Cenozoic delamination of the southwestern Yangtze craton owing to densification during subduction and collision
Cenozoic delamination of the southwestern Yangtze craton owing to densification during subduction and collision
复制标题
俯冲和碰撞致密化导致扬子克拉通西南新生代拆沉
作者：
Wang Jun;Wang Qiang;Xu Chuan-Bing;Dan Wei;Xiao Zhuo;Shu Chutian;Wei Gangjian
人们普遍认为大洋俯冲可以触发克拉通龙骨剥离，但扬子克拉通西南部(中国西南部)在新生代大陆碰撞中失去了下龙骨。变薄的SYC之下的上地幔含有不完全拆离的龙骨，具有高速地震异常。结合地球物理观察和不同深度始新世地幔镁铁质钾质熔岩的地球化学特征，我们认为深部(~130 km)拆离岩石圈比浅部(~55 km)完整岩石圈(Fo=94.2；δ18O=5.2‰)更富饶、致密，具有低镁橄榄岩(Fo；摩尔10 0×mg/[Mg+Fe=91.3)和高O18O(5.9‰)。在早期大洋俯冲过程中，由于富铁玄武岩熔体的加入，深龙骨经历了强烈的参考和致密化作用。俯冲驱动的参考作用和随后的碰撞驱动的冷却由于成分和热致密作用而不是水化或氧化诱导的流变性减弱而导致克拉通龙骨剥离。我们的研究提供了一个碰撞造山带中新生代克拉通龙骨剥离的例子，并强调了成分和热致密作用在俯冲和碰撞过程中的关键作用。
It is widely thought that oceanic subduction can trigger cratonic keel delamination, but the southwestern Yangtze craton (SYC; southwestern China) lost its lower keel during Cenozoic continental collision. The upper mantle beneath the thinned SYC contains its incompletely delaminated keel, which has high-velocity seismic anomalies. Combining geophysical observations with the geochemistry of Eocene mafic potassic lavas derived from the SYC mantle at different depths, we suggest that the deep (~130 km) delaminated lithosphere was more fertile and dense, with low-forsterite (Fo; molar 100 × Mg/[Mg + Fe = 91.3) and high-δ18O (5.9‰) olivine, than the shallow (~55 km) intact lithosphere (Fo = 94.2; δ18O = 5.2‰), although both were rehydrated and oxidized. The deep keel underwent strong refertilization and densification owing to the addition of Fe-rich basaltic melts during earlier oceanic subduction. Subduction-driven refertilization and subsequent collision-driven cooling caused cratonic keel delamination due to compositional and thermal densification rather than hydration- or oxidation-induced rheological weakening. Our study provides an example of Cenozoic cratonic keel delamination in a collisional orogen and highlights the key roles of compositional and thermal densification in delamination during subduction and collision.