早二叠世塔里木陆块的经度：来自大火成岩省的制约

经度的确定是板块重建的难点. 塔里木盆地下二叠统大规模溢流玄武岩已被确定为大火成岩省, 提供了一次根据大火成岩省来定量确定塔里木陆块早二叠世经度的机遇. 核幔边界约2800 km深处地震波低速带与全球300 Ma以来喷发的大火成岩省之间的关系已得到建立: 恢复喷发位置后, 大火成岩省全部分布在核幔边界低速带的边缘之上, 其中大部分在非洲LLSVP和太平洋LLSVP边缘, 个别在规模较小的LSVPs边缘. 在使用塔里木陆块早二叠世古地磁数据来限定其纬度的基础上, 本文利用上述理论方法, 并联系前人的地质结论, 发现塔里木大火成岩省约290 Ma喷发时的位置最可能为20°N, 60°E. 本文提出, 塔里木大火成岩省与西伯利亚大火成岩省相似, 其喷发时并不在两大LLSVPs的边缘带上, 而最可能与非洲LLSVP东侧附近一个单独的、范围较小的LSVP(20°N, 60°E)相关联, 暗示重建之前的假设"塔里木大火成岩省源自核幔边界"是合理的. 如果塔里木、峨眉山和西伯利亚大火成岩省都源自核幔边界, 上述(20°N, 60°E)位置的获得说明三者都不是同一幔源.

Longitude of the ca.290 Ma Tarim Block: Constraint from the Tarim Large Igneous Province of NW China

It is difficult to determine longitudes of a paleo-plate for times before ~130 Ma, the age at the end of the oldest hotspot track. In the Early Permian, the Tarim Block is without strong longitudinal constraints. What we know is that the northern margin of the Tarim craton had collided with Kazakhstan by this period. The 290 Ma Tarim Large Igneous Province (TLIP) can help constrain the longitude of the 290 Ma Tarim Block using the Plume Generation Zone (PGZ) method. The PGZ method concludes that because LIP eruption sites all project downward onto the margins of the African & Pacific Large Low Shear Velocity Provinces (LLSVPs) and other smaller, Low Shear Velocity Provinces (LSVPs) at the core-mantle boundary (CMB), those PGZs have not moved for 300 (even 540) Ma. The 290 Ma TLIP is on the Tarim Block and good paleomagnetic data position it at a certain latitude between 17.6°N and 33.6°N (present-day reference point 39°N, 80°E, estimated center for where the plume impinged the lithosphere according to the sedimentary response to the Early to Middle Permian basaltic magmatism) at a time little affected by True Polar Wander (TPW). If the TLIP erupted above the margins of the LLSVPs, there are five possible longitudinal zones where the 17.6°N to 33.6°N parts of latitude intersects or touches the margins. But the reconstructions with the Tarim LIP erupted above the margins of two LLSVPs would imply unrealistically huge sizes up to 12000~18000 km long of Kazakhstan between Baltica and Tarim, and high mean drift rates of Tarim (27~42 mm/a minimum) for the ~290 Ma interval between the Early Permian and the present. In conclusion, the TLIP eruption site is not related to the margins of two LLSVPs. Interestingly, we found that there is a separate, smaller LSVP (20°N, 60°E; dV_S≈-0.4% in the SMEAN model of Becker and Boschi (2002) while different negative dV_S values in other S-/P-wave velocity anomaly models) near the eastern edge of the African LLSVP, which is just within the latitudinal limits between 17.6°N and 33.6°N and not covered by Pangea either. There is no disagreement with previous geologic data if we reconstruct TLIP above this LSVP. So 20°N, 60°E (with a ~5° uncertainty) is the most realistic alternative as the TLIP eruption site. Not only does this positioning determine the previously unknown width of Kazakhstan between Tarim and Baltica, the northeastern part of Pangea (in the Global Hybrid reference frame) at that time, which was as much as 2000 km, but also demonstrates that the TLIP is very similar with the 251 Ma Siberian LIP (SLIP), whose eruption site also lies vertically above the margin of a separate, small LSVP. This reconstruction of the 290 Ma Tarim Block suggests that the TLIP was probably formed by a deep CMB plume and didn't share the same PGZ with the 258 Ma Emeishan LIP (ELIP, related to the Pacific LLSVP suggested) and the SLIP, which is not consistent with previous suggestions about the plume underneath the 290 Ma TLIP may be linked to similar magmatism that formed ELIP and SLIP, if they all generated from the CMB.