西藏努日晚白垩世埃达克岩:洋脊俯冲的产物

前人对冈底斯带晚白垩世埃达克岩的成因和地球动力学背景存在不同的认识.对努日地区的石英闪长玢岩开展了锆石U-Pb年代学、全岩地球化学及Hf同位素研究.结果表明，努日石英闪长玢岩侵位于96.5±1.3 Ma，以高SiO₂（63.96%~65.75%）、Al₂O₃（14.37%~15.99%）、MgO（2.12%~2.39%）、Sr（362×10-6~575×10-6，平均为467×10-6）含量，低Y（8.94×10-6~11.50×10-6）、Yb（0.81×10-6~1.06×10-6）含量及高Sr/Y比值（33.52~60.65）为特征，显示埃达克岩地球化学特征.岩石属低钾-中钾钙碱性准铝质花岗岩（A/CNK=0.81~0.96），富集LREE、亏损HREE，富集大离子亲石元素（LILE）、亏损高场强元素（HFSE），无明显负Eu异常.锆石ε_Hf（t）值为-0.3~+15.2（主要为+10.0~+15.2），二阶段模式年龄t_DM2为187~1 173（主要为187~516 Ma），表明源区以俯冲洋壳为主，并可能卷入了少量俯冲沉积物.岩石具有较高的Mg#值和相容元素Cr、Ni含量，表明熔体在上升过程中与地幔发生了相互作用.通过与南冈底斯典型埃达克岩对比，认为努日石英闪长玢岩是在洋脊俯冲背景下、穿过板片窗的高热流导致板片窗边缘的洋壳（及少量俯冲沉积物）部分熔融形成的. 

Late Cretaceous Adakite in Nuri Area,Tibet: Products of Ridge Subduction

There are different understandings of the genesis and geodynamic mechanism for the Late Cretaceous adakite of the Gangdese belt.In this paper, we present zircon U-Pb data, geochemical and Hf isotopic data for the quartz diorite porphyry from Nuri area.The results show that the quartz diorite porphyry was emplaced at 96.5±1.3 Ma.These rocks are characterized by high SiO₂(63.96%-65.75%), Al₂O₃(14.37%-15.99%), MgO (2.12%-2.39%), Sr (362×10-6-575×10-6, 467×10-6 on average), low Y (8.94×10-6-11.50×10-6), Yb (0.81×10-6-1.06×10-6) and high Sr/Y ratio (33.52-60.65), implying adakitic geochemical features.These rocks are low to medium-K, calc-alkaline and metalumious.They are enriched in LREE and depleted in HREE, together with enrichment in large-ion lithophile elements (LILE) and depletion in high field strength elements (HFSE), as well as small negative Eu anomalies.ε_Hf(t) values of zircons range from -0.3 to +15.2 (mainly between +10.0 and +15.2), with t_DM2 ranging from 187 Ma to 1 173 Ma (mainly from 187 Ma to 516 Ma), which indicates that the sources of these rocks were derived from subducted oceanic crust probably with a minor proportion of subducted sediments.High Mg# and compatible element Cr and Ni indicate that the melts have interacted with the mantle during ascent.By comparing with adakitic rocks from South Gangdese, we propose that high heat flow through a slab window induced partial melting of the oceanic crust at slab window edges to form the quartz diorite porphyry from Nuri, under the geodynamic setting of ridge subduction.