Archaean tectonics in the Agnew supracrustal belt,Western Australia |
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| Authors: | JP Platt PD Allchurch RWR Rutland |
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| Institution: | 1. Geological Institute, University of Amsterdam, Amsterdam The Netherlands;2. Australian Section Pty, Kalgoorlie, W.A. Australia;3. Department of Geology, University of Adelaide, Adelaide, S.A. Australia |
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| Abstract: | The Agnew supracrustal belt consists of a greenstone sequence (interlayered metabasalt, differentiated gabbroic sills, ultramafic bodies, and black volcanogenic sediment) unconformably overlain by granitoid-clast conglomerate and meta-arkose. The base of the preserved sequence is intruded by grey tonalite with a crudely concordant upper contact, and by small discordant bodies of leucogranite.An early deformation (D1) produced isoclinal folds and a regional penetrative foliation. These structures were probably gently dipping when formed. D2 produced large-scale NNW-trending upright folds, a regional foliation, and a vertical N-trending ductile fault on the west side of the belt. D2 structures indicate a combination of ENE-WSW shortening, and right-lateral shear along the ductile fault. Both D1 and D2 were accompanied by metamorphism under upper greenschist to lower amphibolite facies conditions.The interpreted sequence of tectonic events is (1) deposition of the greenstone sequence on an unknown basement; (2) intrusion of large volumes of tonalite, separating the supracrustal rocks from their basement; (3) erosion of mafic rocks and tonalite to produce the clastic sedimentary sequence; (4) the first deformation; (5) intrusion of small volumes of leucogranite; (6) the second deformation.The bulk of the granitoid rocks were emplaced before the first recognisable deformation. Thus the granitoid magma cannot have been produced by partial melting of previously downbuckled ‘greenstone belt’ rocks, nor can the large-scale upright folds (D2) be a result of forceful emplacement of the magma — two common postulates for Archaean terrains. The D2 folds are closely related to the ductile fault bounding the zone: these structures, which give the present N-trending tectonic belt its form, are the youngest features in the terrain. |
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