Alternating Trench Advance and Retreat: Insights From Paleozoic Magmatism in the Eastern Tianshan, Central Asian Orogenic Belt

Accretionary orogens generally involve trench advance and retreat, recognition of which is critical to understand the thermal evolution and geodynamics of the lithosphere. A systematic study has been conducted on the Silurian to Carboniferous volcanic rocks from the Dananhu-Harlik arc in the Chinese eastern Tianshan. Three distinct episodes of volcanism have been recognized: Late Silurian normal andesite, high Sr/Y andesite and sanukitoid, Late Devonian bimodal high-alumina basalt and dacite, and Early Carboniferous high-alumina basalt. These magmatism experienced variable degrees of interaction between a depleted mantle wedge and subducting slab-/sediment-derived fluid/melt. Combining with previous geological data, we propose that the Dananhu-Harlik arc was formed by the northward subduction of the North Tianshan Ocean (NTO), different from the traditional southward subduction of the Junggar Ocean, which was likely subducted northward to account for the East Junggar arc system. Temporal changes in Ce/Y of basalts and Ho/Yb of felsic rocks in the Dananhu-Harlik arc imply that the arc crust underwent episodic thickening and thinning, which probably reflect the trench advance/retreat processes. The high Ce/Y and Ho/Yb ratios in the Late Ordovician to Devonian and Late Carboniferous correspond to two phases of trench advance. Conversely, the low Ce/Y and Ho/Yb ratios in the Early Carboniferous record a period of trench retreat, which caused the splitting of the Dananhu-Harlik arc and formation of the Bogda back-arc basin. Meanwhile, a southward subduction initiated along the south margin of the NTO, producing the Yamansu arc. Late Carboniferous bipolar subduction of the NTO led to its final closure.