Volcanic–plutonic parity and the differentiation of the continental crust

Abstract

The continental crust is central to the biological and geological history of Earth. However, crustal heterogeneity has prevented a thorough geochemical comparison of its primary igneous building blocks-volcanic and plutonic rocks-and the processes by which they differentiate to felsic compositions. Our analysis of a comprehensive global data set of volcanic and plutonic whole-rock geochemistry shows that differentiation trends from primitive basaltic to felsic compositions for volcanic versus plutonic samples are generally indistinguishable in subduction-zone settings, but are divergent in continental rifts. Offsets in major- and trace-element differentiation patterns in rift settings suggest higher water content in plutonic magmas and reduced eruptibility of hydrous silicate magmas relative to dry rift volcanics. In both tectonic settings, our results indicate that fractional crystallization, rather than crustal melting, is predominantly responsible for the production of intermediate and felsic magmas, emphasizing the role of mafic cumulates as a residue of crustal differentiation.

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

“…More debate has always surrounded the I-type granites, with some arguing for a direct and sometimes major mantle input to the magmas (e.g. DePaolo et al 1992;Douce 1999;Soesoo 2000;Sisson et al 2004;Gray and Kemp 2009;Nandedkar et al 2014;Keller et al 2015) and others pointing out that what could be interpreted as a relatively juvenile mantle isotope signature, or a mixed crust-mantle source, could be explained just as readily in terms of partial melting of crustal igneous rocks, themselves not long extracted from the mantle (e.g. Whalen et al 2002;Waight et al 2007;Clemens et al 2011a, b).…”

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“…The restite continued to sink and settle to the bottom of the felsic layer. This is similar to the fractional crystallization process proposed for forming modern felsic crust [32]. Much of the restite would re-enter the mantle, but some would solidify to form a thermal barrier around the felsic crust thereby allowing a keel to form beneath the Archean crust.…”

Compositional symmetry between Earth’s crustal building blocks

“…3). However, in this case, crystal fractional curves represent globally averaged melting conditions rather than the evolution of a single, co-genetic magma suite (Keller and Schoene, 2012;Keller et al, 2015). Compatible elements yield concave-down patterns, which for younger time slices are displaced to lower values at any given SiO 2 concentration (Fig.…”