A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic–ultramafic layered xenoliths from Beaunit (French Massif Central)
A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic–ultramafic layered xenoliths from Beaunit (French Massif Central)
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DOI：
10.1016/s0009-2541(03)00124-4
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发表时间：
2003-09
期刊：
影响因子：
3.9
通讯作者：
O. Féménias;N. Coussaert;B. Bingen;M. Whitehouse;J. C. Mercier;D. Demaiffe
中科院分区：
文献类型：
--
作者：
O. Féménias;N. Coussaert;B. Bingen;M. Whitehouse;J. C. Mercier;D. Demaiffe
法国中央地块的Puy Beaunit火山口显示出大量的深成基性至超基性捕虏体，通常呈不对称、毫米至厘米厚的分层结构。各层为辉石岩至辉长岩，较少见橄榄岩（二辉橄榄岩、纯橄榄岩、二辉橄榄岩）和斜长岩。这些捕虏体被解释为位于壳幔边界的层状侵入体样品。主要的堆晶相是橄榄石和斜方辉石，其次是单斜辉石和斜长石;罕见的堆晶间辅助相（磷灰石，金红石和锆石）观察到在最分化的层。均质捕虏体，解释为单一堆积层，具有钙碱性地球化学与轻稀土和亲石元素（LILE）相对富集Nb，Ta和Ti。辉石岩的负Eu异常可能与早期的斜长石分馏有关，如在辉长岩层中所观察到的。微量元素激光烧蚀电感耦合等离子体发射质谱（LA-ICP-MS）和二次离子质谱（西姆斯）分析斜长石，斜方辉石和锆石从层状岩石之间的平衡和共生关系的硅酸盐相。对1.5mm的锆石晶体进行U-Pb西姆斯测年，得到岩浆或亚固相线平衡年龄为257±6 Ma。Beaunit层状侵入体属于二叠纪大型板内岩浆幕，通常在欧洲和北非观察到钙碱性地球化学特征。它可能对应于一个基性岩底侵事件空间控制后华力西trans-tensional trans-pressive盆地构造在陆内设置。俯冲相关的地球化学特征的岩浆套件被解释为造成被动再活化的地幔源，这是以前交代在华力西俯冲。
The Puy Beaunit volcano vent, French Massif Central, displays a population of plutonic mafic to ultramafic xenoliths, commonly showing asymmetric, millimetre to centimetre thick, layering. Layers are pyroxenitic to gabbroic, and less commonly peridotitic (lherzolite, dunite, websterite) and anorthositic. These xenoliths are interpreted as samples of a layered intrusion, located at the crust–mantle boundary. Primary cumulate phases are olivine and orthopyroxene, followed by clinopyroxene and plagioclase; rare intercumulus accessory phases (apatite, rutile and zircon) are observed in the most differentiated layers. Homogeneous xenoliths, interpreted as single cumulate layers, have a calc-alkaline geochemistry with LREE and large ion lithophile elements (LILE) enrichments relative to Nb, Ta and Ti. The negative Eu anomaly of pyroxenite can be related to earlier plagioclase fractionation, as observed in the gabbroic layers. Trace element laser ablation inductively coupled plasma emission mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) analyses of plagioclase, orthopyroxene and zircon from layered rocks suggest equilibrium and cogenetic relations between the silicate phases. U–Pb SIMS dating of a 1.5 mm zircon crystal gives a magmatic or sub-solidus equilibration age of 257±6 Ma. The Beaunit layered intrusion belongs to the large Permian within-plate magmatic episode commonly of calc-alkaline geochemical signature observed over Europe and North Africa. It probably corresponds to a mafic underplating event spatially controlled by post-Variscan trans-tensional to trans-pressional basin tectonics in an intracontinental setting. The subduction-related geochemical signature of the magmatic suite is interpreted as resulting from the passive remobilisation of a mantle source, which was previously metasomatised during the Variscan subduction.