Detrital zircon SHRIMP U–Pb age study of the Cordillera Darwin Metamorphic Complex of Tierra del Fuego: sedimentary sources and implications for the evolution of the Pacific margin of Gondwana

The Cordillera Darwin Metamorphic Complex in the southernmost Andes includes a basement of probable Palaeozoic age, a mid-Jurassic and younger volcano-sedimentary cover, and a suite of Jurassic granites, all of which were jointly metamorphosed during the Cretaceous. Detrital zircon ages presented here show that some of the amphibolite-facies metamorphic rocks previously mapped as basement have a Jurassic protolith. Overall the detrital zircon age patterns for samples of the Cordillera Darwin basement differ from those of the Madre de Dios Terrane of the western Patagonian Andes with which they had been correlated; instead, they are more comparable with those from the Eastern Andes Metamorphic Complex, which apparently developed in a passive margin setting. The paucity of Cambrian detrital zircons indicates that the meta-igneous basement of the Magallanes foreland basin of central and northern Tierra del Fuego was not the main source of detritus for the protolith of the Cordillera Darwin Metamorphic Complex. The possibility is envisaged that the Magallanes Fagnano transform fault boundary between the Scotia and South America plates resulted from reactivation of an older, pre-Jurassic suture zone between the basement terranes of north–central Tierra del Fuego and Cordillera Darwin.

位于安第斯山脉最南端的达尔文山脉变质杂岩（Cordillera Darwin Metamorphic Complex），由一套可能形成于古生代的基底、中侏罗世及更年轻的火山沉积盖层，以及一套侏罗纪花岗岩体共同组成，上述所有地质体均在白垩纪发生了同期变质作用。本文报道的碎屑锆石年龄数据显示，此前被划归为基底的部分角闪岩相变质岩，其原岩形成于侏罗纪。整体而言，达尔文山脉变质杂岩基底样品的碎屑锆石年龄谱，与此前被认为可对比的巴塔哥尼亚西部安第斯山脉马德雷德迪奥斯地体（Madre de Dios Terrane）的年龄谱存在显著差异；相反，其与东安第斯变质杂岩（Eastern Andes Metamorphic Complex）的年龄谱更为相似，而后者显然形成于被动大陆边缘环境。寒武纪碎屑锆石的极度匮乏表明，火地岛中北部麦哲伦前陆盆地的变质火成基底，并非达尔文山脉变质杂岩原岩碎屑物质的主要来源。本文还提出一种可能性：斯科舍板块与南美板块之间的麦哲伦-法格诺转换断层边界（Magallanes Fagnano Transform Fault Boundary），是由火地岛中北部基底地体与达尔文山脉变质杂岩之间一条更古老的侏罗纪前缝合带重新活化所形成的。