大兴安岭中部乌兰浩特地区中生代花岗岩的成因——地球化学及Sr-Nd-Hf同位素制约

Petrogenesis of Mesozoic granite in Wulanhaote region,central Da Hinggan Mountains: constraints from geochemistry and Sr-Nd-Hf isotope

大兴安岭中部中生代花岗岩成因和构造背景的确定对讨论该地区及东北地区中生代构造-岩浆演化具有重要意义。乌兰浩特地区中生代花岗岩详细的岩石地球化学及Hf、Nd同位素的研究表明:研究区中晚三叠世查干岩体是A2型花岗岩;早中侏罗世景阳岩体为I型花岗岩,而大石寨岩体是具有特殊稀土元素四分组效应的花岗岩;早白垩世花岗岩体可根据其Sr含量划分为高Sr和低Sr两类,它们具有相同或相似的源岩组成,但其起源深度不同。其中高Sr型花岗岩类似于C型埃达克质花岗岩,起源于压力较高的下地壳,而低Sr型及永和屯花岗岩则是起源于压力较低的中地壳的高分异Ⅰ型花岗岩。全岩Nd和锆石Hf同位素特征及前人的研究结果显示,乌兰浩特地区花岗岩类岩石的源区主要为显生宙新增生的年轻地壳物质,并有老的地壳物质的贡献,并且随着花岗岩侵位时间的逐渐变新,锆石的εHf(t)值逐渐减小,认为地幔上涌导致岩浆底侵以及老地壳物质的折返是造成下地壳源岩组成复杂的原因。

Petrogenesis and tectonic setting of Mesozoic granites in central Da Hinggan Mountains constitute the key to investigating the tectonic and magmatic evolution of Northeast China.Systematical studies of petrology,geochemistry and Sr-Nd and Lu-Hf isotopes of the granites in Wulanhaote-Suolun region indicate that the Middle-Late Triassic Chagan granite is of A2 type.The Early-Middle Jurassic Jingyang granite is an I-type granite and the Dashizhai granite is a very special kind of granite which shows tetrad REE patterns.On the basis of the Sr values,the Early Cretaceous granites can be divided into two types,i.e.,the high-Sr type and the low-Sr type.The two types of granites possess the same or similar magmatic sources but originated in different depths.The high-Sr type granite is similar to C-type adakitic granite which originated from the higher pressure of the lower crust,while the low-Sr type and Yonghetun granite are highly fractionated I-type granites derived from the middle crust of lower pressure.Considering Sr-Nd-Hf isotopic data of granites in the Wulanhaote-Suolun region and the results of previous studies,the authors suggest that crustal growth of the study area occurred in the Phanerozoic.As indicated by the high positive zircon εHf（t） and εNd（t） values and the new young two-stage model age of each of the five intrusions,their magmatic sources must have mainly consisted of new crustal materials during the accretion in Phanerozoic,with the addition of some old crust components.And along with the variation of the emplacement time of the granites gradually towards newness,the εHf（t） values of the zircon change to lowness.The authors consider that magmatic underplating and exhumation of the old crust caused by the mantle upwelling were responsible for the complicatedness of the compositions of the lower crustal source rock.