A two-stage model for the formation of the granite-greenstone terrains of the Kalgoorlie-Norseman area, Western Australia

Details A two-stage model for the formation of the granite-greenstone terrains of the Kalgoorlie-Norseman area, Western Australia A two-stage model for the formation of the granite-greenstone terrains of the Kalgoorlie-Norseman area, Western Australia

Sep 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988e%26psl..90...11c&link_type=abstract

Earth and Planetary Science Letters, Volume 90, Issue 1, p. 11-25.

Physics

55

Scientific paper

Accurate dating of granites and greenstones from the Kalgoorlie-Norseman area shows that there is considerable overlap in time between the formation of granites and greenstones. However, within any given area, the oldest granites are always younger than the oldest greenstones. It is suggested that the long-lived (100 Ma) thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting, leading to the production of granites. The time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust.
Anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma. Where this happens the basaltic magma ponds at the base of the crust, supplying additional heat and accelerating the melting process. Eventually the granitic melt layer becomes unstable and rises within the crust. Areas intruded by granite become topographic highs, with topographic lows forming above areas where light melt has been withdrawn from the lower crust. When the topographic highs protrude above sea-level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones. Thus the change from a basalt-dominated lower greenstone succession to a sediment and felsic volcanic-dominated upper whitestone sequence, seen in many granite-greenstone terrains, results from the intrusion of granitic magma into the upper crust.
The crust of the Kalgoorlie-Norseman area formed in two distinct stages, separated by a time break of up to 700 Ma. The first stage involved the formation of primitive crust, probably above subduction zones. This primitive crust was characterized by low Rb/Sr and high Sm/Nd ratios. The second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents. This led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas. Melting reset the Rb/Sr, Sm/Nd and U/Pb systems, giving the impression that the second event, which occurred at ~ 2.7 Ga, was a major crust-forming event. It was, in fact, a crustal reworking process although some basalt was added to the crust at this time. Although the two-stage history of the Kalgoorlie-Norseman area is clearly seen in the lead isotopes, and in zircons, it is not readily discernible from the Rb-Sr and Sm-Nd systematics, due to mantle-like ratios of those elements in the primitive crust.

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