大兴安岭中段碾子山地区晚三叠世埃达克质侵入岩的发现及其地质意义

Discovery of Late Triassic Adakitic Rocks at Nianzishan in the Central Great Xing’an Range and Its Geological Significance

为加深对大兴安岭中段早中生代构造背景的认识,选取碾子山地区早中生代花岗岩开展岩石学、年代学和元素地球化学研究。结果表明,该地区早中生代花岗岩岩石组合为正长花岗岩和石英二长岩,其LA-ICP-MS锆石U-Pb年龄为225.6~228.9 Ma,形成于晚三叠世。二者的地球化学特征与埃达克岩的地球化学特征非常类似,如高Al_(2)O_(3)、Sr含量和Sr/Y、La/Yb比值,低Y、Yb含量,亏损重稀土,弱负铕异常等。但是二者也存在明显的差别:石英二长岩比正长花岗岩具有更高的MgO、LREE、Cr、V、Sr含量和Sr/Y、K/Rb比值。这种差别可能反映二者成因上的不同:正长花岗岩可能由增厚下地壳的部分熔融形成,而石英二长岩可能由拆沉下地壳部分熔融形成。结合前人发表的相关资料,认为碾子山地区晚三叠世埃达克质侵入岩是在兴安地块与松嫩地块碰撞造山的挤压背景下形成的,其形成受蒙古—鄂霍次克洋板块南向俯冲的影响。拆沉下地壳部分熔融形成的埃达克质侵入岩的发现,显示碾子山及其周边地区具有寻找斑岩型铜钼矿的潜力。

Geochronology and geochemistry of the Early Mesozoic Nianzishan granites were studied to constrain the tectonic evolution in the Central Great Xing’an Range.The studied syenogranite and quartz monzonite samples show LA-ICP-MS zircon U-Pb ages of 225.6-228.9 Ma,indicating that they were both emplaced in the Late Triassic.Both rock types have similar adakitic geochemical,including high Al_(2)O_(3) and Sr contents and Sr/Y and La/Yb ratios,low Y and Yb contents,HREE depletions and slightly negative Eu anomalies.However,the syenogranite has higher MgO,LREE,Cr,V,and Sr contents,and higher Sr/Y and K/Rb ratios than the quartz monzonite.Such differences are likely caused by their different petrogenesis:partial melting of thickened lower crust for the syenogranite and partial melting of a delaminated lower crust for the quartz monzonite.Combined with previous studies,we conclude that the Late Triassic Adakitic rocks at Nianzishan were formed in a compressional orogenic setting related to the collision of the Xing’an and Songnen blocks,which was overprinted by the south-dipping subduction of the Mongol-Okhotsk Ocean.The discovery of adakitic rocks by melting of delaminated lower crust indicates the potential for exploration of porphyry Cu-Mo deposits in the Nianzishan and adjacent area.