Basic Study for a Catalogue of Upper Mantle Xenoliths in the Japan Island Arcs

日本弧下上地幔-下地壳的岩石学特征可以从包体中推断如下。(1)构成日本弧下上地幔-下地壳的岩石在堆积地幔厚度(第I类超镁铁质岩)、地幔橄榄岩的耐火度、年轻富铁、富铝岩石的聚积程度(第II类)和水化程度方面具有很强的不均一性。(2)深部岩石可选择性地并入寄主岩浆中。上升的岩浆可能倾向于选择性地穿过年轻的暗礁，而不是捕捉暗礁的围岩。(3)地幔橄榄岩难熔程度的不均一性可能受控于伴生累积超镁铁质岩，特别是英安岩的丰度。与大量纯橄榄岩包体共生的地幔橄榄岩包体通常为方辉橄榄岩或含高铬尖晶石的难熔二辉橄榄岩。地幔橄榄岩内部形成的英云闪长岩使其周围形成亏损的光环，增强了地幔橄榄岩的非地热成因。(4)日本弧的深部由地幔橄榄岩(方辉橄榄岩和/或二辉橄榄岩)、纯橄榄岩、橄榄辉长岩(变质对应的尖晶石辉长岩)和辉石辉长岩(镁铁质麻粒岩)组成。它们中的一部分后来被水化成角闪岩、角闪辉长岩和角闪辉长岩(或角闪岩)。

Petrological characteristics of the upper mantle-lower crust beneath the Japan arcs can be deduced from xenoliths as follows. (1) Rocks constituting the upper mantle-lower crust beneath the Japan arcs are very heterogeneous in terms of thickness of cumulus mantle (Group I ultramafic rocks), degree of refractoriness of mantle peridotites, degree of accretion of young Fe, Al-rich rocks (Group II), and degree of hydration. (2) Deep-seated rocks can be selectively incorporated into host magmas. Uprising magmas may tend to selectively pass through young dikelets and may not catch wall rocks of the dikelets. (3) The heterogeneity of degree of refractoriness of mantle peridotites may be controlled by abundance of associated cumulative ultramafic rocks, especially of dunites. Mantle peridotite xenoliths which are associated with large amounts of dunite xenoliths are usually harzburgite or refractory lherzolite with high-Cr spinel. Formation of dunites within the mantle peridotites makes depletion aureoles around the dunites, enhancing the heteorgeneity of the mantle peridotites. (4) The deep part of the Japan arcs is composed of, in an an ascending order, mantle peridotites (harzburgite and/or Iherzolite), dunite-websterites, olivine gabbros (oe thier metamorphosed equivalents, spinel websterites), and pyroxene-gabbros (mafic granulites). Part of them have been later hydrated to be hornblendebearing peridotites, harnblende websterites and hornblende gabbros (or amphibolites).