ROLE OF CRUSTAL CONTAMINATION IN HETEROGENEOUS ARCHEAN VOLCANICS FROM THE LEONORA REGION, WESTERN-AUSTRALIA

Archean supracrustal volcanic successions in the Leonora-Laverton region of the Eastern Goldfields Province, Western Australia, are subdivided into two stratigraphic associations: (1) an early succession ten km thick containing homogeneous high-Ti tholeiite basalts displaying nearly flat chondrite-normalized incompatible lithophile element patterns with (La/Sm)N ratios ranging from 0.88 to 1.12, and (2) an unconformably overlying heterogeneous succession four km thick consisting of (a) early siliceous, low-Ti tholeiites and associated intermediate calc-alkaline volcanics, grading to (b) high-Ti tholeiite similar to those of association (1), and (c) late siliceous high-Mg, spinifex-textured basalts. Fractionation models and trace element distribution in high-Ti basalts suggest that all the flows were derived from similar but not identical parental magmas, and that subvolcanic gabbroic fractionation was characteristic. Basaltic end-members of the siliceous, low-Ti suite had identical fractionation patterns, but hornblende joined the assemblage in contaminated intermediate calc-alkaline members, producing low-HREE fractionates. The siliceous basalts exhibit variable enrichment in large-ion lithophile (LILE) and light rate earth (LREE) elements with a continuum of (La/Sm)N ratios up to 2.75. They also have marked negative anomalies for Nb, P, and Ti; several samples also exhibit a positive anomaly for Zr on normalized distribution diagrams. Low Sr concentrations are inconsistent with mantle enrichment processes, and compositions of siliceous volcanics are explained instead by up to 30% contamination of high-Ti tholeiite liquids by Archean crust composed of hornblende tonalite gneiss. Geochemistry of the volcancis is consistent with eruption in a continental back arc basin tectonic setting. © 1990.