ABSTRACT

The Meseta Goaïda magmatism, which occurs as gabbro, dolerite and rhyolitic intrusions in the Neoproterozoic basement and Cambrian platform sedimentary series of the Moroccan Hercynian orogen, has been ascribed to the Variscan orogenesis in the late Palaeozoic. Here, we provide new LA–ICP-MS U-Pb zircon ages – ranging from c. 519 Ma to c. 479 Ma – of this magmatism that reveal, for the first time in the Moroccan Central Meseta, the existence of early Cambrian to early Ordovician bimodal magmatism. Inherited zircons in these magmatic rocks yield dates that range from c. 1.2 to 1.0 Ga and c. 0.63 to 0.50 Ga. This inheritance suggests provenance from Precambrian crustal sources in the Cadomian‒Pan-African of the NW margin of the West African Craton, which is buried beneath the currently exposed Paleozoic Moroccan Meseta. The affinity of this early Palaeozoic magmatism is mainly tholeiite to transitional for the mafic rocks and high-K calc-alkaline for the silicic rocks. The Cambrian–Ordovician age and varying parental magma signature – as shown by trace element and Sr-Nd-Pb isotopic data from this magmatism and its Neoproterozoic granitoid basement rocks – indicate that the Moroccan Meseta early-Paleozoic magmatism might be associated with the weakening of the continental lithosphere during the early stages of rifting during the Cambrian–Ordovician along the NW Gondwana margin. This is supported by the presence of inherited zircons derived from reworking of the Precambrian crust in the early-Palaeozoic magmas.

摘要

Meseta Goaïda 岩浆作用作为辉长岩、辉绿岩和流纹岩侵入体出现在摩洛哥海西造山带的新元古代基底和寒武系台地沉积系列中，被认为是晚古生代的瓦里斯坎造山作用。在这里，我们提供了新的 LA-ICP-MS U-Pb 锆石年龄 - 范围从 c。519 马到 c。479 Ma – 这种岩浆活动首次在摩洛哥中部梅塞塔揭示了早寒武纪到早奥陶世双峰岩浆活动的存在。这些岩浆岩中的继承锆石产生的日期范围从 c. 1.2 至 1.0 Ga 和 c。0.63 至 0.50 Ga。这种遗传表明来自西非克拉通 NW 边缘的 Cadomian-Pan-African 的前寒武纪地壳来源，它被埋在目前暴露的古生代摩洛哥 Meseta 之下。这种早古生代岩浆作用的亲合力主要是拉斑岩向基性岩过渡，而硅质岩则是高钾钙碱性。寒武纪-奥陶纪年龄和不同的母岩浆特征——如来自该岩浆作用及其新元古代花岗岩基岩的微量元素和 Sr-Nd-Pb 同位素数据所示——表明摩洛哥 Meseta 早古生代岩浆作用可能与沿西北冈瓦纳边缘的寒武纪-奥陶纪裂谷早期阶段的大陆岩石圈。这是由早古生代岩浆中前寒武纪地壳再加工产生的继承锆石的存在所支持的。寒武纪-奥陶纪年龄和不同的母岩浆特征——如来自该岩浆作用及其新元古代花岗岩基岩的微量元素和 Sr-Nd-Pb 同位素数据所示——表明摩洛哥 Meseta 早古生代岩浆作用可能与沿西北冈瓦纳边缘的寒武纪-奥陶纪裂谷早期阶段的大陆岩石圈。这得到了从早古生代岩浆中前寒武纪地壳再加工产生的继承锆石的存在的支持。寒武纪-奥陶纪年龄和不同的母岩浆特征——如来自该岩浆作用及其新元古代花岗岩基岩的微量元素和 Sr-Nd-Pb 同位素数据所示——表明摩洛哥 Meseta 早古生代岩浆作用可能与沿西北冈瓦纳边缘的寒武纪-奥陶纪裂谷早期阶段的大陆岩石圈。这是由早古生代岩浆中前寒武纪地壳再加工产生的继承锆石的存在所支持的。