大兴安岭北段新林地区晚石炭世花岗岩的岩石成因及地质意义

大兴安岭北段新林地区晚古生代花岗岩主要出露在大乌苏和富西里附近，岩性主要为二长花岗岩，另有少量花岗闪长岩。对其中二长花岗岩样品进行LA-ICP-MS锆石U-Pb测年表明，大乌苏和富西里岩体侵位年龄分别为（303.7±2.2）和（300.5±0.5）Ma，均为晚石炭世岩浆活动的产物。花岗岩具有富硅（w（SiO₂）为66.77%~75.85%）、富碱（w（Na₂O+K₂O）为7.41%~8.69%）、高铝（w（Al₂O₃）为12.90%~16.22%），低MgO、CaO、TiO₂的特点，属于钙碱性系列；铝饱和指数（A/CNK）为1.06~1.44，为过铝质岩石；镜下未见原生白云母、堇青石、石榴石等富铝矿物，不同于富铝的S型花岗岩；而w（P₂O₅）与w（SiO₂）负相关，呈I型花岗岩特征；富集LREE和Ba、Rb、K等大离子亲石元素，亏损Nb、Ta、Ti等高场强元素，与后造山I型花岗岩特征相似，应形成于拉张的构造环境。花岗岩的⁸⁷Sr/⁸⁶Sr为0.712 938、¹⁴³Nd/¹⁴⁴Nd为0.512 386，（⁸⁷Sr/⁸⁶Sr）_i值为0.704 4，ε_Nd（t）值为-1.09，T_DM2=1 172 Ma，源区物质主要为中-新元古代从亏损地幔增生的地壳物质。结合区域研究成果，大兴安岭新林地区晚石炭世岩浆侵位活动与额尔古纳-兴安地块和松嫩地块碰撞拼合后岩石圈伸展环境有关。

Petrogenesis of Late Carboniferous Granitic Plutons in Xinlin Area,Northern Great Xing'an Range and Their Geological Significance

The Late Carboniferous granites, including Dawusu and Fuxili plutons, consist of monzogranie and granodiorite. The LA-ICP-MS zircon U-Pb date indicates that the emplaced ages of these two plutons are (303.7±2.2)Ma and(300.5±0.5)Ma, formed by the Late carboniferous magmatism. The plutons are characterized by high silica (w(SiO₂)=66.77%-75.85%), alkali (w(Na₂O+K₂O)=7.41%-8.69%) and alumina (w(Al₂O₃)=12.90%-16.22%),with low MgO, CaO,and TiO₂. They are of calcalkaline series and peraluminous as indicated by A/CNK ratio of 1.06-1.44. Different from the aluminum-rich S type granite, no aluminum-rich minerals such as native muscovite, cordierite, and garnet were seen under the microscope. Conversely, the plutons show I type granite characteristics with negative correlation between P₂O₅ and SiO₂. The rocks are enriched in LREE, Ba, Rb and K with negative Nb, Ta and Ti anomalies, which is similar to that of the post-orogenic I type granitoids, indicating the plutons formed in an extensional tectonic environment. The w(⁸⁷Sr/⁸⁶Sr)ratio of the monzogranie is 0.712 938, and w(¹⁴³Nd/¹⁴⁴Nd) is 0.512 386 with the initial w(⁸⁷Sr/⁸⁶Sr)_i and ε_Nd(t)value of 0.704 4 and-1.09 respectively, indicating the source material is mainly the crust material from the depleted mantle. This is supported by the young Nd model age (T_DM2=1 172 Ma). Combined with the previous studies in the regional tectonics, the Dawusu and Fuxili plutons are related to the lithosphere extension after the orogenic collision between the Erguna-Xing'an block and the Songnen block.