冀北崇礼红旗营子岩群中镁铁质岩石地球化学和年代学研究:对华北克拉通新太古代晚期-古元古代早期大地构造演化的启示

Geochemistry and Geochronology of Mafic Rocks from Hongqiyingzi Group in Chongli Area of the Northern Hebei Province: Implications for the Late Neoarchean-Early Paleoproterzoic Tectonic Evolution of the North China Craton

崇礼地区红旗营子岩群位于华北克拉通北缘,该区红旗营子岩群的岩石组合、构造属性及时代归属对于探讨华北克拉通新太古代晚期-古元古代早期大地构造演化具有重要意义.本文对崇礼地区红旗营子岩群中的镁铁质岩石进行了野外、镜下岩相学、全岩地球化学、锆石U-Pb年代学及Hf同位素分析,并讨论其对华北克拉通北缘新太古代晚期-古元古代早期地壳演化的指示意义.研究区红旗营子岩群中的镁铁质岩石以夹层产出于表壳岩中,主要由斜长角闪岩和斜长辉石角闪岩组成.红旗营子镁铁质岩石主量元素中SiO2(40.17%~51.53%)、Na2O(1.52%~3.47%)和K2O(0.43%~1.23%)含量范围较大,具有较高的CaO(9.15%~12.68%)、MgO(5.90%~13.36%)和Al2O3(9.81%~20.92%)含量.镁铁质岩石轻稀土元素轻微富集,重稀土元素配分模式近平坦,大离子亲石元素Ba、U、K、Sr富集,高场强元素Th、Nb、Ta、Ti明显亏损.斜长角闪岩LA-ICPMS锆石U-Pb测年显示207Pb/206Pb加权平均年龄为2 477±23 Ma(MSWD=3.1),上交点年龄为2 468±22 Ma,表明其形成于新太古代晚期-古元古代早期.斜长角闪岩锆石Hf同位素分析结果显示εHf(t)为负值(-5.2~0),一阶段模式年龄(TDM1)为2 673~2 783 Ma,二阶段模式年龄(TDM2)为2 880~3 074 Ma,均大于锆石的形成年龄,指示其来源于古老地壳物质的再循环.镁铁质岩石全岩地球化学分析结果显示其形成于岛弧相关环境.结合华北其他地区研究成果,本研究表明崇礼地区新太古代晚期-古元古代早期可能发生了一期与岛弧有关的岩浆事件.

The Hongqiyingzi Group in the Chongli area is located in the northern margin of the North China Craton. The rock association, structural characteristics and formation age of the Hongqiyingzi Group in the Chongli area are of great significance to study the tectonic evolution of the North China Craton from late Neoarchean to early Paleoproterozoic. In this paper, based on field, petrography, geochemistry, zircon U-Pb geochronology and Hf isotope analysis of these mafic rocks from the Hongqiyingzi Group in Chongli area, we study the origin and genesis of these rocks and discuss their tectonic implications on the late Neoarchean to early Paleoproterozoic crustal evolution of the North China Craton. The mafic rocks of the Hongqiyingzi Group are mainly composed of amphibolite and clinopyroxene amphibolite. The mafic rocks have a relatively wide range of geochemical compositions including SiO2(40.17%-51.53%), Na2 O(1.52%-3.47%) and K2 O(0.43%-1.23%). These rocks have relatively high values of CaO(9.15%-12.68%), MgO(5.90%-13.36%) and Al2 O3(9.81%-20.92%), and are characterized by enrichment of large ion lithophile elements such as Ba, U, K, and Sr, depletion of high field strength elements such as Th, Nb,Ta, and Ti and flat heavy rare earth elements patterns. The LA-ICP-MS zircon U-Pb dating shows that the magmatic zircon of amphibolite was formed at 2 477±23 Ma, with the upper intercept age of 2 468±29 Ma, representing its formation age. The Hf isotopic analyses of amphibolite show that the εHf(t) values are negative(-5.2-0). The one-stage(TDM1) and two-stage Hf model ages(TDM2) are 2 673-2 783 Ma and 2 880-3 074 Ma, respectively, indicating that they were originated from recycling of the Archean crustal materials. In addition, the εHf(t) values of zircons suggest that a crustal accretion event occurred in the Neoarchean in the Chongli area. The geochemical characteristics indicate that these rocks were formed in an arc-related tectonic environment.Based on the research of other areas in the North China Craton, we suggest that a magmatic event related with arc environment may have occurred in the late Neoarchean to early Paleoproterozoic in the Chongli area.