中亚造山带北山下—中二叠统物源分析及古地理意义

Provenances of the Lower-Middle Permian in Beishan region, Central Asian Orogenic Belt and its paleogeographic implication

甘蒙北山地区位于中亚造山带中段南端,该区广泛出露的海相二叠系是中亚造山带最年轻的海相沉积之一,是解析古亚洲洋晚期演化的理想载体。然而,北山地区的物源学数据分布不均衡、缺少统计对比,直接限制了学界对古亚洲洋古地理结构的探讨。本文在贯穿北山地区中部至南部的8条剖面中获得了681个碎屑锆石U-Pb年龄和192个Hf同位素数据;在等时地层对比的基础上,统计计算了北山地区已发表的下—中二叠统碎屑锆石数据,重建了区域古地理面貌。早—中二叠世,红石山洋向南俯冲至旱山地块之下,形成了黑鹰山弧和弧后裂谷盆地。该盆地北侧的物源分别来自于旱山地块的前寒武纪基底(926~775 Ma)和奥陶纪—志留纪岩体(485~421 Ma),而南侧的物源主要来自于马鬃山隆起。马鬃山隆起是早古生代红柳河—洗肠井洋闭合的产物,主要由奥陶纪—泥盆纪岩体(470~367 Ma)组成,包括早古生代马鬃山岛弧和双鹰山地块的大部分地区。北山南部发育柳园裂谷盆地,盆地南北两侧的碎屑物质分别主要来自于石炭纪—二叠纪火山岩(322~270 Ma)和马鬃山隆起的早古生代岩体,这两种碎屑物质在盆地中心交汇。本文古地理重建结果表明:红石山洋可代表早—中二叠世古亚洲洋在北山地区的分支,弧后位置发育多个裂谷盆地。

The Beishan area is located at the southern end of the middle section of the Central Asian Orogenic Belt(CAOB).The widely exposed marine Permian in this area is one of the youngest marine deposits in the CAOB.It is an ideal carrier for analyzing the late evolution of the Paleo-Asian Ocean(PAO).However,the paleogeographic discussion of the PAO is hindered by the unbalanced distribution of the provenance data and the lack of statistical analysis.This study reports 681 U-Pb ages and 192 Hf isotopic data from eight sections from the central and southern Beishan regions.The paleogeographic pattern is reconstructed based on isochronous stratigraphic correlation and statistical analysis of published samples of detrital zircons from the Lower-Middle Permian strata.During the Early-Middle Permian,the southward subduction of the Hongshishan ocean resulted in the Heiyingshan arc and a back-arc rifting basin.The deposits on the northern margin of this rifting basin were fed from the Precambrian basement(926~775 Ma)and Ordovician-Silurian plutons(485~421 Ma)in the Hanshan Block.The sediments on the southern margin were sourced from the Mazongshan Uplift.This uplift composed of the Ordovician-Devonian plutons(470~367 Ma)was caused by the closure of the Hongliuhe-Xichangjing ocean in the early Paleozoic.The realm of this uplift covers the main areas of the Mazongshan arc and northern Shuangyingshan block.Detritus of the southern and northern margins of the Liuyuan rifting basin were respectively sourced from Carboniferous-Permian volcanic rocks(322~270 Ma)and early Paleozoic plutons of the Mazongshan Uplift.The detrital exchange occurred in the depocenter of the Liuyuan basin.The paleogeographic reconstruction indicates that the Hongshishan ocean as the main branch of the PAO lasted in the Early-Middle Permian and several rifting basins developed in the back arc regions.