大兴安岭中段扎兰屯南部花岗质糜棱岩岩石成因及地质意义

Genesis and Geological Significance of Granitic Mylonites in Southern Zhalantun,Central Xing’an Range

大兴安岭扎兰屯南部位于中亚造山带东段,兴安地块(XB)与松嫩地块(SB)之间,是探究兴安地块与松嫩地块构造演化过程,乃至中亚造山带东段微地块聚合过程的关键地区.对该地区出露的花岗质糜棱岩进行地质学、地球化学、锆石U-Pb测年分析,以期查明板块俯冲过程中的岩浆作用特征,并尝试限定兴安地块与松嫩地块碰撞拼合时间.LA-ICP-MS锆石U-Pb测年结果表明,花岗质糜棱岩分为两期:Ⅰ期为早泥盆世(~398.8 Ma),Ⅱ期为晚泥盆世末‒早石炭世(351.6~365.7 Ma).两期岩石均具有高SiO_(2)(68.20%~77.90%),富碱(K_(2)O+Na_(2)O=6.32%~9.67%),贫镁(MgO=0.22%~0.97%),偏铝质,高钾钙碱性系列的特征.稀土元素分布模式均为右倾[(La/Yb)_(N)=4.33~10.77],Eu负异常(δEu=0.03~0.13),富集LILE,亏损HFSE,具有I型花岗岩的特征.两期花岗质糜棱岩可能为俯冲背景下岛弧岩浆活动形成的I型‒分异I型花岗岩.结合区域构造演化和岩石地球化学研究,大兴安岭北段晚古生代早期花岗质岩浆作用与兴安地块与松嫩地块的碰撞拼合作用有关,扎兰屯地区早泥盆世‒早石炭世处于古亚洲洋向兴安地块和松嫩地块俯冲的构造背景下,两个地块的最终碰撞拼合时限可能为晚石炭世.

The southern of Zhalantun in the Great Xing’an Range is located between the Xing’an Block(XB)and Songnen Block(SB)in the eastern section of the Central Asian Orogenic Belt(CAOB),which is of great importance for the study on the collision and combination of blocks during the tectonic evolution process.The exposed granitic mylonites were analyzed by geological and geochemical analysis,and zircon U-Pb dating in this paper.LA-ICP-MS zircon U-Pb dating results show that the granitic mylonites are divided into two phases:Phase Ⅰ is the Early Devonian(-398.8 Ma),and Phase Ⅱ is the end of the Late Devonian to the beginning of the Early Carboniferous(351.6-365.7 Ma).The rocks of the two periods are characterized by high SiO_(2)(68.20%-77.90%),high alkali(K_(2)O+Na_(2)O=6.32%-9.67%)and low magnesium(MgO=0.22%-0.97%),belonging to the aluminous and high potassium calc-alkaline series.The REE distribution pattern is right-leaning[(La/Yb)_(N)=4.33-10.77],with a slight negative Eu anomaly(δEu=0.03-0.13).All samples were enriched in LILE and depleted in HFSE,which have the characteristics of differentiating I-type granites.The two phases of granitic mylonites may be I-typedifferentiated I-type granites formed by island arc magmatism under the background of subduction.Combining regional tectonic evolution and petrochemical research,the early Late Paleozoic granitic magmatism in the northern section of the Great Xing′an Range is related to the collision and combination of the XB and SB.During the Early Devonian-Early Carboniferous,the Zhalantun area was under the subduction environment of the Paleo-Asian Ocean-oriented XB and SB,the collision and closure time of the two blocks here may be the Late Carboniferous.