大兴安岭中北段雅尔根楚地区晚古生代花岗岩的成因及其地质意义

Petrogenesis of Late Paleozoic granites from Yaergenchu area in north-central Great Xing'an Range and its geological implication

为确定大兴安岭地区晚古生代花岗质岩石的成因及其构造背景,笔者对雅尔根楚地区二长花岗岩样品进行了锆石U-Pb年代学与地球化学测试分析。锆石U-Pb年龄结果显示研究区二长花岗岩形成于晚石炭世(312±2 Ma)。二长花岗岩具有较高的SiO_2(75.16%~75.32%)和全碱(8.45%~8.76%)含量,较低的MgO(0.12%~0.14%)含量,以富集大离子亲石元素(Rb、Ba、Th、U、K)和轻稀土元素,亏损Nb、Ta、Ti及P元素为特征,并且其ε_(Hf)(t)值和Hf二阶段模式年龄变化于+3.5^+9.5和1 103~715 Ma,表明其原始岩浆来源于新生地壳物质的部分熔融。结合年代学、地球化学特征与区域地质调查结果,认为研究区晚石炭世二长花岗岩的形成与古亚洲大洋板块向兴安地块下的俯冲作用有直接联系。

To determine the petrogenesis and tectonic setting of Late Paleozoic granitoids in Great Xing＇an Range,NE China,the authors undertook zircon U-Pb dating and geochemical analysis on samples from monzogranites in Yaergenchu area. Zircon U-Pb age dating indicates that the monzogranites in the studied area were emplaced in the Late Carboniferous（ 312 ± 2 Ma）. The monzogranites contain high SiO_2（ 75. 16% - 75. 32%） and Na_2O ＋K_2O（ 8. 45% - 8. 76%）,but low MgO（ 0. 12% - 0. 14%）. Moreover,the samples are characterized by the enrichment of large ion lithophile elements（e. g.,Rb,Ba,Th,U,K） and light rare earth elements,but depletion of Nb,Ta,Ti and P,with ε_（Hf）（t） values of ＋ 3. 5 to ＋ 9. 5 and Hf two-stage model ages of 1 103 - 715 Ma,indicating that the parental magma was derived from partial melting of juvenile crustal material. Integrating the geochronological data,geochemical features and regional geological observations,it is suggested that the formation of Late Carboniferous monzogranites in the studied area was related to the subduction of the Paleo-Asian oceanic plate beneath the Xing＇an terrane.