Metamorphic and geochronological constraints on the final assembly of West Gondwana: an Ediacaran-Cambrian collisional setting in the northern Araguaia Orogen, central Brazil

The Araguaia Orogen, located in Central Brazil, represents an Ediacaran-Cambrian collisional orogen associated with the final assembly of West Gondwana. Two contrasting tectonic models have been proposed for the evolution of this orogen, involving different histories for the Amazonian palaeocontinent drift: (i) subduction of a large ocean and final collision of the Amazonian palaeocontinent with Gondwana between ~ 560 and 500 Ma and (ii) intracontinental orogeny after early amalgamation of the Amazonian palaeocontinent at ca. 620 Ma. Detailed tectono-metamorphic studies in this key locality for the evolution of Gondwana are also still scarce. Hence, in this study we present a detailed metamorphic investigation of two metapelite samples (schists) and one garnet amphibolite aiming to unravel the pressure-temperature-time (P-T-t) paths for the higher grade rocks exposed in this orogen. Pressure-temperature estimates based on thermodynamic modelling using isochemical phase diagrams in the MnNCKFMASHTO system for the schists constrained a single and continuous prograde metamorphic path (burial and heating) from ~ 4–6 kbar and ~ 450–500°C to ~ 7–8 kbar and ~ 580–650°C. The near-peak metamorphic conditions of the garnet amphibolite were estimated from classic geothermobarometry at 7.5 ± 0.8 kbar and 552 ± 40°C. Lutetium-hafnium garnet dating constrained an average age for the prograde metamorphism at around 537–533 Ma. These results show that the Araguaia Orogen records one single metamorphic event during the late Ediacaran-early Cambrian, and likely represents part of a collisional system developed between the Amazonian Craton and Parnaíba Block after the closure of the Clymene Ocean.

位于巴西中部的阿拉瓜亚造山带（Araguaia Orogen），是与西冈瓦纳大陆最终拼合相关的埃迪卡拉纪-寒武纪碰撞造山带。针对该造山带的演化，学界已提出两种截然不同的构造模型，二者对于亚马孙古大陆的漂移演化历史存在分歧：其一为大型大洋俯冲作用，且亚马孙古大陆与冈瓦纳大陆于约560至500 Ma期间发生最终碰撞；其二为亚马孙古大陆于约620 Ma完成早期拼合后，发生陆内造山作用。针对这一冈瓦纳演化关键研究区的详细构造-变质作用研究目前仍较为匮乏。有鉴于此，本研究对两件泥质岩样品（片岩）及一件石榴角闪岩开展了详细的变质作用研究，以期厘清该造山带出露的高级变质岩的温压时间（P-T-t）路径。针对片岩，基于MnNCKFMASHTO体系下的等化学相图热力学模拟所得到的温压估算结果，限定出一条单一且连续的进变质路径（埋藏增温过程），其温压条件从约4–6 kbar、450–500℃升高至约7–8 kbar、580–650℃。通过经典温压计法估算，石榴角闪岩的近峰期变质条件为7.5±0.8 kbar与552±40℃。镥-铪（Lu-Hf）石榴石定年结果限定，进变质作用的平均年龄约为537–533 Ma。上述结果表明，阿拉瓜亚造山带仅记录了埃迪卡拉纪晚期-寒武纪早期的一期变质事件，其很可能代表了克莱门斯洋闭合后，亚马孙克拉通（Amazonian Craton）与帕奈巴地块（Parnaíba Block）之间形成的碰撞构造体系的一部分。