The mid-Variscan suture in the Morais complex of Trás-os-Montes (Portugal): insights from geochemistry and geochronology of oceanic rocks

The Middle Allochthon of the Galicia-Trás-os-Montes Zone (GTMZ) in NW Iberia is an assemblage of oceanic units representing the Variscan suture of one or more peri-Gondwanan oceans. This contribution brings new geochronological and geochemical data from the Middle Allochthon of the Morais Complex, in northern Portugal. There, the oceanic ensemble consists of a stack of five tectonic units which from bottom to top are the Junqueira, Pombais and Izeda oceanic supracrustals and the Remondes and Morais-Talhinhas ophiolitic units. The Junqueira and Pombais units consist of greenschists and metapelites. No age data are available for these two units, but a correlation is established with a Cambro-Ordovician unit in the Órdenes Complex of Galicia (NW Spain) dated at 500 Ma. The Izeda Unit consists of fine-grained, low-grade amphibolites transitional to epidote-amphibolites and greenschists, for which an age of 484 ± 3 Ma has been obtained for a metabasic tuff. In the ophiolitic ensemble, the Remondes and Morais-Talhinhas units consist of fine-grained amphibolites associated with deformed metagabbros, mafic cumulates and serpentinized ultramafics. In this study, two plagiogranite samples from the Remondes Unit yielded a complex spread of age data with Silurian ages representing the dominant population. This suggests that the newly defined Remondes Unit is younger than the Izeda Unit but older than the previously published age for these rocks, which are considered part of the overlying Morais-Talhinhas Unit, dated as Devonian (406–395 Ma) in a previous study. The geochemistry of the basic rocks in the five units is that of NMORB with a subduction-derived component that varies from weak to null. The ages, together with new and previously published whole rock geochemical data obtained from basic igneous samples, are interpreted to date and reflect the formation of igneous protoliths in oceanic ridge settings associated with the Mid-Variscan Ocean, a part of the Rheic oceanic realm that registered magmatic activity between the late Cambrian and the Lower Devonian.

伊比利亚西北部的加利西亚-特拉斯-奥斯-蒙特斯带（Galicia-Trás-os-Montes Zone, GTMZ）的中间外来岩席（Middle Allochthon）是一套大洋单元组合，代表了一个或多个冈瓦纳边缘洋的华力西缝合带。本研究提供了葡萄牙北部莫赖斯杂岩（Morais Complex）中间外来岩席的新年代学与地球化学数据。该区域的大洋组合由5个从下至上叠置的构造单元组成，依次为容奎拉、蓬拜斯、伊泽达大洋表壳岩单元，以及雷蒙德斯和莫赖斯-塔尔伊尼亚斯蛇绿岩单元。容奎拉与蓬拜斯单元由绿片岩和变泥质岩构成，目前尚无这两个单元的年代数据，但已与西班牙西北部加利西亚奥登斯杂岩（Órdenes Complex）中时代为500 Ma的寒武-奥陶纪单元建立了对比关系。伊泽达单元由细粒低级变质角闪岩过渡为绿帘角闪岩与绿片岩，其变基性凝灰岩的测年结果为484±3 Ma。在蛇绿岩组合中，雷蒙德斯与莫赖斯-塔尔伊尼亚斯单元由细粒角闪岩与变形变辉长岩、镁铁质堆晶岩以及蛇纹石化超镁铁质岩伴生组成。本研究中，采自雷蒙德斯单元的两件斜长花岗岩样品获得了复杂的年龄分布数据，其中志留纪（Silurian）年龄为优势群体。这表明新厘定的雷蒙德斯单元时代晚于伊泽达单元，但早于此前针对该类岩石发表的测年结果——此前研究认为这类岩石属于上覆的莫赖斯-塔尔伊尼亚斯单元，时代为泥盆纪（Devonian，406~395 Ma）。五个单元内基性岩的地球化学特征属于带有俯冲成因组分的正常洋中脊玄武岩（NMORB, Normal Mid-Ocean Ridge Basalt），该俯冲组分的强度从弱至无。结合本次新获取及此前已发表的基性火成岩全岩地球化学数据，这些年龄数据被用以限定并反映了与中华力西洋（Mid-Variscan Ocean）相关的洋脊环境中火成原岩的形成时代；中华力西洋是莱茵洋域（Rheic oceanic realm）的一部分，其岩浆活动时限为晚寒武世至早泥盆世。