大兴安岭扎兰屯地区四班岩体岩石成因及构造环境研究

大兴安岭扎兰屯地区四班岩体主要由正长花岗岩和二长花岗岩组成,内部发育细粒闪长质包体。二长花岗岩和正长花岗岩LA-ICP-MS锆石U-Pb定年结果分别为303±3Ma和291±3Ma,属于晚古生代岩浆活动的产物。四班花岗质岩石高硅(67.9~77.5 wt%)、富碱(K_2O+Na_2O=7.55~10.79 w%)、相对高铝(Al_2O_3=12.05~16.33 wt%),富集轻稀土元素(LREE)和大离子亲石元素(LILE),而亏损高场强元素(Nb、Ta、Ti和P等),属于高钾钙碱性I型花岗岩。四班花岗质岩石内部发育的闪长质微粒包体及花岗岩与其伴生的基性岩的"一锅粥"现象,表明四班花岗质岩石具有岩浆混合成因的特征,地球化学特征也支持上述观点。四班岩体显示后碰撞岩浆岩的岩石学及地球化学特征,为后碰撞阶段岩石圈地幔拆沉减薄壳幔相互作用的产物。

A STUDY ON THE PETROGENESIS AND TECTONIC SETTING OF THE SIBAN GRANITE MASS IN ZHALANTUN AREA, GREAT KHINGAN

The Siban rock mass in Zhalantun area of Great Khingan is mainly composed of syenogranites and monzonitic granites with fine-grained diorite enclaves developed inside. LA-ICP-MS zircon U-Pb dating results reveal that the crystallization ages of the monzonitic granite and syenogranite in the Siban rock mass are 291±3 Ma and 303±3 Ma respectively, proving that the Siban rock mass emplaced during the late Paleozoic era. The geochemical results of the whole-rock major and trace elements exhibit that the Siban granite yields high SiO₂ (67.9~77.5 wt%), alkaline components (w (Na₂O+K₂O)=7.55~10.79 wt%) and Al₂O₃ (12.05~16.33 wt%). It is also highly enriched with LREE and LILE while with a depletion of HFSE (e.g. Nb, Ta, Ti and P), which is comparable to high K calc-alkalne I-type granite. The existence of fine-grained diorite enclaves and associated basic rocks developed inside the Siban granlte reveals that the Siban granite has the characteristics of magma mixing between mantle and crustal magmas and subsequently underwent the fractional crystallizations of pyroxene, amphibole, Ti-enriched minerals, plagioclase and apatite, which is supported by their geochemical features. Taken all together, the petrologies, geochemistries and petrogenesises indicate that the Siban rock mass is akin to the post-collision granite and formed by mantle-crust interactions under the delamination of lithospheric mantle environment.