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MarcJDEFANT,$2. 2002. 埃达克岩:关于其成因的一些不同观点. 岩石学报, 18(2): 129-142
埃达克岩:关于其成因的一些不同观点
MarcJDEFANT  $2
[1]DepartmentofGeology,UniversityofSouthFlorida,Tampa,FL33620,USA [2]中国科学院广州地球化学研究所,广州510640,UniversityofSouthFlorida,Tampa,FL33620,USA
基金项目:This work was partially funded by the National Natural Science Foundation of China(grant No.40172028), and the Major State Basic Research Program of the Peoples Republic of China (No.G1999043202).
摘要:
      埃达克岩的概念是十多年前提出来的,指由俯冲的年轻洋壳熔融形成的火成岩。自从最初在现代岛弧近十几个地方报道埃达克岩以来,新近又在几个地方发现有埃达克岩(如日本西南部,外墨西哥火山岩带,等等)。但是,过去十 多年的研究也表明,埃达克岩可以由俯冲期间的其它过程产生(例如,沿俯冲板片的撕裂边,留在上地幔中的板片残余等)。另外,埃达克岩似乎与一些岩石呈共生组合,这些岩石包括高镁安山岩、富Nb的弦玄武岩(NEAB),还可能有玻安岩(几个研究者已在玻安岩中发现有埃达克岩的组分)。高镇安山岩不是来自埃达克与地幔的相互作用(Adak-type),就是来自此相互作用期间地幔的熔融(Piip-type);富Nb的弦玄武岩,据认为是来自一种被埃达我岩广泛交代的地幔的部分熔融。作为一个新的岩套,埃达克岩交代火山岩系列已被建议用来解释各种岩石组合。此外,大量的富Pb弧玄武岩也已被发现包含有超镁铁质的地幔包体,而这些包体有高亏损地幔与埃达克反应的明显证据。关于主要与下地壳熔而不是冲板片有关的埃达克岩的起源已提出几种假说,一个模型认为,下地壳熔融出现在玄武质岩浆底侵下地壳时。但是,有许多理由似乎可以排除这种模式。另一种模型认为,在大陆地壳很厚的区域,下地壳可能变成榴辉岩,从而拆离并下沉到地幔中(拆沉)。这个拆沉过程将导致下地壳下中拆沉的下地壳的上部与相对热的地幔接触,进而可引起下地壳熔融和埃达克岩的形成。这使我们认为,在中国东部发现的与俯冲作用无关的白垩纪埃达克可能是下地壳熔融与拆沉作用的产物。我们 还要强调,如果下地壳熔融与拆沉作用真能形成埃达克岩,那么埃达克岩这一术语不应该仅仅局限于与板片熔融有关的过程,而应包括那些与下地壳熔融有关的过程。太古宙的大陆地壳主要由奥长花岗岩、英云闪长岩和英安岩(TTD)组成。这种大陆地壳是来自板片熔融还是下地壳熔融仍是有争议的。然而,我们认为,太古宙期间地幔的较高温度会导致较多的洋中脊的形成,从而产生比今天“更多”的年轻洋壳的俯冲。据此,我们认为,太古宙TTD大陆地宙主要由板片熔融形成。我们也注意到,太古宙是广泛金矿化的时期。有些研究者还发现,金和铜的矿化与埃达克质交代火山岩系列有关。因此,该火山岩系列可能会寻找金属矿床的一个重要标志。
英文摘要:
      Adakites were proposed over a decade ago to be products of the melting of young subducted oceanic crust. In fact, several new localities have been discovered since the original work documented approximately ten localities in modern arcs (e.g. , southwestern Japan, Trans Mexican Volcanic Belt, etc. ). But work over the past ten years has also shown that adakites can be generated by other processes during subduction (e. g. > along the edge of tears in the subducting slab, remnant slabs left in the upper mantle, etc. ). In addition, adakites appear to be associated with a suite of rocks including high-Mg andesites resulting from either adakite interaction with the mantle (Adak-type) or melting of the mantle during adakite interaction (Piip-type), niobium enriched arc basalts (NEAB) that are believed to be derived from the partial melting of a mantle metasomatized extensively by adakites, and possibly boninites (several researchers have found an adakite component in boninites). A new rock suite, the adakite metasomatic volcanic series, has been proposed to account for the various associations. In addition, a large number of NEAB have been found to contain ultramafic mantle xenoliths with clear evidence of reaction between ultradepleted mantle and adakites. Several alternative hypotheses have been proposed for the generation of adakites primarily involving the melting of the lower crust rather than the subducting slab. One model proposes that the melting of the lower crust occurs when basaltic melts underplate the lower crust. There are many reasons that appear to rule this method out. The other model proposes that in areas where the continental crust is thick, the lower crust can become eclogitic and separate and sink into the mantle (delaminate). This delamination process will bring the lower sections of the lower crust or the upper part of the delaminated lower crust in contact with relatively hot mantle, which could initiate melting, and the production of adakites. This has led us to conclude that many of the Cretaceous adakites not associated with subduction found in East China are the result of lower crustal melting via delamination. We also emphasize that, if true, the term adakite should not be restricted to processes related only to slab melting but must include those involving the melting of the lower crust. The Archean continental crust consists of primarily trondhjemites, tonalities, and dacites (TTD). It remains problematic as to whether this continental crust was derived from slab melting or lower crustal melting. However, we believe that the higher mantle temperatures during the Archean led to more mid ocean ridges, which generated the subduction of "more" younger crust than today. Based on this, we suggest that the Archean TTD continental crust was generated primarily by slab melting. We also note that the Archean was a period of extensive gold mineralization. Several researchers have also found gold and copper mineralization associated with the adakite metasomatic volcanic series. This series may be an important indicator of ore deposits.
关键词:埃达克岩 板片熔融 钠质交代 下地壳熔融 拆沉作用 铜矿化 火山岩 金矿化
  修订日期:2002-01-10
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