中国东部富钾埃达克岩成因的实验约束

Abstract

Adakites are geochemically distinct volcanic and plutonic granitoid rocks found in intraoceanic island arc settings where relatively young, hot oceanic lithosphere has been subducted and melted (termed "slab melting"), and in continental arcs, such as the Andes, where melting has taken place at the base of tectonically- or magmatically-thickened lower crust (termed "lower crustal melting"). In both settings, the distinctive geochemical signature of adakitic granitoids is attributed to an origin by partial melting of a variably-hydrated metabasaltic protolith at sufficient depths for garnet to be stable within the residual crystalline assemblage (z. e. , residues of garnet-amphibolite and/or eclogite). Once generated, "pristine" or "parental" adakite melts may have their composition subsequently modified by processes of assimilation (of either mantle or continental material) and crystal fractionation during transport to and emplacement in the middle-upper crust. Late Mesozoic (early-mid Cretaceous, ~-160—110 Ma) adakites in eastern China are unusually rich in potassium (K20) and other large-ion lithophile elements (e.g. , Ba, Th, U), with low Na20/ K20 ratios (~ l . 0 — 1. 1), in contrast to sodic adakites, found in eastern China and elsewhere, which resemble experimental adakite liquids produced by dehydration melting of basalt in the garnet-amphibolite to eclogite facies. and which formed by either slab melting of oceanic crust, or by partial melting of broadly basaltic, lower crustal protoliths. Despite these compositional differences, their overall geochemical character defines the potassic granitoids of eastern China as adakites. We attribute the unique chemistry of these potassium-rich adakites to either peculiarities in the composition of their source, or to the processes, including assimiliation and fractional crystallization (AFC), that subsequently modified parental adakite magmas. Although the apparent lack of proximity to a subduction zone suggests that adakites in eastern China formed by partial melting of underplated (magmatically-thickened) mafic lower crust, geodynamic scenarios involving "flat slab" subduction in eastern China during the Yanshanian period cannot be ruled out. Show more