小兴安岭东南晚奥陶世鹤林侵入杂岩体成因探讨

Petrogenesis of the Late Ordovician Helin Intrusive Complex in the Southeastern Xiaoxinganling Mountains

小兴安岭东南晚奥陶世鹤林侵入杂岩体岩性变化较大,由角闪辉长岩—(石英)闪长岩—英云闪长岩—二长花岗岩组成,不同岩石类型在野外宏观上呈相互侵入、包裹和渐变的接触关系;在(石英)闪长岩、英云闪长岩和二长花岗岩中均发育微细粒闪长质包体,包体多具明显塑性流变特点的浑圆外形和典型岩浆结构、针状磷灰石及捕获的寄主岩钾长石、石英斑晶,为MME型岩浆混合成因包体;杂岩体相对富集LILE(Ba、Sr、Rb)、HFS(U、Th、Zr、Ce)元素等,显示出壳源特点,而较高的εNd(t)值(-2.38^-3.77)显示以幔源为主的特点。研究表明杂岩体具明显的壳幔岩浆混合成因的岩相学、岩石化学和地球化学特征,形成于基性岩浆底侵作用下的陆缘弧型活动大陆边缘构造环境。

The Late Ordovician Helin intrusive complex in the southeast of Xiaoxinganling Mountains consists of a series of different rocks, including amphibole gabbro, （quartz） diorite, tonalite and monzonitic granite. Intrusive, inclusive and gradual transitional contact relationship has been recognized among different rock types. Some microgranular dioritic enclaves have been discovered in （quartz）diorite, tonalite and monzonitic granite. The enclaves have mainly ovoid-like shape, possibly caused by plastic flow, typically magma structure, acicular apatite as well as K-feldspar and quartz phenocrysts of host rocks, indicating an origin of magma mingling enclaves （MME）. The intrusive complex is characterized by enrichment of large ion lithophile elements LILE （ Ba, St, Rb） and HFS （ U, Th, Zr, Ce） relative to primitive mantle, suggesting derivation of crust. However relatively high εNd （t） values（ - 2. 38 to - 3. 77） reflects a mantle-derived source. In summary, petrographic petrochemical and geochemical characteristics of the Helin complex suggest mixed crust-mantle origin. It can be attributed to underplating of mafic magma in an active continent marginal arc setting.