The P–T evolution of ultra high temperature garnet‐bearing ultramafic rocks from the Saxonian Granulitgebirge Core Complex, Bohemian Massif
The P–T evolution of ultra high temperature garnet‐bearing ultramafic rocks from the Saxonian Granulitgebirge Core Complex, Bohemian Massif
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DOI：
10.1111/j.1525-1314.2010.00876.x
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发表时间：
2010-06
影响因子：
3.4
通讯作者：
E. Schmädicke;J. Gose;T. Will
中科院分区：
文献类型：
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作者：
E. Schmädicke;J. Gose;T. Will
含石榴子石的超镁铁岩（GBUR）包裹在麻粒岩或高级片麻岩中，是阿尔卑斯型碰撞造山带的典型组成部分。欧洲Variscides的波希米亚地块是一个例外，存在大量的地幔衍生岩石，包括GBUR（石榴橄榄岩和石榴辉石岩）。GBUR发生在几个变质单位属于Saxothurigian和Moldanubian区的波希米亚地块。GBUR在波希米亚地块最北端的露头位于萨克森州的Granulitgebirge核心复杂的Saxothuringian区，是本研究的主题。温压测量结果和出溶结构表明，GBUR花岗岩属超高温橄榄岩类。它们经历了由以下四个阶段限制的减压冷却路径：（i）1300-1400 °C和32 kbar，（ii）1000-1050 °C和26 kbar，（iii）900-940 °C和22 kbar，以及（iv）860 °C和12-13 kbar。GBUR透镜体中偶尔出现的石榴石辉石岩层被解释为高压累积物，其在32-36 kbar下通过冷却至1400 °C以下而结晶。GBUR很可能来自上涌的软流圈，并与地壳麻粒岩接触，深度为160 km。板片断离最有可能的原因是：（i）软流圈上涌和冷却，以及（ii）地壳寄主岩石的超高温麻粒岩相变质作用。麻粒岭型橄榄岩与波希米亚地块南部摩尔达努比亚带Gföhl单元的Mohelno型橄榄岩非常相似。相比之下，邻近的厄尔士山脉（也在Saxothurigian带内）的橄榄岩来自次大陆地幔，与Gföhl单元（Moldanubian带）的NoveDvory型橄榄岩非常相似。Saxothuringian和Moldanubian带含有相同变质年龄的相同类型的地幔岩（软流圈和岩石圈），这一事实表明，Saxothuringian和Moldanubian带的经典区分不能得到支持，至少就GBUR的高级单元而言。
Garnet‐bearing ultramafic rocks (GBUR) enclosed in granulite or high‐grade gneiss are rare, yet typical constituents of alpine‐type collisional orogens. The Bohemian Massif of the European Variscides is exceptional for the occurrence of a large variety of mantle‐derived rocks, including GBUR (garnet peridotite and garnet pyroxenite). GBUR occur in several metamorphic units belonging to both the Saxothuringian and the Moldanubian zones of the Bohemian Massif. The northernmost outcrops of GBUR in the Bohemian Massif are situated in the Saxonian Granulitgebirge Core Complex in the Saxothuringian zone and are the subject of this study. Thermobarometric results and exsolution textures imply that the Granulitgebirge GBUR belong to the ultra high temperature group of peridotites. They experienced a decompression‐cooling path being constrained by the following four stages: (i) ∼1300–1400 °C and 32 kbar, (ii) 1000–1050 °C and 26 kbar, (iii) 900–940 °C and 22 kbar, and (iv) 860 °C and 12–13 kbar. Occasional layers of garnet pyroxenite within GBUR lenses are interpreted as high pressure cumulates that crystallized at 32–36 kbar by cooling below 1400 °C. The GBUR were most probably derived from upwelling asthenosphere and came in contact with crustal granulite at ∼60 km depth. Slab break‐off is suggested here as the most probable cause for: (i) asthenosphere upwelling and cooling of the latter as well as (ii) ultra high temperature granulite facies metamorphism of the crustal host rocks. The Granulitgebirge‐type peridotite is very similar to the Mohelno‐type peridotite from the Gföhl unit, Moldanubian zone, in the southern part of the Bohemian Massif. In contrast, peridotite from the adjacent Erzgebirge (also within the Saxothuringian zone) is derived from the subcontinental mantle and much resembles the Nove Dvory‐type peridotite from the Gföhl unit (Moldanubian zone). The fact that the Saxothuringian and Moldanubian zones host the same types of mantle rocks (asthenospheric and lithospheric) of the same metamorphic ages suggests that the classic distinction into the Saxothuringian and Moldanubian zones cannot be supported, at least as far as high‐grade units hosting GBUR are concerned.