北秦岭西段凤县地区草滩沟群龙王沟组酸性凝灰岩年代学、岩石地球化学特征及其构造意义

Age and geochemical characteristics of acidic tuff in the Longwanggou Fromation of Caotangou Group in the Fengxian area,western part of the North Qinling orogenic belt,and their tectonic implications

草滩沟群龙王沟组在北秦岭西段广泛出露,对研究秦岭早古生代构造演化具有重要意义。LA- ICP- MS锆石U- Pb同位素定年结果显示草滩沟群龙王沟组酸性凝灰岩的结晶年龄为457. 4±3. 8Ma。草滩沟群龙王沟组酸性凝灰岩富硅(SiO 2 =64. 55%~73. 07%)和碱(Na 2 O+K 2 O=5. 35%~8. 11%),低铝(Al 2 O 3 =13. 91%~16. 07%);轻稀土元素富集,重稀土元素亏损,具Eu负异常(δEu=0. 34~0. 86);富集Rb、Th等大离子亲石元素(LILE),亏损Nb、Ta、Ti等高场强元素(HFSE),具有较低的Nb/Ta比值和Mg #值。以上显示,草滩沟群龙王沟组酸性凝灰岩具壳源特征,为地壳部分熔融结果。北秦岭造山带草滩沟群龙王沟组酸性凝灰岩和晚奥陶世—早志留世岩浆岩具弧岩浆岩特征,显示它们形成于俯冲环境。结合北秦岭造山带早古生代超高压变质带、岩浆岩分布以及草滩沟群沉积特征显示,早奥陶世北秦岭造山带还处于商丹洋的俯冲阶段。

The Longwanggou Formation of Caotangou Group is widely exposed in the western part of northern Qinling Orogenic Belt, which is of great importance for understanding the early Paleozoic tectonic evolution of the Qinling Orogenic Belt. The acidic tuff in the Longwanggou Formation yields an LA- ICP- MS zircon age of 457. 4±3. 8 Ma. The acidic tuff samples in the Longwanggou Formation have SiO 2 (72. 06%~74. 49%) and Na 2O+K 2O (5. 35%~8. 11%), and low Al 2O 3(13. 91%~ 16. 07%) contents. The rocks are geochemically featured by LILE enrichment and HFSE depletion, low Mg# values and Nb/Ta ratios and obvious negative anomalies (δEu=0. 34~0. 86). The acidic tuff in the Longwanggou Formation has the characteristics of crustal source. The acidic tuff in the Longwanggou Formation and Late Ordovician- Early Silurian magmatic rocks have gecchemical characteristics of arc magmatic rocks, indicating that they were formed in a subduction environment. Combined with the early Paleozoic ultrahigh pressure metamorphic belt, the distribution of magmatic rocks and the sedimentary characteristics of Caotanggou Group, we suggest that the subduction of the Shangdan Ocean was still ongoing in the Early Ordovician.