Lithos, 23 (1989) 63-83 63 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands The Wuluma granite, Arunta Block, central Australia: An example of in situ, near-isochemical granite formation in a granulite-facies terrane W.J. COLLINS ~, R.H. FLOOD 2, R.H. VERNON 2 and S.E. SHAW2 ~ Department of Geology, The University of Newcastle, Newcastle, N.S. IV.. 2308 (Australia) 2School of Earth Sciences, Macquarie University, Sydney, N.S. 141.. 2109 (Australia) LITHOS Collins, W.J., Flood, R.H., Vernon, R.H. and Shaw, S.E., 1989. The Wuluma granite, Arunta Block, cen- tral Australia: An example of in situ, near-isochemical granite formation in a granulite-facies terrane. In: R. Gorbatschev (Editor), Proterozoic Geochemistry. Lithos, 23: 63-83. The Wuluma granite is a small, elongate, relatively undeformed pluton in the Proterozoic Strangways Metamorphic Complex, central Australia. The complex constitutes a supracrustal assemblage that under- went granulite-facies metamorphism 1800 Ma ago. Metamorphism was associated with at least three phases of folding that ultimately produced upright, regional, doubly plunging F3 folds and isobaric cooling en- sued. Generation of the Wuluma granite occurred at ~ 1750 Ma, based on Rb-Sr isotopic data, during syn-D3 regional retrogression and rehydration of the terrane. Contacts between the granite and gneisses are invariably gradational. At the pluton margin, banded gneisses grade along strike into granite containing abundant biotite schlieren that parallel regional struc- tures. Granite and pegmatite dykes cut these rocks. Inwards from the contact, the granite is more homo- geneous, containing diffuse parallel schlieren and small aligned rectangular feldspar crystals, indicating flow of magma. Rafts of unmelted granofels form a ghost layering; they mimic macroscopic F3 folds and show only minor retrogressive metamorphic effects. At the pluton core, the granite is homogeneous and structurally isotropic, containing some subrounded granofelsic inclusions, very diffuse schlieren and dis- aggregated pegrnatite dykes. Thus, it appears that an isoclinallyfolded, vertical body ofquartzofeldspathic gneiss was melted "in situ" to form the pluton, which did not break away from the source. The body resembles a tapered diapir and we term this type of pluton a regional migmatite terrane granite. Geochemical data are consistent with the granite forming by anatexis of quartzofeldspathic migmatitic gneisses with appropriate composition. The chemical similarity of both rock types implies derivation of the granite by either partial melting and retention of residual material in the magma or more complete melting, followed by solidification virtually in situ. The latter interpretation is preferred, although biotite melanosome has been incorporated into the magmas. Migmatitic gneisses, with feldspar proportions cor- responding to ternary minima compositions, are argued to have melted during influx of an externally derived aqueous fluid associated with pegmatites. Field evidence shows that slightly different gneisses melted to produce slightly different granite compositions, even on outcrop scale; the compositions of migmatites and derivative granite both range from 67% to 77% SIO2. Therefore, crystal fractionation did not produce the geochemical variation. Rather, in situ crystallization of the granite resulted in a compo- sition near-isochemical to that of the precursors, but with a coarser, obviously granitic texture. (Accepted February 10, 1989) Introduction Experimental studies have shown that granitic magmas can be derived from a wide range of crustal rock types (e.g., Tuttle and Bowen, 1958; Green, 1976; Clemens and Wall, 1981). Commonly the process envisaged involves the granite magma being formed by 20-30% partial melting; in these in- stances the composition of the source rock is far re- moved from the derived granitic melt and is diffi- cult to infer, unless substantial assumptions are made about the relationship between the granite 0024-4937/89/$03.50 © 1989 Elsevier Science Publishers B.V.