大兴安岭北部兴隆地区寒武纪侵入岩锆石U-Pb年代学、地球化学及其构造意义

Zircon U-Pb Geochronology and Geochemistry of Cambrian Plutons in Xinglong Area of Northern Da-Hinggan Mountains: Implications for Tectonic Evolution

大兴安岭北部兴隆地区位于新林-喜桂图缝合带北部,额尔古纳地块南缘,在大地构造上属于兴蒙造山带北段.然而,由于资料有限,前人对该区寒武纪侵入岩的构造产出环境仍存有争议,从而制约了人们对于新林-喜桂图洋早期构造演化的正确认识.选取兴隆地区出露的寒武纪内河岩体为研究对象,通过岩相学、锆石U-Pb年代学及地球化学研究,以期限定其形成时限,探讨其岩石成因和构造背景,进而为解决上述问题提供新的证据.内河岩体中石英二长岩及侵入其中的辉石闪长岩的LA-ICP-MS锆石U-Pb年龄分别为508±3 Ma和507±5 Ma,证明其形成时代均为中寒武世.石英二长岩和辉石闪长岩样品为高钾钙碱性和钙碱性系列,同时表现出轻稀土元素富集和不同程度的亏损Nb、Ta和Ti.两组样品的Nb/Ta平均值分别为22.41和17.12,较高于原始地幔平均值,MgO的含量(<8%)和Mg#(42~70)值较低.上述地球化学特征为典型俯冲带岩石的特征,暗示内河岩体为经历了俯冲板片析出流体交代作用的原始地幔部分熔融的产物.结合区域已有近同时代侵入岩、蛇绿混杂岩和蓝片岩资料,推测内河岩体可能形成于活动大陆边缘构造环境,为寒武纪期间新林-喜桂图洋向北俯冲的弧岩浆作用的产物.研究结果为兴蒙造山带北段在寒武纪期间仍处于俯冲构造环境提供了重要依据.

The Xinglong area in the northern Da-Hinggan Mountains is located in the north of the Xinlin-Xiguitu suture zone and the southern margin of the Erguna block and belongs to the northern Xingmeng orogenic belt.However,the tectonic settings for the Cambrian plutons have been still in debate due to the absence of available data,which hampers our understanding of the early tectonic evolution of Xinlin-Xiguitu Ocean.In this study,it carries out a detailed investigation on the Neihe pluton from the Xinglong area using zircon U-Pb dating and whole-rock geochemistry to constrain the age and petrogenesis,and provides new evidence for solving the problems.The LA-ICP-MS zircon U-Pb ages for the quartz monzonite and pyroxene diorite are 508±3 Ma and 507±5 Ma,respectively,indicating that they were formed during the Middle Cambrian.Geochemically,the quartz monzonite and pyroxene diorite are high-K calc-alkaline and calc-alkaline series,and both are enriched in light rare earth elements and show variable degrees of negative anomalies of Nb,Ta and Ti.The Nb/Ta ratios for the quartz monzonite and pyroxene diorite are 22.41 and 17.12,respectively,and they are low in the contents of the MgO(<8%)and Mg#values(42-70).These geochemical features are typical of subduction-related rocks,suggesting that the studied rocks may be generated by partial melting of a primitive mantle source modified by subduction-related fluids.Combined with available data from coeval plutons,ophiolitic mélanges,and blueschists,the Neihe pluton should be generated in an active continental margin setting related to the Cambrian northward subduction of the northern Xinlin-Xiguitu Ocean.New findings provide important evidence for the early subductionrelated tectonic evolution of the Xingmeng orogenic belt.