Abstract: Early Cretaceous potassium-riched alkaline volcanic rocks were discovered in the Suhongtu area, Inner Mongolia. These volcanic rocks display higher total alkaline contents (K₂O+Na₂O=5.20%~8.92%), K₂O/Na₂O (K2O/Na2O=0.59~1.03) and TiO₂ (1.59%~2.05%) and characteristics of shoshonites series rocks. Chondrite normalized rare earth element patterns and primitive mantle normalized trace element patterns show slightly positive and negative Eu anomalies (δEu=0.88~1.11) and the enrichment of large ion lithophile elements (LILE), light rare earth elements (LREE) and high field strength elements (HFSE). Suhongtu volcanic rocks show similar geochemical characteristics to those of Siziwangqi and Southern Baiyun’ebo volcanic rocks. These volcanic rocks did not experience obvious contamination of crustal materials. Negative Nb and Ta anomalies probably imply the influence of the subducted slab of Paleo-Asian Ocean crust. Trace elements and isotopic analysis indicate that the Suhongtu and Siziwangqi shoshonites magmas were derived from EMⅠ enrichmental asthenosphere mantle altered by materials from the subducted slab of the Paleo-Asian Ocean crust, and the Southern Baiyun’ebo volcanic rocks was from a less-degree enriched source. However, their petrogenesis is similar to the depleted mantle source with various degree of enrichment. In combination with the regional geology, the middle and western part of the north margin of Northern China Craton (NCC), along which Siziwangqi, Southern Baiyun’ebo and Suhongtu volcanic rocks distributed, probably showed an similar extensional environment after orogenic period in Early Cretaceous. After the closure of the Paleo-Asian Ocean in Late Triassic, a NS-trending compressive process developed in the north margin of NCC, which led the thickening of crust and the delamination of lithosphere. Then the remelting of the subducted slab of the Paleo-Asian Ocean crust, and the melt resulted in enrichment of the depleted asthenosphere mantle, and the upwelling and partial melting of the enriched mantle asthenosphere mentle formed EW-distributed Early Cretaceous shoshonites.

Key words: Suhongtu, shoshonites, enrichmental asthenosphere mantle, delamination

摘要： 对内蒙古阿拉善盟苏宏图地区一套早白垩世富钾碱性玄武岩中主量元素和微量元素进行测试, 结果表明: 岩石以富碱(K₂O+Na₂O含量为5.20%~8.92%)、高K₂O/Na₂O比值(0.59~1.03)和高TiO₂ (1.59%~2.05%, 平均1.82%)为主要特征, 为钾玄岩系列。该火山岩富集大离子亲石元素、轻稀土元素及高场强元素, 有微弱的Eu异常(δEu=0.88~1.11)。岩石地球化学特征表明, 华北板块北缘西段的苏宏图组火山岩与中段四子王旗地区及白云鄂博南部的早白垩世火山岩有较大的相似性。苏宏图组火山岩、四子王旗钾玄岩以及白云鄂博南部玄武岩均为钾玄岩系列, 三地火山岩在喷出过程中都较少经历地壳混染作用, Nb和Ta的负异常更可能是古亚洲洋俯冲物质影响的结果。同位素和微量元素分析结果表明, 苏宏图组火山岩与四子王旗钾玄岩源区为被古亚洲洋俯冲板块交代的EMⅠ型富集软流圈地幔, 白云鄂博南部玄武岩富集程度稍弱于其他两地, 但三地源区有相似的演化历史, 都经历了亏损的软流圈地幔交代富集过程。三地火山岩相似的源区演化历史表明, 华北板块北缘中?西段在早白垩世可能有相似的构造运动, 在古亚洲洋闭合后, 晚三叠世南北向挤压缩短, 地壳强烈加厚, 而后下部岩石圈拆沉, 已俯冲的古亚洲洋板块的物质重熔, 对亏损的软流圈地幔进行交代富集, 最终富集的软流圈地幔上涌, 形成沿华北板块北缘中?西段东西向呈线状分布的早白垩世钾玄岩。

关键词: 苏宏图组, 钾玄岩, 富集地幔, 拆沉