兴凯地块伊曼群的形成时代与物源:碎屑锆石与岩浆锆石U-Pb年代学证据

Formation age and provenance of the Iman Group in the Khanka Massif: Evidence from U-Pb geochronology of detrital and magmatic zircons

本文对出露在兴凯地块南部伊曼群的变沉积岩和侵入其中的火成岩进行了系统的锆石LA-ICP-MS U-Pb定年和Hf同位素研究,旨在查明伊曼群的形成时代与物源,并讨论侵入岩的成因和构造背景。伊曼群二云母片岩中锆石呈自形-半自形,显示典型的岩浆生长环带,暗示其岩浆成因。定年结果表明:100个碎屑锆石分析点给出了555~1322Ma的年龄区间,主要峰值年龄为555Ma、612Ma、700Ma、739Ma、769Ma、839Ma、936Ma;其中约533Ma的锆石εHf(t)值介于-4.1^+3之间,二阶段模式年龄tDM2介于1305~1755Ma之间;700~839Ma的锆石εHf(t)值介于-3.7^+7.8之间,二阶段模式年龄tDM2介于1246~1907Ma之间。侵入伊曼群的白云母二长花岗岩的定年结果显示为443±3Ma,此年龄对应εHf(t)值介于-17.6^-4.2之间,二阶段模式年龄tDM2介于1688~2529Ma之间。结合二云母片岩中碎屑锆石的最小年龄以及穿切其中花岗岩的形成时代,可以判定伊曼群二云母片岩的形成时代介于443~555Ma之间,应为新元古代晚期至晚奥陶世之间。基于伊曼群中碎屑锆石的年龄众数及与区域岩浆事件的对比,伊曼群的沉积物源主要来自松嫩地块和佳木斯地块的新元古代火成岩。此外,白云母二长花岗岩显示出埃达克岩的地球化学属性,岩浆源于加厚下地壳的部分熔融,暗示晚奥陶世研究区可能发生了陆壳加厚事件。

In this paper,we present the zircon LA-ICP-MS U-Pb ages and Hf isotope compositions of detrital and magmatic zircons from the metasedimentary rocks and intrusive rocks within the Iman Group in the southern Khanka Massif to constrain the formation age and provenance of the Iman Group,and discuss the genesis and tectonic background of the intrusive muscovite monzogranites.The zircons separated from two-mica quartz schist in the Iman Group are euhedral-subhedral in shape,and display magmatic growth zones in their CL images,implying their magmatic origin.The dating results indicate that the 100 analytical spots over the detrital zirons yield a range of ages of 555~1322Ma.The main age populations are 555Ma,612Ma,700Ma,739Ma,769Ma,839Ma and 936Ma.TheirεHf(t)values and two-stage model ages(tDM2)of 533Ma vary from-4.1 to+3 and from 1305Ma to 1755Ma,respectively.TheirεHf(t)values and two-stage model ages(tDM2)of 700~839Ma vary from-3.7 to+7.8 and from 1246Ma to 1907Ma,respectively.The zircons from the muscovite monzogranite that intruded into the Iman Group yield a weighted mean 206Pb/238U age of 443±3Ma,itsεHf(t)values and two-stage model ages(tDM2)range from-17.6 to-4.2 and 1688Ma to 2529Ma,respectively.Combined with the youngest age of the two-mica quartz schist and the formation time of the intruding muscovite monzogranite in the Iman Group,we conclude that the Iman Group was formed between 443~555Ma,i.e.,the Late Neo-Proterozoic to Late Ordovician.The age populations of the two-mica quartz schist,together with the regional magmatic events,indicate that the provenance of the Iman Group mainly come from the Neo-Proterozoic igneous rocks in the Songnen and Jiamusi massifs.Additionally,the muscovite monzogranites exhibit a geochemical affinity to adakitic rock,suggesting that its primary magma could be derived from partial melting of a thickened lower crustal material,which implies that the continental crust thickening might occur in the study area in the Late Ordovician.