Incipient backarc magmatism in the Silurian Tumut Trough, New South Wales: An ancient analogue of the early Lau Basin

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Abstract
The Silurian Honeysuckle beds of the ‘Tumut Trough’, southeastern New South Wales comprise a sequence of pillow and massive mafic volcanic rocks, basaltic breccia and fine‐grained sedimentary rocks typically metamorphosed to the lower greenschist facies. The mafic rocks range in composition from tholeiitic basalt to andesite. The primary mineralogy consisted largely of plagioclase, augite and Fe‐Ti oxide, now largely altered to an assemblage of albite, calcite, actinolite, chlorite, epidote, zoisite, sphene and quartz. The abundances of immobile trace and major elements indicate that the mafic rocks were erupted in a plate‐margin setting with compositions spanning the MORB and island‐arc basalts (IAB) fields of most discriminant diagrams. The rocks are enriched in LREE, similar to enriched MORB, IAB and some backarc basin basalts. Enrichment in Th and depletion of Nb and Ti are consistent with modification of the source rocks by subduction‐related fluids. Comparison of the geochemical trends in the Honeysuckle beds with those in modern volcanic suites reveals that they most closely resemble mafic rocks erupted during the initial stages of backarc basin spreading. This is consistent with field relationships; spreading never proceeded far enough to form an oceanic substrate and the mafic rocks probably erupted at the surface via a fracture system in attenuated continental crust. The most appropriate modern analogue is the western Lau Basin in which basalt and andesite, intermediate in composition between IAB and MORB, erupted in a basin‐and‐range‐style setting.