Inferring a deep-crustal source terrane from a high-level granitic pluton: the Strathbogie Batholith, Australia

Abstract

The Strathbogie Igneous Complex is comprised of the ignimbritic rocks of the Violet Town Volcanics and the granitic rocks of the Strathbogie batholith. It is Late Devonian in age and postorogenic-extensional in tectonic setting. The batholith was constructed from peraluminous, metasediment-derived magmas emplaced as several internally heterogeneous plutons. Chemical variation in the magmas was largely inherited from the protolith rather than having been produced by differentiation (crystal-liquid separation) or magma mixing. The Strathbogie magmas formed during a granulite-facies metamorphic event that caused partial melting of the rocks of the Proterozoic Selwyn Block, which forms the basement in this region. The chemistry of the Strathbogie batholith, the Violet Town Volcanics and various other felsic complexes of similar age, implies that the Selwyn Block here originally consisted of andesite, dacite, greywacke and pelite, probably deposited in a back-arc extensional setting.The sedimentary components of this protolith may have been deposited in a basin that was extending and deepening with time, so that the sediments contained progressively higher ratios of clay to volcanic materials. Much later, in the Late Devonian, extensional tectonics allowed the emplacement of mantle magmas into the deep and middle crust, causing the lowpressure granulite-facies metamorphic event that was responsible for the production of the crustal components in the granitic magmas of Central Victoria.

The Tynong pluton, its mafic synplutonic sheets and igneous microgranular enclaves: the nature of the mantle connection in I-type granitic magmas

“…Thus, the Selwyn Block is not only important as a factor relating to the sedimentation and subsequent deformation of the Melbourne Zone during the Tabberabberan, but it is highly likely to contain the protoliths of the crustal components of the granitic magmas (e.g. Clemens et al 2011a, b;Clemens and Birch 2012;Clemens and Bezuidenhout 2014;Clemens and Phillips 2014).…”

“…Recent contributions on the origins of granitic and silicic volcanic complexes in the Victorian sector of the Lachlan Orogen have mainly emphasised source heterogeneity and the role of PAE (e.g. Clemens et al 2011a, b;Clemens and Birch 2012;Clemens and Bezuidenhout 2014;Clemens and Phillips 2014). Most of this work has been on S-type rocks.…”

Post-orogenic shoshonitic magmas of the Yzerfontein pluton, South Africa: the ‘smoking gun’ of mantle melting and crustal growth during Cape granite genesis?

“…Other possible examples include some calc-alkaline to tholeiitic complexes in which larger volumes of mafic to intermediate magmas co-existed with the granitic magmas, commonly in mingling relationships (e.g., Pitcher 1997;Słaby and Martin 2008). However, as stated earlier, in many cases there is little geological sign of the necessarily large mafic heat source (e.g., the very voluminous Late Devonian felsic magmatic province of Central Victoria in Australia; Clemens and Phillips 2014;).…”

Origins and Scales of Compositional Variations in Crustally Derived Granitic Rocks: The Example of the Dartmoor Pluton in the Cornubian Batholith of Southwest Britain

“…Dartmoor granitic rocks. SiO2 contents vary from ~ 71 to 78 wt% and, as is typical in granitic suites, and particularly in S-types (e.g., Sweetman 1987;Clemens and Phillips 2014;Clemens et al 2017c), the major-oxide variations show significant scatter (fig. 3).…”