The geochemistry and crustal origin of the Archaean acid intrusive rocks of the Agnew Dome, Lawlers, western Australia

Foden, J.D., Nesbitt, R.W. and Rutland, R.W.R. (1984) The geochemistry and crustal origin of the Archaean acid intrusive rocks of the Agnew Dome, Lawlers, western Australia. Precambrian Research, 23 (3-4), 247-271. (doi:10.1016/0301-9268(84)90046-9).

Abstract

Acid intrusive rocks of the Agnew Dome intrude a supracrustal sequence, part of the Lawlers-Mt. White greenstone belt. The acid intrusives form a complex batholith with multiple intrusions. Three distinct groups of granitoids have been recognised, both in terms of style of intrusion and on petrological grounds. The oldest intrusion was that of the Lawlers tonalite (granitoid group I) which includes a variety of rock-types from quartz diorite through tonalite to granodiorite. These are characterised by high Na/K, high concentrations of LREE, Ba and Sr, very high La/Yb, low HREE and low Y. Intrusion of the Lawlers tonalite pre-dates the first deformation of the intruded greenstones.

A second granitoid suite (group II) occupies the dominant area of the batholith and comprises a diverse group ranging from granodiorite through adamellite to trondhjemite. These have lower Na/K and La/Yb ratios and a higher SiO2 content than the tonalites. Their intrusion pre-dates the main folding of the greenstones (D2).

The third granitoid group (group III) comprises highly leucocratitic, true granites of near minimum-melt composition, K- and Rb-rich, but with very low concentrations of P, Zr, Ti, Sr and Ba. They have flat REE patterns with Eu-anomalies. These are clearly the latest intrusive phase and form a network of intersecting dykes, sheets and small plugs.

On geochemical grounds the three granitoid suites of Lawlers are not related. The tonalites are likely to have been derived from the fusion of mafic source rocks, while some features of the leucogranites suggest source material must include a K-rich granite component. These features suggest that at least, in this part of the Yilgarn Block, the crust which pre-dates both the 2.65 Ga Lawlers tonalite and the Mt. White-Lawlers greenstone belt must have been of mixed character with mafic and sialic rocks very like the presently exposed Yilgarn Archaean suite.

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