Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire, French Massif Central)
Deep crustal source of gneiss dome revealed by eclogite in migmatite (Montagne Noire, French Massif Central)
复制标题
DOI：
10.1111/jmg.12523
复制
发表时间：
2020-02
影响因子：
3.4
通讯作者：
D. Whitney;Clémentine Hamelin;C. Teyssier;Natalie H. Raia;M. Korchinski;N. Seaton;B. Bagley;A. Handt;F. Roger;P. Rey
中科院分区：
文献类型：
--
作者：
D. Whitney;Clémentine Hamelin;C. Teyssier;Natalie H. Raia;M. Korchinski;N. Seaton;B. Bagley;A. Handt;F. Roger;P. Rey
在世界范围内的造山带和整个地质时代，大量的深陆壳在穹状构造中被挖出。拉张驱动的深部热地壳体的上升是从深部到浅部地壳水平的快速热量和质量转移的一种非常有效的机制，因此是大陆演化的一种重要机制。在被挖出的穹丘中，主要的岩石类型是石英长石质片麻岩（典型的混合岩），它在平衡矿物组合中不记录其以前的高压（HP）条件;相反，它记录了中/浅地壳的侵位和冷却条件。然而，包含在片麻岩中的镁铁质岩石可能包含HP历史的片段记录，并且是它们的寄主岩石也是深源的证据。一个保留部分高压记录的深地壳的很好的例子是在法国中央地块的黑色山穹丘，它在两个位置的混合岩中含有保存良好的榴辉岩（石榴石+绿辉石+金红石+石英）：一个在穹丘核心，另一个在穹丘边缘。这两种榴辉岩在T~700 ± 20 ° C时记录的P~1.5 ± 0.2 GPa，但在全岩和矿物成分、变形特征（形状和晶体学择优取向，CPO）、石榴石和锆石中的变质程度记录以及继承锆石的保存程度方面彼此不同。两种榴辉岩中锆石的边缘年龄与寄主片麻岩的最古老结晶年龄在c。根据榴辉岩锆石中锆石稀土元素丰度，推测高压变质作用的年龄为310Ma。穹缘榴辉岩锆石保留了广泛的原岩年龄记录（c。470 - 450 Ma，与寄主片麻岩原岩年龄相同），而穹窿核榴辉岩锆石具有更少的继承锆石保存。两种榴辉岩的差异可能与原岩基性岩浆成分和历史和/或变质加热的持续时间以及与含水流体相互作用的程度的差异有关，从而影响锆石结晶。高压变形组构的差异可能与榴辉岩相岩石在穹隆发育早期相对于压扭带的位置有关。无论差异如何，这两种榴辉岩在c. 310 Ma，并在华力西造山带末期附近被岩石圈伸展带及其寄主混合岩挖出。这一地区的深部地壳从约50公里到<10公里被迅速挖出，在低P/高T条件下平衡，留下了稀疏但令人信服的记录，表明现在暴露在圆顶中的大部分地壳的深部起源。
In orogens worldwide and throughout geologic time, large volumes of deep continental crust have been exhumed in domal structures. Extension‐driven ascent of bodies of deep, hot crust is a very efficient mechanism for rapid heat and mass transfer from deep to shallow crustal levels and is therefore an important mechanism in the evolution of continents. The dominant rock type in exhumed domes is quartzofeldspathic gneiss (typically migmatitic) that does not record its former high‐pressure (HP) conditions in its equilibrium mineral assemblage; rather, it records the conditions of emplacement and cooling in the mid/shallow crust. Mafic rocks included in gneiss may, however, contain a fragmentary record of a HP history, and are evidence that their host rocks were also deeply sourced. An excellent example of exhumed deep crust that retains a partial HP record is in the Montagne Noire dome, French Massif Central, which contains well‐preserved eclogite (garnet+omphacite+rutile+quartz) in migmatite in two locations: one in the dome core and the other at the dome margin. Both eclogites record P ~ 1.5 ± 0.2 GPa at T ~ 700 ± 20°C, but differ from each other in whole‐rock and mineral composition, deformation features (shape and crystallographic preferred orientation, CPO), extent of record of prograde metamorphism in garnet and zircon, and degree of preservation of inherited zircon. Rim ages of zircon in both eclogites overlap with the oldest crystallization ages of host gneiss at c. 310 Ma, interpreted based on zircon rare earth element abundance in eclogite zircon as the age of HP metamorphism. Dome‐margin eclogite zircon retains a widespread record of protolith age (c. 470–450 Ma, the same as host gneiss protolith age), whereas dome‐core eclogite zircon has more scarce preservation of inherited zircon. Possible explanations for differences in the two eclogites relate to differences in the protolith mafic magma composition and history and/or the duration of metamorphic heating and extent of interaction with aqueous fluid, affecting zircon crystallization. Differences in HP deformation fabrics may relate to the position of the eclogite facies rocks relative to zones of transpression and transtension at an early stage of dome development. Regardless of differences, both eclogites experienced HP metamorphism and deformation in the deep crust at c. 310 Ma and were exhumed by lithospheric extension—with their host migmatite—near the end of the Variscan orogeny. The deep crust in this region was rapidly exhumed from ~50 to <10 km, where it equilibrated under low‐P/high‐T conditions, leaving a sparse but compelling record of the deep origin of most of the crust now exposed in the dome.