Detrital zircon and tectonostratigraphy of the Parautochthon under the Morais Complex (NE Portugal): implications for the Variscan accretionary history of the Iberian Massif

Zircon geochronology using laser ablation inductively coupled plasma mass spectrometry, combined with a new detailed geological map and a stratigraphic study of the Parautochthon at the eastern rim of the Morais Allochthonous Complex, has provided new insights into the evolution of the Iberian Variscan belt. The data confirmed that the Parautochthon is composed of two structural units. The higher and more deformed, called the Upper Parautochthon, consists of recumbently folded pre-Variscan low-grade metasediments deposited in the northern Gondwana passive margin during the opening of the Rheic Ocean. It represents the Variscan tectonic duplication of the NW Iberian autochthonous sedimentary sequence triggered by the advance of a stack of allochthonous units formed previously in an accretionary prism. Below this unit, the Lower Parautochthon is a less pervasively deformed tectonic slice including two lithostratigraphic units (Travanca and Vila Chã formations) composed of Culm-type synorogenic low-grade metasediments deposited in a tectonic trench located between the accretionary prism and a peripheral bulge, with detrital zircon age populations pointing to a Late Devonian–Early Carboniferous age. These sediments were subsequently detached from the Autochthon along mechanically weak Silurian carbonaceous slates. The provenance study on the detrital zircons implies that this basin was fed mainly from the active margin.

采用激光剥蚀电感耦合等离子体质谱法（laser ablation inductively coupled plasma mass spectrometry）开展锆石年代学研究，并结合莫赖斯推覆杂岩（Morais Allochthonous Complex）东缘准原地岩系（Parautochthon）的全新详细地质填图与地层学研究，为伊比利亚华力西造山带（Iberian Variscan belt）的演化提供了新的研究视角。本次研究数据证实，该准原地岩系由两个构造单元构成。其中变形程度更强、构造变形更复杂的上部准原地岩系（Upper Parautochthon），由发育平卧褶皱的前华力西期低级别变沉积岩组成，这些沉积物沉积于冈瓦纳北部被动大陆边缘，形成时期对应Rheic洋（Rheic Ocean）的开启阶段。该单元代表了伊比利亚西北部原地沉积序列的华力西期构造叠加，其成因与增生楔（accretionary prism）中先期形成的推覆体序列的推进作用直接相关。位于该单元之下的下部准原地岩系（Lower Parautochthon）为变形程度相对较弱的构造岩片，包含两个岩石地层单元——特拉万卡组（Travanca formation）与维拉尚组（Vila Chã formation），其岩性为库尔姆型（Culm-type）同造山低级别变沉积岩，沉积环境位于增生楔与周缘隆起之间的构造海沟内；碎屑锆石（detrital zircon）年龄谱系显示其形成时代为晚泥盆世至早石炭世。这些沉积物后续沿力学性质薄弱的志留纪（Silurian）炭质板岩与原地岩系发生剥离。对碎屑锆石的物源分析表明，该沉积盆地的物源主要来自活动大陆边缘。