Oceanic core complexes (OCCs) represent tectonic windows into the oceanic lower crust and mantle; they are key structures in understanding the tectono‐magmatic processes shaping the oceanic lithosphere. We present a petrological and geochemical study of gabbros collected at the Mado Megamullion, a recently discovered OCC located in the extinct Shikoku back‐arc basin. Bathymetry of the Mado Megamullion reveals spreading‐parallel corrugations extending 25 km from the breakaway to the termination. Samples from several locations include peridotites, gabbros, dolerite, and rare pillow basalts. Gabbros range from granular to varitextured olivine gabbros and oxide gabbros. The emplacement of these gabbroic rocks within the oceanic lithosphere was followed by a multiphase tectono‐metamorphic evolution including (i) dynamic recrystallization within shear zones, developed under granulite‐ to upper‐amphibolite‐facies conditions, and (ii) intrusion of highly evolved melts forming felsic segregations. This tectono‐metamorphic evolution recalls that of the lower crust from other OCCs worldwide, demonstrating that this OCC exposes deep‐seated intrusions progressively exhumed by detachment faulting. Nonetheless, the Mado Megamullion lower crustal gabbros show an unusual crystal line of descent, different from what is reported from mid‐ocean ridge lower crustal rocks. We infer that the water‐bearing character of the primary melts in this back‐arc basin triggered the early precipitation of clinopyroxene, soon followed by amphibole and Fe‐Ti oxides. Such modifications in phase saturation are likely to be directly related to the back‐arc setting of the Mado Megamullion. If so, the phase assemblages of oceanic gabbros may be a diagnostic for the tectonic setting of lower crustal rocks in ophiolites.

中文翻译：

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缓慢扩散的弧后盆地的地壳增生：四国盆地Mado Megamullion海洋核心综合体的见解

大洋核心复合体（OCC）代表了进入大洋下地壳和地幔的构造窗口。它们是了解构造海洋岩石圈的构造－岩浆过程的关键结构。我们介绍了在Mado Megamullion收集的辉长岩的岩石学和地球化学研究，Mado Megamullion是一个最近发现的位于灭绝的四国后弧盆地的OCC。Mado Megamullion的测深图显示，平行波状波纹从分离处延伸至终点，延伸25公里。来自几个地点的样品包括橄榄岩，辉长岩，白云石和稀有枕形玄武岩。长颈鹿的范围从粒状到变质地的橄榄石长颈鹿和氧化物长颈鹿。这些辉长岩在海洋岩石圈中的沉积之后是多相构造变质演化，包括（i）剪切带内的动态再结晶，在粒状-上闪石相的条件下发育，以及（ii）高度演化的熔体侵入形成长相分离。这种构造变质的演化使人想起了全球其他OCC的地壳下部，表明该OCC暴露了深层侵扰，这些侵扰是由断层断层逐渐发掘出来的。尽管如此，Mado Megamullion的下地壳辉长岩显示出异常的下降线，这与海洋中脊下地壳岩的报道不同。我们推断，该后弧盆地初级熔体的含水特征触发了斜辉石的早期沉淀，不久之后出现了闪石和Fe-Ti氧化物。这种相位饱和度的修改很可能与Mado Megamullion的弧后设置直接相关。如果这样的话，海洋辉长岩的相组合可能可以诊断蛇绿岩中下地壳的构造环境。