Crustal development in the Agnew region, Western Australia, as shown by Rb/Sr isotopic and geochemical studies

Cooper, J.A., Nesbitt, R.W., Platt, J.P. and Mortimer, G.E. (1978) Crustal development in the Agnew region, Western Australia, as shown by Rb/Sr isotopic and geochemical studies. Precambrian Research, 7 (1), 31-59. (doi:10.1016/0301-9268(78)90004-9).

Abstract

Rb/Sr geochronology on a folded greenstone-granitoid complex in the Agnew area, Western Australia, yields four distinct ages of igneous activity that conform with stratigraphic and intrusive relationships. They are (using View the MathML source, NBS 70A = 522 ppm Rb and 65.3 ppm Sr):

Ma 87Sr/86Sr initial (IR)
(1) Differentiated gabbro-granophyre from a stratigraphically old (Kathleen Valley) greenstone sequence > 2718 ± 50 0.7007 ± 0.0004
(2) Voluminous tonalite, the Lawlers Tonalite 2652 ± 20 0.70152 ± 0.00012
2576 ± 14 0.70218 ± 0.00021
(3) A less voluminous leucogranite, and a large complex pegmatite cutting the Perseverance nickel orebody 2588 ± 18 0.7624 ± 0.0068
(4) Aplitic leucotonalite (very minor volumes but widespread) 2474 ± 14 0.70193 ± 0.00012
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Earlier measurements by Roddick et al. (1976), 70 km to the north, of the Mount Keith Granodiorite (2632 ± 17 Ma, IR = 0.70149 ± 0.00015) and Jones Creek Pegmatites (2481 ± 18 Ma, IR = 0.7253 ± 0.0002) appear to fit groups (2) and (4), respectively. The two principal deformational events in the area are bracketed by groups (2) and (3), and by groups (3) and (4), respectively. Strontium IRs and major and trace-element chemistry suggest that the tonalite-granodiorite and the later leucotonalite were obtained by partial melting of tholeiitic material with garnet as a controlling mineral phase. The leucogranite more likely resulted from remelting of tonalite-granodiorite under similar conditions with garnet again a controlling mineral phase. These four magmatic events constitute a massive addition to the crust from the mantle lasting about 250 Ma. About 90% of the silicic material was emplaced 80 Ma after activity began (tonalite-granodiorite stage) and at least 99% had been emplaced after 140 Ma (leucogranite stage).

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