Abstract of Albany-Fraser Province Synthesis

Record 2001/12

Compiled by Anthony Budd

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• Introduction

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Introduction

The Albany-Fraser province extends along the southern and southwestern margin of the Yilgarn Craton. It consists mainly of orthogneiss and granite but also includes large sheets of metagabbro (including the Fraser Complex), remnants of mafic dykes and widespread metasedimentary rocks. The orthogneisses are derived from Late Archaean and Palaeo- and Mesoproterozoic granitic rocks that were deformed and metamorphosed during Mesoproterozoic orogenic activity.

The province is composed of Archaean and Proterozoic rocks. Proterozoic intrusive activity was accompanied by metamorphism and deformation, and occurred in at least two events. The Biranup Supersuite probably intruded at around 1700 - 1600 Ma, and is mostly composed of heterogeneous orthogneiss. The Nornalup Supersuite appears to include granites intruded at two ages, approximately 1300 Ma and 1190 Ma. It is dominated by heterogeneous ortho- and paragneisses. There is insufficient etailed information to divide the Nornalup Supersuite into two suites as is indicated by these two distinct ages.

Nelson et al. (1995) present a summary of the geological history of the Esperance region (eastern part of the Albany-Fraser province). Widespread granite emplacement in the Yilgarn Craton occurred at 2620 Ma, followed by emplacement of granitic rocks in the Albany-Fraser province at 1700-1600 Ma. Arenaceous sediments were deposited at < ca 1560 Ma. Widespread intrusion of gabbro (Fraser Complex) and granite into thickened crust at a high metamorphic grade occurred between 1300-1280 Ma. Widespread granites were intruded into the southeastern part of the orogen between 1190-1130 Ma.

Geochronological studies in the western part of the province have not found any evidence of the ca 1700-1600 Ma granitic rocks. The post-1300 Ma geological histories of both eastern and western parts of the Albany-Fraser province appear to be broadly similar.

Granites of both the Biranup and Nornalup Supersuites show similar geochemical trends. Analyses of the Nornalup Supersuite include mafic enclaves (Clarke 1995). The granites show little alkali alteration, have a spread of Th/U ratios, and are mostly reduced, becoming oxidised in the most felsic samples. They are metaluminous, have increasing K/Rb (showing that the granites are not K-feldspar-fractionated), and only the most felsic samples show any increase in Rb/Sr. Compositions range from tonalite to granite, and all samples are Sr-depleted, Y-undepleted. Some samples show moderately high HFSE.