Abstract

The Lamboo Complex is one of several Palaeoproterozoic terrains in northern Australia that is intruded by high-K, I-type granites. The oldest granites in the complex consist of the high-K 1865–1850 Ma Paperbark supersuite with initial εNd values between −2.7 and −4.0. Tonalite sheets of the c. 1850 Ma Dougalls suite have high Al2O3, Na2O, Sr, Sr/Y and La/Yb, and low Y and HREE, and initial εNd values between +1.4 and +0.7. The rocks have compositions similar to adakites that form by melting of mafic rocks in the deep crust with garnet in the residue. Granites of the 1835–1805 Ma Sally Downs supersuite range from high-K types through to crustally derived adakites, all with initial εNd values between +0.1 and −1.2. The two supersuites and the Dougalls suite were intruded into different tectonic settings. Previously all of the granites in the Lamboo Complex were interpreted as partial melts of a chemically and isotopically uniform underplate beneath a rifted Archaean craton. However, their spatial and temporal distribution, and their varied compositions, are best explained by large-scale horizontal plate motions and subduction of oceanic crust.

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