摘要
Adakite在地球化学上具明显特征的火山岩和深成花岗岩类岩石 ,见于洋内岛孤环境和大陆孤 ,如安底斯孤。在洋内岛孤 ,由热的消减的年轻大洋岩石圈熔融形成 (叫做“板片熔融”) ,而在大陆孤 ,熔融曾发生在构造或岩浆加厚的下地壳底(叫做“下地壳熔融”)。在这两种产状环境中 ,adakite的鲜明地球化学特征被认为是起因于 ,一种不同程度含水的变质基性原岩在足够深度上的部分熔融 ;这里的足够深度是指可使石榴子石在残余结晶组合 (即石榴角闪石和 /或榴辉石的残余 )中保持稳定的深度。“原始”或“母”adakite熔体一旦形成 ,便可能在其向上运移和侵位到中上地壳期间受到同化作用 (或是地幔 ,或是大陆物质 )和结晶分异作用的改造。中国东部晚中生代 (早中白垩世 ,16 0 - 110 Ma)的 adakite,与见于同一地区和其它地方的钠质 adakite相比 ,通常富含钾 (K2 O)和其它大离子亲石元素 (如 Ba、Th、U) ,有较低的 Na2 O/K2 O比值 (～ 1.0 - 1.1) ,类似于玄武岩在石榴角闪岩 -榴辉岩相含水熔融实验中所产生的 adakite熔体 ,要么是由洋壳板片熔融所形成 ,要么是由不同成分的玄武质下地壳原岩部分熔融所形成。尽管有这些成分差异 ,它们的总体化学特征仍然可将中国东部的富钾花岗岩类岩石定为adakite。我们把这些富钾的
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 (i.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, similar to160-110 Ma) adakites in eastern China are unusually rich in potassium (K2O) and other large-ion lithophile elements (e.g., Ba, Th, U), with low Na2O/K2O ratios (similar to1.0-1.1), 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.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2002年第3期293-302,共10页
Acta Petrologica Sinica
基金
the grant of the National Science F oundation of the United States:Petrogenesis of Archean Granitoidsand Implications for the Geochemical Evolution of Cratonic Lithosphere( EAR-0 0 0 3 63 8)
