Tectonic Controls on Magmatic Tempo in an Active Continental Margin: Insights From the Early Cretaceous Syn‐Tectonic Magmatism in the Changle‐Nan'ao Belt, South China.

The cause of magmatic tempo in an active plate margin remains controversial, partly due to lack of structural analysis. During 130–105 Ma, the magmatism in the South China Block (SCB) was significantly reduced (lull) and restricted around the Changle‐Nan'ao Belt (CNB) while the magmatism was highly active (flare‐up) in North China. For unveiling the tectonic role on magmatism, a multidisciplinary study including field and microscopic structural observations, magnetic fabric measurement (AMS), and zircon/monazite dating was conducted on the plutons with oriented minerals in the CNB. Structural analysis and AMS results show a highly clustered NE‐striking vertical foliation developed during the emplacement indicating a syn‐emplacement NW‐SE shortening regime. The geochronology results confirm that the emplacement occurred during 130–105 Ma. Regional geologic correlation indicates that the collision between the Dangerous Grounds‐West Philippines Block and SCB was responsible for this shortening regime. The collected depleted zircon εHf(t) data suggest that a possible collision‐triggered slab break‐off caused the syn‐collisional magmatic activity around the CNB. This study shows that the magmatic lull in the SCB was coeval with a crustal shortening regime due to the arrival of a microcontinent carried by the subduction slab, while the magmatic flare‐up was coeval with an extensional regime due to the subduction roll‐back and retreat as evidenced by the Cretaceous evolution of the SCB before and after the collision and that of North China which is unaffected by the collision. The cause of magmatic tempo in the continental margin is largely due to the tectonic evolution of underlain subduction slabs. Plain Language Summary: Magmatic activity is closely connected to human life as it provides us with useful metals, releases greenhouse gases, triggers natural hazards. It is necessary to explore what controls a magmatic activity. Several previous studies considered that a shortening regime with thickened crust causes strong magmatism (flare‐up). The South China Block (SCB) featured by alternating magmatic flare‐up and lulls, provides an appropriate research target to test this hypothesis. During the Cretaceous, the SCB was dominated by an extensional regime caused by the Paleo‐Pacific oceanic slab subduction. However, our structural analyses on the SCB document a shortening regime caused by the collision between the SCB and a microcontinent carried by the Paleo‐Pacific oceanic slab in the period of the Cretaceous. During the collision, the SCB was generally featured by a magmatic lull, although magmatic activity was only observed around the Changle‐Nan'ao Belt, probably caused by the break‐off of the subducted Paleo‐Pacific oceanic slab. Before and after the collision, the extensional regime caused by Paleo‐Pacific oceanic slab subduction was coeval with magmatic flare‐up. This study questions the previous claims and proposes that the tectonic evolution of the subducting slab may control the magmatic flare‐up and lull in the overlain active continental margin. Key Points: The 130–105 Ma syn‐tectonic plutons intruded the Changle‐Nan'ao Belt in a NW‐SE shortening regime caused by a microcontinent collisionThe shortening/extension regimes were coeval with magmatic lull/flare‐up in the East Asia Continental Margin during the CretaceousThe collision causing break‐off can account for the plutonism in the Changle‐Nan'ao Belt during the magmatic lull of the South China Block [ABSTRACT FROM AUTHOR]