Melting of Archaean sialic crust under high aH 2O conditions: genesis of 3300 Ma Na-rich granitoids in the Mount Edgar Batholith, Pilbara Block, Western Australia

Abstract

The voluminous 3300 Ma old granitoids of the Mount Edgar Batholith, Pilbara Block, Western Australia, are typically trondhjemites and silicic tonalites with SiO 2 ranging from 65–75 wt%. They have high Sr (average: 425 ppm at 70.6% SiO 2) and Na 2O (3.5–5.0 wt%) contents, with K 2O rapidly increasing (1.0–4.5 wt%) with increasing silica, so that the most fractionated are granodioritic. A lack of restite minerals, calcic plagioclase cores, and mafic microgranitoid enclaves suggests that the granitoids were produced from chemically evolved sources and that they crystallised from melts approximating the bulk composition of the batholith (70.6 wt% SiO 2). Based on experimental data, mass balance and trace element modelling, these silicic melts are considered to have been produced by high degrees of partial melting, involving complete plagioclase dissolution, of tonalitic/dacitic precursors similar in composition to the dacites of the ∼ 3450 Ma greenstone succession. The data are consistent with field and isotopic evidence from the gneiss complex of the batholith, where orthogneisses have formed virtually in situ from older (∼3450 Ma) banded tonalitic gneisses that resemble coeval metamorphosed felsic volcanic rocks of the Duffer Formation, or deeper-level equivalents. A number of granitoids from other batholiths of the Pilbara Block are geochemically similar to the Mount Edgar Batholith, and probably were derived from similar sources. The following three-stage model of crustal growth is envisaged: (1) production of a thick, primitive mafic crust at (∼360 Ma, remnants of which exist as gabbroic anorthosite xenoliths in some granitoids (McNaughton et al., 1988); (2) partial melting of the mafic crust at 3500-3450 Ma to produce tonalitic/ dacitic rocks at shallow mantle depths ( < 60 km), associated with formation of the major greenstone belts of the Pilbara Block; (3) infracrustal melting of the tonalitic/dacitic rocks at 3300 Ma to produce the abundant silicic tonalitic/trondhjemitic (sodic) granitoids. Uniformitarian and non-uniformitarian tectonic models involving arc magmatism, rifting, mantle plumes and delamination are discussed for the three-stage crustal growth process, but the available geological evidence does not favour any particular model. The data indicate that Archaean granitoids are similar to those produced in modern subduction-related environments, but the process of Archaean magmatism might have been quite different.