东乌旗地区早古生代中-基性杂岩锆石U-Pb年代学及地球化学特征

内蒙古东部二连浩特-东乌旗一线出露的早古生代岩浆活动的成因及动力学背景对于揭示恢复区域的构造演化历史具有重要意义。本文选取东乌旗地区早古生代中-基性杂岩进行了锆石U-Pb年代学、岩石地球化学和Lu-Hf同位素分析。定年结果显示, 变辉石闪长岩成岩年龄为465±5Ma, 片麻状角闪辉长岩为456±3Ma, 表明该杂岩形成时限为中-晚奥陶世。地球化学组成上, 东乌旗早古生代中-基性杂岩相对富集轻稀土元素而亏损重稀土元素, Eu负异常相对明显, 相对富集Rb、Sr等大离子亲石元素(LILs), 亏损Nb、Ta、Zr等高场强元素(HFSE), 兼具MORB和岛弧玄武岩的特征。随着时间的推移, 岛弧玄武岩的印记正逐渐减弱, 而MORB成分不断增加。岩浆来源于被俯冲流体为主、熔体为辅交代的地幔部分熔融。杂岩中的变辉石闪长岩ε_Hf(t)=8.27~12.03, 片麻状角闪辉长岩ε_Hf(t)值为-2.12~1.10, 总体呈现出早期亏损成分先出熔, 后期富集成分再出熔的特征。结合区域岩石地层资料, 综合分析表明东乌旗中-基性杂岩形成于弧后盆地初始裂解-不成熟弧后盆地阶段, 兴安地块西部南缘的苏尼特左旗-锡林浩特-大石寨早古生代岩浆岩带和北侧的二连浩特-东乌旗早古生代岩浆岩带共同构成了古亚洲洋北缘早古生代弧盆系, 其演化特征可以与地块东部进行类比。

Geochronology,petrogenesis, and tectonic implications of Early Paleozoic intermediate-basic complex of East Ujimqin Banner area

The origin and dynamic backgrounds of the Early Paleozoic magmatic activities in the Erenhot-East Ujimqin Banner line in eastern Inner Mongolia are of great significance for revealing the tectonic evolution history of the restoration area. In this paper, the Early Paleozoic intermediate-basic complexs in the East Ujimqin Banne area were selected for zircon U-Pb geochronology, rock geochemistry and Lu-Hf isotopic analysis. The dating results show that the diagenetic ages of the meta-pyroxene diorite and the gneissic hornblende gabbro are 465±5Ma and 456±3Ma respectively, indicating that the complex was formed in the Middle-Late Ordovician. In terms of geochemical compositions, the Early Paleozoic intermediate-basic complexes in East Ujimqin Banne are relatively enriched in light rare earth elements and depleted in heavy rare earth elements, with relatively obvious negative Eu anomalies. Furthermore, they are relatively enriched in large ion lithophile elements (LILs) such as Rb and Sr, and depleted in high field strength elements (HFSE) such as Nb, Ta and Zr, showing characteristics of mixed MORB and island arc basalts. The imprint of the island arc basalt is gradually weakening over time, while the MORB composition is increasing. The magma of this complex is originated from the partial melting of the mantle where the subducted fluid is the main source, and the melt is supplemented by the metasomatism. The meta-pyroxene diorite in the complex has εHf(t)=8.27~12.03, and the gneissic hornblende gabbro εHf(t) value is -2.12~1.10. It shows the characteristics that the early deficient components will melt first, and the later enriched components will melt again. Combined with regional rock formations data, comprehensive analysis shows that the East Ujimqin Banner intermediate-basic complex was formed in the back-arc basin initial cracking-immature back-arc basin stage. The Sonid Left Banner-Xilinhot-Dashizhai Early Paleozoic magmatic rock belt on the southwestern margin of the Xing'an Block and the Erenhot-East Ujimqin Banner Early Paleozoic magmatic rock belt on the north side constitutes of the Early Paleozoic arc-basin system on the northern margin of the Paleo-Asian Ocean. The evolution characteristics can be compared with the eastern part of the block.