The Great Xing'an Range is one of the regions with a development of continental volcanic rocks in the Northeastern China.The volcanic rocks distribute widely, their lithofacies assemblages change greatly, and their formation is complex.We conducted a research on the geology, petrography, geochronology, and Hf isotope geochemistry for the Mesozoic volcanics erupted and emplaced in the granite zone in Xiaomoerke area.The results indicate that: the volcanic rocks in this area can be roughly divided into three assemblages: the first assemblage is mainly of dacitic pyroclastic rocks with dacite interlayer, presenting the characteristics of volcano outbreak;the second one is of basaltic trachyte andesite-trachyte andesite, showing the characteristics of overflow of volcanic magma;the third one is of trachy andesitic fused node clastic rock and breccia trachyandesite lithic crystal tuff combination.The studies on the geochemistry of volcanic lava and tuff reveal that they are all rich in alkali (w(Na2O+K2O)=4.88%-7.12%)and belong to the high potassium calcium alkaline series.They possess the similar trace element geochemical characteristics: significant enrichment in large ion lithophile elements (Ba, K etc.) and apparent loss of the HFSE elements (Nb, Ta, Ti, HREE).The fractionation degree of rare earth elements is moderate(LREE/ HREE=8.4-8.5) and the negative Eu anomaly is small(δEu is 0.91-1.02).The Above characteristics indicate that these volcanic rocks are the products of homologous magma chamber crystallization, differentiation, and evolution, and the attribute of the magma source is of a mixed transition type of mantle and crust mantle, or of E-MORB or metasomatism of oceanic crust.The two zircon U-Pb ages obtained from the chips of dacitic crystal tuff and trachyandesitic tuff are (124.8±1.0) Ma and (123.3±1.3) Ma respectively, with Hf isotope(εHf(t)=1.7-9.7,TDM=705-407 Ma, TDMC=1 464-748 Ma).In combination with the research progress of regional geodynamics, we further derive that the volcanic activity occurred in the continental margin magmatic arc environment under the subduction of the Mesozoic Paleo Pacific plate to the eastern continent of China;and this coincided with the peak period (120 Ma±) of northeastern area large-scale magma underplating and lithosphere delamination , the initial magma should be adakitic magmas formed through the Paleo Pacific plate deep subduction p.The transitional rocks of adakitic magma and island arc magma might have been formed from crustal the material occurred with a certain degree of contamination during magma invasion and lithosphere delamination;in respect to the compatibility of ore-forming elements, it is confirmed that this period of magmatism provided Mo, Cu and Ag for ore-forming fluids.%大兴安岭是中国东北部陆相火山岩发育的地区之一,其不仅分布广泛,而且空间岩相组合变化较大、成因复杂.笔者对大杨树火山盆地西缘喷发就位在花岗岩带的小莫尔可地区中生代火山岩的地质、岩相学、年代学、元素和Hf同位素地球化学进行了研究.结果表明,该区出露火山岩可划分3个阶段,第一阶段主要为英安质火山碎屑岩夹英安岩组合,呈爆发式火山喷发特征;第二阶段为玄武质粗面安山岩、粗面安山岩组合,呈溢流式火山喷发特征;第三阶段是玄武粗面安山质熔结火山碎屑岩(部分含角砾).各阶段代表性火山熔岩以及熔结火山碎屑岩的元素地球化学特征揭示:均为富碱(w(Na2O+K2O)=4.88%~7.12%),属高钾钙碱性系列;具有相似的痕量元素地球化学分馏特征,即明显富集Ba、K、LRRE等大离子亲石元素,亏损Nb、Ta、Ti、HREE等高场强元素;稀土元素分馏程度中等(LREE/ HREE=8.4~8.5)、Eu负异常不明显(δEu为0.91~1.02).这些特征表明它们是同源岩浆房结晶分异演化的产物,岩浆源区性质呈现地幔与壳幔混合过渡类型的属性,或呈现E-MORB性质的源区或交代洋壳性质的源区.鉴于获得的英安质岩屑晶屑凝灰岩(HSY-1)和玄武粗面安山质岩屑晶屑熔结凝灰岩(P2121)单颗粒锆石U-Pb同位素定年分别为(124.8±1.0)、(123.3±1.3)Ma,εHf(t)为1.7~9.7、TDM =705~407 Ma、TDMC=1 464~748 Ma,并结合区域地球动力学研究进展,可进一步得出:火山作用背景应属于中生代古太平洋板块向中国东部大陆俯冲的大陆边缘岩浆弧环境,适值东北地区大规模岩浆底侵、岩石圈拆沉作用的峰期(约120 Ma),初始岩浆应是古太平洋板块深俯冲作用过程形成的埃达克质岩浆,而呈现埃达克质岩浆与岛弧性质的过渡岩石学、地球化学特征,可能是在岩浆底侵、岩石圈拆沉过程与地壳物质发生一定程度的混染作用;rn并从成矿元素的相容性角度确认该期岩浆作用具有提供Mo、Cu和Ag成矿流体的可能.
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