中亚造山带北山南部柳园洋晚古生代两阶段古地理演化

甘蒙北山地区位于中亚造山带中段，其间的柳园洋是古亚洲洋的重要分支之一，关于该洋盆的演化过程存在两阶段裂谷盆地和单阶段长期俯冲两种认识。柳园洋位于北山南部石板山地块和双鹰山地块之间，石板山地块的晚古生代沉积序列和物源变化完整记录了洋盆的古地理演化过程，对于重建区域构造演化有重要意义。此次研究聚焦石板山地块独山地区的上古生界火山- 沉积序列，通过砂岩碎屑颗粒组分和碎屑锆石U- Pb- Hf同位素分析，提供柳园洋古地理演化的约束证据。本文数据表明：独山地区下—中泥盆统碎屑锆石年龄为早古生代单峰式分布（~415 Ma），来自于柳园洋向南俯冲在石板山地块边缘而形成的岩浆弧；上石炭统—下二叠统样品中出现了明显的中元古代碎屑锆石年龄（~1426 Ma），来自于北侧的双鹰山地块和中天山地块，指示洋盆闭合事件；下二叠统碎屑锆石年龄主要为晚石炭世—早二叠世单峰式分布（301~290 Ma），来自于裂谷火山活动。此外，本次研究通过对北山南部已发表物源学数据的统计计算，进一步证明并完善了古生代柳园洋两阶段演化模型：中奥陶世—中泥盆世，柳园洋双向俯冲至敦煌- 石板山地块和双鹰山地块之下；晚泥盆世，柳园洋闭合，直至石炭纪晚期，敦煌- 石板山- 双鹰山地块形成了统一的陆缘环境；早—中二叠世，柳园裂谷盆地逐步发育至原洋盆地。

Two- stage evolution of the Late Paleozoic Liuyuan Ocean in the southern Beishan region, Central Asian Orogenic Belt

The Liuyuan Ocean, situated in the Beishan region of the Central Asian Orogenic Belt in northwest China, is one of the critical branches of the Paleo- Asian Ocean. The evolution of the Liuyuan Ocean has been subjected to two divergent views, namely the existence of two stages of rift basins and long- lived subduction. Tectonically,the Liuyuan Ocean is located between the Shibanshan and Shuangyingshan blocks. The sedimentary sequence found in the Shibanshan block, with its provenance shifts, plays an important role in tectonic reconstruction by preserving the paleogeographic patterns of the Liuyuan Ocean. This study focuses on the analysis of sandstone composition and detrital zircon U- Pb- Hf isotopic results from the Upper Paleozoic volcanic- sedimentary sequence in the Dushan area of the Shibanshan block. The analysis of detrital zircons from the Lower- Middle Devonian sample reveals a unimodal spectrum (~415 Ma), suggesting that they originated from magmatic arcs formed by the subduction of the Liuyuan Ocean beneath the Shibanshan block. Detrital zircons from the Upper Carboniferous- Lower Permian exhibit a prominent Mesoproterozoic peak (~1426 Ma), indicating the influx of detritus from the Shuangyingshan and Central Tianshan blocks due to the Liuyuan Ocean contraction. The Lower Permian samples show a significant peak in the Later Carboniferous- Early Permian age distribution, indicating the influence of rifting- induced volcanism. Based on a statistical comparison of published provenance data in the southern Beishan region, a two- stage evolution model is proposed for the Paleozoic evolution of the Liuyuan Ocean. During the Middle Ordovician to Middle Devonian, the Liuyuan Ocean underwent bidirectional subduction beneath the margins of the Dunhuang- Shibanshan and Shuangyingshan blocks.The closure of the Liuyuan Ocean occurred in the Late Devonian, and the Dunhuang- Dushan- Shuangyingshan blocks transitioned into an integrated continental margin environment until the Late Carboniferous. From the Early to Middle Permian, the Liuyuan rift basin gradually developed into a proto- oceanic basin.