冈底斯东段古新世朱拉岩体LA-ICP-MS锆石U-Pb定年和岩石地球化学特征

摘要

Field investigation has found that Zhula intrusion consists of biotite monzogranite with abundant dioritic enclaves, and there exists gradually transitional relationship between the enclaves and their host rock. Zircon LA-ICP-MS U-Pb dating of 3 samples from the granite yielded the ages of 64,6±0.8Ma,64.3± 0.8Ma and 63.9±0,5Ma respectively, with a surface age of 776Ma of inherited zircon. A dioritic enclave samples yielded the age of 66.1±0.3Ma. The dioritic enclaves have rich Fe and high alkalinity as well as poor Si, Mg and Ca, thus belonging to aluminous, high-K cale-alkaline series; they have Nb/Ta ratio of 15.9, Sr/Y ratio of 3.69, LREE/ HREE ratio of 1.92, substantial negative Eu anomalies, higher LILE(Rb,K) and RHE(U,Th) concentrations, lower HFSE (Nb, Ta, Sr, Ti) concentrations relative to the primordial mantle, thus indicating that dioritic magma originated from partial melting of the mantle. The host rocks have rich Si, K, and poor P, belonging to meta-aluminous and aluminous and cale^alkaline to high-K cale-alkaline series, they have Nb/Ta ratio of 15.9, higher LILE(Rb, K), lower HFSE(Nb, Ta, Ba, P, Ti), similar to calc-alkaline rocks in island arc or active continental margin. The host rocks and enclaves might have been derived from the same magmatic processes- The enclave rocks originated from crystallization differentiation of mantle source magma assimilating and contaminating some crust materials, while the host rocks originated from crystallization differentiation of large quantities of crustal magma mixed with some mande sourced magma. In combination with previous research results, the authors hold that the petrogenesis might have been related to the northward subduction of the Yarlung Zangbo Ocean.%朱拉岩体位于冈底斯带东部,主要由黑云母二长花岗岩组成,其中发育闪长质包体,二者呈渐变过渡关系.3件寄主岩石LA-ICP-MS锆石U-Pb年龄加权平均值分别为64.6Ma±0.8Ma、64.3Ma±0.8Ma和63.9Ma±0.5Ma,含776Ma的继承岩浆锆石.1件闪长质包体样品年龄加权平均值为66.1Ma±0.3Ma.闪长质包体低Si,中Mg(Mg#平均41.1),属铝质高钾钙碱性系列,Nb/Ta值为22,Sr/Y值为3.69,稀土元素分馏较低,LREE/HREE平均值为1.92,Eu强烈亏损,富集大离子亲石元素(Rb、K、U、Th),亏损高场强元素(Nb、Ta、Sr、Ti),暗示岩浆成分主要为幔源.寄主岩石富Si、K,贫P,属铝质-偏铝质钙碱性-高钾钙碱性系列,Nb/Ta值15.9,富集大离子亲石元素(Rb、K、Th),亏损高场强元素(Nb、Ta、Ba、P、Ti),具有活动陆缘钙碱性岩系的微量元素分布特征.寄主岩石与闪长质包体具有密切的成生联系,闪长岩形成于富集俯冲带组分的地幔熔体,在上升过程中混染了一定量的壳源物质结晶分异产物,寄主岩石则为底侵作用产生的大量壳源熔体与少量幔源熔体混合并发生一定程度分离结晶后的产物.结合前人的研究成果,认为古新世朱拉岩体与雅鲁藏布江大洋的向北消减有关.