冈底斯南缘桑日地区渐新世花岗质岩石成因及其形成机制  

Lithogenesis and formation mechanism of Oligocene granitic rocks of Sangri area in the southern Gangdese

作　　者：董洪凯 段炳鑫 唐建科 提振海 王硕 刘思林 刘广 DONG Hong-kai;DUAN Bing-xin;TANG Jian-ke;TI Zhen-hai;WANG Shuo;LIU Si-lin;LIU Guang(Institute of Regional Geological Survey of Hebei Province,Langfang 065000,China;The 5th Geological Brigade of Tibet,Golmud 816099,China)

机构地区：[1]河北省区域地质调查院,河北廊坊065000 [2]西藏自治区地质矿产勘查开发局第五地质大队,青海格尔木816099

出　　处：《岩石矿物学杂志》2024年第6期1465-1483,共19页Acta Petrologica et Mineralogica

基　　金：冈底斯-喜马拉雅铜矿资源基地调查项目(DD20160015⁃07)。

摘　　要：桑日地区渐新世花岗质岩石主要由石英二长闪长岩、二长花岗岩和正长花岗岩组成,锆石U-Pb定年结果分别为31.2±0.5 Ma、31.0±0.5 Ma和29.0±0.1Ma,侵位时代为渐新世。该期岩石发育大量不同形态的暗色包体;K_(2)O含量3.24%~5.73%,属高钾钙碱性岩石,具偏铝质向过铝质岩石演化特征(A/CNK值分别为0.90~1.02、0.98~1.07和1.10~1.29);属轻稀土元素富集型,具铕亏损呈由弱到强的演化特征(Eu/Eu^(*)值分别为0.66~0.75、0.40~0.55和0.04~0.09);相对富集Rb和亏损Ba、Sr等大离子亲石元素,富集Th、U和亏损Nb、P、Ti(Ta)等高场强元素并富集(La、Ce)Nd、Sm等轻稀土元素;SiO_(2)与主量、稀土、微量元素形成的拟合曲线耦合程度较高,为同源岩浆不同阶段演化的产物。锆石饱和温度(t_(Zr))平均值为700℃左右,可能代表了熔体形成的初始温度。通过对包体、构造样式、岩石地球化学特征、Hf同位素等分析,认为该期岩石成因类型属I型花岗岩;形成于印度-亚洲大陆陆内碰撞快速抬升的构造环境,其形成机制为印度大陆板片在低角度俯冲过程中发生了拆沉或对流剥离,板片下沉过程中释放少量流体以致地幔局部熔融发生底侵作用并诱发大量拉萨地体下地壳熔融形成混源岩浆,并沿桑日地区北东向张性断裂就位,最终导致青藏高原陆壳进一步增生。The Oligocene granitic rocks in the Sangri area are mainly composed of quartz monzodiorite,monzogranite and syenogranite.The weighted average ages are 31.2±0.5 Ma,31.0±0.5 Ma and 29.0±0.1 Ma,respectively,and the emplacement age is the Oligocene.The rocks are developed abundant melanocratic enclaves of different shapes.The K_(2)O content is 3.24%~5.73%,belonging to the high potassium calc-alkaline rock,with the characteristics of metaluminous to peraluminous rock evolution(A/CNK:0.90~1.02,0.98~1.07,1.10~1.29).The rocks are LREE enriched type and the europium depletion shows an evolutionary characteristic from weak to strong(Eu/Eu^(*):0.66~0.75,0.40~0.55,0.04~0.09).They are relatively enriched in LILEs such as Rb,HFSEs such as Th,U,and LREEs such as(La,Ce)Nd,Sm,but depleted in LILEs such as Ba,Sr and HFSEs such as Nb,P,Ti(Ta).The fitting curves formed by SiO_(2) vs.other major elements,rare earth elements and trace elements are highly coupled and the R2 values are all greater than 0.90,which indicated they were products of the evolution of the same magma at different stages.The average zircon saturation temperature(t_(Zr))is about 700℃which may represent the initial temperature of melt formation.Based on the analysis of enclaves,structural styles,petrogeochemical characteristics and Hf isotopes,etc.,the petrogenetic type of Oligocene granites in the Sangri area belongs to I-type granite.They were formed in the tectonic environment of rapid uplift due to the intracontinental collision of the Indian and Asian continents.The formation mechanism was the delamination or convective removal of the Indian continental slab during low-angle subduction.A small amount of fluid was released during the slab sinking process which caused the partial melted mantle-derived magma underplating and induced lower crust melting of the Lhasa terrane to form mix-derived magma.At the same time,the magma emplaced along the northeast-trending extensional fault in the Sangri area,eventually further leading to the continental crust accreti

关 键 词：桑日地区 渐新世 Ⅰ型花岗岩 混源岩浆 陆壳增生 

分 类 号：P588.121[天文地球—岩石学]