滇西腾冲地块勐连花岗岩及暗色包体锆石U-Pb年龄、地球化学和Sr-Nd-Pb同位素特征

滇西腾冲地块是东特提斯构造带的重要组成部分，广泛发育中生代和新生代岩浆作用。出露于腾冲地块东南部的勐连花岗岩及暗色包体的锆石U-Pb定年显示，勐连花岗岩形成时代为(127±2)Ma，暗色包体为(125±2)Ma。勐连花岗岩主要岩石类型为石英二长岩、花岗闪长岩和黑云母二长花岗岩，属于高钾钙碱性Ⅰ型花岗岩，兼具部分S型花岗岩的过铝质特征。虽然暗色包体与花岗岩形成时代基本一致，但是暗色包体的矿物粒度更小，稀土元素总质量分数((305.2～322.8)×10^-6)更高，同时具有较高的w(Nb)/w(Ta)值(18.2～19.5)、较低的Pb同位素组成和更亏损的ε_Nd(t)值，表明勐连花岗岩内暗色包体应为异源包体，可能是基性岩浆侵入酸性岩浆中淬冷的产物。勐连花岗岩的全岩Sr-Nd同位素随SiO₂并没有明显的变化趋势，暗示地壳物质混染作用并不显著。Ba-Sr图解显示勐连花岗岩主要经历了斜长石和钾长石的分离结晶过程。二阶段Nd模式年龄(1.37～1.58 Ga)和上地壳类似的Pb同位素组成暗示中元古代高黎贡山群是勐连花岗岩的主要物质源区。但与高黎贡山花岗岩相比，更亏损的ε_Nd(t)值暗示勐连花岗岩源区中存在亏损地幔物质的加入。整体上，全岩初始N(⁸⁷Sr)/N(⁸⁶Sr)值(0.710 020～0.712 001)和ε_Nd(t)值(-4.6~-2.1)暗示勐连花岗岩与拉萨地块北部岩浆带具有类似的形成环境。结合岛弧特征的构造环境和高w(Nb)/w(Ta)值的异源暗色包体，认为勐连花岗岩及暗色包体的形成可能是中特提斯洋俯冲-碰撞-闭合过程的产物。

Zircon U-Pb Ages,Geochemistry and Sr-Nd-Pb Isotope Characteristics of Menglian Granites and Their Dark Enclaves in Tengchong Block of Western Yunnan,China

The Tengchong block in western Yunnan is an important component of East Tethys tectonic belt, and the Mesozoic and Cenozoic magmatism is widely distributed. The zircon U-Pb ages of Menglian granites and their dark enclaves in the southeastern Tengchong block indicate that they are formed in (127±2)Ma and (125±2)Ma, respectively. The main rock types of Menglian granites are quartz monzonite, granodiorite and biotite monzogranite, and the geochemical characteristics show that they are high K calc-alkalinity I-type granites with the aluminum saturation index point to S-type granites. Although the dark enclaves and granites show similar zircon U-Pb ages, the dark enclaves display smaller grain size, higher REE contents ((305.2-322.8)×10^-6), higher w(Nb)/w(Ta) (18.2-19.5), lower Pb isotope compositions and depleted ε_Nd(t), indicating that the dark enclaves should be heterogenous, and may be quenched by the mafic and felsic magma mingling or mixing process. The constant Sr-Nd isotope compositions with varying SiO₂ contents suggest the crustal contamination is negligible. The Ba-Sr diagram implies the crystallization of plagioclase and K-feldspar for Menglian granites. The two-stage Nd model ages (1.37-1.58 Ga) and similar upper crust Pb isotope compositions indicate that the main material source of Menglian granites should be Mesoproterozoic Gaoligongshan Group. But in contrast to Gaoligongshan granites, the more depleted εNd(t) indicates that some mantle-derived materials should be involved. As a whole, the initial N(⁸⁷Sr)/N(⁸⁶Sr) (0.710 020-0.712 001) and ε_Nd(t) (-4.6-2.1) of Menglian granites are similar to the granites in the northern magmatic belt of Lhasa block, suggesting the same formation environment. Combining with the island arc tectonic environment and the heterogenous dark enclaves with high w(Nb)/w(Ta), the Menglian granites and their dark enclaves may be formed by the subduction-collision-closure processes of the middle Tethys ocean.