The Lyngen Gabbro: the lower crust of an Ordovician Incipient Arc
The Lyngen Gabbro: the lower crust of an Ordovician Incipient Arc
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林根辉长岩：奥陶纪初期弧的下地壳
DOI：
10.1007/s00410-004-0609-8
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发表时间：
2004
影响因子：
3.5
通讯作者：
R. Pedersen
中科院分区：
文献类型：
--
作者：
A. Kvassnes;A. Strand;Heidi Moen;R. Pedersen
我们提供了挪威北部特罗姆斯奥陶纪林根岩浆杂岩林根辉长岩起源的证据。林根辉长岩的两套岩浆岩走向平行于北北东-南南西向，并具有独特的岩浆特征。我们通过使用主量和微量元素分析以及选定的主量和微量元素矿物分析以及对来自这个大型辉长岩杂岩体的七个详细横截面的辉长岩至英英闪长岩深成岩的 143Nd/144Nd 同位素全岩分析来定义这些特征。林根辉长岩的西部套件是从岩浆中沉淀出来的，这些岩浆可能与相关的火山岩来自同一系统。相对于共存的镁铁质矿物，辉长岩的斜长石 An 含量较高（An>90）。再加上较高的 ɛNd(t) 值 (+6)，这意味着母岩浆是含水拉斑玄武岩，类似于今天弧后盆地中发现的岩浆。另一方面，东部套件由类似于弧前发现的超贫高钙胶质岩浆的熔体沉淀而成的堆积物组成。极高 An 斜长石 (An>95) 与演化的镁铁质矿物和氧化物共存，并且 ɛNd(t) 值比西方套件低 (+4)。东部套房没有火山对应物，但与套房相交的岩堤具有可能代表其母体岩浆的成分。海洋的雷普达伦剪切带通常将北部的两个岩层分开，但向南的几个从胶古岩到拉斑岩的非构造转变表明岩浆作用是同时发生的。岩浆岩层之间的接近性、含水岩浆作用以及不存在硅质或钙碱性成熟弧部分表明，林根辉长岩是在目前在后来作为外弧高位的初期弧中发现的条件下形成于巨卫洋中的。
We present evidence for the origin of the Lyngen Gabbro of the Ordovician Lyngen Magmatic Complex in Troms, Northern Norway. The two magmatic suites of the Lyngen Gabbro strike parallel NNE-SSW, and have distinct magmatic signatures. We define these signatures by using major and trace-element analyses together with selected major- and trace-element mineral analyses and 143Nd/144Nd-isotope whole-rock analyses of gabbroic to tonalitic plutonic rocks from seven detailed cross-sections from this large gabbro-complex. The Western suite of the Lyngen Gabbro precipitated from magma that may have been derived from the same system as the associated volcanic rocks. The gabbros have high An-content (An>90) of their plagioclases relative to co-existing mafic minerals. Together with somewhat high ɛNd(t) values (+6), this implies that the parental magmas were hydrous tholeiites similar to those found in back arc basins today. The Eastern suite, on the other hand, consist of cumulates that were precipitated from melts resembling those of ultra-depleted high-Ca boninitic magmas found in fore-arcs. Extremely high-An plagioclases (An>95) co-exist with evolved mafic minerals and oxides, and the ɛNd(t) values are lower (+4) than in the Western suite. The Eastern suite has no volcanic counterpart, but dikes intersecting the suites have compositions that possibly represent its parental magma. The oceanic Rypdalen Shear Zone generally separates the two suites in the north, but several non-tectonic transitions from boninitic to tholeiitic signatures southwards advocate that the magmatism happened concurrently. The magmatic proximity between the suites, the hydrous magmatism and the absence of a silicic or calc-alkaline mature arc section, suggests that the Lyngen Gabbro formed in the Iapetus Ocean under conditions presently found in incipient arcs later emplaced as outer arc highs.