华北克拉通南缘太古—元古宙界线安沟群火山岩的年龄及地球化学

Age and Geochemistry of Volcanic Rocks of Angou Group at the Archean-Proterozoic Boundary

河南登封地区登封群和安沟群记录了华北克拉通南缘太古宙—元古宙界线两侧地层较完整的演化历史.Krneretal.(1988)曾获得登封群顶部老羊沟组变英安岩中锆石的SHRIMP年龄为(2512±12)Ma.本研究获得不整合覆盖在登封群之上的安沟群同源双峰式拉斑玄武岩和英安岩7个样品全岩SmNd同位素等时线年龄为(2507±96)Ma,MSWD=6.3,初始143Nd/144Nd=0.509560±0.000093,εNd(2.5Ga)=3.27±1.82.上述年龄制约了登封群顶部长英质火山岩与安沟群底部的双峰火山岩之间的年龄相差最多应不超过113Ma.安沟群演化程度较低的拉斑玄武岩La/Nb比值为0.66～1.14,与软流圈地幔的比值接近,而明显不同于岛弧玄武岩(La/Nb<2.0).结合安沟群火山岩的双峰特征,这些说明太古宙末或元古宙初期至少华北克拉通南缘已进入陆内裂谷伸展环境.

The Dengfeng Group and Angou Group from the Dengfeng area, Henan Province preserve rocks at the Archean-Proterozoic boundary. Kroner et al. (1988) yielded an SHRIMP U-Pb zircon age of (2512±12) Ma for a date from the upper Dengfeng Group. Here we report an Sm-Nd whole rock isochron age of (2507±96) Ma (MSWD=6.3) for 7 volcanic rocks from the Angou Group with initial 143Nd/144Nd=0.509560±0.000093, which corresponds to ΕNd(2.5 Ga)=3.27±1.82. This constrains an age difference of less than 113 Ma between the top of Dengfeng and the bottom of Angou volcanic rocks. The less evolved Angou metatholeiites are characterized by asthenospheric La/Nb ratios (<2.0). Together with the bimodal character, it is inferred that the Angou Group formed in a continental rift setting.