Geochronological record of Cadomian exterior orogen reworking in bi-modal igneous protoliths of the Cabo Ortegal Allochthon (NW Iberia): the Cambrian onset of the Palaeozoic plate-tectonic cycle

New geochemical data and radiometric ages of the protoliths of variably reworked metamafic (granulites and gabbros) and felsic rocks (orthogneisses) from the Cabo Ortegal allochthonous Complex are presented. Their igneous precursors were emplaced in ensialic environments and exhibit geochemical signatures transitional to tholeiitic character, probably originating in arc settings (with continental and oceanic components and minor crustal recycling). The new geochronological results (encompassing the Miaolingian and Furongian epochs of the Cambrian) link the aforementioned scenarios to a Cadomian magmatic arc. The 505Ma protoliths of calc-alkaline orthogneisses and mafic granulites would be the oldest in the sequence of intrusion events. The latter and the 498–486Ma protoliths of gabbros (emplaced in sedimentary host sequences later transformed into Variscan high-pressure and intermediate-pressure gneisses) can be considered as snapshots of the ongoing continental arc evolution. The 498–486Ma gabbro intrusives of these gneisses, together with the 505–503Ma orthogneisses, are ascribable to palaeo-geographically separate scenarios before the Variscan amalgamation of their country rocks. They would expand the initiation of rifting of the Gondwana continental margin to the Miaolingian Cambrian epoch. It is inferred that the Cadomian continental-arc (active at least since 750Ma) experienced in the 510–460Ma interval the consequences of a vanishing active subduction. Certain domains flanking the arc underwent forearc sedimentation and deformation, whereas others were affected by an increasing extension (rifting) and/or back-arc basin formation. The latter marked the initiation of the beginning of a Wilson Cycle, which ended in the late Palaeozoicwith the formation of the Variscan interior orogenic belt.

本文报道了采自奥尔特加角推覆杂岩（Cabo Ortegal allochthonous Complex）的、经不同程度改造的变镁铁质岩（麻粒岩与辉长岩）及长英质岩（正片麻岩）原岩的全新地球化学数据与同位素年代学数据。其火成原岩形成于硅铝质环境，地球化学特征呈现向拉斑系列过渡的属性，大概率形成于弧构造背景（兼具陆壳与洋壳组分，并伴随轻微地壳再循环过程）。本次获得的新年代学结果涵盖寒武纪苗岭统与芙蓉统，将上述构造背景与卡多姆岩浆弧（Cadomian magmatic arc）建立了成因联系。505 Ma形成的钙碱性正片麻岩与镁铁质麻粒岩原岩，是该套侵入事件序列中时代最古老的成员。上述镁铁质麻粒岩原岩，以及498~486 Ma形成的辉长岩原岩（其侵位于沉积围岩序列中，后续围岩被改造为华力西期高压及中压片麻岩），可被视为大陆弧演化过程的阶段性记录。该套片麻岩中的498~486 Ma辉长岩体，与505~503 Ma的正片麻岩，在华力西期围岩拼合之前，应属于古地理上相互独立的构造背景。本研究将冈瓦纳大陆边缘裂谷作用的起始时间拓展至寒武纪苗岭统时期。研究推断，至少自750 Ma便开始活动的卡多姆大陆弧，在510~460 Ma期间经历了活动俯冲作用消亡的相关地质过程。弧体周边的部分区域发生了弧前沉积与变形作用，其余区域则经历了持续增强的伸展（裂谷作用）及/或弧后盆地形成过程。后者标志着威尔逊旋回（Wilson Cycle）的启动，该旋回于晚古生代以华力西内部造山带的形成而告终。