大兴安岭北段塔河南部早白垩世侵入岩年代学和地球化学

Age and Geochemistry of the Early Cretaceous Intrusive Rocks in Southern Tahe,Northern Great Xing'an Range

以大兴安岭北段塔河南部基性、中性和酸性侵入岩(Si O2=51. 27%～74. 90%)为研究对象,通过锆石U-Pb年代学、Hf同位素和岩石地球化学分析,探讨了其岩石成因和构造环境。测定的2件样品锆石U-Pb定年结果分别是(126. 9±0. 8) Ma和(123. 4±0. 9) Ma,侵位于早白垩世。在哈克(Harker)图解中,多数样品主量元素和微量元素随Si O2含量增加具有很好的线性演化趋势(除K2O和Rb外)。微量元素总体显示富集Pb和大离子亲石元素K、Rb、Ba等,亏损高场强元素Nb、Ta、Ti、P等,并具有相似的Sr、Nd同位素特征((87Sr/86Sr)i=0. 705 517～0. 706 827,εNd(t)=-6. 82～0. 79)。综合研究认为,塔河南部早白垩世侵入岩的岩浆源区是与俯冲有关的熔流体交代的石榴子石二辉橄榄岩相和尖晶石二辉橄榄岩相的混合源区,且以石榴子石二辉橄榄岩相为主,在岩浆快速上升的过程中主要经历了分离结晶作用,最后形成一套宽谱系的岩浆演化序列。结合中国东北地区火山岩展布方向、古地磁资料、蒙古—鄂霍茨克洋闭合时间以及古太平洋板块俯冲方向等已有成果,认为大兴安岭北段塔河南部的侵入岩产出在与蒙古—鄂霍茨克洋闭合有关的后碰撞环境。

This paper presents zircon U-Pb dating,in-situ zircon Hf isotopic and geochemical data for the intrusive rocks from southern Tahe,northern Great Xing7an Range,to constrain their petrogenesis and tectonic set tings.The rocks consist of a continuous mafic to felsic magmatic suite(SiO2=51.27%-74.90%).Zircon U-Pb dating indicates that the two intrusive rock samples analyzed were emplaced in the Early Cretaceous(126.9±0.8)Ma and(123.4±0.9)Ma).In the Harker diagrams,most of the major elements and trace elements have a good linear evolutionary trend with increasing SiP2 content(except K2 O and Rb).The rocks are gener-aly enriched in Pb and LILEs(K,Rb,Ba),depleted in HFSEs(Nb,Ta,Ti,P),and have similar Sr and Nb isotope characteristics(87Sr/86Sri=0.705,517-0.706,827,εNd(t)=-6-82-0.79).This indicates that the Early Cretaceous intrusive rocks in southern Tahe derived from a mixed source of garnet lherzolite(predomi-nant)and spinel lherzolite,which were metasomatized by subduction-related fluids or melts.During the rapid magma ascent,the magma has experienced fractionation,eventually forming a broad compositional spectrum of rocks.The paper reviews the distribution of volcanic rocks in northeast China,paleomagnetic data,the timing of the Mongolia-Okhotsk ocean closure and the subduction direction of the Paleo-Iaciflc Plate.These data imply that the Early Cretaceous intrusive rocks in southern Tahe were likely formed in a post-collisional setting after the closure of the Mongolia-Okhotsk ocean.