西藏玛日埃错地区花岗斑岩岩石成因及其对班公湖-怒江缝合带中段演化的制约

为进一步了解班公湖-怒江缝合带中段闭合时限及构造演化模式，对西藏双湖县玛日埃错地区花岗斑岩进行了锆石U-Pb定年、岩石地球化学和Sr-Nd-Pb同位素方面的研究.结果显示，玛日埃错地区花岗斑岩形成于晚白垩世晚期（78.3±0.4 Ma）；具有高硅（72.41%~74.06%）、富碱（Na₂O+K₂O=6.66%~7.14%）的特点，属钙碱性系列，A/CNK值介于1.01~1.07，具弱过铝质特征；稀土配分模式为轻稀土富集、重稀土亏损的右倾型，并且富集大离子亲石元素（如Rb、Ba、U、K、Th等），亏损高场强元素（如Nb、Ta、P、Ti），显示其源区残留相为石榴石-角闪岩相.岩石具有相对较低的87Sr/86Sr初始比值（0.705 2~0.706 0）和较低的正ε_Nd（t）值（1.5~2.3），及相对年轻的二阶段模式年龄（t_DM2=692~758 Ma）；Pb同位素组成相对均一，具造山带演化特征.综合分析研究表明，玛日埃错地区花岗斑岩可能起源于伸展背景下的新生地壳部分熔融，源区富含流体，残留相以石榴石-角闪岩为主，不含斜长石.玛日埃错花岗斑岩形成于班怒洋盆闭合、两侧地体碰撞后的伸展背景下，表明班公湖-怒江缝合带中段地区在晚白垩世晚期已经进入后碰撞伸展阶段. 

Petrogenesis of Granite Porphyry in Mariaicuo Area,Shuanghu County,Tibet, and Constraints on the Evolution in the Middle Section of Bangonghu-Nujiang Suture Zone

LA-ICP-MS zircon U-Pb dating, geochemical and Sr-Nd-Pb isotopic data analysis were conducted on the granite porphyry in Mariaicuo area in order to determine its formation time, petrogenesis, structural setting and geological significance. LA-ICP-MS zircon U-Pb dating of the granite porphyry in Mariaicuo yields a crystallization age of 78.3±0.4 Ma, indicating that the rock formed in the late of Late Cretaceous. Petrogeochemically, the samples of the rocks are high in SiO₂ (72.41%-74.06%) and rich in Na₂O+K₂O of 6.66%-7.14%, which belong to calc-alkaline series. Its A/CNK ratios of 1.01-1.07 are of characteristics of weakly peraluminous granites. They are enriched in the large-ion lithospile elements of Rb, Ba, U, K and Th, and strongly depleted in high field strength elements Nb, Ta, P, and Ti. The REE distribution mode is obvious right-leaning, indicating that it remains garnet-amphibolite facies in source area. Their isotopic compositions are characterized by low initial (87Sr/86Sr)_i of 0.705 2-0.706 0 and low positive ε_Nd(t) (1.5-2.3) values and relatively young second-stage model age (t_DM2=692-758 Ma) of crust. It has relatively homogeneous Pb isotope composition with the evolution of orogenic characteristics. Comprehensive analysis shows that the granite porphyry in Mariaicuo area may originate from the new partial melting of crust within extensional background, and its source region is enriched in fluid and remains garnet-amphibolite facies, excluding plagioclase. It formed in the post-collision extensional environment, which occurred after the closure of Bangonghu-Nujiang Tethys Ocean (BTO) and collision between tarranes on both sides, when the middle section of Bangonghu-Nujiang suture zone has entered the extension phase in Late Cretaceous.