Arc Magmas Sourced from Mélange Diapirs in Subduction Zones Horst R
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Arc magmas sourced from mélange diapirs in subduction zones Horst R. Marschall a;b;∗ John C. Schumacher c aDepartment of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA bDepartment of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA cDepartment of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK Abstract At subduction zones, crustal material is recycled back into the mantle. A certain proportion, however, is returned to the overriding plate via magmatism. The magmas show a characteristic range of compositions that have been explained by three-component mixing in their source regions: hydrous fluids derived from subducted altered oceanic crust and components derived from the thin sedimentary veneer are added to the depleted peridotite in the mantle beneath the volcanoes. However, currently no uniformly accepted model exists for the physical mechanism that mixes the three components and transports them from the slab to the magma source. Here we present an integrated physico-chemical model of subduction zones that emerges from a review of the combined findings of petrology, modelling, geophysics, and geochemistry: Intensely mixed metamorphic rock formations, so-called mélanges, form along the slab-mantle interface and comprise the characteristic trace-element patterns of subduction-zone magmatic rocks. We consider mélange formation the physical mixing process that is responsible for the geochemical three-component pattern of the magmas. Blobs of low-density mélange material, so-called diapirs, rise buoyantly from the surface of the subducting slab and provide a means of transport for well-mixed materials into the mantle beneath the volcanoes, where they produce melt. Our model provides a consistent framework for the interpretation of geophysical, petrological and geochemical data of subduction zones. Geochemical studies of volcanic rocks erupted at con- “subduction factory” are unknown. vergent plate margins have identified element and iso- Current models propose that a hydrous component, topic ratios that seem to require material present in their enriched in incompatible trace elements, is released source that is absent in volcanic rocks from other set- from the subducting slab and migrates into the overly- tings, such as mid-ocean ridge basalts [1]. This subduc- ing mantle wedge. The wedge is at much higher tem- tion zone geochemical fingerprint has led to the estab- peratures than the slab due to the ‘corner flow’ of the lishment of three-component mixing models to explain asthenospheric mantle, driven by viscous coupling of the composition of magmatic rocks produced at conti- the mantle to the subducting slab [4]. Mantle-wedge nental and island arcs [2, 3]. Their trace-element charac- peridotite partial melting is thought to be promoted by teristics are believed to have originated from two distinct the influx of slab-derived fluids [5]. Yet, no uniformly sources added to the depleted mantle source: hydrous accepted model exists for the actual physical mecha- fluids derived from subducted altered ocean crust and nism that transports the components from the slab to the a component derived from the thin sedimentary veneer source of arc magmas in the hot corner of the mantle of the slab. However, the details of the physical process wedge. that allows the mixing of these components within the We present a physico-chemical model for subduc- tion zones based on field observations, numerical mod- els and analogue experiments. We suggest that three- ∗ Corresponding author. Tel: +1-508-289-2776. Fax: +1-508-457- component geochemical mixing takes place at the slab- 2183. E-mail: [email protected] mantle interface, that this mixed material is transported Email address: [email protected] (Horst R. Marschall). Preprint submitted to Nature Geoscience 11 October 2012 into the mantle wedge by diapirs and carried directly to The mineral assemblages of these newly formed rocks the source region for the volcanic arc magmas, provid- differ from assemblages of both the ultramafic mantle ing a source for the surface volcanism. and metamorphosed hydrated basalts and sediments of the slab. The importance of such hybrid rock types is twofold [10, 13, 14, 15]: (i) the new mineralogy affects Mélange has it all the redistribution of major and minor elements, creating mixed compositions composed of the three initial com- Extensive fieldwork in areas displaying exhumed ponents. (ii) the newly formed assemblages have signif- high-pressure metamorphic rocks has led to the estab- icant potential for storing and releasing H2O and other lishment of models on the subduction and exhumation volatiles. They are characterised by sets of devolatili- of oceanic crust [6, 7]. One type of mechanism in- sation reactions distinct from each of the initial bulk cludes a narrow channel overlying the subducting slab compositions. Hence, the