大兴安岭北段奥陶系砂岩碎屑锆石U-Pb年代学及其地质意义

Detrital zircon U-Pb geochronology of the Ordovician sandstone and its constrain to the tectonic evolution of the northern Great Xing’an Range

东北陆块群是中亚造山带的主要构造单元,关于其前寒武纪古老基底属性的判别、古生代构造单元划分及增生造山演化过程一直是地质学家研究热点。兴安增生地体被认为是东北陆块群的重要组成部分,由于其前寒武纪沉积-岩浆记录的大量缺失,使得奥陶纪沉积-岩浆事件成为研究其构造演化的关键。本文对出露于兴安增生地体奥陶系的多宝山组进行了碎屑锆石LA-ICP-MS U-Pb定年、锆石Hf同位素及地球化学分析,旨在准确限定多宝山组的沉积时限,揭示其沉积环境及物源区性质。研究结果表明,来自大扬气镇南、花朵山南部及伊尔施西北部三个地区的多宝山组变质砂岩的最年轻锆石年龄分别为481±5Ma (D9088)、462±5Ma (296NJ-1) and 473±11Ma (HDG06),类似于其对应加权平均年龄482±3Ma (n=12)、475±6Ma (n=10)和483±8Ma (n=7),由此限定多宝山组的沉积下限为早-中奥陶世。其中<1.0Ga样品数量最多的锆石年龄为462~520Ma,峰值年龄为516Ma、497Ma和482Ma;次者在790~980Ma,该年龄区间出现969Ma、830Ma、788Ma、760Ma等峰值;> 1.0Ga的具有较弱的峰值(1321~2410Ma),主要为1882Ma和2410Ma两个峰值,以上所有峰值与额尔古纳地块内部同期岩浆岩体完全吻合,说明所研究样品的物源区主要来自额尔古纳地块。对比分析不同区域多宝山组碎屑锆石Hf同位素特征,发现自东向西越靠近额尔古纳地块,多宝山组碎屑锆石ε_(Hf)(t)值越小,二阶段模式年龄t_(DM2)越老,暗示物源区基底古老物质逐渐增多。结合奥陶系砂岩的地球化学特征,我们推测这种变化趋势可能反映了由活动大陆边缘向额尔古纳地块内部过渡的构造环境。

The combined NE China blocks are the major component of the Central Asian Orogenic Belt(CAOB).There has been a continued debate on the derivation of the old Precambrian crustal basements and Paleozoic tectonic division and evolutions of this orogen.Due to the absence of the Precambrian tectono-magmatic events,the Ordovician strata and magmatism play a key role in the reconstruction of the tectonic evolution of the Xing’an accretionary Terrane located in the west of the combined NE China blocks.In this study,we conducted U-Pb dating,geochemical analyses and Lu-Hf isotope analysis for the detrital zircon grains from metasandstone and tuff samples in the Duobaoshan Formation.The youngest ages of 481±5 Ma(D9088),462±5 Ma(296 NJ-1)and 473±11 Ma(HDG06)and similar to the youngest weighted mean^(206)Pb/^(238)U ages of 482±3 Ma(n=12),475±6 Ma(n=10)and 483±8 Ma(n=7),and thus give the lower limit age of sediments,indicating the Duobaoshan Formation developed in the Early-Middle Ordovician.The ages of detrital zircons from the Duobaoshan Formation range(<1.0 Ga)are mainly concentrated in 462~520 Ma(with peaks at ca.516 Ma,497 Ma and 482 Ma),and 790~980 Ma(peaks at ca.969 Ma,830 Ma,788 Ma and 760 Ma).The ages of detrital zircons(>1.0 Ga)have a minor population of 1321~2410 Ma,with the peaks at ca.1882 Ma and 2410 Ma.All the peaks are consistent with the contemporaneous magmatism of the Erguna Block,which reveal that the provenance of the metasandstone and tuff samples is mainly derived from the Erguna Block.By comparing the detrital zircon Hf isotopes of the Duobaoshan Formation in different areas,a rule can be observed that the farther the Duobaoshan Formation are away from the Erguna Block,the smallerεHf(t)values and the older Hf model ages(tDM2)of the detrital zircons in it will be,which suggests that there are more older crustal source from the Erguna Block from east to west.Together with the geochemical characteristics of the Ordovician strata,the Duobaoshan Formation was deposited in an environment related to an active continental margin.