东昆仑五龙沟地区晚志留世A型花岗岩成因:U-Pb年代学、地球化学、Nd及Hf同位素制约

东昆仑造山带东段五龙沟金矿田内首次发现晚志留世A型花岗岩体.对其开展了锆石U-Pb年代学、岩石地球化学、Nd及Hf同位素研究，探讨岩体成因和构造背景.岩体LA-ICP-MS锆石U-Pb定年结果为420±3 Ma，为晚志留世岩浆活动产物.岩石具有高SiO₂（76.0%~78.4%）、K₂O（4.64%~5.22%）和Na₂O（2.93%~3.25%）含量，低FeOT（0.98%~1.45%）、MgO（0.11%~0.22%）和CaO（0.27%~0.79%）含量特征.样品富集大离子亲石元素（Rb、K、La）和LREE，亏损高场强元素（Nb、P、Ti）和HREE，具有强烈的Eu负异常（Eu/Eu*=0.09~0.12）.该岩体104×Ga/Al比值为3.09~3.15，具有A型花岗岩的特征.全岩ε_Nd（t）=-2.5~-2.2，对应的二阶段模式年龄t_DM2（Nd）=1 339~1 365 Ma.锆石ε_Hf（t）=-2.8~+2.1，二阶段模式年龄t_DM2（Hf）=1 269~1 583 Ma.地球化学、Nd及Hf同位素揭示该岩体为软流圈地幔部分熔融形成的幔源岩浆与其诱发的古老地壳物质混合形成.构造判别图解指示岩体具有A₂型花岗岩特征，形成于后碰撞伸展构造环境.结合和勒冈那仁和冰沟A型花岗岩体，认为东昆仑地区至少在晚志留世已进入伸展阶段. 

Origin of Late Silurian A-Type Granite in Wulonggou Area,East Kunlun Orogen: Zircon U-Pb Age,Geochemistry, Nd and Hf Isotopic Constraints

Late Silurian A-type granite is reported for the first time in the Wulonggou gold district, eastern segment of the Kunlun orogenic belt. This paper presents (1) laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb age for the Wulonggou A-type granite to determine precisely the time of the magmatism, (2) geochemical, Nd and Hf isotope data for the Wulonggou A-type granite to constrain the petrogenesis and tectonic setting. The LA-ICP-MS U-Pb analyses of zircon from the Wulonggou A-type granite yielded a weighted mean 206Pb/238U age of 420±3 Ma, indicating that it was emplaced in the Late Silurian. All rock samples are enriched in SiO₂ (76.0%-78.4%), K₂O (4.64%-5.22%) and Na₂O (2.93%-3.25%), but with relatively low concentrations of FeOT (0.98%-1.45%), MgO (0.11%-0.22%) and CaO (0.27%-0.79%). Samples are also enriched in large ion lithophile elements (Rb, K, La) and light rare earth elements, but depleted in high field strength elements (Nb, P, Ti) and heavy rare earth elements with strong negative Eu anomalies (Eu/Eu*=0.09-0.12). The 104×Ga/Al ratios of the Wulonggou A-type granites vary from 3.09 to 3.15. Mineralogy and geochemistry of the rocks show an affinity with aluminous A-type granite. The ε_Nd(t) values of whole-rock range from -2.5 to -2.2 with corresponding two-stage Nd model ages ranging from 1 339 to 1 365 Ma. The ε_Hf(t) values of zircon vary from -2.8 to +2.1 and two-stage Hf model ages range from 1 269 to 1 583 Ma. The Nd and Hf isotopic data imply that different source materials have contributed to the magma genesis. Integrated geological, geochemical and isotopic data suggest that Wulonggou A-type granite is most likely generated via the mixture of asthenosphere-derived magma and the ancient crustal materials. The A-type granite was marked by A₂ type and was formed in post-collisional extensional tectonic environment. In combination with the A-type granites such as those in the Helegangnaren and Binggou, it is concluded that the eastern Kunlun area had begun its post-orogenic extension stage since Late Silurian, and this stage continued at least about 34 Ma.