Toward an integrated model of geological evolution for NE Brazil-NW Africa: The Borborema Province and its connections to the Trans-Saharan (Benino-Nigerian and Tuareg shields) and Central African orogens

Abstract Both the Borborema Province of NE Brazil and the geological provinces of NW Africa (the Trans-Saharan Orogen consisted of the Tuareg and Benino-Nigerian shields and the Central African Orogen of Cameroon, Chad, and Central African Republic) are complex geological regions with superposition of distinct deformational, metamorphic and magmatic events and final structural configuration during the Brasiliano/Pan-African Orogeny (ca. 625-510 Ma). These provinces represent the site of major mountain building processes in the Ediacaran/Cambrian transition that culminated in the amalgamation of West Gondwana after the collision of the West African-São Luís, São Francisco-Congo, and Saharan paleocontinents. In the last years, discovery and characterization of key tectonic units such as ophiolites, eclogites, HP/UHP rocks, and both oceanic and continental magmatic arcs are helping to clarify these processes and propose tectonic models for the geological evolution of NE Brazil-NW Africa. Connections of the marginal belts that frame these provinces, bordering the eastern margin of the West African-São Luís Craton (Médio Coreaú-Dahomeyides-Gourma-West Tuareg Shield) and the northern margin of the São Francisco-Congo Craton (Rio Preto-Riacho do Pontal-Sergipano-Yaoundé-Central African) are progressively better constrained, while correlations within the interior, highly reworked and sectioned portions of both the Borborema Province, the Benino-Nigerian Shield, the Central and East Tuareg Shield, Western Cameroon, and Adamawa-Yadé domains are more complicated and demand further investigation. Some of the questions of prime importance in this context are the continuation or not of the 1000-920 Ma Cariris Velhos Belt of NE Brazil into NW Africa, and if the basement-dominated North Borborema/Benino-Nigerian (NOBO-BENI) and Alto Pajeú-Alto Moxotó-Rio Capibaribe-Pernambuco-Alagoas/Adamawa-Yade (APAMCAPAY) domains could represent major decratonized blocks (such as LATEA in the Central Tuareg Shield), perhaps developed due to hyperextension and detachment of a Greater São Francisco-Congo paleocontinent northern margin. In this case, the Goiás-Pharusian and Transnordestino-Central African oceanic realms along with restricted internal oceans such as the hypothetical Piancó-Alto Brígida/Western Cameroon (PAB-WECA) Seaway probably separated these ancient paleocontinental blocks during the Neoproterozoic. The development of subduction zones and the docking of Neoproterozoic juvenile terranes welded the hyperextended Archean/Paleoproterozoic lithospheric fragments together and they became squeezed and reworked in between the major cratonic landmasses during the Brasiliano/Pan-African Orogeny. The quest for the sites of ancient oceans and continents that once composed NE Brazil and NW Africa goes on and tentative scenarios will surely benefit from novel geological, isotopic, and geochronological data put forward in the near future.

摘要 巴西东北部的博尔博雷马省（Borborema Province）与西北非洲的诸多地质省——包括由图阿雷格（Tuareg）和贝宁-尼日利亚地盾构成的泛撒哈拉造山带（Trans-Saharan Orogen），以及覆盖喀麦隆、乍得与中非共和国的中非造山带（Central African Orogen）——均为复杂地质区域，在巴西利亚诺/泛非造山作用（Brasiliano/Pan-African Orogeny，约625-510 Ma）期间经历了多期变形、变质与岩浆事件的叠加，并最终形成现今的构造格局。上述地质省代表了埃迪卡拉纪（Ediacaran）-寒武纪（Cambrian）转折期重大造山作用的发生地，该造山作用在西非-圣路易斯、圣弗朗西斯科-刚果以及撒哈拉古大陆碰撞后，最终完成了西冈瓦纳大陆的聚合。近年来，诸如蛇绿岩（ophiolites）、榴辉岩（eclogites）、高压/超高压（HP/UHP）岩石，以及大洋与大陆岩浆弧（magmatic arcs）等关键构造单元的发现与研究，正逐步阐明上述造山过程，并为巴西东北部-西北非洲的地质演化提出构造模型。环绕上述地质省的边缘带——即毗邻西非-圣路易斯克拉通（West African-São Luís Craton）东缘的莫迪奥-科雷奥-达荷美地槽-古尔马-西图阿雷格地盾带（Médio Coreaú-Dahomeyides-Gourma-West Tuareg Shield），以及圣弗朗西斯科-刚果克拉通（São Francisco-Congo Craton）北缘的里约普雷托-里奥什杜蓬塔尔-塞尔希培-雅温得-中非带（Rio Preto-Riacho do Pontal-Sergipano-Yaoundé-Central African）——其关联关系正日益得到明确；而博尔博雷马省、贝宁-尼日利亚地盾、中东部图阿雷格地盾、喀麦隆西部以及阿达马瓦-亚德（Adamawa-Yadé）地块内部经强烈改造与肢解的区域，其内部对比研究则更为复杂，有待进一步开展工作。此背景下若干核心问题亟待解答：巴西东北部1000-920 Ma的卡里里斯维卢斯带（Cariris Velhos Belt）是否向西北非洲延伸？以基底为主的北博尔博雷马/贝宁-尼日利亚（NOBO-BENI）地块与阿尔托帕茹-阿尔托莫克索托-里奥卡皮巴里贝-伯南布哥-阿拉戈斯/阿达马瓦-亚德（APAMCAPAY）地块，是否可代表主要的去克拉通化地块（类似中突阿雷格地盾的LATEA地块）？这类地块或许因大圣弗朗西斯科-刚果古大陆北缘的超伸展与拆离作用形成。若此假设成立，则戈亚斯-法鲁辛（Goiás-Pharusian）与特兰斯诺德斯蒂诺-中非洋域，以及诸如假说中的皮安科-阿尔托布里吉达/喀麦隆西部（PAB-WECA）海道等局限内洋，可能在新元古代（Neoproterozoic）时期分隔了上述古老的古大陆地块。俯冲带的发育与新元古代新生地体的拼贴，将经超伸展作用改造的太古宙（Archean）/古元古代（Paleoproterozoic）岩石圈碎片焊接为一体；在巴西利亚诺/泛非造山作用期间，这些碎片被挤压并改造于主要克拉通地块之间。对曾构成巴西东北部与西北非洲的古大洋与古大陆遗迹的探索仍在持续，未来将涌现的全新地质、同位素与年代学数据，无疑将为相关推测性演化情景提供更坚实的支撑。