Crustal thickness control on Sr/Y signatures of recent arc magmas: an Earth scale perspective

Abstract

Arc magmas originate in subduction zones as partial melts of the mantle, induced by aqueous fluids/melts liberated by the subducted slab. Subsequently, they rise through and evolve within the overriding plate crust. Aside from broadly similar features that distinguish them from magmas of other geodynamic settings (e.g., mid-ocean ridges, intraplate), arc magmas display variably high Sr/Y values. Elucidating the debated origin of high Sr/Y signatures in arc magmas, whether due to mantle-source, slab melting or intracrustal processes, is instrumental for models of crustal growth and ore genesis. Here, using a statistical treatment of >23000 whole rock geochemical data, I show that average Sr/Y values and degree of maturation (MgO depletion at peak Sr/Y values) of 19 out of 22 Pliocene-Quaternary arcs correlate positively with arc thickness. This suggests that crustal thickness exerts a first order control on the Sr/Y variability of arc magmas through the stabilization or destabilization of mineral phases that fractionate Sr (plagioclase) and Y (amphibole ± garnet). In fact, the stability of these mineral phases is function of the pressure at which magma evolves, which depends on crustal thickness. The data presented show also that high Sr/Y Pliocene-Quaternary intermediate-felsic arc rocks have a distinct origin from their Archean counterparts.

High-Mg adakitic rocks and their complementary cumulates formed by crystal fractionation of hydrous mafic magmas in a continental crustal magma chamber

“…11). Chiaradia (2015) showed that trends of median Sr/Y values versus MgO in modern arc magmas vary with arc thickness (Fig. 11b).…”

“…A subset of adakitic intermediate-felsic igneous rocks, both subduction-related and collision-related, have been interpreted as the products of crystal fractionation from basaltic magmas in the mid-to lower crust (e.g. Castillo et al, 1999;Chiaradia, 2015;Lee et al, 2007;Lu et al, 2015;Macpherson et al, 2006;Rodríguez et al, 2007). It is generally accepted that most primary magmas in subduction zones are basaltic or basaltic-andesite in composition and are H 2 O-rich (Grove et al, 2012).…”

“…Among these models, fractional crystallization of basaltic magmas, leaving behind a plagioclase-free cumulate assemblage in the deep crust, is recognized as an important mechanism that could be responsible for subduction-related and collision-related adakitic rocks (Lu et al, 2015;Richards and Kerrich, Lithos 260 (2016) [211][212][213][214][215][216][217][218][219][220][221][222][223][224] 2007). A recently statistical treatment (Chiaradia, 2015) of geochemical data on magmatic rocks from Pliocene-Quaternary arcs around the Earth also suggests that intra-crustal fractionation of mafic magmas is the main parameter controlling the development of adakitic signatures of igneous rocks. If so, there should be abundant complementary cumulates/residues of these adakitic rocks and these might play a significant role in the formation and differentiation of the continental crust (Lee, 2014;Lee et al, 2007;Müntener et al, 2001).…”

Initiation of the Wrangell Arc: A Record of Tectonic Changes in an Arc-Transform Junction Revealed by New Geochemistry and Geochronology of the ~29–18 Ma Sonya Creek Volcanic Field, Alaska

“…Melting of some mafic lower crustal garnet-bearing rock (i.e., eclogite or garnetamphibolite) can generate the adakite signature of elevated Sr/Y and La/Yb ratios (Petford and Atherton, 2003;Arculus et al, 1999;Haschke et al, 2002;Karlsi et al, 2011), and characterizing the difference between melting of in situ lower crust and melting of lower crust that has delaminated and sunk into the mantle can be accomplished by examining the MgO, Ni, and Cr contents of the resulting magmas (Xu et al, 2002;Wang et al, 2006;Karlsi et al, 2011 (Chiaradia, 2015). Adakites from China are shown to have been derived via melting of underplated basaltic magmas following delamination and sinking into the mantle (Xu et al, 2002;Wang et al, 2006).…”

Coexisting Early Cretaceous High-Mg Andesites and Adakitic Rocks in the North China Craton: the Role of Water in Intraplate Magmatism and Cratonic Destruction

“…This loose geochemical definition may not be appropriate; the term adakite must be restricted to true slab melt, and other rock types with adakitic signatures should be termed adakitic rocks (Castillo, 2006(Castillo, , 2012Ma et al, 2015). Various models have been proposed to account for the origin of intermediate-felsic igneous rocks with adakitic signatures (Martin et al, 2005;Castillo, 2006Castillo, , 2012Lee et al, 2007;Richards & Kerrich, 2007), with general emphasis on melting depth (Defant & Drummond, 1990;Gao et al, 2004), inheritance from source composition (Qian & Hermann, 2013;Ma et al, 2015) and magma generation processes (Castillo et al, 1999;Chen et al, 2013;Chiaradia, 2015).…”