Relation between archaean high-grade gneiss and granite-greenstone terrain in western Australia

Abstract

Two major regions of Archaean crust occur in Western Australia called the Pilbara and Yilgarn Blocks. All the Pilbara and most of the Yilgarn Block consist of granite-greenstone terrain with metamorphic grades in greenschist or low amphibolite facies, but these terrains developed at different times in the two regions: 3500–3000 Ma ago in the Pilbara and 3000–2600 Ma ago in the Yilgarn. High-grade gneisses are only known to occur in a belt along the western margin of the Yilgarn Block in a region called the Western Gneiss Terrain. Direct observations of the relation between this high-grade gneiss terrain and typical granite-greenstone terrain can therefore only be made in this region. Two kinds of relation are seen. In the north, the high-grade gneiss terrain developed between 3800–3300 Ma ago. It is distinctly older than, and contains a different assemblage supracrustal rocks from, the granite-greenstone terrain. The gneiss terrain could have formed a sialic basement to the greenstones but direct evidence of this relation is not preserved. The gneiss terrain is intruded by granites similar to those of the granite-greenstone terrain and the two terrains are deformed and metamorphosed together. Prograde metamorphism of the granite-greenstone terrain increases from mainly greenschist facies in the east, to high amphibolite facies in the west, where it is associated with retrograde metamorphism of the gneiss terrain from granulite to high amphibolite facies. In the south, a portion of the Western Gneiss Terrain also consists of gneisses which pre-date the granite-greenstone terrain. However, most of the gneiss terrain consists of younger granitoid intrusions, similar to those that form a major component of the granite-greenstone terrain, but metamorphosed in granulite facies. As in the north, a regional eastward decrease in metamorphic grade suggests that the exposed crustal section is tilted eastwards. The tectonic significance of the high-grade gneiss and granite-greenstone terrains remains uncertain. Regional relationships on the scale of the Yilgarn Block suggest three possible models: (1) cratonic basinal model, whereby large greenstone basins formed on gneissic basement; (2) continental marginal model, whereby greenstone terrains developed adjacent to pre-existing protocontinent; or (3) collision involving two unrelated segments of crust.