EXTENSIONAL EMPLACEMENT OF A HIGH-GRADE GRANITE GNEISS COMPLEX INTO LOW-GRADE GREENSTONES, EASTERN GOLDFIELDS, YILGARN-CRATON, WESTERN-AUSTRALIA

A high-grade granite gneiss complex was emplaced into low-grade greens tones after an early episode of Late Archaean deformation (D1) in the Eastern Goldfields of Western Australia. The components of the graniti c 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 comp lex are foliated and gneissic granite with igneous emplacement ages of c. 2675 Ma, that postdate the bulk of greenstone volcanism (c. 2720-2 675 Ma). The complex includes voluminous and largely undeformed monzog ranite 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 comple x was partly accommodated by normal faulting. Final emplacement occurr ed during the c. 2660 Ma granite event, and overlapped with regional e ast-west shortening (D2-D3) in the adjacent greenstones. Uplift of the granitic gneiss complex to higher crustal levels may have resulted fr om preferential injection of granite magma into gentle domes of the ba sal greenstone contacts. These domes formed during earlier regional ex tension of the greenstones. (C) 1997 Elsevier Science B.V.