Geochemistry of the Yandal belt metavolcanic rocks,Eastern Goldfields Province,Western Australia

Archaean felsic metavolcanic rocks occur throughout the Yandal belt in the north of the Eastern Goldfields of Western Australia where they are most abundant in the higher parts of the stratigraphy. With the exception of the Spring Well Sequence at the southern end of the belt, these rocks are typically dacites showing geochemical affinities with Archaean high‐Al trondhjemite‐tonalite‐dacite (TTD) suites. They have high Sr, Al₂O₃, and (La/Yb)_N; low Y, Nb, Zr and heavy rare‐earth elements (HREE); and lack a significant Eu anomaly. In contrast, broadly coeval mafic volcanic rocks have flat REE patterns and trace‐element compositions more typical of modern backarc basin basalts. The Spring Well Sequence is readily distinguished lithologically and geochemically from the remainder of the Yandal belt. Spring Well basaltic andesites are geochemically similar to modern calc‐alkaline arc magmas, i.e. negative Nb–Ta anomalies and enrichment of both large‐ion lithophile elements (LILE) and light rare‐earth elements (LREE). Andesites and rhyolites, both abundant in the Spring Well Sequence, have elevated LILE relative to high field strength elements, and moderate to strong negative Nb, Ta, Sr and Ti anomalies. Rhyolites have low Sr/Y and relatively flat REE patterns ((La/Yb)_N = 4.2–5.0). The chemistry and lithostratigraphic associations of the Yandal belt, with the exception of the Spring Well area, suggest a similarity with the Kalgoorlie Terrane, which is supported by published geochronological data. In contrast, the abundance of rhyolite, distinctive calc‐alkaline chemistry and ca2690 Ma age of the Spring Well Sequence suggests a possible association with ca2692 Ma bimodal calc‐alkaline arc‐rift sequence at Teutonic Bore and similar rocks at Melita and Jeedamya, 150 km south of Spring Well. The abundance of TTD dacite and tholeiitic basalt throughout the Yandal belt suggests magma generation from both decompression partial melting of mantle peridotite to produce backarc tholeiitic magma, and partial melting of subducted oceanic lithosphere to produce high‐Al dacite‐tonalite magma. Based on field relationships of the lithological associations, spatial geochemical patterns and published geochronological data, a shallow, west‐dipping subduction model is postulated for the Yandal belt. In this model, widespread tholeiitic basalt and TTD dacite volcanic sequences are thought to have formed in a backarc basin west of a predominantly submerged continental margin volcanic arc. The dominance of dacite in the upper stratigraphy of the Yandal belt could indicate the development of a secondary volcanic ridge or arc in this basin. The Spring Well Sequence is interpreted to occupy the northern preserved portion of the primary arc, remnants of which now extend south through Teutonic Bore to the Melita and Jeedamya volcanic centres. South of Spring Well, volcanic sequences become distinctly bimodal with basalt and high silica rhyolite suggesting an increasing influence of arc extension toward the south.