Primitive Arc Magmatism and Delamination: Petrology and Geochemistry of Pyroxenites from the Cabo Ortegal Complex, Spain
Primitive Arc Magmatism and Delamination: Petrology and Geochemistry of Pyroxenites from the Cabo Ortegal Complex, Spain
复制标题
DOI：
10.1093/petrology/egw064
复制
发表时间：
2016-10
影响因子：
3.9
通讯作者：
R. Tilhac;G. Ceuleneer;W. Griffin;S. O’Reilly;N. Pearson;M. Benoit;H. Henry;J. Girardeau;M. Grégoire
中科院分区：
文献类型：
--
作者：
R. Tilhac;G. Ceuleneer;W. Griffin;S. O’Reilly;N. Pearson;M. Benoit;H. Henry;J. Girardeau;M. Grégoire
镁铁质岩石，如辉石岩，在上地幔化学不均匀性的发展和地球深部及其外部包层之间的元素再循环中起着重要作用。辉石岩是显着参与的原始弧岩浆的成因有重大影响的大陆形成在后太古代的分化，但罕见的深弧段暴露限制了我们的理解熔体迁移和熔体-岩石相互作用。在伊比利亚西北部的华力西缝合线中，卡波奥尔特加杂岩的Herbeira岩代表了一个独特的公里级的弧下地幔方辉橄榄岩，纯橄榄岩和铬铁矿，以及丰富的辉石岩。然而，负责其丰度和地球化学特征的过程并没有很好地建立，特别是由于一个特别复杂的构造历史。这项工作结合了新的领域和岩相学观察的主要特征，微量元素和放射性同位素（Sr，Nd，Hf和Os）组成的这些辉石。通过研究小尺度的不均匀性来限制次生过程的影响后，结果表明辉石的低Al含量、相容元素的高丰度和斜长石的缺乏与原始含水母体熔体有关。保存纯橄榄岩透镜体的橄榄枝玄岩和单斜辉石岩（1型和它们的叶状3型产物）记录了在相对低压（< 1.2 GPa）下苦橄质熔体侵入后，熔体/岩石比降低时橄榄岩的部分替换，可能在弧根内。大规模的websterite（2型）代表在较高的熔体/岩石比，可能作为静脉和岩墙结晶玻安岩熔体从最初的苦橄质熔体分化的反应的最终产物。这些富硅熔体和纯橄榄岩之间的相互作用产生了罕见的opx丰富的websterite（4型）。色谱再平衡伴随着后期岩浆结晶角闪石迁移或困残余熔体，产生一系列的稀土元素模式，从勺形强烈LREE富集，越来越unradiogenic Nd组合物。卡波奥氏辉石岩中特别高的CaO/Al 2 O3和LILE/HFSE的选择性富集表明，它们的富Ca母体熔体是通过耐火二辉橄榄岩在< 2 GPa下的低度第二阶段熔融，随后是板状流体和/或碳酸盐熔体的渗滤而产生的。放射性同位素系统学表明，一个老的交代地幔域的参与，这表明在弧岩浆作用的大陆边缘的改造，和回收的沉积物。他们的地质年代学表明，卡波奥尔特加辉石形成于500 Ma左右，这意味着以前报道的年龄约390 Ma可能对应于变质事件。高温变形后，辉石岩及其寄主橄榄岩记录了鞘褶和糜棱岩的发育过程。我们认为，这一幕对应于弧根的拆沉作用，它伴随着俯冲变质作用，峰值在1.6- 1.8GPa和780-800°C。折返的弧根侵入俯冲带后，伴随着水化作用，建议丰富的运动后角闪石，并通过添加S和动员Re，建议由Os-同位素系统学。因此，卡波奥尔特加尔复杂的可能保留一个独特的例子脱层弧根，轴承的原始弧岩浆在深度分化过程中的熔体橄榄岩相互作用的作用的证据。
Mafic lithologies such as pyroxenites play a major role in the development of chemical heterogeneity in the upper mantle and in the recycling of elements between the deep Earth and its external envelopes. Pyroxenites are notably involved in the differentiation of primitive arc magmas whose genesis has had a major impact on continent formation in post-Archean times, but the rarity of exposures of deep arc sections limits our understanding of melt migration and melt-rock interaction. In the Variscan suture of north-western Iberia, the Herbeira massif of the Cabo Ortegal Complex represents a unique km-scale section of sub-arc mantle harzburgites, dunites and chromitites, and abundant pyroxenites. However, the processes responsible for their abundance and geochemical characteristics are not well established, notably due to a particularly complex tectonothermal history. This work combines new field and petrographic observations with the characterization of major-, trace-element and radiogenic-isotope (Sr, Nd, Hf and Os) compositions of these pyroxenites. After constraining the effects of secondary processes by studying small-scale heterogeneities, it is shown that the low-Al content of pyroxenes, the high abundance of compatible elements and the absence of plagioclase are related to primitive hydrous parental melts. Olivine websterites and clinopyroxenites preserving dunite lenses (type 1, and their foliated type-3 products) record the partial replacement of peridotites at decreasing melt/rock ratios, following intrusion of picritic melts at relatively low pressure (< 1.2 GPa), potentially within an arc root. Massive websterites (type 2) represent the final products of this reaction at higher melt/rock ratios, potentially as veins and dykes crystallized from boninitic melts differentiated from the initial picritic melts. Interaction between these Si-rich melts and dunites produced rare opx-rich websterites (type 4). Chromatographic reequilibration accompanied the late-magmatic crystallization of amphibole from migrating or trapped residual melts, producing a range of REE patterns from spoon-shaped to strongly LREE-enriched, consistent with increasingly unradiogenic Nd compositions. Particularly high CaO/Al2O3 and the selective enrichment of LILE/HFSE in Cabo Ortegal pyroxenites suggest the generation of their Ca-rich parental melts via low-degree, second-stage melting of a refractory lherzolite at < 2 GPa, following percolation of slab-derived fluids and/or carbonatite melts. Radiogenic-isotope systematics indicate the involvement of an old metasomatized mantle domain, suggesting the reworking of a continental margin during this arc magmatism, and of recycled sediments. Their geochronology suggests that Cabo Ortegal pyroxenites formed at around 500 Ma, which implies that previously reported ages around 390 Ma probably correspond to a metamorphic event. Pyroxenites and their host peridotites then recorded the development of sheath folds and mylonites following high-temperature deformation. We suggest that this episode corresponds to the delamination of the arc root associated with prograde metamorphism peaking at 1.6-1.8 GPa and 780-800°C. Exhumation of the arc root after its intrusion into a subduction zone was accompanied by hydration, as suggested by abundant post-kinematic amphibole, and by addition of S and mobilization of Re, as suggested by Os-isotope systematics. The Cabo Ortegal Complex may thus preserve a unique example of delaminated arc root, bearing evidence of the role of melt-peridotite interaction during the differentiation of primitive arc magmas at depth.