3.18 OneView of the Geochemistry of Subduction-Related Magmatic Arcs, with an Emphasis on Primitive Andesite and Lower Crust P. B. Ke l e m e n Columbia University, Palisades, NY, USA K. HanghÖj Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY,USA and A. R. Greene University of British Columbia,Vancouver, BC, Canada 3.18.1 INTRODUCTION 2 3.18.1.1 Definition of Terms Used in This Chapter 2 3.18.2 ARC LAVA COMPILATION 3 3.18.3 CHARACTERISTICS OF ARC MAGMAS 7 3.18.3.1 Comparison with MORBs 7 3.18.3.1.1 Major elements 7 3.18.3.1.2 We are cautious about fractionation correction of major elements 9 3.18.3.1.3 Distinctive, primitive andesites 11 3.18.3.1.4 Major elements in calc-alkaline batholiths 11 3.18.3.2 Major and Trace-Element Characteristics of Primitive Arc Magmas 18 3.18.3.2.1 Primitive basalts predominate 18 3.18.3.2.2 Are some low Mgx basalts primary melts? Perhaps not 22 3.18.3.2.3 Boninites, briefly 22 3.18.3.2.4 Primitive andesites: a select group 23 3.18.3.2.5 Three recipes for primitive andesite 23 3.18.3.3 Trace Elements, Isotopes, and Source Components in Primitive Magmas 29 3.18.3.3.1 Incompatible trace-element enrichment 29 3.18.3.3.2 Tantalum and niobium depletion 36 3.18.3.3.3 U-series isotopes 37 3.18.3.3.4 Geodynamic considerations 40 3.18.4 ARC LOWER CRUST 43 3.18.4.1 Talkeetna Arc Section 43 3.18.4.1.1 Geochemical data from the Talkeetna section 45 3.18.4.1.2 Composition, fractionation, and primary melts in the Talkeetna section 49 3.18.4.2 Missing Primitive Cumulates: Due to Delamination 52 3.18.4.3 Garnet Diorites and Tonalites: Igneous Garnet in the Lower Crust 56 1 2 One View of the Geochemistry of Subduction-Related Magmatic Arcs 3.18.5 IMPLICATIONS FOR CONTINENTAL GENESIS 57 3.18.5.1 Role of Lower Crustal Delamination in Continental Genesis 57 3.18.5.2 Additional Processes are Required 58 3.18.5.2.1 Andesitic arc crust at some times and places 58 3.18.5.2.2 Arc thickening, intracrustal differentiation, and mixing 59 3.18.6 CONCLUSIONS 59 ACKNOWLEDGMENTS 59 REFERENCES 60 3.18.1 INTRODUCTION processes in the crust including mixing and as- similation. For a more complete view of these This chapter has four main aims.We wish to theories, we refer the reader to the many excel- provide a comprehensive picture of the com- lent review papers and individual studies that position of volcanic rocks from subduction- are, all too briefly, cited below. related magmatic arcs. There are several recent reviews of the geochemistry of arc basalts. This chapter differs in including andesites as well as basalts, in focusing on major elements, as well 3.18.1.1 Definition of Terms Used in This as trace elements and isotopes, and in using Chapter elemental abundance in ‘‘primitive’’ lavas, rather than trace-element ratios, to investigate The following terms are used extensively enrichments of incompatible elements in arc throughout this chapter, and/or in the recent magmas relative to primitive mid-ocean ridge arc literature. We have tried to define each term basalts (MORBs). where it first arises in the text. However, we realize that not all readers will read every sec- 1. We review evidence in favor of the existence tion. Thus, we include the following brief def- of andesitic, as well as basaltic primary initions here. magmas in arcs. While we have recently Accumulated minerals. These are crystals in reviewed evidence for this in the Aleutian lavas that are in excess of the amounts that arc, in this chapter we broaden our data set could crystallize from a melt in a closed system. to arcs worldwide, and concentrate on Similar to cumulates, some cases of accumu- whether mixing of lower crustal melts with lated crystals may be impossible to detect, but primitive basalts offers a viable alternative others stand out because they result in rock to the hypothesis that there are ‘‘primary’’ compositions different from all or most terres- andesites, i.e., andesites in Fe/Mg equilib- trial melts. rium with mantle olivine, passing from the Adakite. This term is justly popular, but un- mantle into the crust beneath arcs. fortunately it means many different things to 2. We present new data on the composition of different people, so we try not to use it. It is arc lower crust, based mainly on our ongoing generally used for andesites and dacites with ex- work on the Talkeetna arc section in south- treme light rare-earth element (REE) enrichment central Alaska. To our knowledge, this is the (e.g., La/Yb49), very high Sr/Y ratios (e.g., first complete ICP-MS data set on an arc Sr/Y450), and low yttrium and heavy REE crustal section extending from the residual concentrations (e.g., Yo20 ppm, Ybo2ppm). mantle to the top of the volcanic section. Andesite. For the purposes of this chapter, 3. We summarize evidence from arc lower andesites are simply lavas (or bulk composi- crustal sections that a substantial propor- tions) with 454 wt.