Devonian ignimbrites of central Victoria: explosive magmas from multiple sources, and deep crustal structure in the Selwyn Block

Abstract

The magmas that formed each Devonian, S-type, volcanic complex in central Victoria were generated through partial melting of a distinct assemblage of rock types in the crustal source region. The more silicic rhyolites were formed independently of the rhyodacitic to dacitic magmas, rather than as differentiates. Within every complex, each ignimbritic unit is also heterogeneous in its chemical and isotopic characteristics. Thus, in this part of Paleozoic Australia, magma heterogeneity occurred on at least three scales—the individual rock unit, the complex and across the region. This means that the Selwyn Block, the logical source of the ignimbrite magmas, is highly heterogeneous, probably in three dimensions. The statistically significant differences in the ages of the various complexes suggest that the heat sources for partial melting of the deep crust were themselves rather local in extent, and probably reflected numerous asynchronous intrusions of mantle-derived mafic magmas, most likely in relatively small batches. Thus, there should be no thick underplated or intraplated layer of gabbroic rock in the deep crust here. Thinner bodies of hot mafic magmas were probably emplaced in different places and at different levels, inducing local high-grade metamorphic conditions. Published radiometric dates for various volcanic units suggest that the magmatic heating of the crust occurred over a period of at least 17 Myr, although maximum intensity occurred between ca 375 and ca 371 Ma. Thus, a model of the deep crust here should involve numerous, perhaps thin metasedimentary units with highly variable ages and original clay contents, as well as sills of Late Devonian mafic rocks scattered both vertically and horizontally and intruded episodically. The dominantly meta-igneous sources of crustally derived I-type magmas generally lie at higher structural levels, where they did not experience metamorphic temperatures sufficient to cause the higher-temperature melting reactions needed to form I-type volcanic magmas. KEY POINTS Each Late Devonian, S-type, volcanic magma in central Victoria formed through melting of a distinct assemblage of crustal source rocks, most likely within the Selwyn Block. The rhyolites formed independently of the rhyodacitic to dacitic magmas, not as differentiates. The mafic magma heat sources for partial melting of the deep crust were emplaced as numerous, relatively small batches, in sheets distributed horizontally and vertically. The dominantly meta-igneous and shallower crustal sources of I-type magmas did not experience the higher temperatures required to form I-type volcanic magmas.