东北地区嫩江东北部早古生代闪长岩的成因探讨:锆石U-Pb年代学和地球化学证据

东北兴安地块与松嫩地块的拼贴演化历史一直存在较大争议,而早古生代岩浆记录的发现无疑对该问题的解决具有重要意义。笔者在黑龙江省嫩江依克特地区识别出了早古生代闪长岩体,其LAICP-MS锆石U-Pb年龄为(435.3±1.1)Ma,该闪长岩体的SiO_2质量分数为55.01%~61.82%,全碱(Na_2O+K_2O)质量分数为4.55%~6.94%,Na_2O/K_2O值变化(2.01~48.60)较大,Al_2O_3质量分数为14.97%~16.67%,具有中等程度的K_2O(0.10%~2.12%)、TiO_2(0.99%~1.42%)和P_2O_5(0.33%~0.49%)以及低的TFeO/MgO(0.51~0.96)值,属于中钾钙碱性系列。其A/CNK值为0.78~1.35,A/NK值为1.63~2.21,大体符合I型花岗岩特点。依克特闪长岩的稀土总量较高(w(ΣREE)=227.02×10-6~289.17×10-6),轻重稀土分异明显(La/Yb)_N=9.86~13.93],并具有弱的铕异常(Eu/Eu*=0.88~1.02);具有明显高场强元素(Nb和Ti)亏损的特征。上述特征表明,岩浆源区为受俯冲流体交代地幔楔的部分熔融。结合区域资料,本文认为依克特闪长岩体形成于活动大陆边缘环境,与嫩江-黑河洋的洋壳持续俯冲相关。

Petrogenesis of the Early Paleozoic Diorite in the Northeast Nenjiang Area:Evidence from Zircon U-Pb Chronology and Geochemistry

NE China was traditionally considered the eastern part of the Central Asian Orogenic Belt (CAOB) and was composed of different mico-blocks in the Paleozoic.There are some debates about the collision age and location between the Xing'an and Songliao-Nenjiang blocks in NE China.The discovery of the records of the Early Paleozoic magmatism is a key to solve the debates.Early Paleozoic diorites are indentified in this study in the Nenjiang area,western Heilongjiang Province.The LA-ICP-MS zircon U-Pb dating indicates an age of (435.3 ± 1.1) Ma.The diorites have 55.01％-61.82 wt％ SiO2,and 14.97％-16.67 wt％ Al2O3,with Na2O+K2O=4.55％-6.94 wt％,and exhibit high Na2O/K2O and low FeOT/MgO (0.51-0.96) ratios,and moderate K2 O (0.10％-2.12％),TiO2 (0.99％-1.42％),and P2O5 (0.33％-0.49 ％).These results indicate that the diorites belong to medium K cale-alkaline series.Their A/CNK and A/NK values range from 0.78 to 1.35 and 1.63 to 2.21 respectively,which are generally consistent with those of type Ⅰ granites.The diorites display right-inclined rare earth element (REE) patterns (La/Yb)N =9.86-13.93,with relatively high total REE abundances (∑REE=227.02×10 6-289.17×10-6),and week Eu anomalies (Eu/Eu * =0.88-1.02).In trace elements,the diorites are characterized by depletion in high field strength elements (HFSEs,such as Nb and Ti).The primary magma of the diorites in the northeast Nenjiang area was generated by partial melting of mantle wedge that had been metasomatized by subduction-related fluids.Integrating regional data and the research result of this study it is suggest that the Nenjiang diorites was formed in an active continental margin setting,resulting from subduction of the paleo Nenjiang-Heihe oceanic plate located between the Xing'an and Songliao blocks.