Extensional emplacement of a high-grade granite gneiss complex into low-grade greenstones, Eastern Goldfields, Yilgarn Craton, Western Australia

Abstract

A high-grade granite gneiss complex was emplaced into low-grade greenstones after an early episode of Late Archaean deformation (D1) in the Eastern Goldfields of Western Australia. The components of the granitic gneiss complex range from migmatitic granitic gneiss to foliated and massive granite. The complex is surrounded by a marginal zone of high-grade (amphibolite facies) greenstones. The oldest phases of the complex are foliated and gneissic granite with igneous emplacement ages of c. 2675 Ma, that postdate the bulk of greenstone volcanism (c. 2720-2675 Ma). The complex includes voluminous and largely undeformed monzogranite that is correlated with widespread granite plutonism at c. 2660 Ma. Fabrics in marginal high-strain zones between the granite gneiss complex and greenstones, and in faults separating high-grade from low-grade greenstones, indicate that emplacement, or uplift, of the complex was partly accommodated by normal faulting. Final emplacement occurred during the c. 2660 Ma granite event, and overlapped with regional east-west shortening (D2–D3) in the adjacent greenstones. Uplift of the granitic gneiss complex to higher crustal levels may have resulted from preferential injection of granite magma into gentle domes of the basal greenstone contacts. These domes formed during earlier regional extension of the greenstones.