uplift exposed some of the layered intrusions as basement on which the Smoke Hill Volcanics were extruded. The uplift may have been accommodated along amphibolite facies shear zones that mark a change to more hydrous conditions in the upper crust. These shear zones are cut by plagioclase-phyric gabbro dykes. The Saturn mafic ring complex and another large granite pluton represent two distinct intrusive bodies that crosscut all of the previous events. The final magmatic event is manifested as rare, NW trending aplites, granites, and pegmatites. These were commonly focused along the same structures earlier utilized by the plagioclase-phyric gabbro dykes and invariably display alteration halos equal to their thickness. Geochemically, the felsic magmas are all intraplate A-type magmas, but Nd-isotopic data shows that they tapped different source regions (crust and mantle) at different stages.

The immense magmatic output from the Giles Event represents a 15 km thick layer of relatively dense material emplaced into the upper crust in a very short time span. Isotopic ages from one of the earliest felsic magmatic events and from the Tollu granite pluton constrain most of the magmatism and deformation to 1080 - 1070 Ma, with the actual time frame likely much less.

 

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