Generation of alkaline magmas in subduction zones by partial melting of mélange diapirs—An experimental study

Arc Andesitic Rocks Derived From Partial Melts of Mélange Diapir in Subduction Zones: Evidence From Whole‐Rock Geochemistry and Sr‐Nd‐Mo Isotopes of the Paleogene Linzizong Volcanic Succession in Southern Tibet

“…Considering the subducted oceanic slab components are usually added to the subarc mantle in the forms of aqueous fluid, sediment components (bulk sediment or sediment‐derived melt; Class et al, ; Codillo et al, ; Cruz‐Uribe et al, ; Marschall & Schumacher, ; Nielsen & Marschall, ), we suggest that the subducted sediment components rather than aqueous fluid was likely incorporated into the magma source of the andesitic rocks, based on the following observations. First, the Dianzhong andesites have significantly higher Th/Nd ratios than oceanic basalts from the Neotethyan ophiolite suites (Liu et al, ; Miller et al, ; Xu et al, ; Zhang et al, ; Figure c), which argues against an input of aqueous fluids into the magma source.…”

“…In the traditional models, the mantle wedge was either metasomatized by aqueous fluids then to serve as a source for arc magmatic rocks (Class et al, ; Kessel et al, ) or by slab‐derived melts (including partial melts of oceanic crust and/or subducted sediment, or a mixture of them) that react with the overlying mantle wedge to generate arc magmas (Liu et al, ; Tsuchiya et al, ). Recently, a synthetic research on mélange rocks, including petrology, numerical modeling, geochemistry, geophysics, and experimental petrology, suggested that materials of subducting slab (including subducted sediments and mafic oceanic crust) and overlying mantle wedge peridotite can be transported into the magma source of arc magmatic rocks (Codillo et al, ; Cruz‐Uribe et al, ; Marschall & Schumacher, ; Nielsen & Marschall, ). First, the metamorphic rocks of mélanges were intensely mixed with mantle in the subduction channel (the interface between the subducted slab and the overlying mantle wedge), where mantle rocks are physically mixed with materials originated from the subducting slab.…”

“…Another is melts of subducted slab (including sediments) reacted with the overlying mantle wedge to generate andesitic magmas that are responsible for the production of new continental crust (Castro et al, , ; Liu et al, ; Tatsumi & Kogiso, ). However, based on recent relevant numerical thermome chanical modeling (Castro et al, ; Gerya & Yuen, ; Gerya et al, ), a new genetic mechanism has been proposed for the formation of andesites: partial melting of mélange diapir (Codillo et al, ; Cruz‐Uribe et al, ; Hao et al, ; Marschall & Schumacher, ; Nielsen & Marschall, ). The mélange, forming a subduction channel along the interface between the subducted slab and the overlying mantle, is considered as a mixture of depleted mantle rocks (mid‐ocean ridge basalt [MORB] and mantle wedge) and enriched sediments (Codillo et al, ; Cruz‐Uribe et al, ; Marschall & Schumacher, ; Nielsen & Marschall, ).…”

Generation of I-type granitic rocks by melting of heterogeneous lower crust: COMMENT

“…According to thermomecanical models of active margins (Gerya and Yuen, 2003;Gerya et al, 2004), sediment-basalt mélanges start to form at the subduction channel and evolve mechanically to Rayleigh-Taylor instabilities and then to silicic diapirs that intrude into the suprasubduction mantle, where they become into granitic magmas that are finally relaminated at the lower arc-crust or continental margin. Melting experiments of such subducted mélanges (Castro et al, 2010;Cruz-Uribe et al, 2018) confirm that large amount of melt, with compositions matching the geochemical and isotopic features of I-type batholiths, can be formed by this mechanism.…”

Melt Segregation and Depletion During Ascent of Buoyant Diapirs in Subduction Zones

“…Diapiric ascent from a sedimentary layer has been used to explain both the ubiquitous presence of a “sediment signature” (enrichment in fluid‐immobile elements such as Ba, Th, Be, Pb, and other light rare earth elements) in arc lavas (e.g., Elliott et al, ; Hawkesworth et al, ; Plank & Langmuir, ), as well as geochemical constraints that indicate this melting occurs dominantly in the mantle wedge as opposed to on the slab surface (Behn et al, ). However, while there is accumulating evidence that diapirs play an important role in subduction zone dynamics (Cruz‐Uribe et al, ; Liu et al, ; Marschall & Schumacher, ; Nielsen & Marschall, ), there have been few quantitative modeling studies that investigate both the solid advection of buoyant material, as well as the physical process of melting, melt segregation, and depletion in these environments. Such models are necessary to better understand the depletion of ultrahigh‐pressure rocks that record the residues of this melting process (Hacker et al, ) and the relationship between melt segregation and the observed “sediment signature” in arc lavas.…”