大兴安岭巴雅尔吐胡硕地区碎斑熔岩锆石U-Pb年龄、地球化学特征及其地质意义

Geochemical characteristics and zircon U-Pb ages of porphyroclastic lava in the Bayaerhushuo area, the south-central segment of Great Xing’an Range, and its geological significance

在大兴安岭中南段巴雅尔吐胡硕地区发现一套碎斑熔岩。根据野外调查,可分为中心相的细微粒斑状石英二长岩(-石英二长斑岩)和边缘相的英安流纹质碎斑熔岩。通过SHRIMP锆石U-Pb测年,获得细微粒斑状石英二长岩的年龄加权平均值为137.4±0.9Ma(MSWD=1.13),英安流纹质碎斑熔岩的年龄加权平均值为135.2±0.8Ma(MSWD=1.17),二者的形成时代均属于早白垩世早期。由元素地球化学分析可知,6件样品均属于过铝质的高钾钙碱性A型花岗岩,且具有完全一致的微量元素蛛网图和稀土元素配分曲线,均具有明显的负Eu异常,均富集大离子亲石元素Rb、Pb和轻稀土元素,亏损高场强元素Nb,Th较富集,Ba、Sr、Eu具有一定亏损,Pb强烈富集,表明岩石来源于地壳部分熔融。在构造环境判别分析的基础上,提出研究区碎斑熔岩属于碰撞后或造山期后的张性构造环境花岗岩(A2型花岗岩),形成于拉张环境,代表了伸展的大地构造背景。结合大兴安岭地区的大地构造演化特征,其形成与蒙古-鄂霍茨克闭合造山后的岩石圈伸展作用有关。

The porphyroclastic lava is located in the Bayaerhushuo area, the south-central segment of Great Xing’an Range, which can be divided into two parts, the central facies of fine-grained quartz monzonites(-quartz monzonitic porphyry) and the margin facies of dacitic rhyolitic porphyroclastic lava. Based on the SHRIMP zircon U-Pb dating, the porphyry monzonitic granite and dacitic ryholititic porphyroclastic lava samples yielded U-Pb ages of 137.4±0.9 Ma(MSWD=1.13) and 135.2±0.8 Ma(MSWD=1.17)respectively, indicating that the porphyroclastic lava samples in the study area were formed during the Early Cretaceous. The results of geochemical analyses show that the six samples are all high-K calc-alkaline A-typed granites and have the same characteristics in the trace element spidergrams and REE patterns. The features of evident negative Eu anomaly, enrichment in the LILEs Rb and Pb,LREE and Th, depletion in HFSE Nb, Ba, Sr and Eu, reveal that these samples may originate from the partial melting of crust. By tectonic environment discriminations, the porphyroclastic lava are plotted into the A2-typed granite,revealling the post-collisional extension environment. Combined with the Mesozoic tectonic evolution of the Great Xing’an Range, the formation of the Early Cretaceous porphyroclastic lava should be related to the lithospheric extensional envrinment caused by the closure of MongoliaOkhotsk ocean.