juxtaposition of disparate rock in which slab-derived rocks are mixed with the mantle- types strongly impacts the trace-element and volatile derived ultramafic rocks and are transported from great fluxes of subduction systems [10, 13, 14, 15]. depths towards the surface [6, 7, 8, 9]. Computer-aided, high-resolution numerical modelling has provided new insight into the dynamics of these subduction channels Composition of mélange rocks and of the fore-arc mantle overlying the subducting slab [9]. These models not only provide an explana- Field studies have identified the dominant newly tion for fast exhumation of high-P rocks from great formed lithologies in a number of exhumed mélange depth [7, 9], but also provide a formation mechanism zones. One recurring rock type that is commonly pro- for high-P mélange zones as the products of mixing of duced where metabasites were juxtaposed with serpen- hydrated mantle rocks with fragments derived from the tinites is jadeitite, a rock that consists dominantly (or subducting slab. entirely) of jadeite [16, 17, 18]. Jadeitites are typically Fieldwork in high-P mélange zones yielded direct massive, isotropic rocks with high mechanical strength observations of various stages of processes that con- that have major-element composition high in Na, Al, trol mixing of the three dominant components (altered and Si, and low in Mg, Fe and Ca. They are resistant to oceanic crust, sediment, mantle wedge). These areas are deformation and stable over a wide range of P-T con- globally distributed [7], and many of these localities ditions. Phengite is a very common phase in jadeitites show centimetre to kilometre-sized blocks of eclogite, and an important host of large-ion lithophile elements metasediment and peridotite/serpentinite embedded in [16, 19] (LILE; e.g. K, Ba, Rb), elements that are char- a schistose matrix of mafic to ultramafic composition acteristically enriched in subduction-related volcanic [7, 8, 10, 11, 12]. It is important to note that not all local- rocks (Fig. 1). In addition, accessory minerals, such ities were interpreted to represent exhumed sections of as zircon, rutile and allanite, are abundant in jadeitites the slab-mantle interface or of the metasomatized man- [16, 17]. These minerals host large concentrations of tle wedge in fossil oceanic subduction zones. Neverthe- trace elements that are employed in geochemical stud- less, these localities can be taken as kilometre-scale nat- ies, such as rare earth elements (REE), Th, U, and ural laboratories that allow us to investigate the mech- high-field strength elements (HFSE; e.g. Zr, Hf, Nb, anisms and products of mixing of mafic, sedimentary Ta). The spatial extent of pure jadeitites is limited in and ultramafic components at a variety of relevant P-T most places and they commonly show a transition into conditions. Pressures recorded in these rocks typically similar, but more Ca-Mg-Fe-rich rock types that con- range from 1.0 to 2:5GPa [7], and field studies have re- tain Ca-amphibole, omphacite and chlorite in various vealed three different mechanisms of mixing that lead proportions [16, 17]. to the formation of hybrid rock types [12, 13]: (i) Me- The second key rock type that has been observed chanical mixing, (ii) advective mixing (fluid and melt to extend to thicknesses from tens to hundereds of metasomatism), and (iii) diffusion (see supplement for metres (or even kilometres) in a number of high-P discussion). mélange localities is chlorite schist [10, 13, 15]. This The juxtaposition of crustal rocks of the subducting rock type is almost monomineralic chlorite along with slab and ultramafic rocks of the mantle wedge with their minor talc and Ca-amphibole. Chlorite schist com- contrasting chemical compositions leads to the forma- monly forms where metabasic rocks are metasoma- tion of entirely new bulk compositions with no sedimen- tised and mechanically mixed with serpentinite. It is tary or magmatic equivalents on the surface [10, 13]. distinct from chlorite-bearing greenschist-facies meta- 2 (a) (b) Mélange average 100 100000 Aleutian Kermadec Central America Izu-Bonin 50000 6.2 Greater Antilles Sunda 20 10000 10 8000 Sample/PRIMA 6000 2 4000 Rb Th K Ta Ce Pr Nd Zr Ti Gd Yb Ba U Nb La Pb Sr Sm Hf Eu Y 2000 (c) mélange mélange rocks rocks 1000 n = 195 n = 224 Ba/Th 1500 arc volcanic 100 N-MORB rocks n = 1184 10 PRIMA Ce/Pb mélange 1000 average 1 AOC GLOSS GLOSS UCC UCC 0.1 arc volcanic 500 rocks (95 %) 0.01 arc volcanic N-MORB rocks (100 %) n = 924 10.6 PRIMA 0 0.001 0 1 2 3 4 5 6 7 0.01 0.1 1 10 100 (La/Sm)N Nb/La Fig. 1. Trace element compositions of mélange rocks in comparison to subduction zone volcanic rocks. (a) Plot of (La=Sm)N versus Ba=Th. (b) Primitive-mantle (PRIMA [47]) normalised multi-element plot arranged according to increasing element compatibility depicting the distinctive anomalies in Pb, Nb-Ta and Ti in both the average mélange composition and average compositions of volcanic rocks from different subduction zones.