摘要:
提要：哈达阳地区出露的镁铁-超镁铁质岩石，分布于嫩江—黑河构造带哈达阳构造混杂岩中，岩石类型主要为角闪辉长岩和角闪石岩，呈异地岩块产出。笔者对其进行了LA-ICP-MS锆石U-Pb年代学及全岩地球化学研究，获得角闪石岩和角闪辉长岩中锆石的加权平均206Pb/238U年龄分别为（362.09±0.55）Ma（n=26）和（363.4±1.2）Ma（n=33），属晚泥盆世，为该构造带内迄今报道的最古老的镁铁-超镁铁质岩年龄。岩石地球化学特征研究表明，角闪石岩低Ti（TiO2=0.65%~0.97%）、低K（K2O=0.04%~0.12%）和高Mg（MgO=20.42%~24.07%）、高Fe（TFeO=10.25%~11.46%）；LREE和HREE分馏较为明显（（La/Yb）N=2.35~3.97），Th、Hf、Ce、Nb富集和Ba、Sr、K、Zr亏损，Zr/Nb=10.04~17.12。角闪辉长岩高Ti（TiO2=1.30%～5.04%）、高Na（Na2O=2.92%~3.14%）、相对高K（K2O=0.31%~0.45%），Mg相对偏低（MgO=5.94%~8.25%），LREE和HREE分馏也较为明显（（La/Yb）N=1.90～3.23），Zr、Nb亏损，明显富集Th、Sr。分析认为，哈达阳镁铁-超镁铁质岩具有火山弧型玄武岩特征，形成于板块消减带之上的岛弧及前弧盆地，大约363 Ma时嫩江—黑河构造带已经进入了板块俯冲阶段，这为贺根山—扎兰屯构造带东延及大兴安岭北段晚古生代的古亚洲洋构造域演化提供了新的年代学证据。
Abstract:
Abstract: The Late Devonian mafic - ultramafic rocks were first discovered in Nenjiang - Heihe area. The rocks outcrop in the Hadayang tectonic mélange in Inner Mongolia. The mafic - ultramafic rocks are allochthonous rock block consisting mainly of amphibole - gabbro and amphibolite. The laser ablation inductively coupled plasma mass spectrometry (LA - ICP - MS) zircon U - Pb geochronology and geochemistry of the rocks were studied. The dating results show that the weighted average 206Pb/238U age of the amphibole - gabbro is （363.3 ± 1.2） Ma (n= 33), and the age of amphibolite is （362.09 ± 0.55 ）Ma (n =26). The ages belong to Late Devonian. The geochemical characteristics of the amphibole - gabbro show high Ti (TiO2 = 1.30%-5.04%), high Na (Na2O = 2.92%-3.14%), relatively high K (K2O = 0.31%-0.45%) and low Mg (MgO =5.94%-8.25%), the fractionation of HREE and LREE is obvious ((La/Yb)N =1.90-3.23), with the depletion of Zr, Nb and enrichment of Th and Sr. The geochemical characteristics of amphibolite show low Ti (TiO2 =0.65%-0.97%), low K (K2O=0.04%-0.12%), high Mg (MgO=20.42%-24.07%), high Fe (FeOT =11.40%-12.74%), obvious LREE and HREE fractionation ((La/Yb) N = 2.35-3.97), enrichment of Th, Hf, Ce and Nb, and depletion of Ba, Sr, K and Zr, Zr/Nb = 10.04-17.12. The characteristics of the Hadayang mafic - ultramafic rocks are similar to the features of the volcanic arc basalts. Combined with existing research results, the authors hold that the rocks probably formed in an island arc and fore arc basin above the subduction zone, and the subduction of the Nenjiang-Heihe tectonic belt already began in Late Devonian. This paper provides new evidence for the study of the tectonic evolution of the eastern part of the Hegenshan - Zhalantun tectonic belt and the Late Paleozoic Paleo - Asian Ocean of the northern Da Hinggan Mountains.
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