Palaeogeography and crustal evolution of the Ossa–Morena Zone, southwest Iberia, and the North Gondwana margin during the Cambro-Ordovician: a review of isotopic evidence

Cambro-Ordovician palaeogeography and fragmentation of the North Gondwana margin is still not very well understood. Here we address this question using isotopic data to consider the crustal evolution and palaeogeographic position of the, North Gondwana, Iberian Massif Ossa–Morena Zone (OMZ). The OMZ preserves a complex tectonomagmatic history: late Neoproterozoic Cadomian orogenesis (ca. 650–550 Ma); Cambro-Ordovician rifting (ca. 540–450 Ma); and Variscan orogenesis (ca. 390–305 Ma). We place this evolution in the context of recent North Gondwana Cambro-Ordovician palaeogeographic reconstructions that suggest more easterly positions, adjacent to the Sahara Metacraton, for other Iberian Massif zones. To do this we compiled an extensive new database of published late Proterozoic–Palaeozoic Nd model ages and detrital and magmatic zircon age data for (i) the Iberian Massif and (ii) North Gondwana Anti-Atlas West African Craton, Tuareg Shield, and Sahara Metacraton. The Nd model ages of OMZ Cambro-Ordovician crustal-derived magmatism and Ediacaran-Ordovician sedimentary rocks range from ca. 1.9 to 1.6 Ga, with a mode ca. 1.7 Ga. They show the greatest affinity with the Tuareg Shield, with limited contribution of more juvenile material from the Anti-Atlas West African Craton. This association is supported by detrital zircons that have Archaean, Palaeoproterozic, and Neoproterozoic radiometric ages similar to the aforementioned Iberian Massif zones. However, an OMZ Mesoproterozoic gap, with no ca. 1.0 Ga cluster, is different from other zones but, once more, similar to the westerly Tuareg Shield distribution. This places the OMZ in a more easterly position than previously thought but still further west than other Iberian zones. It has been proposed that in the Cambro-Ordovician the North Gondwana margin rifted as the Rheic Ocean opened diachronously from west to east. Thus, the more extensive rift-related magmatism in the westerly OMZ than in other, more easterly, Iberian Massif zones fits our new proposed palaeogeographic reconstruction.

人们对寒武纪-奥陶纪（Cambro-Ordovician）古地理格局以及北冈瓦纳大陆边缘的裂解过程仍不甚明晰。针对这一科学问题，本文依托同位素数据，探究北冈瓦纳伊比利亚地块奥萨-莫雷纳带（Ossa–Morena Zone, OMZ）的地壳演化与古地理位置。奥萨-莫雷纳带保留了复杂的构造岩浆演化历史：新元古代晚期卡多造山运动（约650–550 Ma）、寒武纪-奥陶纪裂谷作用（约540–450 Ma）以及华力西造山运动（约390–305 Ma）。结合最新的北冈瓦纳寒武纪-奥陶纪古地理重建成果，已有研究提出伊比利亚地块其他次级构造带曾位于更靠东的位置，毗邻撒哈拉元克拉通（Sahara Metacraton）。为验证这一假说，本文构建了一套全新的大型数据集，整合了已发表的两类数据：一是伊比利亚地块的晚元古代-古生代Nd同位素模式年龄、碎屑锆石与岩浆锆石年龄数据；二是北冈瓦纳地区阿特拉斯西部非洲克拉通、图阿雷格陆盾（Tuareg Shield）以及撒哈拉元克拉通的相关数据。奥萨-莫雷纳带寒武纪-奥陶纪地壳源岩浆作用及埃迪卡拉纪-奥陶纪沉积岩的Nd同位素模式年龄介于约1.9 Ga至1.6 Ga之间，峰值年龄约为1.7 Ga。该特征与图阿雷格陆盾的亲缘性最高，仅接收少量来自阿特拉斯西部非洲克拉通的新生地壳物质贡献。这一亲缘关系得到了碎屑锆石的佐证：其太古代、古元古代与新元古代的测年结果与前述伊比利亚地块次级构造带高度相似。但奥萨-莫雷纳带存在中元古代年龄缺失现象，未出现约1.0 Ga的年龄峰值，这一点与其他构造带不同，但再次与更靠西的图阿雷格陆盾的年龄分布特征一致。上述结果表明，奥萨-莫雷纳带的古地理位置比此前认为的更靠东，但仍比其他伊比利亚地块次级构造带更偏西。已有研究提出，随着古Rheic大洋（Rheic Ocean）自西向东穿时张开，北冈瓦纳大陆边缘在寒武纪-奥陶纪发生了穿时裂谷作用。因此，奥萨-莫雷纳带相较于其他更靠东的伊比利亚地块次级构造带，发育了更为广泛的裂谷相关岩浆作用，这与本文提出的全新古地理重建模型相符。