% SiO2. For brevity, we tion of the dense, lower crustal pyroxenites have not subdivided relatively SiO2-rich mag- and garnet granulites produced by crystal mas into dacite, rhyodacite, etc. fractionation are missing. These lithologies Basalt. In this chapter, basalt means lavas or may have been removed by diapirs descen- bulk compositions with o54 wt.% SiO2. Note ding into less dense upper mantle. that we have eliminated alkaline lavas (nephe- line or kalsilite normative) from the data com- pilation. In a few places, we use terms picrite To achieve these aims and limit the length of and komatiite to refer to basaltic melts with the chapter, we have not provided a detailed more than 15 and 18 wt.% MgO, respectively. review of theories regarding the origin of prim- Boninite. By boninites, we mean andesites itive arc basalts, the mixing of magmatic com- with TiO2o0.5 wt.%, plus lavas identified as ponents derived from the upper mantle, aqueous boninites in the original papers presenting geo- fluids, and sediment melts, or open-system chemical data (includes some basalts). Arc Lava Compilation 3 Calc-alkaline. These are magmas having Primitive. Lavas, melts, and liquids with both high Na þ K at high Mgx (Irvine and Mgx460. Primitive cumulates have Mgx485. Baragar, 1971) and high SiO2 at high Mgx (i.e., REE groups. Light—lanthanum to samarium; high SiO2 at low Fe/Mg; Miyashiro, 1974); middle—europium to dysprosium; heavy— ‘‘tholeiitic magmas’’ have lower SiO2 and holmium to lutetium. Na þ K at the same Mgx, when compared with Subduction-related magmatic arc (or simply, calc-alkaline magmas. arc). Chains of volcanoes on the overthrust Compatible element. An element with equi- plate parallel to, and B100–200 km horizon- librium solid/melt partition coefficient 41. In tally away from, the surface expression of a garnet (and zircon), heavy REE, scandium, subduction zone, together with coeval, under- vanadium, and yttrium are compatible. These lying plutonic rocks. elements are incompatible in all other rock-for- ming minerals involved in igneous fractionation processes discussed in this chapter. 3.18.2 ARC LAVA COMPILATION Cumulate. A rock formed by partial crystal- lization of a melt, after which the remaining Data in this and subsequent sections are from melt is removed. While some cumulates may be the Georoc database (2006) (arcs worldwide), difficult to recognize, others are evident be- our Aleutian arc compilation, including all data cause the cumulate mineral assemblage has available from the database compiled by James major and/or trace-element contents distinct Myers and Travis McElfrish and available at from all or most terrestrial melts, easily under- the Aleutian arc website (2006), supplemented stood as the result of crystal/melt partitioning. by additional data cited in Kelemen et al. Eclogite facies. This term represents (2003b), a new Costa Rica and Panama com- high-pressure and relatively low-temperature, pilation (Abratis and Worner, 2001; Carr et al., metamorphic parageneses with omphacitic 1990; Cigolini et al., 1992; de Boer et al., 1991, clinopyroxene and pyrope-rich garnet. 1988, 1995; Defant et al., 1991a, b, 1992; Drum- EPR. East Pacific Rise. mond et al., 1995; Hauff et al., 2000; Herrstrom Evolved. Lavas, melts, and liquids with et al., 1995; Patino et al., 2000; Reagan and Mgxo50. Gill, 1989; Tomascak et al., 2000), and Central JDF. Juan de Fuca Ridge. American data compiled by Mike Carr and High Mgx. Lavas, melts, liquids with Mgx available at http://www.rci.rutgers.edu/carr/in- from 50 to 60. dex. We also included a very complete data set Incompatible element. Equilibrium solid/melt on lavas from Mt. Shasta in the southern Cas- partition coefficient o1. cades (Baker et al., 1994; Grove et al., 2001). LILEs. Large ion lithophile elements, rubi- We compare compositions of arc lavas to data dium, radium, barium, potassium (and ce- on MORB glasses downloaded from the PetDB sium—but we do not use cesium data in this website (2006). chapter). Although they are not, strictly speak- Lava data come from intra-oceanic arcs ing, LILE, we sometimes group thorium and (Tonga, n ¼ 704; Kermadec, 189; Bismark/ uranium with the LILE when referring to ele- New Britain, 165; New Hebrides, 252; Mar- ments that are highly incompatible. ianas, 834; Izu-Bonin, 878; oceanic Aleutians, Mantle wedge. ‘‘Triangular’’ region under- 1082; South Sandwich, 328; Lesser Antilles, lying arc crust, overlying a subduction zone, 356) and arcs that are, or may be, emplaced extending to perhaps 400 km depth.