辉石岩:高压结晶还是再循环洋壳?
Pyroxenites:High-Pressure Segregates or Recycled Oceanic Crust?
查看参考文献132篇
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文摘
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辉石岩有三种不同的成因:(1)堆晶成因(Ⅰ类辉石岩);(2)再循环洋壳变质成因(Ⅱ类辉石岩);(3)交代成因。I类辉石岩由橄榄岩部分熔融产生的熔体在岩浆通道内上升过程中,在1.5~2.5GPa压力范围内结晶形成。常具有堆晶结构或火成结构,在CaO/MgO-SiO_2/MgO图解中无明显的线性关系,无Eu异常,其Sr-Nd-O同位素组成与幔源岩浆相似。II类辉石岩多数为再循环洋壳的变质产物,常具变晶结构,在CaO/MgO-SiO_2/MgO图解中形成明显的线性关系,具Eu正异常,其ε_(Nd)值与MORB相似,而Sr同位素比值变化范围大;其O同位素组成与原岩有关,如原岩是洋壳下部,δ~(18)O<地幔值;如原岩是洋壳上部,则δ~(18)O>地幔值。交代成因辉石岩是熔体-橄榄岩相互反应的结果,常被方辉橄榄岩或纯橄岩包围,矿物种类相对其他两类辉石岩单一,在CaO/MgO-SiO_2/MgO图解中较分散,其ε_(Nd)值较II类辉石岩低,而Sr同位素比值变化较小,δ~(18)O值低于、高于、近似于地幔值都存在。再循环洋壳在俯冲到地幔深部和随超基性岩体上升的过程中由流体萃取作用和部分熔融作用形成化学成分丰富的流体和熔体,这些熔/流体或交代围岩橄榄岩将其转化为辉石岩,或直接高压结晶形成辉石岩,或者由洋壳变质形成的榴辉岩经退变质形成Ⅱ类辉石岩。上述过程导致了在同一超基性岩体中各类成分、成因不同辉石岩共存的现象。 |
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其他语种文摘
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There are three petrogenetically different pyroxenites,namely cumulate pyroxenite,metamorphosed oceanic crust,and metasomatic pyroxenite.Type I pyroxenites formed as crystallization segregates from mantle-derived melts under pressure of 1.5~2.5 GPa.They commonly show cumulated or igneous textures and do not form perfect linear correlations in a CaO/MgO-SiO_2 /MgO plot.Type I pyroxenits show no Eu anomaly and their Sr-Nd-O isotopes are similar to those of mantle rocks.Type II pyroxenites are metamorphic products of recycled oceanic crust.They commonly show a deformed texture,forming a linear correlation in a CaO/MgO-SiO_2 /MgO plot and show a positive Eu anomaly.While theirε_(Nd) values are identical to those of the MORB,their Sr-O isotopes show distinct features.Metasomatic pyroxenites result from interactions between melts and peridotite.They are commonly surrounded by harzburgite or dunite.Minerals in metasomatic pyroxenites are less variable than other two types of pyroxenites.Dispersed in the CaO/MgO-SiO_2 /MgO plot,theirε_(Nd) values are lower than those of the Type II pyroxenites,and their Sr isotopic ratios show a restricted range.The δ~(18)O values of metasomatic pyroxenites are variable,either lower than,higher than,or similar to the mantle value.To explain the co-existence of pyroxenites of different origins within a single ultramafic massif,the following model is conceived:the oceanic upper crust,including basalts and sediments,produces considerable elementally variable fluids and magmas,which react with surrounding peridotites and convert it to pyroxenites and also form type I pyroxenites by high-pressure crystal segregation along magma conduits in the peridotites.The oceanic lower crust consisting of troctolite and gabbroic rocks retained their original compositions and translated to type Ⅱ pyroxenites without partial melting or other significant metasomatic reaction during subduction and exhumation at subsolidus conditions. |
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来源
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高校地质学报
,2012,18(1):74-87 【核心库】
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关键词
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辉石岩
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高压结晶
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循环洋壳
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交代作用
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鉴别标
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地址
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中国科学院广州地球化学研究所, 同位素地球化学国家重点实验室, 广州, 510640
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语种
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中文 |
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文献类型
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研究性论文 |
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ISSN
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1006-7493 |
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学科
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地质学 |
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基金
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国家自然科学基金项目
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文献收藏号
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CSCD:4502596
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