小兴安岭“古元古代”东风山群早古生代片麻状花岗岩锆石U-Pb定年、Hf同位素特征及地质意义

Zircon U-Pb dating,Hf isotope characteristics and geological implications of Early Paleozoic gneissic granites from“Paleoproterozoic”Dongfengshan Group in Lesser Xing’an Range

对小兴安岭东风山群红林组中黑云母混合片麻岩进行了岩相学、年代学和Hf同位素研究,以确定这套岩石的原岩性质、形成时代以及岩石成因。岩相学特征表明这套混合片麻岩主要由片麻状二长花岗岩和片麻状正长花岗岩两种岩性组成,局部保留的变余半自形粒状结构或变余似斑状结构指示其原岩为花岗岩。片麻状二长花岗岩和片麻状正长花岗岩中的锆石多呈自形-半自形柱状,具有明显的振荡环带结构,Th/U比值较高(0.11~1.16)指示其岩浆成因。对岩浆锆石LA-ICP-MS U-Pb定年结果表明,片麻状二长花岗岩和片麻状正长花岗岩分别形成于479 Ma和499 Ma,即早奥陶世和晚寒武世。片麻状二长花岗岩和片麻状正长花岗岩的Hf同位素组成相似且变化范围小,锆石εHf(t)值分别为-6.6^-10.6和-6.4^-8.6,两阶段模式年龄分别为1 698~1 958 Ma和1 692~1 831Ma,表明片麻状花岗岩的岩浆源区为古老地壳物质,其原始岩浆可能源自于古元古代地壳的部分熔融。结合前人研究成果讨论了东风山群的物质组成及原岩性质,认为东风山群具有构造混杂岩属性,其组成不仅包括新元古代、早古生代和晚古生代等具有地层性质的变质碎屑岩(副变质岩),而且存在新元古代和早古生代由岩浆侵位形成的花岗质岩石(正变质岩)。

Petrography,zircon U-Pb dating and Hf isotope studies were undertaken for Early Paleozoic granites in Honglin Formation of Dongfengshan Group in the Lesser Xing’ an Range to determine the protolith,the formation age and petrogenesis.The petrography features show that this suite of compound gneiss mainly consists of gneissic monzogranites and gneissic syenogranites.Blastohypidiomorphic granular and blastoporphyritic texture remained in parts of these rocks suggest that the protolith of them are granites.The zircons in these gneissic monzogranites and gneissic syenogranites are mostly euhedral-subhedral with fine-scale oscillatory growth zoning and relatively high Th/U ratios( 0.11 ~ 1.16).These features indicate a magmatic origin.The LA-ICP-MS zircon U-Pb dating results indicate that the gneissic monzogranites and gneissic syenogranites are formed in 479 Ma and499 Ma,respectively,which are Early Ordovician and Late Cambrian.The Hf isotope composition of gneissic monzogranites and gneissic syenogranites are similar and vary in a narrow range.The εHf( t) values range from-6.6 to-10.6 and-6.4 to-8.6,respectively,while the two-stage model ages are 1 698 ~ 1 958 Ma and1 692 ~ 1 831 Ma,respectively.These results indicate that the granites were derived from primary magma generated by partial melting of Paleoproterozoic crustal material.Combined with previous studies,the composition and the protolith of the Dongfengshan Group were discussed,and it is concluded that the Dongfengshan Group has the characteristics of tectonic complex,which is not only composed of Neoproterozoic,Early Paleozoic and Late Paleozoic metamorphic clastic rocks( parametamorphites) with sedimentary features,but also the granites( orthometamorphites) formed by the emplacement of magma in the Neoproterozoic and Early Paleozoic.