Geochronological and Lu-Hf isotopic study on detrital zircons of the Jianshan Formation, Bayan Obo Group in Shangdu area, Inner Mongolia: Constraints on Precambrian crustal evolution of the Western Block, North China Craton

Abstract

华北克拉通前寒武纪地壳生长过程是地质学的研究热点。本文对华北克拉通西部陆块北缘的白云鄂博群尖山组进行了碎屑锆石U-Pb定年和Hf同位素分析，并在此基础上对尖山组的沉积时限、物源及西部陆块北缘的前寒武纪地壳生长过程进行了探讨。样品TM33中碎屑锆石年龄主要集中在1872~2100Ma（n=64），并有~1960Ma的主年龄峰值和~2000Ma的次年龄峰值，Hf同位素分析结果显示锆石的ε_Hf（t）值在-2.6~7.9之间（除1个测点为-17.6），Hf同位素二阶段模式年龄（t_DM2）变化范围在2240~4386Ma之间。样品Z1724中碎屑锆石年龄分布于1784~2200Ma（n=44），并有~1965Ma的主年龄峰值和~2080Ma次年龄峰值。谐和线上最年轻一组锆石的加权平均年龄为1794±73Ma（n=2），ε_Hf（t）在-5.4~3.2之间，Hf同位素二阶段模式年龄（t_DM2）变化范围在2584~3203Ma之间。基于以上数据，结合侵入白云鄂博群底部1670Ma的辉长岩以及白云鄂博群底部全岩²⁰⁷Pb-²⁰⁶Pb等时线年龄为1649Ma的沉积碳酸盐岩，尖山组的沉积时间被限定在1800~1650Ma。尖山组的物源主要来自固阳、西乌兰不浪地区新太古代基底以及大青山和卓资地区古元古代晚期孔兹岩带。综合前人对研究区及邻区中新元古代沉积地层中碎屑锆石的Hf同位素研究结果，认为华北克拉通西部陆块北缘可能存在2600~2750Ma、~2500Ma和2000~2100Ma三期主要的地壳生长事件，且最后一次规模可能小于前两次。;The Precambrian crustal growth of North China Craton (NCC) has been a hot topic for geologists for a long time. This paper presents LA-ICP-MS U-Pb dating and in-suit Lu-Hf isotopic analysis results of detrital zircons from Jianshan Formation, Bayan Obo Group in the northern western block of North China Craton to illustrate its depositional time and provenance as well as the Precambrian crustal growth in this area. The ages of detrital zircons from Sample TM33 cluster at 1872~2100Ma (n=64), with a major age peak at ~1960Ma and a minor peak at ~2000Ma. The in-suit Lu-Hf isotopic analysis of the zircons indicate their ε_Hf(t) values vary from -2.6 to 7.9 (except one of -17.6), with two-stage Hf model ages (t_DM2) varying between 2240Ma and 4386Ma (averaged at 2749Ma). The ages of detrital zircons from Sample Z1724 are mainly between 1784~2200Ma (n=44), with a major age peak at ~1965Ma and a minor peak at ~2080Ma. The weighted average age of the youngest concordia ages is 1794±73Ma (n=2). In-suit zircon Lu-Hf isotopic analysis indicate their ε_Hf(t) values vary from -5.4 to 3.2, with two-stage Hf model ages (t_DM2) between 2584~3203Ma. Based on the above data and combined with the 1670Ma gabbro intruding into Bayan Obo Group and the carbonatite with whole rock ²⁰⁷Pb-²⁰⁶Pb age at 1649Ma, the depositional age for Jianshan Formation is between 1800Ma and 1650Ma. The main provenances for Jianshan Formation are the Neo-Archean basement in Guyang, Xiwulanbulang and Kondalite Belt in Daqingshan, Zhuozi. There might exist three crustal growth events in the northern western block of NCC at 2600~2750Ma、~2500Ma and 2000~2100Ma respectively, and the scale of the last one might be much smaller than the other two.