Two types of peridotite in continental orogenic belts—a case study from the North Qaidam UHP metamorphic belt
Two types of peridotite in continental orogenic belts—a case study from the North Qaidam UHP metamorphic belt
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作者：
M. Key
本文简要回顾了大陆俯冲碰撞带地幔橄榄岩的特征和成因，重点讨论了柴北超高压变质带中地幔橄榄岩的成因。这条超高压带中有两种类型的橄榄岩。(1)石榴石橄榄岩由石榴石二辉橄榄岩、含石榴石纯橄榄岩、不含石榴石纯橄榄岩和石榴石辉石岩组成，是陆相俯冲带的重要岩石类型之一。锆石晶体中的钻石包裹体和石榴石和橄榄石中的减压出溶体等观察结果，以及地温气压计算，都令人信服地证明，这块石榴石橄榄岩肯定来自200公里以上的地幔深处。地球化学资料表明，石榴石橄榄岩的原岩为弧下地幔楔形环境中高镁熔体的堆积成因。(2)大洋岩石圈地幔方辉橄榄岩与变质堆积杂岩(包括石榴石辉石岩和蓝晶石榴辉岩)及与MORB有关的榴辉岩一起产出，代表大洋岩石圈的块体。这两类橄榄岩的确定有助于我们更好地了解柴达木北部超高压带的性质和构造过程。
This paper gives a brief review of the characteristics and petrogenesis of mantle peridotite in continental subduction and collision belts, and then discusses in more detail the petrogenesis of peridotites in the North Qaidam ultrahigh pressure (UHP) metamorphic belt. There are two types of peridotites within this UHP belt. (1) Garnet peridotite consists of garnet lherzolite, garnet-bearing dunite, garnet-free dunite and garnet pyroxenite, which could be considered as one of the critical rock types in a continental-type subduction zone. Observations such as diamond inclusion in a zircon crystal and decompression exsolutions in garnet and olivine, together with geothermobarometric calculations, argue persuasively that this garnet peridotite must have derived from mantle depths greater than 200 km. Geochemical data reveal that the protolith of the garnet peridotite was of cumulate origin from high-Mg melts in a sub-arc mantle wedge environment. (2) Oceanic lithospheric mantle harzburgite occurs together with meta-cumulate complex (including garnet pyroxenite and kyanite-eclogite) and MORB-related eclogite; these represent blocks of oceanic lithosphere. The determination of these two types of peridotites allows us to better understand the nature and tectonic processes of the North Qaidam UHP belt.