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- W2043951772 abstract "This chapter provides an overview of the current state of research on orogenic andesites. While their importance as proxies to the evolution of the continental crust has long been recognized, andesite genesis has remained highly controversial with a broader consensus yet to be reached. The controversy is fuelled by the question of whether orogenic andesites are primary melts of slab and mantle materials, or instead derivative products of basaltic mantle melts that differentiate in the overlying crust. These hypotheses are addressed in three sections of the book devoted to slab–mantle processes, the complexities of melt differentiation at crustal levels, and models pertaining to arc crustal growth. We believe that cross-fertilization and discussion among seemingly opposite and irreconcilable hypotheses will smooth the pathway towards a holistic communal model of andesite petrogenesis. Among the terrestrial planets, an andesitic continental crust is unique to Earth. Representing only c. 1 vol% of the silicate Earth, continents cover c. 40% of its surface and, by being permanently emerged above sea-level, constitute the foundation of human life and habitat. While the rate and mechanisms of continental growth have varied through time (Taylor & McLennan 1995; Keller & Schoene 2012), the discovery of Hadean detrital zircons demonstrates that a felsic continental crust must have existed as early as c. 4.4 Ga (Wilde et al. 2001; Bell et al. 2011) and, since then, it has constituted an enduring feature during planetary evolution (Harrison 2009). Yet, and despite its importance for habitability and as an archive of Earth’s history, the formation of andesitic continents has long intrigued geoscientists because their very existence presents a paradox. Andesitic rocks have high abundances of silica, aluminium, sodium and potassium, but are relatively depleted in iron, magnesium, calcium and titanium. Hence, unlike basalts, they cannot be direct partial melts of the Earth’s peridotite mantle. However, the andesitic crust is a major repository of incompatible elements in the silicate Earth and complements the depleted mantle, both of which were presumably separated from the Earth’s primitive mantle (Hofmann 1988; Rudnick 1995; Rudnick & Gao 2003). Yet how did an andesitic continental crust form? By which processes did these elements segregate to form a separate reservoir that is nearly as old as Earth, but fundamentally different from the crust produced by mantle melting? To date, many models of andesite crust formation have been proposed that range from sialic meteoritic infall during the early history of the Earth (Donn et al. 1965) to the gradual or episodic extraction of continental crust from the mantle through geological history (Harrison 2009; Cawood et al. 2013). Much of the early continental crust and its genetic information have been destroyed by tectonism and erosion over the course of geological time. On modern Earth, however, andesitic magmas with remarkable compositional similarities to the average continental crust are currently being produced at convergent margins (Gill 1981; Rudnick & Gao 2003). This observation gave rise to the ‘andesite model’ (Taylor 1967), which postulates continental crust formation by cycles of accretion of andesitic belts that had been created directly above orogenic areas. Over 30 years ago, the significance of orogenic andesites as an offspring of plate tectonics was recognized in the books Orogenic Andesites and Plate Tectonics (Gill 1981) and Andesites: Orogenic Andesites and Related Rocks (Thorpe 1982). Illustrating developments and state-of-the-art scientific thinking in the field of subduction-related igneous rocks, these publications became landmarks for professional geologists and scholars alike, as they provided an From: Gomez-Tuena, A., Straub, S. M. & Zellmer, G. F. (eds) 2014. Orogenic Andesites and Crustal Growth. Geological Society, London, Special Publications, 385, 1–13. First published online October 8, 2013, http://dx.doi.org/10.1144/SP385.16 # The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics elegant explanation that linked old-school petrology with the conceptual revolution of plate tectonics. Since then, much progress has been made in refining this approach, stimulated by novel analytical advances as well as by emerging geophysical tools, both assisted by an invigorated computational power and an increasing availability of global data banks. Most current studies on convergent margins have confirmed the causal link between plate subduction and andesite formation. The re-emergence of the short-lived cosmogenic isotope Be in arc volcanoes provided indisputable proof that subducted sediments are recycled at convergent margins (Tera et al. 1986; Morris et al. 1990). By now, numerous studies have also demonstrated that the budget of fluid-mobile large-ion lithophile elements of arc magmas mostly derive from the various components that constitute the subducted slab, such as oceanic sediments, altered oceanic crust (AOC) and possibly abraded crust from the upper plate (Plank & Langmuir 1993; Miller et al. 1994; Elliott et al. 1997; Kelemen et al. 2003; Plank 2005; Goss & Kay 2006; Tonarini et al. 2011). Subducted serpentinite from either within or beneath the subducted oceanic crust contributes water that plays a vital role in mobilizing the elements from the overlying oceanic crust (Schmidt & Poli 1998; Ranero et al. 2003; Straub & Layne 2003; Hacker 2008). Importantly, quantifications of slab contributions showed that the elemental flux from the voluminous AOC rivals and exceeds the concomitant flux from the highly enriched, but very thin, sediment layer. Because the ratios of radiogenic isotopes of the AOC are similar to those of subarc mantle, a strong AOC flux thus buffers the flux from the sediment and effectively conceals the magnitude of the total slab flux in Sr–Nd–Pb–Hf isotope space (Miller et al. 1994; Straub & Zellmer 2012). The strong link between slab input and arc output is also emphasized by U series studies that suggest recycling from slab to surface within less than a few 100 ka and possibly even within a few thousand years (e.g. Newman et al. 1984; Elliott et al. 1997; Turner et al. 1997; Sigmarsson et al. 1998). While there is little doubt about the efficient recycling of many trace elements, including climatically active volatiles, the origin of the distinct major element composition of arc magmas is less clear (Fig. 1). The major contention is whether andesites are primary melts of the subducted slab or the subarc mantle, or a mixture of both (Kelemen 1995; Straub et al. 2011); or alternatively, whether andesites derive from basaltic partial melts of peridotites (the basalt-input model) and are only created by secondary processing in the overlying crust (Hildreth & Moorbath 1988; Plank & Langmuir 1998; Annen et al. 2006; Reubi & Blundy 2009). Because major elements make up .99% of the melt mass, the question of slab and mantle v. a crustal origin is of key importance for the mode and rate of arc crustal growth. This is illustrated by means of the arc output equation (equation 1): elemental flux = element abundance × arc density× arc growth rate (1) where arc density is the density of the melts, the elemental abundance is the concentration of an element in the primary melt, and the arc growth rate is the mass of melt added per unit time (given in km per km of arc length per myr). Because the range of possible arc densities is limited (c. 2.3– 2.8 g cm), the elemental flux is principally a function of the arc growth rate and the element abundance, and both depend on the composition of the primary arc melts. For example, in a basalt-input model, the calculation of the mode and rate of arc crustal growth requires corrections to lower incompatible element abundances in the basaltic parental ec log i t e + amph ibo l i t e p e r ido t i t e + pyroxen i t e ( ? ) + amph ibo l i t e ( ? ) l i t ho log i e s con t i nen ta l c r u s t d i f f e ren t" @default.
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- W2043951772 date "2013-10-08" @default.
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- W2043951772 title "An introduction to orogenic andesites and crustal growth" @default.
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